Scaling of fracture response in Over-height Compact Tension tests

An experimental investigation into in-plane scaled Over-height Compact Tension (OCT) [45/90/−45/0]_4s carbon/epoxy laminates was carried out to study the scaling of fracture response. The dimensions of the baseline specimens were scaled up and down by a factor of 2. Interrupted tests were carried out for specimens of each size in which the tests were stopped after certain load drops in order to study the failure mechanisms. X-ray Computed Tomography (CT) scanning was applied after the interrupted tests to examine the damage development and its effect on the fracture response. The test results showed that the scaling of the initial propagation of fracture follows Linear Elastic Fracture Mechanics (LEFM), but the development of the damage process zone differs with specimen sizes. The OCT specimens were found to be not large enough to generate a self-similar damage zone during propagation, and so no conclusions could be drawn regarding the R-curve effect.     

Simulation of discrete damage in composite Overheight Compact Tension specimens

Damage progression in laminated Overheight Compact Tension specimens was modeled using discrete representations of individual cracks and delaminations. Matrix cracking and delamination initiation, propagation, and interaction, without any prior knowledge and/or meshing of matrix cracking surfaces, is accomplished by combining stress and fracture mechanics-based constitutive modeling within a mesh independent crack-modeling framework. Simulation results including only matrix damage for specimens with [45₂/90₂/?45₂/0₂]_s and [0₄/90₄]_2s stacking sequences were compared with load–displacement curves and 3D X-ray micro computed tomography results from tested specimens. Excellent correlation was shown between the simulated and experimental load–displacement curves including statistical variations and proper representation of both the curve non-linearity and peak load. Similarly, a high level of correlation between simulated and experimental damage extent was shown. Additionally, a [45/90/?45/0]_2s specimen exhibiting significant fiber fracture was modeled and results compared with experiment. Fiber fracture was simulated using a continuum damage mechanics approach in addition to the discrete cracking and delamination damage representations of matrix damage. The simulated load displacement curve and damage extent compared favorably with experimental results.     

基于损伤累积的爆破振动传播规律试验研究

为进一步揭示基于损伤累积的爆破振动波传播与衰减规律,设计混凝土试块进行损伤累积及爆破振动测试试验。结果表明,测点损伤值随爆破次数增加持续增大,中远区测点损伤有初期缓增及后期突增两阶段;测点单次爆破振动速度随距离增大而减小,近区测点振动速度减小较中、远区测点快;测点爆破振动速度随爆破次数增加总体减小,且爆源近区减小较快,中远区呈小幅度缓慢减小;随爆破次数继续增大各测点振动速度存有突增过程,且衰减系数 增加缓慢, 值减小,两者均存在小幅度畸变过程;多次爆破时确定振动安全阈值及爆破振动波传播衰减规律( 、 值)时应充分考虑损伤累积效应。     

镍基单晶合金紧凑拉伸试样高温断裂特性的实验研究

采用紧凑拉伸（CT）试样对三种晶体取向[001]，[011]，[111]的镍基单晶合金DD3在950，850℃和760℃下的拉伸以及950℃时的蠕变和疲劳性能进行了实验研究。应用光学显微镜（OM）和扫描电子显微镜（SEM）分别对单晶体裂纹扩展路径和断口表面进行了观察和分析。拉伸实验结果表明：单晶体裂纹扩展路径沿着特定的晶体学矢量方向扩展而在试样自由表明呈现Z字型外观，其扩展方向及形状取决于晶体取向。温度对试样的断裂形式影响较为显著，760℃时的断裂特征为剪切型脆性断裂，断裂平面为沿滑移面的光滑斜断口；950℃下试样断裂逐渐转变为微孔聚集型韧性断裂，断口由粗糙的剪切唇和纤维区构成。蠕变和疲劳实验结果表明：镍基单晶具有明显的蠕变和疲劳性能各向异性，蠕变寿命以[011]，[111]，[001]顺序依次减小，疲劳寿命以[111]，[011]，[001]顺序依次减小；蠕变及疲劳裂纹扩展路径均与加载方向垂直，断裂表面均为光滑的平断口。试样断口细观分析显示，蠕变试样断口由杯锥状韧窝组成，而疲劳试样断口则由疲劳裂纹组成。疲劳损伤比蠕变损伤更有利于裂纹的扩展。     

Data-rich characterisation of damage propagation in composite materials

A novel methodology for the synchronised capture of high resolution white-light and infra-red (IR) images during a fatigue test is described. The approach allows digital image correlation (DIC) and thermoelastic stress analysis (TSA) to be applied practically simultaneously without the requirement to pause the cyclic load. The methodology is demonstrated on cross-ply carbon-epoxy specimens that have experienced damage induced by intermediate strain rate loading. Similar undamaged specimens are studied and the results from each compared. Various damage types are identified which include transverse cracking, delaminations and longitudinal splitting. The results are verified using X-ray computed tomography (CT).     

A comparative study of continuum damage models for crack propagation under gross yielding

In this paper, the problem of fracture initiation in an aluminum alloy thin plate containing a central crack is examined by employing several phenomenological continuum damage mechanics models. These models differ mainly in the selection of the kind of tensorial property the damage variable assumes, the nature of the equivalence postulate between damaged and pseudo undamaged material states, and the way damage evolution laws are formulated. Two formulations of damage effect tensor based on the engineering notation and the normative notation of stress and strain, respectively, are compared. In addition, the hypothesis of strain equivalence is compared to that of stress working equivalence. The error in the assumption of isotropic damage development in the crack tip process zone is also checked against that of anisotropic damage. In the numerical algorithm, both updated Lagrangian formulation and small displacement formulation of material non-linearity only are adopted and compared. The influence of non-proportionality in stress histories present in the crack tip region is accounted for by introducing a dynamic coordinate system of principal damage such that the principal direction of damage rotates in accordance with that of the loading. The calculated fracture initiation loads are finally compared with those determined experimentally.     

Experimental investigation on evolution of surface damage and topography parameters during rolling contact fatigue tests

Nowadays, micropitting is perceived as one of the prevalent wear mechanisms, which is responsible for the breakdown of high power density (power throughput/weight) machines. The current work demonstrates the evolution of surface damage and surface topography parameters during micropitting tests. The test surfaces are examined at different load stages utilizing 3D optical profiler, optical microscope, and scanning electron microscope. The effect of slide‐to‐roll ratio (SRR) on surface damage and topography parameters is also presented. It is observed that the severity of micropitting increases with an increase in the number of cycles. It is concluded that surface bearing index (Sbi), kurtosis (S_ku), maximum valley height (S_v), and autocorrelation length (S_al) are positively correlated with surface damage, whereas mean summit curvature (S_sc) and skewness (S_sk) are negatively correlated with surface damage. Surface texture ratio (S_tr) and power spectral density (PSD) of test surfaces are used for analysing surface texture, and it is found that deterioration in surface texture occurs after 6 million cycles. It is also concluded that geometrical isotropy increases with an increase in SRR.     

损伤可控型钢框架边节点的试验研究

该文提出一种损伤可控型钢框架节点（DCSF节点）。在梁端腹板上设置连接钢板,通过高强摩擦螺栓将梁、柱拼接在一起,并在梁内布置预应力筋来提供节点的复位能力。在小震作用下,通过高强螺栓来提供节点的初始刚度和抗弯承载力;在中震作用下,通过螺栓杆在螺栓孔壁内的滑移来改善节点的耗能,以此避免梁、柱主体构件受到损伤。共设计完成了4个足尺试件的低周往复加载试验,分别考虑了连接钢板的不同接触表面、预应力筋的初张拉值以及高强螺栓预紧力对该类节点的初始刚度、损伤特征、滞回性能和自复位能力的影响。研究结果表明：高强螺栓在螺栓孔内的滑移为节点提供了良好的耗能能力;预应力筋能够减少结构的残余变形,提供节点的自复位能力;当层间位移角达2.4%时,主体构件未发现明显损伤,螺栓杆与孔壁发生挤压破坏,更换连接钢板后,节点的承载力和耗能能力基本不变,DCSF节点具有良好的抗震性能;节点的连接无需现场施焊,施工方便、快捷,可在地震区推广应用。     

复合材料层合板准静态压痕实验研究

采用准静态压痕(QSI) 实验方法针对多种材料对集中准静态压痕力的损伤阻抗进行了测试。选取2 个特征载荷下的试样进行了超声C 扫描探伤和扫描电镜(SEM) 显微观察, 分析了QSI 实验中层合板的损伤过程, 针对初始分层接触力f₁和最大接触力f₂对材料的损伤阻抗特性进行了分析。测试了2 种材料体系层压板的最大接触力, 发现该值具有较好的可重复性。研究了测试条件(试样尺寸和支持条件) 和材料特性(不同纤维、基体种类和铺层方式) 对最大接触力的影响。实验结果表明, 试样尺寸和支持条件对最大接触力的影响较小, 在必要时可以采用非标试样测试最大接触力, 而最大接触力与树脂基体、纤维特性及铺层方式都有关, 最大接触力对应的凹坑深度d₂主要由纤维强度和延伸率决定, 树脂特性对d2有影响但影响程度较小。      

Experimental study of I + III mixed mode fatigue crack transformation propagation

In this paper, compact tension specimens with tilted cracks under monotonic fatigue loading were tested to investigate I + III mixed mode fatigue crack propagation in the material of No. 45 steel with the emphasis on the mode transformation process. It is found that with the crack growth, I + III mixed mode changes to Mode I. Crack mode transformation is governed by the Mode III component and the transformation rate is a function of the relative magnitude of the Mode III stress intensity factor. However, even in the process of the crack mode transformation the fatigue crack propagation is controlled by the Mode I deformation.     

Experimental study of unstable crack propagation velocity for X80 pipeline steel

Unstable crack propagation velocity following the failure of pipeline steel has always been a problem. In this study, differences between the existing unstable crack propagation calculation models were analysed, and the deficiencies within the crack propagation velocity models were overcome. The hardening coefficient and hardening index of the ductile material were considered, and a velocity model of unstable crack propagation of pipelines based on the Hutchinson‐Rice‐Rosengren (HRR) theory elastic‐plastic stress‐strain field and the displacement field was established. Consequently, the limit propagation velocity was obtained. In full‐scale burst test, the pipe with diameter of 1422 mm was chosen to simulate the natural gas pipeline transportation environment. Compared with the theoretical result, the error was limited in 13%. The research result solved the problem of quantifying the propagation velocity of the crack in X80 pipeline, which was significant for describing the forecast of velocity and length of unstable propagation and failure characteristics of pipelines.     

Wave propagation in cementitious material containing artificial distributed damage

The propagation of ultrasonic pulses through highly inhomogeneous mortar is discussed in this paper. The inhomogeneity is introduced by light plastic inclusions in different volume contents to simulate distributed damage. Wave propagation in such media becomes dispersive and therefore, although pulse velocity is influenced, other easily measured features are much more indicative of the inclusion content. These features can certainly improve characterization since they include information from the whole waveform and not only the leading edge.     

双环光束在CS_2中传输的研究

双环光束在CS2中传输时,当入射光功率密度超过140 kW/cm2时,光束产生丝状自聚焦现象,传输过程中光斑出现分裂现象;建立双环光束模型,通过数值求解非线性Schr dinger方程,得出光束在一定功率之上会产生丝状自聚焦和光斑分裂现象,并详细讨论了环形斑激光束在非线性克尔介质中的传输特性.     

Experimental study of the propagation of an apertured high-intensity laser beam in Kerr-active CS2

Abstract

Transverse pattern formation along the propagation in Kerractive CS₂ of a circularly and an elliptically apertured high-intensity input beam is studied. Observation is made of the modification in the Fresnel diffraction pattern as a function of the input beam power and the propagation distance. In addition to the well defined ring patterns, at high powers these rings break up into several spots. Also, a thermal-induced nonlinearity was observed to affect the dynamics of the pattern formation. Both forward stimulated Raman scattering (SRS) and backward stimulated Brillouin scattering were observed to occur during the propagation of the beam along the CS₂ cell. An unexpected pattern of fringes was always observed in the Stokes components of the SRS.     

Experimental study on dynamic damage evolution of concrete under multi-axial stresses

The effect of stress state on the dynamic compressive strength and the dynamic damage evolution process of concretes are investigated by use of a Spilt Hopkinson Pressure Bar (SHPB) and the ultrasonic technique. The columned concrete specimen is encircled by a steel sleeve. The multi-axial loading includes the axial and the radial loadings. The axial loading is supplied by the incidence bar, and the radial ones are produced by the steel sleeve. Analysis of the dynamic damage evolution of the samples is based on the measurement of the changes of ultrasonic wave velocities before and after the impact tests. The waveforms in the test bars, the stress strain curves, the confining pressure of the specimen, the dynamic compressive strength and other information about the samples are obtained during the SHPB experiments. The results of the tests show that the loading rate and stress states of the specimen apparently influence the damage evolution process in concretes. The dynamic damage evolutions are accelerated with the increase of the strain rate and are delayed significantly under the confined pressure.     

R125表面张力的实验研究

为适应国际上即将禁用CFC和HCFC类物质的迫切要求,寻找其替代物成为当务之急.R125是一种对臭氧层无破坏作用的新替代物,在目前国际上所提出的R22的混合工质替代物中是必不可少的组分之一.表面张力是分析计算有相变换热问题所需要的基本热物性参数.本文使用毛细管法实测了从248K到323K温度范围内40组R125表面张力实验数据,温度不确定度小于±10mK,表面张力的不确定度为±0.1mN·m-1.综合本文及文献的实验数据,拟合了R125表面张力随温度变化的关联式.