Masahiro Jono

FATIGUE CRACK GROWTH AND CRACK CLOSURE OF STRUCTURAL MATERIALS UNDER RANDOM LOADINGS

ABSTRACT Fatigue crack growth tests were carried out under various programmed and pseudo-random loadings on a carbon steel, a high tensile strength steel and an alminum alloy, over a wide range of growth rate regime and crack closure behavior was investigated. Crack length and closure were measured by a minicomputer aided unloading elastic compliance method. The crack opening point was unchanged during one block of programmed load sequence and also during a period of pseudo-random loading. The crack opening stress intensity Kop was found to be controlled by the maximum stress intensity range-pair and its stress ratio, and furthermore, could be predicted from constant amplitude crack opening data. Cyclic counting methods were compared and the effective stress intensity range-pair counting was found pertinent to assessment of crack growth under irregular load histories. A prediction procedure was proposed for fatigue crack growth rates under stationary variable loadings.

Quantitative Analysis of Deformation near Fatigue Crack Tip Using Image Processing Technique.

A computer image processing system incorporated into specially designed servo-hydraulic fatigue loading facilities operating in a scanning electron microscope has been developed. Direct, real-time observations of fatigue crack growth behavior under both constant amplitude and repeated two-step loadings were made, and quantitative analysis of crack opening displacement and deformation near the fatigue crack tip were made using a newly developed image processing technique.

Mixed Mode Delamination Creep Crack Growth of Unidirectionally Reinforced AS4/PEEK Laminate at Elevated Temperature.

Mixed mode I+II delamination crack growth tests at elevated temperature (473K) were carried out on unidirectionally carbon fiber reinforced thermoplastics (CFRTP) under static creep condition. Asymmetric end loaded split (AELS) specimens were used in order to investigate the effect of mixed mode ratio, GI/GII, on crack growth. The creep crack growth rate, da/dt, became higher in terms of mode I component of total energy release rate, GI, as GI/GII decreased. This implies that the crack propagation rate was accelerated by mode II component, GII. The measurement of the load point displacement rates due to mode I or mode II component of total load revealed that the creep deformation at the crack tip by mode II component was larger than that by mode I component and the size of creep deformation area depended on GI/GII. It is considered that the creep deformation by mode I component, which was perpendicular to carbon fibers, was constrained strongly by fibers, while the creep deformation by mode II expanded along carbon fibers. The crack growth rate under mixed mode loading was governed by the creep J integral, J, regardless of the mixed mode ratio. Fractographic observation showed that matrices were elongated along carbon fibers and fracture occurred at the interface between fibers and matrix.

Fatigue Crack Growth Behavior in Silicon Nitride under Constant and Variable Amplitude Load Sequences

In order to investigate cyclic fatigue crack growth behavior of a gas-pressure-sintered silicon nitride under constant and variable amplitude load sequences, fatigue crack growth tests were carried out using compact type (CT) specimens, The crack length and macroscopic crack closure were measured using an unloading elastic compliance method. Grain interlocking was observed around crack wake in all fatigue test conditions, which implied fatigue crack growth of this material was associated with progressive degradation of the grain interlocking by cyclic loading. The fatigue crack growth rate, da/dn, under constant amplitude loading was controlled not only by the maximum stress intensity factor, K max , but also the stress amplitude, while K max was the most important factor in cyclic fatigue crack growth. The overload and high-level load excursion produced the acceleration of fatigue crack growth and the downward shift of crack closure point under low-level loading. This decrease was thought to result from much severer crash or frictional wear of grain interlocking and the crack closure could explain the acceleration behavior qualitatively.

Fatigue Crack Growth Behavior of Silicon Nitride under Constant and Non-Stationary Variable Amplitude Loadings.

In order to investigate the effect of load variation on the cyclic fatigue crack growth behavior of gas-pressure-sintered silicon nitride, fatigue crack growth tests under constant amplitude loading, multiple peak overloading, Lo-Hi and Hi-Lo two-step loadings were carried out using compact type (CT) specimens. Crack length and macroscopic crack closure were measured using the unloading elastic compliance method. Grain interlocking was observed around crack wake in all fatigue test specimens by SEM. Fatigue crack growth rate, da/dn, under constant amplitude loading was controlled by not only maximum stress intensity factor, Kmax, but also load amplitude. Crack opening stress intensity factor, Kop, decreased as load amplitude increased, as was concerned with breaking of grain interlocking. Overload caused the acceleration of fatigue crack growth rate, which was in contrast to the retardation observed in metallic materials. The acceleration was due to the breaking of grain interlocking by overload. The crack growth rate recovered as crack grew in a relatively short distance after overload, because grain interlocking in crack wake was reconstructed during crack growth.

Prediction Method of Elastic-Plastic Fatigue Crack Growth Rate under Variable Amplitude Loadings.

Load-controlled fatigue crack growth tests were carried out under reversed and pulsating repeated variable amplitude loadings in post-yield region. Crack growth increment and crack closure were measured by the minicomputer-aided unloading elastic compliance method. The effects of load variation on the elastic-plastic fatigue crack growth behavior and the application of range-pair counting method to j-integral, ΔJ*, for estimation of crack growth rates, where both one-directional and cyclic plastic deformation were observed remarkably, was discussed. It was found that fatigue crack growth rates under reversed varying loadings were estimated by the linear summation rule based on ΔJ*, and fracture mechanics parameter ΔJ*/(1-Jmax/C), where Jmax is the maximum J-integral, should be used for estimation of growth rates of fatigue cracks where residual one-directional plastic deformation occurred as was observed under pulsating varying loadings.

DIRECT OBSERVATION AND MICROMECHANISM OF FATIGUE CRACK GROWTH UNDER VARYING AMPLITUDE LOADINGS

Direct observations of growing fatigue cracks under varying amplitude loadings were made by a high-resolution, field emission type scanning electron microscope equipped with a specially designed servo-hydraulic fatigue loading system on a grain oriented 3% silicon iron and fatigue crack growth mechanisms were investigated. Moreover, microscopic parameters which control the fatigue crack growth behaviors were discussed.

Direct SEM observation and mechanism of fatigue crack growth under varying loading conditions.

Fatigue crack growth tests under Hi-Lo two-step, repeated two-step and programmed loadings were performed on grain-oriented 3% silicon iron in a high-resolution, field emission type scanning electron microscope, using a specially designed servo-hydraulic fatigue loading system, and direct, real time observations were made, partly with aid of image processing technique.Under Hi-Lo two step loadings, the relationships of crack growth direction and crack tip opening displacement to crack growth rates were found quite different from those of constant amplitude loading test. On the contrary, the relationship between crack tip opening displacement and effective stress intensity range under repeated two-step loading could be considered similar to that of constant amplitude loading. Moreover, the change of crack tip opening displacement was found to correspond well to the range-pair of stress intensity range. From the above mentioned observation results, microscopic and macroscopic controlling parameters of fatigue crack growth were discussed.

Near threshold fatigue crack growth behavior under repeated two-step loadings in residual stress fields.

In order to make clear the effect of residual stress on fatigue crack growth behavior, fatigue crack growth tests under constant amplitude and repeated two-step loadings were carried out on SM50A steel in the near threshold region by using the specimens with or without longitudinal residual stress. The crack length and crack opening point were measured by the unloading elastic compliance method.The results obtained are as follows.(1) Under varying loadings, a fatigue crack was found to grow in the residual stress field even when the low level effective stress intensity range (ΔKeff)L was less than (ΔKeff)th.(2) However, the fatigue crack growth rate under low level load (da/dn)*L was influenced by the number of cycles of low level load NL. When NL was rather large, the crack growth increment ΔaL became saturated and the averaged growth rate decreased.(3) The modified Miner type linear accumulation law of crack growth in terms of ΔKeff was confirmed to give a conservative estimation of near threshold fatigue crack growth rate under varying loadings even in the residual stress field.