ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00863.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00864.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—A study into microstructural effects and crack growth behaviour of AISI type 316 stainless steel under creep‐fatigue conditions at 550°C within the high strain ranges of 0.9–2.5%, including a 60min hold time, was undertaken on a high‐temperature reverse‐bending rig. Throughout the tests, surface cracks on both the tensile‐hold and the compressive‐hold sides were monitored by means of a plastic‐strip replication technique. Additional investigations were conducted on failed specimens to examine the crack morphology in the depth direction, and to examine the function of oxidation; also to study changes of fracture surface morphology, changes in dislocation structures and precipitate configurations corresponding to the different strain ranges. These detailed analyses revealed that the predominantly intergranular long cracks on the tensile‐hold side and transgranular short cracks on the compressive‐hold side are dominant aspects of the investigation. The dislocation structures under creep‐fatigue conditions are strain‐range dependent, with a clearly defined cell structure at the higher strain ranges and dense dislocation tangles at lower strain ranges. The large reduction in creep‐fatigue endurance can be attributed to early crack growth and grain boundary cracking caused by stress relaxation, oxidation, precipitation and, most importantly, the coalescence of the many

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00865.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—A study has been undertaken into short crack growth behaviour of AISI type 316 stainless steel under creep‐fatigue conditions at 550°C within the high strain range of 0.9 to 2.5% and including a 60 min hold‐time. During the high‐temperature, reverse‐bending tests, surface crack initiation and growth on both the tensile‐hold and the compressive‐hold sides of circular‐section specimens were monitored by means of a plastic replication technique. Detailed analysis revealed that under creep‐fatigue conditions, the initiation and growth behaviour of many individual cracks and their subsequent coalescence to form a major Stage II (tensile) crack was the dominant feature in the failure process. A life prediction model is proposed which incorporates the process of short crack coalescence. Satisfactory predictions of creep‐fatigue lifetimes are de

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00866.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—The behavior of fatigue crack growth for low and medium carbon steels, an austenitic stainless steel and an aluminum alloy under pure Mode II loading was investigated experimentally, using cruciform specimens. The results show that under pure Mode II loading, fatigue crack propagation has three possibilities, namely, bifurcation into two branches, propagation along the original Mode II direction, and the mixture of these two situations, depending on the material. The growth rate da/dN vs. ΔKIIrelation for Mode II propagation is similar to a Pans type law for Mode I growth. Fractographic observations by optical microscopy and SEM were made also on all specimens tested. When a crack branched, striations parallel to the crack front which were often associated with Mode I fatigue crack growth were observed and long marks parallel to the crack propagation direction were also found for slanted fracture surfaces. When a crack propagated along the original Mode II direction, many frictional marks parallel to the crack propagation direction were observ

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00867.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—Fully reversed uniaxial fatigue tests were performed on polished hour‐glass specimens of commercially pure aluminium with three different grain sizes, in order to examine the effect of grain size on fatigue. The growth of surface cracks was monitored by a plastic replication method. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall‐Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. Multiple crack initiation and growth is a feature of the fatigue of aluminium, while the grain size influences the specific detail of crack coalescence. Crack path deviation is greatest in the coarse grained microstructure and crack surface roughness is more pronounced. SEM fractography reveals that crack initiation and early crack growth takes place along crystallographic slip planes, and that fatigue striations, characteristic of stage II cracking, extend to the very edge of the specimen section, suggesting extensive crack tip blu

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00868.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—Advances in manufacturing techniques and improvements in non‐destructive inspection procedures have reduced the size of initial defects in fatigue‐critical components to the scale of the microstructure. Unfortunately, the growth of these so‐called short fatigue cracks is not properly characterised by conventional linear elastic or elastic‐plastic fracture mechanics. However, the Navarro‐de los Rios crack growth model does account for many aspects of their growth and has proved successful in predicting fatigue life of medium carbon and C‐Mn steels. In consequence, the growth of short fatigue cracks in commercially pure aluminium, under fully reversed uniaxial loading, has been studied, using a plastic replication technique and modelled, using the Navarro‐de los Rios crack growth equation. The predicted lifetimes are in good agreement with the actual experimental lifetimes, and compare favourably with predictions based on two previously reported crac

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00869.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—The nucleation and growth of small naturally initiated cracks have been investigated on Ti 6 Al 4V alloys, with bimodal or globular microstructures. Tests have been performed in air at a stress amplitude near 0.75 of the yield stress. The influence of microstructure on fatigue damage is described and the differences in fatigue life of some titanium alloys are explained. The effect of microstructure on fatigue resistance is mainly related to the early stages of damage including initiation and small crack growth. Coalescence processes favoured by a high density of initiated surface microcracks in fine α grain material, or rapid initiation of large cracks in coarse α colonies, explains low fatigue resistance or lifetime scattering in globular alloys. Small crack growth retardation due to α/β barriers associated with a low surface crack density, limiting coalescence processes, explains the highest resistance of bimodal struc

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00870.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract—Under a natural mapping between the standardR‐curve analysis diagram and the failure assessment diagrams of R6 and PD6493 Level 3 theR‐curve becomes the RCI (R‐curve image). It follows that whenever the assessment point moves along the failure assessment line during ductile crack growth, the implication is that the failure assessment line is the RCI. This result is used to test the conservatism of a specific PD6493 Level 3 analysis by two methods. The first calculates the variation during crack growth of the applied elastic‐plastic crack tip opening displacement (CTOD) parameter (or‘driving force') which is implied by the PD6493 analysis and then compares this variation with an independent estimate of it. The second uses an assumed driving force to deduce the CTOD resistance curve implied by the failure assessment line. It is shown by both methods and also by a directR‐curve analysis that this particular PD6493 analysis is conservative relative to anR‐curve analysis which uses a crack driving force estimated by the EPRI (Electric Power Research Institute) procedures. However there is inconsistency between standardR‐curve analysis and PD6493 Level 3 analysis in that the latter implies a material resistance which, for a given material, depends on the geometry of the structure. A similar inconsistency arises in any failure assessment procedures like the Options 1 and 2 with Category 3 of R6, which require the assessment point to move on a geometry independent failure assessment line during crack growth; indeed, even when the failure assessment line is geometry dependent there is full agreement with R‐curve analysis only if the correct RCI is used as the failure assessment line. In a brief discussion it is noted that the new failure assessment diagram studies involving multi‐parameter fracture mechanics may help to amel

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00871.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00872.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY