Temperature effects on quasi-static fracture of PMMA

The effect of temperature on the quasi-static fracture behaviour of PMMA is examined. It is found that the critical stress intensity factor, K ₁₀, at crack growth initiation decreases with temperature up to a certain critical temperature, T _c. Above T _c, K ₁₀ increases rapidly. The speeds with which slow crack growth could be maintained without transition to brittle fracture were found to be significantly higher at high temperatures.     

Temperature effects on the fatigue of highly filled PMMA

The modulus and fracture toughness of an ATH-filled PMMA composite are determined as a function of temperature. The modulus can be modelled as a series addition of the two phases, giving a decreasing modulus with temperature tending to zero at 110°C. The K_1C value remains constant. Fatigue crack growth data in the form of da/dN versus K were obtained as a function of temperature and modelled using the Paris Law. The power index remained constant at 7.5, but the coefficient had a maximum at 50°C. It is suggested that this arises from microcracks generated by interparticle thermal stresses which are shown to have a maximum at the same temperature (50°C). A two-stage zone fatigue crack growth model was also applied to the data and gave a damage stress which correlated with the thermal stress and suggested a criterion based on achieving a constant energy per unit area.     

Temperature dependence of fracture effects in self-bonded SiC

The effect of temperature on the fracture strength and work of fracture of a self-bonded SiC has been measured from room temperature to 1100° C. Over the same temperature range the work of fracture was observed to increase twofold. These phenomena were accounted for in terms of the behaviour of the free-silicon phase. In addition, electron microscopy of the fractured surface was undertaken. Fracture chips showed that dislocations were generated during the failure process in both the secondary SiC and silicon phases. Stacking faults were observed in the SiC phases, and some of these were shown to have formed during the fracture process.     

Temperature and enviromental effects on the fatigue fracture in polystyrene

Fatigue tests have been conducted on polystyrene in air at different mean stress conditions and also over the temperature range–60 to 60°C using notched specimens. In addition, some tests were performed with the specimens immersed in a detergent and in a corn oil. The data were first represented on a conventional log K-log da/dN plot to determine the Paris law parameters and how these varied with mean stress, temperature and environment. Considerable variations were noted but no useful pattern could be discerned. The data were then analysed using a recently proposed two-stage line plastic zone model so that a crack tip zone stress and a fatigue damage factor could be found. These stresses were found to be very high in air and to increase with increasing mean stress, and this is attributed to a high degree of constraint at the crack tip. There was an abrupt change in _c at about 20° C which, it is suggested, is caused by the onset of crazing due to the presence of a loss peak. Associated changes in K _c were also noted. In environments, the stresses were much smaller and agreed with those obtained in static fracture, suggesting that crazes are formed in environments prior to, and not during, fatigue crack growth. The stress reduction factor, , remained constant at about 0.2 with both temperature and environment.     

Time-temperature dependent fracture toughness of PMMA

Some observations are made on the fractography of surfaces obtained by cracking “compact tension” profile testpieces of PMMA over a range of temperatures and crack speeds, both stably and unstably. To a first approximation, it was possible to group and “shift” (as in visco-elastic transformations) characteristic surface markings at various fracture toughness/temperature/crack velocity combinations, particularly in the range where a toughness-biased Ree-Eyring relationship described the experimental toughness data.     

Non-steady,periodic behavior in the dynamic fracture of PMMA

A crack moving dynamically through a sheet of Polymethylmethacrylate at high average velocity is found in reality to propagate in a non-steady, periodic, and perhaps discontinuous fashion. The spatial period of the fracture process in the direction of travel, or banding morphology, is of millimeter size and is consistent both in magnitude and discontinuous nature with a crazing mechanism proposed in fatigue settings. The band size in both the dynamic and fatigue environments scales with the applied stress intensity. This commonality has importance in modeling. It suggests that a single mechanism is fundamental in both dynamic and fatigue loading regimes.     

Investigation of loading rate and plate thickness effects on dynamic fracture toughness of PMMA

To investigate the effects of loading rate and plate thickness on the fracture toughness of PMMA (polymethyl methacrylate) under impact loading, two methods, A method and B method, are applied as follows. In the A method, a dynamic finite element method and a strain gage method are applied to measure the dynamic fracture toughness in the fracture test using an air gun. In the B method, a single axis strain gage method is applied to measure the critical dynamic stress intensity factor, namely dynamic fracture toughness, in the fracture test using a weight dropping type apparatus. The dimensions of the PMMA specimen are L = 140 mm length and W = 30 mm width. Three values of the plate thickness B, 15.0 mm, 10.0 mm and 5.0 mm, are selected to investigate the plate thickness effect in the fracture test. Both results by the A and B methods precisely indicated the minimum value and the loading rate effect on the dynamic fracture toughness.     

Determination of dynamic fracture toughness for PMMA

A dynamic FEM (finite element method) and a strain gage method are applied to analyze the dynamic fracture toughness and SIF (stress intensity factor) for PMMA (polymethyl methacrylate). The analyses are carried out for plates with an edge crack subjected to one-point bending in a plane of the plate. A simple procedure that the present author has proposed is applied to the problem of using a triangular element of assumed constant strain on finite element analysis. The numerical simulation by FEM provides values for the applied forces as measured with the strain gages. Also, a crack initiation time is measured with the strain gage mounted around the crack tip. The dynamic fracture toughness is determined by adapting the crack initiation time to the simulation curve of the dynamic SIF calculated by the FEM. In this study, the usefulness of the method to determine the dynamic fracture toughness is investigated by comparing predictions with the experimental results for dynamic stresses and SIFs.     

Temperature and porosity effects on the fracture of fine grain size MgO

The strength of three fine grain size magnesias (<0.8m) has been studied as a function of porosity and temperature from 20 to 1200° C. In the low-temperature region (T800°C) fracture occurs by the extension of flaws introduced during the specimen machining. In this case the fracture stress can be related to porosity by an exponential law. In the high-temperature region (T>800° C) plasticity increases the size of pre-existing flaws, but this effect is partially annihilated by a rapid increase with temperature in the effective surface energy for fracture initiation. This entails only a slow decrease in fracture stress with temperature. These results are correlated with observations of fracture surfaces by scanning and transmission electron microscopy.     

Temperature dependence of fracture stress in the brittle fracture region

The temperature dependence of the fracture stress of bcc metals in the brittle fracture region is examined. A model taking into account the effect of local plastic deformation of the crack tip on transition to brittle fracture is proposed. The model is then used to derive analytical expression for the temperature dependence of the fracture stress in the material with an initial crack or notch.Translated from Problemy Prochnosti, No. 11, pp. 57–62, November, 1991.     

Effect of modifier concentration on the fracture behaviour of rubber-modified PMMA

Izod fracture surfaces of blends of PMMA with various amounts of a rubber modifier were studied using scanning electron microscopy. Attention was focused on modes of crack initiation and propagation and on the role of the modifier in the fracture process. It was found that the impact strength of this class of materials increased monotonically with an increase in modifier concentration, at least up to 40 wt% modifier. Unmodified PMMA was studied to provide a basis for understanding the morphological features on the fracture surfaces of the rubber-modified blends. It was confirmed that PMMA fractures through the formation and rupture of crazes. This phenomenon was also found to occur in blends containing 10 wt% modifier. However, blends with 20 wt% modifier crazed only in the later stages of the fracture process, when the crack speed had exceeded some critical value. No evidence of crazing was found in blends with 30 and 40 wt% modifier loadings, although extensive plastic deformation was observed on the fracture surfaces.     

Temperature effects on the fracture behavior and tensile properties of silane-treated clay/epoxy nanocomposites

We investigated the effects of clay silane treatment on the fracture behaviors of clay/epoxy nanocomposites by comparing the compliance, critical fracture load, and fracture toughness of silane-treated samples with those of untreated samples. The fracture toughnesses of untreated and silane-treated clay/epoxy nanocomposites were 8.52 J/m² and 15.55 J/m², respectively, corresponding to an 82% increase in fracture toughness after clay silane treatment. Tensile tests were performed at ?30 °C, 25 °C, 40 °C, and 70 °C. Tensile strength and elastic modulus were higher at ?30 °C than at 25 °C for both samples. However, the tensile properties decreased as temperature increased for both samples. In particular, at 70 °C, the tensile properties were less than 10% of the original value at room temperature, independent of surface treatment. The fracture and tensile properties of silane-treated clay/epoxy nanocomposites increased due to good dispersion of the clay in epoxy and improvement in interfacial adhesive strength between epoxy and clay layers.     

Temperature,strain rate and fibre orientation effects in the compressive fracture of SiC fibre-reinforced glass-matrix composites

Continuous SiC fibre-reinforced glass-matrix composites have been tested in compression over a wide range of temperatures and loading rates. Both uniaxial and cross-plied fibre orientations were studied. Strength is found to depend sensitively on orientation and loading rate, while temperatures up to 800°C have less effect. Orientation effects are explained in terms of matrix microfracture and fibre buckling. The latter are also shown to control strengthening at very high strain rates, where it is hypothesized that strength is enhanced by inertial effects which inhibit the development of the localized pockets of intense matrix microfracture and general buckling required for the nucleation of fibre kinks.     

The prediction of dynamic fracture evolution in PMMA using a cohesive zone model

A cohesive zone model was used in conjunction with the finite volume method to model the dynamic fracture of single edge notched tensile specimens of PMMA under essentially static loading conditions. In this study, the influence of the shape of the cohesive law was investigated, whilst keeping the cohesive strength and separation energy constant. Cohesive cells were adaptively inserted between adjacent continuum cells when the normal traction across that face exceeded the cohesive strength of the material. The cohesive constitutive law was therefore initially rigid, and the effective elasticity of the material was unaltered prior to insertion of the cohesive cells. Notch depths ranging from 2.0 to 0.1 mm were considered. The numerical predictions were compared with experimental observations for each notch depth and excellent qualitative and quantitative agreement was achieved in most cases. Following an initial period of rapid crack tip acceleration up to terminal velocities well below the Rayleigh wave speed, subsequent propagation took place at a constant rate under conditions of increasing energy flux to an expanding process region. In addition, attempted and successful branching was predicted for the shorter notches. It was found that the shape of the cohesive law had a significant influence on the dynamic fracture behaviour. In particular, the value of the initial slope of the softening function was found to be an important parameter. As the slope became steeper, the predicted terminal crack speed increased and the extent of the damage decreased.