Void growth to coalescence in a non-local material

The size-effect in metals containing distributed spherical voids is analyzed numerically using a finite strain generalization of a length scale dependent plasticity theory. Results are obtained for stress-triaxialities relevant in front of a crack tip in an elastic-plastic metal. The influence of different material length parameters in a multi-parameter theory is studied, and it is shown that the important length parameter is the same as under purely hydrostatic loading. It is quantified how micron scale voids grow less rapidly than larger voids, and the implications of this in the overall strength of the material is emphasized. The size effect on the onset of coalescence is studied, and results for the void volume fraction and the strain at the onset of coalescence are presented. It is concluded that for cracked specimens not only the void volume fraction, but also the typical void size is of importance to the fracture strength of ductile materials.     

Spatially resolved observations of strain fields at necking and fracture of anisotropic hardened steel sheet material

In this work plastic strain localization, also referred to as necking, of press-hardened ultra-high strength steel is observed using digital speckle correlation. The region of the neck is studied during tensile tests of specimens specially designed to facilitate strain localization at an inner point of the material, thus avoiding edge effects on localization and fracture. By using measurements with a length scale small enough to properly resolve the neck, its growth and shape can be studied. Furthermore, the anisotropy of the material is investigated by examining specimens cut out at different angles to the rolling direction. It is seen that the local fracture strain of specimens cut out along the rolling direction is approximately twice as high as it is for specimens cut out perpendicular to the rolling direction.     

An In-Situ X-Ray Microtomography Study of Split Cylinder Fracture in Cement-Based Materials

In an effort to quantify microstructure-property relationships, three dimensional imaging experiments were conducted on small cylinder specimens subjected to split cylinder fracture. 3-D images were made using synchrotron-based x-ray microtomography, and the experiments were conducted with an in-situ frame such that a specimen could be examined while under load at varying degrees of damage. The specimens were made of fine-grained portland cement mortar and 0.5 mm glass beads, which served as aggregates. The diameter of the specimens was 5 mm. 3-D image analysis routines were developed or adapted to characterize microstructure and internal damage, which could then be related to bulk splitting strength and fracture energy. For fracture energy calculation, crack surface area could be measured in a way that accounted for roughness, branching, and fragmentation. Results showed that, for the specimens tested, aggregate surface roughness had little effect on strength but significant effect on fracture energy. Split cylinder strength showed correlation with specimen porosity, although there was considerable scatter. Strength did not correlate with maximum flaw size, although flaw location was not evaluated.     

在役设备材料断裂力学参数测定方法综述--小冲杆实验力学研究进展之二

20多年以来, 用小型试件的小冲杆实验技术来测量在役设备材料的各种力学参数已经取得了很大进展. 小冲杆实验的试样虽然还没有标准, 但一般均采用直径为3～10mm,厚度为0.1～0.5mm的圆片试样.这个方法已经用来确定材料的弹性模量、屈服强度、塑性性能、抗拉强度、断裂形态转变温度、断裂韧性、蠕变性能以及表示塑性硬化和韧性损伤的微观力学参数和黏塑性性能等各种力学性能.该方法兼具取样方便和几乎无损的优点,非常适用于那些无法取出传统试样或取样不经济的场合,因此引起科技界和工业界的关注.本文综述了小冲杆试验的测量技术及从测量数据来确定材料断裂形态转变温度、断裂韧性和主曲线的移动温度的各种计算方法研究进展.     

不同化学溶液下砂岩I型断裂韧度及其强度参数相关性的试验研究

本文通过长期浸泡的方法,以陕西铜川新区龙潭水库典型的库岸边坡的砂岩为研究对象,研究了浸泡在不同化学溶液下砂岩的力学特性及其损伤劣化机制,分析了经不同化学溶液腐蚀后试样的物理力学特性随化学腐蚀时间的劣化规律。研究发现：化学腐蚀后砂岩的物理力学特性均发生不同程度的劣化,其随化学腐蚀时间的劣化规律基本一致;但不同化学溶液下试样物理力学特性的劣化程度有所差异,酸性溶液（pH=3.0）加剧了砂岩的劣化程度,中性化学溶液对其损伤劣化程度也有一定的影响,试验初期,强碱性溶液下砂岩的劣化程度最小,但随着化学腐蚀时间的加长,强碱性溶液下的劣化程度逐渐加剧,大于中性溶液下的,但仍小于酸性溶液。化学腐蚀后砂岩试样均呈现出明显的弱化趋势,其断裂韧度KIC、抗压强度和抗拉强度的劣化程度显著,但各力学特征的劣化程度存在明显的差异,其中抗压强度的劣化程度相对减小,而其KIC的劣化程度较大;同时,化学腐蚀后砂岩的断裂韧度与抗拉强度、抗压强度间的一致性比较明显,存在明显线性关系。可以利用不同化学溶液下砂岩的裂纹扩展半径r来间接说明其力学特征发生损伤劣化的程度。     

冻融循环和干湿循环作用下砂岩断裂韧度及其与强度特征相关性的试验研究

以三峡库区岸边坡消落带的典型砂岩为研究对象,针对其实际的赋存环境设计了干湿循环和冻融循环2种试验方案,分别研究了2种试验方案过程中砂岩试样的力学特征,分析了浸泡在不同化学溶液中的砂岩试样经历不同循环次数后各力学参数的变化规律。试验结果表明：干湿循环和冻融循环作用下砂岩试样具有明显的“弱化”趋势,其断裂韧度、抗压强度和抗拉强度劣化趋势基本一致,但试样各力学参数劣化程度却存在差异。其中,断裂韧度KIC的劣化程度最大,抗拉强度次之,抗压强度最小。干湿循环和冻融循环作用过程中砂岩试样各力学参数的劣化程度均非常显著,该现象说明在模拟库岸边坡消落带岩体遭受化学溶液作用时,干湿循环和冻融循环作用均是不可能忽视的影响因素。但比较而言,在冻融循环作用下砂岩试样各力学参数劣化程度更加显著。不同的化学溶液对砂岩试样力学特征的损伤劣化程度的影响有所不同,酸性化学溶液加剧了砂岩的劣化程度,而SO42-离子对砂岩劣化的影响大于HCO3-离子的。     

A Simple Methodology to Measure the Dynamic Flexural Strength of Brittle Materials

A simple methodology is proposed for measuring the dynamic flexural strength of brittle materials. The proposed technique is based on 1-point impact experimental setup with (unsupported) small beam specimens. All that is needed is a measurement of the prescribed velocity as a boundary condition and the fracture time for a failure criterion, both to be input in a numerical (FE) model to determine the flexural strength. The specimen was modeled numerically and observed to be essentially loaded in bending until its final inertial failure. The specimen’s geometry was optimized, noting that during the very first moments of the loading, the specimen length does not affect its overall response, so that it can be considered as infinite. The use of small beam specimens allow large scale testing of the flexural strength and comparison between static and dynamic loading configurations. Preliminary experiments are presented to illustrate the proposed approach.     

混凝土断裂过程及尺寸效应分析

为研究混凝土裂纹断裂过程和最大承载力计算方法,通过实验机对四种不同尺寸混凝土紧凑拉伸断裂试件进行了加载过程实验,并对其中一个试件进行应变片跟踪测试.由实验结果分析得到了一系列关系曲线,如试件的载荷-加载点位移关系曲线,断裂损伤区变形随载荷变化曲线;并且计算了不同尺寸断裂试件的应力强度因子.结合计算粘聚裂纹应力强度因子的公式与断裂准则,完成了对承载力理论值的计算,并将其与实验峰值平均值进行对比,其结果是两者相比误差较小,表明此种计算裂纹结构最大承载力方法是可行的.     

玻璃纤维增强型复合材料圆筒高温高压动态冲击断口形貌分析

为了探究埋头弹火炮所用的玻璃纤维增强型(GFR)复合材料药筒在高温高压瞬态冲击条件下的结构强度,分别开展了圆筒静态整体拉伸和动态高温高压冲击实验,从拉伸/瞬态超高压破坏试样断口部分截取断口样品,在扫描电子显微镜下观察断口形貌,得到GFR复合材料在两种不同受力情况下的失效模式。结果表明:室温整体单轴拉伸断裂时,GFR复合材料的断面与轴线夹角接近45°, 失效模式为环氧树脂基体破坏和纤维拔出;在高压瞬态冲击作用下,试样主要失效模式为纤维的脆性断裂,同时由于火药燃烧产生的高温燃气使部分环氧树脂基体碳化,纤维与基体界面结合力降低,少数纤维熔融或软化附着在断口上,部分软化的纤维因瞬态超高压被拉细。     

求解I型裂纹构元J积分的半解析方法

表征裂纹尖端应力应变场程度的J积分是一个定义明确、理论严密的弹塑性断裂力学基础参量. 目前J积分的计算主要是依靠塑性因子法和有限元法,但对各类裂纹构元获得J积分以及载荷-位移关系的解析公式以实现材料断裂韧性理论预测和材料测试是断裂力学的重要和困难的任务. 以J积分为参量的材料断裂测试中应用最广的是I型裂纹试样的断裂韧性测试. 本文在平面应变条件下,针对断裂韧性测试中使用的6种I型裂纹构元,基于能量等效假设,提出了J积分-载荷和载荷-位移的工程半解析统一表征方法,进而结合有限元分析的少量计算获得J积分-载荷和载荷-位移关系的半解析公式待定参数. 分析表明,6种I型裂纹构元的J积分-载荷和载荷-位移统一公式的预测结果与有限元结果吻合良好. 新提出的J积分-载荷工程半解析公式包含了材料的弹性模量、应力强度系数和应变硬化指数,能够广泛适应不同的材料,且运用该公式能够方便获取任意载荷点对应的J积分值. 应用新方法可便于获得各类I型裂纹构元的J积分-载荷和载荷-位移工程半解析公式.      

SHPB冲击作用下层状千枚岩多尺度破坏机理研究

通过分离式霍普金森杆对层状千枚岩施加动态载荷，得到不同层理倾角下层状千枚岩的动态抗压强度与宏观破坏模式。采用三维激光仪获得断裂面细观形貌，引入分形几何定量计算断口面粗糙度；结合SEM观察到的微观尺度下不同层理倾角断口破坏机理，分析了不同层理倾角下层状岩石的动态破坏机制。研究结果表明:动态压缩下层理弱面对岩石的抗压强度影响较大；不同层理倾角千枚岩的断口形貌分形维数随层理倾角增大呈U型变化；从强度与裂纹扩展两方面考虑层理弱面对层状岩石破坏特征的影响，对于层理倾角为0°的试样，强度由岩石基质控制，但层理弱面仍对岩石破坏的裂纹分布与走向产生较大影响；对于层理倾角为22.5°的试样，强度与裂纹走向受岩石基质与层理弱面共同控制；对于层理倾角为45°～67.5°的试样，强度与裂纹走向受层理弱面控制；而对于层理倾角为90°的试样，动态抗压强度受岩石基质的影响较大，在层理弱面较早形成纵向宏观裂纹，导致该层理弱面角度下裂纹受层理弱面的影响较大。     

聚氨酯泡沫塑料的强度与断裂韧性

针对3种密度的聚氨酯泡沫塑料进行了拉伸实验。通过无缺口试件确定了3种密度泡沫塑料的拉伸断裂强度,而利用有缺口试件确定了这些材料的拉伸断裂韧性。为了研究高密度泡沫塑料的拉伸断裂机制,还对破坏后试件进行了扫描电镜分析。此外,还简要讨论了泡沫塑料拉伸断裂力学性能的理论预测问题。     

Failure resistance of intermetallic matrix reinforced by wires

Composite materials with brittle matrices such as ceramics and intermetallic compounds have gained increased importance in application. Ceramics and intermetallic compounds possess unique heat-resistance at high temperatures. They are, however, vulnerable to brittle fracture. This problem can be overcome by reinforcing the intermetallic compounds with wires. NiAl-tungsten composite wire was manufactured by hot diffusion welding of alternate layers of the matrix and wires. These specimens were subjected to a three-point bending in the temperature range from 20° to 1000°C. Temperature dependence of the bending strength exhibited brittle to ductile transition behavior. At room temperature, unstable failure by bending is terminated in a stable fashion. Brittle fracture of the matrix and wire were observed. For text temperatures of 300°, 500° and 700°C, subcritical crack growth occurred where the matrix and wire showed brittle and ductile fracture, respectively. A pronounced necking of the specimen was observed as the temperature is increased. Substantial plastic deformation occurred when the test is performed at 1000°C. The critical stress intensity factor K_1c and specific work of fracture were measured and found to be two to three times larger than the intermetallic compounds without wire reinforcement.     

Interplay of size effects in concrete specimens under tension studied via computational stochastic fracture mechanics

We attempt the identification, study and modeling of possible sources of size effects in concrete structures acting both separately and together. We are particularly motivated by the interplay of several identified scaling lengths stemming from the material, boundary conditions and geometry. Methods of stochastic nonlinear fracture mechanics are used to model the well published results of direct tensile tests of dog-bone specimens with rotating boundary conditions. Firstly, the specimens are modeled using microplane material law to show that a large portion of the dependence of nominal strength on structural size can be explained deterministically. However, it is clear that more sources of size effect play a part, and we consider two of them. Namely, we model local material strength using an autocorrelated random field attempting to capture a statistical part of the complex size effect, scatter inclusive. In addition, the strength drop noticeable with small specimens which was obtained in the experiments is explained by the presence of a weak surface layer of constant thickness (caused e.g., by drying, surface damage, aggregate size limitation at the boundary, or other irregularities). All three named sources (deterministic-energetic, statistical size effects, and the weak layer effect) are believed to be the sources most contributing to the observed strength size effect; the model combining all of them is capable of reproducing the measured data. The computational approach represents a marriage of advanced computational nonlinear fracture mechanics with simulation techniques for random fields representing spatially varying material properties. Using a numerical example, we document how different sources of size effects detrimental to strength can interact and result in relatively complex quasibrittle failure processes. The presented study documents the well known fact that the experimental determination of material parameters (needed for the rational and safe design of structures) is very difficult for quasibrittle materials such as concrete.     

含预制裂纹L形梁柱试件动态断裂过程

针对含预制裂纹L形梁柱试件,为研究预制裂纹动态扩展的力学特征及其对梁柱试件破坏模式的影响,采用数字动态焦散线实验系统,对距节点核心区不同距离l处含有预制裂纹的试件进行落锤冲击实验,得到预制裂纹的扩展轨迹、速度、动态应力强度因子的变化规律。结果表明,l值增大,扩展裂纹在梁下边缘的贯通点与预制裂纹的夹角逐渐增大,曲裂程度变大。裂纹扩展速度随着l的增大振荡性增强,裂纹扩展平均速度逐渐降低。l值为2 mm时,裂尖表现为Ⅰ型断裂,l值增大,裂尖受到剪应力作用增强,Ⅰ型动态应力强度因子减小,Ⅱ型动态应力强度因子增大,断裂逐渐转变为Ⅰ-Ⅱ复合型。     

Estimating the tensile strength of super hard brittle materials using truncated spheroidal specimens

New approaches need to be introduced to measure the tensile capacity of super hard materials since the standard methods are not effective. To pursue this objective, a series of laboratory tests were constructed to replicate the fracture mechanism of diamond-based materials. Experiments indicate that under a certain compressive test condition, stresses normal to the axisymmetric line in truncated spheroidal specimens (bullet-shaped specimens) are in tension contributing to the tensile fracture of the material. From experimental and numerical studies, it is concluded that semi-prolate spheroidal specimens can be used to determine precisely the tensile strength of brittle stiff diamond-like composites.     

多孔材料中应力波的传播

针对多孔泡沫混凝土实验应力应变曲线的基本特点,提出并建立了一种可以考虑孔隙压实过程的材料本构模型及其数学形式.通过典型算例,分析了应力波在多孔材料中传播的耗散效应,指出多孔材料中的孔隙在被压实过程中能有效吸收应力波能量、降低应力波强度,在引起应力波衰减的诸因素中,孔隙压实所起的作用更大更显著.     

An improved experimental method for the determination of the critical stress intensity factor KIc of brittle materials

The critical stress intensity factor K_Ic is determined by a simple and accurate method, using small test specimens and a simple procedure in this paper.Single edge V-notched tension specimens made of PMMA are subjected to a load which is slowly increased until the crack begins to move from the notch tip. During the crack propagation event shadow patterns at the tip of the crack are recorded in a video recorder. Under these loading conditions, the creating real crack propagate slowly until the crack propagation velocity take an abrupt increase and the entire fracture of the specimen takes place. The stress intensity factor which correspond to the transition from the slow to fast crack speed, is the critical stress intensity factor K_Ic and it can be the fracture toughness of the material.The results are accurate and in good agreement with those values of K_Ic which are calculated by approximate theoretical expressions.The purpose of this paper is to introduce an improved, simple and accurate experimental method for the determination of fracture toughness of brittle materials.     

A mesomechanical fracture model for an orthotropic material with different tensile and compressive strengths

The paper addresses a fracture problem for an orthotropic cracked plate made of a material with different tensile and compressive strengths and subjected to biaxial loading. The problem is solved using a micromechanical fracture model proposed earlier by the authors. It is assumed that the fracture of the material in the fracture process zones at the crack front is described by the Gol’denblat–Kopnov failure criterion. Strength curves for an orthotropic cracked plate with different strength and fracture-toughness parameters are plotted