摘要:

Abstract—An experimental investigation of the effects of small surface scratches on the fatigue behavior of Al‐Li alloys has been carried out. Small scratches were generated onto the specimen surface which were then subjected to fatigue testing. These scratches were characterized by three different parameters, scratch angle, scratch tip radius of curvature, and scratch depth. These parameters have exhibited an important influence on the fatigue life of the scratched samples. In particular, it was found that the fatigue life decreased sharply as the scratch depth increased. In addition, the scratch surface and fracture surfaces were examined under a scanning electron microscope. The observations helped in the assessment and identification of the mechanisms responsible for fail

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00250.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—An experimental study of the elastic‐plastic fatigue behavior of small surface cracks in Alloy 718 at 650°C was conducted under conditions of total strain control. During cycling, the crack growth was continuously monitored using a direct current potential drop technique while the influence of crack closure was monitored using a laser interferometry technique measuring the crack mouth opening displacement. The crack tip plastic zone size was also measured using a post‐test delta phase decoration technique. Results show that the growth rates of the small cracks correlate well with long crack LEFM data when using an appropriate elastic‐plastic driving force parameter. The anomalous crack growth rates observed in some experiments were found to be experimental transients dominated by the crack initiation fracture and do not represent an intrinsic behavior of Alloy 718. No contribution to the anomalous small crack growth could be attributed to either crack closure or enhanced crack tip pl

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00251.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The interaction between fatigue damage (i.e., fatigue crack propagation) and internal grain boundary damage (i.e., cavity formation at grain boundaries) has been studied for the Alloy 800H at 750°C for constant plastic strain ranges but different experimental conditions. Most experiments were performed at constant ranges of alternating tensile/compression stresses. Symmetrical as well as asymmetrical tests (with larger compression stresses) were performed. In comparison to the former tests, asymmetrical tests led to shorter cyclic lifetimes mainly due to cavity formation which was not observed for symmetrical tests. It could be shown that a fast compressive and a slow tensile half cycle (at large compressive and low tensile stresses) are ideal conditions for the nucleation and growth of cavities. Based on quantitative measurements of the cavity density from interrupted fatigue tests, a physical model is presented which can predict the number of cycles to failure. This cycle number is determined only by fatigue crack growth which is controlled by (a) athermal plastic deformation, (b) creep deformation and (c) rate enhancement by cavitati

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00252.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—A finite element analysis and fatigue crack growth studies are made of dovetail joints in aeroengine compressor discs. Three aspects are examined: the first deals with the finite element stress analysis of the critical geometrical features and interface conditions of different dovetail configurations, thus enabling an assessment to be made of the critically loaded regions in the disc. The second deals with the prediction of the direction of potential fatigue cracks, which were allowed to initiate in the finite element model at the regions where fretting damage is most likely to occur, using an incremental crack tracking criterion. The third is concerned with the verification of the above modelling techniques with fatigue tests on a uniaxial back‐to‐back arrangement, which attempts to simulate the stress fields of a rotating

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00253.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The fatigue crack growth behavior of Ti‐1100 is analyzed at elevated temperatures to evaluate the effects of mechanical and environmental variables. Experiments conducted over a wide range of frequencies from 0.01 Hz to 200 Hz indicate a strong dependence of the growth rate upon cyclic loading frequency. Superposition of hold time at maximum and minimum loads over a baseline 1.0 Hz cyclic loading frequency produces an insignificant variation in crack growth rate, which may be attributed to the combined effects of enhanced environmental degradation, crack‐tip blunting and increased asperity‐induced closure level in this material. It is deduced that a hold time at maximum load results in an interaction of the environmental effects with a retardation effect due to crack tip blunting as a consequence of creep under maximum applied load, whereas for hold at minimum loads, extensive crack‐branching and micro‐cracking appear to enhance crack closure loads resulting in lower crack growth rates. A linear superposition model is employed to account for the complex interactions due to fatigue, creep and environmental

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00254.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The fatigue behaviour of a Ni‐Cr‐base powder flame‐spray coating on a 0.4% C steel is investigated. Fatigue tests were carried out using mild hour‐glass profile specimens. Cracks were detected and measured using plastic replicas and an image analysis system. Coated specimens showed a slightly lower fatigue endurance than plain specimens under torsion loading, while the opposite was observed for push‐pull loading. Microcracks in coated specimens invariably form at pores.Contrary to the usual case of stage I shear growth for a plain 0.4% C steel in tension or torsion loading, the coated specimens show initial crack growth from pores along directions perpendicular to the maximum tensile stress. The crucial behaviour of short cracks, and their growth rates, relative to the thickness of the coating, are discussed in

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00255.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Fatigue strengths of Sic whisker or Sic particulate reinforced 6061‐T6 aluminum matrix composite (SiCw/Al or SiCp/Al), fabricated by a high pressure infiltration method, were obtained in laboratory air, ion exchanged water and a 3 pct NaCl aqueous solution. A comparison was made with the properties of the matrix aluminum alloy 6061‐T6. The SiCw/Al composite maintained a higher fatigue resistance than the SiCp/Al composite or the monolithic 6061 Al, even in a 3 pct NaCl solution. Good correlation was observed between a deterioration in the fatigue strength and the value of the corrosive potential and current, when changing the environment from laboratory air or ion‐exchanged water to a 3 pct NaCl aqueous s

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00256.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The effect of stress state on the time and strain to failure has been considered in terms of currently accepted models of cavity growth. It is shown that the increasing contributions of compressive stress cause changes in cavity growth mechanisms which lead to increases in ductility. A tensile component of stress is necessary to provide the driving force for cavity growth by diffusion of vacancies and hence only strains in the presence of a tensile stress can lead to creep‐dominated failure in creep‐fatigue.Equivalent stress functions for isochronous stress rupture have been derived in terms of the cavity growth models and their corresponding relationships for calculating damage in terms of strain fractions developed. It is shown that it is difficult to discriminate between the various models on the basis of available experimental data. However, the analysis allows data to be assessed within the framework of physically based mechanisms and suggests methods which lead to conservative lower bound estimates of endurance.It is concluded that the shape of the isochronous creep rupture locus depends on the controlling process of cavity growth and that a detailed analysis of uniaxial creep ductility is necessary to obtain a complete description of the multiaxial behaviour. In many instances such an analysis will prove more valuable than simply performing creep tests over a limited range of stress states. Increasing contribution of principal stress to the failure process leads to a greater value for the equivalent stress in the presence of a compressive component compared with the von Mises equivalent value. However, the equivalent stress is reduced in the tensile quadrant of bi‐axial stress. Hence the degree of conservatism arising from using the von Mises equivalent stress will vary with stress and may become slightly non‐conservative. The relationship between equivalent stress functions for application in a time fraction assessment of creep and the calculation of creep damage by a strain fraction method has been demonstrated. Finally, guidance is given on how a limited data base of uniaxial rupture properties can be used to obtain a conservative estimate of behaviour under multiaxia

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00257.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—This paper examines the determination of the T‐stress by making use of a secondary state and the complex‐variable forms of the work reciprocal and Rice's Jxintegrals in terms of the Muskhelishvili complex stress functions. A simple relationship is found which relates the stress intensity factors and T‐stresses of the two states and is applied to the problem of a crack at the root of

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00258.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—This paper demonstrates a method to determine the elasticT‐stress for a semi‐infinite half‐plane containing a surface‐breaking crack which is loaded by an arbitrarily distributed far‐field tension. The method consists of representing the crack by a continuous distribution of edge dislocations and forming singular integral equations to determine the equilibrium dislocation distributions. By numerically solving the integral equations, stress intensity factors andT‐stresses are obtained for the example case of a crack which is normal and inclined to the free‐surface of a half‐plane and loaded by a uniform

ISSN:8756-758X    

DOI:10.1111/j.1460-2695.1994.tb00259.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY