Fracture surface analysis of fracture mechanics specimens with splits: Determination of the physical crack extension

Abstract

Determining ductile fracture toughness requires evaluating the physical crack extension (Δap) on the fracture surface of fracture mechanics specimens. This step is essential for both the basic and single specimen methodologies. While established procedures exist for determining Δap from various standards, measuring it on fracture surfaces with splits poses challenges due to the lack of guidelines in current standards for treating such delaminations. This investigation delves into measuring Δap on fracture surfaces with and without splits using digital image analysis. The fracture surface characteristics of specimens with splits were analyzed, revealing tortuous crack fronts whose complexity increased with the number of delaminations. Additionally, issues with applying the standard method in materials exhibiting high fracture toughness (ductile behavior) and splits were discussed, highlighting significant errors that can arise from the standard methodology in fracture surfaces with splits.Subsequently, four alternative methodologies were proposed and evaluated: the Adapted Nine-Point method (A9P), the Adapted Eighteen-Point method (A18P), the Adapted Area as Lucon’s proposed method (AL), and the Adapted Area Average method (AAA). These were validated in specimens without splits, establishing the AAA method as the reference one. The A9P exhibited poor performance, similar to the AL method. In contrast, the AAA and A18P demonstrated excellent results, recommending their application in fracture surfaces with splits. The advantages and disadvantages of each proposed method are discussed comprehensively. Furthermore, crack front straightness was assessed using ASTM and BS ISO criteria, revealing 88% and 50% rejection rates, respectively.