Detection of Damage and Fracture of Forging Die by Fractal Property of Acoustic Emission

Abstract

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.