Effect of Displacement Increment on Fracture Toughness (JIc) of HSLA Steel

Abstract

Fracture toughness of low strength tough materials are provided by the J-integral and expresses as JIc. The resistance curve procedure, specified in ASTM E1820, is mostly used to obtain the fracture toughness, since it is a single specimen test, unlike the basic procedure which requires multiple specimens. In this method the specimen undergoes loading, unloading and reloading process repeatedly. The spacing between unload/reload sequence is known as displacement interval or displacement increment. ASTM E1820, specifies the maximum limit of displacement interval/displacement increment between each unload/reload sequence (0.01 times of the remaining ligament). However, it has been observed from experiments that there is variation of JIc values at lower values of displacement interval/displacement increment. Hence in this work the effect of displacement increment on JIc of HSLA (High Strength Low Alloy) steel has been studied through experimental investigations and an attempt has been made to obtain the range of displacement increment where the variation of JIc values is within acceptable limit. Another two parameters which affect the value of fracture toughness is a/W (crack length to specimen thickness) ratio and loading rates/strain rates. It is reported that there is no significant influence of loading rates on fracture toughness. Therefore in this investigation fracture toughness was measured with varying a/W ratio and displacement increment. The tests were carried out with side-grooved full C(T) specimen, following ASTM-E1820. The resistance curve procedure which utilizes unloading compliance (UC) method, is used to evaluate the fracture toughness.