Short Beam Specimens Research Articles

Carbon fiber-reinforced epoxy filament-wound composite laminates exposed to hygrothermal conditioning

This study focuses on the evaluation of the effect of hygrothermal conditioning on tensile, compressive, in-plane and interlaminar shear properties, and also on the viscoelastic characteristics of carbon fiber/epoxy laminates. Flat unidirectional laminates were manufactured by dry filament winding and cured under hot compression. The laminates were later exposed to hygrothermal conditioning in a chamber, following the recommendations of ASTM D5229M. All composite coupons were tested before and after conditioning. An analytical Fickian model was used to fit experimental data, showing very good estimates. Shear strength and modulus reduced to about 30 and 38 %, respectively. All specimens presented acceptable failure modes; shear specimens failed at the gage section with delaminations and fiber/matrix debonding, whereas short beam specimens failed via delaminations at the specimen mid-plane. Moisture penetration through the carbon/epoxy surface lead to interfacial debonding and matrix plasticization. Puck’s failure envelope accurately predicted failure under compressive and shear loading.

Characterization of stress–strain behavior of composites using digital image correlation and finite element analysis

This work presents a cost–effective method taking advantage of the full-field measurement capability of digital image correlation (DIC) for a simultaneous assessment of multiple stress–strain relations for fiber-reinforced composites in a principal material plane. The method is based on combining a finite element model based stress calculation and a full-field surface strain measurement in a custom short-beam shear (SBS) test. The unknown constitutive properties are determined through minimization of the square difference between the DIC-measured and the FEM-calculated strains. The robustness of the proposed method has been evaluated using different initial approximations of material constitutive parameters. Explicit derivation and evaluation of the sensitive matrix are not required in the method. Due to geometric nature of stress distribution in the unidirectional short-beam specimen loaded in a principal material plane, the optimization procedure converges rapidly, and is not sensitive to the formulation and parameters in the initial approximation of the constitutive model.

A method for measurement of multiple constitutive properties for composite materials

Accurate stress–strain constitutive properties are essential for understanding the complex deformation and failure mechanisms for materials with highly anisotropic mechanical properties. Among such materials, glass-fiber- and carbon-fiber-reinforced polymer–matrix composites play a critical role in advanced structural designs. The large number of different methods and specimen types currently required to generate three-dimensional allowables for structural design slows down the material characterization. Also, some of the material constitutive properties are never measured due to the prohibitive cost of the specimens needed. This work shows that simple short-beam shear (SBS) specimens are well-suited for measurement of multiple constitutive properties for composite materials and that can enable a major shift toward accurate material characterization. The material characterization is based on the digital image correlation (DIC) full-field deformation measurement. Two key elements show advantage of using DIC in the SBS tests. First, tensile, compressive, and shear stress–strain relations are measured in a single experiment. Second, a counter-intuitive feasibility of closed-form stress and modulus models, normally applicable to long beams, is demonstrated for short-beam specimens. The modulus and stress–strain data are presented for glass/epoxy and carbon/epoxy material systems.

Computer Modelling of Dynamic Fracture Experiments

In this work the time-domain boundary element method (BEM) is applied to simulate dynamic fracture experiments. The fast fracture is modelled by adding new boundary elements at the crack tip. The direction of crack growth is perpendicular to the direction of maximum circumferencial stress. The time dependent loading of specimens and velocities of crack growth are taken from experiments as input data for computer simulations. The method is used to analyze: a short beam specimen, a special mixed-mode specimen and a three-point bend specimen subjected to impact loads. The dynamic stress intensity factors (DSIF) and the crack paths are compared with the results obtained by other authors who used the finite element method (FEM) and experimental methods.

3420 切り欠き付ショートビーム試験によるFRPの層間破壊靱性の検討(S15-2 複合材料,S15 先端材料システムの強度・機能評価とメゾメカニックス)

This paper describes about interlaminar fracture toughness of FRP. Mode II interlaminar fracture toughness by 4ENF test is compared with interlaminar shear strength by DNT test and Mode II interlaminar fracture toughness by Notched Short Beam test. In addition, fracture surface of DNT specimen and Notched Short Beam specimen is observed by the scanning electron microscope (SEM). It seems that there is no relationship between Mode II interlaminar fracture toughness by 4ENF and interlaminar shear strength by DNT. But there is a good relation between 4ENF test and Notched Short Beam test.

Special virtual fields for the direct determination of material parameters with the virtual fields method. 2––Application to in-plane properties

This paper deals with the direct identification of mechanical parameters that govern the in-plane constitutive law of orthotropic materials. Those parameters are extracted from heterogeneous strain fields that occur in a short beam specimen tested in a Iosipescu fixture. The procedure used is the virtual fields method with special virtual fields. The case of linear elasticity is first addressed. It is shown that the parameters are directly extracted with this method: no iterative calculations are required. The stability is also discussed in different cases. A non-linear shear response is then considered. The parameter that governs this non-linearity is also directly identified with the special virtual fields.

Nondestructive evaluation of thermal stress-induced damage in thin composite laminates

Nondestructive evaluation of thermal stress-induced failure in the free edge region of thin composite laminates (1 mm thick) has been carried out. The damage zone which arose under a cryogenic cooling was examined using the ultrasonic C-scan and optical microscopy. Characteristic behavior of thermo-acoustic emission (AE) detected from the composite specimen during the heating and cooling cycle indicated the degree of damage which had occurred differently for as-moulded and cryogenically-treated specimens with various lay-up angles. Analysis of the vibration spectrum data obtained with short beam specimens exhibited that the failure occurrence caused an obvious decrease in resonant frequency and some considerable increase in flexural damping ratio. Total AE energy showed a feature similar to the flexural damping ratio which increased with increasing crack density. Consequently, it is thought that thermo-acoustic emission in association with flexural damping might be employed as a tool for nondestructive evaluation of thin composite laminates.

ＣＦＲＰ積層材の静的横せん断特性

The first experiment for obtaining the static transverse shear modulus and failure strain of CFRP Laminates is presented in this paper. In order to obtain these values, the difference in vertical displacement of two points on the neutral axis of a short beam specimen is measured by a newly devised electro-optical extensometer. The transverse isotropy is first confirmed by our proposed method. Furthermore, the experimental results of transverse shear modulus and failure strain are compared with those based on a classical lamination theory and a three dimensional FEM code, respectively.

Effects of grain size and specimen geometry on the transformation and R-curve behaviour of 9Ce-TZP ceramics

Transformation and R-curve behaviour have been investigated in 9 mol% Ce-TZP ceramics with different grain sizes. Both single-edge notched beam and short double-cantilever beam specimens were tested to measure the crack-resistance curves. The size and shape of the transformation zone not only depend on grain size, but are also strongly influenced by the specimen geometry. This different transformation behaviour has led to different crack-resistance curves. These experimental results are discussed in terms of the thermodynamics of transformation, the effect of autocatalytic transformation, and fracture mechanics.

Improved quality acceptance procedure for interlaminar shear strength

An efficient and cost-effective test method has been developed for the interlaminar shear strength determination of fibre-reinforced plastics. The new technique is based on the use of a continuous beam specimen instead of the conventional, small, short beam configuration. Excellent correlation between the continuous and the short beam specimens has been obtained for various composite systems.

A finite element study of the short beam test specimen under mode II loading

The finite element technique simulating in-plane shear mode, Mode II, has been used to analyse fracture behaviour in a short shear beam specimen in plain concrete. The analysis incorporates the influence of fracture behaviour resulting from variation in the position of pre-cut slots in the short shear beam specimens. The values predicted by the simulation procedure are found to be in close agreement with those obtained from experiment. The fracture toughness values, K IIC determined for these shear beam specimens using the compliance method are found to be lower than those obtained from the split-cube test which corresponds to tensile mode, Mode I.

Fatigue strength and failure mechanisms in uniaxial carbon fibre reinforced epoxy resin composite systems

Studies on configurations of carbon fibre composites test-pieces, which were expected to display the importance of shear effects on the fatigue failure of the composites, are described. A 3-point loading system was used on standard long-beam flexure and short beam specimens, and a specially constructed, 4-point loading, pure bending rig was used for reversed bending tests. The results show that the severity of the effect of cyclic stresses below the static failure stress value is greatest under conditions where the shear stress parallel to the fibre axis is relatively high, or where a significant interlaminar weakness exists.