Plane-Strain Propagation of a Fluid-Driven Crack in a Permeable Rock with Fracture Toughness

A solution to the problem of a plane-strain fluid-driven crack propagation in elastic permeable rock with resistance to fracture is presented. The fracture is driven by injection of an incompressible Newtonian fluid at a constant rate. The solution, restricted to the case of zero lag between the fluid front and the fracture tip, evolves from the early-time regime when the fluid flow takes place mostly inside the crack toward the large-time response when most of the injected fluid is leaking from the crack into the surrounding rock. This transition further depends on a time-invariant partitioning between the energy expanded to overcome the rock fracture toughness and the energy dissipated in the viscous fluid flow in the fracture. A numerical approach is used to compute the solution for the normalized crack length and crack opening and net-fluid pressure profiles as a function of two dimensionless parameters: the leak-off/storage evolution parameter and the toughness/viscosity number. Relation of this solution to the various available asymptotic solutions is discussed. Obtained mapping of the solution onto the problem parametric space has a potential to simplify the tasks of design, modeling, and data inversion for hydraulic fracturing treatments and laboratory experiments.     

U71Mn和U75V钢轨钢疲劳短裂纹的扩展行为

在扫描电镜下对U71Mn和U75V钢轨钢的Ⅰ型低周疲劳加载和准静态加载下裂纹扩展的过程进行了观察.研究发现,从整体上看材料没有产生明显的宏观塑性变形,属准解理断裂.但从微观上看,裂纹的微观扩展路径曲折、不连续,常伴有分枝裂纹的形成.可观察到局部有韧窝、滑移线等典型的韧性断裂特征.尤其是U71Mn钢轨钢中还伴有局部的撕裂等更强烈的变形.钢轨钢U71Mn比U75V的抗疲劳裂纹扩展性能和韧性略优.     

Effect of Structural State and Temperature on Crack Generation and Propagation Resistance in Pipe Steels

Metallurgist - Comparative research of microstructure and properties of different strength classes of plate steels produced by controlled rolling with accelerated cooling is performed. Standard...     

Effect of Impact Toughness Anisotropy on Brittle Fracture Resistance Characteristics of High-Strength Steels Subjected to Thermomechanical Treatment

Metallurgist - The structure and mechanical properties under tension and impact bending of eleven batches of high-strength manganese-containing steels (Mn-steels) containing 0.09 to 0.14 wt.% Ti...     

Crack Propagation in Flexural Fatigue of Concrete

In this paper the behavior of concrete subjected to flexural fatigue loading is studied. Notched concrete beams were tested in a three-point bending configuration. Specimens were subjected to quasi-static cyclic and constant amplitude fatigue loading. The cyclic tests were performed by unloading the specimen at different points in the postpeak part of the quasi-static loading response. Low cycle, high amplitude fatigue tests were performed to failure using four different load ranges. The crack mouth opening displacement was continuously monitored throughout the loading process. Crack propagation caused by quasi-static and fatigue loads is described in terms of fracture mechanics. It is shown that the crack propagation in the postpeak part of the quasi-static load response is predicted using the critical value of the mode I stress intensity factor (KIC). The ultimate deformation of the specimen during the fatigue test is compared with that from the quasi-static test; it is demonstrated that the quasi-static deformation is insufficient as a fatigue failure criterion. It is observed that crack growth owing to constant-amplitude fatigue loading comprises two phases: a deceleration stage when there is a decrease in crack growth rate with increasing crack length, followed by an acceleration stage where the rate of crack growth increases at a steady rate. 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 peak load of the quasi-static response. Analytical expressions for crack growth in the deceleration and acceleration stages are developed, wherein the expressions for crack growth rate in the deceleration stage are developed using the R-curve concept, and the acceleration stage is shown to follow the Paris law. It is observed that the crack length at failure for constant amplitude fatigue loading is comparable to that of the corresponding load in the postpeak part of the quasi-static response. Finally, a fracture-based fatigue failure criterion is proposed.     

Fracture Toughness to Understand Stretch-Flangeability and Edge Cracking Resistance in AHSS

The edge fracture is considered as a high risk for automotive parts, especially for parts made of advanced high strength steels (AHSS). The limited ductility of AHSS makes them more sensitive to the edge damage. The traditional approaches, such as those based on ductility measurements or forming limit diagrams, are unable to predict this type of fractures. Thus, stretch-flangeability has become an important formability parameter in addition to tensile and formability properties. The damage induced in sheared edges in AHSS parts affects stretch-flangeability, because the generated microcracks propagate from the edge. Accordingly, a fracture mechanics approach may be followed to characterize the crack propagation resistance. With this aim, this work addresses the applicability of fracture toughness as a tool to understand crack-related problems, as stretch-flangeability and edge cracking, in different AHSS grades. Fracture toughness was determined by following the essential work of fracture methodology and stretch-flangeability was characterized by means of hole expansions tests. Results show a good correlation between stretch-flangeability and fracture toughness. It allows postulating fracture toughness, measured by the essential work of fracture methodology, as a key material property to rationalize crack propagation phenomena in AHSS.     

Analytical Prediction of Fatigue Crack Propagation in Metals

An analytical model for fatigue crack propagation of long cracks in metals and metal alloys is presented. The key features of the model are an extension of Griffith’s theory of fracture to include fatigue, a dislocation model for the crack tip opening displacement, and cyclic plasticity-induced closure. The net cyclic stretch of the process zone at the crack tip plays a major role in the fatigue crack propagation under cyclic loading. Only constant amplitude loading is considered in this paper. The model predictions utilize only the readily available material properties, such as Young’s modulus, yield strength, threshold stress intensity factor, and the fracture toughness. There are no empirical fitting constants. The model predictions are validated by an extensive amount of published fatigue crack growth studies. The agreement between the model predictions and the experimental data is good.     

航天用高强高韧钢的优化研究

提出了一种新的优化模式，解决了航天用高强高韧钢需要同时对两个性能指标进行优化的问题。并将模拟退火算法与人工神经网络相结合，建立了双输出优化系统。为解决多变量非线性复杂系统的双指标优化提供了一种新的有效方法。根据对断裂韧性的不同要求给出了３种不同的优化方案。     

Fatigue Strength and Crack Initiation Mechanism of Very-High-Cycle Fatigue for Low Alloy Steels

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5?Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 10⁸ cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with ??fish-eye?? pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K _I ) at different regions of fracture surface were calculated, indicating that the K _I value of fish-eye crack is close to the value of relevant fatigue threshold (??K _th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.     

粉末冶金构件疲劳裂纹的扩展寿命

 针对航空发动机粉末冶金旋转盘件的疲劳裂纹扩展特性,以标准紧凑拉伸试样裂纹扩展数据为基础,基于Paris公式获得了材料典型温度下的裂纹扩展参数,并利用简单件对所采用的裂纹扩展计算方法进行了验证。进行了粉末冶金旋转构件的裂纹扩展试验,继而进行断口分析,通过测量疲劳条带获得了裂纹扩展寿命。同时,采用有限元方法计算出粉末冶金构件的裂纹扩展寿命,与实测值吻合较好,验证了裂纹扩展寿命分析方法的有效性及实用性。     

Fatigue Crack Propagation in Friction Stir Welded and Parent AA6082

The fatigue crack propagation characteristics of a friction stir welded Al‐Mg‐Si alloy, 6082, have been investigated. The electrical potential drop method was used for measurements. A low and a high load ratio (R) level were tested. At low load ratio (R=0.1) and a low stress intensity δK the propagation rate in the weld was higher than in the parent material by a factor of 3 to 5. However, the propagation rates were approaching each other close to fracture. At high load ratio (R=0.8) the propagation rate was similar in the parent material and weld. The weld crack growth rate was about the same at low and high R (except close to fracture), while the parent material growth rate increased at high R. Paris law was used to describe the measured crack propagation rates in the weld. In the case of the parent material, showing an R‐dependence, Forman's law was used.     

缺口小裂纹的疲劳扩展速率

将复型技术应用于疲劳小裂纹扩展试验中的裂纹长度测量。在等载荷比R=0.1、不同平均载荷水平影响的疲劳条件下,板试样V型缺口小裂纹疲劳扩展速率做了试验测试;通过结果分析,提出了缺口小裂纹疲劳扩展速率表达式,并以ε_P为控制参数,求出45^*钢的计算式。     

氢蚀程度对钢疲劳断裂性能的影响

研究了国产钢经不同温度和时间氢暴露后的力学性能、疲劳性能和断裂韧性,用扫描电镜证实了氢蚀后断裂机制发生了变化.研究表明:随氢蚀程度增加,20G钢抗拉强度和塑性降低明显,CrMo钢抗拉强度略有降低,塑性变化不大.氢蚀使20G钢的门槛值有一个最小值,而断裂韧性随氢蚀程度升高而降低,在氢蚀程度较低时,断裂韧性下降程度大;在氢蚀程度较高时,断裂韧性下降程度变缓.碳钢的疲劳性能变化是由于材料损伤作用和氢蚀造成的裂纹表面引起的闭合效应增加二者共同作用的结果.     

钒微合金化钢的热影响区韧性

研究了结构钢单道次焊接接头,尤其是靠近融合线位置的粗晶热影响区(CGHAZ)的组织和力学性能。通常,这个区域的韧性比母材和焊缝金属的韧性要低很多,所以CGHAZ是最容易发生断裂的薄弱位置。重点关注钒微合金化钢板,并且与相同强度级别的铌微合金化钢板进行对比。为了充分利用钒的强化效果,有必要将氮含量提高至0.01%;为优化钢的性能,必须考虑钒与氮的配合。     

Crack Arrest Toughness of Two High Strength Steels (AISI 4140 and AISI 4340)

The crack initiation toughness (K _c ) and crack arrest toughness (K _a ) of AISI 4140 and AISI 4340 steel were measured over a range of yield strengths from 965 to 1240 MPa, and a range of test temperatures from -53 to +74°C. Emphasis was placed onK _a testing since these values are thought to represent the minimum toughness of the steel as a function of loading rate. At the same yield strengths and test temperatures,K _a for the AISI 4340 was about twice as high as it was for the AISI 4140. In addition, theK _a values showed a more pronounced transition temperature than theK _c values, when the data were plotted as a function of test temperature. The transition appeared to be associated with a change in fracture mechanism from cleavage to dimpled rupture as the test temperature was increased. The occurrence of a “pop-in” behavior at supertransition temperatures has not been found in lower strength steels, and its evaluation in these high strength steels was possible only because they are not especially tough at their supertransition temperatures. There is an upper toughness limit at which pop-in will not occur, and this was found for the AISI 4340 steel when it was tempered to its lowest yield strength (965 MPa). All the crack arrest data were identified as plane strain values, while only about one-half of the initiation values could be classified this way.     

Mechanisms of Hydrogen Induced Stress Crack Initiation and Propagation in Super Duplex Stainless Steels

Austenitic and ferritic duplex stainless steels, DSS, have recently suffered from hydrogen stress induced cracking, HISC, in subsea components with a cathodic protection. This paper provides discussions on possible HISC mechanisms. HISC initiation can occur at the ferritic grain boundaries and phase boundaries at a stress lower than the yield strength, but dominantly at phase boundaries at a stress higher than the yield strength. EBSD analysis shows that HISC in DSS results from the interaction between the dynamic plasticity by creep and hydrogen diffusion. A model on the formation of microstresses in these two phases under creep conditions is proposed, which explains why HISC occurs mainly in the ferritic phase. Discontinuous two‐dimensional HISC paths were observed. The austenitic phase acts as obstacles for crack propagation. The fracture covers “valleys” and “peaks” with the cleavage ferrite and the austenite with microfacets or striations due to the hydrogen‐enhanced localized‐plasticity.     

The Significance of Crack Initiation Stage in Very High Cycle Fatigue of Steels

Different stages of the Very High Cycle Fatigue (VHCF) crack evolution in tool steels have been explored using a 20 kHz ultrasonic fatigue testing equipment. Extensive experimental data is presented describing VHCF behaviour, strength and crack initiating defects in an AISI H11 tool steel. Striation measurements are used to estimate fatigue crack growth rate, between 10^?8 and 10^?6 m/cycle, and the number of load cycles required for a crack to grow to critical dimensions. The growth of small fatigue cracks within the “fish‐eye” is shown to be distinctively different from the crack propagation behaviour of larger cracks. More importantly, the crack initiation stage is shown to determine the total fatigue life, which emphasizes the inherent difficulty to detect VHCF cracks prior to failure. Several mechanisms for initiation and early crack growth are possible. Some of them are discussed here: crack development by local accumulation of fatigue damage at the inclusion – matrix interface, hydrogen assisted crack growth and crack initiation by decohesion of carbides from the matrix.     

Effect of Sulfur Contents on Fracture Toughness of Wheel Steels for High Speed Train

The effect of different contents of S on fracture toughness of railway wheel steels for high speed train has been researched.The results show that there are kinds of non-metallic inclusion when O content retaining at about 10×10-6,such as Al 2 O 3 inclusions and Al 2 O 3 +MnS complex inclusions and single MnS inclusions.This is connected with S content level in steel.Fracture toughness of railway steel increase obviously because of Al 2 O 3 covered by MnS around when S content is increase to about 70×10-6.It shows that,after Al 2 O 3 covering by MnS around,stresses around complex inclusion decrease rapidly till disappear when analyzed by tessellated stresses theory.That is the reason of fracture toughness rise.