TiC_p/W复合材料的断裂行为

利用扫描电镜观察了含３０ ％ ＴｉＣ 颗粒（ 体积分数） 的钨基复合材料在室温和高温的微观断裂过程, 讨论了裂纹萌生、扩展条件及其影响因素。室温下的断裂过程受控于裂纹萌生阶段, 相应的应力－ 挠度曲线表现为线性, ＴｉＣ 颗粒和Ｗ 基体在微观上都呈现脆性断裂。高温下的断裂则存在一个亚稳态的初始裂纹长大和合并过程, 使应力－ 挠度曲线呈现出非线性, 在微观上ＴｉＣ 颗粒呈现脆性断裂,Ｗ 基体呈现韧性撕裂。同时也指出了复合材料在室温和高温下的强化机制     

分形下材料裂纹的临界表面能

分形理论在金属断裂分析方面的应用越来越广,针对材料裂纹临界表面能与实际表面能存在较大误差的现象,分析并修正了临界表面能与分形尺度的关系.将裂纹临界表面能准则的裂纹平直路径扩展假设修正为分形路径扩展,采用分形理论建立了沿晶断裂、穿晶断裂、沿晶和穿晶混合断裂的分形模型,并根据自相似性分别计算其Hausdorff维数.最后在...     

金属材料冲裁过程的有限元模拟

介绍了利用弹性有限元对金属冲裁加工模拟的基本原理,包括弹塑性大变形木匠,裂破坏模拟以及网格重划分等技术,利用商业化有限元程度DEFORM－2D^TM对不锈钢（AISI304）板料的冲裁工艺进行了模拟,模具采用刚体模型,工件采用弹塑性模型,材料断裂破坏采用Cockroft-Latham破坏准则,分析了冲裁过程中裂纹生成、扩展以及材料断裂分离的过程,冲载力的变化等,模拟结果中的材料断裂和冲裁力的变化与已发表文献中的实验结果一致。     

基于微观损伤机理的延性裂纹扩展行为分析

在考虑材料内部塑性损伤的基础上,通过有限元数值方法(特殊单元模型法),编制了求解韧性断裂特征参量的程序,模拟材料的韧性断裂过程.通过单轴拉伸试验的模拟结果,得出反映材料微观损伤的控制参量,再根据该参量对同种材料的标准三点弯曲试验进行定量预测,即实现从一种试验结果预测同种材料的其它试验结果.结果表明,在韧性裂纹扩展时,考虑塑性损伤的特殊单元模型能够很好地描述材料的韧性断裂过程,可以实现同种材料不同试验间相互预测的目的.     

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

Micromechanism of Cleavage Fracture of Weld MetalsEI北大核心CSCD

Cleavage fracture is the most dangerous form of fracture. Cleavage fracture usually happens well before general yielding at low nominal fracture stress and strain. Cleavage fracture is often spurred by low temperature and determines the toughness in the lower shelf temperature region. This paper describes a new framework for the micromechanism of cleavage fracture of high strength low alloy (HSLA) steel weld metals. Cleavage fracture not only determines the impact toughness in the lower shelf but also plays a decisive role on the impact toughness in the transition temperature region. The toughness is determined by the extending length of a preceding fibrous crack which is terminated by cleavage fracture. Three non-stop successive stages, i.e. crack nucleation, propagation of a second phase particle-sized crack across the particle/grain boundary, propagation of a grain-sized crack across the grain/grain boundary are explained. The "critical event" of cleavage fracture is emphasized which offers the greatest difficulty during crack formation and controls the cleavage process. The critical event indicates the weakest microstructural component and its critical size which specifies the local cleavage fracture stress sigma(f) for cleavage fracture. In toughness-study it is paramount important to reveal the critical events for various test specimens. Three criteria for crack nucleation, for preventing crack nucleus from blunting and for crack propagation are testified. An active region specified by these criteria is suggested where the combined stress and strain are sufficient to trigger the cleavage fracture. It can be used in statistical analyses. A case study, using the new framework of micromechanism for analyzing toughness of 8% Ni steel welding metals is presented to analyze the experimental results.     

汽车用先进高强钢韧性断裂模型的研究与应用进展

韧性断裂预测对汽车轻量化产品设计与成形工艺优化有着重要的意义.全面综述了强耦合型与弱(非)耦合型韧性断裂模型的发展与研究现状;重点围绕考虑加载路径与应力状态的断裂失效与成形极限曲线预测、各向异性耦合的失效模型拓展、应变速率与温度效应对材料断裂的影响等3个方面的研究现状及应用效果进行了分析介绍,其中,MMC模型、Lou-...     

陶瓷涂层断裂韧性的表征

由于陶瓷本身的脆性,陶瓷涂层的应用受到了极大的制约。断裂韧性是反映材料抵抗裂纹失稳扩展能力的力学性能指标,陶瓷涂层因为脆性和低维特点,对其断裂韧性的表征存在较大困难,目前主要有临界应力强度因子KⅠC、临界裂纹扩展能量释放率GⅠC和裂纹密度β三种表征方法。本文对上述方法进行了总结、分析。     

考虑材料韧性损伤的中厚板半冲孔成形过程分析

为研究半冲孔成形过程中韧性损伤的演化以及部分工艺参数对成形质量的影响规律,本文在ABAQUS有限元模拟软件中建立了半冲孔轴对称有限元模型,并通过VUMAT用户子程序引入GTN(Gurson-Tvergaard-Needle-man)损伤模型,结合同时考虑空穴形状与体积变化影响的韧性断裂准则,进行弹塑性大变形有限元分析.基于该有限元模犁,预测了半冲孔工艺中静水压力、等效应力、等效应变、应力三轴度以及损伤断裂的产生和发展趋势,分析了反顶力、压边力和冲裁间隙对零件的影响规律,并与实验结果进行对比分析,验证了数值模拟的准确性.     

Investigation on ductile fracture in fine-blanking process by finite element method

In order to continuously analyze the whole fine-blanking process.from the geginning of the operation up to the total rupture of the sheet-metal,without computational divergence, a 3-D rigid visco-plastic finite-element method based on Gurson void model was developed.The void volume fraction was introduced into the finite element method to document the ductile fracture of the sheet-metal.A formulation of variation of the rigid visco-plastic material was presented according to the virtual work theory in which both the effects of equivalent stress and hydrostatic pressure in the deformation process were considered.The crack initiation of the sheet was predicted and the crack propagation was geometrically fulfilled in the simulation by separating the nodes according to the stress state.Furthermore.the influences of different state-variables on the deformation process were also studied.     

The characterization and interpretation of ductile fracture mechanisms in AL2024-T351 using X-ray and focused ion beam tomography

This paper describes an experimental investigation into the mechanism of ductile fracture in the aluminum alloy AL2024-T351 using a combination of synchrotron X-ray and focused ion beam tomography. Microstructural features that influence fracture at the micro- and nanoscale were characterized in virgin material in three-dimensions. The nature and volume fraction of ductile damage was then quantified as a function of distance below the fracture surfaces of tested notched and fatigue pre-cracked laboratory specimens. In both specimens the ductile fracture process initiates with the brittle fracture of large irregular intermetallic particles at low levels of plastic strain. With increasing plasticity, the resulting voids grow and combine with pre-existing porosity to increase the overall void volume fraction. Once a critical void volume fraction is attained, final failure occurs by the fracture or decohesion of dispersoids from the matrix, initiating a second population of nanoscale voids, which interlinks the larger voids. The critical void volume fraction for coalescence and the distribution of ductile damage below the fracture surface is markedly different between blunt-notched and pre-cracked specimens, with notched specimens exhibiting a significantly lower critical void volume fraction and a more extensive distribution of ductile damage below the fracture surface than is observed in pre-cracked specimens. This observation, related to the gradients in stress triaxiality and plastic strain in each specimen type, has important implications for the calibration of ductile damage mechanics models against notched-specimen data and their use to predict crack behavior in engineering structures.     

淬火无析出区对Al-Zn-Mg-Cu合金断裂行为的影响

根据超高强铝合金淬火、时效组织透射电镜观察结果,以及合金断裂行为与断口形貌分析,提出了一种Al-Zn-Mg-Cu合金产生延性断裂的物理模型。对平衡相、基体沉淀相、无析出微区的断裂行为进行了分析。研究结果表明,在晶内同步变形条件下,平衡相粒子最先断裂;晶内沉淀区内裂纹的形成与扩展早于无析出区,沉淀区内初始孔洞扩展与聚合,导致沉淀区断裂,这是合金断裂的主要原因;无析出区最后断裂。引起淬火敏感性的平衡相粒子,不是造成合金断裂的直接原因,粒子周围的无析出微区起到了限制裂纹扩展和协调变形的作用,是合金形成延性拉伸断口的原因。     

HIGH-TEMPERATURE TENSILE FRACTURE BEHAVIOR OF DIRECTIONALLY SOLIDIFIED Ni, Cr, Al-TaC EUTECTIC SUPERALLOY

The high-temperature tensile fracture behavior of the Ni, Cr, AI-TaC eutectic superalloy directionally solidified under high temperature gradient is investigated. The hightemperature tensile fracture of this in situ composite has ductile character with lots of ductile nests whose diameters decrease with the increasing solidification, rates. The maximum δb and δ are respectively 668.5MPa and 19.6％. There is a TaC whisker in the center of each nest, and the deformation of γ‘ and TaC is uneven. The hightemperature tensile behavior cannot be explained by the rule of mixtures but is decided by the formation of the plastic deformation band. The crack extension model is given.     

Predictive modeling of transition undeformed chip thickness in ductile-regime micro-machining of single crystal brittle materials

This paper proposes a predictive model to determine the undeformed chip thickness in micro-machining of single crystal brittle materials, where the mode of chip formation transitions from the ductile to the brittle regime. The comprehensive model includes a force model considering the rounded tool edge radius effect and ploughing. Irwin's model for computing the stress intensity factor is adopted here as it gives a relation between the stress intensity and applied normal stress including effects of crack size and crack inclination. The occurrence of plastic deformation is built upon the condition that the shear stress in the chip formation region must be greater than the critical shear stress for chip formation and the stress intensity factor must be less than the fracture toughness of the material. The point of transition takes place when the fracture toughness is equal to the stress intensity factor. The above conditions form the theoretical basis for the proposed model in determining the transition undeformed chip thickness. End-turning experiments have been conducted using a single crystal diamond cutting tool on (1 1 1) single crystal silicon, and the results compared to the model predictions for validation. The proposed model would support the determination of the cutting conditions for the micro-machining of a brittle material in ductile manner without resorting to trial and error.     

Study on fracture behavior of SiCp/A356 composites

The fracture behavior of SiCp/A356 composite at room and high temperatures was studied.Under tensile stress condition at room temperature, the fracture is mostly a combination of the brittle fracture of SiC particles and ductile fracture of A356 matrix.As the tensile temperature increases, the composite changes the main fracture behavior to the separation fracture of the bonding surface between SiC particles and A356 matrix.When the tensile temperature reaches 573 K, the fracture behavior of the composites is almost the whole separation fracture of the bonding surface, which is the main strengthening mechanism at high temperature.Under the cycle stress condition at room and high temperatures, the main fracture behavior of the composites is always a combination of the brittle fracture of SiC particles and ductile fracture of A356 matrix.However, under the cycle stress at high temperature, cycle behavior of the composites changes from cycle hardening at room temperature to the cycle softening at high temperature.     

Loss of crack-tip constraints for shallow cracks in bending

Single-edge notched bars (SENB) in bending with a/W ratios ranging from 0.05 to 0.5 are examined for fracture toughness in terms of the J-integral approach. The corresponding results indicated that, for a a/W ratios less than 0.3, there is a significant loss of J-dominance, which is attributed to the impact of plastic deformation on the cracked face. For larger a/W ratios, J-dominance is maintained into a large-scale yielding regime. According to the recently developed two-parameter criterion (J,Q), compressive Q-stress was interpreted as an indication of low crack-tip stress triaxiality for shallow cracks, whereas positive g-stress was associated with high crack-tip stress triaxiality for deep cracks. For the material properties and specimen geometries considered herein, a fracture toughness locus is constructed in terms of the (J-Q) parameters for each of the a/W ratios. The overall fracture data, which are in agreement with those predicted by other approaches, appear to provide a rigorous framework for interpreting the effect of loss of crack-tip constraint in elastic-plastic fracture analysis.     

<Emphasis Type="Italic">J</Emphasis>-<Emphasis Type="Italic">Q</Emphasis> Characterization of Constraint Effects of a Three-Dimensional Cracked Specimen

Failure assessment of the integrity of a ductile flawed structural component is done currently by a one-parameter fracture mechanics approach. The J-integral is the one-parameter used; it has proven to be useful in order to predict ductile crack initiation. However, when tension loading dominates and/or a fully plastic condition develops around the crack, the J-integral alone does not describe completely the crack-tip stress field and a second parameter is needed. In this work, an accurate modeling of the elastic-plastic stress field around a deep crack in a three-dimensional three-point bend specimen is carried out. Numerical results for the crack-tip stress field are used to evaluate a crack-tip constraint parameter Q, in terms of applied loading, from contained plasticity to large-scale yielding. The parameter Q, measures the degree of stress triaxiality and constraint around the crack-tip. In order to obtain the stresses in the near-crack-tip field with high accuracy, a detailed mesh with higher order three-dimensional finite elements is located around the crack front. The modeling of crack-tip blunting deformation is performed by using a small notch radius in the crack-tip. Large-strain and finite-rotation nonlinear behavior effects around the crack-tip are included. The material, an ASTM A 516 steel, is modeled with incremental theory of plasticity. Numerical results of the Q triaxiality parameter are presented for increasing level loads to obtain an extended yield condition. Additional results of J-integral parameter and crack-tip opening displacement, for different load ratios and for different position across the specimen thickness are shown.     

AISI-1035钢精冲成形与断裂的数值模拟

使用DEFORM2D软件对AISI-1035钢的落料、冲孔精冲工艺进行了弹塑性大变形有限元数值模拟,将McClintock断裂准则应用于精冲韧性断裂的预测中,预测了材料变形过程中静水应力、等效应力和等效应变的分布以及发展趋势、精冲最后阶段微裂纹产生发展和最终断裂。     

In situ observations on effects of hydrogen on deformation and fracture of A533B pressure vessel steel

External hydrogen gas atmospheres enhanced dislocation motion, multiplication of dislocations, and dis-location source activation under applied loading during in situ high-voltage electron microscopy (HVEM) observations of A533B pressure vessel steel. However, in both vacuum and hydrogen atmos-pheres, fracture occurred in a ductile manner in specimen areas transparent in the 1000-keV HVEM. The principal effect of the hydrogen atmosphere was to decrease the stress required for deformation near the crack tip and for crack propagation. Deformation at the crack tip was highly localized in both atmos-pheres, and a yielding strip plastic zone, analogous to the Dugdale-Barenblatt model for crack growth, formed ahead of the crack tip. The crack tip plasticity was confined to this strip. Inside the yielding strip, final cracking occurred through a sliding-off mechanism in the thin areas of the HVEM specimen. In the thicker areas of the specimen, where the yielding strip ahead of the crack was no longer transparent, crack tip blunting and void/microcrack formation ahead of the main crack tip could be observed directly. Crack tip blunting occurred by a two-corner mechanism, and further crack growth initiated by strain lo-calization at one of the crack tip vertices. Also void/microcrack formation ahead of the main crack tip was operative and resulted in coalescence into the main crack tip along the anticipated shear bands. Frac-tography of the thicker areas showed a ductile, dimpled fracture mechanism both in vacuum and hydro-gen atmospheres. Technical Research Centre of Finland, Metals Labo-ratory, SF-02150 Espoo, Finland