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The finite element method was used to simulate indentation tests on a particle embedded in a matrix, to investigate the influence of the properties of the particle and the matrix, and the indentation depth on the measured hardness. The particle’s work-hardening exponent and the mismatch in particle and matrix yield strength have a significant influence on the measured hardness. A particle-dominated indentation depth was identified, within which the measured nanoindentation hardness agrees very well with the true hardness of the particle material. Numerical results from the simulations of a wide range of material properties determined that the measured hardness is within 5% difference of the particle’s true hardness when the indentation depth is less than 13.5% of the particle’s radius. The results can be used in practice as a guideline to measure the hardness of a particle embedded in a matrix and provides the theoretical basis to develop a particle-embedded method to measure the hardness of individual particles. [ABSTRACT FROM PUBLISHER]

Abstract: The application of the indentation method to measure the elastic modulus of particles embedded in a composite is theoretically investigated in this paper by finite element simulation. The Oliver–Pharr method, which is widely used in commercial nanoindentation instruments, is used to probe the elastic modulus of the particle from the simulated indentation curve. The predicted elastic modulus is then compared with the inputted value. Two cases are studied, that of a stiff particle embedded in a soft matrix and a soft particle embedded in a stiff matrix. In both of these cases, there exists a particle-dominated depth. If the indentation depth lies within this particle-dominated depth, the Oliver–Pharr method is able to be applied to measure the particle’s elastic modulus with sufficient accuracy if the real contact area is used. This could lead to an experimentally-convenient method of determining the primary properties of individual particle, providing they can be well dispersed in the polymeric matrix. [Copyright &y& Elsevier]

Abstract: Carbon nanotube (CNT)–hybridized carbon fibre (CF) composite is a new generation composite, where CNTs grow radially on carbon fibres to form a hybrid reinforcing phase. To evaluate the bridging effect of this new reinforcing phase, a numerical method is proposed to theoretically investigate the pullout of a hybrid fibre. There are two finite element models developed in this method, which are applied to simulate a single CNT pullout from the matrix at microscale and the pullout of the hybrid fibre at macroscale. The bridging effect of the CNTs during the hybrid fibre pullout is simulated by spring elements in the macroscale finite element model, where the properties of spring elements are obtained from the microscale finite element simulation. The numerical results indicate that the apparent interfacial shear strength of the hybrid fibre and the specific pullout energy can be significantly increased due to the additional bonding of the CNT–matrix interface. A parametric study indicates that the bridging effect of the hybrid fibre can be further enhanced by improving the interfacial bonding between CNT and matrix and increasing the size or length of CNTs. This study provides a new numerical method to simulate the multiscale CNT/CF hybrid fibre pullout. [Copyright &y& Elsevier]

Abstract Scarf or stepped-lap repairs are desirable for composite structures in aerospace, especially where a flush finish is required. Damage tolerance is a critical issue in the repair design. This paper investigates and compares the damage tolerance of scarf and stepped-lap joints (repairs) under quasi-static loading using finite element analysis (FEA). The damage was represented by a flaw embedded in the bondline between composite adherents. 3D FE models were built for both scarf and stepped-lap joints. Both linear and nonlinear adhesive properties were simulated under room temperature (RT) and hot-wet (HW) conditions. The flaws with varying length and width were imbedded in the bondline, so that the sensitivity of damage tolerance to the flaw size can be analysed and compared between scarf and stepped-lap joints. Other parameters, which may affect the damage tolerance of both joints, include flaw location, number of steps of stepped-lap joint, stacking sequence of laminate adherend, out-of-plane boundary conditions and the presence of an external doubler. The results showed that the stepped-lap joint exhibited better damage tolerance than the scarf joint with linear elastic adhesive. The damage tolerance of both joints became similar when the adhesive became nonlinear under RT or HW conditions. The damage tolerance of both joints can be improved by attaching composite doubler reinforcement or constraining the out-of-plane movement. [ABSTRACT FROM AUTHOR]

A 3D intralaminar continuum damage mechanics based material model, combining damage mode interaction and material nonlinearity, was developed to predict the damage response of composite structures undergoing crush loading. This model captures the structural response without the need for calibration of experimentally determined material parameters. When used in the design of energy absorbing composite structures, it can reduce the dependence on physical testing. This paper validates this model against experimental data obtained from the literature and in-house testing. Results show that the model can predict the force response of the crushed composite structures with good accuracy. The simulated energy absorption in each test case was within 12% of the experimental value. Post-crush deformation and the damage morphologies, such as ply splitting, splaying and breakage, were also accurately reproduced. This study establishes the capability of this damage model for predicting the responses of composite structures under crushing loads. [ABSTRACT FROM AUTHOR]

The failure envelope of the matrix in composite laminates under tensile loads has not received much attention in literature. There are very little to no experimental results to show a suitable failure envelope for this constituent found in composites. With increasing popularity in the use of micromechanical analysis to predict progressive damage in composite structures, it is important that matrix behaviour under tension is modelled correctly. In this paper, the authors present and test a new biaxial specimen design to investigate tensile matrix failure in composite structures. Through the use of micromechanical analysis, the authors developed a method in which the matrix stresses at failure can be extracted. Comparing to the existing off-axis test, it was shown that the presented specimen design and test methodology can improve the accuracy of the obtained matrix failure stresses, i.e., the matrix failure envelope for EP280 resin. Additionally, the results indicate that matrix failure takes place earlier than that predicted by von-Mises failure criterion and that the 1st Stress Invariant criterion can better predict matrix failure under tensile loading. [ABSTRACT FROM AUTHOR]

To avoid high peel stresses around the perimeter of a bonded composite patch repair, the patch thickness often tapers to a fine edge. This paper investigates and compares the damage tolerance of two different configurations of taper design under fatigue loading with different size of initial bondline flaws. The bondline damage at the tip of the doubler repair was simulated by an initial flaw which was created through a Teflon tape. A constant amplitude fatigue loading was applied to all specimens. The crack propagation against the number of cycles was recorded and the fatigue life when the crack reached 100 mm from the doubler tip was reported. Microscope investigations were conducted and provided micro scale evidence on the effects of initial flaw size on the crack initiation pattern of the two doubler design configurations. Finally, the damage tolerance of the two configurations were compared and conclusions were drawn with implications for the design of composite patch repairs. It was found that ply-drop doubler joints showed better damage tolerance than stepped doubler joints under fatigue loading. The experimental results suggested a threshold size of 5 mm for the doubler tip flaw, if any inspection technology should be adopted for damage detection. [ABSTRACT FROM AUTHOR]

Despite the abundance of studies investigating the performance of composite structures under crush loading, disagreement remains in the literature regarding the effect of increased strain rate on the crush response. This study reports an experimental investigation of the behaviour of a carbon–epoxy composite energy absorber under static and dynamic loading with a strain rate of up to 100 s - 1 . Consistent damage modes and measured force responses were obtained in samples tested under the same strain rate. The energy absorption was found to be independent of strain rate as the total energy absorption appeared to be largely associated with fibre-dominated fracture, which is independent of strain rate within the studied range. The results from this study are beneficial for the design of energy absorbing structures. [ABSTRACT FROM AUTHOR]