Off-axis Tensile Loading Research Articles

In-situ characteristic elastic properties of bond-lines in laminated engineered wood products using microscopic digital image correlation

Adhesively laminated engineered wood products (EWPs), such as glue-laminated timber (GLT) and cross-laminated timber (CLT), have gained global popularity due to the increasing development of tall wood buildings. The mechanical performance of a bond-lines is crucial for the structural behavior of EWPs. In this study, a novel method was developed for evaluating in-situ elastic properties of a bond-line in adhesively laminated solid wood products. This method deployed the microscopic digital image correlation (DIC) technique to capture the strain distribution along the bond-line under axial (0°) and off-axis (45°) tensile loading. Three types of adhesives, namely polyurethane (PUR), emulsion polymer isocyanate (EPI), and phenol–resorcinol–formaldehyde (PRF), were used to make specimens. The modulus of elasticity (E) and modulus of shear (G) of a bond-line were accordingly calculated based on the strain along a bond-line and load applied on a specimen. For verification purpose, finite element (FEA) models on CLT and GLT beams were, considering the elastic properties of bond-lines and disregarding the elastic properties, developed to evaluate the effects of the elastic properties of a bond-line on the stress distribution. It was found that the proposed method could be used to accurately determine the in-situ characteristic elastic properties of bond-lines in EWPs. The elastic properties of a bond-line exhibited a large variability, but did not appear to have a significant impact on the overall stress distribution in a specimen. The E and G of bond-lines might be neglected in the structural analysis of large-sized laminated wood products such as GLT and CLT.

Experimental Investigation of Mechanical Properties and Failure Mechanisms in Jute-Polyester Composite Laminates under Off-Axis Tensile Loading

Abstract This study focuses on the experimental investigation of the mechanical properties and failure mechanisms of jute-polyester composite laminates under off-axis tensile loading. Various off-axis tests were conducted to analyze the effects of different fiber orientations (0°, 15°, 30°, 45°, 60°, 75°, and 90°) with respect to the load direction. The experimental results revealed a decrease in both off-axis strength and elastic moduli as the off-axis angle increased. Notably, the 45° off-axis tests exhibited the lowest tensile strength and modulus, indicating a more ductile behavior with the highest failure strain. The tensile stress-strain relationship exhibited a nonlinear behavior during off-axis tension loading. Failure analysis was performed using the Tsai-Wu criteria, considering various values of the interaction coefficient F12. The tensile strength and modulus predicted by the Tsai-Wu criteria demonstrated a good correlation with the experimental results. Consequently, the Tsai-Wu criterion proves effective for analyzing the failure properties of woven fabric composites under multiaxial stress conditions.

Prediction Ability Analysis of Phenomenological Strength Criteria for Composites

The article examines and assesses the phenomenological strength theory of composite materials. A comparative analysis of the theoretical envelopes was conducted for each criterion. A unified form of the phenomenological strength criterion was established. The study specifically examined the effects of altering the interaction parameter on the Tsai-Wu criterion’s theoretical envelope. Based on the available experimental data, the study plotted the failure envelopes of each strength criterion under planar composite stress states. The variation of these envelopes across various stress quadrants was highlighted. As a result of the examinations, four typical phenomenological strength criteria were chosen. The composites’ off-axis tensile and biaxial loading test data were used to evaluate the predictive power objectively. The results showed that not all stress states’ test results agreed with the predictions of the phenomenological strength theory. The criterion proposed by Norris and Tsai-Hill performed better at accounting for the material’s different compressive and tensile characteristics. The other criteria tended to be conservative under particular circumstances. Simultaneously, the Hoffman criterion matched the test data more closely over a broader range of stress states. Overall, this study clarified the limitations and applicability of various strength criteria in composite material strength prediction.

Finite-volume micromechanics-based multiscale analysis of composite structural model accounting for elastoplastic and ductile damage mechanisms

A hierarchical homogenization framework is proposed for nonlinear elastic-plastic and damage simulation of a unidirectional composite laminate. Proper scale transition rules between the macro- and the micro-scale variables were introduced, wherein the finite-element and finite-volume methods are employed for predicting the structural and material point response, respectively. Specifically, at the macroscopic scale, the ABAQUS/CAE structural analysis provides macroscopic strains for the lower-scale unit cell simulation (top-down) at each load increment. Reversely, at the microscopic scale, the unit cell problem is modelled using extended parametric finite-volume direct averaging micromechanics (FVDAM) through a Meta-UMAT subroutine that links the global finite element solver and the constitutive laws (UMAT subroutines). Homogenization of stresses and effective stiffness tensor obtained from the unit cell predictions is passed back to the ABAQUS/CAE for the higher-scale finite-element structural model computation (bottom-up). The stresses and strains at the global and local scales can be tracked during the incremental loading on the macroscopic structures through continuous homogenization and localization. The predictive capabilities of the proposed technique are verified vis-à-vis the experimental response of the Boron/Aluminum (B/Al) laminate obtained from off-axis tensile and shear loading available in the literature.

A consistent formulation for yarn re-orientation in shear and off-axis loading of woven composites RVEs

The mechanical response of woven composites under off-axis tensile loading is strongly affected by the damage mechanism in the yarn-yarn and yarn-matrix interfaces. Due to damage in these interfaces, the yarns will be able to rotate and reorient. For plain weave and satin weaving schemes, the yarns will reorient towards the loading direction in off-axis tensile loading, resulting an increase in the apparent stiffness. The material model used to simulate the reinforcing yarns needs to handle this property as the material axes corresponding to the fiber direction should be updated during loading to reach realistic results. The key ingredient of a material model required to achieve this is a fiber frame stress rate description (Badel et al., 2008), which will be compared with a hyperelastic model (Bonet and Burton, 1998). The definition of the fiber frame is reformulated to properly conform to woven composites and the implementation of the model is extended for an implicit solver. Simulations are performed in ABAQUS/Standard with models implemented in UMAT routines. The results are compared with the standard ABAQUS implementation and are shown to retrieve a realistic stiffness evolution induced by fiber reorientations.

Effect of off-axis angle on mesoscale deformation and failure behavior of plain-woven C/SiC composites with digital image correlation

In this study, the deformation response and failure behavior of a plain-woven C/SiC composite were investigated under on-axis and off-axis tensile loading. Digital image correlation (DIC) was utilized to characterize the full-field deformation and mesoscale strain distribution. The test results indicate a strong influence of the woven architecture on the mechanical properties and strain distribution, and the materials exhibit failure modes dependent on the loading directions or off-axis angles: the fracture positions of different layers are the same under off-axial load, while for on-axil loading, the fracture positions of different layers do not affect each other. SEM results provide direct evidence that the width of the off-axis specimen has a great influence on the mechanical properties. The reduction of the modulus and strength of off-axis specimen, is not only due to the off-axis loading, but also due to the reduction of effective bearing area or effective bearing fiber.

Characterisation of off-axis tensile behaviour and mesoscale deformation of woven carbon-fibre/PEEK using digital image correlation and X-ray computed tomography

The architectural intricacy of multi-ply woven composites, caused by differences in layer alignment, introduces complexity into their mesoscale response to on- and off-axis tensile loading. To better understand the relationship between architecture and deformation response, this work combines digital image correlation (DIC) and 3D micro-computed tomography ( μ CT) to examine deformation for 0°, 15°, 30° and 45° specimens. Tensile tests were conducted with a four-layer plain woven carbon/polyetheretherketone (C/PEEK) laminate. In addition to the evident variance of mechanical properties between orientations, significant mesoscale differences in topography and strain were observed resulting from the layer alignment of individual specimens. Alignment of the top two surface layers induced distinct topographical peaks under extension, whereas nested architectures formed continuous topographical ridges. Matching observed surface strains around microstructural cracks identified in μ CT images revealed both inter-tow and intra-tow shearing during off-axis extension and corresponding fibre reorientation, which effectively illustrates the potential benefits from combining DIC and μ CT. • X-ray computed tomography and digital image correlation investigation of woven C/PEEK. • Influence of architectural layer alignment on surface topography/strain revealed. • Mesoscale off-axis deformation mechanisms and damage characteristics ascertained. • Findings provide means to link future surface DIC results to probable internal structure.

Non-contact strain monitoring of carbon fiber composites using spatial frequency domain eddy current imaging data

A new eddy current testing method for non-contact strain monitoring of continuous carbon fiber-reinforced plastics (CFRPs) was developed. The strain of a CFRP was estimated from a slight change of the fiber orientation measured by eddy current testing. A bidirectional CFRP laminate under off-axis tensile load was scanned by an eddy current probe to obtain the signal change caused by material inhomogeneity. Two-dimensional Fourier transform was applied to the probe signal distribution to identify the fiber orientation of the tested CFRP. The relationship between the fiber orientation and strain was analytically derived using a simple deformation analysis and the classical lamination theory of fiber composites. The strain development predicted by the eddy current method agreed well with that measured by a strain gauge. This method enables non-contact strain monitoring without surface treatment to the tested sample.

An elastic-plastic constitutive model for thermal shocked oxide/oxide ceramic-matrix composites

The oxide/oxide ceramic-matrix composites exhibits superior mechanical properties and complex nonlinear behavior in a high-temperature environment. This paper developed a constitutive model utilising an elastoplastic modeling framework to represent the nonlinear mechanical response of the thermal shocked ox/ox-CMCs under off-axis tensile loading accurately. The model captures main features such as the effects of the thermal shock temperatures on material response. The main interest of the proposed model is the ability to predict the nonlinear mechanical response for thermal shocked ox/ox-CMCs, which is of importance for industrial applications. The validity of the constitutive model is verified by analysing the mechanical behavior of an open-hole specimen and comparing the simulation result with the experimental data.

Characterization and modeling of the creep behavior of fiber composites with tension and compression asymmetry

This paper seeks to present an integrated experimental-theoretical-numerical approach to accurately model the asymmetric tension/compression creep behavior of IM600/Q133 composites. The nonlinear deformation behavior of composites depends on loading time, loading mode, and fiber orientation, and does not satisfy the Boltzmann superposition principle. Therefore, an elastic-viscoplastic constitutive model was developed to characterize this behavior based on an improved one-parameter plasticity model. In addition, the plastic potential function was extended to describe the plastic flow and time-dependent behavior of unidirectional fiber composites with tensile and compressive asymmetry. The proposed model was implemented using the finite element analysis (FEA) in the ABAQUS software via a user-defined subroutine (user material or UMAT). The model was validated against experimental creep curves for unidirectional and angle-ply laminates under off-axis tensile and compressive loading. Finally, the results were discussed to illustrate the applicability of the proposed method.

A non-local stress fracture criterion accounting for the anisotropy of the fracture toughness

A comparative assessment between the maximum circumferential stress criterion and the non-local stress criterion applied to mixed mode fracture problems in wood is presented. Various attributes of the two fracture theories, such as the shear stress component acting on the critical plane, the non-singular term of the crack tip stress field and the angular distribution of the fracture toughness, are investigated for their impact on the mixed mode fracture behavior of wood. Predictions of the direction of crack propagation and the fracture locus are validated against experimental data available in the literature, with emphasis on results obtained from single edge-notched wood specimens subjected to on-axis biaxial loading and off-axis tensile and off-axis shear loading.

Tensile Initial Damage and Final Failure Behaviors of Glass Plain-weave Fabric Composites in On- and Off-axis Directions

Mechanical properties and failure behavior of glass plain-weave fabric composites under on-axis (i.e. 0 and 90° direction) and off-axis (i.e. θ=5, 10, 15, 30, 45 and 60°) tensile loading conditions have been experimentally investigated. Acoustic emissions (AE) monitoring and step-by-step optical observation with interrupted tension tests were adopted to study the character and failure mechanisms of various fiber orientations. The experimental results indicate that both off-axis elastic modulus and strength degrade with increasing off-axis angle in all cases. There is a good agreement for the axial modulus Ex between the experimental results and theory results obtained from orthotropic linear elasticity theory. Adoption of the principal shear strength S45°=28.5 MPa which is evaluated using the tensile strength based on the 45° specimen allows better predictions of the off-axis tensile strengths by means of the Tsai-Hill failure criterion. Off-axis specimens exhibit lower knee point stress than on-axis ones and initial fracture strain monitored by AE (>75 dB) has a good fit to knee-point strain in this study. Tensile strength exhibits an increased tendency toward the delay of initial fracture behavior.

The effect of off-axis angles on the mesoscale deformation response and failure behavior of an orthotropic textile carbon-epoxy composite

The deformation response and failure behavior of an orthotropic textile carbon-epoxy composite were investigated under off-axis tensile loading. Digital image correlation (DIC) was utilized to effectively capture the full-field and mesoscale strain distribution. The macroscale mechanical performance was strongly sensitive to the fiber bundles orientation relative to the loading direction. Based on the experimental data, a quantitative relation between the rotation angle and off-axis angle was established, and a negative correlation between the failure strength and the rotation angle was observed. The underlying failure mechanisms of the specimens with different off-axis orientations were analyzed using scanning electron microscopy (SEM) and DIC techniques. The load-bearing mechanisms were different between the on- and off-axis cases. High local shear strain eventually resulted in the brushy shear-type fracture in the off-axis case, and the local micro cracks developed during the loading caused the reduction and imbalance of local load-bearing capacity.

Tensile Creep Behavior of Quasi-Unidirectional E-Glass Fabric Reinforced Polypropylene Composite.

The present work addressed the creep behavior of quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading. A series of creep tests were performed on the composite at three different loading stress levels. The creep response of off-axis samples of quasi-unidirectional composites under a constant loading level can be clearly observed. A phenomenological viscoplasticity model was built for describing the creep behavior of the composite. To improve the accuracy of prediction, cyclic loading-unloading tests were adopted to determine the material constants in the model. The predicted results in terms of the strains after a load over a period of time were found to be satisfactory, compared with the experimental results. In addition, same failure mechanism was found in off-axis samples under quasi-static and creep loading cases.

Experimental research on the mechanism of non-monotonic characteristic between curing temperature and mechanical behaviors of Kevlar/epoxy composite

An experimental procedure is conducted to investigate the mechanism of the non-monotonic characteristic between curing temperature and mechanical behaviors of the Kevlar/epoxy composite in macro and micro levels. Different specimens are fabricated at four different curing temperatures and tested with ±45° off-axis tensile loading on a universal test machine coupled with digital image correlation (DIC). Moreover, the environmental scanning electron microscope (SEM) was used to obtain the micrographs and reveal their mechanism. The tested results show that the tensile mechanical behaviors are sensitive to the curing temperature and the relationship is non-monotonic. Also, as the temperature increases, the thicknesses of the specimens are significantly enlarged. By analyzing the SEM micrographs of the matrix grooves in the damage zone and DIC strain contours, it is concluded that the non-monotonic relationship is dominated by the properties of the Kevlar/epoxy interfaces and deformation of the distorted fibers.

Strain rate-dependent mechanical behavior of quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading

To make a better use of quasi-unidirectional fabrics in structural application, the strain rate-dependent tensile behavior of quasi-unidirectional E-glass fabric reinforced polypropylene composites was studied. Monotonic tensile tests were firstly performed on 10°, 20° and 90° off-axis samples of composites. The experimental results show that both mechanical properties and nonlinear stress-strain behavior of composites are dependent of strain rate. A regression function is adopted to describe the variation in overall mechanical properties with the imposed strain rate. A strain rate-dependent coupled damage-plasticity model is built for quasi-unidirectional composites. The damage variables and plastic parameters in the proposed model are determined through cyclic loading-unloading tensile tests and can be represented as a function of the imposed strain rate. In addition, the damage mechanism of composites is observed to evolve with strain rate. Increasing strain rate can hinder the initiation and evolution of damage in composites.

Temperature-dependent response of quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading

Abstract The temperature-dependent off-axis tensile behavior of quasi-unidirectional E-glass fabric reinforced polypropylene composites has been investigated both experimentally and constitutively in this paper. The nonlinearity in composites under off-axis loading is shown to be a combination of damage and plasticity regardless of temperature. A nonlinear constitutive model is established to predict the temperature-dependent nonlinear stress-strain curves of composites, in which the accumulation of plastic strain and evolution of damage in composites are described by the plasticity and damage theories, respectively. A full suite of temperature-dependent plastic and damage parameters in the model are determined through cyclic loading-unloading tests.

A multiscale experimental approach for correlating global and local deformation response in woven composites

A multiscale experimental approach is introduced that facilitates correlating globally applied stress to the local material response at yarn scale. Using a high-resolution optical digital image correlation, low and high strain zones in off-axis woven composite samples loaded in uniaxial tension are identified and the corresponding full-field strains are quantified. The local strain response quantified over low and high strain domains are used along with the continuum scale deformation to establish a correlation between global and local deformation behavior as a function of fiber orientation. In addition, global stress-local strain curves are extracted for low and high strain domains at different fiber orientation angles. A simple plasticity model in conjunction with the obtained stress-strain curves is implemented and utilized to develop a phenomenological model that enables correlating global stress with the locally developed strain at different locations in a composite specimen subjected to off-axis tensile load.

Characterizing the off-axis dependence of failure mechanism in notched fiber metal laminates

Fiber metal laminates (FMLs) are usually more complicated in the failure aspect as compared to their constituents, especially when complex load is applied. In this paper, notched FMLs and the constituents, i.e., the aluminum and glass fiber reinforced plastic (GFRP) were subjected to off-axis tensile loading to evaluate the failure mechanism. Initial attention was paid on the off-axis dependence of mechanical responses, especially on its relation to the notch size. As expected, off-axis dependence of notched strength and notch sensitivity were closely related to the notch size, but different for GFRP laminates and FMLs due to their different damage behavior. Finite element model was employed to predict accurately the mechanical responses of FMLs, particularly for elucidating the damage mechanism. Failure of notched FMLs under off-axis loading was a combined transverse fracture and shear-off, while it was shear-dominated in notched GFRP laminates. Transverse fracture in aluminum appeared first, followed by the fiber breakage, which was postponed by the extensive subcritical damage. Thereafter, shear-failure in fiber and aluminum layers were encountered instantaneously at the descending part. Thus, the critical failure of off-axis notched FMLs was still tension-dominated as that in the on-axis case, but aluminum played the critical role.

Characterization of nonlinear response in quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading

The present paper addressed the nonlinear stress-strain response in quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading. A series of monotonic and cyclic loading-unloading tensile tests were carried out. Both irreversible strains and stiffness degradation were observed in cyclic loading-unloading tests, which indicate that the nonlinear response of composites was induced by a combination of damage and plasticity. A coupled damage-plasticity model was employed to describe the nonlinear off-axis tensile stress-strain relation of materials. In this model, a plastic potential function together with associated plastic flow rule were adopted to assess the evolution of plastic strains. The damage variables in forms of stiffness degradation were expressed as a Weibull function of the effective stress. A full suite of model parameters was experimentally determined from cyclic loading-unloading tensile tests. The stress-strain curves predicted by this model agreed well with experimental results.