Micromechanisms of ductile fracturing of DH-36 steel plates under impulsive loads and influence of polyurea reinforcing

Micromechanisms of ductile fracturing of DH-36 steel plates subjected to blast-induced pressure loads are studied experimentally using specially designed dynamic testing facilities. The sample geometry is such that it undergoes controllable fracturing, generally initiated near its center and grows first circumferentially and then radially. Selected fractured samples are sectioned normal to the fracture surfaces, at various intervals along the length of the fracture which extends into the necked and finally to the uniformly-deformed material. The thickness profile of the sample at each cross section is analyzed using optical and scanning electron microscopy. In this manner, we have been able to examine the microstructural evolution that has taken place, from necking inception to fracture initiation and growth, leading to a better understanding of the underpinning mechanisms of the fracturing of this material. The observed ductile fracture involves void nucleation, and void growth and coalescence, creating dimpled fracture surfaces. The examination of the microstructure of the deformed steel samples also revealed that the microstructure does not change significantly during the deformation and that there are no shearbands developing in and around the fracture zones. Based on the observed micro-scale deformation, a finite-element model was developed to further study the fracturing process, using a physics-based and experimentally-supported temperature- and rate-sensitive constitutive model for DH-36 steel. The finite-element model was capable of predicting the fracture process of the steel plates rather well. Additional finite-element simulations are performed to investigate the effect of the polyurea coating on the fracturing and fracture resistance of the plates. These also correlated well with the experimental results.