Simulations of embedded lateral stress gauge profiles in shocked targets

Abstract

In principle, stress gauges mounted to measure lateral stresses in a shocked matrix allow the shear strength of the material to be determined. However, interpreting the resistance profiles from lateral stress gauges is hindered by the fact that the stress field in the vicinity of the insulating layer in which the gauges are embedded can differ significantly from the stress field that would be generated in the sample if no gauge were present. A series of high resolution Eulerian hydrocode simulations have been run which suggest that the stresses in the insulating layer vary with distance and time in a way that depends on the thickness of the layer, the shock strength and the elastic and plastic properties of both the layer and the matrix. In particular, if the shock velocity in the matrix material is high the stress at a typical gauge position initially rises to a sharp peak then falls with time, but when the shock velocity in the matrix is low the stress rises relatively gradually throughout the time of interest. The shapes of the stress versus time profiles predicted by the hydrocode compare well with the results of lateral gauge experiments on several different materials. It is concluded that lateral gauges can be used to measure the dynamic strength of materials provided high resolution computer simulation is used to take account of the perturbation of the stress field in the shocked sample caused by the gauges.

Experimental and computational investigation of lateral gauge response in polycarbonate

“…Firstly, the gradient behind the shock appears constant and independent of the nature of gauge encapsulation. This is consistent with the work by Appleby-Thomas et al [5], and suggests that matrix/encapsulation dispersion [7][8][9] has no influence on the measured lateral stress at the gauge. Further, the magnitude of the gradients in Fig.…”

“…Secondly, it is notable that an overshoot in stress on shock arrival occurs for the two dry-joint targets; whereas no such feature is apparent for the glued-joint arrangements. Combined with the negative gradient behind the shock, such behaviour would be consistent with the phenomenon of Mach stem formation (attributable to a lower interlayer shock velocity) proposed by Winter and Harris [7,8] and Winter et al [9]. However, if this were the case, the lack of a shock in the air filled layer should result in a significantly steeper gradient in the dry-joint case.…”

“…However, inclusion of this interlayer is inherently invasive, complicating interpretation. Recent computational modelling results [7][8][9] have identified a potential link between perceived changes in strength behind the shock and shock dispersion between the encapsulation and target material. These studies, plus work by various other authors [10,11], serve to highlight the on-going debate over lateral gauge interpretation.…”

On the importance of encapsulation environment for lateral gauges

“…However, this is an inherently invasive approach, complicating interpretation. Recent computational modelling by Winter and Harris [8,9] and Winter et al [10] has highlighted on-going debate within the shock physics community [2][3][4][5][6][8][9][10][11]. In these works, the complexity of the embedded gauge environment was studied and a potential link between perceived changes in strength behind the shock and shock dispersion between the encapsulation and target material was demonstrated.…”

“…Interestingly, recent extensions of the models discussed in Refs. [8][9][10] have suggested that this equilibration time may also be influenced by extrusion of the central encapsulation material under high-rate loading. To this end, here, the effect of variation of the geometric environment of embedded gauges on their response in tantalum (Ta), which is known to show variation in lateral stress behaviour behindthe-shock [12], was experimentally interrogated.…”