Showing papers by "Alan J. Lesser published in 2000"

Intercalated clay nanocomposites: Morphology, mechanics and fracture behavior

Abstract: Intercalated nanocomposites of modified montmorillonite clays in a glassy epoxy were prepared by crosslinking with commercially available aliphatic diamine curing agents. These materials are shown to have improved Young’s modulus but corresponding reductions in ultimate strength and strain to failure. These results are consistent with most particulate filled systems, The macroscopic compressive behavior is unchanged, although the failure mechanism in compression varies from the unmodified samples. The fracture toughness of these materials is investigated and improvements in toughness values of 200% over unmodified resis are demonstrated. The fracture surface topology is examined using SEM and tappin-mode AFM and showm to be related to the clay morphology of the system.

Fracture behavior of dynamically vulcanized thermoplastic elastomers

Abstract: 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 material's 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. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 763–770, 2000

Yield behavior and failure response of an aliphatic polyketone terpolymer subjected to multi-axial stress states

Abstract: The yield behavior of an engineering thermoplastic under biaxial stress states has been investigated. The material considered is an aliphatic polyketone terpolymer. Multiaxial testing was performed on thin-walled hollow cylinders at four different temperatures and three strain rates. Various stress states were applied in order to develop failure envelopes. Within each envelope, the nominal strain rate along the octahedral shear plane, γoct, was held constant. These tests were performed at 0, 20, 50 and 80°C at γoct = 0.05 min−1. At 20°C, samples were also tested at γoct = 0.005 and 0.5 min−1. Below the Tg of 12°C, failures in all stress states investigated, except axial compression, were brittle. At temperatures of 50 and 80°C, all failures were ductile. At 20°C, both ductile and brittle failures were observed. Although the rate affected the yield strength of the material, it had little effect on the mode of failure. In contrast, the temperature had a significant effect on the yield strength and mode of failure. While the effect of strain rate on yield strength was greater in the hoop direction than axial, the opposite was true for the effect of temperature. It was also observed that the state of stress played a significant role in the material failure.

Yield modeling of an aliphatic polyketone terpolymer in multiaxial stress states

Abstract: A continuum yield model for pressure dependent anisotropic materials that accounts for the stress state has been combined with a thermally activated model to account for the temperature and rate dependence of yield stress. The resulting model has been applied to describe the yield behavior of a semicrystalline aliphatic polyketone terpolymer. Utilizing biaxial data from this thermoplastic, best-fit values for loading along principal directions were determined. With these principal values, predictions for the yield strength under biaxial loading conditions were made. These predictions have been compared to experimental data obtained from the material at four different temperatures and three different strain rates. The experimental and theoretical results appear to compare quite well with each other.

In situ orientation of linear low density polyethylene films subjected to Mode I fracture load

Abstract: An in situ technique has been developed to study the deformation and orientation near a crack tip during Mode I fracture. The technique employs a pair of rotating crossed polar films positioned on either side of the sample. Images gained from transmitted light show kidney shaped process zones ahead of the crack tip that increase in size with deflection, primarily by isotropic expansion. Extinction bands, which are regions of either unoriented material or material oriented in the direction of one of the two polars, are observed as a function of the angular position of the polars with respect to the loading direction. A set of extinction bands at various crossed polar positions provides orientation direction information within the sample. Continuous flow of the molecules around the crack tip is observed at several stages of deformation and for all films tested. The orientation field around the crack tip evolves with increasing radius of curvature of the crack tip. A transition in orientation is observed when the cracks are coincident with the orientation direction of the film but not when they are normal to the orientation direction. The technique of rotating crossed polars is successfully used to determine the orientation direction in the vicinity of a crack tip for linear low density polyethylene during several stages of Mode I loading. The advantage of the technique is that in situ data may be collected quickly when compared to techniques such as X-ray scattering, which rely on data collection through scanning and not the parallel data collection utilized herein. The authors acknowledge that this technique is limited to material that transmits light.