Thermoplastic composites are suitable alternatives to metals in some load-bearing applications such as in the automotive industry due to a large number of advantages they present. These include light weight, ease of processing for complex geometries at high production rate, outstanding cost to performance ratio, ability to reprocess, and corrosion resistance. Addition of fillers such as talc or reinforcements such as short glass fibers can improve the mechanical performance of unreinforced thermoplastics to a high degree.
Components made of thermoplastic composites are typically subjected to complex loadings in applications including static, cyclic, thermal, and their combinations. These applications may also involve environmental conditions such as elevated temperature and moisture which can dramatically affect their mechanical properties.
This study investigated tensile, creep, fatigue, creep-fatigue interaction, and thermo-mechanical fatigue (TMF) behaviors of five thermoplastic composites including short glass fiber reinforced and talc-filled polypropylene, short glass fiber reinforced polyamide-6.6, and short glass fiber reinforced polyphenylene ether and polystyrene under a variety of conditions. The main objectives were to evaluate aforementioned mechanical behaviors of these materials at elevated temperatures and to develop predictive models to reduce their development cost and time.
Tensile behavior was investigated including effects of temperature, moisture, and hygrothermal aging. Kinetics of water absorption and desorption were investigated for polyamide-6.6 composite and Fickian behavior was observed. The reductions in tensile strength and elastic modulus due to water absorption were represented by mathematical relations as a function of moisture content. In addition to moisture content, aging time was also found to influence the tensile behavior. A parameter was introduced for correlations of normalized stiffness and strength with different aging times and temperatures. Higher strength and stiffness were obtained for re-dried specimens after aging which was explained by an increase in crystallinity. Mechanisms of failure were identified based on fracture surface microscopic analysis for different conditions.
Creep behavior was investigated and modeled at room and elevated temperatures. Creep strength decreased and both creep strain and creep rate increased with increasing temperature. The Larson-Miller parameter was able to correlate the creep rupture data of all materials. The Monkman-Grant relation and its modification were successfully used to correlate minimum creep rate, time to rupture, and strain at rupture data. The Findley power law and time-stress superposition principle (TSS) were used to represent non-linear viscoelastic creep curves. Long-term creep behavior was also satisfactory predicted based on short-term test data using the TSS principle.
Effect of cycling frequency on fatigue behavior was investigated by conducting load-controlled fatigue tests at several stress ratios and at several temperatures. A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in dynamic mechanical analysis tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson-Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together.
Effects of temperature and mean stress on fatigue behavior were also investigated by conducting load-controlled fatigue tests under positive stress ratios and at room and elevated temperatures. Larson-Miller parameter was used and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures. Effect of mean stress on fatigue life was significant for some of the studied materials, however, for the polyphenylene ether and polystyrene blend no effect of mean stress was observed. Modified Goodman and Walker mean stress equations were evaluated for their ability to correlate mean stress data. A general fatigue life prediction model was also used to account for the effects of mean stress, temperature, anisotropy, and frequency.
Creep-fatigue tests were conducted using trapezoidal load signal with hold-time periods. Effects of temperature, frequency, load level, mean stress, and hold-stress position on creep-fatigue interaction behavior were studied. In-phase TMF tests were conducted on polyamide-based composite for the temperature variation between 85 to 120 °C. Significant non-linearity was observed for the interaction of creep and fatigue damage. The applicability of Chaboche non-linear creep-fatigue interaction model to predict creep-fatigue and TMF lives for thermoplastic composites was investigated. A frequency term was added to the model to consider the beneficial effect of increased frequency observed for some the studied materials. The Chaboche model constants were obtained by using pure fatigue, pure creep, and one creep-fatigue interaction experimental data. More than 90% of life predictions by the Chaboche model were within a factor of 2 of the experimental life for both creep-fatigue and TMF test conditions.