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Abstract: The indentation test is a popular experimental method to measure a material’s mechanical properties such as elastic modulus and hardness, and the Oliver–Pharr method is commonly used in commercial indentation instruments to obtain these two quantities. To apply the Oliver–Pharr method correctly in all of these cases, it is essential to know the limitations of this method. The present study focuses on the applicability of the Oliver–Pharr method to measure the mechanical properties of particles in composites. The finite element method is used to undertake virtual indentation tests on a particle embedded in a matrix. In our numerical studies, the indentation “pile-up” phenomenon is generally observed in our numerical case studies, which indicates that the contact area used for predicting the elastic modulus should be measured directly, not be estimated from the indentation curve. The Oliver–Pharr method based on the real contact area is applied to estimate the elastic modulus of the particles by using the indentation curve from the numerical simulation, with the estimated elastic modulus being compared with the input value. Applying the real contact area value (not the one predicted from the indentation curve) we show that the Oliver–Pharr method can still be applied to measure the elastic modulus of the particle with sufficient accuracy if the indentation depth is smaller than the particle-dominated depth, a value defined in this work. The influences of the matrix and particle properties on the particle-dominated depth are studied using a dimensional analysis and parametric study. Our results provide guidelines to allow the practical application of the Oliver–Pharr method to measure the elastic modulus of particles in composites. This could be particularly important where particles are formed in situ in a matrix (as opposed to being preformed and subsequently incorporated in a matrix), or when the modulus of individual performed particles is required such as for subsequent modelling, but the modulus of individual material particles (or its material) cannot readily be determined. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental investigation on mode I delamination of z-pinned double-cantilever-beams (DCB) and associate z-pin bridging mechanisms. Tests were performed with three types of samples: big-pin with an areal density of 2%, small-pin with an areal density of 2% and small-pin with an areal density of 0.5%. The loading rates for each type of samples were set at 1mm/min and 100mm/min. Comparison of fracture load under different loading rates shows the rate effects on delamination crack opening and delamination growth. Optical micrographs of z-pins after pullout were also presented to identify the bridging mechanisms of z-pins under different loading rates. [Copyright &y& Elsevier]

This paper presents an experimental study on the evaluation of bridging law for a z-pin. The relationship between the z-pin bridging force and its displacement was measured by z-pin pullout tests. The tests were carried out using three types of samples with: single small pin; 3×3 small-pins (three columns×three rows) and 3×3 big-pins. For 3×3 small-pins samples, a typical pullout curve with initial bonding, debonding and frictional sliding was obtained. A high peak value of the debonding force was reached before z-pin debonding started. After debonding was initiated, the pullout force dropped rapidly to a lower value, the pins were then pulled out steadily against friction. However, for samples with 3×3 big-pins, it was difficult to discern the peak debonding force. The major results of this study are expected to provide a better physical understanding of the mechanics and mechanisms of z-pin bridging, aside from an efficient and accurate methodology to measure the crack-bridging law. [Copyright &y& Elsevier]

Mode II delamination toughness of z-pin reinforced composite laminates is investigated using finite element (FE) method. The z-pin pullout process is simulated by the deformation and breakage of non-linear springs. A critical shear stress criterion based on linear elastic fracture mechanics is used to simulate crack growth in an end-notched-flexure (ENF) beam made of z-pinned laminates. The mode II toughness is quantified by the potential energy release rate calculated using the contour integral method. This FE model is verified for an unpinned ENF composite beam. Numerical results obtained indicate that z-pins can significantly increase the mode II delamination toughness of composite laminate. The effects of design variables on the toughness enhancement of z-pinned laminates are also studied, which provides an important technological base and useful data to optimize and improve the z-pinning technique. [Copyright &y& Elsevier]

A finite element (FE) model is developed to investigate mode I delamination toughness of z-pin reinforced composite laminates. The z-pin pullout process is simulated by the deformation of a set of non-linear springs. A critical crack opening displacement (COD) criterion is used to simulate crack growth in a double-cantilever-beam (DCB) made of z-pinned laminates. The toughness of the structure is quantified by the energy release rate, which is calculated using the contour integral method. The FE model is verified for both unpinned and z-pinned laminates. Predicted loading forces from FE analysis are compared to available test data. Good agreement is achieved. Our numerical results indicate that z-pins can greatly increase the mode I delamination toughness of the composite laminates. The influence of design parameters on the toughness enhancement of z-pinned laminates is also investigated, which provides important information to optimise and improve the z-pinning technique. [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]

Carbon nanotubes (CNTs) were coated on short glass fibres (SGFs) through a one-step flame synthesis technique. Then, these CNT coated SGF (CNT-SGF) reinforced polylactic acid (PLA) composite filaments were processed as a new fused deposition modelling (FDM) 3D printing feedstock. Tensile tests were conducted on FDM 3D-printed specimens with two raster angles (±45° and 0°/90°) each containing pure PLA filaments, SGF/PLA and CNT-SGF/PLA composite filaments separately. Compared to the pure 3D-printed PLA samples, experimental results showed that the 3D-printed specimens using SGFs (≥5 wt%) and CNT-SGFs (1–5 wt%) reinforced PLA filaments exhibited higher Young's modulus and tensile strength values due to the enhanced interface adhesion. Furthermore, the FDM printing raster angles (±45° and 0°/90°) did not noticeably affect the tensile properties of the samples made of the same material, which could be attributed to the complete fusion between adjacent rasters during printing. Scanning electron microscopy (SEM) images on the fracture surfaces of the tensile specimens indicated that the filler distribution within the printed samples was random. Theoretical calculations of the Young's modulus and computational fluid dynamics (CFD) simulation on SGF reinforced PLA samples further verified a random distribution of the fillers within the matrix due to the high printer nozzle diameter to SGF length ratio compared to the short SGF aspect ratio. [Display omitted] [ABSTRACT FROM AUTHOR]