ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00952.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00953.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—An experimental study of the surface evolution during controlled plastic strain amplitude single‐step and two‐step loading tests reveals the important damage mechanisms for 316L stainless steel. In the first stage, the cyclic plastic strain is concentrated into persistent slip bands (PSBs) and a surface relief is formed consisting of extrusions and intrusions. The frequency of occurrence and the total density of PSBs has been assessed using systematic observations in a scanning electron microscope. The relative volume occupied by PSBs determines the fatigue damage in this stage. Two‐step loading has only a small effect on the PSB damage evolution and nearly equal saturated values (corresponding to the applied plastic strain amplitude) were achieved in the single‐step and the two‐step loading inv

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00954.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—The evolution of the surface crack distribution in single‐step and two‐step plastic strain amplitude controlled cycling was studied. The frequency of crack occurrence and total crack density were evaluated together with crack growth rates of the largest cracks. The crack density increases, reaches a maximum, and then decreases during fatigue life. The maximum value of crack density partitions the whole life into a “crack generation period” and a “crack propagation period”. In the first period the crack growth rate is approximately constant and is determined by the applied plastic strain amplitude. The crack growth rate increases with crack length in the second period, but is basically determined by the crack growth rate in the “crack generation” period. Hence the crack growth rate in the “crack generation” period is the li

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00955.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—Constant amplitude, plus high and low and repeated two‐step loading fatigue tests were conducted on a structural steel and a titanium alloy. Detection and measurement of very small crack sizes were possible by using a specially designed small‐sized test specimen. By improving the unloading elastic compliance technique, the crack closure behaviour was continuously monitored without interruption of the test. It was found that the growth process of a small crack is a transition process from the incipient, ideal crack to the conventional long fatigue crack. Fatigue cracks longer than 0.2 mm were shown to have a residual tensile deformed region in the wake of the crack tip and exhibited retardation after load reductions due to differences in closure levels. However, for cracks smaller than 0.1 mm, closure is insufficient to control the crack growth behaviour, and the high level loading may give rise to an acceleration of crack g

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00956.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—The effect of variable amplitude loading on the initiation and propagation of fatigue cracks from shot peened and unpeened surfaces was investigated. The results confirm the well‐known effect of shot peening, i.e. the increase of fatigue life in comparative tests under constant amplitude loading. Shot peening increases the time to crack initiation and also decreases crack propagation rate, particularly in the earlier stages of growth.Variable amplitude tests were of various kinds: (i) half‐life at one stress level, followed by testing at the second stress level until failure, (ii) sequence loading at two stress levels, and (iii) a different number of overload cycles (1, 10, 100) following a given number of base amplitude cycles.The results of the two amplitude tests showed different behaviour depending on whether the first half‐life cycles were of higher or lower stress levels. These results are explained in terms of crack density and crack coalescence. The extent of damage accumulation during block loading depends on the stress levels employed. The benefits of shot peening are compromised by high stress levels.Results from the overload tests showed that the interpretation of results is not straight forward. A combination of the theories of crack retardation and microstructural‐fracture mechanics seems a necessary prerequisite in an explanation of the resultant

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00957.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—The effects of specimen thickness, stress intensity levels andRratio following single tensile overloads, multiple overloads and overload/underload events have been investigated in a BS4360 Grade 50D steel. The amount of fatigue crack growth retardation increased with both decreasing applied baseline ΔKand increasing overload block size. Smaller increases in retardation were obtained for overload/underload block events compared with block overloads. The data suggests that crack flank plasticity resulting in crack closure adequately accounts for much of the observed behaviour. Near crack tip plasticity was thought to play a more important role in generating crack closure than that remote from the crack t

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00958.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—Experiments on the propagation behaviour of physically short fatigue cracks were conducted on an axle‐steel, using a direct current potential technique. Various overload ratios (OLR = 1.5, 1.7, 1.9 and 2.1) and different overload positions were employed. The experimental results show that physically short cracks are more sensitive to the overload retardation effect than long cracks. The overloads produce a beneficial effect when they are introduced early. Later overloads, although giving an increase in life, produce a smaller effect. The delay cyclesNDdue to the application of a single overload increase with increasing overload ratios. The experimental data, when plotted on a log‐log graph ofND/NCAL(whereNCALis the number of cycles under constant amplitude loading) versus overload ratio OLR, gives a straight line relationship, namelyND/NCAL= 2.81 × 10−3(O

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00959.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—The load interaction effects that occur under variable amplitude loading conditions following single peak overloads, low‐high blocks and high‐low blocks have been investigated on a commonly used structural steel. A detailed investigation has been carried out on long cracks which aims at developing and assessing the efficacy of a simple predictive model based on closure and plastic zone effects.The material was tested in a quenched and tempered condition. All the tests were carried out in the four point bending mode, in both the Paris regime and near the threshold, with overloads ranging from 10% to 100%.An attempt to relate crack closure to crack growth rates in the transient growth rate region (which follows a load change) was made.The trends observed for a wide range of loading parameters are pres

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00960.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract—Fatigue crack growth rates in corner notched specimens of forged Ti‐6A1‐4V, used in the manufacture of rotating aero‐engine components, have been measured for fatigue loadings that combine major and minor stress cycles. The loadings are simple representations of the flight cycle and the potential in‐flight vibrations, each loading block consisting of high‐frequency minor cycles superimposed on the major cycle dwell at maximum load. The crack growth rates are dependent on the number and stress ratio of the minor cycles, but only when they individually contribute to the growth of the crack. Estimates of the fatigue threshold values and near‐threshold growth rates associated with the minor cycles have been made, all potential load history effects having been ascribed to the minor cycle component of the loading. Using this data, satisfactory crack propagation life predictions have been demonstrated for a wide range of test conditions involving the conjoint action of major and minor

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1996.tb00961.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY