Effects of hygrothermal aging and a silane coupling agent on the tensile properties of injection molded short glass fiber reinforced poly(butylene terephthalate) composites

Displacement controlled experiments on nylon 66, poly(etherether ketone), and poly(ether imide) at room temperature suggest that nonlinear elastcity is not a good model for these polymers. Rather, qualitative evidence is presented that a state variable model shows promise. In this model, the rate of deformation is the sum of the elastic and the inelastic rates of deformation. The elastic rate of deformation is given by an objective formulation of Hooke's law, and the inelastic deformation is an increasing function of the overstress, the difference between the Cauchy stress and the equilibrium stress. The equilibrium stress is a state variable, and represents the stress that can be sustained at rest following deformation. Load controlled tests, intended to verify or falsify the model, show that the creep rate at the same stress level can be different on loading and unloading, and that the creep rate need not increase with an increase in creep stress level. These anomalous results can easily be explained by the introduction of the overstress concept, and by proper evolution of the equilibrium stress. They confirm the usefulness of the overstress concept for the modeling of these polymers.

A state variable model for high strength polymers

In this study, the effects of processing parameters on the mechanical properties of injection molded thermoplastic polyolefin (TPO) foams are investigated. Closed cell TPO foams were prepared by injection molding process. The microstructure of these foamed samples was controlled by carefully altering the processing parameters on the injection molding machine. The foam morphologies were characterized in terms of skin thickness, surface roughness, and relative foam density. Tensile properties and impact resistance of various injection molded TPO samples were correlated with various foam morphologies. The findings show that the mechanical properties are significantly affected by foam morphologies. The experimental results obtained from this study can be used to predict the microstructure and mechanical properties of cellular injection molded TPO foams prepared with different processing parameters.

Effect of Processing Parameters on the Mechanical Properties of Injection Molded Thermoplastic Polyolefin (TPO) Cellular Foams

The development of a finite element program specifically designed to simulate thermoforming in complex three-dimensional geometries is described. The large strain finite element model developed for simulating thermoforming processes is based on a total Lagrangian formulation which results in a nonlinear system of equations that must be solved iteratively. The nonlinear material behavior and contact between the polymer and mold surf aces ieads to additional complications in the numerical solution of the thermoforming simulation problem. In an effort to verify the accuracy of the finite element model developed in this study, analyses are compared with measurements obtained from three-dimensional three-formed parts.

Three-dimensional finite element simulation of thermoforming

Injection molded short glass fiber reinforced poly(butylene terephthalate) was subjected to hygrothermal aging at two different relative humidities—81.2% and 100% RH. A single free phase model of diffusion has been used to analyze the data obtained from the kinetics of moisture absorption study. The diffusion coefficient and the equilibrium moisture content were found to be dependent on the volume fraction of fibers and relative humidity. Incorporation of short glass fibers into a poly(butylene terephthalate) matrix has led to a significant improvement in the retention and recoverability of the tensile properties. Examination of fracture surfaces using a scanning electron microscope (SEM) has revealed some evidence for the hydrolysis of the polymer matrix. The hydrolysis resulted in the formation of microvoids, the absence of a plastic deformation process, and degradation at the fiber-matrix interface.

Effect of moisture absorption on the tensile properties of short glass fiber reinforced poly(butylene terephthalate)

A noncontacting experimental procedure has been used for characterizing the high-temperature large-strain uniaxial behavior of polycarbonate (PC), polyetherimide (PEI), and poly(butylene terephthalate) (PBT), through controlled tests on flat specimens cut from extruded sheet. While each of these polymers exhibits stable necking over a broad temperature range, the transition to the necked state is shown to be more gradual at higher temperatures, resulting in a less sharply defined neck transition region during the drawing process. These trends in necking behavior, and other phenomena such as double necking, which only occur at high temperatures, are illustrated through contour plots of the Cauchy-Green deformation tensor. Elevated temperature true-stress versus stretch data are given for each of these polymers.

High-temperature large-strain behavior of polycarbonate polyetherimide and poly(butylene terephthalate)

The through-thickness variation in the porosity of structural foam material is known to result in different “material properties” when mechanics based on homogeneous materials is used to interpret data from standard tensile and bend tests. Unresolved issues relating to structural design include the specification of the most useful test specimen, the identification of useful material properties, and the application of these properties to part design and analysis. This paper develops procedures for determining the mechanical properties of rigid thermoplastic structural foams, and for the application of these properties to the design of load-bearing components. Rather than molded specimens, it is suggested that specimens cut from large, edge-gated plates be used for determining mechanical properties of structural foams. By modeling foams as continuous but nonhomogeneous materials, it is shown that data from simple tensile and flexural tests can be used in structural analysis to systematically account for the through-thickness variation of material properties.

Mechanical properties of rigid thermoplastic foams—Part I: Experimental considerations

The through-thickness variation in the porosity of structural foam material is known to result in different “material properties” when mechanics based on homogeneous materials is used to interpret data from standard tensile and bend tests. Procedures for determining the mechanical properties of rigid thermoplastic structural foams and for the application of these properties to the design of load-bearing components were developed in a companion paper. This paper reports the mechanical properties of modified polyphenylene oxide foams, such as elastic moduli, ultimate stress and strain, as determined by tests on specimens cut from large, edge-gated foam plates. Tests were conducted to study plate-to-plate variations in properties and to evaluate the effect of specimen thickness. Correlations of tensile and flexural data with the average specimen density are also discussed.

D. A. Ysseldyke

Papers

High-temperature large-strain behavior of polycarbonate polyetherimide and poly(butylene terephthalate)

Mechanical properties of rigid thermoplastic foams—Part I: Experimental considerations

Mechanical properties of rigid thermoplastic foams—Part II: Stiffness and strength data for modified polyphenylene oxide foams