Papers by Mohammadreza Eftekhari
Variable amplitude fatigue behavior of injection molded thermoplastics was investigated. A neat t... more Variable amplitude fatigue behavior of injection molded thermoplastics was investigated. A neat thermoplastic (polypropylene impact co-polymer) and a thermoplastic composite made of polybutylene tereph-thalate (PBT) with 30 wt% short glass fibers were used for the experimental study. Fatigue tests were conducted on both un-notched and notched specimens using plate-type specimens. For the notched specimens , a circular hole was drilled in the center of the specimens. Two-step loading (high-low and low-high) tests at two cycle ratios of 0.2 and 0.5 and stress ratios of R = 0.1 and À1 were conducted to investigate load sequence and mean stress effects, as well as the accuracy of linear damage rule. Periodic overload and service load history tests were also conducted to investigate the effect of overloads, cycling frequency, small stress cycles, and maximum stress level. A strengthening effect of increased frequency was observed for both materials and the effect of small stress cycles was found to be negligible. A general fatigue damage model along with the rainflow cycle counting method and linear damage rule were used for life predictions of variable amplitude service load history tests. More than 89% of the life predictions based on the modeling procedure used were within a factor of 3 of the experimental lives for both materials.
Creep behavior of neat, talc-filled, and short glass fiber reinforced injection molded thermoplas... more Creep behavior of neat, talc-filled, and short glass fiber reinforced injection molded thermoplastic composites were investigated and modeled at room and elevated temperatures. Creep strength decreased and both creep strain and creep rate increased with increasing temperature. The temperature effect was more significant for samples with glass fibers in the transverse to the load direction, as compared with the longitudinal direction. The LarsoneMiller parameter was able to correlate the creep rupture data of all materials. The MonkmaneGrant 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 timeestress superposition principle (TSS) were used to represent nonlinear viscoelastic creep curves. Long-term creep behavior was also satisfactory predicted based on short-term test data using the TSS principle.
Creep-fatigue interaction and thermo-mechanical fatigue (TMF) behaviors of injection molded therm... more Creep-fatigue interaction and thermo-mechanical fatigue (TMF) behaviors of injection molded thermoplastic composites were investigated. Talc-filled polypropylene and short glass fiber reinforced polypropylene, polyamide-6.6, and a blend of polyphenylene-ether and polystyrene were used for the experimental study. Trapezoidal load signal with hold time periods was used for the creep-fatigue testing. Effects of temperature, frequency, load level, and hold-stress position on creep-fatigue interaction behavior were studied. In-phase TMF tests were conducted on polyamide-based composites for the temperature variation between 85 and 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 thermoplastics. More than 90% of the life predictions by the Chaboche model were within a factor of 2 of the experimental life for both creep-fatigue and TMF test conditions.
An experimental study was conducted to investigate the effect of temperature, moisture, and hygro... more An experimental study was conducted to investigate the effect of temperature, moisture, and hygro-thermal aging on the tensile behavior of thermoplastic composites. Four different composites including talc filled and short glass fiber reinforced polypropylene, short glass fiber reinforced polyamide-6.6, and a blend of polyphenylene ether and polystyrene with short glass fibers were used for the study. Kinetics of water absorption and desorption were investigated for polyamide-6.6 composites and Fickian behavior was observed. The reductions in tensile strength and elastic modulus due to water absorption are 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 is introduced for correlation of normalized stiffness and strength with different aging times and temperatures. Higher strength and stiffness obtained for re-dried samples after aging was explained by an increase in crystallinity.
An experimental study was conducted to evaluate the variable amplitude fatigue behavior of a neat... more An experimental study was conducted to evaluate the variable amplitude fatigue behavior of a neat polymer (polypropylene impact
co-polymer) and a polymer composite made of polybutylene terephthalate (PBT) with 30 wt% short glass fibers. Fatigue tests were
conducted on un-notched and notched specimens at room temperatures. Plate-type specimens were prepared in the transverse direction
with respect to the injection mold flow direction and a circular hole was drilled in the center of notched specimens. Two-step loadings
(high-low and low-high) tests at two damage ratio of 0.2 and 0.5 at stress ratios of R = 0.1 and -1 were conducted to investigate load
sequence effects and prediction accuracy of the linear damage rule. Different behaviors were observed for unreinforced and short glass
fiber reinforced polymers under the two-step loading tests. Periodic overload tests were also conducted to evaluate the effect of
overloads during cyclic loading on fatigue life and this effect was found to be less pronounced for polymeric materials, as compared to
metallic materials.
Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinf... more Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinforced injection molded polymer composites was investigated by conducting load-controlled fatigue tests at several stress ratios (R = À1, 0.1, and 0.3) and at several temperatures (T = 23, 85 and 120°C). 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 DMA 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. An analytical fatigue life estimation model was also used to consider the strengthening effect of frequency in addition to mean stress, fiber orientation, and temperature effects on fatigue life.
A B S T R A C T Polymer composites are suitable alternatives to metals in some applications as th... more A B S T R A C T Polymer composites are suitable alternatives to metals in some applications as they are cost effective, lightweight and corrosion resistant. Short fibre reinforced polymer composites (SFRPCs) are typically subjected to complex loadings in applications, including static, cyclic, thermal and their combinations. These applications may also involve harsh environmental conditions such as elevated temperature and moisture which can dramatically affect mechanical properties. In this paper, a broad survey of the literature on mechanical behaviour of SFRPCs at elevated temperatures is presented. The mechanical behaviours included consist of tensile, creep, isothermal fatigue, thermo-mechanical fatigue and creep-fatigue interaction. Environmental effects such as moisture and ageing at elevated temperatures are also included. The studies reviewed include experimental works, modelling works and failure mechanisms studies. A critical assessment of the information from the literature presented for each type of behaviour is also provided.
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Papers by Mohammadreza Eftekhari
co-polymer) and a polymer composite made of polybutylene terephthalate (PBT) with 30 wt% short glass fibers. Fatigue tests were
conducted on un-notched and notched specimens at room temperatures. Plate-type specimens were prepared in the transverse direction
with respect to the injection mold flow direction and a circular hole was drilled in the center of notched specimens. Two-step loadings
(high-low and low-high) tests at two damage ratio of 0.2 and 0.5 at stress ratios of R = 0.1 and -1 were conducted to investigate load
sequence effects and prediction accuracy of the linear damage rule. Different behaviors were observed for unreinforced and short glass
fiber reinforced polymers under the two-step loading tests. Periodic overload tests were also conducted to evaluate the effect of
overloads during cyclic loading on fatigue life and this effect was found to be less pronounced for polymeric materials, as compared to
metallic materials.
co-polymer) and a polymer composite made of polybutylene terephthalate (PBT) with 30 wt% short glass fibers. Fatigue tests were
conducted on un-notched and notched specimens at room temperatures. Plate-type specimens were prepared in the transverse direction
with respect to the injection mold flow direction and a circular hole was drilled in the center of notched specimens. Two-step loadings
(high-low and low-high) tests at two damage ratio of 0.2 and 0.5 at stress ratios of R = 0.1 and -1 were conducted to investigate load
sequence effects and prediction accuracy of the linear damage rule. Different behaviors were observed for unreinforced and short glass
fiber reinforced polymers under the two-step loading tests. Periodic overload tests were also conducted to evaluate the effect of
overloads during cyclic loading on fatigue life and this effect was found to be less pronounced for polymeric materials, as compared to
metallic materials.