Crack deflection during fatigue and monotonic fracture of a SiC whisker reinforced 2009 aluminium alloy

Fatigue crack growth under Mode I loading, and static fracture in both symmetrical and asymmetrical notched four point bend specimens, have been examined in SiC whisker reinforced 2009 aluminium alloy. In the fatigue tests a range of orientations of the starter notch, with respect to the extrusion direction, L, was examined. Slanted crack propagation was observed in all specimens except that in the T-L configuration. For the monotonic tests the specimen orientation (L-T) remained constant whilst the ratio of Mode I to Mode II loading was varied. Again crack deflection was observed in all cases apart from the L-T specimen under pure Mode I loading. Whisker debonding was found to be the dominant factor controlling crack deflection, independent of the mixity of the loading mode. Under mixed-mode static loading, the deflection angle was controlled by the average orientation of the whiskers subject to the asymmetrically distributed maximum principal stress. In fatigue loading, however, the crack tended to follow the most frequently observed whisker orientation. These contrasting observations are interpreted in terms of the different sampling volumes at the crack tip in monotonic fracture and in fatigue crack growth.     

Corrosion Fatigue Crack Initiation in a Mode II Notch Specimen

The stress intensityK _II of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3, and 10.2 MPa m^1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular facets at low ΔK _II values but of quasi-cleavage at higher ΔK _II values and were different from those in air.     

Influence of residual stresses and loading frequencies on corrosion fatigue crack growth behavior of weldments

The effect of residual stresses and loading frequencies on corrosion fatigue crack growth behavior under synthetic seawater with a free corrosion potential was examined using center-cracked tension (CCT) and single edge-cracked tension (SECT) specimens machined from mild steel butt-welded joints and the parent material. A series of fatigue crack growth tests were carried out with a sinusoidal loading wave form at a stress ratio of 0.05 with a loading frequency of 0.017 to 6.7 Hz. The results show that the crack growth resistance of a weld metal in the SECT specimen is higher than that in the CCT specimen regardless of testing conditions. The discrepancy is attributed to the differences in residual stress distribution at the crack tip in the two specimen geometries. The crack growth rate of the weld metal in the CCT specimen in seawater increased with decreasing loading frequency. The acceleration of the crack growth rate may be related to the occurrence of brittle striation or cleavage due to hydrogen embrittlement. It was found that the corrosion fatigue crack growth rate of a welded joint with tensile residual stress can be predicted using the effective stress intensity factor range, which takes into account both the residual stress and the loading frequency effects.     

The influence of water vapor on the fatigue crack propagation kinetics in pure aluminum single crystals

In general, water vapor accelerates the fatigue crack propagation rate in pure aluminum (99.999 pct) single crystals under Mode II or Mode I–II loading. This effect can be explained by the fact that hydrogen generated during corrosion fatigue enters into the specimen and is transported by dislocation sweeping during fatigue deformation from the surface to the interior of the crystal for a much longer distance than by diffusion. It is hydrogen which accelerates the fatigue crack propagation rate. However, for certain orientations, the reverse effect termedunusual retardation was observed, which can be explained by the blunting effect due to the presence of numerous microcracks along the secondary PSB.     

A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展及其数值模拟

为探究不同加载角度下A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展机理,在MTS疲劳试验机上采用紧凑拉伸剪切试件（CTS）对A7085铝合金进行不同加载角度的疲劳实验;用有限元分析计算不同裂纹扩展长度的裂纹尖端应力强度因子,通过七点递增多项式法对数据进行处理,计算出A7085铝合金Paris公式中的参数C和m.结果表明不同加载角度的裂纹基本沿着与外载荷垂直的方向扩展,裂纹扩展路径近似为一条直线,裂纹扩展角测量结果基本符合最大环向拉应力理论;Ⅰ-Ⅱ复合型裂纹一旦发生扩展,Ⅱ型应力强度因子K_Ⅱ所占比例急剧减小,Ⅰ型应力强度因子K_Ⅰ不断增大,此后K_Ⅱ远远小于K_Ⅰ,有效应力强度因子（K_Ⅰ和K_Ⅱ的组合）基本等于K_Ⅰ,相当于裂纹扩展主要受Ⅰ型应力强度因子控制,研究结果有助于对Ⅰ-Ⅱ复合型疲劳裂纹扩展机理的理解.      

Near-threshold fatigue crack growth behavior of 2195 aluminum-lithium-alloy—prediction of crack propagation direction and influence of stress ratio

Tensile properties and fatigue crack propagation behavior of a 2195-T8 Al-Li alloy were investigated at different stress ratios, with particular emphasis on their dependence on specimen orientation. Specimens with orientations of 0, 15, 30, 45, and 90 deg to the rolling direction were tested. The alloy contained a strong brass-type texture and a profuse distribution of platelike precipitates of T ₁ (Al₂CuLi) phase on {111} matrix planes. Both tensile strength and fatigue thresholds were found to be strongly dependent on the specimen orientation, with the lowest values observed along the direction at 45 deg to the rolling direction. The effect of stress ratio on fatigue threshold could generally be explained by a modified crack closure concept. The growth of fatigue crack in this alloy was found to exhibit a significant crystallographic cracking and especially macroscopic crack deflection. The specimens oriented in the L-T + 45 deg had the smallest deflection angle, while the specimens in the L-T and T-L orientations exhibited a large deflection angle. The dependence of the fatigue threshold on the specimen orientation could be rationalized by considering an equivalent fatigue threshold calculated from both mode I and mode II values due to the crack deflection. A four-step approach on the basis of Schmid’s law combined with specific crystallographic textures is proposed to predict the fatigue crack deflection angle. Good agreement between the theoretical prediction and experimental results was observed.     

Fatigue Fracture of Concrete Subjected to Biaxial Stresses in the Tensile C–T Region

In this paper cyclic quasi-static and constant amplitude fatigue responses of concrete subjected tensile compression–tension (C–T) biaxial stress are presented. In the tensile C–T region within the biaxial stress space, magnitude of the principal tensile stress is larger than or equal to that of the principal compressive stress. An experimental program consisted of subjecting hollow, cylindrical concrete specimens to torsional loading. Failure in both quasi-static and fatigue is due to crack propagation. It is shown that the crack propagation resulting from the biaxial loading can be predicted using Mode I fracture parameters. The fatigue crack growth is observed to be a two-phase process: an acceleration stage that follows a deceleration stage. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the quasi-static peak load. Analytical expressions for crack growth in the deceleration and acceleration stages are developed in terms of the mechanisms that influence quasi-static crack growth. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the biaxial fatigue response. Finally, a fracture-based fatigue-failure criterion is proposed, wherein the fatigue failure can be predicted using the critical Mode I stress intensity factor.     

Mode III fatigue crack propagation in low alloy steel

To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects, showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 10⁴ cycles at nominal yield to 10⁶ cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to be slower than in Mode I, although still power-law related to the alternating stress intensity(△K _III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements(△CTD _III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTD_III per cycle. Formerly with Massachusetts Institute of Technology, Cambridge, MA Formerly with M.I. T.     

Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

Very high cycle fatigue behavior(107-109 cycles)of 304 Laustenitic stainless steel was studied with ultrasonic fatigue testing system(20kHz).The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation.It was found that micro-plasticity(slip markings)could be observed on the specimen surface even at very low stress amplitudes.The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure.By detailed investigation using a scanning electron microscope and an infrared camera,slip markings appeared at the large grains where the fatigue crack initiation site was located.The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack.Finally,the coupling relationship among the fatigue crack propagation,appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.     

High-cycle fatigue properties of the ODS-alloy MA 6000 at 850 °C

The high cycle fatigue (HCF) and cyclic crack growth rate (CCGR) properties of the dispersion strengthened ODS-alloy MA 6000 were investigated with smooth bars and with fracture mechanics samples at 850 °C. The material was very coarse grained with the grains elongated in the rolling direction. Fatigue crack initiation and crack propagation were studied parallel and perpendicular to the rolling direction and a pronounced influence of orientation was found. The fatigue limit of sam-ples cut parallel to the grain elongation direction (p-samples) was almost a factor of 2 higher than the one of samples cut transverse to the elongation direction (t-samples). Inclusions were found to be responsible for crack initiation. For p-samples a reasonable agreement between particle size, fatigue limit, and crack growth behavior was found. For t-type samples such an agreement also exists provided differences in the crack growth behavior of short cracks and long cracks are taken into consideration. The low fatigue strength of t-samples could be linked with low Young's modulus in this direction. The crack propagation rate of long cracks is lower in t-samples than in p-samples due to crack branching along the grain boundaries. HCF-strength of MA 6000 is high compared to conventional cast alloys mainly because of reduced size of crack nucleation sites and higher fatigue threshold stress intensity range ΔK_th, as a result of higher Young's modulus.     

GH4169合金高温疲劳裂纹扩展的微观损伤机制

空气环境对高温合金在高温下的损伤行为有显著影响.为了研究标准热处理态GH4169合金在高温疲劳裂纹扩展过程中的微观损伤机制,在空气环境中进行650℃、初始应力强度因子幅ΔK=30 MPa·m1/2和应力比R=0.05的低周疲劳裂纹扩展试验.使用扫描电镜(SEM)及能谱(EDS)对试样的断口、外表面和剖面进行观察和分析.实验结果表明:疲劳主裂纹以沿晶方式萌生并扩展,随后沿晶二次裂纹出现,并且其数量和长度沿主裂纹方向逐渐增加,进入快速扩展阶段后,断口呈现韧窝组织形貌;在裂纹扩展过程中,δ相与基体的界面发生氧化,使得沿晶二次裂纹沿界面扩展并产生偏折,从而起到阻碍二次裂纹扩展的作用;试样外表面的主裂纹周围出现晶界氧化损伤区,其尺寸和晶界开裂程度沿主裂纹扩展方向逐渐增大.      

Microstructural Characterization and Fatigue Performance Evaluation of MIG‐Welded Ship Hull Steel

The effectiveness of MIG welding with Argo‐shield gas & ER70S‐6 electrode in joining LRS (Grade‐B) steel was investigated through structure–property correlation of the joint region. Microstructure, tensile and fatigue properties, and mode of fracture (SEM fractograph) were correlated. Fatigue behavior has been investigated in air and sea water with thin specimen at near‐endurance stress amplitude up to 10⁵ cycles. The crack growth rate (da/dN) maintained a non‐linear relationship with logarithm of stress intensity factor range (logΔK) for the near‐threshold values of ΔK. Considerable hardness and microstructural variation was observed across the weldment. Weld with more pearlite content was found to possess higher hardness and strength than the parent steel. Though, both in weld and in parent steel, either in air or in sea water, fatigue crack propagated at very slow rate with significant intermittent crack arrest, weld provided much higher resistance to crack growth in air. However, sea water accelerated the crack growth in weld and brought it closer to that in the parent steel. The morphologically complex microstructure of weld suffered much faster crack propagation in sea water than in air. While fatigue fracture in parent steel (both in air and sea water) and weld in air was found to occur through dimple rupture via microvoid coalescence, weld in sea water exhibited a mixed mode of failure.     

Experimental Investigation on High-Cycle Fatigue of Inconel 625 Superalloy Brazed Joints

The high-cycle fatigue performance and crack growth pattern of transient liquid phase-brazed joints in a nickel-based superalloy Inconel 625 were studied. Assemblies with different geometries and types of overlaps were vacuum-brazed using the brazing paste Palnicro-36M in conditions such as to generate eutectic-free joints. This optimal microstructure provides the brazed assemblies with static mechanical strength corresponding to that of the base metal. However, eutectic micro-constituents were observed in the fillet region of the brazed assembly due to an incomplete isothermal solidification within this large volume of filler metal. The fatigue performance increased significantly with the overlap distance for single-lap joints, and the best performance was found for double-lap joints. It was demonstrated that these apparent changes in fatigue properties according to the specimen geometry can be rationalized when looking at the fatigue data as a function of the local stress state at the fillet radii. Fatigue cracks were nucleated from brittle eutectic phases located at the surface of the fillet region. Their propagation occurred through the bimodal microstructure of fillet and the diffusion region to reach the base metal. High levels of crack path tortuosity were observed, suggesting that the ductile phases found in the microstructure may act as a potential crack stopper. The fillet region must be considered as the critical region of a brazed assembly for fatigue applications.     

Three-Orthogonal-Direction Stress Mapping around a Fatigue-Crack Tip Using Neutron Diffraction

Quantitative determination of the stress fields around the crack tip is a challenging and important subject to understand the fatigue crack-growth mechanism. In the current study, we measured the distribution of residual stresses and the evolution of the stress fields around a fatigue crack tip subjected to the constant-amplitude cyclic loading in a 304L stainless steel compact-tension (CT) specimen. The three orthogonal stress components (i.e., crack growth, crack opening, and through thickness) of the CT specimen were determined as a function of distance from the crack tip with 1-mm spatial resolution along the crack-propagation direction. In-situ neutron-diffraction results show that the enlarged tensile stresses were developed during loading along the through-thickness direction at a localized volume close to the crack tip, resulting in the lattice expansion in all three orthogonal directions during P _max. The current study suggests that the atypical plane strainlike behavior observed at the midthickness position might be the reason for the mechanism of the faster crack-growth rate inside the interior than that near the surface.     

超声冲击对P355NL1钢焊接接头超高周疲劳性能影响

 通过超声疲劳试验探究超声冲击对P355NL1钢焊接接头超高周疲劳性能的影响。结果表明,由疲劳[S-N]曲线可知,在105～109寿命区间内,冲击态试样的疲劳性能要高于焊态试样,在1.0×108的疲劳寿命下,焊态试样的疲劳强度为139 MPa,冲击态的疲劳强度为217 MPa,冲击态疲劳强度相较于焊态提高了56%,这表明超声冲击可以明显提高焊接接头的疲劳强度。利用扫描电镜（SEM）观察断口形貌可以发现,裂纹源位于焊接接头焊根区表面。P355NL1钢焊接接头疲劳断裂为准解理断裂,超声冲击可以提高焊接接头的疲劳强度,但不会改变其疲劳失效机理。超声冲击可以降低焊根处应力集中,引入有益压应力和表面晶粒细化,从而提高焊接接头的疲劳强度。     

Growth of small fatigue cracks in PH 13-8 Mo stainless steel

The growth of small fatigue cracks in PH 13-8 Mo (H1050) stainless steel under constant amplitude loading at different mean stresses (R=0.1 and −1) under generally high cycle fatigue conditions was investigated. Small cracks were allowed to initiate naturally at the root of a single edge notch specimen and were monitored using a surface replicating technique. It was found that the initiation and growth of surface cracks up to 100 μm encompassed 70 to 90 pct of the total fatigue life at stress amplitudes just above the fatigue limit. Cracks of length less than 100 μm were subject to strong influences of the microstructure and exhibited stage I (shear-dominated) growth, which was manifested in oscillatory crack growth rates. The oscillations diminished as the crack transitioned to stage II growth. The higher stress ratio (R=0.1) resulted in a more rapid transition from stage I to stage II growth in comparison to R=−1. After transitioning to stage II, the crack growth could be well characterized by conventional long crack tools even when the crack was still physically small. The small crack growth behavior is shown to be similar to that of a quenched and tempered AISI 4340 steel having a comparable strength.     

Residual Stress Measurement and Fatigue Crack Growth Prediction after Cold Expansion of Cracked Fastener Holes

The cold expansion technique is often used to introduce beneficial compressive stress at fastener holes, and can be used for remedial work where cracks already exist. In this paper, results are presented showing the effect of preexisting cracks on the residual stress field produced by cold expanding a fastener hole, and on subsequent fatigue crack growth. The effect on the residual stresses was experimentally evaluated in two ways: indirectly, in terms of retained expansion and directly, by measurement of the stresses using the X-ray and neutron diffraction techniques. The retained expansion ratio showed that cold expansion is more sensitive to the existence of precracks at lower levels of applied interference, and the inlet and outlet faces have different behavior. The stress measurements showed that preexisting cracks reduce the compressive residual stresses more on the mandrel inlet face than on the outlet face and in the middle of the specimen. The effect on fatigue crack growth rates was modeled using a linear-elastic fracture mechanics approach. It was found that cold expansion of a hole containing a preexisting crack longer than 1 mm introduces little benefit for subsequent fatigue crack growth behavior.     

The micromechanics of fatigue crack growth at 25 °C in Ti-6Al-4V reinforced with SCS-6 fibers

Micromechanics parameters for fatigue cracks growing perpendicular to fibers were measured for the center-notched specimen geometry. Fiber displacements, measured through small port holes in the matrix made by electropolishing, were used to determine fiber stresses, which ranged from 1.1 to 4 GPa. Crack opening displacements at maximum load and residual crack opening displacements at minimum load were measured. Matrix was removed along the crack flanks after completion of the tests to reveal the extent and nature of the fiber damage. Analyses were made of these parameters, and it was found possible to link the extent of fiber debonding to residual COD and the shear stress for fiber sliding to COD. Measured experimental parameters were used to compute crack growth rates using a well-known fracture mechanics model for fiber bridging tailored to these experiments.