Design of a biaxial extensometer for measuring strains in cruciform specimens

A biaxial extensometer for measuring strains in flat cruciform specimens is presented. This extensometer allows for the use of two displacement or strain transducers to control and measure strains along mutually orthogonal directions in the plane of the specimen. The measurement and control of strains along each direction are completely independent of each other. The resolution and the measurable strain range of the biaxial extensometer depend on the particular displacement transducers employed. Independently adjustable gage lengths are incorporated along the two directions such that different strain ranges can be measured with the same transducer displacement range. The use of the biaxial extensometer with cruciform specimens and other applications, including the possibility for measuring triaxial strains in a plate specimen, are discussed.     

Mixed numerical–experimental technique for orthotropic parameter identification using biaxial tensile tests on cruciform specimens

This paper presents a mixed numerical–experimental method for the identification of the four in-plane orthotropic engineering constants of composite plate materials. A biaxial tensile test is performed on a cruciform test specimen. The heterogeneous displacement field is observed by a CCD camera and measured by a digital image correlation (DIC) technique. The measured displacement field and the subsequently computed strain field are compared with a finite element simulation of the same experiment. The four independent engineering constants are unknown parameters in the finite element model. Starting from an initial value, these parameters are updated till the computed strain field matches the experimental strain field. Two specimen geometries are used: one with a centered hole to increase the strain heterogeneity and one without a hole. It is found that the non-perforated specimen yields the most accurate results.     

Development of an apparatus for fatigue testing in vacuum

The development and use of a small, light-weight, remotely controlled fatigue apparatus are described. The particular application was for fatigue tests of 7075-T6 aluminum alloy in vacuums of 8.6×10^?8 to 4×10^?9 Torr. The fatigue life in vacuum was 1.8 times greater than the fatigue life in air based on the upper 95 percent confidence limit for tests in air and the lower 95 percent confidence limit for tests in vacuum.     

Development of viscoplastic constitutive equation through biaxial material testing

Internal-state variables have been used to represent the deformation historyin the recently proposed viscoplastic constitutive equations. In the current study, creep tests under nonproportional loadings were used to study the relative roles played by the internal-state varaibles in the constitutive equation by tracing the strain trajectory in strain space for a given stress trajectory in stress space. An experimental approach to studying the evolution rule for the tensorial state variable is also proposed. The experimental results on 2618-T61 aluminum alloy suggest that the scalarstate variable should play a much more dominant role than the tensorial state varaible in the constitutive modeling of 2618-T61 aluminum alloy.     

New equipment for cyclic biaxial testing

Methods of achieving biaxial-stress states in fatigue tests are reviewed. A new design of equipment that produces five distinct biaxial-stress states by simultaneous direct pressurization and axial loading of thinwalled cylindrical specimens is described. Four variations of stress state are obtained by the use of two sizes of specimen and by reversal of the pressurizing connections. The fifth state is obtained by direct pressurization without axial load. The actual magnitudes of stress in the specimen are computed from the output of the load cell in the reaction frame in which the biaxial-testing device is mounted. Additional stress ratios are obtained using standard uniaxial and torsional cyclic-testing arrangements. The initial program using this equipment was to investigate the effects of biaxiality on the cyclic properties and low-cycle fatigue behavior of normalized 1018 mild steel, under fully reversed constant-amplitude strain control. Control was achieved using a servo-controlled, electrohydraulic testing system with one of the two clipon strain transducers, that were mounted on the specimen, providing the controlling electrical signal. The system allowed direct recording of the stress-strain hysteresis loops in both principal directions. The behavior of the equipment and the modes of failure of the specimens are described and some test data are presented. The range of application and limitations of the equipment for further cyclic biaxial testing are discussed.     

Biaxial testing of fiber composites using tubular specimens

The failure of advanced continuous fiber laminates is not well understood. Stresses on a ply level are usually multiaxial, and reliable rules for the prediction of laminate failure are not generally available. Experiments to measure laminate-failure properties are difficult to perform and to relate to composite structures, being complicated for example by stress concentrations at the free edges of laboratory specimens. A multiaxial laminate specimen based on internal pressure and axial-force loading of a cylinder has been developed that appears to give reliable response and failure data. The purpose of this paper is to present in detail the design and show the supporting analysis of the specimen. In addition recent results on the failure of carbon/epoxy laminates are reviewed.     

A test apparatus for measuring the biaxial strength of brittle materials

Experimental Techniques -     

Miniature load-cell instrumentation for finite-deformation biaxial testing of elastomers

Accuracy of biaxial-test equipment to examine the nonlinear mechanical behavior of thin-sheet specimens of elastomeric materials has been hampered by lack of precise determination of the force distribution along the sides of the specimen. It has been necessary to use an effective width established by approximate means to obtain the stress from the overall force applied along the edges. To avoid this experimental difficulty, individual miniature proof-ring load cells utilizing semiconductor strain gages have been designed and applied to the support hooks for the thin-sheet specimens. Typical results are shown for time-dependent stress distributions for all degrees of biaxiality. An accurate evaluation of the effective specimen width is now possible, a fact which offers improvement in both accuracy and economy for future testing.     

大当量浅埋地下爆炸抛掷成坑效应的缩比模拟实验装置

针对当前地下爆炸物理模型实验无法模拟大当量地下爆炸抛掷弹坑和疏松鼓包现象的难题,基于相似理论,采用地下爆炸效应真空室模型实验方法,研制了考虑重力影响的大当量地下爆炸效应模拟实验装置。整套装置由容器罐体、快开门密闭机构、爆源系统、真空泵组、量测控制系统等组成,提出的新型爆源模拟装置可以实现精确起爆控制。该装置可模拟0.1~100 kt TNT、埋深20~400 m范围内不同比尺的地下核爆炸成坑和隆起实验,同时也能够模拟不同装药配置方案、不同地质条件下的大当量地下浅埋化爆抛掷实验。典型的核爆抛掷成坑模型实验结果表明,装置实验参数精确可调,实验过程可控,实验结果可信,为钻地核武器地下爆炸毁伤效应分析和大型工程爆破效果预测预报提供了实验室模拟和科学研究设备,填补了爆炸离心机无法模拟大当量地下爆炸抛掷成坑效应的空白。

     

A simple testing technique for fracture under biaxial stresses

Evidence is accumulating to suggest that the fracture toughness of and cyclic crack-propagation rates in a material may be affected by stresses acting parallel to the crack plane. This effect contradicts the justifiable assumption, implicit in fracture-mechanics theory, that only loads causing a stress singularity at the tip of a crack can affect its behavior. More extensive investigation of this important problem involves the development of special testing equipment and specimens. This article offers a simple design for such a system, which has proved in practice to be highly reliable and of adequate accuracy. Preliminary tests on polymethylmethacrylate (PMMA) under biaxial fatigue and ramp loading are described, to demonstrate the technique itself and the phenomena under investigation. The results suggest that, for this material at least, the effects of transverse stresses are indeed slight.     

A new direct biaxial testing machine for anisotropic materials

A screw-driven new biaxial testing machine for the realization of experimental investigations on anisotropic sheet materials, such as composite plates or rolled sheet metals, is presented. The described mechanical concept and servocontrol system allow cruciform specimens to be subjected to large strain biaxial tensile and compressive tests without kinematic incompatibilities. Moreover, for the proper implementation of biaxial tensile tests, the specific problems linked to the anisotropic properties of the investigated materials are taken into account; therefore, for the first time, the biaxial machine is supplied with the original ‘off-axes testing device,’ consisting of hinged fixtures with knife-edges at each arm of the cruciform specimen. A recently developed optimization method for the optimal design of flat tensile cruciform specimens is shortly reviewed. Numerical simulations illustrate the decisive superiority of the optimized specimen compared with specimen designs proposed in the literature, as well as the necessity to use the ‘off-axes’ testing technique in biaxial tests on anisotropic materials.     

A new method for the biaxial testing of cellular solids

Commercial cellular solids such as metal foams and honeycombs exhibit deformation and failure responses that are dependent on specimen size during testing. For foams, this size dependence originates from the fabrication-induced material and structural inhomogeneities, which cause the uncontrolled localization of deformation during the testing of foam cubes. Different peak loads and failure modes are observed in honeycomb specimens in the plate-shear configuration depending on specimen height. This size dependence causes difficulty in obtaining a more representative constitutive behavior of the material. It has recently been established that the size dependence under uniaxial compression can be eliminated with tapered cellular specimens, which enable controlled deformation at a given region of the specimen. This concept is extended in this paper to the biaxial testing of butterfly-shaped cellular specimens in the Arcan apparatus, which focuses deformation at the central section of the specimen. The Arcan apparatus has been modified such that all displacements at the boundaries of the specimen could be controlled during testing. As a consequence of this fully displacement controlled Arcan apparatus, a force perpendicular to that applied by the standard universal testing machine is generated and becomes significant. Thus, an additional load cell is integrated on the apparatus to measure this load. Example responses of butterfly-shaped specimens composed of aluminum alloy honeycomb, aluminum alloy foam and hybrid stainless-steel assembly are presented to illustrate the capabilities of this new testing method.     

An apparatus for tensile testing of concrete

This paper describes a test fixture and alignment jig used for testing small concrete or rock cylinders in uniaxial tension. The fixture may be easily modified to test cube specimens and is adaptable for use in any standard universal testing machine. A testing procedure is described and results presented for 3 in. × 6 in. (76 mm × 152 mm) concrete cylinders. The main virtues of the fixture are its ease of construction, simple alignment procedure for the test specimen, and accuracy comparable to that obtained from the standard compression test of concrete cylinders.     

On testing of charpy specimens using the one-point bend impact technique

This paper reports our methodology and results for the assessment of the dynamic fracture energy of notched Charpy A508 steel specimens. The fracture tests consist of one-point bend impact applied to the specimen in contact with an instrumented bar. Fracture is caused by the inertia of the unsupported specimen only. The fracture energy is determined from the incident, reflected and single wire fracture gage signals. High-speed photographic recordings show that for all the specimens investigated in the “lower shelf” temperature regime, fracture occurs relatively early and prior to “taking off” of the bar by rigid body motion. It also confirms that the fracture gage readings indeed coincide with the formation of a crack from the notch tip. The present methodology is relatively easy to implement, and it allows the investigation of the fracture properties of materials at loading rates (and velocities) that are substantially higher than those achieved in a conventional Charpy test. Moreover, this test is attractive for modeling purposes since its boundary conditions are simple and well defined.     

The preparation of powder specimens for shear cell testing

Conclusion The main aim of the present work was to provide a reliable method for preparing powder specimens for testing in a shear cell, and the method described above is recommended for use in cases where the simpler method described byJenike is found not to work.The discussion of what happens during the compaction process, and whether or not it is necessary to shear the specimen almost to the point of failure during preparation, must be regarded as tentative. More exact information depends on a knowledge of the shape of the failure surfaces for industrial powders; work along these lines is at present being planned.Paper presented at a meeting of the British Society of Rheology, University of Nottingham, April 6–8, 1965.     

Plane-stress fracture testing of finite sheets under biaxial loads

A procedure which combines the Williams series-type stress- and displacement-field expressions at the crack-tip neighborhood with a suitable numerical scheme away from the crack-tip was employed in the determination of the plane-stress fracture properties of four finite 7076-T6 aluminum sheets containing cracks emanating from a circular hole under four biaxial loads. The compatibility of the analytical and numerical displacements at the nodal points along the boundary of the crack-tip neighborhood was utilized in formulating displacement-continuity expressions containing some undetermined constants which solution depends on the nature of the boundary loading conditions. By linear superposition of the displacement due to remote uniaxial load and the displacements due to remotely applied transverse load in the neighborhood of the crack-tip, biaxial-displacement-continuity expressions containing these important fracture properties—namely, the opening Mode I stress-intensity factorK, the nonsingular stress term associated with the stresses in the direction parallel to the plane of cracksA and the integration termB associated with the displacement in this direction—were evaluated. Because no known biaxial testing of this geometry had been reported prior to this research, the analytical procedure was used to select the optimum geometry required in a biaxial fracture test of a finite-sheet specimen containing cracks emanating from a circular hole. This geometric optimization of the specimen guaranteed uniformity of stress all over the volume of specimen and also made the alteration of the existing MTS test fixtures unnecessary. Four square sheets of 7075-T6 aluminum alloy containing a central hole with two collinear cracks emanating radially at the edge of the hole were then fabricated in accordance with the analytically determined geometric requirements. The biaxial fracture test was then conducted under four biaxial load factors (λ) of 0.0, 0.5, 1.0 and 1.5. The fracture toughness obtained in this research was compared with those reported for uniaxial loading of large panels. It was found that there is a good correlation between the reported fracture toughness and this work.     

On the biaxial testing and strength of coated fabrics

In applying coated fabrics as roofing material for lightweight structures like tents, air-inflated structures and convertible roofs, the material is stressed in two dimensions. The different possibilities for biaxial testing of fabrics were examined and a suitable specimen shape was designed which allows the measurement of the true biaxial breaking strength. Experimental results showed that the biaxial strength is equal to the uniaxial strength. Stress-strain relations are given and compared with the uniaxial case.     

A dropped-weight apparatus for low-speed impact testing of composite structures

A dropped-weight test apparatus has been developed that can be used to perform low-speed impact tests on composite aircraft structures. This vertical drop-weight test apparatus is simple, compact, inexpensive and has precision impact and self-arresting design features similar to the more sophisticated, expensive test machines. The test apparatus has been used to perform low-speed impact response studies on laminated composite plates to understand the influence of impactor and target parameters on structural response and to develop a validated analysis method. Some of the experimental results generated by using this test apparatus for composite laminated plates are presented in the present paper and compared with the corresponding analytical results.