ORAL REFERENCE: ICF100557OR DUCTILE TEARING FROM INTERACTING SURFACE BREAKING DEFECTS B. Bezensek and J.W. Hancock Department of Mechanical Engineering, University of Glasgow, Glasgow, Scotland, UK ABSTRACT The interaction of coplanar surface breaking defects has been studied numerically and experimentally. Attention has been focussed on the behaviour of the re-entrant sectors which form as adjacent cracks coalesce in fatigue. Fatigue studies and numerical modelling shows that such sectors exhibit enhanced values of the stress intensity factor. The line spring technique [9,10] was used to analyse a series of evolving configurations, which originate from the interaction of two adjacent semi-elliptical cracks in monotonic loading. Under elastic-plastic conditions amplified values of J-integral were found in the re-entrant sectors, which also exhibit loss of crack tip constraint, as parameterised by T [7,8]. The numerical work is compared with ductile tearing experiments performed on a plain carbon–manganese steel. The experiments show that ductile tearing initiates from the re-entrant sector and the crack develops towards a bounding shape. The evolution of the shape of the coalescing defect in ductile tearing is initially similar to the development in fatigue. The conservatism of codified recharacterisation procedures is demonstrated for defects with re-entrant sector(s) in fatigue and ductile tearing conditions. KEYWORDS fatigue, fracture, crack interaction, coalescence, ductile tearing, recharacterisation INTRODUCTION Defects in real engineering structures frequently have complex shapes. Defect assessment procedures such as ASME, Section XI [1] and BS 7910 [2] recommend recharacterising the defect with a simple shape, which is amenable to analysis. Fracture mechanics assessments are performed for the recharacterised defect and structural integrity is assured when the recharacterised defect meets specified safety criteria. The validity of the procedure has been extensively studied in fatigue [3-5]. In fatigue adjacent surface breaking defects interact and form a single defect with a re-entrant sector, which exhibits accelerated crack growth rates [3]. The number of cycles in this stage may constitute a significant portion of fatigue life of the component, before the crack evolves into a bounding shape. The recharacterisation process is thus inherently conservative for fatigue. However for defects which fail in a brittle manner on the lower shelf, the presence of the amplified stress intensity factor values in the re-entrant sector presents significant concerns for the recharacterisation process [6]. The current work investigates the evolution of complex defects exhibiting a re-entrant sector in ductile tearing. Numerical analysis used the line spring concept [9,10] to evaluate crack tip parameters, including the J-integral and the non-singular T stress [7,8]. A related experimental programme studied the coalescence process in fatigue, ductile tearing and in brittle fracture [6] for defects with re-entrant sectors.
RkJQdWJsaXNoZXIy MjM0NDE=