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1.
Various compression test methods for composite materials have been developed during the past few years incorporating specimens of different gage lengths and modes of load introduction. Most of these test methods are not recognized as standards at the present time. Thus the question arises as to whether these various test methods produce similar results.Two commonly used test fixtures are compared in detail here, viz., the ASTM Standard D 3410 IITRI compression test fixture, and another often referred to as the Boeing-Modified D 695 compression test fixture. The IITRI fixture loads a specimen of 12.7-mm gage length via shear through end tabs and wedge grips. The Boeing-Modified D 695 fixture also uses a tabbed specimen, but with only a 4.8-mm gage length and loaded directly on its ends.Hercules S2/3501-6 glass/epoxy and AS4/3501-6 carbon/epoxy unidirectional composites were used as the primary material configurations in the present comparison. Highstrength, highly orthotropic materials such as these are among the most difficult composites to test successfully in compression. However, a carbon/epoxy quasi-isotropic laminate was tested as well.The results obtained are compared with similar data available in the literature. It is concluded that there is relatively little influence of specimen gage length as long as it is sufficiently small so that gross buckling does not occur. Also, the differences between shear loading and end loading of the specimen are small, as long as the ends of the end-loaded specimen do not crush, and that in either case the tabs do not debond.  相似文献   

2.
Failure loads of spot weld specimens are investigated under impact combined loading conditions. A set of test fixtures was designed and used to obtain failure loads of mild steel spot weld specimens under combined opening and shear loading conditions. Three different impact speeds were applied to examine the effects of separation speed on failure loads. Micrographs of the cross-sections of failed spot welds were obtained to understand the failure processes in mild steel specimens under different impact combined loads. The experimental results indicate that the failure mechanisms of spot welds are very similar for mild steel specimens at various impact speeds. These micrographs show that the sheet thickness can affect the failure mechanisms. For 1.0 mm specimens, the failure occurs near the base metal in a necking/shear failure mode. For 1.5 mm specimens, the failure occurs near the heat-affected zone in a shear failure mode. Based on the experimental results, the effects of the inertia force, the separation speed, and the loading angle on the failure loads of spot welds are investigated. Failure criteria are proposed to characterize the failure loads of spot welds under impact combined opening and shear loads for engineering applications. The failure load can be expressed as a function of the tensile strength of the base metal, the nugget size, the sheet thickness, the maximum separation speed, the loading angle, and empirical coefficients for a given welding schedule.  相似文献   

3.
Simple shear tests are widely used for material characterization especially for sheet metals to achieve large deformations without plastic instability. This work describes three different shear tests for sheet metals in order to enhance the knowledge of the material behavior under shear conditions. The test setups are different in terms of the specimen geometry and the fixtures. A shear test setup as proposed by Miyauchi, according to the ASTM standard sample, as well as an in-plane torsion test are compared in this study. A detailed analysis of the experimental strain distribution measured by digital image correlation is discussed for each test. Finite element simulations are carried out to evaluate the effect of specimen geometries on the stress distributions in the shear zones. The experimental macroscopic flow stress vs. strain behavior shows no significant influence of the specimen geometry when similar strain measurements and evaluation schemes are used. Minor differences in terms of the stress distribution in the shear zone can be detected in the numerical results. This work attempts to give a unique overview and a detailed study of the most commonly used shear tests for sheet metal characterization. It also provides information on the applicability of each test for the observation of the material behavior under shear stress with a view to material modeling for finite element simulations.  相似文献   

4.
Fatigue Loading and Life Prediction in Three Fretting Fatigue Fixtures   总被引:1,自引:0,他引:1  
Three fixtures for conducting laboratory fretting fatigue tests are described and their respective testing methods and the results of the analysis are compared. Each of these fixtures has been used to investigate the effects of various parameters of interest in fretting fatigue. These fixtures include a unique apparatus in which all load applied to the specimen is transferred to the fretting pads, an apparatus similar to many found in the literature where partial load transfer occurs across the pads, and a simplified dovetail fixture in which the clamping load, P, and the shear load, Q, are varied in phase. Select test conditions from prior experiments performed on identical material and resulting in similar lives ranging from one to ten million cycles from these fixtures are identified. The various testing conditions were used to compute the unique stress field for each case. The resulting contact stresses were used to calculate crack initiation based criteria, and to calculate stress intensity factors. The three fixtures were shown to be able to accommodate a range of loads, fretting pad contours, and specimen geometries that produced a variety of stress fields. A crack-initiation-based criterion was shown to predict the failure lives of thinner specimens accurately. The stress intensity factor calculations showed the possibility of a crack arresting for a stress field that decays rapidly and the possibility of a local minimum for K as a function of depth. The fixtures are shown to be complementary in generating data for development of robust fretting fatigue models that use these criteria.  相似文献   

5.
The numerical simulation of dynamic structural failure by localized shear is quite complex in terms of constitutive models and choice of adequate failure criteria, along with a pronounced mesh-sensitivity. As a result, the existing numerical procedures are usually quite sophisticated, so that their application for design purposes is still limited. This study is based on the implementation of a simple energy-based criterion, which was developed on experimental considerations (Rittel et al., 2006), and uses a minimal number of adjustable parameters. According to this criterion, a material point starts to fail when the total strain energy density reaches a critical value. Thereafter, the strength of the element decreases gradually to zero to mimic the actual structural behavior. The criterion was embedded into commercial finite element software and tested by simulating numerically four typical high-rate experiments. The first is the dynamic torsion test of a tubular specimen. The second concerns the failure mode transition in mode II fracture of an edge crack in plain strain. The last two involve dynamic shear localization under high rate compression of a cylindrical and a shear compression specimen. A very good adequation was found both qualitatively and quantitatively. Qualitatively, in terms of failure path selection, and quantitatively, in terms of local strains, temperatures and critical impact velocity. The proposed approach is enticing from an engineering perspective aimed at predicting the onset and propagation of dynamic shear localization in actual structures.  相似文献   

6.
The shear properties of brittle and highly porous carbon (graphitic) foam cannot be measured reliably with most standard test methods, such as single rail, double rail, Iosipescu shear, etc. A new testing device has been developed to accurately measure the shear stiffness and strength of carbon foam or other porous materials. Specimens of cylindrical cross section are used to reduce the high stress concentration that normally occurs in the vicinity of the grip section. Since strain gages could not be installed on the specimen surface (due to porosity), the shear strain is determined from the specimen end rotation. A high resolution in the rotational measurement is achieved by using a stepper motor with multiple gear reduction. In view of testing low modulus material, the load cell of the fixture was mounted onto an axial roller to relieve the axial constraint while twisting the specimens. The accuracy of the measurement and calibration of the test fixture has been demonstrated by measuring the shear modulus of two plastic (polyvinyl chloride (PVC) and urethane).  相似文献   

7.
Behavior of Pin-loaded Laminated Composites   总被引:1,自引:0,他引:1  
In this study, an investigation was carried out to determine the effects of joint geometry and fiber orientation on the failure strength and failure mode in a pinned joint laminated composite plate. Behavior of pin-loaded laminated composites with different stacking sequence and different dimensions has been observed experimentally. E/glass–epoxy composites were manufactured to fabricate the specimens. Mechanical properties of the composites were characterized under tension, compression and in-plane shear in static loading conditions. Laminated composites were loaded through pins. Single-hole pin-loaded specimens were tested for their tensile response and width-to-hole diameter (W/D) and edge distance-to-hole diameter (E/D) ratios evaluated. A series of experiments was performed with six different material configurations ([0/±45]s–[90/±45]s, [0/90/0]s–[90/0/90]s and [90/0]2s–[±45]2s), in all, over 120 specimens. E/D ratios and W/D ratios of plates were changed from 1 to 5 and 2 to 5, respectively. Failure propagation and failure type were observed on the specimens. The influence of the joint geometry on the strength of the pin-loaded composites was assessed. When laminated composite plates were loaded to final failure, three basic failure modes consisting of net-tension, shear out and bearing failure were observed for the different geometric dimensions. All the connections tested showed that the fiber orientations have a definite influence on the position around hole circumference at which failure initiated. Net-tension failure occurred for specimens that had small width and large end distance. When the width was increased, the specimens which had small end distances failed in the shear-out modes. When the end distance was increased, bearing failure developed in addition to shear-out failure. The experimental results showed that the ultimate load capacities of E/glass–epoxy laminate plates with pin connection were increased by increasing W and E. However, increasing the E/D and W/D ratios beyond a critical value has an insignificant effect on the ultimate load capacity of the connection.  相似文献   

8.
In-plane unidirectional shear properties of carbon/epoxy and glass/epoxy materials were measured using unidirectional and cross-ply specimens utilizing the Wyomingmodified two-rail shear test fixture. Simple test specimens of trapezoidal and rectangular geometries, and specimens with tabs were tested. Various specimen sspect ratios were also considered. Untabbed unidirectional specimens exhibited premature shear failures but cross-ply specimens produced reasonably reliable results with acceptable failure modes. Both bonded tab and integral tab 0° specimens were found to give comparable results. Unidirectional and cross-ply PEEK specimens, and angle-ply and quasi-isotropic AS4/3501-6 specimens, were also tested.  相似文献   

9.
This paper documents an experimental study that was conducted to demonstrate the sensitivity of the shear gage to the presence of normal strains. The shear gage is a specially designed strain gage rosette that measures the average shear strain in the test section of notched specimens such as the losipescu, Arcan and compact shear specimens. These specimens can have complicated stress states with high shear and normal strain gradients. To evaluate the sensitivity of the shear gage to normal strains, shear gages were tested on an Arcan specimen. The Arcan specimen is a notched specimen that can be loaded in pure shear (90 deg), pure tension (0 deg) and at intermediate 15- deg increments. The shear modulus for an aluminum specimen was determined at each of these loading angles. It was found that the gages display nearly zero sensitivity to normal strains ( x, y). Moiré interferometry was used to document the shear and normal strain distributions in the test section and to provide an independent method for determining the average shear strain. These results reinforce the robust nature of testing with the shear gage.  相似文献   

10.
评估TiN薄膜与基材结合的划痕试验及有限元模拟   总被引:1,自引:0,他引:1  
通过有限元模型模拟划痕试验得到的结果表明∶切应力的起伏变化?膜/基界面处切应力差值?接触区附近膜层表面张应力?高载下的几种应力集中等,对膜/基体系的失效都有重要的作用.通过模型计算临界载荷下的膜/基界面处切应力差值,可用来评价膜层与基材的结合强度;提出了划痕试验中膜/基体系失效的2种机制.不同性能基材的TiN膜/基体系划痕试验结果,可验证本文有限元模拟的有效性,并表明临界载荷是膜/基结合强度?体系承载能力?内聚结合性能等的综合反映;低载往复摩擦磨损试验的结果进一步证实,用划痕试验的临界载荷评估膜/基结合强度具有局限性.  相似文献   

11.
The statistical models of the strength of thin ceramic substrates with surface defects under piston-on-3-ball loading conditions are formulated using Batdorf’s statistical theory and Kirstein and Woolley’s moment equations. These models possess the form of a Weibull distribution function, making it possible to process the piston-on-3-ball biaxial flexural strength data using a Weibull treatment. During this study, it was noted that the thickness of the specimen had no effect on the failure distribution. Therefore, it was deemed that a reasonable thickness of the specimen disk could be selected for the piston-on-3-ball test in the case where the thickness is so small that the deflection of the center of the specimen exceeds half of the thickness (this thickness would invalidate the strength evaluation equation specified in ASTM F 394-78). The strengths of seven different compositions of 8YSZ with dopants were tested using the piston-on-3-ball method. The results were then processed using the derived models. The failure distributions of the different thickness groups of 8YSZ specimens were similar, verifying that the thickness, indeed, has no effect on the failure distribution.  相似文献   

12.
兰天宝  苏飞 《力学与实践》2014,36(4):442-446
在采用搭接剪切试件进行的剪切强度实验中,试件失效往往是由于试件两端的剥离应力过大,而不是切应力造成的,这往往造成错误的实验结果. 通过有限元和光弹实验相结合的方法研究在搭接结构中引入凹槽以使试件所受的切应力均匀分布并减少两端的剥离应力,从而提高实验精度的可行性. 研究表明,只要凹槽的形状和位置合适,完全可以达到上述效果,从而实现了搭接剪切试件的优化设计.  相似文献   

13.
缺口件疲劳寿命分布预测的有效应力法   总被引:1,自引:0,他引:1  
论文提出了一种由光滑件疲劳寿命试验数据预测缺口件疲劳寿命分布的有效应力法.该方法中缺口件的裂纹可能萌生表面被分解成一个个微元,整个表面可看成是这些微元组成的一个串联模型,按照串联概率失效模型,缺口件的疲劳强度失效概率就可以由各微元的疲劳强度失效概率计算得到,其中微元的疲劳强度失效概率是由光滑件的疲劳强度失效概率通过最弱环节理论计算得到的.在缺口件的疲劳强度失效概率表达式中,引入了有效应力的概念,用它查取光滑件的疲劳寿命试验数据就可以直接得到缺口件的疲劳寿命分布.该方法可以同时考虑到应力梯度和试件尺寸对缺口件疲劳寿命分布的影响.进行了材料LY12CZ的带中心孔缺口件的寿命算例分析,预测结果和试验结果吻合良好,表明该方法是有效的.  相似文献   

14.
Novel Technique for Static and Dynamic Shear Testing of Ti6Al4V Sheet   总被引:1,自引:0,他引:1  
Few shear test techniques exist that cover the range of strain rates from static to dynamic. In this work, a novel specimen geometry is presented that can be used for the characterisation of the shear behaviour of sheet metals over a wide range of strain rates using traditional tensile test devices. The main objectives during the development of the shear specimen have been 1) obtaining a homogeneous stress state with low stress triaxiality in the zone of the specimen subjected to shear and 2) appropriateness for dynamic testing. Additionally, avoiding premature specimen failure due to edge effects was aimed at. Most dimensional and practical constraints arose from the dynamic test in which the specimen is loaded by mechanical waves in a split Hopkinson tensile bar device. Design of the specimen geometry is based on finite element simulations using ABAQUS/Explicit. The behaviour of the specimen is compared with the more commonly used simple shear specimen with clamped grips. Advantages of the new technique are shown. The technique is applied to Ti6Al4V sheet. During the high strain rate experiments high speed photography and digital image correlation are used to obtain the local shear strain in the specimen. Comparison of experimental and numerical results shows good correspondence.  相似文献   

15.
A discrete element method (DEM) called particle flow code (PFC2D) was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of rock-like material, a set of micro-parameters in PFC2D that reflected the macro-mechanical behavior of rock-like materials were obtained. And then PFC2D was used to simulate Brazilian splitting test for jointed rock mass specimens and specimen containing a central straight notch. The effect of joint angle and notch angle on the tensile strength and failure mode of jointed rock specimens was detailed analyzed. In order to reveal the meso-mechanical mechanism of crack coalescence, displacement trend lines were applied to analyze the displacement evolution during the crack initiation and propagation. The investigated conclusions can be described as follows. (1) The tensile strength of jointed rock mass disc specimen is dependent to the joint angle. As the joint angle increases, the tensile strength of jointed rock specimen takes on a nonlinear variance. (2) The tensile strength of jointed rock mass disc specimen containing a central straight notch distributes as a function of both joint angle and notch angle. (3) Three major failure modes, i.e., pure tensile failure, shear failure and mixed tension and shear failure mode are observed in jointed rock mass disc specimens under Brazilian test. (4) The notch angle roles on crack initiation and and joint angle play important propagation characteristics of jointed rock mass disc specimen containing a central straight notch under Brazilian test.  相似文献   

16.
Ductile failure experiments on a double notched tube (DNT) specimen subjected to a combination of tensile load and torque that was applied at a fixed ratio is presented. The experimental results extend those in Barsoum and Faleskog (2007a) down to zero stress triaxiality. A new and robust evaluation procedure for such tests is proposed, and a simple relation for the equivalent plastic strain at failure for combined normal and shear deformation, respectively, is developed. Tests were carried out on the medium strength medium hardening steel Weldox 420, and the high strength low hardening steel Weldox 960. The experimental results unanimously show that ductile failure not only depends on stress triaxiality, but is also strongly affected by the type of deviatoric stress state that prevails, which can be quantified by a stress invariant that discriminates between axisymmetric stressing and shear dominated stressing, e.g., the Lode parameter. Additional experiments on round notch bar (RNB) specimens are recapitulated in order to give a comprehensive account on how ductile failure depends on stress triaxiality, ranging from zero to more than 1.6, and the type of stress state for the two materials tested. This provides an extensive experimental data base that will be used to explore an extension of the Gurson model that incorporates damage development in shear presented in Xue et al. (2013) (Part II).  相似文献   

17.
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.  相似文献   

18.
Numerical and experimental direct shear tests for coarse-grained soils   总被引:3,自引:0,他引:3  
The presence of particles larger than the permissible dimensions of conventional laboratory specimens causes difficulty in the determination of shear strength of coarse-grained soils. In this research, the influence of particle size on shear strength of coarse-grained soils was investigated by resorting to experimental tests in different scale and numerical simulations based on discrete element method (DEM). Experimental tests on such soil specimens were based on using the techniques designated as "parallel" and "scalping" to prepare gradation of samples in view of the limitation of laboratory specimen size. As a second approach, the direct shear test was numerically simulated on assemblies of elliptical particles. The behaviors of samples under experimental and numerical tests are presented and compared, indicating that the modification of sample gradation has a significant influence on the mechanical properties of coarse-grained soils. It is noted that the shear strengths of samples produced by the scalping method are higher than samples by the parallel method. The scalping method for preparing specimens for direct shear test is therefore recommended. The micromechanical behavior of assemblies under direct shear test is also discussed and the effects of stress level on sample behavior are investigated.  相似文献   

19.
An analysis for an impact system is presented. The results are then used to interpret the test data from dynamic separation of resistance spot welded joints. In this Part I of the investigation emphasis is placed on the design consideration, development of a test system and verification of the design from actual test data obtained from the test system. In addition, the inertia effect of a generic dynamic system is analyzed using the principle of rigid body dynamics. It is shown that the load recorded by a load cell could include both the load experienced by the test specimen and the inertia force generated from the mass and acceleration between the specimen and the load cell, when the load cell is placed on the fixed side of the test specimen. Impact fixtures designed for spot weld strength testing are then studied for the inertia effect.  相似文献   

20.
刘振国  金涛  树学峰 《实验力学》2014,29(6):760-768
通过压缩具有一定倾斜角(0°,10°,15°,20°和25°)试件和双剪切模型试件,实现了单轴压缩、压缩-剪切复合应力以及纯剪切三种应力状态,得到PMMA(聚甲基丙烯酸甲酯)在相应应力状态下的应力-应变曲线,同时对不同应力状态下试件的破坏模式进行了分析。结果表明:在不同受力环境中材料的强度和破坏的机理不同;同单轴压缩状态下相比,材料在压缩-剪切复合应力状态下屈服极限、强度极限以及破坏应变均不同程度的增大,呈现明显的"剪切增强"现象。单轴压缩与压缩-剪切应力状态下试件的破坏模式均为在试件短对角面上出现明显的剪切屈服带,由应力分析得出试件剪应力在短对角面上达到最大,引起在此平面上分子链间滑动从而产生应变软化形成剪切屈服带;双剪切试件的破坏模式为与剪切面呈45°的斜面。  相似文献   

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