Monotonic Plastic Zone Size Research Articles

Ｘ線によるＳＭ５０Ａ及びＨＴ８０鋼の疲労破面解析

An X-ray fractographic study was made on the fatigue fracture surface of SM50A and HT80 steels. The residual stress and the half-value breadth of X-ray diffraction profile were measured at and beneath the fracture surface. The correlation was examined between the parameters obtained from the X-ray measurements and the fracture mechanics parameter of Kmax or ΔK at which the fracture surface had formed.The residual stress on the fracture surface was found to increase at first, reach to a maximum value at a certain value of Kmax or ΔK and then decrease. It was also found that the residual stress was controlled by Kmax in a low Kmax or ΔK region, while it was governed by ΔK in a high Kmax or ΔK region. The reasons of this behavior were discussed by using the data from constant Kmax and constant ΔK tests which were additionally planned and made in this study. The half-value breadth, B, on the fracture surface was found to increase with Kmax in the two steels used in this study. The value of B was influenced by stress ratio in HT80, although it was not influenced in SM50A. Both of the distributions of residual stress and half-value breadth beneath the fracture surface were found to be useful for the prediction of monotonic plastic zone size or Kmax.

Fatigue crack growth retardation under constant amplitude and variable mean stress

Fatigue crack growth retardation under stress spectra with constant amplitude and variable mean stress, respectively, was studied. Flat specimens with a central through crack were tested under tension-tension load. The specimens were made with low alloy steel 4 mm thick, with yield strength of 625 Mpa and ultimate tensile strength of 784 MPa. Overload affected crack length increments Δa ∗ were studied. The best correlation was obtained between monotonic plane stress plastic zone size 2r y and Δa ∗. The cyclic plastic zone size 2r pc correlated with crack length increment of minimum crack growth rate after overload. Forman's equation and Willenborg's model of fatigue crack growth retardation were used for theoretical prediction of fatigue crack propagation life. The best agreement between theoretical and experimental results was also obtained using monotonic plastic zone size instead of monotonic plastic zone radius or cyclic plastic zone size. The agreement is reasonably good, even though in the case of one spectrum, cracks were arrested for several thousand cycles.

Cyclic crack opening displacement behavior during high-amplitude block loading

Load-crack opening displacement hysteresis behavior was monitored for fifty cycles of highamplitude loading which followed fatigue pre-cracking at low stress intensity factor levels. The material studied was quenched and tempered (400°C) AISI 4140 steel which showed pronounced cyclic softening. Despite this softening behavior, cycle-to-cycle decreases in load-COD hysteresis were observed during the initial cycles of high-amplitude loading. Steady state load-COD hysteresis behavior was attained by fifty loading cycles in each case and the fifty-cycle hysteresis loop widths agreed well with those for continuously cycled (non-pre-cracked) samples for equivalent loading conditions. The cycles during which the load-COD hysteresis decreased most dramatically represented fatigue crack growth distances equal to approximately 30% of the calculated plane strain monotonic plastic zone size. Greater percentage reductions in load-COD hysteresis were observed for lower stress intensity factor ranges. The observed behavior was in general agreement with that predicted by finite element fatigue crack closure models in the literature. In addition, the level of prior loading was found to have a pronounced effect on subsequently measured fracture toughness values for this material.

Delayed retardation phenomena of fatigue crack growth in various steels and alloys

The delayed retardation phenomena of fatigue crack growth resulting from a single application of overload were investigated for five steels, two aluminium alloys and a titanium alloy. As long as the small scale yielding condition was satisfied at the overloaded crack tips, the retardation behaviour of these materials was expressed consistently by four parameters; the peak/baseline stress ratio, r, the exponent in the Paris equation, m, the overload-affected zone size, ωD, and the crack distance at the minimum rate of crack growth, ωB. Then the parameters, ωB and ωD, characterizing the retardation phenomena for these materials were determined. The retardation of aluminium alloys was stronger than that of the other materials. This is attributed to the lower value of ωB/ωD in aluminium alloys than in the other materials. In the case of r=2, the overload-affected zone sizes, ωD, were nearly equal to 1.5ω0 in HT80 steel and aluminium alloys, slightly lower than 1.5ω0 in SNCM8 steel and much larger than 1.5ω0 in A553 steel and the titanium alloy, where ω0 is the monotonic plastic zone size calculated according to the Dugdale model. The dependence of retardation on baseline stress intensity, ΔK1, appeared somewhat complicated. In the cases of A553 steel and A2017 aluminium alloy the amount of retardation increased with increasing ΔK1 value, while in the cases of HT80 steel and Ti-6A1-4V titanium alloy the tendency appeared in the reverse direction. The former behaviour was related to the change in the stress state from plane strain to plane stress at the overloaded crack tips and the latter was related to the threshold of stress intensity.