Experimental study of I + III mixed mode fatigue crack transformation propagation

In this paper, compact tension specimens with tilted cracks under monotonic fatigue loading were tested to investigate I + III mixed mode fatigue crack propagation in the material of No. 45 steel with the emphasis on the mode transformation process. It is found that with the crack growth, I + III mixed mode changes to Mode I. Crack mode transformation is governed by the Mode III component and the transformation rate is a function of the relative magnitude of the Mode III stress intensity factor. However, even in the process of the crack mode transformation the fatigue crack propagation is controlled by the Mode I deformation.     

Weld root crack propagation under mixed mode I and III cyclic loading

This study examined fatigue propagation behaviour and fatigue life of weld root cracks under mixed mode I and III loading. Fatigue tests were performed on butt-welded joints with a continuous lack-of-penetration (LOP) inclined at angles of 0°, 15°, 30° or 45° to the normal direction of the uniaxial cyclic load. Branch and/or co-planar crack propagation was observed, depending on the initial mode I stress intensity factor (SIF) range. Co-planar crack propagation predominated when the SIF range was large. The fatigue crack propagation mode affected fatigue life; the life of branch crack propagation was longer than that of co-planar crack propagation. Using an initial equivalent SIF range based on a maximum strain energy release rate criterion, the results obtained from the 0°, 15°, 30°, and 45° specimens indicated almost the same fatigue lives, despite the different inclination angles.     

Shear mode fatigue crack growth in 1050 aluminium

The shear mode crack growth mechanism in 1050 aluminium was investigated using pre‐cracked specimens. A small blind hole was drilled in the centre section of the specimens in order to predetermine the crack initiation position, and a push–pull fatigue test was used to make a pre‐crack. Crack propagation tests were carried out using both push–pull and cyclic torsion with a static axial load. With push–pull testing, the main crack grew by a mixed mode. It is thus apparent that shear deformation affects the fatigue crack growth in pure aluminium. In tests using cyclic torsion, the fatigue crack grew by a shear mode. The micro‐cracks initiated perpendicular and parallel to the main crack's growth direction during the cyclic torsion tests. However, the growth direction of the main crack was not changed by the coalescence of the main crack and the micro‐cracks. Shear mode crack growth tends to occur in aluminium. The crack growth behaviour is related to a material's slip systems. The number of slip planes in aluminium is smaller than that of steel and the friction stress during edge dislocation motion of aluminium is lower than many other materials. Correlation between the crack propagation rate and the stress intensity factor range was almost the same in both push–pull and cyclic torsion with tension in this study.     

Path prediction of I + III mixed mode fatigue crack propagation

In this paper, compact tension specimens with tilted cracks under monotonic fatigue loading were tested to investigate I + III mixed mode fatigue crack propagation in the material of No. 45 steel with the emphasis on the propagation rate expression and the path prediction. It is found that during the mode transformation process, the crack propagation rate is still controlled by the mode I stress intensity factor; and Paris equation also holds for the relationship between and ΔK_I . Crack propagation path can be predicted only when both the crack mode transformation rate and propagation rate are available.     

Fatigue crack growth of a railway wheel

Typically, fatigue crack propagation in railway wheels is initiated at some subsurface defect and occurs under mixed mode (I–II) conditions. For a Spanish AVE train wheel, fatigue crack growth characterization of the steel in mode I, mixed mode I–II, and evaluation of crack path starting from an assumed flaw are presented and discussed.Mode I fatigue crack growth rate measurement were performed in compact tension C(T) specimens according to the ASTM E647 standard. Three different load ratios were used, and fatigue crack growth thresholds were determined according to two different procedures. Load shedding and constant maximum stress intensity factor with increasing load ratio R were used for evaluation of fatigue crack growth threshold.To model a crack growth scenario in a railway wheel, mixed mode I–II fatigue crack growth tests were performed using CTS specimens. Fatigue crack growth rates and propagation direction of a crack subjected to mixed mode loading were measured. A finite element analysis was performed in order to obtain the K_I and K_II values for the tested loading angles. The crack propagation direction for the tested mixed mode loading conditions was experimentally measured and numerically calculated, and the obtained results were then compared in order to validate the used numerical techniques.The modelled crack growth, up to final fracture in the wheel, is consistent with the expectation for the type of initial damage considered.     

STRESS-RATIO EFFECT OF FATIGUE CRACK PROPAGATION IN A BIAXIAL STRESS FIELD

Fatigue crack propagation tests were conducted under conditions of equibiaxial, uniaxial and shear loading by using a cruciform specimen in a servo hydraulic testing machine. The effect of non-singular stress cycling on the fatigue crack propagation rate was examined based on the observation of crack opening behavior. The crack propagation rate was significantly influenced by the non-singular stress parallel to the crack when it was correlated to the stress intensity range. The crack closure behavior was greatly affected by the non-singular stress. The crack propagation rate was uniquely correlated to the effective range of the stress intensity factor except for the case of completely reversed shear loading where significant plasticity was detected. The crack opening displacement range was concluded to be a parameter controlling the crack propagation rate for all the stress conditions examined in the present experiments. Some discussion is made on the effect of material anisotropy on fatigue crack propagation in a biaxial stress field.     

ESTIMATION OF TORSIONAL FATIGUE STRENGTH OF MEDIUM CARBON STEEL BARS WITH A CIRCUMFERENTIAL CRACK BY THE CYCLIC RESISTANCE-CURVE METHOD

Near-threshold fatigue crack propagation tests were performed on circumferentially precracked round bars of a medium carbon steel under torsional loading. The crack propagation rate decreased with crack extension, because of the shear contact of crack faces. The crack propagation rate without the influence of crack-surface contact was determined by extrapolating to zero crack extension the relationship between the crack propagation rate and crack extension. The applied stress intensity factor range was divided into two parts: one was the effective value responsible for crack growth and the other was the value corresponding to crack-tip shielding. The resistance-curve method was used to predict the fatigue limit for crack initiation and fracture. The R -curve was constructed using the experimentally determined threshold value of the stress intensity range, which was the sum of the threshold effective stress intensity range and the threshold shielding stress intensity range. The threshold effective stress intensity range was constant. The R -curve was independent of the precrack length and specimen dimensions. The predicted values agreed well with the experimental results.     

Fatigue of shot peened 7075-T7351 SENB specimen – A 3-D analysis

As‐received or shot peened 7075‐T7351 single‐edged notch bend (SENB) specimens, 8.1‐mm thick, were fatigued at a constant maximum load and at stress ratios of R= 0.1 and 0.8 to predetermined numbers of fatigue cycles or to failure. The SENB specimens were then fractured by overload and the tunnelling crack profiles were recorded. The crack‐growth rate, da/dN, after crack initiation at the notch was determined by crack‐profile measurement and fractography at various fatigue cycles. The shot peened surface topography and roughness was also evaluated by three‐dimensional (3‐D) laser scanning microscopy. Residual stresses in the as‐received specimens and those generated by shot peening at Almen scales of 0.004A, 0.008A, 0.012A and 0.016A, were measured by an X‐ray diffraction stress analyser with an X‐ray target, CrK, every 0.1 mm to a depth of 1 mm. The 3‐D stress intensity factor of the curved crack front was determined by the superposition of the 3‐D finite element solutions of the stress intensity factor of the loaded SENB specimen without the residual stress and the stress intensity factor of the unloaded SENB specimen with a prescribed residual stress distribution. da/dN versus the resultant stress intensity factor amplitude, ΔK_I, plots showed that while the residual stress locally retarded the crack‐growth rate it had no effect on the overall crack‐propagation rate.     

Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.     

The preferred fatigue crack propagation mode in a M250 maraging steel loaded in shear

Macroscopic torsional fatigue cracks are shown to propagate in shear, in plain tubular specimens, in the M250 maraging steel, for stress ranges from 90% down to 40% of the yield stress. This cannot be explained in terms of microcrack coalescence for the smallest stress range, for which microcracks are scarce. The kinetics and mechanisms of mode II fatigue crack growth are thus investigated, using precracked CTS or tubular specimens. For a high Δ K _II , slowly decelerating mode II propagation takes place for a distance that increases with Δ K _II before branching occurs. Friction stresses along the crack flanks shield the applied load and explain this deceleration. An inverse analytical procedure is used to derive the effective stress intensity factor, allowance being made for friction effects, from displacement profiles measured from microgrids using a scanning electron microscope. The measured crack growth rates correlate much better with the effective stress intensity factor than with the nominal Δ K _II value. The crack paths observed in torsion are discussed in terms of maximum crack velocity.     

Crack propagation in notched specimens of porous silicon carbide under cyclic loading

Notched specimens of porous silicon carbide with porosity 37% were fatigued under four‐point bending at frequencies of 30 and 0.3 Hz. The fatigue life expressed in terms of time was rather insensitive to the test frequency, while that expressed in terms of cycles was much shorter for the case of 0.3 Hz than for 30 Hz. A time‐dependent mechanism of stress corrosion cracking was mainly responsible for crack propagation, and stress cycling enhanced the crack‐propagation mechanism. The crack length was estimated from the change in compliance of the specimen. The crack‐propagation curve was divided into stages I and II. In stage I, the crack‐propagation rate decreased even though the applied stress intensity factor became larger with crack extension, and then turned to increase in stage II. The transition from stage I to II took place at a crack extension of around 0.8 mm. This anomalous behaviour is caused by crack‐tip shielding due to microcracking and asperity contact. Fractographic observations showed that the fracture path was along the binder phase between silicon carbide particles, or more precisely along the interface between particles and binders.     

Effect of microstructure on crack propagation in high-temperature fatigue of directionally solidified Ni-based superalloy

In order to clarify the crack propagation properties of an anisotropic material (Ni‐based directionally solidified superalloy), longitudinally loaded specimens (L‐specimens) and transversely loaded specimens (T‐specimens) with a crack are subjected to high temperature fatigue. The crack propagation rate is reasonably well correlated with the effective stress intensity factor range regardless of the propagation direction (specimens L and T), the stress range and the stress ratio. However, the crack propagation rate shows a notable fluctuation particularly in the T‐specimens. It is at most about five times faster than the average. The fracture surface features can be classified into four types with three transgranular and one intergranular types. In the former, though the crack is along the {100} or {110} planes on a macroscopic scale, it threads through the {111} or {100} planes on a microscopic scale. Crack propagation is notably accelerated in the intergranular region, while deceleration is caused by crack branching.     

A mixed mode crack growth model taking account of fracture surface contact and friction

Fracture surface interactions, of whatever origin, can significantly affect the stress intensity factor, and consequently, can also be relevant to fatigue crack propagation. In the occurrence of interaction between fracture surfaces, the effective loading cycle experienced by material near the crack tip may be very different from that evaluated on the basis of the external loadings only. The purpose of the work described in this paper is to obtain the effective mode II stress intensity factor, k _IIeff, in a surface cracked elasto-plastic plate with a factory roof fracture surface subjected to an in-plane shear (mode II) loading. A new model estimating the magnitude of the frictional mode II stress intensity factor, k _f, arising from the mismatch of the fracture surface roughness during in-plane shear, is developed. Furthermore, the results of this study are employed in modeling the fatigue response of the surface cracked plates subjected to mixed mode loading.     

Threshold and growth mechanism of fatigue cracks under mode II and III loadings

ABSTRACT The fatigue crack growth behaviour of 0.47% carbon steel was studied under mode II and III loadings. Mode II fatigue crack growth tests were carried out using specially designed double cantilever (DC) type specimens in order to measure the mode II threshold stress intensity factor range, ΔK_IIth. The relationship ΔK_IIth > ΔK_Ith caused crack branching from mode II to I after a crack reached the mode II threshold. Torsion fatigue tests on circumferentially cracked specimens were carried out to study the mechanisms of both mode III crack growth and of the formation of the factory‐roof crack surface morphology. A change in microstructure occurred at a crack tip during crack growth in both mode II and mode III shear cracks. It is presumed that the crack growth mechanisms in mode II and in mode III are essentially the same. Detailed fractographic investigation showed that factory‐roofs were formed by crack branching into mode I. Crack branching started from small semi‐elliptical cracks nucleated by shear at the tip of the original circumferential crack.     

CRO试样的疲劳裂纹扩展行为试验方法研究

考虑到如紧凑拉伸和三点弯曲等标准试样的大尺寸要求以及高试验成本等突出问题,该文发展了含外侧径向裂纹C形环小试样（C-ring with an outer radial crack,CRO）的疲劳裂纹扩展行为试验方法。利用有限元分析建立了CRO小试样的高精度应力强度因子算式以及基于柔度法理论的裂纹长度预测公式。采用5083-H112铝合金分别完成了CRO试样和标准CT试样的疲劳裂纹扩展速率试验,获得了相应的Paris方程中的幂指数。通过对比发现,CRO和CT试样的疲劳裂纹扩展规律基本一致,验证了新方法的有效性。基于上述方法对C250钢两种厚度CRO试样的疲劳裂纹扩展行为进行了应用研究。     

The effect of T‐stress on crack‐tip plastic zones under mixed‐mode loading conditions

In this paper, the influence of T‐stress on crack‐tip plastic zones under mixed‐mode I and II loading conditions is examined. The crack‐tip stress field is defined in terms of the mixed‐mode stress intensity factors and the T‐stress using William's series expansion. The crack‐tip stress field is incorporated into the Von Mises yield criteria to develop an expression that determines the crack‐tip plastic zone. Using the resultant expression, the plastic zone is plotted for various combinations of mode II to mode I stress intensity factor ratios and levels of T‐stress. The properties of the plastic zone affected by T‐stress and mixed‐mode phase angle are discussed. The observations obtained on plastic zones variations are important for further fatigue and fracture analyses for defects in engineering structures under mixed‐mode loading conditions.     

7050铝合金Ⅰ-Ⅱ复合型疲劳裂纹研究

为研究7050铝合金在Ⅰ-Ⅱ型复合加载下疲劳裂纹扩展规律,在Amsler HFP5000高频试验机上利用Richard加载装置,完成紧凑拉剪(CTS)试样疲劳裂纹扩展试验,利用有限元对Ⅰ-Ⅱ复合型裂纹进行数值模拟,采用APDL命令流计算不同裂纹长度的应力强度因子,并引入最大周向应力准则计算裂纹扩展角,用有限元计算等效应...     

Mode I and mixed mode fracture of polysilicon for MEMS

An experimental study was carried out to investigate the local and effective fracture behaviour of polycrystalline silicon for microelectromechanical systems (MEMS). The apparent mode I critical stress intensity factor was determined from MEMS‐scale tension specimens containing atomically sharp edge pre‐cracks, while local deformation fields were recorded near the crack tip, with high resolution by the in situ Atomic Force Microscopy (AFM)/Digital Image Correlation (DIC) method previously developed by this group. The effective mode I critical stress intensity factor varied in the range 0.843–1.225 MPa√m. This distribution of values was attributed to local (in grain) cleavage anisotropy and to enhanced grain boundary toughening. The same sources resulted in very different local and macroscopic (apparent) stress intensity factors, which, combined with the small grain size of polysilicon (0.3 μm,) were the reason for subcritical crack growth that was evidenced experimentally by AFM topographic and AFM/DIC displacement measurements. The effect of local in‐grain anisotropy and granular inhomogeneity was stronger under mixed mode loading of edge cracks inclined at angles up to 55° with respect to the applied far‐field load. The K_I–K_II locus was characterized by scatter in the K_Ic values but on average it followed the curves calculated by the maximum tensile stress and the maximum energy release rate criteria calculated assuming isotropy.     

Influence of effective stress intensity factor range on mixed mode fatigue crack propagation

ABSTRACT The behaviour of fatigue crack propagation of rectangular spheroidal graphite cast iron plates, each consisting of an inclined semi‐elliptical crack, subjected to axial loading was investigated both experimentally and theoretically. The inclined angle of the crack with respect to the axis of loading varied between 0° and 90°. In the present investigation, the growth of the fatigue crack was monitored using the AC potential drop technique, and a series of modification factors, which allow accurate sizing of such defects, is recommended. The rate of fatigue crack propagation db/dN is postulated to be a function of the effective strain energy density factor range, ΔS_eff. Subsequently, this concept is applied to predict crack growth due to fatigue loads. The mixed mode crack growth criterion is discussed by comparing the experimental results with those obtained using the maximum stress and minimum strain energy density criteria. The threshold condition for nongrowth of the initial crack is established based on the experimental data.     

Fracture mode identification of low alloy steels and cast irons by electron back-scattered diffraction misorientation analysis

The fracture modes of low alloy steels and cast irons under tensile and fatigue conditions were identified by electron back-scattered diffraction(EBSD) misorientation analysis in this research. The curves of grain reference orientation deviation(GROD) distribution perpendicular to the fracture surface were obtained by EBSD observation, and the characteristics of each fracture mode were identified. The GROD value of the specimen fractured in tension decreases to a constant related to the elongation of corresponding specimen in the far field(farther than 5 mm away from the fracture surface). The peak exhibits in GROD curves of two smooth specimens and a notched specimen near the fracture surface(within 5 mm away from the fracture surface), and the formation mechanisms were discussed in detail based on the influences of specimen geometries(smooth or notched) and material toughness. The GROD value of fatigue fractured specimen is close to that at undeformed condition in the whole field, except the small area near the crack path. The loading conditions(constant stress amplitude loading or constant stress intensity factor range K loading) and the EBSD striation formation during fatigue crack propagation were also studied by EBSD observation parallel to the crack path.