Major solutions needed in frlcture analysis are (d) simple and accurate matedal characterization and (ô) easy transferring of malerial data lo cracked structures. ln !h€ proposed methodology. constitutive relationships. including cavity growth and coalescence, are used. Material characterization is based on the simple notched tension lest. Several structural steels have been characterized. especially 4508 steel. and the direct transferring of material data hâs been demonstrated wilh tests on circumf€rentially cracked tension specimens.In addition. the extrapolation of material data io different inclusion conl€nls and temperatures was attempted, with favorabl€ rcsults for the first factor through a specific parameter of the model. Th€ temperature dcpendence of 4508 steel ductiliiy is related to an inverle strain rale effect on the flow curv€: the modeling of this effect gives encoùraging results, but it must be refined to producc an effeclive prediction. KEY WORDSI ductile fracture. damâge mechanics. local approach to fracture, crack iniriation, slable crack growth, 4508 steel. inclusion contenls, notched tension test. fracture mechanics. nonlinear fracture mcchanics Some of the major problems engineers have to cope with in fracture analysis are the following:(a) material characterization, that is. the generation of adequate data from specimen testing, and(ô) the transferring of fracture mechanics data (o the struclural analysis of components.As a mattcr of fact, the generation of matedal data can be money and time consuming: for example, the determination ofJ-Ad resistance curves is still a toilsome task, though some progress was gained with partial unloading cornpliance methods. Frequently, existing material data do not correspond to the specific application (in reference to temperature, strain rate, irradiation. aging, and so forth), and hazardous extrapolations are necessary.
Considering a material at the ductile plateau, fracture tests on small specimens in generalized yielding conditions are common. We need to extract from them adequate information to characterize the fracture resistance properties of the material. We also need to predict initiation, crack growth, and maximum load for a crack found in a ductile structure. But the real problems are three-dimensional; for instance, semi-elliptical surface cracks or through-cracks in “small” thicknesses (tunneling and mixed-mode fracture). Moreover, they are not only in the symmetrical Mode I case (angled crack extension). The common denominator of all these phenomena is the ductile fracture processes in the material at the crack border; these micromechanisms extend over some characteristic length which needs to be introduced at a crack tip because of the very intense strain gradient. We thus need a ductile fracture damage function belonging to the continuum mechanics frame and related to the history of stresses and strains averaged over such a characteristic volume. In this numerical feasibility study, using elastic-plastic finite-element computations and guided by a ductile fracture model in three stages—void nucleation, void growth, and coalescence—we tried such a differential damage history, in a most simplified form. We integrated this during the whole stress and strain history in each finite element along the crack path, and we studied the influence of the mechanical and numerical parameters playing a role in this methodology. We describe herein the evolution, which results from this criterion, of some parameters used in the literature as initiation and crack growth criteria.
After a short review of different experimental methods used for studying the plastic zone ahead of a crack tip, the capabilities of the microhardness method are analyzed in detail. A finite-element calculation, which takes the cyclic hardening of 316 stainless steel into account, permits a thorough study of plastic deformation at the crack tip. The microhardness technique is a good method of studying the shape of the plastic zone and the deformation around the tip of the crack.
A local criterion based on the simulation of hole growth by plastic deformation has been evaluated. Fracture of a material volume is reached for an assumed critical value of cavity growth. This critical value is determined from notched tensile tests. When dealing with cracked geometries, a process zone is introduced at the crack tip. This zone is modeled as the first mesh element (a)c at the crack tip in a finite-element code. The size of this element, which is a material constant, is measured from a conventional compact tension (CT) test. Different tests with cracked geometries were carried out on side-grooved CT specimens of different sizes (25 and 50-mm width) and on axisymmetrically cracked tensile bars (TA) with 15, 30, and 50-mm outer diameters. In all cases the fracture was flat with no shear lips. Keeping the parameters of the fracture local criterion constant, crack initiation and crack propagation were modeled using the node release technique. The numerical procedure and results are described in detail. The model results are shown to be in good agreement with the experimental results.
The local approach to fracture is applied to analysis of the influence of fabrication and operation history on the safety margins for fracture of a pressurized water reactor vessel with an underclad reheat crack in the belt line, subjected to a severe pressurized thermal shock (PTS) transient at end-of-life. The elastoplastic analysis begins with numerical simulation of the manufacturing of the belt-line cladding, then continues with the creation of the crack during stress relief heat treatment, proof testing, loading-to-operating conditions, and, finally, occurrence of the pressurized thermal shock. Computation of the behavior of laboratory specimens at the temperature occurring at the crack tips at maximum load during the PTS transient makes it possible to translate the results obtained in terms of cleavage or tearing damage values, respectively, into the scales of the conventional KI and J toughness parameters. This analysis, which accounts for all effects of the vessel fabrication, and the operation history, discloses much larger safety margins than the conventional analysis presently used for vessel integrity assessments.
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