Interlaminar shear test by hybrid sandwich specimen under distributed load

A new method is proposed for the determination of the interlaminar shear strength of composites. The method is particularly pertinent to composites of high interlaminar shear strengths, where the ratio of tensile (compressive) strength to shear strength is relatively low. In such materials, including unidirectional composites with improved fiber/matrix bond strength and angle-ply laminates, an analysis based on a short beam interlaminar shear test is highly problematic and may, in fact, be erroneous. The test method is based on the use of a sandwich composite structure with a core made of layers of the tested composite and skins made of an elastic, strong unidirectional composite. A proper design procedure determines the choice of the skin material and of the relative thicknesses, so that flexural testing under distributed load leads to the intended core failure in shear. Calculations of the stress profile in a hybrid sandwich beam in bending and of the stress ratios under distributed load are presented. Also presented are experimental results recorded with sandwich hybrids made of unidirectional carbon-fiber-reinforced epoxy skins and a ±θ aramid-fiber-reinforced epoxy angle-ply core.     

Short‐beam and three‐point‐bending tests for the study of shear and flexural properties in unidirectional‐fiber‐reinforced epoxy composites

The bending properties of composite materials are often characterized with simply supported beams under concentrated loads. The results from such tests are commonly based on homogeneous beam equations. For laminated materials, however, these formulas must be modified to account for the stacking sequence of the individual plies. The horizontal shear test with a short‐beam specimen in three‐point bending appears suitable as a general method of evaluation for the shear properties in fiber‐reinforced composites because of its simplicity. In the experimental part of this work, the shear strength of unidirectional‐glass‐fiber‐reinforced epoxy resin composites was determined in different fiber directions with the short‐beam three‐point‐bending test. Also, the elastic constants and flexural properties of the same materials were determined from bending experiments carried out on specimens in the 0, 15, 30, 45, 60, 75, and 90° fiber directions with high span–thickness ratios. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 63–74, 2004     

Characterization of the interlaminar shear fatigue properties of SMC R-50

The interlaminar shear fatigue behavior of the sheet molding compound SMC-R50 has been studied. A thick-laminate, short beam shear test was employed to characterize S-N behavior for the material at 21 and 90°C. The shear modulus (G_xy) was determined at 21 and 90°C and the effect of fatigue on modulus at both test temperatures is discussed. Scanning electron microscope (SEM) and optical photomicrographs of pristine and post test specimens were studied to assess the relationship between material microstructure and the observed fatigue results for strength and modulus. The experimental evidence suggests that the fatigue life for this material is determined by a single flaw growth mechanism, rather than a global “wear out” process.     

Characterization of Interlaminar Shear Strength of Ceramic Matrix Composites

The interlaminar shear strengths of three ceramic matrix composites have been characterized using a double-notch shear (DNS) test. The material systems investigated are plain woven C/SiC, plain woven SiC/SiC, and cross-plied SiC/calcium aluminosilicate-II. The use of the double-notch shear test for measuring the interlaminar shear strength of ceramic matrix composites is evaluated first. Numerical stress analyses are performed to investigate the effect of DNS specimen length, notch distance, and specimen supporting jig on the stress distribution in the expected fracture plane and the interlaminar shear strength. The numerical findings are then compared with an analytical model proposed elsewhere and correlated with the experimental results. The validity of this test technique has been established.     

Physical and mechanical properties of pultruded composites containing fillers and low profile additives

This article deals with the effect of fillers and additives content on the physical and mechanical properties of unidirectional pultruded glass/polyester composites. The physical characterization consisted of determining the void volume fraction (Vv), density, shrinkage ratio, coefficient of thermal expansion (CTE), and dynamic mechanical properties. The mechanical tests consisted of three‐point‐bending tests under static, impact, and fatigue loading. The low profile additive (LPA) has been found to compensate the cure shrinkage by microvoid formation. Dynamic mechanical analysis measurements show that the LPA slightly lowers the glass transition temperature Tg and increases the internal damping Tan δ. The transverse coefficient of thermal expansion was found to be sensitive to the LPA content. Three‐point bending tests show that the interlaminar shear strength (ILSS) is slightly sensitive to the fillers and LPA content, but the flexural strength is not affected. Impact test results on short beam shear and flexural specimens show the same behavior as in the static tests except that the LPA content has a detrimental effect on the flexural impact properties. The fatigue tests performed show that the effect of fillers is not significant, while the LPA effect is mixed. It seems that there is an LPA content for which the fatigue resistance is maximized. Finally, the wide range of behaviors and span‐to‐depth ratios investigated suggest that the ILSS as measured according to the ASTM 2344 recommendations can be largely misleading because of the unavoidable compressive yielding under the loading nose. For the materials investigated, higher span‐to‐depth ratio are more representative of the ILSS. POLYM. COMPOS., 27:71–81, 2006. © 2005 Society of Plastics Engineers     

Bend Strength versus Tensile Strength of Fiber-Reinforced Ceramics

The bending strength of fiber-reinforced glasses and ceramics is often observed to be higher than their tensile strength; the difference varies, however, from one material to another. To gain an understanding of the relationship between these two measure of strength, we have carried out an analysis of bending which accounts for the deviations from linearity that occur on the tensile side of the beam. The results of this analysis indicate that the strength ratio (bending strength/tensile strength) depends most sensitively on the rate at which the stress drops after the ultimate tensile strength. In particular, composites failing gracefully (with a gradual decay in stress) tend to have comparatively higher strengths in bending. A method of inferring the: tensile strength from simply the load-deflection curve in bending is proposed. In addition, by accounting for the weakness in interlaminar shear, we can predict the variation in bend strength with beam aspect ratio. The various theories are compared with experimental data.     

Development and experimental validation of explicit dynamics simulation of composite structures using a stacked thick-shell methodology

The stacked thick-shell modelling approach is investigated in the frame of explicit dynamics FE method for the simulation of composite structures. The methodology is developed for static and dynamic loading conditions and demonstrated in the case of three-point bending of laminated strips. For the validation of the stacked thick-shell modelling approach, experimental testing using laminated short beam shear coupons of the AS4/8552 composite material system is performed and the interlaminar shear strength under impact loading is determined. The specimen dynamic tests were performed using a drop tower apparatus and a specially designed three-point loading fixture. In parallel, conventional three-dimensional solid models are also analysed for comparison purposes. Test results correlate well to the respective numerical predictions, demonstrating the accuracy of the stacked thick-shell approach and the efficiency it provides in interlaminar stresses prediction, which makes the proposed approach suitable for large-scale composite structures simulation, with emphasis in delamination damage propagation.     

The influence of velocity on the impact strength of glass reinforced polypropylene

The influence of impact velocity, between 1 and 8.7 meters per second (m/s) (2.2 to 19.5 mph), on the impact behavior of polypropylene, reinforced with 20 volume percent continuous glass fibers, was investigated in a 3-point bend test at 21 °C. The ratio of specimen span to thickness, which has a profound effect on the observed results, was varied between 5.3 and 26. An attempt to apply simple beam theory for the analysis of the initial specimen response to the high loading rate was successful, except for the lower than expected values of shear modulus. The stress to break and the tensile and shear moduli were found to increase along with velocity. The dependence of impact energy on velocity was observed to be affected by the span to thickness ratio: a positive dependency was observed at low ratios and none at high ratios. This is different from the negative dependency reported for polypropylene reinforced with short fibers, and is attributed to the influence of the continuous glass fibers on the impact behavior of the composite.     

Interlaminar shear stress distribution between nested layers of plain weave composites

During the manufacture of woven fabric composites, fabric layup and lateral compression may cause fabric layers to nest with each other and compact altering the mechanical behavior of the composite. In this work, nesting and layer compaction were geometrically defined and their mechanistic impact on the interlaminar shear distribution was analyzed. Neighboring layers of plain weaves were geometrically modeled to nest with one another via a relative in‐plane horizontal translation, an out‐of‐plane vertical translation, forcing them into valleys of neighboring layers, and vertical compaction. The normal stress distribution within each layer was mapped and used to obtain the interlaminar shear stress distribution between layers utilizing a strain energy density approach. Four‐point flexural tests were carried out and the average value of the normal stress in different layers at failure was used to evaluate the interlaminar shear strength. It was found that nesting reduces the interlaminar shear stress between layers, while compaction of nested layers reduces the variation in the stress distribution within the layer itself. Failure occurs when the interlaminar shear stress exceeds the shear strength of the matrix material between the layers. POLYM. COMPOS., 31:1838–1845, 2010. © 2010 Society of Plastics Engineers     

Measurement of adhesive shear properties by short beam shear test based on higher order beam theory

This paper refers to the measurement of the shear properties of adhesive bonding by a new beam theory using the short beam shear (SBS) test configuration. A novel higher-order sandwich beam theory has been developed to analyze the adhesive bonded beam that consists of two adhered laminates and a single layer of adhesive in between. The closed form analytical solution for the SBS test model of the adhesively bonded beam is obtained in terms of deflection and stress distribution. The present theory has been used for calculating the adhesive shear modulus from the structural compliance. The initiation of stiffness degradation for the short beam shear test model was used as the critical load value for deriving the adhesive shear strength. A finite element model is built for validating the present model, and to evaluate its suitability for measuring adhesive shear properties. The present theory shows better accuracy for measuring the shear modulus than existing theories for both thin and thick adhesive layers. The measured strength values are more accurate than those obtained from the single lap joint shear test model. This theory can be used for adhesive materials with linear elastic deformation behavior.     

Avoidance of fabricational thermal residual stresses in co-cure bonded metal-composite hybrid structures

In this work, a smart cure cycle with cooling, polymerization and reheating was devised to nearly completely eliminate thermal residual stresses in the bonding layer of the co-cure bonded hybrid structure. In situ dielectrometry cure monitoring, DSC experiments and rheometric measurements were performed to investigate the physical state and the cure kinetics of the neat epoxy resin in the carbon fiber/epoxy composite materials. From the experimental results, an optimal cooling point in the cure cycle was obtained. Also, process parameters such as cooling rate, polymerization temperature and polymerization time in the curing process were investigated. Then, the thermal residual stresses were estimated by measuring the curvatures of co-cure bonded steel/composite strips and their effects on the static lap-shear strengths of co-cure bonded steel/composite lap joints were measured. Also, the effects of thermal residual stresses on the tensile strength, the interlaminar shear strength and the interlaminar fracture toughness of the composite material itself were measured using tensile, short beam shear and double cantilever beam tests. From these results, it was found that the smart cure cycle with cooling, polymerization and reheating eliminated the thermal residual stresses completely and improved the interfacial strength of the co-cure bonded hybrid structures, as well as the tensile strength of the composite structures.     

The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems

The interfacial bond strength in glass fibre-polyester resin composites has been investigated using various experimental techniques. These included blocks of resin containing fibre (in which, depending on the geometry of the specimen, failure occurs in either a shear or tensile mode) the pullout of a fibre from a disc of resin and a short beam shear test for interlaminar shear strength determination.

Low power optical microscopy and optical retardation measurements of stress induced birefringence were used to detect the difference between intact and debonded fibre resin interfaces. The shear modulus and shear strength of the resin were obtained from torsion tests on cylindrical rods of the resin.

The single fibre shear debonding specimen and the short beam shear test are shown to be the most viable test methods but interpretation of the results is complicated by the various modes of failure possible and by the different stress states which exist in the area of the specimen where debonding starts. Stress concentration factors obtained by finite element analysis and photoelastic analysis have been applied to the results from these tests and the corrected interfacial bond strengths are in close agreement.

The real interfacial bond strengths of well bonded glass-fibre polyester resin systems is shown to be of the order of 70 MN m^?2.     

The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems

The interfacial bond strength in glass fibre-polyester resin composites has been investigated using various experimental techniques. These included blocks of resin containing fibre (in which, depending on the geometry of the specimen, failure occurs in either a shear or tensile mode) the pullout of a fibre from a disc of resin and a short beam shear test for interlaminar shear strength determination.

Low power optical microscopy and optical retardation measurements of stress induced birefringence were used to detect the difference between intact and debonded fibre resin interfaces. The shear modulus and shear strength of the resin were obtained from torsion tests on cylindrical rods of the resin.

The single fibre shear debonding specimen and the short beam shear test are shown to be the most viable test methods but interpretation of the results is complicated by the various modes of failure possible and by the different stress states which exist in the area of the specimen where debonding starts. Stress concentration factors obtained by finite element analysis and photoelastic analysis have been applied to the results from these tests and the corrected interfacial bond strengths are in close agreement.

The real interfacial bond strengths of well bonded glass-fibre polyester resin systems is shown to be of the order of 70 MN m^-2.     

Effects of Matrix Porosity on the Mechanical Properties of a Porous-Matrix, All-Oxide Ceramic Composite

The effects of matrix porosity on the mechanical properties of an all-oxide ceramic composite are investigated. The porosity is varied through impregnation and pyrolysis of a ceramic precursor solution. Mechanical tests are performed to assess the role of the matrix in both matrix-dominated and fiber-dominated loading configurations. The results demonstrate a loss in damage tolerance and tensile strength along the fiber direction as the porosity is reduced. Concomitantly, some improvements in interlaminar strength are obtained. The latter improvements are found to be difficult to quantify over the entire porosity range using the standard short beam shear method, a consequence of the increased propensity for tensile fracture as the porosity is reduced. Measurements of interlaminar shear strength based on the double-notched shear specimen are broadly consistent with the limited values obtained by the short beam shear method, although the former exhibit large variability. In addition, effects of precursor segregation during drying on through-thickness gradients in matrix properties and their role in composite performance are identified and discussed. An analysis based on the mechanics of crack deflection and penetration at an interphase boundary is presented and used to draw insights regarding the role of matrix properties in enabling damage tolerance in porous-matrix composites. Deficiencies in the understanding of the mechanisms that enable damage tolerance in this class of composites are discussed.     

Advanced characterization of laminated electrical steel structures under shear loading

ABSTRACT

The thermomechanical and mechanical behavior of electrical steel laminates was investigated by dynamic mechanical analysis (DMA) and a short beam test. These tests were implemented to evaluate the effect of various steel alloys on the adhesion properties of epoxy. DMA showed a dependency of curing state affected by the thermal conductivity of the steel alloy. Strain evaluation by digital image correlation showed that interlaminar shear failure occurred in a predominately cohesive manner in the epoxy layer at a shear angle of about 5°. This critical angle was not affected by the yield strength of the steel alloy used to create the laminate.     

Determination of core shear properties of three-dimensional woven sandwich composites

Abstract

The core shear characteristics of a relatively new material have been investigated by two methods: three point bending tests and a shear test. The material is based on woven sandwich fabric preform, which offers important advantages related to the integral core–skin structure. The core shear moduli results from the block shear test and ASTM three point bending tests show very little difference. The results from the shear test obtained by measuring the crosshead displacement were verified by conducting additional shear tests, in which two linear variable differential transformers were used for strain measurement in the longitudinal and transverse directions. The ASTM block shear test and three point bending test were used to determine the core shear strength of the material. To obtain pure core shear failure leads to a correct core shear strength result; the three point bending test was modified by increasing the skin thickness and stiffness of the panel.     

Experimental and numerical investigations of the interlaminar shear properties of carbon/carbon composites

Experimental tests and numerical simulations were implemented to investigate the interlaminar shear properties of carbon/carbon composites (C/Cs). A unit‐cell model, according to the microstructure of the C/Cs, was used to predict material properties of the C/Cs. A three‐dimensional finite element model was established to investigate the damage behavior of C/Cs on the basis of Linde failure criterion and damage evolution. Good agreement, in terms of the load force history and failure modes, was observed between the experimental and numerical results; this provided the applicability of the numerical simulation. The test results show that the interlaminar shear strength of the C/Cs was 10.52 MPa and the value of the simulation result was 10.89 MPa, with the relative error being less than 4%. Damage contours and stress distribution analysis of the simulation results are discussed. Fiber damage occurred at the bottom of the specimen, and matrix damage was found in the upper half of the specimen; this was similar to the appearance of the tested specimens. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44783.     

The effect of curing additive on the mechanical properties of phthalonitrile-carbon fiber composites

Unidirectional carbon fiber-reinforced phthalonitrile composite panels were fabricated by prepreg consolidation with bis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS) as the phthalonitrile curing additive. Rheometric measurements and elevated-temperature, short beam shear tests were used to evaluate the cure of the composite as a function of the cure and postcure conditions. These techniques revealed that a fully cured phthalonitrile composite was obtained when the composite was heated at 375°C for 8 hours. Room-temperature mechanical properties of the cured composite were then evaluated using short beam shear, tension, and flexural tests. The results are compared with those obtained by curing the phthalonitrile with 1,3-bis(3-aminophenoxy)benzene (m-APB). The data indicate that substitution of p-BAPS for m-APB has little effect on the mechanical properties of the cured composite. Elevated-temperature, short beam shear studies up to 371°C show that the cured phthalonitrile composite retains approximately 70% of its room-temperature apparent interlaminar shear strength. The composite also retains 70% of its room-temperature storage modulus up to 450°C. Polym. Compos. 25:554–561, 2004. © 2004 Society of Plastics Engineers.     

Comparison of the Adhesive Shear Modulus in Bulk and Bonded States

A method is proposed for determining the in situ shear modulus of a structural adhesive from a sandwich beam loaded in 3-point bending in which the adhesive is contained as a thin layer. Expressions for calculating the elastic shear modulus of the adhesive layer from compliance data on the beam are derived, and experimental tests to validate the theory are conducted. To verify the test results, tensile tests are also conducted, and the shear modulus for bulk adhesive is determined using the constitutive equation for an isotropic material relating tensile modulus and Poisson's ratio to shear modulus.

However, the bulk shear modulus as traditionally determined from a tensile test was up to an order of magnitude greater than the in situ shear modulus obtained from the 3-point bend test. A finite element simulation and sensitivity study replicated the experimental results of the 3-point bend tests, and showed that using the shear modulus obtained from the tensile tests would result in significant errors in predicting material and joint behavior. In addition, torsion tests were conducted on bonded cylinders to measure directly the shear modulus. The shear modulus from the torsion test was in agreement with the in situ modulus obtained from the 3-point bend test. This combined experimental-computational approach validated the 3-point bend test as a means to determine the in situ adhesive shear modulus. Finally, micrographs of the interface of the 3-point bend specimen indicated that adhesion occurred by the extension of adhesive pillars to the surface of the adherends. This pillaring phenomenon may have resulted in a lack of bonding along significant portions of the interface, and may explain the compliance of the 3-point bend specimens and, subsequently, the lower shear modulus. The repeatability of the experiments and the substantiation of the results of the experiments by finite element analysis suggest that this pillaring phenomenon may be a mechanism of adhesion.     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.