Akihiko Hirano

Relationship Between Rockwell C Hardness and Inelastic Material Constants

This paper describes the relationship between Rockwell C hardness and elastic-plastic material constants by using finite element analyses. Finite element Rockwell C hardness analyses were carried out to study the effects of friction coefficient and elastic-plastic material constants on the hardness. The friction coefficient and Youngs modulus had no influence on the hardness but the inelastic materials constants, yield stress, and strain hardening coefficient and exponent, had a significant influence on the hardness. A new equation for predicting the hardness was proposed as a function of yield stress and strain hardening coefficient and exponent. The equation evaluated the hardness within a ±5% difference for all the finite element and experimental results. The critical thickness of specimen and critical distance from specimen edge in the hardness testing was also discussed in connection with JIS and ISO standards.

Fatigue. Fatigue Characteristics of High Speed Steel P/M Alloys.

In order to reveal the mechanical performance of powder metallurgical (P/M) alloys, several kinds of mechanical tests were carried out on three sorts of HIPed high speed steel P/M alloys. Three ingot metallurgical (I/M) alloys with almost the same chemical compositions as those of the P/M alloys were also used as reference materials. Dependence of mechanical properties on chemical composition and fablication process was discussed in details. The P/M alloys show higher bending strength than I/M alloys irrespective of the difference in additive content. Fracture toughness of both P/M and the I/M alloys depends on Co content. Fratigue strength of the P/M alloys is higher than that of the I/M alloys. Fatigue crack growth rate of P/M alloys shows clear dependence on additive content; the greater the additive content, the higher the crack growth rate. Such a crack growth behavior of the P/M alloys can be correlated well with the size and the density of carbides distributed in the matrix of each P/M alloy.

Retardation of Fatigue Crack Growth Rate due to Impact Over-Load

As one of the on-going studies to investigate the fatigue crack growth characteristics in impact fatigue, the fatigue crack growth behavior resulting from the single application of impact over-load was investigated by using Dual-Phase steel with ultimate tensile strength of about 800MPa as an experimental material. Discussions were made on the retardation of crack growth rate, crack opening behavior after over-loading and fractographical aspect in comparison with the results obtained through the application of single static over-load with peak stress hold time of 3min.The major conclusions obtained are summarized as follows:(1) Retardation of crack growth rate after impact over-loading was not so distinct when compared with the case after static over-loading.(2) The values of crack opening ratio after impact over-loading were higher than after static over-loading.(3) Fractographical observations showed that abrasions were caused on the fractured surfaces after impact over-loading, but not so after static over-loading. The following speculation on the crack tip plastic deformed zone was needed to explain the peculiar crack growth behavior after im- pact over-loading as mentioned above. In comparison with the condition of the plastic zone constructed by static over-load, the size of the zone constructed by impact over-load is smaller, but the strength of this small plastic zone is higher, in other words, the higher density of slip bands is attained under impact over-load.