Abstract
Dynamic fracture behavior of syntactic foams made of thin walled microballoons dispersed in epoxy matrix is studied. Monotonic reduction in dynamic Young’s modulus with increasing volume fraction of microballoons is observed using ultrasonic pulse-echo and density measurements. The measurements are also in good agreement with the Hashin-Shtrikman lower-bound predictions for elastic porous solids. Dynamic crack initiation toughness and crack growth behaviors are examined using instrumented drop-tower tests and optical measurements. Crack initiation toughness measurements show a linear relationship with Young’s modulus over the entire range of volume fraction of microballoons studied. A proposed model based on a simple extension of micro-mechanics prediction agrees well with the measurements. The optical method of Coherent Gradient Sensing (CGS) has been used along with high-speed photography to record crack tip deformation histories in syntactic foam samples subjected to impact loading. Pre- and post-crack initiation events have been successfully captured and apparent dynamic stress intensity factor histories are extracted from the interferograms. Optically measured stress intensity factors at crack initiation and during crack growth also decrease with increasing volume fraction of microballoons. Increasing crack speeds with volume fraction of microballoons has become evident. No significant dependence of dynamic fracture toughness on crack speed is seen in any of the volume fractions studied.