Toshinori Yokomaku

EFFECT OF MICROSTRUCTURE ON FATIGUE PROPERTIES IN LOW AND ULTRA-LOW CARBON STEELS

Ultra-low carbon steel containing phosphorus and copper (P-Cu steel) has both a higher fatigue limit and better crack propagation resistance than conventional low carbon steels with the same tensile strength. In this paper, the mechanism for improving the fatigue properties of P-Cu steel is discussed on the basis of microscopic observations by electron microscope and measurements of crack closure behaviour for small and long fatigue cracks. The excellent fatigue limit and small-crack propagation resistance in P-Cu steel can be attributed to solution hardening caused by phosphorus and precipitation hardening caused by epsilon-Cu. On the other hand, the superior resistance to long-fatigue crack propagation was caused by grain coasening which occurs with reduction of carbon content, leading eventually to roughness-induced crack closure.

The Relationship between Fatigue Crack Propagation Property and Fatigue Life at Elevated Temperatures on P/M Ni-base Superalloy, AF 115. Through Fatigue and Creep Strain Energy Parameters, .DELTA.Wf and .DELTA.Wc.

Low cycle fatigue properties under pure fatigue and creep/fatigue conditions were evaluated using a powder metallurgy superalloy, AF 115, containing a variety of defect-types and defect-sizes. The relationship between life, defect size, and loading condition were analyzed on the basis of crack growth rules and strain energy parameters. Following results were obtained ; (1) Inclusions, micro porosities and prior particle boundaries were observed at the origin of fatigue specimens. (2) By treating these defects as either cracks or notches, the lower or upper boundary of fatigue life can be obtained from the elastic-plastic strain energy parameter, lWf, and the fatigue crack growth rule. (3) The introduction of a tension hold period to the stress cycle or a decrease in the strain rate in the strain cycle significantly affects the fatigue life. The linear summation rule for partitioned lives, based on the elastic-plastic and creep strain energy parameters, lWf and lWc, and fatigue and creep crack growth rules, is suitable for life estimation under creep/fatigue conditions.

Fretting Fatigue Behavior of an Aluminum Alloy.

Fretting fatigue tests were carried out using a 2618 aluminum alloy to investigate the effect of contact pressure and relative slip range between the specimen and the contact pad on fretting fatigue strength. Irrespective of contact pressure and relative slip range, the fretting fatigue strength was reduced to one quarter of the plain fatigue strength under the stress ratio R=0.0. However, a slight reduction in fretting fatigue strength occured with a decrease in contact pressure and with an increase in relative slip range, respectively. Friction force came to a constant value beyond a certain relative slip range (9 μm) under a pad-span of 30 mm and a contact pressure of 20 MPa. MoS2+PTFE coating improved fretting fatigue strength. And using fracture mechanics models, a prediction method for fretting fatigue endurance was assessed. The predicted fretting fatigue endurances were in good agreement with the experimental results of fretting fatigue tests.

Microstructural Design for Steels by Fatigue-Crack-Growth and Arrest Simulation.

The fatigue limits of single-, dual-and tri-phase steels were estimated using the crack-growth and arrest simulation, which was based on a model for continuous distribution of dislocations ahead of a small fatigue crack tip. The effects of microstructural parameters such as grain size, hardness and volume fraction of the three phases on the fatigue limit were systematically analyzed using this simulation. The simplified estimation equation for fatigue limit was derived from the simulation results as follows : δw=HV1·(k1+k2/√(D1))+Σj=2, 3(HVj-HV1)·Vj·(k3j+k4j/√(Dj)), where HVj is the hardness, Dj is the grain size, and Vj is the volume fraction of the j-th phase. Microstructural design for high fatigue strength steels is discussed with reference to this equation. Also, a more accurate simulation method is outlined including the effects of cyclic hardening and softening behavior of materials.