Through-thickness Compressive Stress Research Articles

Measurement of Interlaminar Shear Properties of Pultruded CFRP Composites Based on Modified DNS Specimens

A double-notch shear (DNS) test was modified for a measurement of interlaminar shear properties of pultruded carbon-fibre-reinforced polymer (CFRP) composites under moderate through-thickness compressive stresses, which were applied by torqued bolts. A range of failure criteria were presented to compare with experimental results. An attempt was made to predict the shear stress-strain relationship in the combined stresses with through-thickness compressive and shear stresses. Results indicate that the modified 2DNS specimen makes the shear surface of the specimen more uniform and reduces the peel stress at the notch tip. The 2DNS fixture made it easier to study the influence of composite interlaminar shear strength and shear stress-strain relationship under moderate through-thickness compression. The initial shear stiffness was not affected by moderate through-thickness compressive stresses. Moderate through-thickness compressive stresses on pultruded CFRP materials can increase interlaminar shear strength and ductility. Of all the failure criteria, the Mohr-Coulomb criterion was preferred to predict the enhancement of the shear strength under moderate through-thickness compression. Based on the Mohr-Coulomb criterion and the inverse solution of Ramberg-Osgood model, a unified equation was proposed and can effectively characterise the interlaminar shear stress-strain relationship under moderate through-thickness compression.

Revealing the interlaminar shear failure behavior of unidirectional laminate under combined compression–shear loads

Due to the coupling effects between stresses in different directions, the mechanical behavior of an advanced composite material under multiaxial loading is extremely complex. In this study, the influence of through-thickness compressive stress on the interlaminar shear performance of a carbon fiber-reinforced composite was experimentally investigated. Hollow cylindrical unidirectional laminate specimens were fabricated to conduct combined compression–shear tests, and the fracture morphologies of the specimens were characterized to reveal their failure behavior. The results indicate that a moderate compression load significantly enhanced the shear properties of the laminate by inhibiting crack propagation and improving the friction effect. The shear strength and modulus of a laminate specimen subjected to combined stresses improved up to a maximum of 76% and 231%, respectively, over those of an equivalent specimen subjected to pure shear. However, as the applied through-thickness load approached the compressive strength of the laminate, the specimen shear capacity began to decline owing to the transition of fracture mechanisms. Indeed, the specimens exhibited mixed failure modes corresponding to the different stress states, which were induced by the combined effects of through-thickness compressive and shear stresses. As the applied through-thickness compressive stress increased, the dominant failure mode of the laminate specimen changed from fiber–matrix debonding to fiber shearing and then to fiber breakage, resulting in various shear performances.

Predicting interlaminar damage behaviour of fibre-metal laminates containing adhesive joints under bending loads

This study includes experimental and numerical investigations on fibre-metal laminate structures containing adhesive joints under static bending loads. Experimental tests were carried out on Glare® 4B specimens manufactured in-house and containing doubler joint features. Numerical analyses were performed using Abaqus software including damage in the glass fibre reinforced polymer layers, ductile damage in the resin pockets (FM94 epoxy) and plasticity in the metal layers. A new cohesive zone model coupling friction and interfacial shear under through-thickness compressive stress has been developed to simulate delamination initiation and growth at the metal/fibre interfaces with the adhesive joint under flexural loading. This model is implemented through a user-defined VUMAT subroutine in the Abaqus/Explicit software and includes two main approaches, firstly, combining friction and interfacial shear stresses created in the interlaminar layers of the fibre-metal laminate as a result of through-thickness stresses and secondly, considering elastic-plastic damage behaviour using a new cohesive zone model based on the trapezoidal law (which provides more accurate results for the simulation of toughened epoxy matrices than the commonly used bilinear cohesive zone model). Numerical results have been validated against experimental data from 4-point bending tests and a good correlation observed with respect to both crack initiation and evolution. Delamination and shear failure were noted to be the predominant failure modes under bending stresses as expected. This is due to the higher mode-II stresses introduced during bending which cause different damage evolution behaviour to that seen for axial stresses. Finite element results revealed that both friction and shear strength parameters generated from through-thickness compression stresses have a significant effect in predicting damage in fibre-metal laminate structures under this type of loading.

Influence of metal/composite interface on the damage behavior and energy absorption mechanisms of FMLs against projectile impact

This paper focuses on the interface failure in metal/GFRP laminates on account of the high-velocity impact phenomenon by a hemispherical projectile. The study considers three laminates in which the failure inside the 8-layer 0/90 GFRP laminate is compared with the other two laminates that include metal layers in their layup configuration. The metal layers were placed on the top and bottom on one type of laminates while in the other additional metal layers are placed symmetrically inside the layup as well. They were subjected to high-velocity impact by a hemispherical projectile at different energy levels and the idea is not to perforate the laminate configuration instead to account for the damage incurred in these laminates and the role of metal layers in providing resistance to damage within these laminates. The study utilizes experimental findings and proposes a rate-dependent Finite Element (FE) model consisting of the Hashin-Puck failure scheme for composite and the Johnson-Cook damage model for metal layers. The results of the model satisfactorily agree with their experimental counterparts and provide valuable insight into the damage resistance inside the laminates. It has been observed that the 8-layer GFRP laminate was good in terms of elastic recovery and prevention of propagation of damage inside the laminates only, till the impact energy was lower. For higher impact energy, they show poor damage resistance as the fiber failure is triggered in them. However, laminates with metal layers are shown to protect the laminate by dissipating energy in the delamination of metal/GFRP interface, shear failure of the metal layer, and on account of metal plasticity. The study further shows that the through-thickness compressive stresses were responsible for the failure of laminates and also triggering the delamination in them. A damage energy study was performed to investigate the amount of energy dissipating in various failure modes like delamination, matrix cracking, fiber failure, etc.

A delamination failure criterion considering the effects of through-thickness compression on the interlaminar shear failure of composite laminates

Through-thickness compressive stress of composite laminates can inhibit the interlaminar shear failure which has been proved by experiments. However, common delamination damage initiation criterion cannot well describe this mechanism. For this issue, in this article, based on the insight of the failure mechanism, an interlaminar failure function is constructed at first. Then, a delamination failure criterion considering the effect of through-thickness compression on interlaminar shear failure is established based on logical deductions from a polynomial expansion of the failure function. In the criterion, a constant coefficient with physical meaning is used to describe the effect of through-thickness compression on the interlaminar shear failure, and the estimation method of the coefficient is also given in this study. Here, this failure criterion can be considered as a general form to describe the delamination between laminae, by defining different coefficient, it can be converted to other criteria such as the conventional secondary stress failure criterion or the Christensen failure criterion. Finally, by comparing the numerical calculation and experimental results, the delamination failure criterion proposed in this study exhibited a good prediction ability for the interlaminar damage under different loading conditions.

The effect of through-thickness compressive stress on mode II interlaminar crack propagation: A computational micromechanics approach

A micromechanics framework for modelling the mode II interlaminar crack onset and propagation in fibre-reinforced composites is presented with the aim of i) modelling the micro-scale failure mechanisms that underlie interlaminar crack propagation, and ii) determining the effect of the through-thickness pressure on mode II fracture toughness. An algorithm for the generation of the fibre distribution is proposed for the generation of three-dimensional Representative Volume Elements (RVEs). Appropriate constitutive models are used to model the different dissipative effects that occur at crack propagation. Numerical predictions are compared with experiments obtained in previous investigations: it is concluded that the proposed micromechanical model is able to simulate conveniently the interlaminar crack propagation and to take into account the effect of the through-thickness pressure on mode II interlaminar fracture toughness.

The effect of through-thickness compressive stress on mode II interlaminar fracture toughness

The effect of through-thickness compressive stress on mode II interlaminar fracture toughness is investigated experimentally and replicated numerically. The modified Transverse Crack Tensile specimen recently proposed by the authors is used, together with an experimental device designed to apply a constant transverse compressive stress on the surface of the specimen. Experiments are conducted using IM7/8552 specimens for different compressive stresses, ranging from 0 to 100MPa, covering all the practical applications commonly encountered in the aeronautical industry (e.g., tightened filled holes or bolted joints). It is shown that mode II interlaminar fracture toughness increases with the applied compressive through-thickness stress. Finally, experiments are replicated using appropriate numerical models based on cohesive elements that take into account frictional effects. A good agreement between numerical predictions and experiments is found.

A cohesive zone model taking account of the effect of through-thickness compression

Experiments in the literature show that through-thickness compressive stress significantly enhances interfacial fracture resistance. Most existing cohesive zone and interface element models which consider the enhancement only introduce friction in the model. In this paper, a new method is proposed to include both friction and enhanced interfacial shear strength in a cohesive zone model. Contact and friction at micro/macro crack closure is added to the cohesive constitutive law. A traction-based failure function and an energy-based failure function are employed and combined to construct a damage surface. An enhancement of interfacial shear strength due to through-thickness compressive stress is introduced into the traction-based function which governs the damage initiation, while the energy-based function controls the damage growth. The damage surface shrinks in the traction space as damage develops and leads to a softening cohesive constitutive law. The model is employed to simulate shear failure of symmetric double notch specimens and delamination failure in specimens with cut- and dropped-plies. Numerical predictions are in good agreement with available experimental data in the literature. Parametric studies show that both the friction at crack closure and enhancement of the interfacial shear strength play an important role in enhancing fracture resistance.

A finite element analysis of bolted joints loaded in tension: protruding head and countersunk fastener

PurposeThe purpose of this paper is to investigate the effect of fastener geometry (protruding head and countersunk fastener) and friction coefficient on the stress distributions around the hole of the double-lap single bolted aluminium alloy joints.Design/methodology/approach3D finite element analyses of double-lap bolted 7075-T6 aluminium joints were carried out. An elastic-plastic multi-linear kinematic hardening material behaviour was assumed for the Al alloy. Contact was defined using an augmented-Langrange contact algorithm, including the friction effect. Bolt clamping force and remote axial tensile loading were applied in two load steps and their separate and combined effects on the joint behaviour were investigated for two types of fastener configurations.FindingsIt was observed that bolt clamping reduces the axial tensile stress at the hole edge by introducing a through-thickness compressive stress. This reduction in stress concentration may have a beneficial effect on the fatigue life of the joint. Second, bolt clamping reduces the bearing stress at the fastener hole by creating a frictional force between the joint plates. Results showed that the joint with protruding head fastener shows lower tensile stress concentration, and lower bearing stress, near the bolt hole of the middle plate.Originality/valueBolt clamping force reduces both the stress concentration near the hole edge and the bearing stress at the hole by creating a frictional force. Joint with a protruding head fastener may lead to higher load carrying capability and improved fatigue life. Friction coefficient affects the stress levels around the bolt hole.

Effect of through-thickness compression on in-plane tensile strength of glass/epoxy composites: Experimental study

The effect of through-thickness compression on in-plane tensile strength of glass/epoxy composites with random microstructure was investigated experimentally. The studied composite laminates were manufactured with a self-regulating Resin Transfer Moulding device. Their mechanical behaviour was assessed in pure in-plane tensile and through-thickness compressive tests, followed by biaxial tests combining both loading modes; indenters with a radius ranging from 5 to 25 mm were used to impose a compressive mode. The obtained results demonstrate a nonlinear decreasing trend for the in-plane tensile strength under the growing through-thickness compressive stress. All the failed specimens showed catastrophic brittle failure with a specific fracture orientation that mainly exhibited a tensile mode of fibre fracture for smaller radii of indenters and a combination of matrix crack, fibre fracture and typical shear failure for larger radii.

The effect of normal stress on the formability of sheet metals under non-proportional loading

Theoretical and experimental studies have shown that when a compressive stress is applied normal to the sheet surface its formability can be significantly improved. In forming processes such as hydroforming, the normal stress imposed at the surface of the sheet or tube can be very significant in specific locations and it is therefore necessary to account for this stress state when assessing the forming severity of industrial parts. It is also well known that the forming limit curve (FLC) can vary significantly in strain space as a result of non-linear strain paths. In this work, a numerical code based on the Marciniak and Kuczynski (MK) analysis (1967) was developed to predict the FLC of sheet metals by simultaneously accounting for the effects of strain path non-linearity and the normal stress. The FLC predicted with this code can be used to assess the forming severity of parts that are deformed under complex loading conditions in industrial metal forming processes. The predictive FLC model was validated using experimental data in which linear and bi-linear strain paths were applied to different steel and aluminum sheets or tubes under plane stress and three-dimensional stress conditions. This research work has two specific purposes: first, to investigate the influence of the magnitude of the prestrain in bi-linear loading on the sensitivity of the FLC to the normal stress, and second, to study the effect of the normal stress on the path dependence of the FLC. This work showed that, with increasing magnitudes of prestrain the influence of the normal stress on the formability of sheet metal will decrease, regardless of the loading path. Moreover, it was observed that even a significant normal stress will practically not affect the path dependence of the FLC. • Forming Limit Curves (FLC) were predicted in 3D stress states and non-linear loading paths. • The effect of prestrain magnitude on the sensitivity of the FLC to the through-thickness compressive stress was investigated. • The effect of normal stress on the path-dependence of FLC was studied. • It was shown that higher prestrain magnitudes will reduces the sensitivity of FLC to the normal stresses. • In most cases, the normal pressure on the sheet metal surface will not affect the path-dependence of FLC.

Effect of high through-thickness compressive stress on fibre direction tensile strength of carbon/epoxy composite laminates

The fibre-direction tensile strength of carbon/epoxy laminates under the influence of through-thickness compressive stresses has been experimentally investigated. In a unidirectional (UD) laminate, the through-thickness compressive stresses will cause premature longitudinal fibre-splitting, masking the effect of transverse stresses on the fibre-direction strength. Here a cross-ply laminate has been used. With the addition of 90° plies preventing the 0° fibres from splitting, it effectively allows the dependence of fibre-direction tensile strength on high through-thickness stresses to be studied. The severity of the through-thickness loads has been varied using cylindrical indenters of different radii, up to the loads near the through-thickness compressive failure stress of the cross-ply laminate. The results show that there is a linear decrease in fibre-direction strength with the mean through-thickness stress. In all the test cases, the specimens failed in a catastrophic brittle manner, with scanning electron micrography showing primarily a fibre tensile fracture mode. The detailed stress state in the specimens has been calculated via finite element analysis. Two failure criteria are proposed, which can be used as conservative design criteria concerning fibre-dominated failures in multiaxial load scenarios.

Predicting the effect of through-thickness compressive stress on delamination using interface elements

The failure of interface elements is typically based on interactive mixed mode criteria for both initiation and propagation of damage. The effect of tensile through-thickness stress is normally taken into account in combination with interlaminar shear stress. When the through-thickness stress is compressive, however, its effect is usually ignored and the failure of interface elements is considered to be pure mode II. Experiments on single-lap, cut-ply and dropped-ply specimens however show that the compressive through-thickness stress can greatly increase the delamination failure stress and cannot be simply neglected. The influence of compressive stress on mode II damage evolution is investigated numerically based on the cut-ply and dropped-ply experiments. A new interfacial failure model with modified failure initiation and propagation criteria is proposed to take the effect of compression on matrix shear strength and mode II critical fracture energy, G IIC, into account. The new model uses one independently determined parameter to relate the compression to the increase in interlaminar shear strength and G IIC. With the new failure criterion applied, two types of cut-ply models and two types of dropped-ply models, using the same input parameter, all produce excellent correlation with experimental delamination stresses. As a validation case the single-lap model was run with the new criterion and the same input parameters and this also achieves very good correlation with the experimental failure stress.

Through-thickness stress sensing of a carbon fiber polymer–matrix composite by electricalresistance measurement

Through-thickness stress self-sensing in a quasi-isotropic carbon fiber epoxy–matrixcomposite by in-plane electrical resistance measurement is effective. The resistancedecreases reversibly upon through-thickness compression conducted up to 67 MPa, due toan increase in the proximity between adjacent laminae. The sensing can be attainedby measuring the surface resistance in the direction of the surface fibers or bymeasuring the volume resistance in essentially any in-plane direction. The sensingis ineffective if the transverse surface resistance is the quantity measured, dueto the dominance of the surface fibers in governing the surface resistance. Inthe case of the longitudinal surface resistance, the decrease in resistance uponcompression has a slight irreversible component, due to an irreversible increase in theproximity between adjacent laminae and the consequent increase in the degree ofcurrent penetration. This effect is smaller for the longitudinal or transverse volumeresistance. The variability of the resistance from area to area in the same laminate islarger for the surface resistance than the volume resistance, due to its highersensitivity to current spreading. The sensitivity of stress sensing, as described by thefractional change in resistance per unit through-thickness compressive stress, is−10−5 MPa−1. The magnitude of the effectiveness is lower for the resistance away from the stressedregion than that at the stressed region.

Stamp Thermo-Hydroforming: A New Method for Processing Fiber-Reinforced Thermoplastic Composite Sheets

The stamp thermo-hydroforming process involves supporting the thermoplastic sheet with a bed of heated viscous fluid that applies a hydrostatic pressure across the part throughout the forming process. This hydrostatic pressure produces a through-thickness compressive stress that delays the onset of delamination, reduces the formation of wrinkles due to frictional traction forces, and results in better-formed parts. The overall goal of this research is to verify, through experimentation and numerical modeling, that the stamp thermo-hydroforming process provides a suitable alternative to conventional thermoforming methods, such as bag molding and stamp forming, as a means for processing glass-mat fiber-reinforced thermoplastic materials. The goal of this paper is to solely address whether the use of a counteracting pressure can be beneficial during the forming of fiber-reinforced thermoplastic sheets more specifically; this paper addresses the experimental setup that was developed and built to conduct small-scale stamp thermo-hydroforming experiments with a hemispherical punch, and discusses some results obtained using this setup. Experiments were performed on a random oriented continuous glass-mat fiber-reinforced polypropylene with hydrostatic pressure applied locally on the top surface of the composite sheet, as it was being stretched by a hemispherical punch. An increase of up to 10% in draw depth was achieved by applying local hydrostatic pressures up to 207 kPa (30 psi). In addition to the draw depth improvements, the application of the counteracting hydrostatic pressure also delayed the onset of delamination.

The Behavior of Compressively Loaded Stiffener Runout Specimens – Part II: Finite Element Analysis

The termination of stiffeners in composite aircraft structures give rise to regions of high interlaminar shear and peel stresses as the load in the stiffener is diffused into the skin. This is of particular concern in co-cured composite stiffened structures where there is a relatively low resistance to through-thickness stress components at the skin–stiffener interface. In Part I, experimental results of tested specimens highlighted the influence of local design parameters on their structural response. Indeed some of the observed behavior was unexpected. There is a need to be able to analyse a range of changes in geometry rapidly to allow the analysis to form an integral part of the structural design process. This work presents the development of a finite element methodology for modelling the failure process of these critical regions. An efficient thick shell element formulation is presented and this element is used in conjuction with the Virtual Crack Closure Technique (VCCT) to predict the crack growth characteristics of the modelled specimens. Three specimens were modelled and the qualitative aspects of crack growth were captured successfully. The shortcomings in the quantitative correlation between the predicted and observed failure loads are discussed. There was evidence to suggest that high through-thickness compressive stresses enhanced the fracture toughness in these critical regions.