Determining fracture toughness from cutting tests on polymers

A cutting test method has been developed for polymers and analysis schemes derived for the determination of the fracture toughness, G_c from the cutting data. The experimental scheme requires the measurement of forces for a cut of width b, in both the cutting direction, F_c/b and the transverse direction, F_t/b. Depths of cut were varied from 0.025 mm to 0.25 mm and the tool rake angle was varied from ?20° to 30°. Cutting was performed at a speed of 10 mm s^?1 on three polymers (HIPS, PA 4/6 and LLDPE). In addition, values of fracture toughness and yield strength were determined for the polymers using standard tests for comparison with the values obtained from cutting. Three analysis methods were derived to analyse the cutting data with the most favoured scheme based on an energy balance and using Merchant’s force minimisation criterion to determine the shear plane angle. This avoids the need to measure the cut chip thickness. Results for HIPS and PA 4/6 gave values of G_c in good agreement with the values determined via LEFM. However, the cutting method is intended for materials such as LLDPE which has a low yield stress and moderately high toughness, i.e. materials which cannot be tested using standard LEFM fracture mechanics tests. The cutting analysis appeared to give valid values of G_c for LLDPE in that they were independent of rake angle. There were some complications when analysing this polymer due to visco-elastic recovery effects in the chip and these have been considered. Finally, the cutting analyses always determined high values of yield stress which would appear to indicate work hardening.     

Determination of impact fracture toughness of polyethylene using arc-shaped specimens

This paper describes a methodology for the determination of the impact radial fracture toughness, G_IC, of cylindrical polymeric molded parts using arc-shaped specimens. The proposed methodology is an extension of the ISO/DIS 17281 Standard which states that for brittle behavior, a basically linear relationship exits between the fracture energy, U; and the energy calibration factor, φ. This relationship allows calculating the critical strain energy release rate from the slope of the U vs. φ plot. An expression for the energy calibration factor, φ, for the arc-shaped specimen is proposed in this work, by combining tabulated functions and finite element results. The methodology is applied to test high density polyethylene arc-shaped specimens taken from cylinder walls.     

A mechanistic approach for determining plane-stress fracture toughness of polyethylene

A new mechanistic approach is used to characterize resistance of polyethylene to deformation and fracture in double-edge-notched tensile test. The new approach considers all three mechanisms involved in the fracture process, i.e. for fracture surface formation, shear plastic deformation, and necking, and can be used to determine values of specific energy consumption for each mechanism. This is different from the conventional approach, known as essential work of fracture (EWF), which does not consider the difference between shear plastic deformation and necking. Results from the new approach for a polyethylene copolymer show that specific energy density for fracture surface formation is about half of that determined from the EWF approach, and specific energy density for necking is very close to that determined from simple tensile test. The latter provides some support for validity of the new approach in characterizing fracture behaviour of polyethylene when accompanied by large deformation and necking. The paper also points out crack growth conditions that have to be met for valid application of the EWF approach and shows that such conditions are not met when deformation and necking occur in polyethylene.     

Vickers压入识别氮化硅断裂韧性

依据仪器化Vickers压入氮化硅断裂韧性实验获得的有关压痕裂纹参数,通过有限元数值分析方法识别出氮化硅的弹性模量和屈服强度,进一步采用虚拟裂纹闭合法确定其裂纹尖端的应力强度因子KI。以此为基础,与氮化硅断裂韧性标准值对比,分析有限元仿真KIC结果和基于L-E-M模型建立的3种典型陶瓷断裂韧性压入测试方法的准确度。结果表明:基于Vickers压入有限元数值分析结果的最大误差仅为2.38%,Anstis公式最大识别误差为2.65%,而Lawn公式和Miyoshi公式的识别误差的绝对值均超过10%,因此Vickers压入测试具有较高测试准确度。     

Determination of fatigue fracture toughness by conventional rotary bending specimen

Determination of fatigue fracture toughness,K _fc, is made by rotary bending specimen considering partial contact of fatigue cracked surfaces in the compression side of the beam specimen. It is shown thatK _fc is a material constant independent of the nominal stress at the notch section, the specimen geometry, and the shape of the final fracture area.

---

Résumé On détermine la ténacité à la rupture par fatigueK _fc à l'aide d'éprouvetes soumises à flexion rotative, en considérant le contact partiel entre les surfaces fissurées par fatigue, au cours de la phase de compression. On montre queK _fc est une constante du matériau, indépendante de la tension nominale au droit de la section entaillée, de la géométrie de l'éprouvette, et de la forme de la surface finale de rupture.

     

Crack morphology models for fracture toughness and fatigue strength analysis

Planar cracks represent an approximation, largely adopted in fracture mechanics and fatigue problems, of the physical reality, where cracks feature complex geometric morphologies related to material microstructure, residual stresses, material properties dispersions and so on. In the present paper, firstly a model to describe the influence of roughness and friction of the crack surfaces is reviewed in relation to the resulting near‐tip stress field and the fracture resistance under monotonic loading. Such a model is based on the Distributed Dislocation Technique, and considers a periodic profile of the crack. Then, some approximate theoretical models describing periodically kinked cracks are reviewed in their application to the estimation of fatigue strength of materials. In particular, the influence of crack path meandering on fatigue propagation is analysed by modelling the crack profile as a piecewise linear periodic curve in two dimensions. The same type of model is discussed within the framework of self‐similar fractal geometries. In the paper, emphasis is given to the effect of crack size on the fracture resistance and fatigue strength, where such an effect depends on the ratio between the characteristic length of the crack morphology and the nominal length of the crack itself.     

Accelerated fatigue fracture mechanism of medium density polyethylene pipe material

Fatigue crack propagation studies were performed in medium density polyethylene pipe to elucidate the damage mechanism associated with pipe failure. Past pipe testing methods required up to several years to produce failures which mimicked those observed in the field. However, by fatiguing a specially designed test specimen, brittle failure, resembling that observed under service conditions, was produced in only three days. It was determined that the method of loading and the crack plane orientation greatly affect the degree and extent of brittle crack propagation. In some specimen geometries, the initial brittle fracture may undergo a transition to a more ductile failure mode. The damage which precedes the crack tip during brittle cracking is a root craze and two smaller side crazes; these crazes are primarily composed of yielded membranes which are oriented normal to the crack propagation direction, rather than being composed of fibrils. The number and length of these crazes was shown to be dependent on the chosen test geometry.     

Effect of particle morphology and polyethylene molecular weight on the fracture toughness of hydroxyapatite reinforced polyethylene composite

Fracture toughness testing has been performed on hydroxyapatite-polyethylene composites. Sintered and unsintered grades of hydroxyapatite and two grades of high-density polyethylene were used to make 40 vol % hydroxyapatite composites. Compact tension testing was performed at both room temperature and at 37 degrees C and at three strain rates. The effect of increasing the loading rate from 2 to 200 microm s(-1) was to increase the fracture toughness. Increasing the testing temperature or decreasing the surface area of the reinforcing particles also increased the fracture toughness. However, using a lower molecular weight, injection moulding, grade of polyethylene reduced the fracture toughness. Thus for higher fracture toughness, a low surface area sintered hydroxyapatite in a high-molecular weight polyethylene is required.