No abstract

ABSTRACT. In this work, the use of circumferentially cracked bar (CCB) sample to determine material fracture toughness in the upper shelf regime for carbon steels has been investigated. Since high fracture toughness materials are known to exhibit extensive crack tip blunting before ductile crack initiation, accurate specimen design is required to provide realistic fracture toughness measurement. Here, a CCB was designed to have similar loss of constraint as for SENT sample. Continuum damage mechanics was used to predict the occurrence of ductile crack initiation and propagation. Finite element analysis was performed to predict specimen response and to compare computed J-integral crack driving force with measured CTOD. Finally, experimental tests were performed on X65 carbon steel and the measured critical CTOD was compared with available fracture data obtained with SENT.

Codes and standards for oil&gas industry, as OS-DNV-F101, recommend the use of single edge cracked plate in tension (SEN(T)) for the experimental determination of the material critical CTOD. For clad pipe welds, this specimen geometry is difficult to be obtained for the weld material or clad corrosion resistance alloy due to the specimen shape and minimum dimensions. Alternatively, circumferential cracked bar geometry, CCB(T) could be used. This geometry configuration can be machined when limited material quantity is available and used for both quasi-static and dynamic fracture characterization. In this paper, an extensive elastic-plastic finite element investigation has been carried out on both SEN(T) and CCB(T) geometries in order to select equivalent configurations in the J-Q space. Ductile crack initiation and growth has been simulated using continuum damage mechanics model. Numerical simulation results indicate that CCB(T) with a crack depth ratio r/a = 0.2 realizes constraint loss similar to that of SEN(T) with a crack depth ratio a/W = 0.5. Similar crack resistance curves have been obtained for these two configurations confirming the equivalence of the selected sample geometries.

Abstract. At high strain rates, deformation processes are essentially adiabatic and if the plastic work is large enough dynamic recrystallization can occur. In this work, an examination on microstructure evolution of OFHC copper in Dynamic Tensile Extrusion (DTE) test, performed at 400 m/s, was carried out. EBSD investigations, along the center line of the fragment remaining in the extrusion die, showed a progressive elongation of the grains, and an accompanying development of a strong <001> + <111>dual fiber texture. Discontinuous dynamic recrystallization (DRX) occurred at larger strains, and it was showed that nucleation occurred during straining. A criterion for DRX to occur, based on the evolution of Zener-Hollomon parameter during the dynamic deformation process, is proposed. Finally, DTE test was simulated using the modified Rusinek-Klepaczko constitutive model incorporating a model for the prediction of DRX initiation.

Abstract. Hat-shaped specimen geometries were developed to generate high strain, high-strain-rates deformation under prescribed conditions. These geometries o↵er also the possibility to investigate the occurrence of ductile rupture under low or negative stress triaxiality, where most failure models fail. In this work, three tophat geometries were designed, by means of extensive numerical simulation, to obtain desired stress triaxiality values within the shear region that develops across the ligament. Material failure was simulated using the Continuum Damage Model (CDM) formulation with a unilateral condition for damage accumulation and validated by comparing with quasi-static and high strain rate compression tests results on OFHC copper. Preliminary results seem to indicate that ductile tearing initiates at the specimen corner location where positive stress triaxiality occurs because of local rotation and eventually propagates along the ligament.