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 mu m. 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 mu m 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. (C) 2000 John Wiley & Sons, Inc.