Monotonic and Fatigue Testing of Polymer and Composite Materials Used in Heavy Duty Trucks

Improved material models for engineered polymer and composite materials including both monotonic and fatigue characteristics are necessary for creating more accurate digital simulations for heavy duty trucks. Unlike steel and other alloys that are commonly included in truck designs, these advanced polymer materials do not have pre-existing fatigue characteristic data. Additionally, there are no individual standard test procedures that can be commonly cited and followed during a research program. These materials are found in hoods, dashboards, body panels and splash shields of trucks, and are subject to cyclic loading conditions at various amplitudes and durations throughout the entire use or “duty cycle” of the vehicle. The applied loads vary between truck models, as some trucks will be used for vocational purposes and others will remain on the highway. This paper describes the testing of isotropic non-reinforced, and anisotropic glass-fiber-reinforced polymers and the subsequent calculation of the monotonic and fatigue properties that are needed to describe their behavior under various loading conditions. Material characteristics are measured using a series of constant amplitude strain-controlled fatigue tests that follow standard practices from ASTM D638 (Standard Test Method for Tensile Properties of Plastics), ASTM E606 (Standard Practice for Strain-Controlled Fatigue Testing) methods, and SAE J1099 (Technical Report on Low Cycle Fatigue Properties of Ferrous and Non-Ferrous Materials). The ASTM D638 Type 1 coupon geometry is used for all materials, with a varied sample thickness and length. An axial extensometer is incorporated to measure strain data through the duration of all tests, and an anti-buckling fixture is installed during cyclic tests to eliminate any bending in the specimen during the compressive portion of the fully-reversed waveform. A transverse extensometer is also installed on the gauge length of the material coupons to measure instantaneous cross-sectional area as well as Poisson’s ratio during monotonic testing. The data collected through the monotonic testing procedure is used to calculate Young’s Modulus, Poisson’s ratio, ultimate tensile strength, elongation (% strain), yield strength and strain, and true fracture strength and strain. The fatigue testing procedure yields data that can be used to calculate the fatigue strength coefficient (σf′), fatigue strength exponent (b), fatigue ductility coefficient (εf′), and fatigue ductility exponent (c). These parameters provide accurate stress-strain, cyclic stress-strain, and strain-life curves for the materials in question. A method will also be suggested for calculating the stress-life fatigue parameters, stress range intercept and slope, from the strain-controlled data. Furthermore, mold-flow analysis is applied to predict general orientation of the reinforcement fibers induced by the direction of material flow as a part is injection-molded. The calculated monotonic and fatigue parameters in conjunction with mold-flow analysis can immediately be applied within digital s imulations, allowing improved accuracy in life-expectancy estimations for truck parts.