Curing kinetics and thermomechanical properties of latent epoxy/carbon fiber composites

In this work, resins based on diglycidyl ether of bisphenol A (DGEBA) epoxy and a latent hardener, dicyandiamide (DICY), as well as carbon fiber (CF) composites based on them, were prepared with three commercial accelerators: a methylene bis (phenyl dimethyl urea), a cycloaliphatic substituted urea, and a modified polyamine. The curing kinetics of the three DGEBA/DICY/accelerator systems were investigated by chemorheology and differential scanning calorimetry (DSC), in isothermal and over temperature change conditions. Differences in the reaction onset temperature, and in the glass transition temperature (Tg) were highlighted. For curing of thick resin samples, a slow curing cycle at the lowest possible temperature was used, followed by high temperature (160 – 180 °C) post-curing. Indeed, fast curing at higher temperatures caused the formation of hot spots and led to local burning of the samples. The obtained thermomechanical properties, assessed by ultimate tensile testing and dynamic mechanical analysis (DMA) in single cantilever configuration, were all in the expected range for epoxy resins, with tensile moduli close to 3 GPa and Tg > 140 °C. The longterm stability of these resins at room temperature was verified by DSC. Composite samples were prepared by hand lay-up by manually impregnating four layers of 5-harness satin CF textile, and curing in vacuum bag. Impregnation quality and void content were assessed by optical microscopy. The flexural properties of the post-cured composites were assessed by three-point bending test at room temperature and showed no relevant differences, all composites having bending moduli of 45 - 50 GPa. Finally, composites cured with a faster high temperature curing cycle (20 min at 140 °C) were prepared with the DGEBA/DICY/ methylene bis (phenyl dimethyl urea) system, obtaining similar properties as with the slower curing cycle, showing that the prepreg system allowed more flexibility in terms of curing cycle than the bulk resin samples

Curing Kinetics and Mechanical Properties of a Composite Hydrogel for the Replacement of the Nucleus Pulposus

A polymer material system has been developed to propose an injectable, UV and in situ curable hydrogel with properties similar to the native nucleus pulposus of intervertebral disc. Neat hydrogels based on Tween® 20 trimethacrylates (T3) and N-vinyl-2-pyrrolidone (NVP) and composite hydrogels of same composition reinforced by nano-fibrillated cellulose were synthesized with different T3 concentrations and their curing kinetics was investigated by photorheology using UV light. The T3 concentration has an influence on the time of curing and final shear stiffness of the material. NFC does not alter the time of curing but increases the final mechanical performance of the hydrogels for a same chemical composition. Hydrogel samples, neat and composite, were then tested in unconfined compression at different hydration stages and in confined compression and their elastic modulus was determined. The amount of fluid present in the network is mostly responsible for the mechanical properties and NFC fibres proved to be an efficient reinforcement. The elastic modulus ranged from 0.02 to 8 MPa. Biocompatibility studies showed that cells are confluent at 90% and do not show any morphology change when in contact with the hydrogel. The present hydrogel can therefore be considered for NP replacement

Tailoring swelling to control softening mechanisms during cyclic loading of PEG/cellulose hydrogel composites

One of the novel approaches for discogenic lower back pain treatment is to permanently replace the core of the intervertebral disc, so-called Nucleus Pulposus, through minimally invasive surgery. Recently, we have proposed Poly(Ethylene Glycol) Dimethacrylate (PEGDM) hydrogel reinforced with Nano-Fibrillated Cellulose (NFC) fibers as an appropriate replacement material. In addition to the tuneable properties, that mimic those of the native tissue, the surgeon can directly inject it into the degenerated disc and cure it in situ via UV-light irradiation. However, in view of clinical applications, the reliability of the proposed material has to be tested under long-term fatigue loading. To that end, the present study focused on the characterization of the fatigue behavior of the composite hydrogel and investigated the governing physical phenomena behind it. The results show that composite PEGDM-NFC hydrogel withstands the 10 million compression cycles at physiological condition. However, its modulus decreases by almost 10% in the first cycle and then remains constant, while cyclic loading does not affect the neat PEGDM hydrogel. The observed softening behavior has similar characteristics of the Mullins effect. It is shown that the reduction of modulus is due to the gradual change of NFC network, which is highly stretched in the swollen state. Moreover, the swelling degree of the matrix is correlated to the extent of softening during cyclic loading. Consequently, softening can be minimized by lowering the swelling of the composite hydrogel

Pre- and Post-Transition Behavior of Shear Thickening Fluids in oscillating Shear

The dynamic shear-thickening behavior of concentrated colloidal suspensions of fumed silica in polypropylene glycol has been investigated. Dynamic frequency sweeps showed that, for any given solids concentration, the complex viscosity at different imposed strain amplitudes followed a unique power-law-type behavior up to the onset of strain thickening. Moreover, similar behavior was also observed in the post-transition state, i.e., the viscosities again superimposed at frequencies beyond the transition frequency. In an attempt to develop a parametric description of this behavior, both the Delaware–Rutgers rule and the concept of a critical shear stress for the onset of shear thickening in steady-state experiments were considered. However, neither approach could account for the observed trends over the entire range of strains and frequency investigated. Plots of the critical shear strains for the onset and the end-point of the transition as a function of frequency were, therefore, used to describe the state of the suspensions for an arbitrary combination of strain and frequency. Finally, Fourier transform (FT) rheology was used to evaluate the extent of non-linearity in the response of the suspensions to dynamic shear, and it was shown that the observed behavior was not significantly influenced by wall slip at the tool–specimen interface

How plate positioning impacts the biomechanics of the open wedge tibial osteotomy; a finite element analysis

A numerical model of the medial open wedge tibial osteotomy based on the finite element method was developed. Two plate positions were tested numerically. In a configuration, (a), the plate was fixed in a medial position and (b) in an anteromedial position. The simulation took into account soft tissues preload, muscular tonus and maximal gait load.The maximal stresses observed in the four structural elements (bone, plate, wedge, screws) of an osteotomy with plate in medial position were substantially higher (1.13-2.8 times more) than those observed in osteotomy with an anteromedial plate configuration. An important increase (1.71 times more) of the relative micromotions between the wedge and the bone was also observed. In order to avoid formation of fibrous tissue at the bone wedge interface, the osteotomy should be loaded under 18.8% (approximately 50 kg) of the normal gait load until the osteotomy interfaces union is achieved

Simultaneous observations of the quasar 3C 273 with INTEGRAL, XMM-Newton and RXTE

INTEGRAL has observed the bright quasar 3C 273 on 3 epochs in January 2003 as one of the first observations of the open programme. The observation on January 5 was simultaneous with RXTE and XMM-Newton observations. We present here a first analysis of the continuum emission as observed by these 3 satellites in the band from 3 keV to 500 keV. The continuum spectral energy distribution of 3C 273 was observed to be weak and steep in the high energies during this campaign. We present the actual status of the cross calibrations between the instruments on the three platforms using the calibrations available in June 2003.Comment: 4 figures, accepted for publication in A+A letter

Polylactic acid-phosphate glass composite foams as scaffolds for tissue engineering

Phosphate glass (PG) of the composition 0.46(CaO)-0.04(Na(2)O)- 0.5(P(2)O(5)) was used as filler in poly-L-lactic acid (PLA) foams developed as degradable scaffolds for bone tissue engineering. The effect of PG on PLA was assessed both in bulk and porous composite foams. Composites with various PG content (0, 5, 10, and 20 wt %) were melt-extruded, and either compression-molded or foamed through supercritical CO(2). Dynamic mechanical analysis on the bulk composites showed that incorporating 20 wt % PG resulted in a significant increase in storage modulus. Aging studies in deionized water in terms of weight loss, pH change, and ion release inferred that the degradation was due to PG dissolution, and dependent on the amount of glass in the composites. Foaming was only possible for composites containing 5 and 10 wt % PG, as an increase in PG increased the foam densities; however, the level of porosity was maintained above 75%. PLA-T(g) in the foams was higher than those obtained for the bulk. Compressive moduli showed no significant reinforcement with glass incorporation in either expansion direction, indicating no anisotropy. Biocompatibility showed that proliferation of human fetal bone cells was more rapid for PLA compared to PLA-PG foams. However, the proliferation rate of PLA-PG foams were similar to those obtained for foams of PLA with either hydroxyapatite or beta-tricalcium phosphate