Fracture Toughness Of Polymers Research Articles

Particulate Composite Damage: The Influence of Particle Shape on Crack Path

Using stiff particles mixed into polymer matrix may significantly improve global mechanical response of the composite. Unfortunately, this process leads to other side effects, for example, presence of stress concentration at the particle-matrix interface or negative influence on the fracture toughness. The paper presents an approach to estimate the influence of particles on the micro-crack propagation. Material properties of matrix and particles were estimated experimentally. A two-dimensional computational model was proposed and all calculations were done in software ANSYS. On the base of linear elastic fracture mechanics, the influence of the particle shape on the micro-crack propagation paths was analyzed via numerical studies. The results of numerical simulations show that the shape of the particles can significantly influence the micro-crack path as well as the stress intensity factor on the crack tip, which corresponds to fracture toughness of polymer composite filled with rigid particles. The conclusions of this paper could contribute to better understanding of the behavior of the polymer composites.

Investigation of fracture toughness parameters of epoxy nanocomposites for different crack angles

Abstract The effects of single-walled carbon nanotubes (SWCNTs) on the mechanical properties of nanocomposites with epoxy matrix were studied, with the emphasis on fracture toughness under tensile loading conditions. It has been demonstrated that adding CNTs into polymer-based materials can improve the mechanical properties of this material. CNTs possess a certain potential to improve the fracture toughness of epoxy systems due to their mechanical properties and increase the fracture toughness of nanocomposites. Since the fracture toughness parameters were best manifested in the scaling properties and were the main parameters, the angles of different cracks have been simulated in a 3D finite element framework and the effects of different angles of crack, on the fracture toughness of polymers, have been modeled and investigated. The simulations are run for different bias angles. The influence of angle, the crack lengths and the variations of different lengths of nanocomposite in different volume fractions (vol%) are investigated. That is to say, at first, nanocomposites had a significantly higher fracture toughness compared to the pure epoxy. We found that nanocomposites, in the presence of SWCNTs, had a greater effect on fracture toughness of nanocomposites in a greater volume of fractions. Also, the nanocomposites exhibited a significant increase in fracture toughness, with zero angle of crack compared with greater angles. In addition, it is found that at a constant volume fracture, fracture toughness, increases by increasing crack lengths.

Electron-beam irradiation of polymer bracket materials

Electron-beam irradiation can be used to influence the properties of polymers. Electron beams cause cross-linking that enhances the molecular mass of the polymer; this leads to branched chains until, ultimately, a 3-dimensional network is formed. The aim of this study was to evaluate the effect of electron-beam irradiation on polymer bracket materials. Since polymers are commonly used materials for brackets, polyoxymethylene, polycarbonate, and polyurethane were chosen for this study. The acceleration voltage of the electron-beam device was 10 MeV, and the energy dose was 100 kGy with an electron accelerator (BGS beta gamma service, Rhodotron, Bruchsal, Germany). Three-medium wear, fracture toughness, and Vickers hardness tests were performed. The irradiated samples were compared with untreated control groups. Polycarbonate and polyurethane bracket materials have enhanced fracture toughness and Vickers hardness after electron-beam irradiation of 100 kGy and 10 MeV. Polyoxymethylene bracket materials showed significantly lower fracture toughness values after irradiation compared with the untreated control group. Polyoxymethylene had the best mechanical properties, followed by polycarbonate and polyurethane. Almost the same effects could be measured during the 3-medium wear test. Electron-beam postcuring improved Vickers hardness and fracture toughness of polymers with lower mechanical properties (polycarbonate and polyurethane). Polyoxymethylene, with sufficient hardness and fracture toughness, could not be improved with electron-beam postcuring.

Modelling of the carbon nanotube bridging effect on the toughening of polymers and experimental verification

The effect of aligned and randomly oriented carbon nanotube (CNT), with respect to the crack growth plane, on the fracture toughness of polymers is modelled in this paper using the Elastic Plastic Fracture Mechanics. According to a critical length, two dominant toughening mechanisms for CNT-modified polymers are presented, i.e. CNT pull-out and CNT rupture. The model is then used to identify the effect of CNTs geometrical and mechanical properties on the enhancement of fracture toughness in CNT-modified polymers. The key CNT properties are the CNT radius, average length, ultimate strength, elongation before failure, interfacial shear strength between CNTs and the polymer and nanotube volume fraction. Finally, experimental results are compared with the model predictions. The correlation shows that processing of long, aligned CNTs remains the main barrier in achieving major fracture toughness enhancement.

Fracture toughness of polymers in the ductile‐to‐brittle transition region: Statistical approach and lower bound determination

AbstractA method available in literature was adapted and proposed for treating scatter and nonlinearity effects in fracture toughness of polymers in the ductile‐to‐brittle transition regime. The materials used were polypropylene homopolymer (PPH) and a polypropylene‐elastomeric polyolefin blend (PPH/POes 20 wt %), at room temperature and at 20‐mm/min test rate. Under such conditions, the fracture toughness presents a large scatter and a mean value can not be used as a design parameter because it leads to toughness overestimation. Then, there is a need to find a threshold of toughness, as a safe characteristic value for design. The toughness was evaluated by using the J‐integral approach. Large sets of specimens, 53 samples per each material, were tested with the purpose to reveal a reliable tendency in fracture behavior. As the toughness was considered nonuniform throughout the material, a weakest link model was assumed, and then results were analyzed statistically by means of a three‐parameter Weibull model (3P‐W). The PPH responded well to this 3P‐W model, whereas some deviations from the original model were observed in the PPH/POes blend. However, lower‐bound toughness values could be determined for both materials by censoring nonvalid data (Δa > 0.1b0). From an engineering point of view, the results are very encouraging, since this methodology allows to obtain a threshold of fracture toughness from a given population, that is suitable to characterize the material fracture toughness at a given temperature and strain rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3674–3684, 2005

A New Approach to Toughness Evaluation for Engineering Thermoplastics

The interest in obtaining a standard method of measuring the fracture toughness of polymers is related. Descriptions are given of previous failed attempts. The crack layer model originated by Chudnovsky is outlined. It is noted that this model has been shown to accurately imitate slow crack growth in polymers. The equations of the crack layer model are used to perform a numerical simulation of a single edge notch specimen loaded under displacement control, for each of three values of the displacement rate. A load vs. displacement curve is generated from each simulation. Each curve is used to calculate the JR curve and Ji c, according to ASTM E8 13-81. Each JR curve is shown to be different from the other two curves, and each value of Jic is shown to be different from the other two values. The following proposals are then made. The JR curve and Jic are rate dependent, and so cannot be material properties. The JR curve and Jlc are not measures of material fracture toughness, and no universal measure of material fracture toughness exists. Along with material properties, fracture toughness depends on the specific conditions of usage, i.e., specimen geometry and type and magnitude of loading. The material properties, specimen geometry, and loading type and magnitude, are all specified for each numerical simulation of crack growth using the equations of the crack layer model. For an individual set of these toughness parameters, a numerical simulation can provide a measure of fracture toughness in the form of the time to failure, or lifetime.

The Effect of Precracking Techniques on Fracture Toughness of Polymers

The technique of introducing an initial crack in the specimen for fracture toughness tests has a significant effect on the fracture toughness. This study is mainly concerned with the technique of pressing a razor blade to make a pre-crack in PMMA, GPPS and PC which were commercial grades. A pre-crack of 5mm depth was introduced using a home-made notching machine at various temperatures. Then, three point bending tests were carried out to obtain the fracture toughness. For PMMA and GPPS, the crack length was difficult to be control at room temperature and 50°C. When the temperature was raised near Tg a pre-crack of 5mm length without crazes could be introduced which reduced fracture toughness. In such a case, the shape of the pre-crack was like a natural crack, but residual stress remained around the crack. A pre-crack of 5mm length could be introduced in PC even at room temperature. However, plastic deformation and residual stress were also generated around the crack. The fracture toughness was increased by the presence of this plastic deformation around the crack.

高分子材料の強度と破壊靭性

高分子材料の強度と破壊靭性

An evaluation of the short rod technique to measure the fracture toughness of polymers

The investigation of the fundamental variables which influence the fracture toughness of structural plastics is greatly hampered by a large amount of scatter and uncertainty associated with the fracture toughness measurement. A major part of the problem is due to a lack of adherence to ASTM Standard E399, mainly with regard to the requirement for a fatigue crack. A razor-blade arrested crack, which is often blunted, is common practice in the plastics field. It is also common to ignore size (plane strain) and precise machining requirements. The short rod (SR) method was evaluated as a potentially more precise and simpler fracture toughness measurement. This toughness measurement is made on a slowly moving and presumably sharp crack, and the geometry of the sample enforces plane strain conditions. Toughness measurements on compact tension (CT) specimens via ASTM E399 were performed on one-inch (25 mm) samples of poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and polysulphone (PSO). Also, a constant compliance method using a contoured double cantilever beam (CDCB) was used to evaluate the toughness of PS, PC, and PSO, but in general we did not achieve stable crack growth. The used samples were then fabricated into SR specimens and their toughness measured. The CT and CDCB methods agreed with each other for PSO and PC, but for PS the CDCB method gave high values. It is argued that the SR method should be compared to the other methods without using a plasticity correction. Then the SR method agrees well with the CT method for PSO and PS and is 15% higher for PC. The PMMA SR results were invalid. Differences between the methods are explained in terms of crack blunting, rate effects, non-homogeneity, residual stresses and the global nature of the crack front. The SR method has promise for polymer evaluation but more experience and evaluation is needed. The method is unique in the ability to study the effects of thermal history and of the environment on fracture toughness.

The effect of rate on the impact fracture toughness of polymers

The determination ofG cfor a polyethylene and PTFE from a conventional energy measuring impact test is described, together with the use of plastic zone and slow crack growth corrections. The rate of the test is changed by two decades by varying the specimen dimensions andG cas a function of loading time is determined. A remarkably strong time dependence (t −0.42) is observed and this is discussed in terms of loss processes in the craze region at the crack tip.