Fracture of shear bands in atactic polystyrene

Thick shear bands in polystyrene formed by compression could cause fracture or the formation of cracks by intersecting with themselves, by relaxing after the removal of the load, by propagating all the way to the side surfaces and by subsequent tensile deformation. The microstructural mechanisms involved in all these fracture processes are discussed.     

DSC and TEM investigations on multiple melting phenomena in isotactic polystyrene

The multiple melting behavior and morphologies of isotactic polystyrene (iPS) isothermally crystallized from the glassy state have been investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The combination of thermal analysis and morphological results indicates that two lamellar populations are responsible for the so-called double melting behavior in iPS. The low-temperature melting peak is attributed to the melting of less perfect (thinner or defect containing) subsidiary lamellae formed in the framework of the dominant (thicker or more perfect crystalline) lamellae upon isothermal crystallization. The high-temperature one is mainly due to the melting of the dominant thicker lamellae, and to some less extent, the melting of a recrystallized population coming from the melted defect lamellae during the heating process in DSC.     

Fracture phenomena of microdrills in static and dynamic conditions

The mechanical drilling of microholes with diameters below 0.5 mm is executed using miniature drills, which are exposed to loads of relatively large cutting forces, frequently causing the failure of the fragile tools. This paper describes the experimental investigations into the deformation, strength, and fracture of the cutting parts of microdrills in static and dynamic conditions. Firstly, microdrill behaviour owing to bending, compression, and torsion was investigated separately in static tests. In addition, torsion was applied together with compression. The values of deflections, forces, and torque were measured and the formation of microcracks and their propagation observed. The tests indicated the characteristic forms of the fracture sections for the kinds of microdrills and loads investigated. In dynamic conditions, the start of the drilling plays an important part. The surface irregularities on the work part cause the drill tip to be pushed aside and changes of its axial position. A typical reason for tool destruction during drilling is excessive stress from cutting forces. The cases of microdrill failure in dynamic conditions collated with various drilling trials. The results show distinctions in the behaviour of microdrills in both circumstances as well as the similarities and differences in the forms of fracture sections.     

Fracture mechanisms in polystyrene/laponite nanocomposites prepared by emulsion polymerization

Transmission electron microscopy (TEM) has been used to investigate the effect of laponite clay on microdeformation in thin latex-based polystyrene (PS) films, in which the laponite was concentrated at the original interfaces between the PS particles. At room temperature, a transition was observed from crazing in pure PS to a coarser fibrillar deformation mode as the laponite content increased. Moreover, whereas pure PS showed increasingly homogeneous deformation as T approached T_g, the fibrillar deformation zones observed in the nanocomposites persisted up to T just below T_g, and there was some evidence for yielding behaviour at even higher T in the presence of laponite. The macroscopic fracture resistance of the films, as assessed from double edge-notched tensile specimens, initially increased with laponite content, but decreased for laponite contents greater than 5 wt% with respect to the styrene monomer. This was attributed to a decrease in local ductility, consistent with the observation of reduced deformation ratios in the deformation zones by TEM, and to the intrinsic weakness of the laponite stacks and/or the PS-laponite interface. Thus, specimens with laponite contents comparable with the estimated threshold for percolation of contacts between the laponite stacks showed extremely brittle behaviour, associated with crack propagation along the interfaces between the latex particles.     

Fracture phenomena in a cracked plate subjected to a concentrated load

The critical load for crack extension and the corresponding angle which the fracture path forms with the initial crack propagation direction of a cracked plate subjected to a concentrated load at an arbitrary point are determined. The strain energy density theory developed by Sih is used. Two different directions of the load parallel and perpendicular to the crack axis are considered. From the whole study useful results concerning the dependence of the above two quantities on the direction and location of the applied load are derived.     

Fracture characteristics of high impact polystyrene under impact fatigue loadings

This study analyses the impact properties of high impact polystyrene (HIPS). HIPS is one of the well-known toughened polymers. The high toughness is given by the rubbery phase. The impact fatigue behavior of HIPS was studied with a Ceast pendulum type tester (Resil 25). The fracture mechanisms were examined with a scanning electron microscopy. The nature of crack initiation and propagation was investigated for small impact angles and three different spans. The impact angles of charpy hammer were chosen as 5°, 10°, 15°, 20°, and 25°. The fracture characteristics varied with the impact angle, the number of impacts, and the distance between supports. The rate of crack propagation was high at higher impact angles with lower endurance, and low at lower impact angles with higher endurance.     

Fracture kinetics and relaxation phenomena in a composite plate under transient loads

The interaction between a cylindrical steel particle and a plate made of asbestos-fabric textolite and aluminum, is investigated numerically by using the method of finite elements. The kinetic fracture model is used in calculations. The fracture kinetics in a two-layer plate and the causes of the failure observed are analyzed.Translated from Problemy Prochnosti, No. 8, pp. 48–51, August, 1992.     

Toughening of polystyrene by natural rubber-based composite particles: Part III Fracture mechanisms

Impact testing has allowed the toughness of PS blends to be correlated with the morphology of the dispersed rubber phase, which was a natural rubber (NR) in particle form, coated with a shell of polystyrene (PS) or polymethylmethacrylate (PMMA). PS subinclusions were also introduced into the NR core. The impact resistance of the prepared PS blends began to rise steeply at a particle content of about 18 wt %. Transmission electron microscopy (TEM) in combination with osmium tetroxide staining techniques, allowed direct analysis of the crazing and cavitation processes in the composite natural rubber particle-toughened PS blends. Bulk samples were studied at high and slow deformation speeds. Different deformation mechanisms were effective, depending on the location of the observed stress-whitened zone relative to the notch tip. The apparent fracture mechanisms in rubber-toughened PS blends were also studied by scanning electron microscopy. PS blends containing polydisperse natural rubber-based particles or monodisperse poly(n-butylacrylate)-based particles, and commercial high-impact polystyrene, were compared. This revised version was published online in November 2006 with corrections to the Cover Date.     

Crack initiation in polystyrene

A technique is described for demonstrating the flaws at which fracture initiates in injection-moulded polystyrene. The primary flaws are shown to be the same features as occur at the foci of secondary fractures in this polymer.     

Critical phenomena in solids

In general a solid-solid phase boundary does not terminate in a critical point due to symmetry requirements. However, there exists systems like pure cerium metal and rare earth compounds like samarium sulphide where an isostructural solid-solid phase boundary terminates at a critical point. The critical point for the pressure-induced semiconductor-metal phase transition in SmS has been determined for the first time using the thermoelectric power data collected upto 12 kbar pressure and 850° C. These results are discussed on the basis of some theoretical models developed for the mixed valent compounds.     

Craze intersections in biaxially-stressed polystyrene

Crazes are produced on two orthogonal planes in both thin film and macroscopic samples of polystyrene by sequentially applying two orthogonal tensile strains ₁ and ₂. Although many crazes produced by the second strain ₂ (secondary crazes) are stopped when they meet a primary craze, some intersections occur. The fraction of craze meetings resulting in intersection increases from 20% at ₁= ₂=3% to 55% at ₁= ₂=5%; intersections also occur preferentially in thin regions of primary crazes. The craze fibril structure in the intersection has a much lower fibril volume fraction, v _f, than either of the two crazes from which it formed. The fibril volume fraction in the intersection is approximately given by the product of the fibril volume fractions of the two crazes, in agreement with a prediction based on the surface drawing mechanism of craze thickening. At higher strain levels the v _fs of the intersections are lower, leading to higher fibril stresses and enhanced fibril fracture; an increasing fraction of intersections breaks down to form large voids at these higher strain levels. Fractography of macroscopic samples containing intersecting crazes demonstrates that voids formed at the intersections can act as nuclei for cracks causing premature fracture of the material.     

Bead fusion in polystyrene foams

A quantitative technique has been developed to measure the extent of fusion between expanded beads in molded polystyrene foams. Experiments were conducted with ASTM D638 tensile test specimens that were molded under conditions to produce various levels of bead fusion in the foam. The tensile properties of the foam for various levels of bead fusion were measured according to ASTM D638 standards. The fracture surfaces of the tensile specimens were analysed by quantitative image analysis techniques to measure the degree of bead fusion in the sample. This technique was then used on a commercial pattern to map the variations in bead fusion at numerous locations in the molded part. The results indicate that there is a good correlation between the tensile properties and the measured bead fusion. Significant variations in bead fusion may be present in a single molded part.     

Reflection losses in polystyrene fibres

The reflection losses, which occur during light guidance through polystyrene fibres of 30 μm diameter at their core-cladding interface, have been determined in order to evaluate their contribution to the total light attenuation of scintillating fibres. For the deviation (1 - q) of the reflection coefficient q from unity we obtained: 6 × 10⁻⁵ (at 458 nm), 5.36 × 10⁻⁵ (at 514 nm) and 5 × 10⁻⁵ (at 633 nm). In addition, we found light losses in the polystyrene core, which we attribute to Rayleigh scattering on submicron density variations in polystyrene. The resulting scattering lengths are: 3.6 m (at 420 nm) and 9.5 m (at 530 nm).     

Fatigue fracture processes in polystyrene

Fatigue crack growth characteristics in polystyrene were studied as a function of stress intensity factor range and cyclic frequency. Precracked single edge notched and compact-tension type specimens made from commercially available polystyrene sheet (mol.wt. =2.7×10⁵) were cycled under constant load at frequencies of 0.1, 1, 10 and 100 Hz, producing growth rates ranging from 4×10^–7 to 4×10^–3 cm/cycle. For a given stress intensity level, fatigue crack growth rates were found to decrease with increasing frequency, the effect being strongest at high stress intensity values. The variable frequency sensitivity of this polymer over the test range studied was explained in terms of a variable creep component. The macroscopic appearance of the fracture surface showed two distinct regions. At low stress intensity values, a highly reflective, mirror-like surface was observed which transformed to a rougher, cloudy surface structure with increasing stress intensity level. Raising the test frequency shifted the transition between these areas to higher values of stress intensity. The microscopic appearance of the mirror region revealed evidence of crack propagation through a single craze while the appearance of the rough region indicated crack growth through many crazes, all nominally normal to the applied stress axis. Electron fractographic examination of the mirror region revealed many parallel bands perpendicular to the direction of crack growth, each formed by a discontinuous crack growth process as a result of many fatigue cycles. The size of these bands was found to be consistent with the dimension of the crack tip plastic zone as computed by the Dugdale model. At high stress intensity levels a new set of parallel markings was found in the cloudy region which corresponded to the incremental crack extension for an individual loading cycle.     

Anomalous phenomena

The laws of large departures from equilibrium and the laws governing structural transitions indicated previously by the author are used as the basis for discussing anomalous phenomena. Their main properties are described, a classification is put forward, a simple model is proposed to describe an anomalous phenomenon, and several families of possible phenomena are indicated. Pis’ma Zh. Tekh. Fiz. 25, 57–63 (April 12, 1999)     

Rubber-crumb modified polystyrene

The strain energy release rate,G _c, of polystyrene (PS) containing rubber crumb has been examined. It was found that for unmodified crumb, addition of small amounts (5%) leads to 100% increase inG _c. This is attributed to crazing in the PS. However, further addition of crumb leads to reductions inG _c, as the crumb-PS adhesion is low and interfacial failure results. If the crumb is modified with PS its adhesion to the matrix PS increases and internal rupture of the rubber occurs.G _c for these composites increases linearly with crumb loading, and is due to matrix crazing as well as rupture of the rubber phase.     

Rubber-crumb modified polystyrene

Peroxide crosslinked polybutadiene and styrene butadiene rubber powders were converted to semi-interpenetrating networks by swelling in styrene monomer and subsequent homopolymerization. Two initiator types were selected, one causing bonding between polystyrene and the rubber (benzoyl peroxide), the other allowing independent polymerization (azobisisobutyronitrile). The polystyrene modified powders were then incorporated into a polystyrene matrix and tensile properties of the resulting composite determined. Improvements in performance over untreated crumb-modified composites were observed, with increased breaking strains due to crazing. Little difference could be distinguished in composites derived from different initiator usage, indicating that polystyrene interpenetration in the rubber, rather than grafting, is the predominant factor.     

Rubber-modified polystyrene compositions

The physical and mechanical properties of polystyrene compositions, prepared by blending high impact polystyrene (HIPS) with different types of elastomer, are studied. The results obtained show a considerable increase in HIPS impact strength when modified with isoprene and butadienestyrene elastomers. This increase in impact strength is due to factors such as lowT _g and the flexibility of macromolecules of the elastomer phase and adhesion at the interphase boundary. An optimal elastomer concentration of 12.5 parts per hundred of resin is observed. A multi-directional influence of the elastomer additive on the impact strength and on the tensile and flexural strength has been found, due to different mechanisms of fracture.