Mode II fracture behavior of a Zr-based bulk metallic glass

Shear band formation and fracture are characterized during mode II loading of a Zr-based bulk metallic glass. The measured mode II fracture toughness, K_IIc=75±4 MPa√m, exceeds the reported mode I fracture toughness by ∼4 times, suggesting that normal or mean stresses play a significant role in the deformation process at the crack tip. This effect is explained in light of a mean stress modified free volume model for shear localization in metallic glasses. Thermal imaging of deformation at the mode II crack tip further reveals that shear bands initiate, arrest, and reactivate along the same path, indicating that flow in the shear band leads to permanent changes in the glass structure that retain a memory of the shear band path. The measured temperature increase within the shear band is a fraction of a degree. However, heat dissipation models indicate that the temperature could have exceeded the glass transition temperature for less than 1 ms immediately after the shear band formed. It is shown that this time scale is sufficient for mechanical relaxation slightly above the glass transition temperature.     

New experimental and numerical techniques to study the arrest and the restart of a crack under impact in transparent materials

The determination of relevant constitutive crack propagation laws under dynamic loading is a rather challenging exercise. In dynamic impact cases, the variations of propagation parameters and the extractions of crack positions are difficult tasks. This paper focuses on a methodology for assessing dynamic crack propagation laws under impact loading for transparent materials. Dynamic brittle fracture experiments are performed on polymethyl methacrylate (PMMA) in which several crack arrest phases occur. Then, these experiments are numerically reproduced by using the eXtended Finite Element Method (X-FEM) in order to validate the algorithms and the criteria assumed.     

一种基于SHTB的Ⅱ型动态断裂实验技术

冲击剪切载荷作用下动态断裂韧性的测定是材料力学性能和断裂行为研究中重要组成部分.为了测定材料的Ⅱ型动态断裂韧性,许多学者采用不同的试样与实验方法进行了实验,但限于实验条件,裂纹断裂模式往往是I+Ⅱ复合型,而不是纯Ⅱ型,因而不能准确测得材料的Ⅱ型动态断裂韧性.鉴于此,本文基于分离式霍普金森拉杆(split Hopkinson tension bar,SHTB)实验技术,提出一种改进的紧凑拉伸剪切(modified compact tension shear,MCTS)试样,通过夹具对MCTS试样施加约束,从而保证试样按照纯Ⅱ型模式断裂.采用实验-数值方法对MCTS试样动态加载过程进行分析,将实验测得的波形输入有限元软件ANSYS-LSDYNA,得到了裂纹尖端应力强度因子-时间曲线,并与紧凑拉伸剪切(compact tension shear,CTS)试样进行了对比.同时采用数字图像相关法进行了实验,验证了有限元分析结果.结果表明,MCTS试样在整个加载过程中K_I K_Ⅱ,裂纹没有张开;而CTS试样在同样的加载过程中K_IK_Ⅱ,出现裂纹张开现象.这说明MCTS试样能够准确地测定材料的Ⅱ型动态断裂韧性,为材料动态力学测试提供了一种有效的实验技术.     

加载速率对40Cr钢Ⅱ型动态断裂特性的影响

采用新型Ⅱ型动态断裂测试技术,对高强钢40Cr在高加载速率下的Ⅱ型动态断裂特性进行了测试研究。基于新设计的Ⅱ型动态断裂试样和分离式霍普金森压杆（split Hopkinson pressure bar, SHPB）技术,通过实验-数值方法确定了裂尖在加载过程中的应力强度因子曲线。采用应变片法确定了试样的起裂时间,最终得到40Cr的Ⅱ型动态断裂韧性值,并对其加载速率相关性和材料的失效机理进行了研究。结果表明,在1.08～5.53 TPa·m1/2/s的加载速率范围内,40Cr的Ⅱ型动态断裂韧性基本表现为与加载速率成正相关的变化趋势。通过对试样断口形貌的分析,确定了材料的失效模式及机理,发现随着加载速率的增加,存在拉伸型失效向绝热剪切型失效模式转变的现象。     

Strip model plasticity analysis of mixed mode crack opening displacement in aluminum alloy LY12

Under mixed mode loading, the crack tip blunts and undergoes displacements in two directions, the normal and shear component corresponding to Mode I and Mode II loading, respectively. These local displacements are determined by the duplicated film method and used to analyze the behavior of mixed mode fracture in aluminum alloy LY12. The mixed mode resultant crack opening displacement (COD) at fracture initiation tends to increase more rapidly with increase of the Mode II shear component. The fracture initiation value of COD for pure Mode II loading is six (6) times greater than that for Mode 1 loading. The same applies to the maximum effective plastic strain crack growth near the crack front. Observed are two typical morphologies, the equal-axes dimples and the parabolic dimples with evidence of slippage as dominated, respectively, by Mode I and Mode II loading.     

DYNAMIC FRACTURE ANALYSIS OF AGED GLASSY POLYCARBONATE BY THE METHOD OF CAUSTICS

The purpose of this work is to investigate the dynamic fracture properties of glassy polycarbonate （PC） with different aging times. The optical method of caustics is adopted in which the shadow spot patterns are recorded by a high speed camera during the dynamic fracture process. Then, the dynamic crack propagation, the stress intensity factor （SIF） and the dynamic fracture toughness of aged PC are obtained through an analysis of the characteristic size of caustic pattern. Moreover, by combining with the investigation of the fracture surface and the energy release rate analysis, the influence of aging time on the dynamic fracture behavior is discussed. Results show that the dynamic fracture toughness and critical energy release rate of PC decreases with aging time for short aging times, whereas they have little change or even increase for longer aging times. Therefore, aging modifies the mechanical properties especially the dynamic fracture properties of PC nonlinearly, not linearly as generally thought of.     

Ductile–brittle transitions in the fracture of plastically-deforming,adhesively-bonded structures. Part I: Experimental studies

Rate effects for adhesively-bonded joints in steel sheets failing by mode-I fracture and plastic deformation were examined. Three types of test geometries were used to provide a range of crack velocities between 0.1 and 5000 mm/s: a DCB geometry under displacement control, a wedge geometry under displacement control, and a wedge geometry loaded under impact conditions. Two fracture modes were observed: quasi-static crack growth and dynamic crack growth. The quasi-static crack growth was associated with a toughened mode of failure; the dynamic crack growth was associated with a more brittle mode of failure. The experiments indicated that the fracture parameters for the quasi-static crack growth were rate independent, and that quasi-static crack growth could occur even at the highest crack velocities. Effects of rate appeared to be limited to the ease with which a transition to dynamic fracture could be triggered. This transition appeared to be stochastic in nature, it did not appear to be associated with the attainment of any critical value for crack velocity or loading rate. While the mode-I quasi-static fracture behavior appeared to be rate independent, an increase in the tendency for dynamic fracture to be triggered as the crack velocity increased did have the effect of decreasing the average energy dissipated during fracture at higher loading rates.     

INFLUENCE OF LOADING RATE ON DYNAMIC FRACTURE BEHAVIOR OF FIBER-REINFORCED COMPOSITES

The effect of loading rate on the dynamic fracture properties and the failure mechanisms of glass fiber-reinforced composite materials under mode I fracture is studied. Dynamic reflective caustic experiments are carried out for two loading rates. By measuring the characteristic dimensions of the shadow spots during the caustic experiments, the dynamic SIFs are calculated for different loading rates. The experimental results indicate that the dynamic fracture toughness Kid increases remarkably with increasing loading rate, and the crack grows faster under the high-velocity impact. Moreover, by examining the crack growth routes and the fracture surfaces, it is shown that the loading rate also greatly affects the failure mechanisms at micro-scale.     

Fracture of notched polymeric beams due to central impact

Narrow beams of polymethyl methacrylate and polycarbonate with central cracks or Vee notches of various depths were subjected to central transverse impact by pneumatically propelled steel spheres and the hammers of a commercial impact tester. The transient loading was monitored by strain gages, and the stress-intensity factor at the tip of the discontinuity was determined sequentially by a shadowgraphic technique using a Cranz-Schardin camera. The fracture phenomena were studied with the aid of these methods.     

Experimental validation of the tip asymptotics for a fluid-driven crack

This paper provides experimental confirmation of the opening asymptotes that have been predicted to develop at the tip of fluid-driven cracks propagating in impermeable brittle elastic media. During propagation of such cracks, energy is dissipated not only by breaking of material bonds ahead of the tip but also by flow of viscous fluid. Theoretical analysis based on linear elastic fracture mechanics and lubrication theory predicts a complex multiscale asymptotic behavior of the opening in the tip region, which simplifies either as or as power law of the distance from the tip depending on whether the dominant mechanism of energy dissipation is bond breaking or viscous flow. The laboratory experiments entail the propagation of penny-shaped cracks by injection of glycerin or glucose based solutions in polymethyl methacrylate (PMMA) and glass specimens subjected to confining stresses. The full-field opening is measured from analysis of the loss of intensity as light passes through the dye-laden fluid that fills the crack. The experimental near-tip opening gives excellent agreement with theory and therefore confirms the predicted multi-scale tip asymptotics.     

动、静裂纹作用偏置效应的动焦散冲击实验

为了研究冲击荷载作用下脆性材料中运动裂纹与静止裂纹的相互作用,选取动态载荷下断裂行为与岩石材料类似且本身光学特性较好的有机玻璃（PMMA）作为实验材料,试件尺寸为220 mm×50 mm×5 mm,采用激光切割制作长度5 mm的预制裂纹和长度10 mm的静止裂纹,预制裂纹位于试件的底部边缘中心,静止裂纹的中心位于试件水平轴线。将静止裂纹偏置距离作为单一变量,采用数字激光动态焦散实验系统对含不同缺陷的PMMA进行三点弯曲实验,并结合几何分形理论研究不同偏置距离下运动裂纹的分形规律。实验结果表明:存在预制裂纹与静止裂纹的临界偏置距离（6 mm）,该条件下裂纹轨迹对应的分形维数值最大,裂纹轨迹的规则程度最低,裂纹破坏形态最复杂。当预制裂纹与静止裂纹的偏置距离在0～6 mm时,裂纹Ⅰ起裂后垂直向上扩展一段距离,与静止裂纹交汇,并停滞一段时间后发生二次起裂,直至贯穿试件,偏置距离和交汇点竖向坐标值呈近似线性函数关系。偏置距离的存在不会影响裂纹Ⅰ的起裂时间和应力强度因子,但会显著减小裂纹Ⅱ的动态应力强度因子,且停滞时长随偏置距离的增大而逐渐缩短。当偏置距离大于临界偏置距离时,运动裂纹不再与静止裂纹交汇而是呈拱状向试件上边缘扩展直至贯穿,裂纹的起偏时间、起偏位置也会出现明显的滞后现象。     

表面粗糙度对6061铝合金薄壁管冲击膨胀断裂性能的影响

采用分离式霍普金森杆实验技术,对表面加工后不同粗糙度的6061铝合金薄壁圆柱管进行动态膨胀断裂冻结回收实验,并对薄壁金属圆柱管动态膨胀断裂过程中裂纹萌生、扩展情况以及最终断裂模式等进行了研究。结果表明:相同冲击压力条件下,薄壁金属圆柱管表面粗糙度越大,材料越容易发生膨胀破裂;裂纹萌生于外壁面,由外向内扩展,并且裂纹的扩展主要受裂纹处应力状态的影响;薄壁金属圆柱管的断裂模式由拉伸和剪切断裂机制起主导作用,其断口为拉剪混合型断口。     

Mode I Fracture at Spot Welds in Dual-Phase Steel: An Application of Reverse Digital Image Correlation

Strain fields in 600 grade dual-phase steel V-notch tensile specimens, both with and without a spot weld, were measured after mode I fracture initiation. Starting with the final image of a fully developed crack, a novel reverse digital image correlation (DIC) analysis was used to determine the path that the crack followed at the onset of fracture as well as the crack tip deformation field. This gave the pixel coordinates of grid points on both sides (i.e. fracture surfaces) of the crack path in the undeformed image. Strain fields that develop in the base material regions surrounding the two fracture surfaces were subsequently measured with forward DIC analysis. Steady state values of the crack tip opening displacement (CTOD) and crack tip opening angle (CTOA), which are important fracture parameters, were measured for the base DP600 metal. Notch tip opening displacement (NTOD) and notch tip opening angle were also measured. It was found that steady state values of the CTOD and CTOA are reached within 2 mm or so of crack growth following completion of the flat-to-slant transition of the fracture surface and stabilization of the crack tunneling effect.     

三维冲击荷载弹塑性弯曲裂纹张开位移

主要研究冲击载荷作用下的三维弹塑性弯曲裂纹尖端的张开位移问题．综合考虑了冲击作用应力,三维塑性区域边界上正应力与剪应力,利用二阶摄动方法计算了三维弹塑性弯曲裂纹尖端的张开位移．用数值解法计算出三维弹塑性弯曲裂纹尖端张开位移,作图分析了三维弹塑性弯曲裂纹尖端张开位移与三维裂纹体几何尺寸之间的变化关系．三维弹塑性弯曲裂纹尖端张开位移随着三维裂纹体厚度的增大而减小,随着三维裂纹体厚度的均匀增大,三维弹塑性弯曲裂纹尖端张开位移尺寸不断减小,减小的幅度越来越小,最终趋于平面应变状态下的弹塑性弯曲裂纹尖端张开位移尺寸．当三维裂纹体几何尺寸相同时,三维弯曲裂纹尖端动态张开位移随外部冲击载荷的不断增大而逐渐增大,三维弯曲裂纹尖端动态张开位移随动荷系数的增大而迅速增大,建立了一个计算三维弹塑性弯曲裂纹尖端动态张开位移的崭新理论模型．     

Dependence of crack acceleration on the dynamic stress-intensity factor in polymers

The caustics method in combination with high-speed photography was employed to study velocity effect on the dynamic-stress-intensity factor of fast cracks in polymethyl methacrylate and in Araldite D. The specimen geometry was so determined that both the accelerating and decelerating crack propagation occurred noticeably in one fracture event. Instantaneous crack velocity as well as its acceleration were expressed as a function of the crack length by using polynomials of the ninth order which were given on the basis of the least-square method. The results show that the dynamic-stress-intensity factor depends not only on the crack velocity but also on crack acceleration, and that the accelerating crack has a smaller value stress-intensity factor than the decelerating crack at the same velocity.     

A dynamic analysis of modified compact-tension specimens using homalite-100 and polycarbonate plates

The dynamic fracture and crack-arrest responses of a modified compact-tension specimen (M-CT) machined from Homalite-100 and polycarbonate sheets were studied by dynamic photoelasticity, dynamic finite-element analysis and streaking photography. In contrast to some of the published results involving steel M-CT specimens, substantial dynamic effects were observed during rapid crack propagation in the Homalite-100 and polycarbonate M-CT specimens. The dynamic crack-arrest toughnesses.K _1a , were relatively constant and were about 80 percent and 50 percent of the corresponding fracture toughness,K _1c , of Homalite-100 and polycarbonate, respectively.     

Three-dimensional modeling of intersonic shear-crack growth in asymmetrically loaded unidirectional composite plates

An anisotropic cohesive model of fracture is applied to the numerical simulation of Coker and Rosakis experiments (2001). In these experiments, a unidirectional graphite–epoxy composites plate was impacted with a projectile, resulting in an intersonic shear-dominated crack growth. The simulations account for explicit crack nucleation––through a self-adaptive remeshing procedure––crack closure and frictional sliding. The parameters used in the cohesive model are obtained from quasi-static fracture experiments, and successfully predict the dynamic fracture behavior. In keeping with the experiments, the calculations indicate that there is a preferred intersonic speed for locally steady-state growth of dynamic shear cracks, provided that sufficient energy is supplied to the crack tip. The calculations also show that the crack tip can attain speeds in the vicinity of the longitudinal wave speed in the direction of the fibers, if impacted at higher speeds. In addition, a double-shock which emanates from a finite size contact region behind the crack tip is observed in the simulations. The predicted double-shock structure of the near-tip fields is in close agreement with the experimental observations. The calculations additionally predict the presence of a string of surface hot spots which arise following the passage of the crack tip. The observed and computed hot spot structures agree both in geometry as well as in the magnitude of the temperature elevation. The analysis thus suggests intermittent friction as the origin of the experimentally observed hot spots.     

A photoelastic study of energy loss during a fracture event

Static and dynamic photoelastic experiments were conducted to evaluate the energy lost due to damping in a thick-walled-ring specimen during a run-arrest fracture event. Short starter cracks were machined into a series of ring specimens fabricated from Homalite 100 and the specimens were loaded by a specially designed mechanical deformeter to giveK _Q/K_Imranging from 1.76 to 2.15. The crack was initiated and high-speed photographs of the isochromatic-fringe loops at the tip of the running crack were recorded. The data were analyzed to obtain the instantaneous stress-intensity factorK(t), the normalized crack positiona/w, and the crack velocity \(\dot a\) . A comparison ofK, as a function of positiona/w, was made between static and dynamic crack growth. Average values ofK were determined from these curves and estimates of initial strain energy and energy lost in forming the fracture surface were made. An energy balance was used to evaluate the energy loss due to damping in all the experiments. The energy loss during the run-arrest event was approximately 50 percent of initial strain energy.     

金属材料中低加载速率下的动态韧脆转变及断裂韧性测量

金属材料在冲击下的韧脆转变现象和动态断裂韧性的测量是金属材料冲击力学性能研究的重要组成部分.针对金属材料在冲击下的韧脆转变现象认识不足和韧性材料在较低加载率下动态J-R阻力曲线难以测量的现状,提出了采用高速材料试验机,设计专用试验夹具,测量15MnTi钢和11MnNiMo钢在不同加载速率下的韧脆转变过程,以及裂尖约束对...     

平面应变下紧凑拉伸试样的动态断裂韧性的实验研究

材料的动态断裂韧性是衡量材料在动载荷作用下抵杭裂纹扩展能力的重要指标,以往的材料动态断裂韧性测试多采用三点弯曲试样,而针对紧凑拉伸试样的动态断裂韧性研究很少.本文将紧凑拉伸试样(即CT试样)简化成等效弹簧质量模型,得到了CT试样动态应力强度因子的近似表达式.对Hopkinson压杆装置进行了改进,利用改进后的实验装置进...