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It is known that the ductility of steel lowers, when the prestrain is given to steel product. In the other, it has been clarified that the generation of the ductile crack of virgin steel can be evaluated by ductility limit curve presented by the relationship between stress triaxiality and equivalent plastic strain. In this paper, by using As-Rolled steel and TMCP steel for the sample material, the effect of the prestrain on the ductile fracture behavior was investigated. In the As-Rolled steel, the lowering of the ductility limit curve by the prestrain was confirmed. And, local cleavage crack was observed only in the fracture surface of test piece in which ductility limit curve lowers. From these results, it was clarified that the main factor of the ductility limit curve lowering was the generation of local cleavage crack. Then, we developed the technique which could uniformly evaluate the generation of the ductile crack by ductility limit curve of the virgin material by considering local cleavage crack.

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<div class="section abstract"><div class="htmlview paragraph">In this research, a material model was developed that has orthotropic properties with respect to in-plane damage to support finite element strength analysis of components manufactured from a randomly oriented long-fiber thermoplastic composite. This is a composite material with randomly oriented bundles of carbon fibers that are approximately one inch in length. A macroscopic characteristic of the material is isotropic in in-plane terms, but there are differences in the tension and compression damage properties. In consideration of these characteristics, a material model was developed in which the damage evolution rate is correlated with thermodynamic force and stress triaxiality. In-plane damage was assumed to be isotropic with respect to the elements. In order to validate this material model, the results from simulation and three-point bending tests of closed-hat-section beams were compared and found to present a close correlation. However, in the case of axial compression of double-hat-section beams, it was found that although the initial peak load corresponded closely, the peak loads after the second peak did not match. As a cause for this, it is conceivable that the damage applied in the transverse direction differs from that of axial compression direction. A test that would identify the relationship between these different instances of damage was devised, and the damage characteristics were measured. It was found from the results of this test that the in-plane damage evolution would need to be given orthotropic properties. The orthotropic damage characteristics were additionally implemented in the material model and the load-stroke curves from axial compression testing of closed-hat-section beams and from simulation were compared. It was confirmed that the loads increased from the second load peak and on, suggesting that the conceptual approach of the material model was correct.</div></div>