Synthesis procedure and the fracture toughness of highly cross-linked resins

Experimental values of the plane strain fracture toughness of cross-linked unsaturated polyester resins are presented. It is demonstrated that these values depend not only on nominal resin composition (e.g. nature of diol, or molar ratio, R, of unsaturated to saturated acid) but also on the conditions under which polycondensation occurs. It is necessary to define rather precisely the molar ratio of total diol to total acid, and also to assess the efficiency of diol utilization. These considerations become more important as R increases. Also, as the value of R increases, toughness falls. When samples are immersed in hot water, a reduction in toughness is found. The effect of crack-tip radius on apparent toughness values is more marked with a resin of low than of high cross-link density.     

Fractal characterization of the fracture surface of a high-strength low-alloy steel

The fractal dimension of fracture surfaces of a high-strength low-alloy steel (ASTM A710) has been measured using different ruler lengths. The fractal dimension increases with increasing impact toughness, E, and can be expressed by the quantitative relationship, E=E0e25D, where E0=1.9 J representing the impact toughness in Euclidean space, and D is the fractal dimensional increment, defined by D=DS–2=DL–1 and DS and DL are respectively, the surface and linear fractal dimensions. © 1998 Kluwer Academic Publishers     

Synthesis and characterization of a novel dimethacrylate based on adamantane as possible dental resins

In this study a novel dimethacrylate based on adamantine (AB-GMA) was synthesized and proposed as possible dental monomers for dental resin mixtures. The monomer was prepared by the reaction of glycidyl methacrylate with dicarboxylic acid ester obtained from adamantanediol and maleic anhydride. The addition reaction of glycidyl methacrylate and the acidic compound was carried out in the presence of basic catalyst—tetraethylammonium bromide. AB-GMA was copolymerized with TEGDMA by varying the curing conditions: monomer composition, photoinitiator concentration and light intensity. A Real-time FTIR technique was employed to monitor the double bond conversion and the rate of polymerization. The prepared polymers were also subjected to dynamic mechanical studies (DMA). AB-GMA exhibited high degree of double bond conversion (up to 77%) and its thermo-mechanical properties were better than those of the commonly known dimethacrylates. AB-GMA showed no cytotoxicity toward growth of L929 cells and had good in vitro biocompatibility.     

Fractal dimension of metallic fracture surface

In this study, a complete method of determination of the fractal dimension for fracture surfaces of ferrous alloys has been proposed. This dimension is determined for the vertical profile obtained by the profile technique cross-section. The image of the profile, seen through the microscope coupled with a camera, is recorded in a computer, where numerical processing is performed. For calculation of the same fractal dimension, the fd3 program has been used, which is available through the Internet. The essential element of the method is optimisation concerning microscopic magnification (scale of a length), resolution of the recorded image and selection of the grey level threshold at binarization. The tests for the stability of discretization, which enable minimization of the error of the measurement, have also been carried out. These tests consist in checking the difference in fractal dimensions for the same profile obtained in two different methods of contouring as well as the difference between capacitive, informative and correlative dimensions. In both cases, too big difference suggests that the determined dimension is not reliable. This method allows determination of the fractal dimension with an absolute accuracy of 0.05 in non-dimensional units. The method has been employed in many studies. In this paper the following tests have been presented: a “fractal map” of the fracture surface was made, an influence of the mechanical notch radius in a compact specimen on the fractal dimension of the fracture surface, an influence of the distortion rate on the fractal dimension, an effect of fatigue crack propagation rate on the fractal dimension and influence of the stress-intensity factor on the fractal dimension of the fracture surface. The following materials were examined: Armco iron, P355N steel and 41Cr4 steel in different states after the heat treatment. The measurements have been made for the specimens of the compact type. There was considered an influence of location of the place of measurement on the fractal dimension being determined. The dimension was determined on the profiles lying longwise and crosswise the crack propagation direction. It has been found that the fractal dimension of the fracture surface does not depend on a place of measurement. This suggests, among other things, that a distinction between the places, which were created under conditions of the plane stress, and the places, which were created under conditions of the plane strain state, cannot be made with the help of the fractal dimension. When testing an influence of the radius of the mechanical tip notch on the fractal dimension of a fracture surface, this dimension was determined in the places located at different distances from the tip of the mechanical notch. With respect to the radii up to 1.0 mm, no significant differences in fractal dimensions have been found. The fractal dimensions of the fracture surface for all examined materials were practically the same and they ranged from 2.02 to 2.10. However in some ranges of da/dN rate the dimension was changing inversely proportional to da/dN. Obtained results confirm that fractal dimension do not depend on the investigated material.     

木质碳纤维原丝表面的分形表征

对不同固化条件下制备的木质碳纤维原丝表面进行电镜扫描(SEM),再对所得图像用分形软件进行识别,最后用投影覆盖法(PCM法)对原丝表面的分形维数进行计算和分析。结果表明:(1)各种不同固化工艺下得到的原丝表面:PCM法求得的分形维数变化范围为2.141～2.170,表明木质碳纤维原丝的表面更接近二维平面特征,而且比较光滑。(2)木质碳纤维原丝表面具有分数维度,说明存在分形特征,采用分形维数描述其表面在几何意义上的微观粗糙程度是可行的。     

Influence of concrete composition on the characterization of fracture surface

An experimental method based on laser triangulation was used to measure the 3-D profile of fracture surface of concrete. The projective-covering method was established to determine the fractal characteristics of fracture surface. Based on the experiment, the influence of composition of concrete on the fractal dimension, including water–binder ratio, maximum aggregate size and aggregate type, was investigated. The variation of fractal dimension with the material composition was also analysed to make further understanding of fracture mechanism of concrete.     

Material characterization of blended epoxy resins related to fracture toughness

The present study describes the effect of the macromolecular modifications on the fracture toughness of an epoxy system. We synthesized epoxy networks by the reaction of diglycidyl ether of bisphenol A (DGEBA) with methyl-tetrahydro-phthalic anhydride (MTHPA), initiated by a tertiary amine. Several materials were obtained by adding a high molecular weight monomer to one with low molecular weight (both based on DGEBA) at different concentrations. In every case, a stoichiometric amount of MTHPA was employed. The glass transition temperature and the Angell’s fragility index, derived from thermo-viscoelastic properties, were used to characterize the materials. Relationship between these two parameters and the fracture properties, including the fracture toughness and the microscopic roughness of the fracture surfaces observed by atomic force microscope (AFM), was then investigated. We found that there were direct correlations among the glass transition temperature, the fragility, the fracture toughness, and the roughness. This study revealed that both the glass transition temperature and the fragility are effective for characterizing material in relation to the fracture toughness of the blended epoxy resins.     

Yield and fracture behaviour of cross-linked epoxies

The yield stress and fracture energies of a series of cross-linked epoxy resins were studied in order to correlate the macroscopic mechanical properties with the polymer microstructure. Five networks with varying cross-link densities were synthesized by reacting a homologous series of epoxy resins with stoichiometric quantities ofm-phenylenediamine. For all the networks, the yield stress decreased with increasing temperatures in accordance with the predictions of the Eyring theory of viscosity. At constant temperatures, the yield stress decreased with increasing molecular weight between cross-links. The fracture studies revealed two distinct types of crack propagation behaviour above and below approximately 0 °C. Below 0 °C the cracks propagated in a stable and continuous manner, while the crack propagation behaviour changed to an unstable stick-slip mode as the test temperature was increased above 0 °C. For unstable crack growth, the fracture energies for crack initiation increased with increasing temperature, while the fracture energies for crack arrest were, within the limits of experimental error, independent of temperature. The crack arrest fracture energies were similar in magnitude to the fracture energies for stable crack propagation. An empirical power-law type correlation was observed between the glassy arrest fracture energies and the average molecular weight between cross-links. Micrographs of specimens which failed by the unstable, stick-slip mode revealed characteristic plastic deformation zones which highlighted the positions of crack initiation and arrest along the crack path. The deformation zone widths were observed to increase with increasing test temperatures, providing evidence of greater localized plastic deformations and higher fracture initiation energies at higher temperatures.     

The fracture toughness and fracture morphology of polyester resins

The change in fracture toughness of an orthophthalate based medium reactivity polyester resin with change in resin flexibility, catalyst content, cure temperature and liquid environment, has been investigated. Various specimen geometries including simple double cantilever beam and tapered double cantilever beam have been used and the results obtained with the different geometries are in good agreement. The critical flaw size for the resin has been determined and an estimate obtained of the radius of the plastic zone under plane strain conditions. The solvent absorption characteristics of the resin were found to be effected by catalyst content, resin flexibility and by residual strains. Observations of the fracture behaviour, using both fracture toughness and tensile specimens have revealed a fracture surface morphology very similar to that of glass but with limited plastic deformation occurring.     

Cells behaviors and genotoxicity on topological surface

To investigate different cells behaviors and genotoxicity, which were driven by specific microenvironments, three patterned surfaces (pillars, wide grooves and narrow grooves) and one smooth surface were prepared by template-based technique. Vinculin is a membrane-cytoskeletal protein in focal adhesion plaques and associates with cell–cell and cell–matrix junctions, which can promote cell adhesion and spreading. The immunofluorescence staining of vinculin revealed that the narrow grooves patterned substrate was favorable for L929 cell adhesion. For cell multiplication, the narrow grooves surface was fitted for the proliferation of L929, L02 and MSC cells, the pillars surface was only in favor of L929 cells to proliferate during 7 days of cell cultivation. Cell genetic toxicity was evaluated by cellular micronuclei test (MNT). The results indicated that topological surfaces were more suitable for L929 cells to proliferate and maintain the stability of genome. On the contrary, the narrow grooves surface induced higher micronuclei ratio of L02 and MSC cells than other surfaces. With the comprehensive results of cell multiplication and MNT, it was concluded that the wide grooves surface was best fitted for L02 cells to proliferate and have less DNA damages, and the smooth surface was optimum for the research of MSC cells in vitro.     

Viscoelastic phenomena in the fracture of thermosetting resins

Comparisons of stress–strain and fracture behaviour of thermosetting resins have shown viscoelastic non-linearity to be compatible with the complex fracture behaviour of thermosets. Fracture on a load controlled test machine has shown additional evidence that non-linearity is present in the material. This phenomenon is linked to previous studies of the fracture surface that have shown evidence of viscoelasticity in the formation of the surface texture. The proposed mechanism is consistent with a crack tip blunting model of stick–slip behaviour. Parallels are shown to exist between known thermoset fracture behaviour and phenomena observed in composite delamination studies.     

On the velocity-dependent fracture toughness of epoxy resins

Journal of Materials Science -     

Crosslink density and fracture toughness of epoxy resins

A series of networks of diverse crosslink density were prepared using bifunctional epoxide prepolymers of different molecular weight, crosslinked with diamine diphenyl sulphone, and their fracture behaviour investigated. The same set of resins was also modified with a reactive rubber. The fracture toughness regularly decreased on increasing the crosslink density for all formulations. The addition of the rubber gave rise to a marked increase in toughness, though it magnified the influence of the molecular weight of the prepolymer. Tests performed with blunt notches showed that the fracture toughness was maximum at medium crosslink densities. A dispersion of rubber particles caused a toughness increase through the formation of microcavities ahead of the crack tip. Failure was preceded by a rapid volume increase caused by void coalescence.     

Fractal aspects of ductile and cleavage fracture surfaces

The aim of this paper is to evaluate and interpret the three-dimensional variational fractal dimension of a ductile and a cleavage fracture surface. The fracture surface is acquired by fracturing Charpy impact and static loaded specimens of a low alloy steel in ductile-to-brittle transition temperature range, and reconstructed by a stereoscopic technique. The three-dimensional variational method for measuring fractal dimension is improved by shifting algorithm and tested on the Takagi surface using the local fractal dimension. We find very good fractal behaviour in the ductile area, however, fractal characteristics in the cleavage area are less noticeable. The results are discussed in thermodynamical terms and promote the idea that fractal behaviour reflects the quasi-static process and that the fracture mechanisms in the ductile fracture are independent of strain rate (at least up to 10³ s⁻¹).