Conclusion 1

1
Stepped Isothermal Method for Creep Rupture
Studies of Aramid Fibres
Nadun Alwis and Chris Burgoyne
2
Overview

• Introduction to stress-rupture
• Accelerated testing methods
• - Time Temperature Superposition (TTSP)
• - Stepped Isothermal Method (SIM)
• Results and discussion
• Conclusions

3
Stress-rupture - Creep until failure.
Conventional creep testing - long durations to
cause failure at low loads.
4

• Test data
• Lifetime models to predict long-term
  stress-rupture
• Probabilistic predictions for design (5 95)

Applied stress
ln(time to failure)
5
Multiple models
Different models give very different results
6

• Reliability of lifetime extrapolations is
  questionable.
• Need Stress-rupture data at low stress levels.
• A long times needed for low stress tests.
• Accelerated creep tests have been suggested.

7
Time temperature superposition principle (TTSP))

• Uses multiple specimens at the same load
• Each specimen creeps at a different temperature
  for short time

• At highest temperature specimen creeps to
  failure.
• A reference temperature is selected (Tr).
• Other creep curves are shifted to superpose to a
  master curve at Tr .

8
Shift on log (time) scale if Arrhenius applies
9
TTSP result
Master curve at 70 ABL
10
Stepped Isothermal Method (SIM)

• Uses one specimen.
• Load applied
• Temperature increased in steps
• Specimen fails due to creep
• Complete creep curve can be obtained from one
  specimen.
• Curves need to be adjusted for-
• 1. Variations in thermal expansion (Vertical
  shift).
• 2. Thermal history (Rescaling)
• 3. Shifting as for the TTSP

11
Vertical shift (v)

• Thermal expansion (or contraction) occurs while
  the temperature is being increased.
• The specimen continues to creep.

The adjusted strain just after the temperature
step can be found by
(a) by adding the thermal contraction, so
(b) by adding the creep over temperature step, so
12
Coefficient of thermal expansion (CTE) not
accurately known. Difficult to calculate
Method (b) uses measured values. Changes of the
creep rate over time tc can be found by
conducting separate creep tests from temperature
T1 to various temperatures T2 and T3
13
Perform two tests - T1 to T2 T1 to T3
Measure creep rates before and after
Known temperature rate
Hence
14
Rescaling (r)

• At higher temperatures specimen did not start
  from zero
• Rescaling accounts for the thermal history.
• Shift in the linear time scale
• PQ should be same as a single TTSP curve

15
P3
i represents each data point j represents
each adjustment
16
Material and experimental set-up

• Kevlar-49 yarns are used in TTSP and SIM tests
• Average breaking load was found to be 445 N

17
Results and discussion
Master curves with conventional creep data at 50
ABL
18
Master curves with conventional creep data at 70
ABL
19
All SIM master curves at 50 ABL
20
All SIM master curves at 70 ABL
21
Stress-rupture data
22
Conclusions

• TTSP and SIM can be used to obtain the long-term
  stress-rupture data.
• SIM can be automated.
• SIM is better than TTSP and conventional creep
  tests.
• SIM curves show repeatability.
• Many stress-rupture data points can be generated
  using SIM and included in the lifetime models to
  improve the reliability.
• The Reverse curve in the SIM master curve is
  notable and further investigation is necessary.

23
Question

• Is the resulting Master Curve the same as the
  curve that would have been obtained from a very
  long conventional test?

• How can we tell?