Experimental investigation of quasistatic and dynamic fracture properties of titanium alloys

Abstract

The goal of this work is to investigate the quasistatic and dynamic fracture properties of three titanium alloys: 6Al-4V titanium, 6Al-4V titanium ELI, and Timetal 5111. While standard tests exist for measuring quasistatic fracture toughness, the dynamic investigation requires that several measurement techniques are employed including Coherent Gradient Sensing (CGS), Crack Opening Displacement (COD), and the use of strain gages. The use of these methods with difficult engineering materials in the dynamic loading regime requires methodologies to be advanced beyond that previously required with model materials having properties ideal for experimental measurements techniques. After a description of each measurement technique is given, stress intensity factor measurements made on 12.7 mm thick pre-cracked 6Al-4V titanium specimens are compared. These specimens were dynamically impacted in three point bend in a drop weight tower. Specimens with and without side-grooves were tested as each measurement technique allows. Side-grooves are useful to increase the degree of plane strain experienced in proximity of the crack tip, allowing plane strain (geometry independent) fracture toughnesses to be obtained from specimens that may be otherwise too thin in cross section. Resulting stress intensity factor-time histories from the different techniques are compared to verify that their results mutually agree. Advancements in employing CGS, a shearing interferometric technique, are described in more detail. First, the analysis of CGS interferograms is extended to allow experimental fringe data to be fit to very general analytical asymptotic crack tip solution to determine mixed mode stress intensity factors. As formulated in this work, the CGS technique can be used to measure stress intensity factors for non-uniformly propagating dynamic mixed mode cracks moving along arbitrary paths in homogeneous linear elastic isotropic materials. Other advancements are also detailed which improve analysis accuracy, objectivity, and efficiency. Finally, with the equivalence of the three measurement technique results established, tests were performed on 8--17 mm thick pre-cracked three point bend specimens of the three materials to measure critical stress intensity values for crack initiation. Side-grooves are necessary for the more ductile 6Al-4V titanium ELI and Timetal 5111 materials to obtain plane strain fracture toughness values. It is found that both the 6Al-4V titanium ELI and Timetal 5111 alloys are 50-70% tougher than the 6Al-4V titanium, and for all three materials their initiation toughness does not vary significantly with loading rate over the domain tested.