Mitsugu Todo

Deformation analysis of the periodontium considering the viscoelasticity of the periodontal ligament

OBJECTIVES The aim of the present work was, by means of a combined experimental and numerical approach, to investigate the full-field distributions of displacement, stress and strain, and their evolution with loading in the entire fresh periodontium under an externally applied force. METHODS In situ intrusion tests were performed to identify the nonlinear, viscoelastic behavior of the periodontal ligament (PDL) of a pig mandible; a digital image correlation method was applied to examine the full-field deformation patterns in the entire periodontium. The finite element (FE) model was created based on the actual anatomic profiles of individual constituents of the tooth structure; the nonlinear and time-dependent viscoelastic properties of the PDL were input into the FE model to fit the numerical computations with the experimental measurements. RESULTS The nonlinear, viscoelastic behavior of the PDL was identified and characterized quantitatively. The simulation results were validated by the experiments. The results showed the tilting of tooth and the movement of cervical bone toward the mid-tooth in the studied periodontium under vertical compressive loading. Major strain was concentrated in the PDL, with the maxima near to the tooth apexes, at the tooth-root bifurcation and also at the sides of the tooth roots, and maintained a slight rise during holding of the applied displacement. High stress in the tooth was located mainly at the sides of tooth roots, in the bone it was concentrated near the apexes and the root bifurcation, and these stresses decreased gradually during the holding period. SIGNIFICANCE The combined approach of experiments that apply the digital image correlation method and FE analyses that take into account the nonlinear and time-dependent viscoelasticity of the PDL enables the acquisition of a full picture of detailed, realistic stress/strain fields and deformation patterns of the entire fresh periodontium, being of essence in orthodontics and dentistry.

Effect of annealing on the mechanical properties of PLA/PCL and PLA/PCL/LTI polymer blends

The effects of annealing on the mechanical properties of polymer blends of poly(lactic acid) (PLA) and poly(ε-caprolactone) (PCL) were investigated. The bending strength and modulus of PLA/PCL tend to increase due to crystallization of the PLA phase by annealing. The mode I fracture energy, J(in), of PLA/PCL decreases dramatically due to the suppression of the ductile deformation of the spherical PCL phase by annealing. The immiscibility of PLA and PCL can be improved by adding lysine triisocyanate (LTI) as a result of additional polymerization. The phase transformation due to LTI addition reduces the size of the spherical PCL phase, resulting in higher fracture energy. An annealing process applied to PLA/PCL/LTI further strengthens the microstructure, resulting in effective improvement of the fracture energy.

Strain-rate dependence of the tensile fracture behaviour of woven-cloth reinforced polyamide composites

Abstract Tensile testing of plain-weave-cloth-reinforced polyamide composites has been carried out at various strain rates ranging from 1×10−2 to 4×101 s−1 in a servohydraulic testing apparatus. Four different polyamide composites studied were composed of two kinds of reinforcements, carbon fibre (CF) and glass fibre (GF), and two kinds of matrices, polyamide-6 (PA6) and modified polyamide-6 (mPA6). The experimental results showed that the tensile strength and failure strain of the composites tend to increase with increase in strain rate except in the case of GF/mPA6 whose fracture properties were stabilized at high rates (>1×100 s−1). Damaged regions of the fractured specimens were observed in order to study the micromechanisms of tensile failure by means of polarized optical and scanning electron microscopes. The microscope studies showed that in the composites with the mPA6 matrix, extensive microcracking occurred in the matrix and transverse threads region prior to the final failure. The effects of strain-rate on the tensile fracture behaviour of these types of composite systems are discussed on the basis of these experimental results.

Study of mode-I interlaminar crack growth in DCB specimens of fibre-reinforced composites

Abstract Two- and three-dimensional finite-element analyses have been carried out to investigate interlaminar crack growth in composite double-cantilever beam (DCB) specimens. The results revealed that crack-tip bluntness causes multiple stress concentrations along the crack contour. By increasing the bluntness, the highest stress point moves from the tip towards the corner along the crack contour. This increases possibility of crack growth towards the fibre/matrix interface. The results also show relatively high tensile stress along the interface near the insert film, suggesting that for DCB specimens with a weak fibre/matrix interface, a crack is likely to start at the interface, not from the insert film. Experimental evidence supporting the analytical results was obtained by scanning electron microscopy of the fracture of glass-fibre composites. The study concludes that crack-tip bluntness has an important role in initial crack growth in DCB specimens, and should be considered for design of interlaminar fracture testing of composites.

Effects of Moisture Absorption on the Dynamic Interlaminar Fracture Toughness of Carbon/Epoxy Composites

Effects of moisture absorption on the dynamic mode II interlaminar fracture behavior of two different kinds of carbon/epoxy composites at a low impact rate of 0.9 m/s were studied. ENF specimens were preconditioned under three different environmental conditions, that is, dry, 80°C-90% RH and 80°C-wet. Mode II interlaminar fracture toughnesses, G IIC , of the composites were then evaluated under quasi-static and dynamic loading. The results showed that moisture absorption had an influence on the GIIC values of the composites. The experimental results also exhibited that the impact loading condition tended to decrease the G IIc values. Scanning electron micrography was also performed to characterize the micromechanisms of mode II interlaminar fracture. Effects of moisture absorption on the mode II interlaminar fracture behavior were then discussed on the basis of the results of the interlaminar fracture testing and microscopic studies.

Initiation of a mode-II interlaminar crack from an insert film in the end-notched flexure composite specimen

Two- and three-dimensional finite-element analyses have been carried out in order to investigate the initiation of an interlaminar crack in the end-notched flexure (ENF) composite specimen. It is believed that the current practice of using an insert film as a starting defect generates a blunt defect which creates a stress pattern that attracts the crack growth towards the fibre/matrix interface. Results from finite-element (FE) modelling support this concept. The FE results also indicate that with a sufficiently low bond strength at the fibre/matrix interface, the crack can be initiated from the interface instead of from the starting defect. The critical interfacial bond strength for transition of the location for crack initiation from the starting defect to the fibre/matrix interface is 14% of the matrix strength for the two-dimensional model, and 28% for the three-dimensional model. Although the two crack-initiation mechanisms are very different, the fracture surfaces generated are similar. The FE models indicate that after the crack initiation its further growth is always along the fibre/matrix interface. The above conclusions from the FE work were verified experimentally by using a glass-fibre/vinylester composite with a fibre volume fraction similar to that used in the models. The overall conclusions from the study are that the crack-tip bluntness plays an important role in the initiation of the interlaminar fracture in the ENF specimens, and that the interlaminar fracture toughness measured from the ENF specimen can depend strongly on the interfacial bond strength.

Effects of proliferation and differentiation of mesenchymal stem cells on compressive mechanical behavior of collagen/β-TCP composite scaffold.

The primary aim of this study is to characterize the effects of cell culture on the compressive mechanical behavior of the collagen/β-tricalcium phosphate (TCP) composite scaffold. The composite and pure collagen scaffolds were fabricated by the solid-liquid phase separation technique and the subsequent freeze-drying method. Rat bone marrow mesenchymal stem cells (rMSCs) were then cultured in these scaffolds up to 28 days. Compression test of the scaffolds with rMSCs were conducted periodically. Biological properties such as cell number, alkaline phosphatase (ALP) activity, and gene expressions of osteogenetic bone markers were evaluated during cell culture. The microstructural changes in the scaffolds during cell culture were also examined using a scanning electron microscope. The compressive elastic modulus was then correlated with those of the biological properties and microstructures to understand the mechanism of variational behavior of the macroscopic elastic property. The composite scaffold exhibited higher ALP activity and more active generation of osteoblastic markers than the collagen scaffold, indicating that β-TCP can activate the differentiation of rMSCs into osteoblasts and extracellular matrix (ECM) formation such as type I collagen and the following mineralization. The variational behavior of the compressive modulus of the composite scaffold was affected by both the material degradation and the proliferation of cells and the ECM formation. In the first stage, the modulus of the composite scaffold tended to increase due to cell proliferation and the following formation of network structure. In the second stage, the modulus tended to decrease because the material degradation such as ductile deformation of collagen and decomposition of β-TCP were more effective on the property than the ECM formation. In the third stage, active calcification by formation and growth of mineralized nodules resulted in the recovery of modulus. It is concluded that the introduction of β-TCP powder into the porous collagen matrix is very effective to improve the mechanical and biological properties of collagen scaffold prepared for bone tissue engineering. Furthermore, the compressive modulus of the composite scaffold is strongly affected by the material degradation and the ECM formation by stem cells under in vitro culture condition.

Local application of fluvastatin improves peri-implant bone quantity and mechanical properties: A rodent study

Statins are known to stimulate osteoblast activity and bone formation. This study examines whether local application of fluvastatin enhances osteogenesis around titanium implants in vivo. Ten-week-old rats received a vehicle gel (propylene glycol alginate (PGA)) or PGA containing fluvastatin (3, 15, 75 or 300 microg) in their tibiae just before insertion of the implants. For both histological and histomorphometric evaluations undecalcified ground sections were obtained and the bone-implant contact (BIC), peri-implant osteoid volume and mineralized bone volume (MBV) were calculated after 1, 2 and 4 weeks. Using the same models mechanical push-in tests were also performed to evaluate the implant fixation strength. After 1 week the MBV and push-in strength were significantly lower in the 300 microg fluvastatin-treated group than in the other groups (P<0.01). At 2 weeks, however, the BIC and MBV were both significantly higher in the 75 microg fluvastatin-treated group than in the non-fluvastatin-treated groups (P<0.01). Similar tendencies were observed at week 4. Furthermore, the data showed a good correlation between the MBV and the push-in strength. These results demonstrate positive effects of locally applied fluvastatin on the bone around titanium implants and suggest that this improvement in osseointegration may be attributed to calcification of the peri-implant bone.

The effect of bimodal distribution on the mechanical properties of hydroxyapatite particle filled poly(L-lactide) composites

The effects of a bimodal distribution of micro-HA and nano-HA particles on the mechanical properties such as bending strength, modulus and mode I fracture energy of HA/PLLA composites were investigated. The bending properties were effectively improved by a bimodal distribution compared to a monomodal distribution. It is considered that in the bimodal HA/PLLA, the PLLA molecular chains are constrained by the existence of well distributed nano-particles, resulting in the reduction of the deformation of molecular chains. The mode I fracture energy was also effectively improved by a bimodal distribution with additional energy dissipation mechanisms, that is, dense ductile deformation of the PLLA matrix and debonding of the particle/matrix interfaces in the crack-tip region.

Effect of Annealing on Fracture Mechanism of Biodegradable Poly(lactic acid)

Effect of annealing on the fracture behavior of poly(lactic acid) (PLA) was investigated. Fracture toughness of PLA samples prepared under different annealing conditions was measured under static and dynamic loadings. Microstructure and crack growth behavior were characterized by polarizing microscopy (POM). Crystallinity was determined by DSC analysis. Fracture surface morphology was also studied by scanning electron microscopy. It was shown that the static fracture toughness increased with increase of crystallinity, while the dynamic toughness increased as crystallinity increased. POM exhibited that craze formation played an important role in the fracture mechanism of amorphous samples. Macroscopic fracture toughness and microscopic crack growth mechanism were correlated on the basis of these experimental results, and effect of annealing on the toughness and mechanism were discussed.