A SEM-Fractographic Study of Dynamic Crack Propagation Effects in Particulate Epoxy Systems under Impact Loading Conditions

In this complex study an attempt was made on the one hand to analyze and understand in a systematic way the exact nature of the formation of certain characteristic energy dissipation-induced fractographic features and patterns/markings revealed by scanning electron microscopy (SEM) in particulate epoxy systems under impact (dynamic) loading conditions, and on the other hand to correlate these patterns and features with relevant crack propagation effects. For this scope a combined approach consisting of a qualitative as well as a semiquantitative analysis was employed. In the qualitative analytical approach it was shown that depending on the actual velocity and direction of crack propagation the above observed fractographic entities can be correlated to certain highly localized energy dissipative processes at front-failures as well as to local inertial molecular mass effects. Depending on the changes in the velocity and direction of propagation, the associated effects may be controlled by two basic processes: the single crack front and the multiple crack front splitting. The first process seemed to be governed by a shear toughness-biased system, whereas the second one used a critical strain energy release rate subcracking mechanism. Under certain conditions both processes may be influenced by inertial molecular effects in promoting the formation of relative-smooth fracture surfaces. The increased presence of particles tends to restrict an increase in the surface roughness due to energy dissipation-induced crack retardation effects. The presence of the notch tends to lower the fracture surface roughness compared to notch-free specimens and also to suppress the occurrence of certain elastic as well as viscoelastic-plastic crack delay effects observed in notch-free specimens in function of particle volume fraction. Based on relevant kinematics-aided modeling and impact energy measurements it seems possible to explain, by a gross semi-quantitative approach, the above particles and notch effects. In this context it seems plausible that the existence of a defect-induced fracturing time spectrum of the propagating crack front, in combination with the `notch-induced shift' behavior of this spectrum, can be valuable for some approximating explanations of the above notch effects and in general the `kinematics' of the surface roughness formation.