C.P. Tsui

Effects of two damage mechanisms on effective elastic properties of particulate composites

This study focuses on development of a method for predicting effective elastic properties of damaged particulate polymer composites. Two damage parameters were introduced and defined by volume fraction of both the void-damage and the debonding-damage. The former parameter was determined by 3-dimensional strain measurement of damaged composite samples, while the latter were estimated from a typical experimental data of the repeatedly load-unload tensile tests. Based on M-T micro-mechanics theory, the equations for estimation of the effective elastic properties of the damaged particulate composites in response to the longitudinal true uni-axial strain were formulated The effects of two damage mechanisms and glass-bead contents on the effective elastic properties of glass bead filled polyphenylene oxide (GB/PPO) composites were investigated by this approach. The predicted results for the effective elastic modulus were close to the experimental data.

Modelling of non-linear stress-strain behaviour of HIPS with craze damage in tensile loading-unloading process

Abstract Experimental and theoretical works have been done to study the nonlinear mechanical deformation response of high impact polystyrene (HIPS) under the condition of uni-axial tensile loading processes which include both loading and unloading stages with different constant strain rates. A damage variable, which is defined by the volume dilatation due to crazing, is used to characterize the effect of craze damage on the true stress and further on the visco–plastic mechanical response of HIPS. The damage variable and the plastic strain have been determined from the experimental data. The results also show that the effect of strain rate on the maximum tensile strength and the viscosity coefficient is very significant. A visco–elasto–plastic constitutive equation of polymeric material that takes account of the effect of craze damage, has been established. The solution of this constitutive equation can be used to simulate the non-linear stress–strain curves in tensile loading–unloading process, which agree well with those obtained experimentally.

Parameter optimization for the visco-hyperelastic constitutive model of tendon using FEM

Numerous constitutive models describing the mechanical properties of tendons have been proposed during the past few decades. However, few were widely used owing to the lack of implementation in the general finite element (FE) software, and very few systematic studies have been done on selecting the most appropriate parameters for these constitutive laws. In this work, the visco-hyperelastic constitutive model of the tendon implemented through the use of three-parameter Mooney-Rivlin form and sixty-four-parameter Prony series were firstly analyzed using ANSYS FE software. Afterwards, an integrated optimization scheme was developed by coupling two optimization toolboxes (OPTs) of ANSYS and MATLAB for estimating these unknown constitutive parameters of the tendon. Finally, a group of Sprague-Dawley rat tendons was used to execute experimental and numerical simulation investigation. The simulated results showed good agreement with the experimental data. An important finding revealed that too many Maxwell elements was not necessary for assuring accuracy of the model, which is often neglected in most open literatures. Thus, all these proved that the constitutive parameter optimization scheme was reliable and highly efficient. Furthermore, the approach can be extended to study other tendons or ligaments, as well as any visco-hyperelastic solid materials.

Fabrication and Characterization of HA-ZrO2-MWCNT Ceramic Composites

In this article, a new kind of hydroxyapatite-zirconia-carbon nanotube (HA-ZrO2-MWCNT) ceramic composites was fabricated to enhance the mechanical properties of a hydroxyapatite (HA) ceramic for satisfying various requirements in bone rehabilitation. A new dispersing process was proposed to ensure a homogeneous distribution of multi-wall carbon nanotubes (MWCNTs) in the HA ceramic matrix. The flexural strength and fracture toughness of the HAZrO2-CNT composites were enhanced by about 126% and 124%, respectively, as compared with those of the unmodified HA ceramics. The X-ay diffraction analysis revealed that a small quantity of HA decomposed in the composites with introduction of strengthening phases. The in vitro test results showed that the sample surfaces were covered with a new apatite layer after immersion in simulated body fluid for 10 days, indicating good biocompatibility of the composites.

Mechanical Properties and Morphology of Poly(l-Lactic acid)/Nano-CaCO3 Composites

The tensile and impact properties of poly(l-lactic acid) composites filled with nanometer calcium carbonate (nano-CaCO3) were measured at room temperature. The results showed that the tensile elastic modulus increased roughly linearly while the tensile yield strength, tensile fracture strength and tensile elongation at break (δb) decreased nonlinearly with increasing the nano-CaCO3 weight fraction (Wf); when Wf was constant, the δb increased with increasing tensile rates. Both the V-notched Izod impact strength and V-notched Charpy impact strength showed the non-linear increase with increasing Wf. The impact fracture surface was observed by means of a scanning electronic microscope to understand the toughening mechanisms.

Surface characterisation of PLLA polymer in HAp/PLLA biocomposite material by means of nanoindentation and artificial neural networks

Abstract In this paper, the mechanical properties of polymer matrix phase (modulus of elasticity, yield stress and work hardening rate) have been determined using combined methods such as nanoindentation, finite element modelling and artificial neural networks. The approach of neural modelling has been employed for the functional approximation of the nanoindentation load-displacement curves. The data obtained from finite element analyses have been used for the artificial neural networks training and validating. The neural model of polymer matrix phase of poly-l-lactide (PLLA) polymer in hydroxyapatite (HAp)/PLLA mechanical behaviour has been developed and tested versus unknown data related to the load-displacement curves that were not used during the neural network training. Based on this neural model, the nanoindentation matrix phase properties of PLLA polymer in HAp/PLLA composite have been predicted.

Fabrication and Characterization of Bioglass-Modified HA–ZrO2 Biocomposites

Hydroxyapatite (HA) being the main mineral constituent of human hard tissues is highly bioactive. Good chemical bonds can be generated between HA and natural bone. However, the low strength and inherent brittleness of HA restrict its application usually to non-load-bearing conditions. In this work, production of a new kind of HA–ZrO2 composite by hot-press sintering method is described. Bioglass which has been widely used in reconstruction of damaged or diseased tissues was added into HA–ZrO2 composites. Comparing with pure HA ceramic, this type of composite possesses better mechanical strength and retains the bioactivity of HA as well. The liquid phase generated by bioglass has been effective in improving the sintering process of HA–ZrO2 composites. The phase composition of HA composite was characterized by XRD and their fracture surfaces were observed by SEM. The XRD results show that introducing a small amount of bioglass into HA–ZrO2 composite cannot enhance decomposition of HA. The SEM images show that there were fewer pores on the fracture surfaces of HA–ZrO2–bioglass composite than in the HA–ZrO2 composite. The flexural strength and toughness of HA–ZrO2 composite containing 2 wt% bioglass were 157 MPa and 1.63 MPa·m1/2, respectively.

Experimental study on stable growth of crack and craze damage in HIPS under tension at room temperature

This study focuses on investigating the stable damage and crack growth process of high impact polystyrene (HIPS) by using the damage mechanics approach and the moire method. Under excessive tensile loading, HIPS will be damaged due to multiple crazing that causes volume dilatation. A damage variable is defined by the dilatation of the representative volume element to characterize the craze-damage. The relationship between the dilatation and the total strain has been determined experimentally. A specimen with a sharp initial discontinuity is tested under tensile force to study the damage and crack growth process. It is observed that an elliptical damage zone is formed in front of the crack tip. The crack propagation is stabilized by the damage zone. Moire fringe patterns show that the deformation within the damage zone is non-uniform. A dynamic configuration of deformation, which comprises the elastic zone, the transition zone, the damage processing zone, and the residual damage zone, has been established. The non-linear strain and stress fields inside the transition zone and the damage processing zone are analyzed. The experimental result also shows that a critical plastic strain criterion may be used to predict the stable growth of macrocracks for HIPS.

Effects of Filler Content on Mechanical Properties of Macroporous Composites

The mechanical properties of hydroxyapatite related macroporous biocomposites (MPBs) are influenced by a number of factors, such as the pore size, the filler content and the properties of the matrix and the inclusion. Failure often occurs when the strength of the implant cannot bear the applied mechanical load. In this study, the effects of filler content on the mechanical properties of MPBs have been investigated. A finite element (FE) unit cell model of a macroporous hydroxyapatite–polyetheretherketone (HA–PEEK) biocomposite structure with uniform and interconnected pores has been constructed. In the FE model, the HA particles were assumed to have random distribution, and particle volume fraction would be varied in the PEEK matrix. The material behaviours of both HA and PEEK have been implemented in the ABAQUS finite element code. HA was modelled to exhibit elastic behaviour and undergo plastic softening to a residual strength when a critical stress was reached, while the PEEK matrix would follow elastic–plastic behaviour. The macroscopic compressive stress–strain relations of the macroporous biocomposite structures have been predicted. Increasing particle volume fraction could lead to an increase in the compressive elastic modulus of the structures but a reduction in the compressive strength. The von Mises stress distribution and the effect of stress concentration in the structures with different filler content are also discussed. The proposed model could provide macro-structural and microscopic information of the macroporous biocomposite structure to the designers in order to facilitate the fabrication of this kind of structure with optimum mechanical properties.

Voxel-based approach to generate entire human metacarpal bone with microscopic architecture for finite element analysis

With the development of micro-computed tomography (micro-CT) technology, it is possible to construct three-dimensional (3D) models of human bone without destruction of samples and predict mechanical behavior of bone using finite element analysis (FEA). However, due to large number of elements required for constructing the FE models of entire bone, this demands a substantial computational effort and the analysis usually needs a high level of computer. In this article, a voxel-based approach for generation of FE models of entire bone with microscopic architecture from micro-CT image data is proposed. To enable the FE analyses of entire bone to be run even on a general personal computer, grayscale intensity thresholds were adopted to reduce the amount of elements. Human metacarpal bone (MCP) bone was used as an example for demonstrating the applicability of the proposed method. The micro-CT images of the MCP bone were combined and converted into 3D array of pixels. Dual grayscale intensity threshold parameters were used to distinguish the pixels of bone tissues from those of surrounding soft tissues and improve predictive accuracy for the FE analyses with different sizes of elements. The method of selecting an appropriate value of the second grayscale intensity threshold was also suggested to minimize the area error for the reconstructed cross-sections of a FE structure. Experimental results showed that the entire FE MCP bone with microscopic architecture could be modeled and analyzed on a personal computer with reasonable accuracy.