Interlaminar Shear Stresses Research Articles

Micromechanical analysis of interlaminar crack propagation between angled plies in mode I tests

This paper presents a micromechanical finite element model to study interlaminar damage propagation and relocation, known as delamination migration, between angled plies, consisting of a double-ply θ/0° Unit Cell (UC) in-between homogenised unidirectional 0° plies. Random fibre distributions and appropriate constitutive models are used to model the different dissipative phenomena that occur at crack onset and propagation. Varying the upper ply fibres orientation, θ, and ply thickness, it is possible to assess their influence on the damage migration mechanism. Different features associated with delamination migration are analysed, such as the distribution of interlaminar shear stresses at the crack tip and the kink angles. When comparing the results of the micromechanical model with previously conducted experimental observations, similar trends are obtained. It is concluded that the computational framework is able to simulate mode I interlaminar damage propagation and delamination migration in multidirectional laminates, providing a sound tool to better understand the conditions behind interlaminar crack migration.

Study on irradiation effect of insulating materials for fusion superconducting magnet: change in electric insulation performance by irradiation

In the fusion reactor, hybrid composite material composed of glass fiber reinforced plastic (GFRP) and polyimide film is used for insulation material of superconducting magnets. The insulation material is used in severe environments of fast neutron fluence of 1022 n/m2 (E > 0.1 MeV), cryogenic temperature, large mechanical stress (43 MPa of interlaminar shear stress) and high voltage of 20 kV. Thus, insulation material is required to maintain its insulation performance even in such environments. In the previous study about irradiation effect on insulation material, interlaminar shear strength was used to evaluate the radiation resistance. However the insulation performance can be degraded by microscopic cracks even if it is not fractured. In order to investigate the effect of mechanical deterioration on the insulation performance, a dielectric breakdown test was conducted under shear stress at 77 K on irradiated hybrid composite of GFRP and polyimide film.

English

The main objective of this study is to investigate the effect of meteorites and debris impact on thermal fatigue life of unidirectional T700 carbon fiber/epoxy composite material. This composite material may be applied in space structures for space missions. These structures are exposed to thermal cycles as they go in and out of the planet’s shadow. If in this environmental condition, meteorites and debris impact with the structure, they may have effect on the thermal fatigue life of the structure. It means that the impact may decrease or increase the thermal cycles to failure of the structure in the presence of meteorite and/or debris. In this study, by applying experimental results and using analytical method attempt was made to estimate the thermal fatigue life in the presence of meteorite and/or debris impact, and thermal cycle condition. The results of this research can contribute to predict the thermal fatigue life in the presence of debris and/or meteorites impact to structure in possible future space missions. Furthermore, the results of this study have shown that the impact of debris and/or meteorites can increase the interlaminar shear stress between the T700 carbon fiber and epoxy which may result in crack formation and decrease the thermal fatigue life. Key words: T700 carbon fiber, meteorite and debris, space structures, thermal fatigue life, impact effect, unidirectional composite. &nbsp

Durability of GFRP composite exposed to outdoors weathering

Abstract Twelve ply glass fibre-reinforced plastic (GFRP) composite samples, hung in the open for out-doors ageing, were characterised in terms of moisture intake, inter laminar shear stress (ILSS) (using 3-point bend test) and glass transition temperature (Tg) (using low temperature differential scanning calorimetry; DSC) as a consequence of the exposure to out-doors. The local relative humidity and the temperature fluctuations were recorded. The responses so obtained were compared with those pertaining to 18 ply GFRP composite samples fabricated with same constituents adopting parallel methods but exposed to higher fixed levels of relative humidity and temperature in a humidity chamber under laboratory conditions (95% of relative humidity at 60 °C). Expectedly, moisture intake was greater for the laboratory-conditioned samples, exposed to higher fixed levels of relative humidity and temperature for longer durations. However, the ILSS decrease and Tg depressions for the out-doors conditioned samples were more severe. More importantly, SEM fractographs revealed relatively smoother broken surfaces for the outdoors treated samples indicating a brittle mode of failure that could be catastrophic.

Reinforcing effects of aminosilane‐functionalized graphene on the tribological and mechanical behaviors of epoxy nanocomposites

ABSTRACTIn this study, aminopropyl trimethoxysilane as an interfacial modifier was introduced on the surface of graphene (Gr) nanoplatelets. The effects of the silane‐modified graphene (SGr) loading (0, 0.05, 0.1, 0.3, and 0.5 wt %) and silane modification on the tensile, compressive, interlaminar shear stress (ILSS), and tribological properties of the epoxy‐based nanocomposites were investigated. Out of these specimens, the highest values of ILSS and compressive strength were related to the 0.3 wt % SGr–epoxy nanocomposite. The addition of SGr enhanced the tensile strength and strain to failure only at low contents (i.e., 0.05 wt %). Also, the tensile and compressive moduli were improved, and the highest values were observed at a 0.5 wt % SGr loading. In addition, decreases of approximately 40 and 68% in the coefficient of friction and wear rate, respectively, were observed at a 0.3 wt % SGr loading. Enhanced tensile, compressive, ILSS, and wear properties in the SGr–epoxy specimens were observed compared to those in the Gr–epoxy specimens. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47410.

English

The main objective of this research is to estimate the critical temperature for triple-walled carbon nanotubes (CNTs)/epoxy resin. Critical temperature in this study is the temperature that inter-laminar shear stress in interfaces of triple-walled CNTs and epoxy resin is the maximum value. At maximum inter-laminar shear stress, the stress concentration between triple-walled CNTs and epoxy resin is the maximum. This phenomenon may cause crack initiation, propagation and fracture in fibers/matrix interfaces. In the presented study, this temperature is derived by applying analytical method. The results of this research showed that maximum inter-laminar shear stress could occur at 85°C, which appears to be the critical temperature. The results of this research could be applied in any industry dealing with thermal stress and thermal cycles. Key words: Critical temperature, triple-walled carbon nanotube (CNT), nanocomposites, thermal analysis, inter-laminar shear stress, coefficient of thermal expansion.

A meshfree method for static and buckling analysis of shear deformable composite laminates considering continuity of interlaminar transverse shearing stresses

A meshfree radial point interpolation method is presented for static and buckling analysis of thick laminated plates, taking into account the continuity of interlaminar transverse shearing stresses and zigzag variation of the displacement field through the plate’s thickness. The kinematics of the deformation field is based on the refined theory of Wang and Shi (2015). The corresponding weak-form for static bending and buckling analyses of plates based on the Wang and Shi’s theory are derived through minimum potential energy principle. The discretized systems of equations for static bending and buckling analyses are derived and the associated numerical integration is performed by the novel CTM quadrature. This integration method can exactly model the problem geometry and thus is very fast and accurate. Several numerical examples are solved to demonstrate the accuracy and efficiency of the plate theory adopted in this paper. Additionally, the examples are also analyzed using the FSDT, TSDT and Shi’s plate theory (2007). Because the four theories considered in this paper use five field variables and they also have some other similarities, the influence of higher order terms of the displacement field, transverse shearing strain energy consistency, and imposition of transverse shearing stress continuity on results is investigated.

Trigonometric zigzag theory for static analysis of laminated composite and sandwich plates under hygro-thermo-mechanical loading

In the present work, the static response of symmetric and anti-symmetric laminated composite and sandwich plates is investigated for elevated temperature and moisture conditions. Analysis is carried out using recently developed zigzag model which is based on a trigonometric (secant) shear deformation in the plate. The theory incorporates inter-laminar shear stress continuity along with the traction free boundary conditions at the top and bottom surfaces. The finite element method is employed for analysis, using the eight noded isoparametric serendipity element, accounting for the C0 continuity. Numerous examples covering the various features such as effect of thermal and moisture coefficients, material anisotropy, boundary and loading conditions and span to thickness ratio are solved for symmetric as well as anti-symmetric plates. Results in the form of deflection and stresses are presented and validated with the available results in the existing literature. Further, few new results are also provided in this work, to set a benchmark study for the future research.

Influence of openings and additional mass on vibration of laminated sandwich rhombic plates using IHSDT

In this article, investigations on the influence of openings and additional mass on free vibration analysis of laminated composite sandwich skew plates using improved higher order shear deformation theory (IHSDT) have been done. The IHSDT model satisfies the interlaminar shear stress continuity at the layer interfaces and also ensures zero transverse shear stress conditions at the top and bottom of the plate. The piecewise parabolic shear stress variation across the thickness of each layer is considered. No shear correction factors are required. The 2-D C0 finite element (FE) model has been developed by authors based on IHSDT. FE model based on IHSDT has been coded in FORTRAN. The problem of C1 continuity requirement associated with the IHSDT is overcome using an appropriate C0 FE formulation. The free vibration frequencies of laminated composite and sandwich plates obtained using the present 2-D FE model are in good agreement with the 3-D elasticity results. The influence of the side-to-thickness ratio, skew angles, boundary conditions, and mode shapes is taken into consideration for the present study.

An efficient two-dimensional shear-lag model for the analysis of patched laminates

Fiber patch placement (FPP) is a manufacturing technique for variable stiffness composites. In the FPP approach, a structural component is assembled from a multitude of discrete fiber patches, thus allowing for an easy tailoring of the layup to the local load state. However, due to the discontinuous fibers at the patch edges, complex stress distributions occur in patched laminates. To date, an efficient method for the analysis of patched laminates on macro-scale does not exist. This article introduces a 2D planar shear-lag model (SLM) based on thin-plate mechanics and a simplified interlaminar shear stress formulation. It is shown how the governing partial differential equation system is assembled and how boundary conditions are set. The model is solved numerically using the Finite Element Method. For verification the 2D SLM is compared to a full 3D linear-elastic solution. It can be shown that the SLM allows for accurate prediction of stress fields disturbed by interrupted fibers with considerably improved numerical efficiency. An application example shows that the SLM resolves the effects of discontinuities significantly better than a state-of-the-art shell element modeling approach. As a consequence, a substantial progress in the design of patch laminated structures is achieved.

Numerical prediction of interlaminar stresses in laminated composites

Laminated composite materials are extensively utilised in aerospace, aviation and automobile industries because of their high strength and low weight. When laminates are composed of layers with different orientations, classical lamination theory implies boundary tractions on free edges. Edge delamination in laminated composites has become significant in structural reliability. Due to more inter-laminar shear stresses nearer to the free edges of the laminates, more failure modes are developed in the laminates. Because of the complex fibre/matrix micro structure of laminated composite materials, an evaluation of these stresses becomes very important in composite materials design. Normally free edge inter-laminar stresses arise from the mismatch of elastic properties between two layers. Therefore it is necessary to predict the inter-laminar stresses of laminated composite plates. In this study, the free edge inter-laminar stresses of laminated composite materials are predicted numerically. Numerical prediction of inter-laminar stresses of non-hybrid and hybrid laminated composite plates are predicted. In hybrid category, carbon fibres and glass fibres are combined together and analysed. Numerical analysis of laminates is performed in ANSYS. The inter-laminar shear forces and stress distribution are plotted along thickness.

Effect of heterogeneity at the fiber–matrix scale on predicted free-edge stresses for a [0°/90°]s laminated composite subjected to uniaxial tension

The effect of the free-edge on the interlaminar stresses that develop in a thin-ply [0°/90°]s laminated composite under uniaxial tension was explored using finite element models that directly modeled the random heterogeneous microstructure and a model that treated the plies as homogeneous, orthotropic materials. The deformed cross-sections were compared for the two cases, showing that the homogeneous model generally captured the displacement field well but the heterogenous model exhibited local perturbations due to the microstructure. The interlaminar normal stress distributions along the ply interfaces were very different for two models. The heterogeneous model exhibited a complex pattern of stresses that were sensitive to nearby fibers. A comparison of the interlaminar shear stress distributions for the two models showed better agreement, though the heterogeneous model greatly differed from the homogeneous model when fibers or matrix pockets were close to the ply interface. After separating the effect of the fibers and the free-edge effect, the stresses that develop due to the free-edge effect matched well between the two models, except very close to the free-edge. It was shown that the Poisson’s ratios of the fibers and matrix significantly affect the stress distributions along the ply interface, and a matrix Poisson’s ratio could be selected that reduces the effect of the microstructure on the interlaminar normal stress. The study highlighted that a full 3D analysis can provide new insights for classical problems, and more optimal design of composites will require consideration of microstructural effects.

Fatigue damage simulation of tension-tension loaded glass/polyester fiber composites with thickness tapering effects

This study evaluates the capability of a progressive damage method in predicting the fatigue failure in glass/polyester fiber composite materials. Residual material properties in different failure modes have been obtained from testing fatigue-loaded specimens to their ultimate limits. A numerical tool utilizes the experimentally obtained values to simulate the process of damage in more complex models with high interlaminar shear stresses. The tool accounts for fatigue damage through stiffness and strength degradation rules without relying on prior calibration/modifications. Benchmark ply-drop problems under constant and variable amplitude fatigue loadings are simulated and compared against experimental results. Verification case studies show that the numerical tool can predict damage initiation and final fatigue life, successfully.

Damage initiation in composite materials under off-centre impact loading

The effect of off-centre impact loading on damage initiation in a woven glass fibre reinforced epoxy resin was studied experimentally. Low velocity impact tests were conducted, in which the incident impact energy was increased until damage was observed in the laminates. It was shown that multiple impacts, with increasing incident energy at the same location, did not greatly influence the critical force for damage initiation, Pcrit. Subsequent testing on a range of panel sizes showed that the critical force is highest for central impacts, decreasing slowly as the impact location moves towards the boundary. It was also shown that, for off-centre impact loading, Pcrit, follows a t3/2 (t = laminate thickness) relationship that has previously been established for central impact. The slope of the plot of Pcrit versus t3/2 decreases as the impact location moves away from a central location, suggesting that the effective interlaminar shear stress also decreases with increasing offset. Tests at energies well above the damage threshold confirmed that off-centre impact is more serious than central impact loading.An energy-balance model was used to predict the off-centre impact response of the panels. Agreement between the energy-balance model and the measured impact response was good at energies that did not generate significant damage. Finally, it is suggested that the energy-balance model can also be used to predict a lower bound on the damage threshold energy in composite plates.

Experimental Characterization of Hybrid and Non-Hybrid Polymer Composites at Elevated Temperatures

Glass and carbon fibre composites and hybrid glass/carbon composites are materials often used in wind turbine blades. In Africa wind turbines have to operate in warm climates. The present study presents the results of an experimental study on the mechanical properties of these composites at elevated temperatures. For this purpose, the composite specimens are fabricated by hand lay-up process to investigate their static and dynamic properties at high temperatures. The properties studied include Inter-laminar Shear Stress (ILSS), Dynamic Mechanical Properties (DMA) such as storage modulus, loss modulus and loss factor using ASTM standards. The inter-laminar shear failure strength of carbon fibre and hybrid glass-carbon fibre composites are found to be close whereas the glass transition and damping behavior of the hybrid composites are higher making them suitable for wind turbine blades operating at high temperatures. Short Beam Shear (SBS) test data are evaluated using chi-square goodness of fit statistical tests and the correlation coefficients from linear regression analysis are determined. Normal, lognormal and 2-parameter Weibull statistical distributions are used at 5% significant level.

Shear stress and interlaminar shear strength tests of cross-laminated timber beams

The interlaminar shear stresses of the three-layer, five-layer, and seven-layer cross laminated timber (CLT) and those of the oriented laminated beams were calculated according to Hooke’s law and the differential relationship between the beam bending moment and shear force. The interlaminar and maximum shear stresses of the CLT beam are related to the number of CLT layers and to the elastic modulus ratio EL/ET (or EL/ER) of the parallel and perpendicular layers. The interlaminar shear strength of the Hemlock CLT was positively correlated with the elastic modulus of its parallel layer. The results showed that the CLT short-span beams had three failure modes when subjected to a three-point bending test, namely perpendicular layer rolling shear failure, CLT interlaminar shear failure, and parallel layer bending failure. The shear stress of the oriented laminated beam followed a parabolic distribution along the height of the section, while the shear stress of the orthogonally laminated beams tended to be balanced, rather than parabolically distributed along the height of section. The short beam three-point bending method was able to effectively test the interlaminar shear strength of CLT due to its stable and readable load.

Ultimate Strength Analysis of Laminated Composite Sandwich Plates

The ultimate strength analysis of laminated composite sandwich plates is presented via ultimate failure load prediction along with modes of failure for laminated composite sandwich plates using mathematical model based on improved higher order shear deformation theory (IHSDT). The proposed IHSDT mathematical model satisfies the inter-laminar shear stress continuity at each layer interface and also ensures zero transverse shear stress conditions at the plate top and bottom. The piecewise parabolic shear stress variation across the thickness of each layer is considered. No shear correction factors are required. The suitable C0 FE formulation of IHSDT model is developed by authors.

In-situ monitoring of woven glass fiber reinforced composites under flexural loading through embedded aligned carbon nanotube sheets

In practical situations, the flexural loading of fiber reinforced composites is one of the most common types of loading for structures. Nonetheless, carbon nanotubes, which have been used extensively to study in-plane strain sensing in composites, have rarely been employed to monitor flexural induced strains. With this motivation, this paper introduces a novel method for localizing aligned carbon nanotube sheet layers in three selected locations inside a laminated composite structure. These top, middle, and bottom locations are more prone to damage since they experience maximum flexural induced tension, compression, and interlaminar shear stresses. The composite structural performance is monitored by establishing an electromechanical coupling between the developing strain and the three carbon nanotube sensing elements’ in-plane electrical resistance changes that are measured simultaneously. The results of the monotonic and dynamic flexural loading tests suggest that carbon nanotube sensing materials exhibit a high level of sensitivity. The results also show that the resistance change of the three embedded carbon nanotube layers appeared to track the mechanical state of the host structure well. The carbon nanotube layer embedded in the middle section showed a piezoresistive behavior in response to the growing complex stress state with a few electrical resistance change spikes corresponding to damage developing inside the laminated structure. This flexural sensing behavior may considered as useful for real applications because in addition to sensing strain, this technology may help in predicting the failure of the composite component before the actual end of service life.

Exploration relation between interlaminar shear properties of thin-ply laminates under short-beam bending and meso-structures

Carbon fiber reinforced polymer matrix composite laminates with standard thickness plies (0.125 mm) usually have weak interlaminar shear strength, meanwhile, for thin-thickness laminate structures such as aircraft wing skin, it is difficult to design a balanced laminate with the standard plies. It is a possible way to improve the interlaminar shear performance of carbon fiber reinforced polymer composite laminates and enlarge the design space of the thin-thickness structures by using thin-plies technology. In this paper, the interlaminar shear strength of carbon fiber/epoxy laminates with thin prepreg thickness subjected to short-beam bending is investigated. Unidirectional, cross-ply and quasi-isotropic laminate specimens were prepared by using prepregs with different ply thicknesses. Results show that, with decreasing of the ply thickness, higher interlaminar shear strength and smaller coefficient of variation of the data are obtained. Compared to laminates made by standard thickness prepreg, the laminates with thin-thickness prepreg exhibit more homogeneous microstructures and more regularly interlaminar shear stress distribution. This indicates that inherent anisotropy of the laminate composites is weakened in the thin-ply laminates and show pseudo-isotropic behavior. Especially in the case of ply thickness less than 0.020 mm, the interlaminar shear stress distributions of the cross-ply and quasi-isotropic laminate are almost the same with that of isotropic materials according to the classic laminate theory. On the other hand, as expected, the design space of the thin-thickness laminate structures will be increased since more ply number are allowed and superior interlaminar properties can be obtained due to the pseudo-isotropic behavior of the thin plies.

Interlaminar shear stress function for adhesively bonded multi-layer metal laminates

A new analytical model is proposed for the estimation of interlaminar shear stress in adhesively bonded metal laminates consisting of an arbitrary number of layers. The interface shear stress in the laminates is related to the difference in average axial strain and elongation between adjoining layers through a newly proposed interlaminar shear stress function (ILSSF). The parameters of the ILSSF are determined from finite element simulations using a data fitting procedure. The accuracy of the model is investigated by comparing experimental measurements of average elongation in three-layer aluminum laminates to values obtained using the model. Good agreement with the experimental results is achieved for several types of adhesives and for different ratios of adhesive-to-layer thicknesses. The influence of Young's modulus of the adhesive on the efficiency of load transfer in three-layer laminates is investigated.