The effect of glass-resin interface strength on the impact strength of fiber reinforced plastics

The effect of glass-resin interface strength on the impact energy of glass fabric (style 181) reinforced epoxy and polyester laminates has been determined. The interface strength was altered by surface treatment of the fabrics with silane coupling agents and with a silicone fluid mold release and the interlaminar shear strength was determined as a means to evaluate the interface strength. An instrumented Charpy impact test was used on unnotehed specimens and thus both initiation and propagation energies could be determined as well as dynamic strength. It was found that the initiation energy for both polyester and epoxy laminates increased with increasing interlaminar shear strength, The propagation energy and thus the total energy for polyester laminates displays a minimum at a critical value of interlaminar shear strength (ILSS). Below this critical value, the total impact energy increases with decreasing shear strength and the dominant energy absorption mode appears to be delamination. Above the critical value, the impact energy increases with increasing values of ILSS and the fracture mode is predominantly one of fiber failure. In all cases, even with mold release applied, the shear strength of epoxy laminates was above this critical value and-thus the total impact energy increases with Increasing values of ILSS. The maximum energy absorbed for the epoxy laminate and the polyester laminate is nearly identical. However, the maximum for the epoxy laminate occurs when the shear strength is maximized while for the polyester laminate the shear strength must be minimized. For the polyester laminate when delamination is predominant, it was found that the glass surface treatment affects the amount of delamination as opposed to the specific value of delamination fracture work.     

Effect of Joint Geometry on the Behavior and Failure Modes of Sandwich T-Joints Under Transverse Static and Dynamic Loads

Due to wide spread application of adhesive T-joints in various industries, a review of properties and strength of their fracture modes under static and dynamic loadings is required. By defining the ability of failure in the joint material and fracture of adhesive in the numerical model, fracture modes of sandwich T-joints have been investigated. This paper presents numerical results on the performance of sandwich T-joints subjected to static tensile, static transverse, and dynamic transverse loading. The results of available experiments in the literature have been used to validate the detailed numerical models capable of simulating the damage processes observed. In general, the failure load predicted by the finite element (FE) analysis is within 5% of the experimental results.     

Fracture behavior of polyetherimide (PEI) and interlaminar fracture of CF/PEI laminates at elevated temperatures

To investigate the effects of environmental temperature on fracture behavior of a polyetherimide (PEI) thermoplastic polymer and its carbon fiber (CF/PEI) composite, experimental and numerical studies were performed on compact tension (CT) and double cantilever beam (DCB) specimens under mode‐I loading. The numerical analyses were based on 2‐D large deformation finite element analyses (FEA). Elevated temperatures greatly released the crack tip triaxiality (constraint) and promoted matrix deformation due to low yield strength and enhanced ductility of the PEI matrix, which resulted in the greater plane‐strain fracture toughness of the bulk PEI polymer and the interlaminar fracture toughness of its composite during delamination propagation with increasing temperature. Furthermore, the high triaxiality was developed around the delamination front tip in the DCB specimen, which accounted for the poor translation of matrix toughness to the interlaminar fracture toughness by suppressing the matrix deformation and reducing the plastic energy dissipated in the plastic zone. Especially, at delamination initiation, the weakened fiber/matrix adhesion at elevated temperatures led to premature failure of fiber/matrix interface, suppressing matrix deformation and preventing the full utilization of matrix toughness. Consequently, low interlaminar fracture toughness was obtained at elevated temperatures. POLYM. COMPOS., 26:20–28, 2005. © 2004 Society of Plastics Engineers.     

Controlled energy dissipation in fibrous composites I. Controlled delamination

Delamination mechanisms in continuous fiber reinforced composites were investigated. The concept of controlled interlaminar bonding (CIB) is proposed as a guideline for preparing fiber-epoxy composite laminates with enhanced fracture toughness without significant degradation in strength properties. The interlaminar bonding was manipulated by several specialized techniques including insertion of delamination promotors and surface modification of laminae. Results indicated that the plane-strain fracture toughness of E-glass-epoxy laminates could be improved by inserting perforated interlaminar films of aluminum, paper, polyester and polyimide, and fabrics. Such interlayers were used to promote delamination which dissipate strain energy by blunting and diverting a propagating crack. The fracture resistance of a laminate was found to be dependent on the degree of delamination. The competition between the growth of delamination cracks and the propagation of a main crack is controlled by the relative magnitude of the interlaminar bonding strength and the lamina cohesive strength. The interlaminar bonding is controlled by the degree of interlayer perforation and the adhesion between interlayer and lamina. The loading direction was found to be very important in dictating the failure processes. Experimental results from several composite systems are presented and discussed along with post-failure analysis data.     

Interlaminar fracture toughness and fatigue fracture of continuous fiber-reinforced polymer composites with carbon-based nanoreinforcements: a review

ABSTRACT

This review critically examines the recent developments in the use of carbon-based nanofillers as additional reinforcement to enhance the interlaminar properties of FRP composites. The low interlaminar strength of FRP composites results in delamination failure. The various nanoreinforcement strategies and their effect on fracture toughness, interlaminar shear strength (ILSS) and interlaminar fatigue are discussed in detail to prevent this delamination failure. Important findings on various factors that influence the interlaminar properties of multi-scale composites are presented by discussing various intrinsic and extrinsic toughening processes. Moreover, an overview of simulation techniques is provided to predict the delamination onset and propagation.     

Curing Characteristics and Mechanical Properties of Oil Palm Wood Flour Reinforced Epoxidized Natural Rubber Composites

Abstract

The paper reports on the curing characteristics and mechanical properties of oil palm wood flour (OPWF) reinforced epoxidized natural rubber (ENR) composites. Three sizes of OPWF at different filler loadings were compounded with a two roll mill. The cure (t ₉₀) and scorch times of all filler size decrease with increasing OPWF loading. Increasing OPWF loading in ENR compound resulted in reduction of tensile strength and elongation at break but increased tensile modulus, tear strength and hardness. The composites filled with smaller OPWF size showed higher tensile strength, tensile modulus and tear strength. Scanning electron microscope (SEM) micrographs showed that at lower filler loading the fracture of composites occurred mainly due to the breakage of fibre with minimum pull-out of fibres from the matrix. However as the filler loading is increased, the fibre pull-out became very prominent due to the lack of adhesion between fibre and rubber matrix.     

Time and temperature dependence of fracture in a unidirectional glass-reinforced epoxy

The results of a study on the time- and temperature-dependent behaviour of unidirectional glass fibre-reinforced epoxy are described and analysed. The fracture parameters examined are the fracture strength, the work of fracture and the apparent fracture toughness. It is shown that the fracture strength decreases with increasing temperature and decreasing loading rate; the work of fracture exhibits a sharp minimum in the vicinity of room temperature, and the fibre pull-out length increases by a factor of 4 at 76K as compared with the room temperature length; the fracture toughness is found to be independent of the crack length and only dependent on the fracture strength; thus its trend with loading rate and temperature follow those of the fracture strength.     

Influence of the interfacial resin layer on delamination initiation in broken ply composite laminates

The present study has investigated the influence of a resin layer on the delamination initiation at the interface of broken and continuous plies in the case of GR/E (graphite/epoxy) laminates with broken central plies. A full three-dimensional (3D) finite element (FE) analysis was performed with each layer of the laminate modelled as homogeneous and orthotropic. The interface between the broken and the continuous plies was modelled with a thin resin-rich layer. Eight-noded isoparametric layered elements were used to model the laminate specimen. Also, 3D contact elements were used to prevent inter-penetration of the delaminated faces at the interface. Based on the results of the 3D FE analysis, strain energy release rates were calculated at the delamination front using Irwin's 'crack closure integral'. Using the concepts of linear elastic fracture mechanics (LEFM), the strain energy release rate was used as a parameter for assessing delamination initiation. The effects of various factors such as resin layer stiffness, resin layer thickness, and fibre orientation at the interface on the three components of the strain energy release rates, namely G_I, G_II and G_III, were studied for laminates with various crack sizes of the broken ply, and the influence of the resin layer in the delamination initiation was established. It was observed that delamination initiation is a mixed-mode phenomenon even in the case of uniaxial loading and the dominance of the mode of delamination is governed by the resin layer stiffness, thickness, and lamina orientation at the interface. The present work also concludes that an increase in the resin layer modulus leads to an increase in the probability of mode I delamination while the probability of mode II delamination decreases. A 0/90 interface exhibits a higher chance of delamination in modes I and II, while mode III delamination is maximum for 0/30 and 0/60 fibre orientation interfaces. It was also observed that the larger the crack width, the greater the probability of delamination initiation at the interface.     

Mixed-mode failure of interfaces studied by the 2D linear elastic–brittle interface model: Macro- and micro-mechanical finite-element applications in composites

Macro-scale delamination and micro-scale fiber–matrix debonding events may notably affect the mechanical performance of fibrous composite elements. This article presents a two-dimensional finite-element (FE)-based formulation of interface of a small but finite thickness relying on the so-called linear elastic-brittle interface model (LEBIM) to be applied for simulation of an adhesive interface debonding and fiber–matrix decohesion failures. This modeling strategy is implemented in the commercial FE package ABAQUS by means of the user-defined subroutine UMAT. The practicability of the developed interface model is assessed through the comparison of the computational results with experimental data and with previous boundary element method (BEM) analyses using the LEBIM formulation. Specifically, LEBIM results for the interlaminar fracture toughness test showed an excellent agreement with experimental results (adhesive saw-tooth post-peak response was captured). Besides, studies of several micro-mechanical fiber–matrix configurations showed that fiber–matrix debonding events are the predominant failure mechanisms for moderate transverse loading values. The developed tool will certainly contribute to elucidate several open aspects regarding the interface crack behavior in fiber-reinforced composite materials.     

Interlaminar fracture behaviour of re-formed bamboo/aluminium sheet laminates

Sandwich laminates containing re-formed bamboo core and aluminium face sheets were produced using two different types of adhesive: an epoxy and a modified polyethylene. The interlaminar fracture behaviour of the laminates was characterized based on peel and lap-shear tests. It was shown that the laminates bonded with polyethylene had much higher peel and shear strengths than those bonded with epoxy. For the polyethylene-bonded laminates, the major failure mechanisms were a combination of cohesive and interfacial failure, whereas for the epoxy-bonded laminates, the fracture occurred almost exclusively along the aluminium/epoxy interface. There was a significant dependence of the failure mechanism and interlaminar strength on the loading direction relative to the bamboo fibre axis and on whether the aluminium sheets were bonded to the inner or outer bamboo surface.     

风电叶片复合材料层间剪切破坏声发射监测

采用声发射技术对含分层缺陷风电叶片多轴向复合材料的层间剪切破坏实验进行实时监测,研究分层缺陷对复合材料层间力学性能的影响规律及其损伤破坏过程的声发射响应特征.结果表明,具有不同分层面积的两类复合材料试样破坏载荷相近,当分层缺陷位于剪切面中间位置时,分层缺陷大小对界面承载能力影响不大,损伤演化主要集中在剪切面上偏离中心两...     

Development and characterisation of high-density oxide fibre-reinforced oxide ceramic matrix composites with improved mechanical properties

A low cost and reliable ceramic matrix composite fabrication route has been developed. It involves the coating of 2D woven ceramic fibres (Nextel? 720) with oxide nano-size ceramic particles by electrophoretic deposition (EPD) followed by impregnation of the coated fibres with ceramic matrix and warm pressing at 180 °C to produce the “green” component ready for pressureless sintering. The effects of two different weak interface materials, NdPO₄ and ZrO₂, on the thermomechanical properties of the composites are also examined. Damage mechanisms, such as debonding, fibre fracture, delamination and matrix cracking within the composite plates subjected to tensile loading are analysed using acoustic emission technique and correlated with microstructure. It is shown that the composites with NdPO₄ interface, 10% porosity and 40 vol.% fibre loading have superior themomechanical properties in terms of strength and damage-tolerant behaviour in multilayer plate form. The improved sinterability and microstructure stability at moderate temperatures ensure both the fibre integrity and load transfer efficiency resulting in high strength damage-tolerant composites. The final components produced are considered to be suitable for use as shroud seals and insulating plates for combustor chambers in aircraft engines.     

Interlaminar fracture toughness of selectively stitched thick carbon fibre reinforced polymer fabric composite laminates

Abstract

Carbon fibre reinforced polymer fabric specimens prepared from selectively stitched thick laminates have been tested under mode I (tension) and mode II (shear) loading, similar to already established tests used for thin unidirectional specimens. The respective interlaminar fracture toughness characteristics were derived for laminates of different stitching configurations. Results indicated significant interlaminar fracture toughness increase for all stitched samples compared with non-stitched samples, especially under mode I loading. It was concluded from parametric investigations that carbon thread stitching is more effective than its aramid counterpart in improving interlaminar fracture toughness. This is attributable to its higher stiffness and better bonding to the carbon fibre reinforced polymer system compared with the aramid thread.     

Experimental and numerical investigations of adhesively bonded CFRP single-lap joints subjected to tensile loads

In this study, both experimental tests and numerical simulation are implemented to investigate the tensile performance of adhesively bonded CFRP single-lap joints (SLJs). The study considers 7 different overlap lengths, 5 adherend widths and 3 stacking sequences of the joints. Three-dimensional (3D) finite element (FE) models are established to simulate the tensile behavior of SLJs. The failure loads and failure modes of SLJs are investigated systematically by means of FE models and they are in good agreement with those of experiments, proving the accuracy of finite element method (FEM). It is found that increasing the adherend width can improve the load-carrying capacity of the joint better than increasing the overlap length does. Moreover, choosing 0° ply as the first ply is also beneficial for upgrading joint's strength. With respect to failure modes, cohesive failure in adhesive and delamination in adherend take dominant, while matrix cracking and fiber fracture only play a small part. With overlap length increasing or adherend width decreasing, cohesive failure takes up a smaller and smaller proportion of whole failure area, but the opposite is true for delamination. SLJs bonded with [0/45/-45/90]_3S adherends are prone to cohesive failure, and [90/-45/45/0]_3S adherends are easy to appear delamination. Both shear and peel stress along the bondline indicate symmetrical and non-uniform distributions with great stress gradient near the overlap ends. As the load increases, the high stress zone shifts from the end to the middle of the bondline, corresponding to the damage initiation and propagation in the adhesive layer.     

Low‐velocity impacts in continuous glass fiber/polypropylene composites

The low‐velocity impact behavior of a continuous glass fiber/polypropylene composite was investigated. Optical microscopy and ultrasonic scanning were used to determine the impact‐induced damage. At low impact energy, the predominant damage mechanism observed was matrix cracking, while at high energy the damage mechanisms observed were delamination, plastic deformation, which produced a residual specimen curvature, and a small amount of fiber breakage at the edge of the indentation on the impacted face of the specimens. The impact load vs. time signals were recorded during impact and showed that the load corresponding to the onset of delamination was independent of the impact energy in the range tested. The load at which the onset of delamination occurred corresponded to the values obtained by performing a linear regression of the delaminated area, obtained by ultrasonic scanning, as a function of the impact force. Tensile and flexural tests performed on impacted specimens showed that the tensile and flexural residual strengths and the flexural modulus decreased with increasing incident impact energy, while the post‐impact residual tensile modulus remained constant. The dynamic interlaminar fracture toughness was evaluated from the critical dynamic (impact) strain energy release rate of specimens with a delamination simulated by an embedded insert. The results are compared with the interlaminar fracture toughness values obtained during subcritical steady crack growth.     

Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

SiC_f/SiC composites with BN interface were prepared through isothermal-isobaric chemical vapour infiltration process. Room temperature mechanical properties such as tensile, flexural, inter-laminar shear strength and fracture toughness (K_IC) were studied for the composites. The tensile strength of the SiC_f/SiC composites with stabilised BN interface was almost 3.5 times higher than that of SiC_f/SiC composites with un-stabilised BN interphase. The fracture toughness is similarly enhanced to 23 MPa m^1/2 by stabilisation treatment. Fibre push-through test results showed that the interfacial bond strength between fibre and matrix for the composite with un-stabilised BN interface was too strong (>48 MPa) and it has been modified to a weaker bond (10 MPa) due to intermediate heat treatment. In the case of composite in which BN interface was subjected to thermal treatment soon after the interface coating, the interfacial bond strength between fibre and matrix was relatively stronger (29 MPa) and facilitated limited fibre pull-out.     

Mode I Fracture Load Predictions of Adhesive T-joints

This paper presents fracture data and a finite element analysis for adhesive T-joints, It is shown that fracture loads of T-joints, bonded with two different structural epoxies and subjected to either tensile loading or three-point bending, can be predicted using a fracture mechanics approach. Fracture loads were predicted by calculating the applied energy release rate, G, using finite element methods, and comparing that with a critical value, G_c, determined experimentally using double-cantilever-beam specimens. By recording the failure sequence of the bondline with a video camera attached to a microscope, it was seen that subcritical crack propagation took place prior to final fracture of the bondline. Accounting for the observed subcritical crack propagation in the finite element analysis gave a good agreement between the actual and the calculated fracture loads.     

Progressive failure analysis of bolted joints in composite laminates

Abstract

A three-dimensional progressive failure analysis methodology was developed to predict the strength of double lap bolted joints in [0°/90°/±45°]_2s carbon fibre reinforced plastic laminates. An experimental programme was conducted to verify and validate the proposed computational model. Good agreement was obtained between the experimental data and predictive model. A parametric study was conducted for varied clamping torque and friction coefficient values. The well known effect of those variables on the joint strength was captured. Although the in-plane mode of failure of each individual layer around the fastener hole was predicted, X-ray radiographs have shown that delamination failure is particularly dominant around the washer’s outer edge. At present, the proposed model does not account for delamination onset and propagation. Future work will involve implementing cohesive zone elements in regions of interest to capture this interlaminar cracking, a work in which the authors are currently engaged in.     

Damage assessment of alumina fibre-reinforced mullite ceramic matrix composites subjected to cyclic fatigue at ambient and elevated temperatures

The damage evaluation behaviour of alumina fibre-reinforced mullite ceramic matrix composites subjected to cyclic fatigue was investigated by means of acoustic emission (AE) monitoring and forced resonance techniques. AE technique provided sufficient information about the damage initiation and progression in real time whilst the forced resonance (FR) technique allowed the detection of changes in elastic modulus (E) and internal friction (Q⁻¹) that occurred with increasing number of cyclic fatigue at room temperature. From the two non-destructive detection techniques results combined with microstructural observations, it is concluded that the composite cyclic fatigue damage evolution begins with multiple crack formation within the matrix and is followed by delamination (interfacial failure). Final failure of the composite is caused by fibre fracture and extensive cyclic sliding along the fibre/matrix interface. The strong bonding between mullite matrix and alumina fibre caused by the glassy phase within the mullite matrix determined the fatigue performance of the composite at 1350°C. Regions with glassy phase failed catastrophically as a result of early fibre fracture.     

Microstructure and damage evolution of SiCf/PyC/SiC and SiCf/BN/SiC mini-composites: A synchrotron X-ray computed microtomography study

SiC_f/PyC/SiC and SiC_f/BN/SiC mini-composites comprising single tow SiC fibre-reinforced SiC with chemical vapor deposited PyC or BN interface layers are fabricated. The microstructure evolutions of the mini-composite samples as the oxidation temperature increases (oxidation at 1000, 1200, 1400, and 1600?°C in air for 2?h) are observed by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction characterization methods. The damage evolution for each component of the as-fabricated SiC_f/SiC composites (SiC fibre, PyC/BN interface, SiC matrix, and mesophase) is mapped as a three-dimensional (3D) image and quantified with X-ray computed tomography. The mechanical performance of the composites is investigated via tensile tests.The results reveal that tensile failure occurs after the delamination and fibre pull-out in the SiC_f/PyC/SiC composites due to the volatilization of the PyC interface at high temperatures in the air environment. Meanwhile, the gaps between the fibres and matrix lead to rapid oxidation and crack propagation from the SiC matrix to SiC fibre, resulting in the failure of the SiC_f/PyC/SiC composites as the oxidation temperature increases to 1600?°C. On the other hand, the oxidation products of B₂O₃ molten compounds (reacted from the BN interface) fill up the fracture, cracks, and voids in the SiC matrix, providing excellent strength retention at elevated oxidation temperatures. Moreover, under the protection of B₂O₃, the SiC_f/BN/SiC mini-composites show a nearly intact microstructure of the SiC fibre, a low void growth rate from the matrix to fibre, and inhibition of new void formation and the SiO₂ grain growth from room to high temperatures. This work provides guidance for predicting the service life of SiC_f/PyC/SiC and SiC_f/BN/SiC composite materials, and is fundamental for establishing multiscale damage models on a local scale.