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Problem Statement Consumer Products warehoused in Georgia experienced
insulation degradation Actual degradation, remaining insulation life if damaged
and expected life of undamaged insulation all needed evaluation
The product in question: a thermoplastic polyurethane (TPU) but unknown if it is ester or ether based.
2© 2009 Ops A La Carte
Sample Undamaged Specimen
Sample has visible surface oily residue
TPU wire is 1/8” ϕ Most samples are
cracked or cracks are being initiated
Millions at stake
3© 2009 Ops A La Carte
Properties of TPU Understand properties
helps to know how to accelerate testing
Unknown whether it is ether or ester based
Polyether based TPU resistant to hydrolysis
Polyester based TPU is resistant to temperature
Standard Properties
Range
Max Operating Temperature
-40C to 80C
Proper Storage Temperature
23 C / 74 F
Proper Storage Rel. Humidity
50%
Activation Energy
0.2-1.0 eV
Heat of Combustion
25,000-29,000 J/g
4© 2009 Ops A La Carte
Strength vs Weaknesses
Polyester Based: Highest heat resistance Highest mechanical
properties Highest resistance to
mineral oils
Polyether Based: Highest hydrolysis
resistance Best low-temperature
flexibility Resistance to
microbiological degradation
5© 2009 Ops A La Carte
Failure Parameters Based on material properties and trends, the failure
parameters need to be identified Analysis evaluates possible acceleration factors and will
provide some foundation for forensics Temperature, hydrolysis and other environmental factors
specific to storage location have to be isolated Eliminate the outliers such as biological factors, potential
chemical exposure and mechanical stresses
7© 2009 Ops A La Carte
Types of Reliability Testing
Types of reliability testing:1. HALT (AFR)2. ALT3. RDT
Choose Accelerated Life Testing because MTTF is desired
8© 2009 Ops A La Carte
Acceleration Factors, Af for Temperature and RH
Af refers to the amount of energy required to initiate an internal chemical change
As identified the activation energy for TPU is 0.2-1.0 eV
Af depends on choice for activation energy
Test Temp ©
Relative Humidity
Accel.Factor
20 75% 2.740 75% 12.160 75% 46.320 80% 3.240 80% 14.760 80% 56.220 85% 3.840 85% 17.760 85% 67.420 90% 4.640 90% 21.060 90% 80.020 95% 5.440 95% 24.760 95% 94.120 100% 6.340 100% 28.860 100% 109.8
9© 2009 Ops A La Carte
Forensic ALT
Objective is to determine the MTTF using major stressors, adequate sample size and confidence level
25 test samples 50, 100, 150 and 240
test hours 75%, 80%, and 90%
confidence levels Combined Af of 3, 15,
21
10© 2009 Ops A La Carte
Test Methodology
High confidence requirement means 90% level Combined stressors (temperature and humidity Not all TPU will have aged at the same rate Stepped temperature approach, incremental
changes allow for early failures of TPU Let the Customer decide the desired temperature,
suggested between 40°C-60°C
12© 2009 Ops A La Carte
Test Methodology (cont’d) 10°C increments for 10 minutes up to the upper
functional operating temperature limit. At the end of each temperature step perform a visual check
for cracks or crack initiation of the insulation. If a failure is visually found, note the failed temperature
and quantity of failure. Remove the sample and replace with another sample.
Add the relative humidity to 90% wait for temperature to stabilize, since there will be a drop with water vapor
The expected acceleration and hence degradation of the TPU insulation will have been reached.
13© 2009 Ops A La Carte
Visual Checks and Balances
The specimens were to be visually checked every hour over a normal working day.
Failure will likely be rapid with the onset of an initial crack and subsequent propagation.
After reaching the first 50 hours of testing, a MTTF can be determined from calc Table
Could continue to the next milestone of 100 hours and so on up to 200 hours
14© 2009 Ops A La Carte
Some Calculations
50 hours at 40 C / 80% RH we had seven (7) failed out of 25 specimens1. MTTF = 1593 @ 90 % confidence2. MTTF = 1832 @ 80 % confidence3. MTTF = 1936 @ 70 % confidence
15© 2009 Ops A La Carte
Conclusions Isolated failure parameters, identified two as possible
stressors and determined acceleration factors Provided MTTF table with acceleration factors for relative
humidity and temperature with sample size of 25 Customer decided on temperature 40C, relative humidity
of 80% giving us an acceleration factor of 15 7 of 25 samples failed after only 50 hours of testing ALT provided MTTF of 1593 at a confidence of 90% Forensically, testing confirmed stressors, failure
mechanisms (cracking and propagation through the TPU) Limited test time and cost (2.5 days of lab time)
16© 2009 Ops A La Carte