Experimental evaluation of the Wyoming-modified two-rail shear test method for composite materials

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.     

An experimental investigation of losipescu specimen for composite materials

A detailed experimental evaluation of the losipescu specimen tested in the modified Wyoming fixture is presented. Moiré interferometry is employed to determine the deformation of unidirectional and cross-ply graphite-epoxy specimens. The results of the moiré experiments are compared to those from the traditional strain-gage method. It is shown that the strain-gage readings from one surface of a specimen together with corresponding data from moiré interferometry on the opposite face documented an extreme sensitivity of some fiber orientations to twisting. A localized hybrid analysis is introduced to perform efficient reduction of moiré data, producing whole-field strain distributions in the specimen test sections.     

On short-beam shear tests for composite materials

Interlaminar beam tests in the form of three-point and four-point flexure are examined both experimentally and analytically. Experimental data are obtained on unidirectional composites. Photomicrographs of actual failure modes and results of a stress analysis based on classical theory of elasticity are utilized to supplement the experimental data. Complex failure modes in the presence of extremely high combined stress gradients are observed and cast serious doubts on the usefulness of interlaminar-beam experiments for characterizing the delamination resistance of composite materials. Further difficulties are encountered with ductile-matrix-resin composites.Paper was presented at V International Congress on Experimental Mechanics held in Montreal, Quebec, Canada on June 10–15, 1984.     

Analytical and experimental evaluation of double-notch shear specimens of orthotropic materials

A finite-difference analysis of the state of stress in a double-notch interlaminar shear strength specimen is developed. The effects of geometry and material parameters on the stress distributions are investigated. It has been found that, in agreement with previous determinations,^1–7 a uniform distribution of shear stress on the fracture plane does not exist. The shear stress distribution becomes more uniform for increased material anisotropy and for small (L/T) ratios, whereL is the distance between the notches andT is the specimen thickness. Also, it has been determined that the notch size (W) and the distance from the notches to the loaded ends of the specimen (h) do not influence the stress distributions significantly. The effects of variations in the (L/T) ratio, the notch size (W), and the length (h) were investigated experimentally. For a graphite/epoxy laminate of 0/90-deg square wave it has been found that the apparent shear strength determined by double-notch shear tests decreases significantly with an increase in (L/T) ratio. The decrease in the apparent shear strength with an increase inh, however, is very small. Also, the apparent shear strength is not affected significantly by increasing the notch sizeW.     

Cutting process of composite materials: An experimental study

This paper focuses on experimental research of milling process of the epoxide-polymer matrix composite reinforced carbon fibers (EPMC—carbon composite). An influence of two control parameters, namely feed and rotational speed, on cutting forces is investigated. The experiment is conducted on a CNC machine with feed rate ranging from 200 to 720 mm/min and rotational speed from 2000 to 8000 rpm. The experimental time series are analysed by means of the delay coordinates method in order to find stable cutting regions and to recognize the kind of behaviour. Using this information, a new model for the cutting forces is proposed that can be used to build a new regenerative vibration model for EPMC milling.     

An experimental and numerical investigation of different shear test configurations for sheet metal characterization

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.     

A tensile impact test apparatus for composite materials

A tensile impact test apparatus capable of applying a pure axial tensile loading to even a highly orthotropic composite material, e.g., a unidirectionally reinforced composite, was designed and constructed. Existing impact test methods such as Charpy, Izod and plate impact induce very complex stress states, making the interpretation of results difficult. Details of the apparatus design, and instrumentation problems which had to be overcome, are discussed.was Graduate Student, Composite Materials Research Group, P.O. Box 3295, University of Wyoming, Laramie, WY 82071.     

The shear gage: For reliable shear modulus measurements of composite materials

A special strain gage called the shear gage was developed for composite materials testing with notched shear specimens. The shear-gage records the average shear strain across the entire test section between the notches of the losipescu and compact shear specimens rather than just sampling the shear strain over a small region in the center of the test section. Hence, the shear stress/strain response is obtained by dividing the average shear stress (load divided by the cross-sectional area between the notches) by the average shear strain. By placing gages on both faces of the specimen, accurate and repeatable shear-modulus measurements can be made without prior knowledge of the shear strain or stress distributions. This scheme essentially integrates the shear strain through the entire test section. Knowledge of other material properties is not required to accurately determine shear modulus values. The shear gage was tested on a variety of composite and isotropic materials resulting in more reliable shear modulus determination and less scatter than previously possible.     

An experimental and analytical investigation of the rail shear-test method as applied to composite materials

This report presents the results from an experimental and analytical investigation of the stress distributions occurring in a rail shear test. The effects of nonuniform stresses induced by differential thermal expansion, rail flexibility and specimen aspect ratio on measured shear modulus and ultimate strength of composite laminates are shown. A two-dimensional linearly elastic finite-element model was used to analytically determine how various geometric parameters influenced the magnitude and distribution of inplane normal and shear stresses in a tensile-rail-shear specimen. Rail shear tests were conducted at room temperature and 589 K (600°F) on selected graphite-polyimide composite laminates using two titanium rail configurations. The analysis and test methods are discussed, and the results of the effects of the various parameters on shear modulus and ultimate strength are presented.     

Determination of in-plane and out-of-plane shear moduli of composite materials

A general closed-form relationship was derived between torque and angle of twist for a prismatic composite specimen in terms of the geometric parameters and shear properties of the laminae. In the case of unidirectional laminates, relations are expressed in terms of three principal shear moduli, G₁₂, G₂₃ and G₁₃. An experimental method was developed for determining these moduli by measuring surface and edge strains with strain gages. Unidirectional coupons of graphite/epoxy and silicon carbide/glass ceramic were tested in torsion and the three shear moduli were determined in each case.     

Conception of an experimental device for measuring the in-plane shear modulus of film materials

A freely oscillating torsional pendulum was designed and built for measuring the in-plane shear modulus of film materials. To make the work easier for the experimenter, an online computer application was also developed with the help of numeric tools (Fourier and Hillbert transforms) for data acquisition and processing. The experimental part was performed on various materials. Experiments on a brass foil (isotropic material) validated measurements achieved with this experimental device. Experiments on orthotropic materials (base papers with different basis weights) gave prominence to the good reproducibility of measurements and to the ability for using this torsional pendulum. For this kind of material (large basis weight base paper), the in-plane shear modulus seemed independent of the fiber orientation.     

Reexamination of the solder ball shear test for evaluation of the mechanical joint strength

Ball shear tests were investigated in terms of the effects of test parameters, i.e., shear height and shear speed, with an experimental and non-linear finite element analysis for evaluating the solder joint integrity of area array packages. Two representative Pb-free solders were examined in this work: Sn–3.5Ag and Sn–3.5Ag–0.75Cu. The substrate was a common solder mask defined (SMD) type with solder bond pad openings of 460 μm in diameter. The microstructural investigations were carried out using scanning electron microscopy (SEM), and the intermetallic compounds (IMCs) were identified with energy dispersive spectrometry (EDS). Shear tests were conducted with the two varying test parameters. It was observed that increasing shear height at a fixed shear speed has the effect of decreasing shear force for both Sn–3.5Ag and Sn–3.5Ag–0.75Cu solder joints, while the shear force increased with increasing shear speed at fixed shear height. Shear heights that were too high had some negative effects on the test results such as unexpectedly high standard deviation values or shear tip sliding from the solder ball. Low shear height conditions were favorable for screening the type of brittle interfacial fractures or the degraded layers in the interfaces.     

An experimental investigation of composite repair

This study investigates the repair of a glass/vinylester composite material with damage caused by impact loading and bending. The repair technique is based on the “standard procedures” established in a previous study. In addition to the damage due to bending, the repair of composite plates with straight cutting-line damage is also investigated due to its similarity to the bending fracture and good repeatability for evaluation. It is found that two glass reinforcing patches of plain weave—that is, (0, 90)₂—on each side of the specimen can restore the original load-bearing capability of the composite material of concern. The investigation of the cutting-line damage can also be viewed as a study of bond-line angles for composite joining. It is concluded that a bond-line angle greater than 60° can restore the undamaged composite strength. In maintaining a large bond-line angle as well as a large bonding surface, various bond-line configurations are presented. Results from the five joints of V, W, WW, U, and UU shapes further verify this conclusion.     

An evaluation of the arcan specimen for determining the shear moduli of fiber-reinforced composites

Moiré interferometry was used to examine the strain distributions in Arcan-type specimens under shear loading. The compact geometry allowed longitudinal and transverse shear behavior to be considered. The best results for longitudinal moduli were obtained with fibers running from notch to notch. As with the losipescu configuration, strain averaging or correction factors were required for modulus determination in all fiber orientations. None of the bending problems that have been encountered in the losipescu specimen were observed. Some twisting was noticed, but its extent (less than 2 percent) was much less than has been noted in shear testing with losipescu specimens.     

Development of a portable shear test fixture for low modulus porous (foam) materials

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).     

An experimental study of rock compression shear fracture nucleus

This paper presents the research results concerning the microcracking zone near the notch tip in rock by compressive shear fracture specimen. By the white light speckles method, photoelastic coating test and microscopic observation on rock slices, it is has been found that the said microcracking zone is of asymmetric mulberry leaf shape and the rock grains in the zone are broken so as to form many cracks. In addition, a full field displacement pattern is obtained. The tests also demonstrate that the principal fracture plane of the rock in the compressive shear state is to propagate along the boundary plane of the microcracking zone. Finally, this paper gives a shape equation which describes the microcracking zone, of fracture nucleus for the rock under compression and shear.The project Supported by National Natural Science Foundation of ChinaThe engineers involved in this projcet are Shao-Ping Gong, Shan-Yu Zhang, Xiang-Gui Long, Zhi-Jun Li, Cui-Yun Zhou, Chun-Ming Hu, Zhong-Jian Chen and Ru-Qiang Zhang.     

Effects of transverse shear on the nonlinear bending of rectangular plates laminated of bimodular composite materials

This paper investigates the application of Dynamic-Relaxation (DR) method to the problems of nonlinear bending of rectangular plates laminated of bimodular composite materials. The classical lamination theory and a shear deformation theory of layered composite plates, taking account of large rotations (in the von Karman sense) are employed separately to analyze the subject. It has been found here that the estimation of the fictitious densities which control the convergence and numerical stability of nonlinear DR solution considering transverse shear effect still needs to be further investigated. In this paper, a procedure to calculate fictitious densities has been presented; hence the numerical stability of this topic has been ensured. In this paper the main steps of solving the nonlinear bending of bimodular composite laminates by means of DR method are outlined. The numerical results are given for simply supported, two-layer cross-ply rectangular plates made of mildly bimodular material (Boron-Epoxy (B-E)) and highly bimodular materials (Aramid-Rubber (A-R) and Polyester-Rubber (P-R)) under sinusoidally distributed and uniformly distributed transverse loads. The results obtained have been compared with linear results and those obtained for laminates fabricated from conventional composite materials, the elastic moduli of which are identical with the tensile moduli of the bimodular materials. In addition, the effect of transverse shear deformation on the nondimensionalized center deflection has been studied.The main contents of this paper were presented at the International Symposium of Composite Materials and Structures (June 1986, Beijing).The authors thank Prof. Zhou Li for his guidance.