摘要:

AbstractIn an attempt to understand the fatigue crack initiation and growth behavior of planar slip alloys, studies have been carried out in strain control on Cu‐16 at.% A1 single crystals oriented for easy glide. Alloy single crystals of Cu‐16 at.% A1 do not show saturation like copper crystals, but harden slowly and steadily throughout life, for the whole range of amplitudes applied. Cracks, the initiation and growth of which were studied by the sharp corner technique, were found to initiate when the peak stress reached 32 MPa irrespective of strain amplitude. The distribution of crack sizes in Cu‐16 at.% A1 single crystals was found to be more skewed to small cracks than that of copper. This behavior is attributed to the frequent migration of the bands of localized slip in this material. The fatigue lives of this alloy at low strain amplitudes (ypm<6‐7 × 10−3) were found to be longer than those of copper single crystals because the strain is distributed more homogeneously than in copper. At high strain amplitudes, however, they become shorter because of the higher peak stress. The fatigue lives of Cu‐A1 crystals are predicted from those of copper on the basis of the different volume fractions of localized strain in the two materials; the prediction agrees well with the actual lives of Cu‐Al crystals. By geometrical consideration of secondary slip planes with respect to a primary crack, it was found that the critical system (A6) is most favored for secondary cracking adjacent to a

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00650.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe influence of microstructure and processing method on the initiation and growth of fatigue cracks in an aluminum‐magnesium‐silicon alloy has been studied with extruded and squeeze‐cast material cycled in strain control. Using the replication technique, initiation and early growth of short cracks was found to be dependent upon the applied strain level. At low strains decohesion at the particle/matrix interface occurred whereas at high strain levels both particle cracking and decohesion were observed. The micromechanisms of subsequent short crack growth were also influenced by strain amplitude. In the case of the squeeze‐cast material cycled at low total strain amplitudes subsequent growth took place from particle to particle along the soft precipitate‐free interdendritic regions, in the direction of maximum shear. Notable deflection occurred at triple points. Short cracks in the extruded material cycled at all total strain amplitudes were found to initiate by debonding of particles in banded zones. Early crack growth at low strain amplitudes over the first 20μ along the surface was significantly impeded by subgrains and some second phase particles, whose effect was much greater than for the squeeze‐cast material due to a finer microstructure (the average subgrain size was 6 μ) and absence of soft interdendritic regions. The fracture surface was also rougher which presented a greater degree of resistance to propagation. In both types of material, Stage I crack growth extended to a depth of about 250‐350 μ. During this portion of life short fatigue crack growth behaviour was observed. Long crack behaviour coincided with the crack switching to Stage II growth. Short crack propagation behaviour and the crack depth at which Stage I was complete could be expressed using a crack growth model based on the strength of slip bands emanating from the crack tip and the strength of barriers which were in th

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00651.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe behaviour of small fatigue cracks in the A1‐Li‐Cu‐Mg‐Zr alloy 8090 has been studied. Growth rates of small fatigue cracks in these alloys were found to be proportional to ΔK2. This relationship is compared to that suggested by Murakami where crack growth rate was proportional to ε2pa. The difference between the equations is attributed to the relative importance of plastic strain for the two sets of data. It is suggested that the two equations are equivalent if crack growth rate is related to total strain. The common observation of a Paris law exponent of two in different alloys suggests that there is a single common mechanism for small fatigue crack growth. This contrasts with long fatigue crack growth, where a variety of Paris law exponents are observed. Other characteristics of this fatigue mechanism include crack propagation on {111} slip bands, insensitivity of crack propagation rate to precipitate distribution and a plastic zone comparable in size to the cra

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00652.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA study has been performed relating to fatigue damage accumulation in Armco‐iron with a mean ferrite grain size of 40 μ. Two constant strain amplitudes, εa= 1 × 10−3(Nf= 250,000 cycles) and εa= 6 × 10−3(Nf= 3050 cycles) were employed to derive base data for cumulative fatigue loading where unit blocks of low (1000 cycles) and high (10 cycles) strains were mixed until failure occurred. Observations in a scanning electron microscope permitted a periodic evaluation of surface damage to be recorded. Two regimes of damage were noted. During the early crack initiation phase, crack density increased and crack growth rate was constant. During the later phase, or crack propagation stage, new cracks did not nucleate, the smallest cracks arrested, crack density decreased, and the largest cracks grew in an exponential manner, which included linkage with smaller cracks. A damage accumulation model is presented which permits a prediction of fatigue lifetime under variable amplitu

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00653.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractOpening and closure of short cracks has been examined experimentally using small strain gauges fixed to unnotched specimens very close to short fatigue cracks. The results of completely reversed tension‐compression constant amplitude tests are that crack opening stresses decrease with increasing stress amplitude and that crack closure occurs at nearly the same strain as crack opening. In variable amplitude loading the crack is subjected to a low crack opening level resulting from larger cycles. Based on the results of this experimental investigation some simple approximation formulas are proposed with which decrease as well as increase of crack opening levels can be described. Using these formulas it is possible to compute crack opening strains for any load sequence. The findings of the investigations have been merged to build an algorithm for the prediction of fatigue lives to initiation of cracks of technical sizes in case of variable amplitude loading. The improvement in accuracy of life predictions compared to results of an existing damage accumulation concept is demonstrated for two materials and two load sequence

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00654.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIn the presence of a notch, fully compressive loading has been shown to develop fatigue cracks, typically less than 1 mm in length, from the notch root in metals, cemented carbides and ceramics. The crack length is dependent on factors like load range, stress ratio, frequency and environmental influences. This paper examines the influence of these factors on crack growth in β‐alumina under compression fatigue. Results obtained support the constitutive microcracking model advanced by Brockenborough and Suresh, but are influenced by crack tip shielding and, at low frequencies, a small stress corrosion component of grow

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00655.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIn this paper, the conformal mapping method is used to solve the plane problem of an infinite plate containing a central lip‐shaped notch subjected to biaxial loading at a remote boundary or a surface uniform pressure on the notch. The stress intensity factorsKIandKIIare obtained by the derived complex stress functions. The simple analytical expressions can be applied to the situation of cracks originating from a circular or an elliptical notch. The plastic zone sizes for such notch cracks are subsequently evaluated in light of the Dugdale strip yield concept. The results are consistent with available numerical dat

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00656.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractAn elastic‐plastic fracture mechanics study on the effects of biaxial stress fields is presented relating to various centre and edge notches containing short cracks under plane strain. The elastic stress intensity factor and the non‐singular elasticT‐stress term are determined by finite element analyses. Comparisons of these results with small scale yielding plastic zone size predictions are given for a wide range of applied stress intensity factors, revealing an improved correlation when considering theT‐stress effect. The influence of positive biaxial stress‐induced crack closure on the stress intensity factor is also

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00657.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA theoretical model which describes the microstructural aspects of short fatigue cracks is further developed to quantify the strength of microstructural barriers. The effectiveness of a barrier to decelerate or arrest short fatigue cracks in single phase materials depends on the relative orientation of the grains ahead of the barrier. A method of calculating the progression of orientation factors of the grains traversed by a fatigue crack is presented. Introducing the orientation factor into the crack growth equation allows the model to be used in the prediction of crack growth rate and of fatigue life. Comparison with experimental results, obtained with an A1‐Li alloy, establishes the accuracy of the predictions. The sequence of grain orientation factors ahead of the crack is also used to predict the stress boundary conditions for short fatigue crack propagation in the form of a Kitagawa‐Takahashi diagram which is compared with the experimental results due to Lu

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00658.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractTorsional tests under different mean shear stresses were conducted at room temperature using a 1.99% NiCrMo steel. A technique was developed to control both the mean and the alternating shear stresses. It is shown that the mean shear stress plays a fundamental role in promoting stage I, mode II short crack growth. The effect of mean shear stress on the plastic deformation at stage II crack tips is discussed, which illustrates the mean shear effect on the transition between stage I and stage II crack growth. The polarity of mean shear stress is found to have no effect on the fatigue crack growth rate and the fatigue lifetime of an isotropic material. A model is proposed to account for the effect of mean shear stress on the fatigue lifetime under torsional loading.

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1991.tb00659.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY