Particle cavitation in rubber toughened epoxies: the role of particle size

Rubber toughened epoxies are used in a wide range of applications including adhesives when toughness is a crucial property. It is well known that the cavitation of the rubber particles is an important process to optimise the toughness of such materials. This article describes the development of a predictive model to describe the dependence of rubber particle cavitation on particle size. The model is developed using a combination of experimental observations and finite element simulations. Predictions have been obtained for both uniaxial loading conditions and the triaxial loading conditions expected ahead of a crack. The model has been extended to consider the cavitation of nano-sized ‘rubber’ particles.     

Environmentally-affected indentation fracture in poly(ethersulphone) and poly(methyl methacrylate)

The effect of plasticization on the ductile-to-brittle fracture transitions (DBFT) in the ball indentation of poly (ethersulphone) (PES) and poly(methyl methacrylate) (PMMA) has been studied. The DBFT in various organic liquids are governed by a size effect related to the indenter radius, in accordance with the Puttick theory of fracture transitions. In addition, abrasive wear rates of the polymers in these liquids are shown to correlate with the critical indenter radius needed to produce a fracture transition, owing to the connection with fracture toughness.     

垂直梯度凝固法生长半绝缘GaAs单晶的工艺研究

在国产三温区VGF单晶炉上，研究了非掺GaAs半绝缘单晶的液封垂直梯度凝固法生长技术。讨论了引晶、坩埚设计与质量、PBN坩埚的剥落、B2O3的水份控制等因素对单晶生长的影响，基本解决了VGF的工艺问题。通过多炉生长，得到了Φ2″=VGF非掺半绝缘低位错全单晶。     

Plastic fracture in poly(vinyl chloride)

The strain fields around diamond-shaped cavities in cold-drawn rigid PVC have been determined by the application of fine grids to the specimen surface. An element of material adjacent to the diamond tip deforms predominantly in simple shear with a direction of strain parallel to the draw direction. Each element attains a maximum shear strain before the next element begins to shear. This process, possibly analogous to neck propagation in tensile tests, produces the characteristic diamond shape. Simple extension and simple shear tests on cold-drawn PVC confirm that under the stress system around a cavity, simple shear in the draw direction is a favourable mode of deformation.     

A fracture mechanics study of the fatigue of rubber in compression

Repeated deformation of rubber cylinders in compression causes cracks to initiate at tensile stress concentrations (eg. bond edges). The cracks grow to remove the rubber which at maximum deformation bulges outside the original profile of the cylinder. The results are analysed in terms of fracture mechanics. The tearing energy, T is substantially independent of crack length. The relation between crack growth rate dc/dN and T appears to be a material property, independent of testpiece geometry.     

Plastic deformation mechanisms in poly(acrylonitrile-butadiene styrene) [ABS]

Thin films of two poly (acrylonitrile-butadiene-styrene) [ABS] resins have been strained in tension, and the ensuing deformation has been characterized by transmission electron microscopy. To enhance contrast of the rubber particles, some of the specimens were stained with OsO₄. Films containing only solid rubber particles 0.1 m in diameter show little tendency for crazing. Instead, cavitation of the rubber particles occurs, together with localized shear deformation between the particles along a direction nearly normal to the tensile axis. For specimens containing a mixture of the same small particles plus larger (1.5m diameter) particles containing glassy occlusions, some crazing does occur. Crazes tend to nucleate at the larger particles only. When crazes encounter the smaller particles these cavitate without appearing to impede or otherwise affect the craze growth. The occluded particles also show significant cavitation, with voids forming at their centres at sufficiently high levels of strain. These voids do not seem to lead to rapid craze break-down and crack propagation. In commercial ABS, which typically has both large and small rubber particles, both crazing, nucleated by the large particles, and shear deformation, encouraged by the cavitation of small rubber particles, can be expected to make important contributions to the toughness of the polymer.     

Characterization of toughness variation due to intrinsic defects in high-thermal-resistant poly(acrylonitrile-butadiene-styrene) (ABS)

A mechanical testing method, named TACL test, is proposed to characterize the effect of gel-like particles on mechanical properties of high-thermal-resistant poly(acrylonitrile-butadiene-styrene) (ABS). The TACL test introduces cracks in the vicinity of gel-like particles by cyclic loading. Mechanical properties are then measured using the cyclically loaded specimens under monotonic tension. Preliminary results show that the mechanical properties, especially the maximum elongation and the total absorbed energy, are very sensitive to the cyclic loading. The study suggests that the TACL test can serve as a means to semi-quantitatively characterize number and distribution of the gel-like particles, which is useful for monitoring batch-dependent toughness variation of the ABS.     

Improvement of fracture properties of hydroxyapatite particle filled poly(l-lactide)/poly(ε-caprolactone) biocomposites using lysine tri-isocyanate

Effects of LTI addition on the mode I fracture energy of HA/PLLA/PCL were examined and the micro-structural modification due to LTI addition was investigated. Both the mode I energy release rate, G _in, and averaged fracture energy, E _f, are improved dramatically due to LTI addition. The reason is considered to be the improvements of the interfacial structure connecting HA particles with PLLA/PCL matrix and the miscibility between PLLA and PCL. These changes of blend morphology and interfacial structure reduce the stress concentration and lead to the ductile deformation and resulted in the increase of those fracture properties.     

The fracture of poly(hydroxybutyrate)Part I Fracture mechanics study during ageing

Linear elastic fracture mechanics techniques have been used to study the fracture behaviour of the biodegradable thermoplastic poly(hydroxybutyrate). Both the compact tension and double cantilever beam test geometries have been used and give toughness values in good agreement. The variation in fracture toughness with age after moulding has been monitored. It has been found that the strain energy release rate is reduced during ageing, but the stress intensity factor is increased. The primary change during ageing is found to be in the polymers stiffness rather than its resistance to fracture. © 1998 Chapman & Hall     

Thermoplastic vulcanizate based on poly(vinylidene fluoride) and methyl vinyl silicone rubber by using fluorosilicone rubber as interfacial compatibilizer

To successfully fabricate a thermoplastic vulcanizate (TPV) based on poly (vinylidene fluoride) (PVDF) and high loading level of methylvinyl silicone rubber (MVSR), stabilizing the phase structure during blending is a challenge due to the distinct interface properties of the two materials. Herein, we report a feasible method to fabricate a desired PVDF/MVSR TPV by using fluorosilicone rubber (FSR) as an interfacial compatibilizer. We found that the FSR was self-assembly migrated from MVSR phase toward PVDF phase and finally located at the interface between PVDF and MVSR, forming core–shell-like spherical particles with a rough surface, and surprisingly, the crosslinked rubber particles were connected by fiber-like elastomeric materials which bonded onto the surfaces of the particles. The excellent property of repeat processing of the PVDF/MVSR/FSR TPV with such novel phase morphology makes it a potential alternative of fluorosilicone rubber in future.     

The fracture of poly(hydroxybutyrate)Part II Fracture mechanics study after annealing

The fracture behaviour of poly(hydroxybutyrate) (PHB) after high-temperature annealing has been studied using linear elastic fracture mechanics techniques. The effect of the annealing temperature on the polymers' fracture toughness both initially and after re-ageing is examined. Annealing at temperatures of 120°C or above is found to result in an improvement in both the critical stress intensity factor and the strain energy release rate measurements of fracture toughness which is largely maintained on re-ageing. A more detailed study of the re-ageing behaviour after annealing at 130°C finds Gc to remain approximately constant but Kc to increase slightly with age. © 1998 Chapman & Hall     

Effect of rubber particle size on the impact tensile fracture behavior of MBS resin with a bimodal particle size distribution

We performed impact tensile fracture experiments on methylmethacrylate–butadiene–styrene (MBS) resin with small and large particles in a bimodal size distribution, and examined the effects of particle size on fracture behavior by fixing the total rubber content (28 wt%) and the small particle size (about 140 nm), and varying the size of large particles (about 490 nm or 670 nm). Dynamic load P′ and displacement δ′ of single-edge-cracked specimens were measured using a Piezo sensor and a high-speed extensometer, respectively. A P′−δ′ diagram was used to determine external work U _ex applied to the specimen, elastic energy E _e stored in the specimen, and fracture energy E _f for creating a new fracture surface A _s. Energy release rate was then estimated using G _f = E _f/A _s. Values of G _f were correlated with fracture loads and mean crack velocity v _m determined from load and time relationships. We then examined the effect of particle size on G _f and v _m, and results indicated that particle size plays an important role in changing the values of G _f and v _m.     

Evaluation of fracture toughness of ABS polymers via the essential work of fracture (EWF) method

The essential work of fracture (EWF) method was employed to determine the fracture toughness of SAN/PB-g-SAN blends with the compositions of 65/35–0/100. It was found that the plane stress EWF approach is applicable for different SAN/PB-g-SAN blends. During EWF tests, three different types of load–displacement curves were recorded, depending on the blend composition. For the samples containing rubbery phase of 35–65 wt% crack growth occurred immediately after full ligament yielding. Further increase in rubber content and for the samples with 75 and 85 wt% of rubbery phase, the onset of crack growth was preceded by the formation of necking zone in the form of a sharp load drop after full ligament yielding. For the sample with the composition of 0/100, strain hardening behavior was observed without any sign of neck formation. The specific essential work of fracture (EWF) w _e increased at first with the increase of PB-g-SAN wt% and then decreased with further increasing amount of rubbery phase. The maximum value of w _e was observed for the blend with 75 wt% of PB-g-SAN. The specific non-essential work of fracture βW _p increased with increasing of rubbery phase concentration. The analyzing of yielding and necking/tearing components of essential and non-essential parameters showed that for the samples containing 55 wt% and higher of rubbery phase, w_\texte,nt \succ w_\texte,y w_{\text{e,nt}} \succ w_{\text{e,y}} and b_\textnt w_\textp,nt \succ b_\texty w_\textp,y \beta_{\text{nt}} w_{\text{p,nt}} \succ \beta_{\text{y}} w_{\text{p,y}} , indicating that a majority of fracture energy was dissipated in the necking and tearing stages of fracture process. Finally, it is shown that w _e values can be predicted via COD values.