Experiments on dynamic fracture and friction

Dynamic fracture and friction under dynamic loading conditions are examined through direct observations in carefully controlled experiments. An electromagnetic loading device is used to generate a compressive stress wave and the full-field optical technique of dynamic photoelasticity and high-speed photography are used as diagnostic tools. In addition, a new optical method to determine both principal stresses and their orientations simultaneously is developed. In this dissertation, the results from experiments aimed at investigating fracture and frictional sliding under shear loading conditions are presented. For dynamic fracture problems, we examine shear cracks in homogeneous materials by introducing a groove in the specimen and trapping the crack to grow within it. The groove does not affect the fracture mechanisms inherent to the material, but influences the energy flux and loading symmetry. Such shear induced cracks growing at speeds in the intersonic regime are demonstrated. Furthermore, it is shown that the main mechanism of the shear crack growth is the sequential nucleation, growth and coalescence of echelon cracks. The spacing and angle relative to the groove plane of the echelon cracks are measured directly from the experimental specimen. Numerical simulation shows that the echelon cracks are well aligned perpendicular the maximum principal (tensile) stress generated in this specimen. The spacing is interpreted as an intrinsic characteristic of the failure process. These experiments also enable the determination of the dynamic failure stress at which microcracks are nucleated. For frictional sliding along an interface, a novel apparatus has been constructed for the understanding of the nature of dynamic friction and studying the slip pulse propagation under extremely high rates of loading. In experiments, dynamic slip along the frictional interface are triggered either by a compressional planar wave, or a compressional cylindrical wave. In both methods, the stress state across a frictional interface is brought to the critical state behind the wave. When slip occurs across the interface, it is forced to run along the interface at the speed of this wave. Several interesting results on frictional sliding are presented. First, the stress drop across the slip interface is characterized from isochromatic fringe patterns. Evaluation of the fringe pattern yields a description of the evolution of the shear stress both ahead and behind the slip event. The shear stress is seen to build up gradually to a maximum at the leading edge of a slip pulse and to decay rapidly over a few mm slip length. Second, slip pulses can be generated from frictional interfaces through interaction with propagating stress waves and are observed to propagate at a speed controlled by the wave that generates slip. Accumulation of fringes near the slip pulse and the orientation of the Mach lines suggestion the slip pulse propagates at a speed close to the dilatational wave speed. Finally, new optical method in which the classical methods of photoelasticity and Mach-Zehnder interferometry are used in a combined arrangement is presented. In dynamic problems the measurement is made with a high-speed photodetector at very high temporal resolution at a single point or a small array depending on the detector array and recording device; this eliminates the need for a high-speed photographic system, but more importantly provides complete, time-resolved evolution of all stress components. Examples of application of the method are demonstrated.Aerospace Engineering and Engineering Mechanic