Alan J. Lesser

Deformation and yield of epoxy networks in constrained states of stress

A series of epoxy networks were made with molecular weights between crosslinks, Mc, ranging from 380 to 1790 g mol-1. Resins were cast into thin walled hollow cylinders and tested in stress states ranging from uniaxial compression to biaxial tension. These tests elucidated the effects of stress state, strain rate, and Mc on the yield and fracture response of epoxy networks. Throughout the study, the strain rate along the octahedral shear plane, γoct, was maintained constant independent of stress state, for each failure envelope. The hollow cylinder tests showed that the yield behaviour of epoxy networks can be described by a modified von Mises criterion, τocty=τocty0−μσm where τoctg is the octahedral shear stress at yield, τocty0 is the octahedral shear stress at yield in pure shear, μ is the coefficient of internal friction and Vm is the hydrostatic tensile stress imposed on the sample. Furthermore, these tests showed that changes in γoct and Mc only affect τocty0, while μ remains constant. Standard tensile and compression tests were run to confirm the hollow cylinder result and to test the effect of temperature on the yield and brittle response. Tensile tests showed that changes in Mc only affect the glass transition temperature, Tg, of the materials, and the glassy modulus remained independent of Mc. With regard to the yield strength, changes in Mc cause a shift in the Tg of the materials, and the yield strengths of all the materials collapse together at a constant temperature relative to Tg. Finally, yielding of these epoxies was shown to follow an Eyring type flow model over the range of temperatures and strain rates tested.

Morphological study of fatigue-induced damage in isotactic polypropylene

This article reports initial results of an investigation whose aim is to characterize fatigue damage induced in semicrystalline polymers subjected to uniaxial high cycle fatigue. Herein we report results obtained from fatiguing tensile bars of high molecular weight compression-molded alpha-phase iPP. Samples were fatigued for up to one million cycles at a frequency of 2 Hz. During fatigue, in situ measurements of dynamic mechanical response and energy densities were recorded. Postmortem morphological studies were also conducted using SEM of etched surfaces and TOM. The results show that damage formation occurs in a regularly spaced array of crazes. This damage, its evolution, and energetics are discussed as they relate to the overall fatigue life of the material. A methodology to isolate the energy consumption for the formation of a single craze is given.

Solid-state processing of polymer in the presence of supercritical carbon dioxide

Open cell microporous bisphenol-A polycarbonate (PC) films are made by a novel, solvent-free processing: drawing PC in supercritical carbon dioxide (scCO2). The pore size is less than 1.0 m, and the porosity is in the range of 20-70% and is highly tunable. The porous film has mechanical properties nearly as high as the original film. The influence of temperature and pressure on drawability, porosity, and mechanical integrity are systematically studied.

Fracture behavior of dynamically vulcanized thermoplastic elastomers

The energetics and micromechanisms of fracture in model dynamically vulcanized thermoplastic elastomers have been studied. Their fracture toughness values have been quantified under mode 1 loading conditions using both the critical J-integral approach and an essential work-of-fracture method. Additional studies evaluating the effect of specimen geometry are reported. For these studies it was found that center-notched and double edge-notched test geometries were equivalent under J-integral test conditions. The effect of elastomer composition was also studied with regard to fracture resistance. Increasing the weight percentage of both elastomer and processing oil caused a considerable decrease in both the materials resistance to both fracture initiation and fracture propagation. Increasing the molecular weight of the thermoplastic phase caused a smaller reduction in fracture resistance. The phase morphology of one model compound, TPE6114, consists of an isotactic polypropylene-rich matrix containing discrete elastomer-rich domains of a diameter of 1–3 μm. A process zone was associated with fracture in this material. The process zone consists of an array of voids and crazes that were 10–30 μm in diameter, an order of magnitude larger than the elastomer-rich domains. These were characterized by scanning electron microscopy (SEM) and optical microscopy. The crazes were found to grow at an angle oblique to the overall crack growth direction. Ruthenium stained SEM samples showed that these crazes and voids occur in both the polypropylene and elastomer domains, and that at least some of the craze fibrils are composed of the elastomeric phase.

Morphological Study of Fatigue Induced Damage in Semicrystalline Polymers

Publisher Summary At high frequencies and/or stress levels, the fatigue life of the polymer is dominated by hysteretic heating. As the stress level and/or test frequency are reduced, another failure mode is observed. This failure mode is more brittle, and postmortem fractographic analyses have shown the failure mechanism is associated with the nucleation and growth of flaws in the material to a critical size. This regime is referred to as the mechanically dominated regime or the high-cycle regime. The total lifetime in the mechanically dominated regime is usually attributed to the nucleation and growth of defects to a critical size. Crack growth kinetics has been measured on a wide range of polymers and has generally shown to follow a Paris-type law once the flaw grows beyond the threshold flaw size. The kinetics of damage evolution is described on the nanometer scale by a thermally activated process and on the rnillimeter scale by a Paris-type law. This chapter summarizes results from the first phase of an investigation to qualitatively and quantitatively describe fatigue damage and its evolution in a commonly used semicrystalline polymer. For this study, isotactic polypropylene (iPP) was selected as a model polymer characteristic of many semicrystalline thermoplastics. Results from an experimental investigation, whereby in situ measurements of dynamic visco-elastic behavior and energy density evolution are complemented by a comprehensive microscopic investigation to describe the kinetics and energetics of damage evolution on the micrometer scale are reported.

Evaluation of Impact Damage Resistance in Laminated Composites Using Resins with Different Crosslink Densities

It is generally recognized that fiber-reinforced laminated composites are susceptible to damage resulting from low-velocity impacts. Over recent years, many strategies have been devised to increase the fracture toughness of resin matrix materials with the aim of improving the composites overall resistance to impact damage. One popular strategy for enhancing the fracture toughness of thermosets involves increasing its molecular weight between crosslinks which, in turn, enhances the resins ductility. In this paper, we investigate the efficiency of this toughening approach with regard to resisting damage in composite laminates subjected to lowvelocity impacts. Generic damage characteristics and mechanisms are reviewed and it is shown that two different events occur during the impact process. First, the laminate experiences a local failure which resembles a Hertzian fracture process followed by subsequent delamination between the plies. Results are presented illustrating the effects that systematically increasing the molecular weight between crosslinks of the resin has on each of these mechanisms. Also, the residual compressive strength (Compression After Impact) of the laminates made with these resins is presented.