A Novel Approach to Dynamic Equi-Biaxial Testing of Thin Flexible Materials Using the Ring-on-Ring Test Method

Abstract

Background

The current ASTM formulation for determining dynamic ring-on-ring test method is applicable for thick plates and is not suitable for thin plates that can undergo large flexural deformation where membrane stresses dominate.

Objective

The objective is to design and develop a new dynamic ring-on-ring test method with the ability to accurately measure load and visually access the tensile surface of a specimen for tracking failure. It is also aimed to develop a scientifically robust test procedure and analysis method to validate this new design for obtaining accurate biaxial flexural strength of thin flexible plates.

Methods

A unique load-cell assembly that houses a doughnut-shaped loadcell and capable of preloading the loadcell to a desired force level while simultaneously providing an unobstructed line-of-sight for a high-speed camera to capture the evolving damage modes in the specimen is developed. This loadcell assembly is used in a Hopkinson bar setup to test thin glass specimens and determine their dynamic biaxial flexural fracture strength. A new calibration procedure is proposed that accounts for the delay in the force sensed by the loadcell and provides a more accurate measure of the applied dynamic load on the specimen surface. An analysis method that accounts for membrane stresses under axisymmetric loading is developed to determine the biaxial failure strength of thin glass specimens that undergo large flexural deformation.

Results

A loadcell calibration method, an experimental procedure to dynamically test thin flexible specimens, and an analysis method that accounts for membrane stresses were developed. The Experimental results for three types of thin transparent materials reveal that the dynamic flexural failure strength is 40% more than their corresponding quasistatic strength. Radial cracks evolve from a preexisting defect during the biaxial loading and the damage growth rate was determined to be 1570 m/s.

Conclusions

The results reveal that the formulation suggested by the ASTM standard overpredicts the failure strength of thin glass specimen by several times the strength determined by the developed analytical method that accounts for the membrane stress. The analysis procedure provides a repeatable measurement of dynamic biaxial failure strength of flexible thin plates.