Title: Fatigue Life Prediction for Polymeric Composite Materials Subjected to Bending Loads at Elevated Tem
1Fatigue Life Prediction for Polymeric Composite
Materials Subjected to Bending Loads at Elevated
Temperatures
- C. Mahieux, J. Jackson, S. Pipik, S. Case,
- and K. Reifsnider
- Materials Response Group
- Virginia Tech
- Blacksburg, VA 24061-0219, USA
2Outline
- Motivation for the research
- Modeling philosophy
- Development of experimental inputs
- Modeling predictions and comparison to data
- Conclusions
- Ongoing and future work
3Use of flexible pipe in offshore oil industry
(Wellstream Inc.)
- Advantages of Polymer Composite material in
flexible pipes - 30 weight reduction - greater depths, lower deck
loads - Corrosion resistance - longer life, more fluid
options
4Typical loading environment
- Mechanical loads
- Tensile loads due to hanging weight
- Cyclic Bending loads due to wave-motion
- Positive and negative internal pressures
- Environmental loads
- Exposure to elevated temperatures
- Exposure to aggressive chemicals
5Material
- Carbon Fiber/Polyphenylene Sulfide (PPS)
- Manufactured by Baycomp, Ontario Canada
6The use of remaining strength as a state variable
- Track remaining strength of the critical element
during the fatigue process - Define a scalar failure function based upon
tensor strength and stresses use this failure
function for calculations - May include the effects of changing loading
conditions
Sult
Stress or Strength
Life Curve
t1
t2
Time
7Mathematical representation
- Some possible choices
- Maximum stress/strain
- Tsai-Hill/Tsai-Wu
- Define a failure criterion, Fa, and a remaining
strength in terms of that failure criterion, Fr - From kinetics we have the change in remaining
strength over the interval - Fa is constant over
- For the special case in which is equal to zero
8Mathematical representation
- For step loading, introduce the concept of
"pseudo-time" based on the idea of equivalent
damage
9Mathematical representation
- Calculate change in remaining strength over the
interval
10Approach for variable loading with rupture and
fatigue acting
- Divide each step of loading into time increments
- Treat each increment as a stress rupture problem
(constant applied stress and temperature) - Reduce residual strength due to time dependent
damage accumulation - Refine number of intervals until residual
strength converges - Input next load level
- Check for load reversal. If load reversal,
increment by 1/2 cycle and reduce residual
strength due to fatigue damage accumulation
11Prediction of combined rupture and fatigue on
coupon level
- Characterize elevated temperature effect with
bending rupture tests at temperature - Characterize fatigue effect with room temperature
fatigue tests - Combine effects using analysis and compare to
experimental results
12Characterize temperature effect
- Bending rupture tests at 75C and 90C
13Characterize temperature effect(time to rupture)
End-loaded bending rupture test results at 90C
14Characterize temperature effect(remaining
strength)
End-loaded bending rupture test results at 90C
15Characterize fatigue effect(cycles to failure)
End-loaded bending fatigue tests
16Characterize fatigue effect(cycles to failure)
End-loaded bending fatigue tests
17Predict of elevated temperature fatigue behavior
18Predict of elevated temperature fatigue behavior
19Conclusions
- A life prediction method for composites based
upon remaining strength has been developed. The
general approach is - Conduct characterization tests and model behavior
(under a single condition) - Combine effects using life prediction analysis
- Validate life prediction using controlled
experimental results - Poor prediction at 90C
- Better prediction at 75C
20Ongoing/Future work
- Refine analysis to eliminate discrepancies
between model/experiments - Further understanding of the failure process
less reliance on phenomenological expressions - Conducting additional experiments to
validate/repudiate the approach
21Acknowledgements
- Air Force Office of Scientific Research
- National Science Foundation
- NASA Langley Research Center