ABSTRACT
The fracture properties and susceptibility to crack-divider delamination (or splitting) of three commercially produced high-toughness X70 pipeline steels are evaluated with Charpy impact test samples modified to incorporate side grooves. Temperature-dependent impact data are compared with standard Charpy V-notch (CVN) and drop weight tear test data (DWTT). It is shown that the modified geometry prevents the accumulation of plastic deformation at upper shelf energy temperatures and improves the accuracy of impact properties measurements. It also promotes splitting, mirroring the splitting behavior assessed with DWTT samples. To demonstrate the effects of splitting on fracture characteristics and impact energies, steels with similar tensile properties but different splitting susceptibilities are considered. Splitting severity is maximum close to the ductile-brittle transition temperature. However, the effect of splitting on impact energy is minimum at such temperature, as this type of delamination increases energy absorption at lower temperatures and decreases it by a similar extent at higher temperatures. This finding is discussed by examination of force-displacement curves from the instrumented impact tests.
ABSTRACT
The Direct Current Electrical Potential Difference (DCEPD) technique has been used for many years in fatigue and fracture testing for monitoring crack propagation in metallic materials. The principle of DCEPD methods is that when a constant current flows through a cracked specimen, the voltage change measured across the crack plane can be analytically related, empirically related, or both, to the change in crack size. In fatigue testing, performed within the limits of Linear Elastic Fracture Mechanics, crack propagation is the sole source of potential change. In ductile fracture testing, additional contributions from specimen dimension changes and crack-tip plastic deformation (blunting) have to be accounted for and distinguished from the DCEPD increase caused by crack growth. The ASTM E08.07.09 Task Group, formed in 2013 and chaired by the author, has been developing an annex for ASTM E1820, Standard Test Method for Measurement of Fracture Toughness, which focuses on the use of DCEPD measurements for the prediction of crack size and crack extension in ductile fracture toughness tests. This article presents the analysis of an analytical round-robin, in which 8 participants analyzed 24 existing fracture toughness data sets using two different approaches, based on the analysis of the displacement versus DCEPD and force versus DCEPD, respectively. The comparison between these two approaches and the implications for the ASTM E1820 annex being developed are the focus of this article.
ABSTRACT
Powder quality in additive manufacturing (AM) electron beam melt (EBM) of Ti-6Al-4V components is crucial in determining the critical material properties of the end item. In this study, we report on the effect of powder oxidation on the Charpy impact energy of Ti-6Al-4V parts manufactured using EBM. In addition to oxidation, the effects on impact energy due to hot isostatic pressing (HIP), specimen orientation, and EBM process defects were also investigated. This research has shown that excessive powder oxidation (oxygen mass fraction above 0.25 % and up to 0.46 %) dramatically decreases the impact energy. It was determined that the room temperature impact energy of the parts after excessive oxidation was reduced by about seven times. We also report that HIP post-processing significantly increases the impact toughness, especially for specimens with lower or normal oxygen content. The specimen orientation effect was found to be more significant for low oxidation levels.
