Initiation Of Short Cracks Research Articles

Growth behavior of short fatigue cracks in a SCS-6/Ti composite

The initiation and growth behavior of short fatigue cracks from the interfacial micro-notches in a SCS-6/Ti composite at various maximum applied stress levels were quantitatively characterized. Micro-notches in the interfacial reaction layer were induced by applying monotonic tensile loading prior to fatigue testing. The propagation and arrest behaviors of each individual crack were carefully monitored. An interface-controlled fatigue cracking model was used to predict several critical parameters which govern the growth behavior of fatigue cracks in the composite.

Micromechanisms of fatigue crack nucleation and short crack growth in a low carbon steel under low cycle impact fatigue loading

An experimental investigation has been conducted to study micromechanisms of short crack initiation and propagation in a low carbon steel under low cycle impact fatigue loading. The results indicated that the most favorable site for crack nucleation was grain boundaries. On the other hand grain boundaries were the effective obstruction for short crack growth. The progressive process of crack growing across a grain boundary involved crack tip blunting, plastic flow zone forming, crack branching and only one of the branches growing continuously. Crack nucleation and propagation along slip bands were difficult in the grains with wavy or cross slip bands, and they were easy in the grains with straight slip bands. The short crack growth rate fluctuated periodically until the crack length reached a critical value and then crack growth accelerated.

Structural sensitivity of steels to corrosion-mechanical fracture under conditions of fatigue and cavitation

We present experimental estimates of the resistance of 30KhGSNA steel after heat treatment in various modes to corrosive-erosive wear in inhibited water. We established the existence of threshold amplitudes under cavitation, below which wear of the steel is not observed. They vary more significantly in the case of inhibiting the medium than owing to a change in the steel structure from perlite to martensite. We compared the results obtained with the literature data concerning the structural sensitivity of the steel under study to corrosion fatigue in the range of high- and low-amplitude loading. We conclude that corrosive-erosive fracture is controlled by the process of initiation and growth of short corrosion cracks.

Collective evolution characteristics and computer simulation of short fatigue cracks

Fatigue testing was conducted using a kind of triangular isostress specimen to obtain the short-fatigue-crack behaviour of a weld low-carbon steel. The experimental results show that short cracks continuously initiate at slip bands within ferrite grain domains and the crack number per unit area gradually increases with increasing number of fatigue cycles. The dispersed short cracks possess an orientation preference, which is associated with the crystalline orientation of the relevant slip system. Based on the observed collective characteristics, computer modelling was carried out to simulate the evolution process of initiation, propagation and coalescence of short cracks. The simulation provides progressive displays which imitate the appearance of experimental observations. The results of simulation indicate that the crack path possesses a stable value of fractal dimension whereas the critical value of percolation covers a wide datum band, suggesting that the collective evolution process of short cracks is sensitive to the pattern of crack site distribution.

EFFECT OF BIAXIAL MEAN STRESS ON CYCLIC STRESS‐STRAIN RESPONSE AND BEHAVIOUR OF SHORT FATIGUE CRACKS IN A HIGH STRENGTH SPRING STEEL

Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour‐glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.

Behaviour of initiation and growth of short fatigue cracks: Wu, Z. and Xu, H. Acta Metall. Sinica (English Lett) (1996) 9 (1), 65–70

The objective of this study is to evaluate the reliability of through-aluminum-nitride-via (TAV) substrate by comparing those experimental results with the finite element simulation associated with measurements of aluminum nitride (AlN) strength and the thermal deformation of Cu/AlN bi-material plate. Two reliability tests for high-power LED (Light emitting diode) applications are used in this study: one is a thermal shock test from − 40 °C to 125 °C, the other is a pressure cook test. Also, the strength of AlN material is measured by using three-point bending test and point load test. The reliability results show that TAV substrates with thicker Cu films have delamination and cracks after the thermal shock test, but there are no failure being found after the pressure cook test. The determined strengths of AlN material are 350 MPa and 650 MPa from three-point bending test and point load test, respectively. The measurement of thermal deformation shows that the bi-material plate has residual-stress change after the solder reflow process, also indicating that a linear finite element model with the stress-free temperature at 80 °C can reasonably represent the stress state of the thermal shock test from − 40 °C to 125 °C without considering Cu nonlinear effect. The further results of the finite element simulation associated with strength data of AlN material have successfully described those of the reliability test.

Effect of testing atmosphere on low cycle fatigue of hot work tool steel at elevated temperature : Tsuhii, N., Abe, G., Fukaura, K. and Sunada, H. ISIJ Inn. (1995) 35 (7), 920–926

The influence of an aggressive environment (0.6 M, aerated NaCl solution) on short fatigue crack initiation and growth behaviour has been studied. The study involved three major test series, namely: air fatigue, corrosion fatigue, and intermittent air fatigue/corrosion fatigue. The above tests carried out under fully reversed torsional loading conditions at a frequency of 5 Hz, showed that it was the non-metallic inclusions which took part in crack initiation resulting from debonding at metal matrix/inclusion interface and pitting of inclusions in both air and corrosove environments, respectively. Short fatigue crack growth results in these two environments obtained by using plastic replication technique, indicated a large effect of microstructure i.e. prior austenite grain boundaries. The stage/stages at which the environmental contribution was dominant has been discussed by considering the results achieved from intermittent tests. However, the mechanisms involved in corrosion fatigue short crack growth have also been described in the light of results obtained from futher investigations carried out by conducting corrosion fatigue tests under applied cathodic potential conditions and tests on hydrogen pre-charged specimens under air fatigue and uniaxial tension conditions.

Influence of environment on low cycle fatigue damage in Ti [sbnd]6Al [sbnd]4V and Ti 6246 titanium alloys

An experimental study of the initiation and propagation of short cracks naturally obtained on low cycle fatigue (LCF) smooth specimens has been carried out on β and αβ forged titanium alloys. Tests were done at room temperature in ambient air and in vacuum. Three different microstructures have been used. Regularly interrupted tests showed that crack initiation is delayed when tests are performed in vacuum. Surface crack densities were lower in vacuum than in air. Crack growth rates can be either strongly slowed down or unaffected by inert media, depending on the microstructure. Crack initiation sites were modified by air environment for the β forged alloy only. For one αβ forged alloy, an internal failure origin occurred in vacuum. The influence of environment on both crack initiation and short crack propagation is discussed in terms of effective slip length and stress-assisted hydrogen embrittlement.

Corrosion fatigue short crack growth behaviour in a high strength steel

The influence of an aggressive environment (0.6 M, aerated NaCl solution) on short fatigue crack initiation and growth behaviour has been studied. The study involved three major test series, namely: air fatigue, corrosion fatigue, and intermittent air fatigue/corrosion fatigue. The above tests carried out under fully reversed torsional loading conditions at a frequency of 5 Hz, showed that it was the non-metallic inclusions which took part in crack initiation resulting from debonding at metal matrix/inclusion interface and pitting of inclusions in both air and corrosove environments, respectively. Short fatigue crack growth results in these two environments obtained by using plastic replication technique, indicated a large effect of microstructure i.e. prior austenite grain boundaries. The stage/stages at which the environmental contribution was dominant has been discussed by considering the results achieved from intermittent tests. However, the mechanisms involved in corrosion fatigue short crack growth have also been described in the light of results obtained from futher investigations carried out by conducting corrosion fatigue tests under applied cathodic potential conditions and tests on hydrogen pre-charged specimens under air fatigue and uniaxial tension conditions.

Fatigue damage and its localization in particulate metal matrix composites

A series of high and low cycle fatigue tests have been conducted on Al 2O 3 particulate-reinforced 6061 aluminum alloy composite(Al 2O 3/6061 Al). Fatigue damage in the composite, as observed by optical and scanning electron microscopes, is primarily in the form of particle debonding, fractured particles and matrix cracks. Mesoscale reinforcement defects, such as a clump of large particles, cause damage localization which may lead to short crack initiation and extension. The cyclic plastic strain amplitude appears to be an appropriate low cycle fatigue damage parameter as indicated by the hysteresis loop variation of strain-controlled cylindrical specimens.

Inclusion induced anisotropy of short fatigue crack growth in steel

In this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions was investigated by generating and evaluating the short crack data under different loading conditions for the six principal specimen orientations. Short cracks showed a strong anisotropic behaviour in which they grew muchfaster in the transverse and short transverse orientations than in the longitudinal orientations in all fatigue processes. This was due to the relatively high content of the elongated inclusions. Inclusion induced anisotropy is also observed in the percentages of fatigue life of the initiation and growth periods of short cracks which may result in inaccurate fatigue life predictions. Inclusion induced anisotropy was the result of strain intensification at the matrix/inclusion interface.MST/3250A

Inclusion induced anisotropy of short fatigue crack growth in steel

In this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions was investigated by generating and evaluating the short crack data under different loading conditions for the six principal specimen orientations. Short cracks showed a strong anisotropic behaviour in which they grew muchfaster in the transverse and short transverse orientations than in the longitudinal orientations in all fatigue processes. This was due to the relatively high content of the elongated inclusions. Inclusion induced anisotropy is also observed in the percentages of fatigue life of the initiation and growth periods of short cracks which may result in inaccurate fatigue life predictions. Inclusion induced anisotropy was the result of strain intensification at the matrix/inclusion interface.MST/3250A

Initiation and early growth of short fatigue cracks at inclusions

In this study, the initiation and early growth behaviour of short fatigue cracks in En 7A steel with a high content of elongated MnS inclusions was investigated, by generating and evaluating data o...

A STUDY OF CUMULATIVE FATIGUE DAMAGE UNDER VARIABLE LOADING‐MODE CONDITIONS

Abstract— The accumulation of fatigue damage was investigated by studying deformation processes and the kinetics of short crack initiation and propagation under variable loading‐mode conditions. Fatigue tests were carried out under push‐pull and torsion, both separately and sequentially, that is specimens were subjected to push‐pull loading prior to torsion. The results show that specimens tested in push‐pull followed by torsion have a much longer fatigue life than virgin specimens. The mechanisms of this phenomenon were explored by experimental observations and theoretical analyses of crack initiation and propagation. The effect of stress state on cumulative damage is described in some detail.

A comparison of short and long fatigue crack growth in steel

In this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions has been investigated by generating and evaluating short crack growth data for the six principal specimen orientations. The anomalous initiation and growth behaviour of short cracks was also compared with that of long crack data for the same material and loading conditions determined before. This comparison revealed the sub-threshold initiation and faster growth rate of short cracks compared with long cracks under the same stress intensity factor range. In order to simulate this discrepancy between short and long cracks, a crack closure model for long crack data was used.

Prediction of Notch Fatigue Threshold Based on R-Curve Method.

Single-edge-notched specimens of a structural carbon steel and a low-alloy steel were fatigued under axial loading. The initiation and propagation behavior of short fatigue cracks emanating from a sharp notch was investigated. The resistance-curve (R-curve) method was used to predict the fatigue thresholds of notched members. The R curve was constructed in terms of the experimentally determined threshold value of the maximum stress intensity factor which was the sum of the threshold effective stress intensity range ΔKeffth and the opening stress intensity factor Kopth. The value of ΔKeffth was constant and the R curve was independent of crack length and notch geometry. The predicted values of the fatigue limit for crack initiation, the fatigue limit for fracture, and the nonpropagating crack length agreed well with the experimental results. The effect of the material property or Vickers hardness on the fatigue notch sensitivity was discussed on the basis of the R-curve method. A simplified method to determine the R curve using Vickers hardness or the tensile strength was proposed for design applications.

Numerical methods for evaluation of service life of gear

AbstractThe service life of mechanical parts, of which the gears are most important, is influenced to the greatest extent by defects occurring as cracks. In case of cracks the most important stage is the initiation and propagation of short fatigue cracks; further, the propagation of these crack up to their critical and/or permissible length also must be taken into account. As these are quite different areas we have introduced into calculations a completely new factor, i.e. the gear stress intensity factor Z. On the basis of the factor Z we have developed appropriate models, e.g. the programe STATFAG which ensures calculation of the service life of gears with and without cracks.

Acoustic emission in dynamic compression and its relevance to tribology

In the present work an acoustic emission (AE) technique is used to detect the initiation and propagation of short cracks in compressive cyclic loading. A mild steel surface was subjected to cyclic compressive stress of 100 cycles s −1 by an electromagnetic exciter system. The compressive load applied was in the range of 2.5 to 25 N in each cycle. The stressing was done with an En31 steel ball in the presence of turbine oil. Acoustic emission was detected by placing an AE sensor 1.5 cm away from the point of contact of ball and plate. The surface damage was observed to be negligible until 10 4 cycles. Microcrack nucleation and/or growth were inferred until 0.6 × 10 5 cycles. Thereafter the acoustic emission activity decreased, suggesting possible arrest mechanisms were operating. The emission activity again became very significant beyond 6.0 × 10 5 cycles, which may be due to possible subsurface crack growth. This information is related to previous one-pass sliding experiments.

The initiation and propagation behaviour of short fatigue cracks in Waspaloy subjected to bending: Yates, J.R. Zhang, W. and Miller, K.J. Fatigue Fract Eng Mater Struct (Mar. 1993) 16 (3), 351–362

Classical definitions of boundary layer mass and momentum flux deficiency thicknesses can lead to gross errors when applied to measurements near a trailing edge where the flow curvature in the free stream is appreciable. This paper presents a double vortex sheet model as a development from the single vortex sheet model of Helmholtz and others. Two bound vortex sheets define a potential function which can describe a flow with the same mass and momentum flux deficiencies as the viscous regions. The bound nature of these sheets allows the modelling of the integral properties of these regions while retaining the advantages of a potential flow. The application to the flow near the trailing edge of a lifting aerofoil is given

Fatigue Crack Propagation from Notch in SiC-Particulate-Reinforced Aluminum Alloy.

The initiation and propagation behavior of short fatigue cracks emanating from a sharp notch in an aluminum alloy (2024-T6) reinforced with 20 volume percent of SiC particles was investigated. The development of crack closure with crack propagation was measured through the compliance method. The propagation rate, dc/dN, of short cracks was higher than that predicted using the dc/dV-ΔK relation for long cracks. For the case of a low stress amplitude, the propagation of short cracks decelerated and finally stopped. The relationship between dc/dN and the effective stress intensity range, ΔKeff, for short cracks at notches was identical to that obtained for long cracks. The relationship between dc/dN and ΔKeff/E(E=Young's modulus) was nearly identical for short and long cracks in reinforced and unreinforced alloys. The roughness of the fatigue fracture surface was determined using a computer image processing method. The roughness of the fracture surface of composites was larger than that of unreinforced aluminum alloy.