Aircraft Wire Degradation Study

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Meiner, Noel Walz, and Cesar Gomez* 8. Performing Organization Report No. 9. Performing Organization N Raytheon Technical Services Company LLC 6125 East 21 st Street Indianapolis, IN219-2058 *Federal Aviation Administration William J. Hugh Technical Center Airport and Aircraft Safety R&D Division Air Worthiness Assurance Branch Atlantic City International Airport, NJ 08405 10. Work Unit No. (TRAIS) ame and Address 46 es 11. Contract or Grant No. DTFA 03-02-C-00040 12. Sponsoring Agence and Address U.S. Departmentf Transportation Federal Aviation Administration Air Traffic Organization Operations Planning Office of Aviati Research and Development Washington, DC 20591 13. Type of Report and Period Covered Final Report 08/01-04/05 y Nam o on 14. Sponsoring Agency Code ANM-111 15. Supplementary Notes 16. Abstract The purpose of this initial research program was to evaluate the aging characteristics of three types of aircraft electrical wi re: polyimide, polytrafluoroethylene/polyimide composite, and polyvinyl chloride/nylon. In addition, predictive models for the aging of these wire types were developed. These wire types were chosen because of their widespread use in commercial aircraft and the amount of reported incidents concerning them. The factors that cause the wire insulation to degrade were examined and techniques to determine when a wire will no longer be capable of transfer of electrical current were evaluated. The results in this study provided n aircraft. The results founommittee Intrusive Inspec te a platform to evaluate existing and new test methods that could be used to monitor the aging of wire i d were similar to the aging samples found from the Aging Transport Systems Rulemaking Advisory C tion Report. 17. Key Words Aged aircraft, Wire degradation, Electrical interconnect wire, Intrusive inspection, Electrical distribution, Aged wire 18. Distribution Statement This document is available to the U.S. public through the National Technical Information Service (NTIS), Springfield, Virginia 22161. 19. Security Classif. (of this report) Unclassified 20. Security Classif. (of this page) Unclassified 21. No. of Pages 275 22. Price Form DOT F 1700.7 (8-72) Reproduction of completed page authorized ACKNOWLEDGEMENTS members: Tim Baer 4Qualstat Services a National Laboratories echanical Design Corp. Jim Meiner 4Raytheon Technical Services Company n Technical Services Company, retired ical Design Corp. al Aviation Administration William J. Hughes Technical Center tributors included the following: The Boeing Company e Safety Board Raychem Wire Products, Tyco Corp. United Airlines ited States Air Force, Wright-Patterson Laboratories . Werner 4Sandia National Laboratories D. Lee 4Naval Air Systems Command D. Johnson 4United States Air Force S. Zingheim 4Tyco P. LaCourt 4DuPont ing group 4Raytheon Technical Services Co The core team included the following Robert Bernstein 4Sandi Bill Linzey 4Lectrom Robert Lofaro 4Brookhaven National Laboratories Joe Kurek 4Raytheon Technical Services Company Ron Peterson 4Raytheo Dr. Noel Turner 4Lectromechan Mike Walz 4Feder The aircraft industry con Airbus Industries Airtran Airlines Bombardier Aerospac DuPont n National Transportatio Naval Air Systems Command Northwest Airlines QinetiQ Tensolite Un Assistance was necessary from the following: R. Pappas and C. Gomez 4Federal Aviation Administration A. Bruning, M. Traskos, and S. Mishra 4Lectromechanical E. Grove, M. Villaran, and L. Gerlach 4Brookhaven National Laboratories Kathy Alam and P Wirmpany David Puterbaugh 4Analog Interfaces iii/iv TABLE OF CONTENTS Page xv 1 1 1 4 4 7 eriment Setup 8 t Methods and Procedures 11 ng Process 13 evelopment 13 ST RESULTS 15 g Data 15 erature 16 24 echanical Stress Cycles 24 et and Dry 28 3.6.3 Insulation Tensile and Elongation 31 33 34 34 36 39 41 AD TEST RESULTS 41 4.1 The PTFE/Polyimide Composite Aging Data 41 4.2 Temperature 42 4.3 Oxidation 46 4.4 Electrical Stress 47 4.5 Mechanical Stress Cycles 47 EXECUTIVE SUMMARY 1. INTRODUCTION 1.1 Purpose 1.2 Background 2. EVALUATION APPROACH 2.1 Test Program 2.2 Evaluation Method 2.3 Wire Samples 7 2.4 Exp s 2.5 Te 2.6 The Agi 2.7 Model D 3. POLYIMIDE AGING AND TE 3.1 Polyimide Agin 3.2 Temp 3.3 Oxidation 23 3.4 Electrical Stress 3.5 M 3.6 Testing Results 25 3.6.1 Visual Examination 25 3.6.2 Insulation Resistance W 3.6.4 Inherent Viscosity 3.6.5 Dynamic Cut-Through 3.6.6 Weight 3.6.7 Thermogravimetric Analysis 3.7 Model Development 3.8 Discussion of PI 4. THE PTFE/POLYIMIDE COMPOSITE AGINGN v 4.6 Testing Results 47 4.6.1 Visual Examination 48 4.6.2 Insulation Resistance Wet and Dry 51 tion Tensile and Elongation 53 4.6.4 Inherent Viscosity 54 Cut-Through 55 4.6.6 Weight 56 ogravimetric Analysis 5 4.7 Model Development 60 6 5. E/POLYAMIDE AGING AND TEST RESULTS 62 /Polyamide Aging Data 62 6 69 5.6 Testing Results 69 5.6.1 Visual Examination 70 tance Wet and Dry 73 Tensile and Elongation 75 ic Cut-Through 77 alysis 78 form Infrared Spectroscopy 5.7 5.8 6. CONC 7. RECOM 92 . REFERENCES 93 9. Pro B 4Discussion 4.6.3 Insula 4.6.5 Dynamic 4.6.7 Therm7 4.8 Discussion of CP Wire 2 POLYVINYL CHLORID 5.1 Polyvinyl Chloride 5.2 Temperature 3 5.3 Oxidation 68 5.4 Electrical Stress 69 5.5 Mechanical Stress Cycles 5.6.2 Insulation Resis 5.6.3 Insulation 5.6.4 Dynam 5.6.5 Weight 77 5.6.6 Thermogravimetric An 5.6.7 Fourier Trans83 Model Development 87 Polyvinyl Chloride/Nylon Discussion 88 LUSIONS 89 MENDATIONS 8 RELATED DOCUMENTS 93 APPENDICES A 4cedure vi table Failures (Uncontrolled Perturbations) to the Wire Aging Process and Aircraft Wiring Terminology D 4Te E 4Qu F 4Samization and Router for Group 10 Setup 2PI70H G 4Te C 4Single-Event Nonpredic st Plan ality Plan ple Work Author st Methods Details and Discussion H 4Model Development vii LIST OF FIGURES Figure Page 2 2 Types of Wire Failures 3 5 Inverse Temperature Arrhenius Relationship of PI Wire 17 6 Arrhenius Relationship of PI Wire 18 7 Temperature Arrhenius Relationship of PI Wire 18 8 Comparison of PI Dynamic Stressors and Static Stressors 20 9 Additive Effect of PI Dynamic and Static Stressors 21 10 Polyimide Stressor Relationships at Multiple Temperatures 22 11 Life as Log of Hours for All PI Data Points 23 12 Failure Time of PI Specimens at Different Airflow Rates 24 13 Average Cycles to Failure vs Temperature 25 14 Progression of Insulation Damage, Aged at 250°C 26 15 Progression of Insulation Damage, Aged at 300°C 26 16 Unaged PI Wire and Aged Wire (Static) 27 17 Unaged PI Wire and Aged Wire (Dynamic) 27 18 Unaged PI Wire and Aged Wire (75 Hours) 28 19 Unaged PI Wire and Aged Wire (180 Hours) 28 20 Wet IR Results for PI 29 21 One-Minute Dry IR Results for PI 30 22 Ten-Minute Dry IR Results for PI 30 23 Tensile Strength Results for PI Wires 31 1 Wiring Conditions From Intrusive Inspection 3 Stressors Found in Aircraft 3 4 Oven Loaded for Testing 13 viii 24 Instron Elongation Results for 32 Mandrel Elongation Results for PI 32 3 esults for PI Wires 34 35 raight PI Life Specimens 36 re Type 37 al Plot 38 or 44 le Temperatures 46 cted to a Dynamic Stressor 48 -Times Dynamic Bend Test 49 est 50 ) 50 sults for CP Wire 52 for CP Wires 53 P Wires 54 s 55 PI Wires 25 26Inherent Viscosity Results for PI Wires 3 27Dynamic Cut-Through R 28Weight Results for PI Wires 29Weight Loss Curves for St 30Differential Scanning Calorimetry (Melt Point) for PI Wi 31Unaged PI Wire TGA Isoconversion 32Aged PI Wire TGA Isoconversional Plot 38 33Arrhenius Relationship of CP Wire 43 34Comparison of CP Dynamic Stressor vs Static Stress 35 Additive Effect of CP Dynamic and Static Stressors 45 36 The CP Stressor Relationships Across Multip 37 Failure Time of CP Specimens at Different Airflow Rates 47 38 Unaged CP Wire and Aged Wire, not Subje 39 Unaged CP Wire and Aged Wires Subjected to a 10 40 Unaged CP Wire and Aged Wire (Example 1) 49 41 Unaged CP Wire and Aged Wire, DWV T 42 Unaged CP Wire and Aged Wire (Example 2 43 Wet IR Results for CP Wires 51 44 One-Minute Dry IR Results for CP Wire 52 45 Ten-Minute Dry IR Re 46 Insulation Tensile Strength Results 47 Insulation Elongation Results for C 48 Inherent Viscosity Results for CP Wire ix 49 Dynamic Cut-Through Results for CP Wires 56 erature of 490 ° C (in Nitrogen) 57 re of 490 ° C (in Air) 58 Hours for CP Wire 59 of PV Wire 64 Data for PV 65 tic Stressors 66 ssors 67 eratures 68 20 Hours 72 000 Hours 72 5200 Hours 74 75 res 77 50 Weight Results for CP Wires 56 51 The TGA Curves at an Isothermal Temp 52 The TGA Curves at an Isothermal Temperatu 53 The OIT Final wt.% vs Aging 54 The TGA Isoconversional Plot for Unaged CP Wire 59 55 The TGA Isoconversional Plot for Aged CP Wire 60 56 Inverse Temperature Arrhenius Relationship 57 Time-to-Failure Curve Compared to IEEE 58 Comparison of PV Dynamic and Sta 59 Additive Effect of PV Dynamic Stressors and Static Stre 60 The PV Stressor Relationships Across Multiple Temp 61 Failure Time of PV Specimens at Different Airflow Rates 69 62 Unaged PV Wire and Aged Wires for 560 and 640 Hours 70 63 Unaged PV Wire and Aged Wires for 400 and 380 Hours 71 64 Unaged PV Wire (White) and Wire Aged for 570 Hours 71 65 Unaged PV Wire (White) and Wire Aged for 7 66 Unaged PV Wire (White) and Wire Aged for 1 67 Unaged PV Wire and Aged Wires for 4200 and73 68 Wet IR Results for PV Wires 74 69 One-Minute Dry IR Results for PV Wires 70 Ten-Minute Dry IR Results for PV Wires 71 Insulation Tensile Strength Results for PV Wires 76 72 Insulation Elongation Results for PV Wires 76 73 Dynamic Cut-Through Results for PV Wi x 74 Weight Results for PV Wires 78 75 Differential Scanning Calorimetry for PV Wire 79 ) s at 135°C for PV Wire 81 tion 84 86 76 The TGA Curves at an Isothermal Temperature of 250 ° C (Unaged Wire79 77 The TGA Curves at an Isothermal Temperature of 250 ° C 80 78 The OIT Final Percent Weight Loss vs Aging Hour 79 The TGA Isoconversional Plot for Unaged PV Wire 82 80 The TGA Isoconversional Plot for Aged PV Wire 82 81 An FTIR Spectrum of PV Wire Insulation Cross Section 83 82 An FTIR Spectrum of Partially Aged PV Insulation 84 83 An FTIR Spectrum of Polyamide From PV Insula 84 Two Areas of the PV Spectra to Quantitate 85 85 An FTIR of PV Immersed in Water at 70°C 86 86 Heat-Aged, Unaged-Control, and Humidity-Aged PV Wire xi LIST OF TABLES le Pa 2 Test Setup Matrix 9 4 Comparisons of PI Aging Data With Original Estimated Failure Times 15 m 40 timated Failure Times 41 ilure Data by the Algorithm 61 ed Failure Times for PV 63 A Method 80 ata 87 Tabge 1 Aircraft Wiring Stressors 5 3 Test Procedures 11 5 Comparison of Actual Failure Data to Predicted Failure Data by the Algorith 6 The CP Comparisons of Aging Data With Originally Es 7 Final Weight Loss for CP Wire Using TGA 58 8 Comparison of Actual Failure Data to Predicted Fa 9 Comparisons of Aging Data With Original Estimat 10 Final Weight Loss for PV Wire Aged at 135°C, Using TG 11 Comparison of Actual Failure Data to Predicted Failure D xii LIST OF ACRONYMS Alternating current sport Systems Rulemaking Advisory Committee p assembly gm V E Electrical specimens EWIS Electrical Wiring Interconnect System FAA Federal Aviation Administration FEP Fluorinated ethylene propylene FTIR Fourier transform infrared spectroscopy HFIP Hexafluoroisopropanol IIR Intrusive Inspection Report IPAM 3 Identer Polymer Aging Monitor IR Insulation Resistance L Life MSDS Material Safety Data Sheet NDT Nondestructive test NTSB National Transportation Safety Board OAM Original aircraft manufacturer ODA 4,4 9-diamino-diphenyl ether OEM Original equipment manufacturer OIT Oxidation induction time P Property PC Personal computer PI Aromatic Polyimide Tape-Wrapped Isulated Wire PMDA Pyromellitic dianhydride PTFE Polytetrafluoroethylene PV Polyvinyl chloride/nylon PVC Polyvinyl chloride QA Quality assurance RH Relative humidity S Fit TDR Time Domain Reflectometry TGA Thermogravimetric Analysis THF Tetrahydrofuran AC AI Analog Interfaces ATSRAC Aging Tran CCA Cable clam CP Polytetrafluoroethylene/polyimide composites DC Direct current dl/ Intent viscosity units DLO Diffusion-limited oxidation DPA Dielelectric phase angle DS Dynamic stressor DSC Differential scanning calorimetry DW Dielectric Withstand Voltage E A Activation energy xiii UV-Vis Ultraviolet vis WAMW Weight average molecular weight on Analysis System ible WIDAS Wire Insulation Deteriorati Z Control Specimens xiv EXECUTIVE SUMMARY e life depends on the safe ignals between aircraft electrical components. his in turn requires that the physical integrity of electrical wire and its insulation be maintained. s aircraft increase in age and cycle time, the wire insulation may be degraded to the point that it no longer capable of ensuring the safe transfer of electrical current. The purpose of this initial search program was to evaluate the aging characteristics of three types of aircraft electrical ire: polyimide (PI), polytetrafluoroethylene/polyimide composite (CP), and polyvinyl hloride/nylon (PV). In addition, predictive models for aging of these wire types were developed nd evaluated. hese three wire types were chosen because of their widespread use in commercial aircraft and e amount of reported incidents concerning them. The factors that cause the wire insulation to egrade were examined and techniques to determine when a wire will no longer be capable of ansfer of electrical current were evaluated. The results of this study provided a platform to valuate existing and new test methods that could be used to monitor the aging of wire in aircraft. The results found were similar to the aging samples found in the Aging Transport Systems Rulemaking Advisory Committee Intrusive Inspection Report. A multivariable test program to assess the aging of the selected wire types was developed, which included dynamic bending, thermal cycling, vibration, chemical exposure, electrical stress, static stress, temperature, humidity, and airflow. The variables included results from previous test programs. The research program used accelerated aging techniques following a modified version of the cStandard Test Methods for Hook-Up Wire Insulation d (ASTM D 3032) and other industry-accepted methods, such as humidity and fluid exposure, static wrap conditions, and thermal cycling. The effects of nonpredictable, single-event failures were also assessed as part of this program. A quality assurance program to control the test procedures and results was implemented. The test results were tabulated and analyzed using statistical regression techniques to create the aging predictive models. They were continuously updated through the progression of the research program as data became available. The models were used to estimate when aircraft wire would fail due to degradation in multistressor environments in a laboratory setting. The results from this program predicted a median time-to-failure of the actual for PI from -25% to +30%, for CP from -20% to +20%, and for PV -16% to 20% for transformed (nonlogarithmic) time data. Additional data can be implemented into the models to improve on the confidence levels of the results as more data becomes available. The results demonstrate that PI and PV aircraft wires that are present in high-moisture areas will have a higher risk of aging or degradation. Single events such as cut-through or improper handling during maintenance can be more detrimental to the wire than aging from temperature and humidity exposure. Wires not subjected to dynamic and static stressors will last longer if they are undisturbed. Aircraft wiring systems should be designed to minimize wires being subjected to a tighter than 10-times dynamic bend (wrapping) either through a designed flex application or during maintenance and repair actions. Aged wire is more susceptible to these forces than a pristine wire, and the risk of failures to the insulation increases with age. The continued safe operation of aircraft beyond their expected servic and effective transfer of power and electrical s T A is re w c a T th d tr e xv Unpredictable single events such of the harness dominated as the main failure mechanism. Visual precursors for wire failure in PV, such as color change, crack the various zones of the aircraft over its operational life, the environmental and stressor r nvironments. This research study serves as a preliminary step to better understand and predict as movement and handling formation, and flaking, provided important evidence that the wire aged. These properties are an indication of increased risk of physical or electrical failure when a maintenance action is performed. Property tests such as insulation elongation, viscosity, dynamic cut-through, and visual inspection were identified as effective tools to monitor the degradation of wire. The inclusion of tests such as (1) visual for insulation cracking or color change, (2) insulation elongation, (3) inherent viscosity, and (4) dynamic cut-through can help to evaluate the age of the wiring. Other property tests have the potential to monitor degradation with further development. In conditions to which wiring is subjected is often not completely understood. Current wire specifications do not include qualification requirements for various wire characteristics that would better define wire performance in a multistressor aircraft environment. Wire specifications should be revised to incorporate resistance to cut-through, abrasion, hydrolysis, and longer-term heat aging, as applicable. Predictive models, such as the ones developed under this study, can be a great resource for electrical wiring interconnect system designers to better understand how wire ages and to estimate how a wire may perform in certain multistresso e the degradation of different wire types. Future studies should look into additional wire types and use their respective data to update these models and thus increase their level of confidence and reliability as a design tool. xvi 1. INTRODUCTION . It has been an accepted industry standard practice to expect the Electrical Wiring Interconnect System (EWIS) to last for the full design life of the aircraft. The risk associated with this practice increases with the continued use of aircraft beyond the original design life. The Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) Intrusive Inspection Report documented the presence of wire deterioration in different zones of aged aircraft [1]. The quantitative aging of the wire could not be determined because an original wire of the same age was not available for a direct comparison to understand the deterioration of the wire 9s physical characteristics. A number of different factors did appear to affect the condition of the wire, cluding the specific aircraft age, type, maintenance, and aircraft zone. The ATSRAC report ecialized areas of the aircraft, such as the engine compartment, were not evaluated in the study ecause special types of wire are required in these areas. Also, aging stressors that could not be controlled in a laboratory setting were identified as perturbations and were not included in the test plan. The test plan, however, did attempt to consider the wire 9s ability to withstand some of the uncontrolled conditions, such as elongation. It is known that many of the uncontrolled stressors play a large role in the aging of wire, and some may overshadow the normal aging process due to the environmental and mechanical stresses of routine service application. 1.1 in indicated that the inspected wire age could not be related to its environmental exposure except in extreme instances. A description of the findings can be found in appendix A. A test plan was developed with various aging stressors to determine the relationships between them and wire degradation. Aging stressors are the specific environmental, chemical, mechanical, and electrical factors that impose a stress on the wire installed in an aircraft. Every wire type is expected to have different aging characteristics based on the various stressors to which it is exposed. Every condition that places a stress on the wire will have some effect on the aging. Due to the large number of factors that impact aging wire characteristics, only the most predominant and general factors were examined in this study to define the majority of the aging characteristics of the wire type. Sp b PURPOSE . This initial research program evaluated the aging characteristics of three types of aircraft electrical wire: polyimide (PI), polytetrafluoroethylene/polyimide composites (CP), and polyvinyl chloride/nylon (PV). Predictive models were developed for the aging of these wire types. The aging process and a preliminary predictive technique was defined to determine when a wire subjected to certain known conditions will not be able to transfer electrical current. 1.2 BACKGROUND . There are many physical, chemical, and electrical mechanisms that affect the degradation of the wire insulation polymers and conductors. These include thermal oxidation, chemical oxidation, photo-oxidation, ultraviolet exposure, and hydrolysis. Results from the Intrusive Inspection Report [1] regarding the condition of wires from various examined aircraft are shown in figure 1. These conditions define some of the stressors that were present in the aircraft, such as heat, 1 vibration, and chemical contamination, while other conditions present a consequence of the stressors that may have been present, such as cracked and abraded insulation. Fluid/Chemical Contamination Cracked/Abraded Insulation Broken Shield/Conductor Exposed Shield/Conductor Corrosion Other Heat/Vibration Damage s s Indirect Damage Previous Repairs important to know how the condition of the wire may be degrading in normal ber of failures due to poor design, installation, or maintenance in order to Exposed Shield/Conductors Broken Shield /Conductors Figure 1. Wiring Conditions From Intrusive Inspection Failures from design, installation, and maintenance issues create stresses that are much more difficult to control and model. Many of these wire failures are due to physical and mechanical damage and are often exacerbated the wire age. Aircraft service data from the National Transportation Safety Board Accident and Incident database, the Aircraft Service Reporting ystem, Service Difficulty Reports database, and the Navy safety and maintenance data were S evaluated. It is service and the num select a wire for an application and ensure that it is installed and maintained properly. A query of these service databases show many accidents and incidents reports were caused by the wire 9s inadequate performance in normal service environments, and by application issues related to the design, installation, and maintenance, as shown in figure 2. 2 Insufficient Data Design, Maintenance, etc. Related Wire Performance Related Insufficient Data Design, Maintenance, etc. Related Wire Performance Related 55 % 33 % 12 % Figure 2. Types of Wire Failures A Federal Aviation AdmiAA) researchintenance evaluated multiple aircraft fro operatober of stressors that were present. These stressors shown in figure 3 were reviewed for implementation into the research study. nistration (F m multiple commercial program on aircraft ma rs and identified a num Figure 3. Stressors Found in Aircraft 3 2. EVALUATION APPROACH . The cStandard Test Methods for Hook-Up Wire Insulation d (ASTM D 3032) method was modified to allow the aging program to evaluate a multitude of environmental stresses. The testing was performed using strict procedural guidelines for ensuring the validity of the data. The Dielectric Withstand Voltage (DWV) test was used as the final criteria to determine when a wire can no longer safely carry the required current. Other stre a part of the multivariate design of experiments, were examined separately. Many of these additional stressors were deemed single-point, nonpredictive events (perturbations to the normal aging process) that could not be effectively modeled in an aging program due to the complexities of modeling degradation for each variable. Analysis of these events was primarily qualitative and attempted to assess how these perturbations affected the normal degradation equations. A more detailed discussion can be found in appendix B. ASTM D 3032 was used to determine the temperature rating of wire based on oxidation degradation; it uses a combination of thermal, mechanical, and electrical stresses to define the life of a wire sample. Changing the level of the stress factors affects the wire temperature rating, which is typically the maximumlation for a specific period of me, often 10,000 hours. Meral accelerated temperatures, ssors, not directly exposure temperature of the insu asurement of the wire life at seve ti based on the DWV failure, allows analysis of the data to make predictions on the potential life of the wire at the rated and lower temperatures. These lower temperatures are often more typical of the actual temperatures to which the wire is exposed or operated. .1 2 TEST PROGRAM . The test program was designed to generate and analyze data that would facilitate the development of models designed to predict the time-to-failure of aircraft wiring. Different stressor combinations were tested at multiple temperatures and were fitted by a line to approximate the Arrhenius relationship. A list of aging stressors is shown in table 1. Median life estimates for any specific temperature can be computed for the wires subjected to any dynamic-static stressor combination using the models developed. Separate aging models were developed for each wire type tested in this program to enable the extrapolation of median life for the wire subjected to combinations of these dynamic and static stressors as well as temperature and relative humidity. Development of the aging models required the generation of data points for time-to-failure for each wire type with combinations of the various stressors over various temperature and humidity environments. The detailed test plan can be seen in appendix D. 4 Table 1. Aircraft Wiring Stressors Stressor Levels in Aircraft Notes Test Program Temperature, High (Life) Up to 260°C One of the central stressors for the thermal oxidative aging of aircraft wire. Yes, up to 300°C Temperature, Cold (Cold Bend) -40°C Very low temperatures do not affect the aging, but do affect the properties due to the increased rigidity of the insulation ( No maintenance, operation). Temperature Cycling and Shock Typically -40° to +85°C Stress of continually cycling temperatures during periods of operation at altitude and idling on the ground may directly affect abrasion insulation integrity. Yes, down to -55°C Chemical Resistance Humidity/Moisture Depends on Insulation Type Evaluated many potential fluid types: comm Yes, selected a high High/Low pH Fluids/Cleaners Corrosion Preventative Compounds Fuels, Lubricants Deicing Fluid Others certain insulation types and corrosion preventive compounds very similar to fuels and lubricants. on aircraft fluids as well as fluids known to affect pH cleaner, jet fuel, deicing fluid, and hydraulic fluid Pressure, Barometric High Altitude Some insulations are known to outgas, creating mass loss, increased rigidity, etc. No Bending, Flexing tress Ten times bend to straight. Three times Stress seen during installation and Yes S allowed in certain applications. Flexing per application or during maintenance maintenance actions. Design allows for a certain bend radius in the wire (static strain), while maintenance actions may flex wire. A notch or other insulation flaw will be magnified by this stress. 5 Tableued) StrLevels N 1. Aircraft Wiring Stressors (Contin essor in Aircraftotes Test Program Vibration Stress Sine, High Frequency Force tha or chafin or may ca Random, t can cause abrasion use flexing. g, Yes Shock, High-G Force By airframe Mechanic No (Landing) al force acting on the wire. Abrasion or Chafing With or Without Debris Wire to Wire Wire to Structure One of thl stressors.and vibration insulationintegrity. Yes e most important mechanica Directly affected by shock . Direct affect of the 9s mechanical Debris Sand, Drill i and L Directly ion, may holdsulation nd may No was evaluated in the FAA Mixed Wire Program Shavngs, Dust int a affects the severity of abras fluids closer to the in create a flame hazard. , This parameter Current Stress Loads High, Overload High current causes resistive current as temperatu See high temperature re increases. Lightning DO-1 rtu Can weak pertieper groundin on the wirin 60 rbation pro pe en or damage the dielectric s of the insulation. Pro g should minimize impact g. No Ozone, Oxidative Pollutants 168 hours at 0.5 ppm Expected insulation exposure minimal. Yes to force the aging of s due to oxidation, but in aircraft is suspected to be Arcing Perturbation Not seen as an aging stressor. No Corona Perturbation Not seen as an issue with lower voltages. bove 10 micro- corona si aircraft p No See voltage stress A00 volts may produce tes in dielectric. Typical ower <300 V. Ultraviolet Perturbation A definitr to certain er insulations. Most wire e from iole No Radiation e aging stresso polym consider ultrav d to be protected t exposure in service. Thermal, Humidity, and Mechanical Strain Comb above levels This come past on pw a direct synergistic effect. May apply to ut expected to apply to fluoropolymer insulation Yes inations of bination has been tested in th olyimide materials to sho lation types as well, but no s. other ins 6 2.2 EVALUATION METHOD . Theocol ipal insulation degechanism is oxi degrads ieation and zation degm ASTM D 3032 test method. This knerallyowevt does not s stressors have specific thermal, mechanical, and electrical characteristics. By se strore r a bpredictive elo of the aging stressors areely proportional to the service life of a wire. The higaster the material ted. In ge of ine representative levy typically designed to exceed 10,000 hours of servicperatures with specific mechanical and electrical factors. Therefore, to induce wire de a shorter period of time, the stress levels were increased to accelerate the aginghe vethedatcting e oal tress fcreasetressors and by combining them; ide inn the aging process that may radically affect the rate of degradation. In other words, as a catas faster. Eaessor was s staticnvvarious levwere then o test fractions. ressors arefine straight applns or in a ition. Thdefine the aging Dynamic actionnd chemical contamination regardless of tal she specific conditions under which a samlude varvels and s of and as use the final n of w 2.3 test protassumes the princradation mdation and the secondary volatili ation mechanism radation mechanis nclude hydrolysis and volatilization. Th s are addressed by the oxid method is well address the impact of the m The aging nown and ge any stressors that ma accepted in the aircraft industry; h y affect these aging mechanism er, i or hydrolysis. changing the model was dev The levels essors to be meflective of aircraft wiring applications, important factors and are invers etter ped. useable is affec her the level of stress on a material, the f the various aging stressors were determ the wire in the aircraft. The wire types being studied are e life at rated tem neral, the levels els experienced b d based on the when stressed terioration in process. T models were de the performanc loped to provide f wire under norm most appropriate method to extrapolate operating conditions. a for predi Particular sactors may in they may prov the susceptibility of a wire to other s sight into the presence of interactions i the presence of stress factor A fact may act classified a combined t lyst causing stres , dynamic, and e or inte factor B to age the wire much ironmental. These stressors, at ch str els, Static ste those that dwhether a wire is installed in icatio bent pos process. e bend radii stressors are the strain that a specimen is subjected to during s that can occur on the wire such as flexin the static stressor applied. Environmen ple will age. These stressors inc g, abrasion, a tressors are t ying le combination determinatio temperature ire failure. humidity. The wet DWV test wd as WIRE SAMPLES . ace, being used for w Thesire types could be the future. All the wire s The Aromatic Polyimide Tape-Wrmilar to other wire specifications such as MIL-W-81381BMS 13-51 and has been commonly used in 1970s. The wire tested was a nickel-coated copper conductor rapped with two layers of fluorinated ethylene propylene (FEP)-coated polyimide N film, followed by a thin topcoat of polyimide/polyamide. The FEP provides adhesion between the The aging char airframe wiring evaluated in teristics of three w ere evaluated. ire types that have been, or currently ar e provide a framework to which other w amples were 22 gauge. apped Insulated Wire (PI) is si and Boeing transport aircraft since the w 7 layers of polyimide, which themselves cannot be easily fused together within temperature limits that would not damage the finished wire. he extruded polyvinyl chloride (PVC) is a tin-coated copper conductor with a polyvinyl The Aromatic Polyimide Tape Wrap With Fluorocarbon Bonding Layers and a polytetrafluoroethylene (PTFE) outer wrap composite (CP) is a nickel-coated copper conductor wrapped with multiple layers of fluorocarbon-coated polyimide N film in accordance with Boeing BMS 13-60, which is similar to specifications such as the initial MIL-W-22759/80 and /92 construction and Airbus. This wire type is often referred to as TKT wire and has been commonly used on large transport aircraft since the mid 1990s. T chloride extrusion followed by a polyamide extrusion. The wire type was commonly used on large commercial and military transport aircraft from the early 1960s to the late 1970s [1]. Similar constructions include Boeing type BMS 13-13 and Douglas type 7616964 and are commonly referred to as PV. 2.4 EXPERIMENT SETUP . A multivariate test program using stressors and environments was developed for each wire type evaluated. Time was the independent variable throughout the test program. The dynamic ressors were randomly assigned an identifier number, and identifier letter codes defined the ative humidity (RH) at up to 95ºC. ded to be secured, except for flex applications. osed to a straight, 1-time, 6-times, and 10-times static strain. Typical wire installation guidelines recommend 10-times strain or less; however, higher strain is allowed e wire samples were also subjected thermal cycling of 100 cycles at -55º to 85ºC after each aging cycle. Four aircraft fluids, a st specific environmental and static stressors. A list of definitions for the stressors and environments can be found in appendix G. Several of the stressors selected for this program were varied in severity. For PI and CP wires, the test temperature was elevated beyond what wire normally experiences on aircraft, with an elevated temperature of 300ºC. For PV wire, the elevated temperature was 135ºC. Humidity exposure was also varied in certain setups with some samples being exposed to 100% rel Wire samples were subjected to 4 cycles of bend per aging cycle, totaling between 40-60 cycles. This interval is estimated to be in the range of what may be expected from maintenance actions or modifications for a typical aircraft wire, but not in a flex application. Wire radii bend dynamic tests were varied from 3-times radii to 10 times. The 3-times radii bend is more severe than what would be expected from a maintenance action, while the 10-times radii bend may be experienced periodically during maintenance, but is usually less severe. Generally, wire is typically not moved and is inten Wire samples were exp in certain situations. In addition, the samples were subjected to a vibration abrasion test, approximately 0.032 lb/linear inch for 2400-3000 cycles of 0.9 inch length, using a flat 6061 T6 aluminum plate with a 24- to 30-microinch surface finish. Th to high pH cleaner, jet fuel, de-icing fluid, and hydraulic fluid were used in the test program. The wire samples were exposed to 8-12 hours per fluid type, which may be less than what is experienced in actual applications. 8 A test matrix showing the tests performed for each wire type is shown in table 2. The numbers within each cell refer to the temperature in degrees Centigrade for each setup run at that nvironmental, dynamic, and static stressor combination. Setups marked with a c+ d have an e additional electrical stress variation. The setups selected for this program were designed to evaluate the selected critical variables and to model their effects. The total number of setups tested for each wire type was: PI 39, CP 28, and PV 26 setups. Additional experiments were done to PI to quantify the known degradation mechanism of hydrolysis. Table 2. Test Setup Matrix Conditions A/A + B C6/C1 D E6/E1F G H I J 0% RH 3 Ovens 85%- 25% 70% RH RH, Cycled85% RH 100% RH (Immersion) Wire Type Stressors Straight (°C) Static Strain (°C) Static Strain (°C) Static Strain (°C) Static Strain (°C) Static Strain (°C) Straight (°C) Static Strain (°C) Straight (°C) Static Strain (°C) 10- imes 6-/1- Times 10- Times 6-/1- Times 10- Times 10- Times 10- Times T PI No stressor protocol (only DWV test) 260 + , 280, 300 + 260, 280, 300 300/300 95 95/95 95 PI Dynamic bend (roll up/down x 250 + ,270, 280, 300 +* 250, 280, 300 70, 95 70 95 70, 95 95 45, 70, 95 2) 10-times mandrel PI Dynamic bend (roll up/down x 2) 3-times mandrel 250, 280, 300 280 PI Temp shock (100 cycles, 260 -55° to +85°C) PI Vibration (abrasion) 260 9 Table 2. Test Setup Matrix (Continued) Conditions A/A + B C6/C1D E6/E1F G H I J 0% RH 3 Ovens 70% RH 85%- 25% RH, cycled85% RH 100% RH (Immersion) Wire Type Stress Straight (° Static Strain Static C) tat tra °C) mes atic ain ) Straight ) 10- Times Static Strain ) Straight ) 10- Times Static Strain ) 10- Times 6-/1- Times 10- imes 6-/1- Times 10- Ti ors C) (°C) Strain (° S ( T Sic in Static Strain St St ) (°C r (°C (°C (°C (°C (°C PI Fluid soak preceded by 10-times mandrel bend 300 300 PI/PTFE No stressor protocol (only DWV test) 260 + , 280, 300 + 260, 280, 300 PI/PTFi p el E Dynam (roll u c bend /do times wn x 2) 10- mandr 260 + , 2 60, 2 300 80, 95 70, 95 280, 300 +* PI/Pic bend -tim rel 2 280, 280 TFE Dynam (roll up/ 2) 3 mand down x es 60, 300 PI/P cles, ° to 260 TFE Temp shock (100 cy -55+85° C) PI/PTFE Vibr (abra ati sion) 260 300 on PI/PTFE Fluid soak ded by es l ben 300 300 prece 10-tim mandred aded cells are the reference conditions to the ASTM D 3032 test method. Some setups are t expected to fail within the testing time available. ns with additional electrical stress variable samples will be run at the setups with res identified by a superscript +. * Will be used to evaluate oxidation rate and will be run at low, medium, and high oven air exchange rates. Notes: 1. Letters in the Conditions columns for a particular stressor represent undetermined temperatures at which that combination will be run. Two- and three-digit numbers represent actual temperatures in degrees centigrade (°C). 2. A blank indicates that no tests will be performed in that condition. Sh no + Conditio temperatu 10 The effects of two additionugh damage, although not predictable, were evaluated. These are refernonpredictable failures. Unfortunately, not all stressors can be quantitatively con measured. For example, a wire that is stressed duringlla ncby anrrant drill bit may be damaged and fail immediately. An example of the resultingge is a mechanical gouge in the insulation that exposes the conductor. This cannot be effectively modeled because the damage is so severe and so quick, completely overwhelmingg process and rendering aging algorithms useless. Ts, however, where the wire can con without failing if an exposed condtth a con surface short circuit. Wire abrasion againss due tosign, br primary support, or drill s pro we do perturbations and their effects on each wire type are cond and complete descriptions, including tests performed, testing frequencies, and types of specimens are in appe 2 al stressors, fluid exposure and cut-thro red to as single-event trolled and instationor during maintenae e dama the agin here are instance does n ucture undle tinue to age uctor t the str in a b ot mak or oth are all e e con er com other act wi ponent perturb ductive poor de at do t and oken se an havings il ations th me not cau ii immediate blem, butl manifest over tim if undetect tae d. A in ore dailed scussn of in appendix C. The test protocols ndix D. .5 TEST METHODS ANDCRE PROEDUS . Several insocumeveloped to define the specific quality assurance aspects o qp conainedn appndix .) Staard cure aging and property tests were used when possible. Where no previous procedure existed for aging and property test, new proented. The referenced aging procedures and property tests ted in tabe 3. Table 3. Test Procedures o Num 100 Environmental Series Industry Standard Methods tructional dents wre de f this testprogram. (Theuality cedures were developed and docum lan ist ieEndtest proeds for are lisl Test Prcedure ber AWD-TPn aging ASTM D 3032, SAE AS4373 method 804, modified -101 Ove AWD-TP-102 Temperature shock MIL-STD-810 AWD-TP-103 HumiSAE AS4373 method 603, modified dity AWD-TPersion SAE AS4373 method 602, modified -104 Water imm AWD01, modified -TP-105 Fluid immersion SAE AS4373 method 6 AWD1 -TP-106 Flammability SAE AS4373 method 80 AWD-TP-107 WIDAS Lectromec Proprietary 200 Physical/Mechanical Series AWD procedure -TP-201 Visual inspection Standard laboratory AWD-TP-202 Dynamic bend test SAE AS4373 method 71 modified 2, AWD-TPRTSC-developed procedure -203 Vibration (Abrasion) AWD-TP-205 Indenter developed AI/FAA 11 Table 3. Test Procedures (Continued) Test Procedure Number 200 Physical/Mechanical Series Industry Standard Methods AWD-TP-206 Weight measurement SAE AS4373 method 902, modified AWD-TP-207 Insulation tensile and elongation SAE AS4373 method 705, modified AWD-TP-208 Conductor tensile and elongation SAE AS4373 method 402, modified AWD-TP-209 Dynamic cut-through SAE AS4373 method 703 AWD-TP-210 Static cut-through Lectromec Method AWD-TP-211 Density Standard laboratory procedure AWD-TP-212 Modulus profiling Per Intrusive Inspection procedure 300 Electrical Series AWD-TP-301 Wet Dielectric Withstand Voltage SAE AS4373 method 510, modified AWD-TP-302 Insulation resistance (wet) SAE AS4373 method 504 300 Electrical Series AWD-TP-303 Insulation resistance BNL/RTSC-developed procedures AWD-TP-304 Dielectric phBNL/RTSC-developed procedures ase angle ATime domain reflectometry BNL/RTSC-developed p WD-TP-305 rocedures AWD-TP-307 Conductor resistance SAE AS4373, method 403 400 Materials/Miscellaneous Series AWD-TP-401 Thermogravimetric analysis cedure NAWC-developed pro AWD-TP-402 Inherent viscosity DuPont/Lectromec-develop procedure ed AWD-TP-403 Oxidation induction time BNL/RTSC-developed procedure AWD-TP-404 Ultraviolet-visible spectroscopy Sandia-developed procedure AWD-TP-405 Fourier transform infrared spectroscopy Sandia-developed procedure, Standard laboratory procedure Rhnicalircraft wi B Nation AI = Analog interfaces TSC = Raytheon Tec NL = Brookhaven Serv al Laboratory ices Company AWD = Aring degradation 12 2.6 THE AGING PROCESS . Thes were thermally aged using the oven aging method from ASTM D 3032. This mvides a meempera iire inshe life specimens were placed into the aging cycle along with all the specimens in the property testing setup in the heating ovens. The second and tspecimr approxirst cycles, respectively, were comin order trd Ast proal definitfe specim eonditioype were aged toget In snd CPn the same chamber.ens wace fo wire specimen ethod proans for developing time versus tture curves and temperature ndices for the wulation . One third of t hird sets of life ens were placed in the ovens afte pleted. This was done mately 1/3 and 2/3 of the fi o improve upon the standa STM D 3032 te ens. Due to the large number of setups, the samples that were aged with common cedure since this provided additionion of failure times for the li nvironmental cns of the same wire t d i her in the same chamber. ded ov ome cases, PI a samples were place lation. Figure 4 shows loa ith plenty of spr air circu Figure 4. Oven Loaded for Testing A of agied and were s tmplesectrically with Insulation saltwater solution, and other test rired foed for each test s Ilysis o appropri t. Fo w problems seen early in the testing. Aging times for a cycle were also modified as the testing progressed in order to focus on the period when the life specimens would begin to fail. fter each cycleng, the specimens were removtressed in accordance with the est plan. The sa were then tested el Resistance (IR), DWV in 5% methods, as defined by the test plan. The specim r the property tests schedul ens were etup. emoved as requ ntermediate anaf the time-to-failure data allowedate adjustments to be made to he test programr example, the stress level of vibrationas reduced due to specimen 2.7 MODEL DEVELOPMENT . T multim he models were developed assuming that a single or coordinated thermally based echanism reaction was occurring and the overall effective activation energy (E A ) can be estimated and used to effectively model thermal oxidative aging. When all samples failed, the median life was calculated using the standard log average life approach. If some of the samples did not fail (censored data), the median failure time was calculated based on a probability/hazard 13 plotting approach. The lognormal distribution represented each setup 9s failure distribution well and was used throughout the data analysis. The models were developed to predict the median is possible that estimated life values would not be logical (e.g., life > 1,000,000 hours). These illogical estimates may occur on setups that had no failures, and thus, had no data to be used in the model development, or were outside the valid window for performing good extrapolations. One disadvantage to not having test data on all possible variants is that the model is not built around those conditions, and may not address, or may even deviate in those areas. For this reason, attempts were made to use simplified relationships to describe how different stresses and stress combinations behave. Variants of the multiple stressor models were used to develop the best fitting degradation model. In the first iteration, a simple additive model, based on the Arrhenius relationship, was evaluated. For this model, each of the stressors was expected to shift the baseline up or down, but not affect the mechanism, resulting in the same slope. The overall addition of energy into a system by molecular energy or periodic mechanical energy (nonthermal) in order to lower the required E A for the reaction to proceed is described by Campbell and Bruning [2]. This would result in shifts of the curve downward, based on the energy imparted on the system. The periodic stress does not change the mechanism of gy into the system to initiate e breakdown if the applied energy is greater than E eff . The resulting model defined the shift, up proceed under certain onditions. eds to be efined. Every possible stressor would need to have data generated to fully develop a good nd the indication that the E A should not change significantly within one wire type, additional relationships were examined based on the data. Parallel lines life of any setup based on the multifactor testing conditions of aging temperature, aging humidity, continuous strain during aging, and a periodic dynamic stress. A number of basic assumptions were made during the development to allow the Arrhenius model to be modified. The activation energy was assumed to be based on the sum of the activation energies from the various chemical/molecular reactions that take place, affecting the degradation of the wire. Therefore, the slopes of various stressor degradation lines were assumed to be similar when the same basic mechanism took place. Temperature (T) rather than (1/T) provided better fitting data in the models. For this reason, all models used degrees Celsius (°C) rather than inverse Kelvin (1/°K). It degradation, but rather, imparts ener th or down, for each stress and provided improvements to the Arrhenius model. Some curvature was apparent (versus temperature and relative humidity), and some of the baseline linear slopes were different. However, multiple reactions may occur simultaneously, and based on the need for certain thresholds of energy to be met, some reactions may not c If it is assumed that the slopes can change, in effect changing the E A , a model can be built that uses the E A related to the presence of each of the different stressors. However, it is not possible to model the stressors for which there was only single temperature data, since a slope ne d model. Due to this drawback, a with the same slope indicate that the same mechanism is occurring, but to a different total energy. Lines with a different slope indicate that the mechanism itself or the ratios of multiple mechanisms may be different. 14 The following sections provide a description of the aging and property test results for each wire type. The test data generated in this program included aging time to DWV failure, electrical measurements, physical property measurements, and visual observations. The time-to-failure data was analyzed from which aging models for each wire type were developed. In the figures r property tests, the final data point for each test setup generally represents the final aging fo cycle, which was typically when the last life specimens failed the DWV test. 3. POLYIMIDE AGING AND TEST RESULTS . 3.1 POLYIMIDE AGING DATA . The aging data for the 1 onsidered complete upo 1 life specimens from each test setup was recorded at each cycle and n the final DWV failure. Table 4 shows the median time-to-failure for a straight sample would be expected to have a longer median me-to-failure than a 10-times static-wrapped sample. The complete aging data can be found in c each of the test setups, as well as estimated failure times based upon previously generated aging data. The failures were generally accompanied by cracking of the wire insulation. The dynamic wrap around a mandrel 10 times the diameter of the wire and no static strain during oven aging exhibited consistently longer times-to-failure than those documented by Elliot [3] for the same stressor conditions. The median time-to-failure of the samples varied due to the stressor combinations. In most cases, the time-to-failure mirrors the generally accepted view of how detrimental a stressor or stressor combination is to a wire. However, Group 1 setup 9 differed from Group 1 setup 13, where ti appendix H. Table 4. Comparisons of PI Aging Data With Original Estimated Failure Times Group Setup Temp. (°C) RH (%) Dynamic Stressor Static Stressor Estimated Failure Time (hr) Median Failure Time (hr) 1 9 250 0 10 times Straight 5821 7,276 1 13 250 0 10 times 10 times 7,695 1 16 250 0 3 times Straight 3,485 2 4 260 0 None 10 times 7,732 2 21 Temp 260 0 Cycling 10 times 8,805 3 5 280 0 None 10 times 3,291 3 11 280 0 10 times Straight 1226 2,662 3 14 280 0 10 times 10 times 2,245 3 17 280 0 3 times Straight 970 4 3 300 0 None Straight 2,977 4 6 300 0 None 10 times 932 4 7 300 0 None 6 times 932 4 8 300 0 None 1 time 2,546 5 12 300 0 10 times Straight 474 843 5 15 300 0 10 times 10 times 564 5 18 300 0 3 times Straight 335 15 Table 4. Comparisons of PI Aging Data With Original Estimated Failure Times (Continued) Group Setup Temp. (°C) RH (%) Dynamic Stressor Static Stressor Estimated Failure Time (hr) Median Failure Time (hr) 5 19 300 0 3 times 10 times 441 7 28 300 0 Fluid Straight 875 7 29 300 0 Fluid 10 times 752 8 34 70 70 10 times 10 times 7,456 9 30 95 70 None 10 times 6,239 9 35 95 70 10 times 10 times 4,274 10 38 70 85 10 times 10 times 1,766 11 41 45 100 10 times 10 times 1,908 12 42 70 100 10 times 10 times 349 13 33 95 100 None 10 times 90 14 40 95 100 10 times Straight 2,316 15 43 95 100 10 times 10 times 136 16 36 70 85-25 10 times 10 times 5,755 17 37 95 85 10 times Straight 7,371 17 39 95 85 10 times 10 times 488 2 1 260 0 None Straight >10,150* 2 24 260 0 Vibration Straight >10,150* 3 2 280 0 None Straight >4,444* 9 64* 31 95 70 None 6 times >2,8 9 32 95 70 None 1 time >3,537* 18 10 270 0 10 times Straight 2016 >800* etdo failspecimal hou when stop 3.2EMPTU *These s ups stoppe prior ture of ens. Acturs of agingped. TERARE . The aging data from each setup was analyzed using techniques defined by Relative Thife and Temperature Index (SAE AS4851). When al failed, the median life values were calated g std logveraethoe of the samples did not fail, the man fai tim calcuted bro, 5, an]. Thognodistriion throughout the data analysis since it represented each setup failure distribution The analysis was based on up to 11 life specimenre aged to faure within eatup. Thirty-seven sample setups were aged at various conditions. Ten to 11 specimens failed in 24 of thtups, while 6sures in the specimens. One additional setup had two fa specates of the time-to-failure and the 90 percent of expected life were developed for each of the 37 setups. Finally, a comprehensive model was developed to prct theian f any up, be m test conditions. ermal L l samples lcuusin le andar age life m n a probability and hazard plotting app d. If som edi d 6 ur e e was rmal la but ased o was us ach [ [4 led . s that weilch se e se setups had at leat 4 fail iledimens. Estimmedian edi medlife osetased on thultifactor 16 Ht ultiple temperatures were fitted by a line to approximate the Arrhenius curve. Median life estimates for any specific temperature, which was similar to the al regbe computed for anynatif dynsor, tresso h dro Arrs plovatio (E A ) can be determined as well as the estimaen thousanhours is typically used to determine wiaxim temre ratinitary purpose. Insulation a hr actin eepeus slope) an a higher teure index would be preferred for better longevity in a general application with thermal oxidative environmeince leadsn inme-to-failure at lower temperatures. The concept of desiring a high Er temtured hiature index be extended to desiring a high humy slopd aidity index. The E A , as classically defined, could n detened f the ms d The Arrhenius plot for the 11 samples that failed ath ofthreeup teatuwn figure 5. An approximation in the activation energy (E A 5.1) the teatuoulated at a speific time frolot. Aparison to the IEEE [3 isfigu umidity/static, strain/dynamic stressor combinations that were applied to the specimens a m experiment r, and relative ion, can umidity from combi m the on o heniu amic stres t, the acti static s n energy the fitte line. F tion of the temperature index for a specific time. Td e thre 9s mumperatug for mils withighevationergy (ster Arrhenidmperat nt, sthis to acreased ti A fopera slope angh-temper can also idite an high hum ot bermiromodeleveloped. ho eac the setmperres is s = 2and mperre index cd be estimcm the p com] data shown in re 6. [1/Temp(°K)] Log(Hrs) 0.001925 0.001900 0.001875 0.001850 0.001825 0.001800 0.001775 0.001750 4. 4. 3. 3. 25 00 75 50 3.25 3.00 2.75 2.50 S0. R-Sq R-Sq(adj) 0824947 95.7% 95.5% Regres 99.7% P1, Hrs) + 5p(°K)] sion PI I: DS=2 RH=0% Log( = - 6.565474 [1/Tem Figure 5. Inverse Temperature Arrhenius Relationship of PI Wire 17 (1/T) Log(Hrs) 0 . 0 0 1 9 2 5 0 . 0 0 1 9 0 0 0 . 0 0 1 8 7 5 0 . 0 0 1 8 5 0 0 . 0 0 1 8 2 5 0 . 0 0 1 8 0 0 0 . 0 0 1 7 7 5 0 . 0 0 1 7 5 0 4.00 3.75 3.50 Scatterplot of Log(Hrs) vs (1/T) - PI: 10x.S FAA Study 1972 IEEE 3.25 3.00 2.75 2.50 Figure 6. Arrhenius Relationship of PI Wire When plotting the individual log life values from each sample against the direct temperature (°C) at each of the three setups, a simplified Arrhenius relationship can be seen. The linear fit of the failure data is shown in figure 7. Traditional approaches plot log life against inverse Kelvin temperatures (1/°K). An extrapolation of the log life versus temperature fit from the figure 7 results in a temperature rating of 244°C at 10,000 hours and 200°C at 60,000 hours. While figure 5 uses the traditional Arrhenius model approach, the extrapolation results in the same temperature rating of 245°C at 10,000 hours, but a slightly higher 209°C at 60,000 hours. Temp Log(Hrs) 300 290 280 270 260 250 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 S0.074551 R-Sq96.5% R-Sq(adj)96.4% 5 Regression 99.7% PI PI: DS=21, RH=0% Log(Hrs) = 8.492 - 0.01839 Temp Figure 7. Temperature Arrhenius Relationship of PI Wire 18 Clearly, within the region of the temperatures tested, it is simpler to model directly against temperature instead of adding the complexity of using inverse temperature. Outside the testing envelope, the confidence decreases and the models diverge. The use of temperature in the models fit the data better for all stressors of each wire type, even though the theoretical basis is to use inverse temperature (1/T). For comparison, using the PI model resulted in temperatures of 245°C for 10,000 hours and 206°C for 60,000 hours. A general rule of thumb for extrapolation is to stay within 20°C for a decent extrapolation. Sixty thousand hours is beyond this, and the estimate of time-to-failure should be viewed with that perspective. The solid prediction line in figure 7 is bounded by dashed 99.7% prediction interval lines. These 99.7% PI lines are similar to ±3S control chart limits and should contain approximately 99.7% (almost all) of the future individual failure times. Any individual failures outside these PI limits would be considered a statistical outlier. Figure 8 shows the comparison of the main effects of each dynamic stressor and static stressor for DWV failure to occur. This comparison averages the values across temperatures and the logarithmic mean of hours to failure increases when a stressor is less stressful. For exam dynamic stressor 1 (no dynamic stress) shows a 1000% longer mean time-to-failure, while dynamic stressor 3 (3-time baseline of stressor 2 STM baseline with 10-times wrap). al the ire into a new form, allowing the insulation to reduce its effective strain [7]. This infers that combinations of stressors may have a significant effect on the mean time-to-failure. ple, es wrap) exhibits 2/3 the average life over th (A This comparison also shows that a 10-times static strain exhibits roughly 20% of the average life as the ASTM baseline setup. The 10-times and 6-times bends reduce the mean failure time by half. This would indicate that if the wire was used in service with a static bend, the estimated service life for that wire would be half of what would otherwise normally be used. Previous testing has shown that the presence of a static strain in the wire will increase the aging of wire. However, it has also been shown that the temperature at which a wire ages can also anne w 19 Comparison of PI Dynamic Stressors and Static Stressors Figure 8. Comparison of PI Dynamic Stressors and Static Stressors Figure 9 depicts the additive effects from each of the dynamic and static stressors for each test setup. The black points are the means of aging, based on the actual test results determined in this test program, while the red points are the predicted means, based on the predictive model that was developed. As the figure shows, the model tracks the actual aging fairly well. The data analysis was performed using the pooled data from all the individual PI specimen failures. The final model combines the additive effects of the discrete dynamic/static stressors with the gradual trend effects that temperature and relative humidity have on the expected life of the samples. As temperature and/or relative humidity increases, the expected life systematically decreases. Interactions between some of these factors are also incorporated. For example, the presence or absence of humidity has a significant impact on how much a 10-times static wrap sample will reduce life versus a straight sample aged without strain. At 0% RH, straight and 10- times static strain samples have similar expected lives, but the 10-times static samples fail much earlier with humidity. Additional interactions and some temperature/humidity curvature were incorporated into the model. 6 4 3 2 1 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 4 3 2 1 Dyn Sta Main Effects Plot (fitted means) for Lo g (Hrs) Dynamic Stressors Static Stressors Mean of Lo g ( Hrs ) 1 3 no stressor, 2 3 10-times bend, 3 3 3-times bend, 4 3 Temperature cycling, 6 3 fluid exposure 1 3 none; 2 3 10-times wrap; 3 3 6-times wrap; 4 3 1-time wrap 20 4.0 3.0 2.0 3.5 Data 2.5 62 61 42 32 31 22 21 14 13 12 11 Setup Variable DynSta. Add(Dyn,Sta) Time Series Plot of DynSta., Add(Dyn,Sta) - PI Across all setups, a total of 301 PI samles eventually failed the DW early failure

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