Plastic Fatigue and Creep

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Fatigue of Plastic

Fatigue here refers to the decrease in stiffness or rupture resulting from cyclic loading, typically at a constant stress, below the yield stress.

Experimentally, a cyclic stress is applied to a plastic test specimen and held. The number of cycles until a failure is achieved is recorded. One data point is recorded for each test. A table of stress levels and cycles to failure results are collected for multiple tests. Typically, the test plan is based on the desire to generate a table of cycles to failure at particular values. This presents a challenge to laboratory operations because only the stresses are controlled, the cycles to failure are material based. This makes it difficult to plan the time on instruments.

When materials are cyclically loaded, there is some level of heat generation in the material due to viscoelastic heat generation. The temperature of the test specimen may increase and melt the specimen. This depends on the material, the stress, and the frequency of the applied cyclic loading. Some testing procedures call out a surface temperature increase that is acceptable. In general, most fatigue testing of plastics is performed in the range of 1 to 5 Hz. As such, the duration of plastic fatigue experiments may be long compared to less viscoelastic metals.

Some plastic fatigue test plans are performed by observing the growth in a pre-cut crack in a plastic specimen. In this case, the growth of the crack is measured and reported throughout an experiment.

Fatigue test Instron instrument with plastic tensile specimen in an environmental chamber.
Fatigue Test Instrument
Fatigue cracked plastic test specimen.
Crack Growth Specimen
Plastic fatigue crack growth test data.
Crack Growth Data in a Plastic with a Pre-Cut
Plastic fatigue crack growth test specimen in grips.
Plastic Compact Tension Specimen Growing a Crack
Fiber filled plastic fatigue s-n test data in 3 flow directions.
Fatigue Test Data for 3 Flow Directions of a Fiber Filled Plastic

Creep Of Plastic

As used here, creep refers to the increase in strain resulting from a constant stress. 

Experimentally, stress is applied to a plastic test specimen and held constant. The resulting strain change is measured over time. These experiments are typically performed at the highest temperatures anticipated in use. Creep measurements can be 10 minutes in duration or 2000 hours in duration.

The test plan design depends on the material and most importantly, the application use of the plastic material. For example, automotive powertrain applications where structural plastics replace metals, even a 0.1% strain change over 500 hours may be unacceptable. These creep tests will require very precise and stable strain measuring and force control. On the other hand, decorative plastic covers in consumer products may strain as much as 5% strain over 500 hours and have acceptable performance.

Some test plans measure strain for a set amount of time. Creep rupture experiments measure strain but end when a particular maximum strain or complete rupture occurs. Creep rupture experiments present a challenge to laboratory operations because the duration time on the instruments in use is somewhat unpredictable.

At Axel, we use electromechanical instruments, servohydraulic instruments and dead weight instruments. Any of these may use mechanical extensometers, video extensometers or simple grip displacement measuring.  The instrumentation is based on the test plan requirements.

Plastic creep test data at 4 stress levels at 120C.
Creep Test Data at 4 Different Stress Levels
Servohydraulic test instrument configured for high strain resolution creep measurement.
A Servohydraulic Instrument with a Chamber and Camera to Capture Creep Straining
Dead weight creep test instruments with high resolution video strain measuring.
Dead Weight Stands Equipped with High Resolution Strain Imaging