Reduction In Fracture Resistance Research Articles

Fracture behavior of high-strength, high-conductivity copper alloys

The fracture behavior of three copper alloys, namely, one dispersion-strengthened alloy: GlidCop™CuAl25, and two precipitation-hardened alloys: Hycon3HP™CuNiBe and Elbrodur CuCrZr, was investigated in the temperature range 20–300°C in vacuum. The results show that all these three alloys experienced a loss of fracture resistance with increasing test temperature. In the case of the CuNiBe alloy, the fracture resistance drops very rapidly as test temperature increases, and the other two alloys also experience drops in toughness, but not quite to the same extent. In fact, the fracture resistance of CuCrZr is affected only moderately by test temperature. The reduction of fracture resistance with increasing temperature in vacuum shows that the environment is not the only factor responsible for poor toughness. Further, microstructural analysis of the CuNiBe alloy shows that changes in grain boundary microstructure resulted from discontinuous precipitation. This is assumed to have a significant effect on the fracture behavior of this alloy.

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 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

Testing the tensile properties of ceramic-matrix composites

This article reports the development of a mechanical testing methodology (including fixtures, extensometry, temperature control, and calibration) and procedure (including control mode options, and analysis) for ceramic-matrix composites (CMCs). Six different CMCs were tensile tested at room temperature (RT) and 1,000°C. An appreciable reduction in fracture resistance was measured for most of the systems at the high temperature, but there was little effect of an order-of-magnitude change in strain rate at either temperature. The reduced properties at 1,000°C could be essentially duplicated at RT for several of the composites by prior exposure in air at 1,000°C. The same exposure in vacuum was much less damaging; thus, oxygen would appear to be the culprit. Fractographic evidence indicated that fiber pullout in any system was a function of test temperature rather than fracture strength. Moreover, the incidence and magnitude of energy-release events during testing varied among the systems and revealed no unique connection with the state of damage induced by the prior exposures. These observations do not fit readily into any simple theoretical model. However, they do serve to identify potential limitations for this class of composite in terms of sensitivity to thermal transients in a non-inert environment.