Stages Of Cyclic Loading Research Articles

繰返し負荷の初期におけるアルミニウムの応力‐ひずみ曲線

The effect of plastic strain history on the stress-strain hysteresis curve in the initial stage of cyclic loading was analysed for commercially pure aluminum subjected to various strain controlled push-pull cyclic tests: constant strain cycling with or without a mean strain, and constant strain cycling after various prestraining. Each test was repeated until a stable stress-strain hysteresis loop was obtained. The stress magnitude for each half cycle was plotted against the cumulative plastic strain and compared with the monotonic tension curve. It was found that in the initial stage of cycling the stress-strain curves for different prestrains were not similar nor had such shapes that coincide with each other by a suitable translation. The stress-strain curve for each half cycle was expressed in terms of two quantities: (1) the value of permanent softening, which is the stress difference between the monotonic tension curve and the cyclic curve at the parallel point on the stress-cumulative plastic strain plot and (2) the transient hardening curve. The former depends strongly on the plastic strain range and cycle number but weakly on the prestain, while the latter depends only on the plastic strain occurred in the just preceding half cycle. A new parameter which is related to the monotonic tension curve was introduced to identify the stable behaviour under cyclic loading.

Fatigue Crack Propagation: New Tools for the Study of an Old Problem

New information on fatigue has been obtained by a cyclic-loading stage for the SEM which allows dynamic observation of the fatigue-crack propagation process. A detailed response of the material characteristics to loading can be determined through measurement of the in-plane elements of the strain tensor to within 1 μm of the crack tip. In addition, the mode of crack-tip opening and its displacement can be determined very accurately. Electron channeling, the only known technique for determining plastic-strain distribution on a wide variety of metals from the interior of the specimen can be used to observe subgrains; from measurements of their size and distribution, and stress and strain distribution, around the fatigue-crack tip can be inferred. Selected-area electron channeling provided an understanding of the mechanisms by which spike overloads retard fatigue-crack propagation. Further research using these techniques will result in better understanding of the mechanism of fatigue and the development of fatigue-resistant materials.

Some features of work hardening and softening of molybdenum single crystals in cyclic strain

1. Molybdenum single crystals subjected to a static tensile load are extremely sensitive to the strain rate and very quickly work harden when this rate is increased. This effect is caused by low mobility of screw dislocations in molybdenum at room temperature with the sources blocked. 2 Work hardening of molybdenum under cyclic load is determined by the loading rate and low mobility of the screw components of strain. 3. The kinetics of the change of inelastic strain under cyclic load is associated with the kinetics of the changing dislocation structure in molybdenum crystals in fatigue: a) a considerable increase in Δe in the early stages of cyclic loading is associated with the multiplication of dislocations in conditions of multiple accumulation and with subsequent regrouping of dislocations; b) the stabilization of Δɛ is caused by simultaneous actuation of two opposing processes: work softening (regrouping of dislocations) and work hardening (screening of slip by the surface layer). 4. The absolute amount of inelastic strain in the cyclic loading process is determined by the number of active slip systems and the intensity of slip at each of the systems in conjunction with the features of multiplication and regrouping of dislocations in the bcc lattice.

A low-frequency cyclic loading stage for the SEM

A cyclic-loading stage for the ETEC Corporation SEM is described. The stage uses servo-controlled electro-hydraulic loading with positive feedback Specimens are loaded symmetric to the specimen centre line. Maximum load is 4893 N, with a maximum loading rate of 4 Hz. Resolution on the specimen is about 50.0 nm for static observation and 100.0 nm for dynamic (TV rate) observation in the secondary-electron mode. Observation of the specimen may also be made using backscattered electrons and in the channelling mode (channelling patterns and channelling contrast). Maximum specimen motion during cyclic loading is 15 mu m.

Inhomogeneity of deformation in the initial stage of cyclic loading of armco iron

Inhomogeneity of deformation in the initial stage of cyclic loading of armco iron

Stress-Strain Curve of Mild Steel in the Initial Stage of Cyclic Loading

Cyclic uniaxial tension-compression tests with various constant strain amplitudes were performed on mild steel to investigate the change of stress-strain curve in the initial stage of cyclic loading. Strain amplitude was nearly equal to a half of the corresponding prestrain. In order to apply the result obtained to a cyclic elastoplastic analysis, the stress-strain curve in each half cycle was fitted by a polynomial, whose coefficients were examined in relation to the number of half cycles and the magnitude of prestrain. An approximate expressioin for the curve in any half cycle was obtained as a function of the number of half cycles, magntidue of prestrain and coefficients in the expression for the first half cycle after the initial loading.