摘要:

AbstractHeavily deformed two‐phase materials, often referred to as in‐situ composites, have been shown to exhibit strengths which approach the theoretical limit. This paper considers the mechanical response of these materials from both a macroscopic and microscopic point of view in terms of the competing processes of continued plastic Bow and fracture. The results from a wide variety of composite systems and experimental techniques are examined to suggest a number of areas requiring further study in the fields of plasticity and fracture of engineering materi

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00900.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA multiaxial fatigue strain energy density parameter has been formulated which normalizes fatigue data obtained under a variety of mean stress levels and loading combinations. This parameter represents that proportion of the overall strain energy contributed by the stresses and strains on the critical or fracture plane. It is shown that multiaxial fatigue life data may be accurately correlated by applying this parameter to the experimental results of Inconel 718 alloy subjected to a variety of mean normal and shear stress levels, as well as to SAE 1045 steel tested under tension, torsion and simultaneous tension and torsion.

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00901.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe present study aims to assess the additional fatigue life enhancement obtained by coldworking a previously cold expanded plate hole. Two different methods of performing the second coldworking were considered, i.e. moving the mandrel in the same direction as for the first coldworking or moving it in the opposite direction. A three dimensional finite element analysis for establishing the residual stress field induced by two successive coldworkings (5.58% then 4.8%) was carried out. Al 7475‐T7351 specimens with a central hole were cold expanded at 5.58%, subjected to cyclic loading at constant amplitude for a predetermined life fraction (on the basis of 5.58% cold expanded hole fatigue life) and then re‐coldworked at 4.8%. After this rework, the specimens were again subjected to the same fatigue loading conditions until failure. During cycling, the fatigue crack extension was monitored using a video‐camera in order to determine the coldworking effect on both the initiation period and the propagation life.The fatigue test results have shown that a second coldworking may enhance the fatigue life of an already coldworked hole. For a given cyclic loading level, the beneficial effect depends upon the applied life fraction before reworking. The direction of the mandrel movement during the second coldworking has no noticeable influence on the additional fatigue life improvement. These experimental results have been analyzed with reference to the calculated residual stress

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00902.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe loads for yield of plates and cylinders with semi‐elliptical surface flaws have been evaluated by a weight‐function method incorporating stress intensity factors using results from finite element calculation. A new weight function for loads partially distributed along a chord is proposed. The yield load is defined as that at which the plastic zone first reaches the back surface. The results show that the effects of the cylinder geometry parametert/Ron yield load is small, and it is proposed that the plate results be used also for cylinders over the range of geometries 0 ≤t/R

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00903.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA study was completed on irradiated and unirradiated (offcut) material from Pickering Nuclear Generating Station (NGS) A Unit 3 pressure tubes which were fabricated from ingots of 100% recycled material (quadruple‐vacuum‐arc‐melted). The toughness of the material is shown to be generally higher than that of previous material fabricated from<100% recycled material (double‐vacuum‐arc‐melted). Fractographic and microstructural studies demonstrate that variations in toughness of the low chlorine (<1 ppm) material are predominantly due to preferential void nucleation at particles, in particular aligned zirconium phosphides. In the absence of such particles, very high toughness levels can be maintained after

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00904.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA study has been conducted on the initiation and growth of fatigue cracks in the three principal directions of an aluminium alloy 2024‐T351 plate tested under stress control (R, minimum to maximum stress =– 1). Early and multiple fatigue crack nucleation from broken Al7Cu2Fe second phase particles resulted in shorter lives for the longitudinal direction specimens in the medium to long life regime. Although fatigue cracks nucleated in large surface grains, rather than at broken particles, the lives of the short‐transverse direction specimens were marginally longer. Cracks also nucleated in large surface grains in the transverse direction specimens, yet the average fatigue lives were about twice as long. This was the consequence of wider slip bands and fewer initial micro

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00905.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractVarious residual stress conditions were introduced into the surface of a 7075‐T6 aluminium alloy by shot peening. This resulted in a considerable alteration in subsequent fatigue crack growth behaviour. A systematic investigation into the effects of a variety of conditions on fatigue crack propagation behavior was conducted. The degree of shot peening was established using the Almen scale and calibration strips. Changes occurred in crack re‐initiation lives, crack growth rate and hardness across the thickness profile. Residual stresses were introduced by various peening operations (such as different peening positions and loading specimens prior to and during peening). By measuring the crack opening level of peened specimens, changes in fatigue crack closure effect with respect to the crack extension in CT specimens are reported. The relations of re‐initiation life versus crack opening level and Almen levels, and of the crack closure effect versus Almen scales, were addressed experimentally. It was found that the improvement in re‐initiation life and the retarding effect of crack growth, both influenced by the Almen scales, are associated directly with the elevation of crack opening level, particularly at the stage of crack re‐i

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00906.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractMonte Carlo simulations of fatigue in welded joints have been performed using an approach which combines a model for initiation with a multiple surface crack propagation model. The results have been compared with experiments on T‐plate welded joints in which initiation and propagation of surface cracks were monitored using potential drop techniques. Predictions of initiation life using a local strain approach were conservative. Despite this underprediction of initiation life, predictions of total fatigue life were very good as a consequence of accurate simulations of propagation life and the fact that initiation represented on average only 12 to 22% of total life, depending on stress level. The initiation model considered variability due to the local weld toe angle and radius, and material strain–life behaviour. The only variability considered in the propagation analysis was the position and timing of initiation events, which leads to variability in coalescence. The underprediction of the variability in propagation and total lives was attributed to the underprediction of initiation life and the fact that out‐of‐plane coalescence effects were

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00907.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractConventionally, the fatigue threshold of a long‐crack is obtained by load shedding using a constant normalizedK‐gradient, as recommended by ASTM E‐647. However, this load shedding procedure often causes the crack opening displacement to decrease with increasing crack length, which may trigger crack closure. In this study, load shedding tests were conducted in load control following a power‐law load shedding schedule such that the crack opening displacement was kept at a relatively constant level. Using this new testing procedure, it is shown that crack closure is not always as high as that associated with the ASTM recommended procedure at a given ΔK.Comparisons of fatigue crack growth rates under identical testing parameters, but with different closure levels produced by the two load shedding procedures, have been made for several structural alloys. The extrinsic shielding of the crack tip zone via crack closure has also been examined using an energy approach for these alloys. On the basis of these analyses, the true effective stress intensity range is evaluated for fatigue crack propagation and the role of crack closure in creating a fatigue threshold is re

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00908.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe dislocation structures formed in the plastic wake of a long fatigue crack have been characterized by TEM for polycrystalline copper tested in air and in vacuum. In order of increasing distanceDfrom the fracture surface, typical structures are cells, walls, PSBs and veins, and finally loop patches and dislocation tangles. The outer boundaries of these different regions depend on the stress intensity factor range ΔK.By comparison with the dislocation structures developed in LCF specimens fatigued at a constant strain amplitude, the structures in the plastic wake appear much distorted and more than one set of slip systems are often found to be activated. This feature is the consequence of the complex stress history occuring in a unit volume of material located at a distanceDfrom the fracture surface. Due to this fact, the plastic strain gradient deduced from the size of dislocation cells can only be approximated. The dislocation structures formed near the fatigue crack tip in vacuum appear to be somewhat different from those formed in air; the cell boundaries are much denser, sharper and straighter, the misorientations between them are more marked and, at a given distanceDand for a given applied ΔK, the cell size is also smaller than that formed in air. These effects are the consequence of the larger number of cycles that a unit volume of material located at a given distanceDcan sustain during fatigue cracking in vacuu

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1995.tb00909.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY