Interlaminar Shear Stresses Research Articles

Image-Based Inertial Impact (IBII) Tests for Measuring the Interlaminar Shear Moduli of Composites

The image-based inertial impact test has previously shown that inertial effects generated with high-strain-rate loading can be used to measure the dynamic constitutive properties of composites at strain rates on the order of 1600,{rm s}^{-1}. This work represents an important next step in exploring the potential of this concept with two tests presented where loading heterogeneity is exploited to measure the interlaminar shear modulus and stress–strain behaviour at high strain rates. The first test configuration used a short-beam with asymmetric loading to activate the shear behaviour. The virtual fields method was used to directly identify the interlaminar shear modulus from heterogeneous full-field maps of strain and acceleration. Simulated experiments were used to optimise the test configuration, select optimal smoothing parameters, and quantify uncertainty from grid rotation on the shear modulus identifications. The test was validated experimentally with three different virtual fields identifying an average shear modulus ranging from 5.7 to 5.9 GPa measured at 1600,{rm s}^{-1}, representing a 16–19% increase compared to quasi-static measurements. The shear modulus could also be identified from shear introduced into specimens tested in the standard, end-on interlaminar IBII configuration from slight in-plane misalignments of the impactor. The identified value of 5.6 GPa validates measurements from the first configuration and also demonstrates the capability to identify multiple interlaminar stiffness parameters from a single test.

Effect of Temperature on the Mechanical Properties of Carbon Composites

The purpose of this research is to investigate and analyze the effect of temperature on the mechanical properties of carbon composites, aluminum foam, and carbon nanotube reinforced aluminum foam. For this analysis, fatigue behavior of aluminum foam has been discussed, and interlaminar shear stress between single-walled carbon nanotube reinforced aluminum foam and multiwalled carbon nanotube reinforced aluminum foam has been compared. The comparison has shown that the interlaminar shear stress within the interface of single-walled carbon nanotube and aluminum foam is higher than that within the interface of multiwalled carbon nanotube and aluminum foam. Hence, the probability of stress concentration and crack initiation within the interface of single-walled carbon nanotube and aluminum foam is higher than that within the interface of multiwalled carbon nanotube and aluminum foam. Furthermore, thermal fatigue lives of different single-walled carbon nanotube reinforced matrix nanocomposites have been evaluated. Additionally, interaction between carbon and molten aluminum has been analyzed. Finally, a new relation for the thermal interlaminar shear stress intensity factor to predict the crack initiation sites on fiber/matrix interface has been introduced.

Influence of fabric orientation and compression factor on the mechanical properties of 3D E-glass reinforced epoxy composites

3-D E-glass fabric reinforced epoxy composites at 6mm thickness were fabricated for various orientations of the binder yarn viz. 0°, 30°, 45°, 60° and 90° respectively. Tensile, flexural, interlaminar shear stress tests were conducted to ascertain the influence of binder yarn orientation on the mechanical properties of the composites. The composites with 0° binder yarn orientation showed the best strength followed by 90° whilst the others showed highly depleted traits in comparison. Shear stress induced at the interface of each lamina was seen as the major reason for drop in the strength. A secondary study was carried out to explore the effect of compression factor during fabrication on the mechanical properties of the composites. Laminates with varying thickness namely, 4mm, 5mm and 7mm but, with same number of plies of 3D E-glass fabric at 0° orientation were fabricated. The test results were compared with the results of 6mm composites from the primary study. The results showed that, compression factor affected the mechanical properties of the composites and had a direct relation with increasing compression factor up to a certain value beyond which a drop in properties was seen. Composites pressed to a thickness of 5mm showed the best properties. Drop in properties was attributed to close packing of reinforcement and crushing of fibres leading to inefficient stress transfer. Scanning electron microscopy was employed to understand the modes of failure. The major failure modes observed were delamination, matrix cracking and debonding. Based on the results obtained, these composites can be seen as a material system for applications like ballistic armours, structural renovations and automobile components.

The Influence of CNT Structural Parameters on the Properties of CNT and CNT-Reinforced Epoxy

The main objective of this research is to review and investigate the influence of carbon nanotube structure on the properties of carbon nanotube and carbon nanotube-reinforced epoxy. Carbon nanotube and carbon nanotube-reinforced epoxy are currently being frequently used in many applications such as aerospace, automotive, and electronics industries due to their excellent properties such as high tensile strength, high Young’s modulus, and electrical and thermal conductivity. In this study, the obstacles to apply carbon nanotubes as fibers within the matrix have been introduced and discussed. Additionally, the epoxy properties and application have been cited, and failure mechanisms of carbon nanotube-reinforced epoxy and geometries of carbon nanotubes have been reviewed. Furthermore, with using experimental data and applying an analytical method, the effect of carbon nanotube diameter on interlaminar shear stress within the carbon nanotube-reinforced epoxy interface has been evaluated. Additionally, the effect of temperature variation on the value of interlaminar shear stress within the single-walled carbon nanotube-reinforced epoxy interface has been discussed. Finally, the influence of the number of hexagons in the unit cell on the Young’s modulus of zigzag and armchair single-walled carbon nanotubes has been evaluated.

A simple analytical method for determining interlaminar shear stresses in symmetric laminates

An analytical procedure for the analysis of interlaminar shear stresses in symmetric laminates is presented. The approach is based on the fact that the coupling effects of each part of the laminate are compensated by the adjacent parts. In a first step, a half of the laminate is considered for determining forces and moments per unit length that prevent the global deformation of the whole laminate. These forces and moments are obtained by solving a homogeneous second order differential equation. Then, the interlaminar shear stresses are computed by applying the stress equilibrium equations and by imposing the continuity of the stresses in the layer interfaces. The analytical results are checked by means of Finite Element Method in the case of quasi-isotropic laminates.

Predicting the relative magnitude of interlaminar stresses due to edge effects in thin angle-ply laminates using macroscopic finite element modeling

Carbon-epoxy and other composite materials are widely used in the design of aircraft and other structural components, but can be susceptible to delamination at free edges due to the presence of out-of-plane stresses. Finite element (FE) modeling can be used to predict the magnitude and location of these edge stresses, but the often used macroscopic approach to representing laminates can result in predictions that vary considerably due to the numerical singularities that are created. In this study, two macroscopic FE approaches are tested for thin angle-ply laminates in tension, and recommendations are made for the most effective method for evaluating the edge-effect phenomenon early in the design process. It has been determined that perfect interface models cannot be used for predicting maximum interlaminar stresses at free edges. Perfect interface models can effectively predict the relative magnitude of the interlaminar shear stress, τxz but resin interface models must be used for the interlaminar normal stress, σz.

An Investigation of the End-Notched Flexure and End-Loaded Split Tests Applied to the Mode II Interlaminar Fracture of a SiC/SiC Ceramic Matrix Composite

Abstract Delamination is a common failure mode observed in ceramic matrix composites (CMCs) and occurs as a result of applied interlaminar tensile and shear stresses exceeding the interlaminar strength. As CMCs are further implemented into aero engines, the need to understand their interlaminar failure becomes increasingly important. While significant contributions have been made toward understanding the mode I fracture toughness of CMCs, limited work exists on mode II. Several test methods for measuring the mode II fracture toughness have been proposed in the literature, namely, the end-notched flexure (ENF) and the end-loaded split (ELS) tests. This work investigates the mode II fracture toughness of a melt-infiltrated SiC/SiC CMC at ambient temperature using the ENF and ELS test methods. Acoustic emission (AE), direct current potential drop (DCPD), and digital image correlation (DIC) are implemented as health monitoring techniques to monitor crack initiation and propagation. Results show reasonable correlation between the two test methods and that the ELS test method is better suited for characterizing R-curve behavior.

Characterization of Plasma Cured E-Glass-Epoxy composite

Abstract Glass fiber (E-Glass) reinforced polymer Epoxy-resign, composite seats were fabricated in the laboratory by hand-lay-up method with 6,12 and 18 layers of laminates. Short-beam-shear (SBS) specimens were machined out of the fabricated seats. The 6 layer SBS specimens were exposed to low temperature plasma for 5, 10, 15 and 20munits. The specimens developed the maximum ILSS (Inter Laminar Shear Stress), for the 6 layered samples with 15 minute of exposure to the low temperature plasma due to post-cure strengthening. on the basis of the above, 12 and 18 layered SBS specimens were also exposed to the same low temperature plasma for 15 minutes duration. The observed ILSS values for the 12 layered specimens were somewhat higher than the same for the 18 layered specimens while being less than the 6 layered once. The least ILSS values were recorded for the 18 play samples. Thus, the radial migration of low temperature plasma inside the core of the specimens, responsible for post cure strengthening was inferred to bea function of the no of layers in the composite specimens, decreasing with the increase in the no of layers responsible for an increase in the thickness of the specimens. Glass transition temperature (Tg) for these plasma cured samples with 15 minute exposure (optimum) also decreased with the increase in the no of layers in the specimens. The mode of failure, as reviled from SEM fractographs, included fiber-pull-out, fiber-matrix debonding, matrix cracking/ crazing and fiber breaking

Effect of Ply Lay Up Sequence on Interlaminar Shear Strength of Symmetric and Asymmetric GFRP Composite

Abstract Fibre reinforcement polymer matrix composite are being used in aerospace, aircraft, and automobile industry etc .These material has high strength, high modulus and low density. Woven Fibres used is in experiment. Reinforcement throughout thickness is better in woven fibres and it show similar properties in warp and weft direction. The composite material properties like load carrying capacity ,stiffness, ILSS etc. is affected by ply orientation and stacking sequence of laminate. Asymmetric laminates have almost similar stiffness characteristics properties to that of symmetric laminates. Symmetric and asymmetric laminate were fabricated by hand layup press moulding process. In the concerned method initially cut plies are wetted by the resin and are placed over each other according to the required stacking sequence. Once the proper layup is achieved, the wet laminate is squeezed under press moulding machine. The magnitude of load defines the final thickness of the laminate. Final thickened laminates’ samples were tested on fully computer controlled universal testing machine using short beam strength method and with the help of ASTM- 2344 ILSS was calculated. This paper show the effect of ply layup sequence on interlaminar shear strength on symmetric and asymmetric laminates of GFRP composite and it was found that interlaminar shear stress is higher for symmetric laminates that of asymmetric laminates. It was also observed that interlaminar shear stress decreases as fibre volume fraction in laminates increases.

Thermoelastic effect on inter-laminar embedded delamination characteristics in Spar Wingskin Joints made with laminated FRP composites

This paper presents two sets of full three-dimensional thermoelastic finite element analyses of superimposed thermo-mechanically loaded Spar Wingskin Joints made with laminated Graphite Fiber Reinforced Plastic composites. The study emphasizes the influence of residual thermal stresses and material anisotropy on the inter-laminar delamination behavior of the joint structure. The delamination has been pre-embedded at the most likely location, i.e., in resin layer between the top and next ply of the fiber reinforced plastic laminated wingskin and near the spar overlap end. Multi-Point Constraint finite elements have been made use of at the vicinity of the delamination fronts. This helps in simulating the growth of the embedded delamination at both ends. The inter-laminar thermoelastic peel and shear stresses responsible for causing delamination damage due to a combined thermal and a static loading have been evaluated. Strain energy release rate components corresponding to the Mode I (opening), Mode II (sliding) and Mode III (tearing) of delamination are determined using the principle of Virtual Crack Closure Technique. These are seen to be different and non-self-similar at the two fronts of the embedded delamination. Residual stresses developed due to the thermoelastic anisotropy of the laminae are found to strongly influence the delamination onset and propagation characteristics, which have been reflected by the asymmetries in the nature of energy release rate plots and their significant variation along the delamination front.

Effect of Stacking Sequence on Interlaminar Shear Strength of Multidirectional GFRP Laminates

Abstract Glass fiber reinforced polymer matrix composites are widely used in many applications such as aerospace, automobile, marine, electrical etc. because of its superior mechanical property coupled with minimum weight requirements of the component. The performance of a composite part gets mostly affected by the residual stresses, stacking sequences due to relation with in-plane stresses. This study investigates performance of multidirectional laminates using bi-directional plain-weave glass fabric, the alteration in stacking sequence is done in, in-between plies, keeping inner and outer ply orientation constant. Laminates are made by hand layup and compression moulding technique. Five class of laminate, keeping stacking sequence of [0°/(0°)2/0°]S, [0°/(15°)2/0°]S, [0°/(30°)2/0°]S, [0°/(60°)2/0°]S, [0°/(75°)2/0°]S are manufactured and tested for interlaminar shear stress using ASTM D2344-16 standard on fully computerised UTM with strain rate of 1 mm/s. It is seen that laminate performance exhibits dependency on orientation of in between plies. The Laminates with all plies oriented in 0° position depicts lowest ILSS whereas the laminate having 60° ply in between possesses highest ILSS

Effect of fibre orientation on damage resistance of composite laminates

The present study deals with the behaviour of angle ply composite laminates subjected to low velocity impact. The laminates were prepared by E-glass fibres as reinforcement and epoxy as resin. Experiments were performed on these laminates by using low velocity drop weight set-up machine at three energy levels produced by using two masses. At energy level it was noticed that, [0/90]s laminate was most damage resistant and [0/15]s laminate was least resistant while the contact duration of impactor on laminate(s) was found to be sensitive to mass level. Finite Element (FE) analysis was performed to quantify the damage in laminates and investigate energy absorbed in intra-layer failure modes. During the post-processing FE analysis helps to mark the critical load points on the load-displacement history and quantification of distinct failure modes. The size of delamination profile was seen to be governed by fibre orientation and inter-laminar shear stresses at the interface.

Finite Element Modelling of Delamination Onset in Polymeric Composite Material

Delamination is a well-known issue in polymeric composite materials. Interlaminar Shear Stress (ILSS) plays an important role in delamination initiation and thus its value needs to be monitored when designing a composite component. In this paper a Short Beam Shear experimental test of a carbon/epoxy specimen reproducing a Formula 1 component laminate is represented through a Finite Element analysis of two different models, one featuring 2D shell elements and the other 3D elements. ILSS results from both models are compared to experimental data in order to assess whether a 2D shell model can be used instead of its 3D counterpart, in favour of design versatility.

Study on mechanical characteristics of woven cotton/bamboo hybrid reinforced composite laminates

In this study, two fabrics namely cotton/cotton woven fabric having cotton yarn in both warp and weft direction; and cotton/bamboo woven fabric with cotton yarn (warp direction) and bamboo yarn (weft direction) were selected. Compression moulding method has been used to fabricate cotton/cotton and cotton/bamboo woven fabric reinforced composites with epoxy resin as a matrix material. The mechanical properties of cotton/cotton and cotton/bamboo reinforced composites had been compared under five different fiber loading conditions (30, 35, 40, 45 and 50wt.%) and the fractured morphology was analyzed using scanning electron microscope. It was noted that cotton/bamboo reinforced composite with 45wt.% fiber loading exhibited the best mechanical properties namely tensile, flexural, impact, compression, and inter laminar shear stress (ILSS), due to its weft direction of bamboo yarn.

Meshfree investigation of the vibrational behavior of pre-stressed laminated composite plates based on a variationally consistent plate model

The vibrational behavior of pre-stressed laminated composite plates is numerically investigated based on a variationally consistent shear deformation plate theory. The numerical analyses are performed by the use of meshfree radial point interpolation method (RPIM). The plate model utilized in this investigation is the Wang and Shi's [1] plate theory which accounts for continuity of interlaminar transverse shearing stresses and zigzag variation of displacement field through the plate's thickness. Firstly, the governing equations for the free vibration of pre-stressed laminated and sandwich plates based on the Wang and Shi's model are derived through the Hamilton's principle. Subsequently, the weak-form of the governing equations is obtained, and the meshfree RPIM is utilized for construction of the discretized system of equations. Several numerical examples with different Young's modulus ratios, side to thickness ratios, boundary conditions, and lamination schemes are presented in order to illustrate the accuracy of the plate theory adopted in this paper. These examples are also analyzed using the first order shear deformation theory (FSDT), the third order shear deformation theory (TSDT), and also the Shi's [2] plate theory in order to investigate the effect of different factors of plate theories on the accuracy of results in various situations.

Electrical conductivity and mechanical property improvement by low-temperature carbon nanotube growth on carbon fiber fabric with nanofiller incorporation

Carbon fiber reinforced polymer composites offer many advantages due to their great combination of high strength and modulus combined with low density and light-weight; however, they typically feature low electrical conductivity. These advanced composites are often unsatisfactory compared to conventional conductive metals that are vulnerable to lightning strike damage. To develop a new generation of highly conductive carbon fiber composites that address these shortcomings, carbon nanotubes (CNTs) were grown on nickel-coated carbon fiber using low-temperature growth to create a fuzzy fiber surface. An additional conductive filler, graphene nanoplatelets (GNPs), was then dispersed on the fuzzy fiber surface to form synergistic physical interactions between two different low-dimensional carbon-based nanostructures. The results reveal a synergistic enhancement in both functional conductivity and mechanical properties. The electrical conductivity of composites with the inclusion of GNPs resulted in approximately 40%, 300%, and 190% enhancement in the fiber, surface, and through-thickness direction compared to the fuzzy fiber composites. Mechanical properties, including flexural, work of fracture, impact, and interlaminar shear stress properties were further enhanced. These results reveal that the presence of GNPs creates more electron transfer pathways, and also promotes a synergistic effect in physical interactions. Altogether, these enhancements provide avenues for future high-performance conductive carbon fiber composites in aircraft structures.

Creep in interlaminar shear of an Hi-Nicalon™/SiC–B4C composite at 1300℃ in air and in steam

Creep behavior in interlaminar shear of an advanced SiC/SiC composite with a self-healing matrix was investigated at 1300℃ in laboratory air and in steam. The composite was processed via chemical vapor infiltration (CVI). The composite has a self-healing oxidation-inhibited matrix comprising alternating layers of silicon carbide and boron carbide and is reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms were coated with pyrolytic carbon followed by a boron carbon overlay. The interlaminar shear properties were evaluated at 1300℃. The creep behavior was examined for interlaminar shear stresses ranging from 13 to 20 MPa in air and in steam. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. Creep run-out (defined as 100 h at creep stress) was achieved at 13 MPa in air and in steam. Presence of steam had little influence on creep strain rates and creep lifetimes. However, larger creep strains were accumulated in steam than in air. The retained properties of all specimens that achieved creep run-out were characterized. Composite microstructure and damage and failure mechanisms were investigated.

The Failure Mechanism of Composite Stiffener Components Reinforced with 3D Woven Fabrics.

Composite industry has long been seeking practical solutions to boost laminate through-thickness strengths and interlaminar shear strengths (ILSS), so that composite primary structures, such as stiffeners, can bear higher complex loadings and be more delamination resistant. Three dimensional (3D) woven fabrics were normally employed to render higher transverse and shear strengths, but the difficulty and high expense in producing such fabrics make it a hard choice. Based on a novel idea that the warp yarns that interlock layers of the weft yarns might provide adequate fiber crimps that would allow the interlaminar shear or radial stresses to be transferred and borne by the fibers, rather than by the relatively weaker matrix resin, thus improving the transverse strengths, this work provided a two point five dimensional (2.5D) approach as a practical solution, and demonstrated the superior transverse performances of an economical 2.5D shallow-bend woven fabric (2.5DSBW) epoxy composites, over the conventional two dimensional (2D) laminates and the costly 3D counterpart composites. This approach also produced a potential candidate to fabricate high performance stiffeners, as shown by the test results of L-beams which are common structural components of any stiffeners. This study also discovered that an alternative structure, namely a 2.5D shallow-straight woven fabric (2.5DSSW), did not show any advantages over the two control structures, which were a 2D plain weave (2DPW) and a 3D orthogonal woven fabric (3DOW) made out of the same carbon fibers. Composites of these structures in this study were conveniently fabricated using a vacuum-assisted resin infusion process (VARI). The L-beams were tested using a custom-made test fixture. The strain distribution and failure mode analysis of these beams were conducted using Digital Image Correlation (DIC) and X-ray Computed Tomography Scanning (CT). The results demonstrated that the structures containing Z-yarns or having high yarn crimps or waviness, such as in cases of 3DOW and 2.5DSBW, respectively, were shown to withstand high loadings and to resist delamination, favorable for the applications of high-performance structural composites.

Failure mode analysis of laminated composite sandwich plate

Failure mode prediction of laminated composite sandwich plates is presented using improved higher order shear deformation theory (IHSDT) mathematical model and its C0 finite element (FE) implementation in FORTRAN. The proposed 2D IHSDT satisfies the inter-laminar shear stress continuity at each adjacent layer and nil transverse shear stress conditions at top and bottom of the plate are imposed. The parabolic shear stress variation is piecewise across thickness of each layer; hence no need of shear correction factors. The possible modes of failures are transverse matrix cracking, fiber breakage and inter-fiber shear failure of the matrix. The failure mode along with its location for laminated composite sandwich plates are obtained and are found in good agreement with the available 3D elasticity results.

Parameter analysis of damaged region for laminates with matrix defects

Considering the influence of bridge fibers, a representative volume element (RVE) with damaged regions of vanishing thickness is developed to study for estimation of laminates with transverse matrix cracks. For the open and closed cracks, the tensile force, the compressive force and the shear force are mainly studied by the stress-based variational approach with the pending partition loads at the statically indeterminate boundary, with considering not only three cases of the ideal transverse cracks, the smooth plane-surface open crack, the smooth plane-surface closed crack and the perfect meshing open crack, but also two cases of the non-ideal transverse cracks, the open cracks and closed cracks with the resistance resisting the relative sliding of crack surfaces. The expressions of the stress components in the RVE are obtained by means of assumption of the function of inter-laminar shear stresses. The governing equations are derived from the variational analysis based on the principle of minimum complementary energy. Both the effective mechanical properties and the distributions stresses of the damaged laminates with the ideal transverse cracks and those of the laminates with the non-ideal transverse cracks are estimated on the basis of the analysis of deformation on surface of the RVE.