lowered from 110 J in the peak-aged condition (5 hours) to but not in high-cycle fatigue. This difference is also reflected 70 J in the overaged condition (10 hours). Since the fracture in the S-N curves, where lines with different slopes are occurred in a mostly transgranular manner, [8,9] the embrittleneeded to fit the high-cycle and low-cycle data. ment was not severe, as compared to the intergranular The martensitic transformation is the basis of the superembrittlement, which produced very low impact toughness elastic and shape-memory properties. Yet, most existing values below 10 and 5 J, even at 300 ЊC, in the Mo and studies do not explore the influence of the martensitic trans-MoW steels, respectively. In the overaged condition, since formation on fatigue properties. The study by Miyazaki [4] the presence of cementite [9] indicates that there are sufficient shows that the martensitic transformation can influence the amounts of W available to tie up the P, no severe intergranufatigue properties, but does not explicitly examine it. We lar embrittlement could occur.propose to examine the influence of the martensitic transformation on the fatigue properties of NiTi by exploring meanstrain effects on the fatigue properties. Mean stress and mean-strain determine the phase of the material in superelas-This work was partly supported by the POSCO. tic specimens. Depending on the mean strain, a specimen may contain austenite (A), martensite (M), or both phases (AM), and different deformation modes are associated with REFERENCES each of these cases. In numerous applications, loads cycle 1.
A pure-bending apparatus is used to measure the constitutive relationship between applied pure bending moments and the resulting curvatures of a few superelastic alloy wires. The sample nickel-titanium alloy (NiTi) wires change phase when ample bending moments are imposed. Like the material’s uniaxial tension stress-strain relationship, the measured moment-curvature relationship shows plateaus of constant moment and hysteresis. The bent shape is circular, except in the mixed phase region where it is composed of a phase mixture of circles. An example of the applications of the measured moment-curvature relations is presented in Part II of this paper where the three-point bending problem is considered.
The constitutive relationship between applied pure bending moment and the resulting curvature of a few superelastic alloy wires is applied to the three-point bending problem. Three-point bending experiments on hard and soft loading machines are described. The relationship between the applied deflection and the resulting force in three-point bending is calculated from a nonlinear Euler-Bernoulli rod theory. A numerical procedure used to solve the three-point bending problem for both loading and unloading is briefly described and numerical results are compared with experiment.
The higher-order asymptotic crack-tip fields are considered for a Mode I in power-law plastic and creeping materials under plane strain conditions. Using an asymptotic expansion and separation of variables for the stress function, a series solution is obtained for stress components at a crack tip. In addition, full-field finite element analysis based on a modified layer approach is employed to model the effects of biaxial loading on nonlinear behavior. Loadings were applied related to a range of biaxial stress ratio (−2, +2). The radial and angular stress distributions and higher-order amplitude coefficients for plastic materials are obtained. Crack tip higher-order fields under various conditions of biaxial loading and spanning the range of times from small scale creep to extensive creep are presented. Account is taken of the radial distance accompanying crack tip blunting. The phenomenon of stress redistributions along crack plane on creep time as a function of biaxial stress ratio is stated. The regions of dominance of the HRR-type field under various biaxial stress ratio, crack distance, hardening exponent, and creep time are found. Good agreement analytical findings with finite element results conforms that HRR-solution corresponds only to the equi-biaxial tension which is a particular case of biaxial loading. It is further demonstrated that the higher-order both for plastic and creeping fields are controlled through constraint parameter A2 by biaxial stress ratio. By fitting numerical results for plastic and creeping materials, two empirical formulas were obtained to describe the higher-order terms amplitude coefficients A2 distributions depending on biaxial stress ratio, crack distance, hardening exponent, and creep time.
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