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The local approach of fracture based on continuum damage mechanics (CDM) combined with the finite element method (FEM) was applied to establish a more precise method of estimation of service life of cold forging tools by simulating the initiation and growth of a fatigue crack. After a brief explanation on the local approach of fracture based on CDM combined with FEM, modeling of elastic-plastic-damage behavior of tool steel was conducted in the framework of thermodynamics. In the modeling, the salient unilateral property of the tool material is considered by introducing a modified stress tensor. The proposed constitutive equations can be found to describe the experimental behaviors under uniaxial tension and compression properly. Then, the initiation and propagation of a fatigue crack in the cold forward extrusion die was analyzed by the local approach of fracture. The initiation of the fatigue crack in the vicinity of the die radius was found to occur when the number of extrusion increased. Then, the crack propagated along the direction perpendicular to the surface of the die radius. Furthermore, the proposed approach was found to estimate the actual behavior of the fatigue crack growth in good agreement with the calculated change of the rate of crack propagation to the number of extrusion. The calculated crack propagation rate also tended to decrease due to a decrease in tensile principal stress at the crack tip as the crack propagates.

The characteristics of various metals bonded by DC pulse resistance heat pressure welding using spark plasma sintering (SPS) process are investigated. As the results of the experiments, the SPS process is found to complete heat pressure welding in a much shorter processing time than the conventional hot pressing process because the SPS process supplies the electric resistance energy directly to the specimens. In addition, the following conclusions are obtained: (1) Bonding strengths of 130$200 MPa between Al-11.5 mass%Si alloy and Al-20.0 mass%Si alloy including SiC particles can be obtained with the application of titanium powder at the interface. (2) For the combinations of similar metal, complete bonding, that is, a bonding ratio of approximately 100%, can be obtained. However, sufficient bonding strength cannot be obtained for the combination of cemented carbide. (3) Dissimilar steels can be bonded with a bonding ratio of more than 70%. (4) A bonding strength is reduced for the combinations of titanium alloy and other metals, because of the formation of some intermetallic compounds at the interface.

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The detection of damage to and fracture of cold forging tools during forming operation by fractal property of acoustic emission (AE) is performed. First, a tensile test on tool steel is conducted to elucidate the fractal dimensions of deformation-induced and fracture-induced AEs. The resultant fractal dimensions are 1.97 and 1.44. Next, the change in the fractal dimension of AE from a die insert is investigated under cold forward extrusions. Workpieces with a conversion coating film are used to eliminate the effect of friction on AE. After 300 extrusions, no damage and wear is observed on the die surface, and the fractal dimension is almost constant at 2.04 on average. Then, another series of cold forward extrusions under high-friction conditions is performed with mineral oil VG2 and stearic acid to promote the onset of damage to and fracture of the die. A defect on the surface of the workpiece is observed at the 101st extrusion, which resulted from the onset of crack on the die radius. The former average fractal dimension, 2.01, of the onset of the crack changes to 1.52 after the onset of the crack. From the results, the fractal dimension can be concluded to be one of the most effective indicators of the progress of damage to a cold forging tool. Finally, a method of separating the die-induced AE from the total AE based on the Kaiser effect is proposed.