摘要:

Abstract—The crack initiation lives of peened specimens of aluminium alloys 7010 and 8090 are shorter than those of unpeened specimens. This is caused by the acceleration of crack initiation due to stress concentration in the rough peened surface, especially at fold‐like defects. The crack growth rate in peened specimens is significantly reduced with increasing ΔK, i.e. with increasing crack length. At a crack length of approximately 0.3 mm this trend is reversed and the crack growth rate rapidly increases and attains the same level of crack growth rate as that in unpeened specimens. The point of smallest crack growth rate roughly corresponds to the point of maximum residual stress. The crack growth rate in a peened specimen has been modelled by assuming the effect of residual stress reduces to the equivalent stress ratio. The predicted results agree well with the experimental

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1987.tb00205.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

Abstract—Near‐threshold fatigue crack growth and crack closure were investigated in a nodular cast iron. Fracture surface roughness was promoted by spheroidal graphites. The spheroidal graphites are partially crushed to form powder which accumulates within the crack and thus enhance crack closure. The marked influence of stress ratio on near‐threshold crack growth is due to graphite‐induced crack closure. When the contribution of graphite‐induced crack closure is excluded, the crack growth characteristics are insensitive to stress ratio, and the threshold behaviour for crack growth tends to disappear. In this case, Young's modulus becomes a controlling material parameter for the power law relationship between crack growth rate and stress intens

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1987.tb00206.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

Abstract—The growth characteristics of small fatigue cracks were investigated under rotary bending in a high tensile strength steel, and the critical crack length above which linear elastic fracture mechanics (LEFM) is applicable was evaluated systematically from the results of the present study and previous studies in which two steels having different microstructures and strengths were used. It is found that the critical crack length extends over two regions each exhibiting different growth mechanisms. These regions of microstructurally small cracks and mechanically small cracks are found to be a unique function of the microstructural unit size and the yield strength of the materials, respectively. Therefore, using these relationships, it is possible to estimate the critical crack length for a given materia

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1987.tb00207.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

Abstract—The influence of moisture on the fatigue crack growth behaviour near threshold of a 2NiCrMoV rotor steel has been investigated. At a high stress ratio (R= 0.6), moisture enhances fatigue crack growth rates by approximately 60% compared with the growth rate in dry air. The effect would appear to be due to the influence of hydrogen which is confined to a volume of material at the crack tip considerably smaller than the plastic zone. At a low stress ratio (R= 0.14), the growth rates in moist air can be very much lower than in dry air. This difference is closely associated with the formation of oxides on the fracture surface—moisture modifying the type and extent of oxidation which is observed. Observations of transient crack growth following environmental changes, suggest that fracture surface oxides within approximately 0.3 mm of the crack tip exert a strong retarding influence on crack growth although oxides up to at least 3 mm from the tip may also have some retarding eff

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1987.tb00208.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

Abstract—The conditions for non‐propagating LEFM type fatigue cracks were investigated on an Inconel, 617 Alloy in the range ofKmaxbetween 10 and 50 MPa m1/2under four different types of fatigue loading conditions. In all tests,Kmaxwas held constant during the fatigue cycling prior to determining the non‐propagation condition. It was found that with decreasing range of appliedKthe fatigue tolerance range ΔKeff, thincreases. Furthermore, there is slight increase of ΔKeff,thwith decreasingKmaxof approximately 20–30% whenKmaxis decreased from 50 to 10 MPa m1/2. The results of the four types of tests are considered in respect to the damage in the near‐region of the crack front, i.e. increasingKmaxincreases the damage zone and therefore decreases the fatigue tolerance ran

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1987.tb00209.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

Abstract—Fatigue crack propagation behaviour under variable amplitude loading is significantly influenced by load history induced sequence effects. Because the understanding of the causes of such sequence effects is still rather limited, crack propagation predictions become difficult. In the present investigation an exploratory test program with simple variable amplitude load sequences was performed, which was designed to improve the general understanding of sequence effects and to offer a basis both for an evaluation of existing crack propagation prediction models and for the development of improved methods.The investigations were performed on center cracked 8 mm thick Al 7475‐T7351 specimens. In the first part of the study constant amplitude tests were performed over a wide range of R‐ratios. The test results and the transition of the fracture mode during tests could be represented on the basis of the effective ΔK‐value.In the second part of the paper tests with simple systematically varied sequences of high loads, low loads and underloads were performed and the crack propagation behaviour was measured. In order to obtain detailed information on the crack propagation increments per cycle, fractographic analyses were also performed. It was found that also under variable amplitude loading the crack propagation approximately followed the effective ΔK‐values, except in one case.A linear (non‐interaction) prediction model could not describe the observed crack propagation behaviour. The results of the application of a model assuming a constant crack opening level are also given. Finally, some general viewpoints regarding further developments of models

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1987.tb00210.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

Abstract—The near threshold fatigue behavior of a nickel base superalloy and a wrought 2024 aluminum alloy is examined as a function of specimen geometry. Experimental results revealed that for a given value of ΔKin Region I, crack growth rates were observed to increase as the specimen loading became more symmetric with respect to the load line. Compact tension type specimens exhibited lower crack growth rates than the more symmetrically loaded center cracked tension specimens. Consideration of the observed discrepancies is given in terms of the near field crack tip stress distribution and deformation behavior of the material affecting crack advan

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1987.tb00211.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1987.tb00203.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY