Effect of plastic deformation on magnetic fields around fatigue crack tips of carbon tool steel (JIS,SKS93)

Small cracks form in steel and then grow to cause failure. It is important to observe the plastic deformation around cracks in order to understand the fatigue level of all kinds of steel components. However, at present, it is not easy to observe the plastic deformation at the crack tip during crack growth. In this study, in order to find a new, non-destructive testing method which is related to the cracks, we investigated the relationship between magnetic fields and plastic deformation around the crack tip. A scanning Hall probe microscope (SHPM) equipped with three-dimensional GaAs film equipment was used to observe the magnetic fields around a crack tip and the plastic deformation induced by Vickers indentations in tool steel specimens (SKS93, JIS B 4404: 2006, equivalent to AISI W4 tool steel). We found that the magnetic fields around the crack tip changed during the crack growth, and also that the decrease in the magnetic fields depends on the plastic deformation size.     

THE ELECTRON CHANNELLING CONTRAST TECHNIQUE APPLIED TO THE CHARACTERISATION OF DISLOCATION STRUCTURES IN THE VICINITY OF A FATIGUE CRACK

Abstract— In this investigation the Electron Channelling Contrast (ECC) technique in scanning electron microscopy (SEM) was applied to reveal the dislocation structures in the vicinity of surface fatigue cracks in comparison to those of cyclically-deformed recrystallized polycrystalline copper. The plastic zone around a fatigue crack was found to consist of an innermost region containing cells, followed by a region containing dense veins and PSBs, surrounded by a structure of loose veins, bundles and loop patches typical of the cyclically deformed matrix. A relation between plastic strain amplitude values deduced from cyclic stress-strain investigations and the dislocation structures near fatigue cracks are given. Typical regions of damage accumulation were identified and plastic strain contours for surface fatigue cracks established. The essentially non-destructive ECC technique is particularly suited to identify the changes in mesoscopic dislocation structures from surface layers to the interior of specimens over large specimen areas.     

Changes in magnetic flux density around fatigue crack tips

Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components, it is important to find the cracks, which eventually grow to cause failures. However, at present, it is not easy to distinguish, in the early stages of growth, the cracks that will grow fast and cause failure.
We hypothesized that it may be possible to distinguish them by comparing changes in the magnetic flux density around the tips of those cracks that grew to failure. In order to measure these changes in magnetic flux density, we developed a scanning Hall probe microscope and observed the fatigue cracks growing from artificial slits in soft bearing steels. Note that we did not magnetize the specimens artificially but succeeded to measure the changes in magnetic flux density during the fatigue tests. We also compared the changes in magnetic flux density around crack tips, which grew under different loads, and found that there is a strong correlation between the magnetic flux density, crack growth and stress intensity factors. In order to understand this, we looked into the relation between stress field, residual strain and magnetic flux density, and concluded that the changes in magnetic flux density are caused not only by the residual strain occurring around the crack tips but also by the increase in the elastic stress.     

Short crack growth and fatigue life in austenitic-ferritic duplex stainless steel

The kinetics of short crack growth has been studied in austenitic‐ferritic 2205 duplex stainless steel. Smooth cylindrical specimens and specimens with shallow notch were subjected to constant plastic strain amplitude loading. The crack growth was studied in notched specimens. The notch area has been mechanically and electrolytically polished to facilitate the observation of crack initiation and growth. The initiated cracks were observed in an SEM (scanning electron microscope). The crack growth was studied using long distance QUESTAR optical microscope equipped with high‐resolution camera. In constant plastic strain amplitude loading the microcracks were initiated and their growth kinetics has been studied. The characteristic features of the crack growth at different plastic strain amplitudes were recorded. Two approaches to analyse the crack growth rates were adopted. The comparison of the prediction of the fatigue life using the plastic‐strain‐dependent crack growth rate was compared with Manson–Coffin law and the relation between parameters of this law and parameters of the short crack growth law were established.     

2219铝合金TIG和FSW接头力学及疲劳性能

目的 研究钨极氩弧焊(TIG)和搅拌摩擦焊(FSW)对2219铝合金(母材)力学及疲劳性能的影响。方法 通过拉伸试验,得到了母材、TIG和FSW接头的抗拉强度和伸长率;通过疲劳性能试验测试了母材、TIG和FSW接头在不同应力下相应的疲劳寿命,根据疲劳试验结果绘制了其试样的S-N曲线;使用扫描电子显微镜观察并分析了疲劳断口的形貌特征。结果 未焊接的铝合金母材抗拉强度和伸长率最高,分别为506 MPa和15.92%;TIG接头抗拉强度和伸长率分别为330 MPa和7.65%,FSW接头抗拉强度和伸长率分别为310 MPa和8.74%。母材、TIG和FSW接头等3种疲劳试样在2×10⁶次循环下的疲劳强度分别为129、108、115 MPa,其疲劳断口均可分为裂纹源区、裂纹扩展区和瞬间断裂区,疲劳裂纹分别起始于试样表面的局部变形区、第二相夹杂物和“吻接”缺陷。疲劳裂纹扩展区的主要形貌为疲劳辉纹和二次裂纹,瞬间断裂区以脆性断裂为主。结论 TIG和FSW等2种焊接工艺均导致了2219铝合金的强度、塑韧性和疲劳性能降低,其接头表面的第二相夹杂物和“吻接”缺陷促进了疲劳裂纹的萌生。     

A new mechanism in hydrogen-enhanced fatigue crack growth behavior of a 1900-MPa-class high-strength steel

This paper presents a new mechanism controlling the acceleration of fatigue crack growth of a hydrogen-charged high-strength steel (bearing steel SAE52100, ?? _ult?>?1, 900MPa, HV =?569). Three- dimensionally complicated shape of a primary crack and secondary cracks were observed in hydrogen- charged specimens. Marked acceleration of fatigue crack growth in the presence of hydrogen was observed particularly at low test frequency, and was attributed to the initiation and successive coalescence of secondary cracks formed ahead of primary crack. These secondary cracks were produced along prior-austenite grain boundaries and carbide boundaries, or by direct cracking of carbides. Surprisingly, secondary cracks were observed outside the ordinary plastic zone ahead of the crack tip. TEM observation elucidated that the secondary cracks outside the crack tip plastic zone were produced by hydrogen-induced deformation twins impinging on grain boundaries and carbides. These results suggest a new mechanism of the acceleration of fatigue crack growth rates in high-strength steels caused by hydrogen-induced deformation twins, rather than due to hydrogen- enhanced localized plasticity. The phenomena associated with time dependent fatigue crack growth are presumed to be correlated with the initiation and coalescence of secondary cracks in the presence of hydrogen.     

THE INITIATION AND PROPAGATION BEHAVIOUR OF SHORT FATIGUE CRACKS IN WASPALOY SUBJECTED TO BENDING

The fatigue crack growth behaviour of a nickel base superalloy, Waspaloy, has been studied using four point bending specimens in laboratory air. Two groups of tests, for which the span/width ratios were 1 and 2.5, were conducted and the results compared. Surface crack lengths were measured from plastic replicas of the surface. Equations which describe both short and long cracks have been derived and used to predict the fatigue life for the Waspaloy specimens. From plastic replication studies and scanning electron microscope examinations, a physical understanding of the relationship between crack growth and microstructural features was enhanced.     

THE SIZE OF PLASTIC ZONES AND FATIGUE CRACK GROWTH BEHAVIOUR OF THREE FORMS OF A Ti–6Al–2.5Mo–1.5Cr ALLOY

The effect of microstructure on the fatigue properties of Ti–6Al–2.5Mo–1.5Cr alloy was investigated. The experimental results for both the fatigue crack initiation and propagation behaviour, as well as the dynamic fracture toughness ( K _Id ) showed clearly that a lamellar microstructure is superior to two other structures. It was found that, as in the case of steels, the initiation and subsequent growth of cracks in the titanium specimens with a sharp notch may also occur on loading levels below the threshold values of the K factor (Δ K _th ) determined for long fatigue cracks. In addition, measurements by interferential-contrast of the plastic zone size on the surface of specimens revealed that the different rate of crack growth at identical values of Δ K in individual structural states can roughly be correlated with the size of the plastic zone. A general relationship between the fatigue crack growth rate and plastic zone size, the modulus of elasticity and the role of crack tip shielding is discussed.     

MODELING THE LONG-LIFE FATIGUE BEHAVIOR OF A CAST ALUMINUM ALLOY

As-cast specimens and smooth specimens of a AA 319 cast aluminum alloy containing casting porosity were fatigue tested with special attention given to the long-life region ( N 1.25 × 10⁸ cycles). Fatigue cracks were observed to initiate from the near-surface casting pores or from discontinuities resulting from the as-cast surface texture. The observed fatigue lives were strongly dependent on the size (√area) of these casting defects.
The effect of casting defects on the fatigue life was modeled assuming the fatigue life to be the sum of the crack nucleation and the crack propagation life (including both the growth of short and long cracks). The crack growth behavior of (mechanically) short cracks was considered in detail by a developed crack-closure-at-a-notch (CCN) model. The CCN model predicted the fatigue lives for both as-cast and machine-notched specimens. Extension of the CCN model to reliability-based design was attempted using the measured size distribution of the fatigue-initiating casting pores.     

STATISTICAL INVESTIGATION OF THE BEHAVIOUR OF SMALL CRACKS AND FATIGUE LIFE IN CARBON STEELS WITH DIFFERENT FERRITE GRAIN SIZES

Abstract— In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propagation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks.     

Fatigue crack growth from small defects under out-of-phase combined loading

A modified linear elastic fracture mechanics analysis is presented for the evaluation of the crack growth and threshold behavior of small cracks initiated from small defects under combined loading fatigue. For the detailed evaluation of the behavior of small fatigue cracks, the Kitagawa effect, the elastic–plastic behavior of cracks in biaxial stress fields and crack closure effects are taken into account. In-phase and out-of-phase combined tension and torsion fatigue tests were conducted using annealed carbon steel specimens containing small holes. The direction of crack propagation, S–N curves and fatigue limits were found to be in good agreement with the theoretical predictions.     

Detection and measurement of fatigue crack in HSLA steel with a dedicated ultrasonic pulse transmission method

A dedicated ‘ultrasound transmission’ method was used for detection of fatigue cracks. The measurements were done in-situ on hourglass-shaped specimens of HSLA steel that had piezoelectric transducers embedded in each end. Time-limited pulses were emitted from one transducer and received by the other. Longitudinal and surface waves resulted from each pulse and were monitored with a storage oscilloscope. During fatigue cycling between zero and a constant tensile load, the changes in the time-of-flight and in the transmitted amplitudes of the ultrasonic waves were used to monitor the elastic and plastic strains in order to detect fatigue damage and crack initiation. During the initial fatigue cycles, a decrease in the transmitted wave amplitude occurred due to plastic deformation. In subsequent cycles, during which the stress-strain hysteresis loops indicated essentially zero plastic strain, the transmitted wave amplitudes remained constant. Eventually, a fatigue crack nucleated and grew. This was detected by a decrease of the transmitted wave amplitudes. From here on, further decrease of the transmitted amplitudes measured the crack growth. Crack growth was also measured from marked crack fronts in the final fracture surface. The entire history of fatigue damage evolution from initial strain hardening, through strain saturation, crack nucleation and growth could be monitored with the present technique. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluation of fibre bridging stress of short fatigue cracks in SCS-6/Ti-15-3 composite

Single‐edge notched specimens of a unidirectional SiC long fibre reinforced titanium alloy, were fatigued under four point bending. The propagation behaviour of short fatigue cracks from a notch was observed on the basis of the effects of fibre bridging. The branched fatigue cracks were initiated from the notch root. The fatigue cracks propagated only in the matrix and without fibre breakage. The crack propagation rate decreased with crack extension due to the crack bridging by reinforced fibres. After fatigue testing the loading and residual stresses in the reinforced fibres were measured for the arrested cracks by the X‐ray diffraction method. The longitudinal stresses in the reinforced fibres were measured using high spatial resolution synchrotron radiation. A stress map around the fatigue cracks was then successfully constructed. The longitudinal stress decreased linearly with increasing distance from a location adjacent to the wake of the matrix crack. This region of decreasing stress corresponded to the debonding area between the fibre and the matrix. The interfacial frictional stress between the matrix and the fibre could be determined from the fibre stresses. The bridging stress on the crack wake was also measured as a function of a distance from a notch root. The threshold stress intensity factor range, corrected on the basis of the shielding stress, was similar to the propagation behaviour of the monolithic matrix. Hence the main factor influencing the shielding effect in composites is fibre bridging.     

Effect of test frequency on fatigue strength of low carbon steel

Ultrasonic fatigue tests (test frequency: 20 kHz) and conventional tension–compression fatigue tests (10 Hz) have been conducted on annealed and 10% pre-strained specimens of 0.13% carbon steel. Small holes were introduced on the specimen surface to investigate the effect of test frequency on small crack growth. The dynamic stress concentration factor and the stress intensity factor under ultrasonic fatigue tests were checked to be almost the same as those of conventional tension–compression fatigue tests. However, the fatigue properties were dependent on the test frequency. Ultrasonic fatigue tests showed longer fatigue life and lower fatigue crack growth rate for the annealed and 10% pre-strained specimens. Slip bands were scarce in the neighbourhood of cracks under ultrasonic fatigue tests, while many slip bands were observed in a wide area around the crack under conventional fatigue tests. In order to explain the effect of test frequency on fatigue strength, dynamic compression tests with Split Hopkinson bars were carried out. The stress level increases substantially with the strain rate. Thus, the increase in fatigue strength might be, to a large extent, due to a reduction in crack tip cyclic plasticity during ultrasonic fatigue tests.     

CRACK GROWTH BEHAVIOUR IN A THERMAL FATIGUE TEST. EXPERIMENTS AND CALCULATIONS

Abstract— This study is concerned with the results of experiments in which thermal cycles have been repeatedly applied through the wall thickness of axisymmetrically cracked tubular specimens. The investigated material is a Cr–Mo steel used for the moulds when fabricating centrifugally cast iron pipes. Crack growth rates have been measured by using the interrupted tests technique. A methodology is proposed to model the crack growth rates under such thermal fatigue loadings. The elastic and plastic stress-strain fields are calculated on the uncracked specimen by means of a finite element code. Special attention was paid to reach a mechanical steady state regime. Fatigue crack growth rates data were obtained, both under isothermal and anisothermal conditions, on CT and SEN specimens. The latter specimens were tested under large-scale yielding in order to obtain the data appropriate to the cyclic stress-strain field calculated in the thermal fatigue specimens. An effective stress intensity factor, which takes into account both plastic strains and crack closure effect, was used to correlate the results of isothermal tests on CT and SEN specimens and to calculate the thermal fatigue crack growth rates in tubular specimens. It is shown that the use of the effective stress intensity factor gives a satisfactory agreement between the observed and the calculated crack growth rates.     

A STUDY OF RESIDUAL CAUSTICS GENERATED FROM FATIGUE CRACKS

Abstract— The method of caustics was used to determine the stress intensity factor of fatigue cracks in steel compact tension specimens. Under zero load a residual caustic was observed at the tip of a fatigue crack indicating the presence of a residual stress field. Caustics were generated at increasing static loads and the stress intensity factors were compared with those predicted by theory. It was found that the difference between each measured stress intensity factor and its corresponding theoretical value was a constant for the range of loads. This difference was shown statistically to be equal to the stress intensity factor determined from the residual caustic. The proposed mechanism for the formation of this residual caustic was probably due to crack tip plasticity effects and not due to crack closure. It was concluded that residual caustics can be measured to quantify crack tip behaviour in fatigue cracks and have been shown to be a useful tool in the measurement of residual stress fields.     

Fatigue crack propagation in tensile shear stainless steel spot welded specimens

Fatigue tests were carried out on 4 mm thick spot welded joints; the material was stainless steel AISI 301, quarter hard. Some specimens were instrumented with a strain gauge bonded in correspondence with one of the edges of the spot weld. Strain gauge output was demonstrated to be a reliable instrument to monitor the nucleation and propagation of fatigue cracks. A good correlation was found between strain gauge output and spent fatigue life. Some fatigue tests were suspended when the strain gauge output was equal to pre-fixed values, corresponding to fatigue life in the range from 15 to 85%. Subsequently, the specimens were dissected to observe fatigue cracks. The same correlation existed between crack depth and fatigue life. Small cracks were observed in specimens fatigue tested up to 15% of the mean fatigue life; fatigue cracks in the joints under examination would be nucleated between 5 and 10% of fatigue life.
Finite Element calculations were carried out, introducing in the models cracks similar to those observed in the fatigue tests. Calculated strain at the external surface compared well with the measured strain as a function of crack depth. Calculations demonstrated that small errors in strain gauge position can be tolerated without appreciable deterioration in crack dimension prediction.     

Plasticity spread in mild steel pre-cracked charpy bars

During dynamic service loading, small fatigue cracks are normally seen to emanate from the root of sharp machined stress concentration region. In a recent authors' paper at ICF4, the fracture strength of a charpy type notched beam has been studied in three different engineering materials, when a small fatigue crack emanates from the notch root. Fracture tests on these medium and high strength materials demonstrate the presence of a large size plastic zone near the crack tip [1–6]. To understand the mechanism of fracture for such complex geometry. it is important to know the size and shape of these plastically yielded regions. The present paper is mainly on the experimental measurement of plasticity spread as well as the stress intensity factor (S.I.F.) for such short cracks. Firstly, the S.I.F. is approximately measured by plain transmission photoelasticity on model castolite specimens. Secondly, plastic zones around crack tip are measured for a wide range of notch root radii and crack-length, by using photo-stress PS-3B coating on mild steel pre-cracked charpy type notched specimens. It is observed that for small scale yielding at the crack tip, the plasticity spread is around 60–65° angle to the line of crack-extension. On the contrary, as the gross applied stress approaches the yield strength of the material, the maximum plasticity spreads around 45° angle. Finally, it is noticed that at high stress level, the plastic zone boundary (for short crack) touches the free machined notch surface. These experimental observations explain the nature and degree of non-linearity in a load—C.O.D. diagram during the fracture test of a short cracked-notched specimen. These data are also useful to predict the crack-extension load for an elastic-plastic material.     

Direct observation of tensile and fatigue cracks

Crack growth in aluminum foil specimens 1 mil thick has been studied under monotonie tension and low cycle fatigue. Crack tip regions were observed during loading by optical microscopy. Following final fracture, the crack path and fracture surface were examined by scanning electron microscopy.

Both tensile and fatigue cracks are typically preceded by a narrow necked region. Voids develop in the neck, crack growth proceeding by their growth and coalescence. As the voids form and grow, the foil necks down to zero thickness along the path of the crack.

Crack growth in both tension and fatigue results from concentrated plastic strain. The plastic flow is heterogeneous to an extent that voids precede the crack. In monotonic tension new voids are continually nucleated in the region of high strain ahead of the growing crack, while in fatigue, the voids result from plastic instabilities under cyclic loading.     

THERMAL FATIGUE UNDER MULTIAXIAL STRESSES

Thermal fatigue under multiaxial stresses has been investigated. Circular cylindrical specimens were tested under thermal fatigue which was in-phase with axial mechanical fatigue loading. The axial forces on the specimens were varied throughout the test programme, but all the temperature cycles were identical, so various biaxial stress and strain ratios were obtained. Straight thermal fatigue cracks occurred in different directions and also net-like crazes in various patterns were observed on the surfaces of the specimens. The transient temperature, stress and strain fields have been calculated with a thermal elasto-plastic Finite Element Method. Comparing test results with calculations, it appears that the patterns of thermal fatigue cracks are dependent on the stress state and the plastic strain state, not on the strain state. The direction of cracks is perpendicular to the maximum principal stress and the maximum plastic strain. Net-like thermal fatigue crazes will occur when one principal stress is about the same as the other one and one plastic strain component is approximately equal to another.