Toughness of high-density polyethylene in shear fracture

This paper evaluates the validity of a new test methodology for measuring shear fracture toughness (mode II) of high density polyethylene (HDPE). The methodology adopts Iosipescu test for the shear loading, and determines the toughness based on the essential work of fracture (EWF) concept. The results show that even under the Iosipescu loading, tensile deformation (mode I) is still involved in the fracture process, possibly due to the significant work hardening that HDPE develops during the plastic deformation. The study found that the mode II fracture toughness can be determined through data analysis using double linear regression, i.e., by extrapolating specific work of fracture to zero ligament length and zero ligament thickness. The paper demonstrates that the new test methodology can be used to evaluate mode II fracture toughness of ductile polymers like HDPE in which significant work-hardening may be involved in the fracture process. The paper also provides quantitative comparison of the fracture toughness for HDPE in mode II with its mode I counterpart.     

An assessment of fracture toughness in the ductile to brittle transition regime using the Euro fracture toughness dataset

The effect of specimen size on the fracture toughness of a ferritic steel in the transition regime has been investigated in a joint European Project. The project involved the testing of 25, 50, 100 and 200 mm wide compact specimens over the temperature range −154°C to 20°C with the aim of evaluating techniques for assessing the fracture toughness data.This paper evaluates the data at, or close to, the onset of stable tearing instead of at cleavage. The approach, which is applicable to structural assessment procedures, results in a temperature shift of less than 12°C between the specimen widths. The approach also enables simplified recommendations to be made for fracture toughness testing in the transition regime and the onset of upper shelf behaviour to be quantified.     

Simple bond energy approach for non-destructive measurements of the fracture toughness of brittle materials

A simple bond breakage model for computing the fracture surface energy and toughness of a wide variety of brittle materials is presented and correlated against values reported in the literature for the single crystalline forms of these same materials. The correlation shows that this simple model can provide an accurate estimate for both the fracture surface energy and toughness of these materials. It is further shown that this simple model can be extended to amorphous materials with reasonable accuracy by normalizing the fracture surface energy of the crystalline material by the ratio of the density of the amorphous material vs. the density of the single crystalline material. Applications to thin film low-k materials and capabilities for non-destructive measurements are also discussed.     

An asymptotic approach to size effect on fracture toughness and fracture energy of composites

Size effect on fracture toughness and fracture energy of composites is investigated by a simple asymptotic approach. This asymptotic analysis based on the elastic/plastic fracture transition of a large plate with a small edge crack is extended to study fracture of composite. A reference crack length, a^*, is used in the model, which indicates an ideal elastic/plastic fracture transition defined by the yield strength and plane strain fracture toughness criteria. Experimental results of cementitious materials available in literature are analyzed and compared. It is shown that the common K_R-curves can also be obtained by the current asymptotic model. Furthermore, a local fracture energy distribution concept is also discussed and compared with the present asymptotic approach.     

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.     

Measuring toughness and the cohesive stress-displacement relationship by the essential work of fracture concept

The complete fracture behaviour of ductile double edge notched tension (DENT) specimen is analysed with an approximate model, which is then used to discuss the essential work of fracture (EWF) concept. The model results are compared with the experimental results for an aluminium alloy 6082-O. The restrictions on the ligament size for valid application of the EWF method are discussed with the aid of the model. The model is used to suggest an improved method of obtaining the cohesive stress-displacement relationship for the fracture process zone (FPZ).     

The effect of fiber inclusions in toughened plastics—part I: fracture characterization by essential fracture work

The effect of fiber inclusions on toughened plastics was studied by means of the technique of essential fracture work. The major advantage of the technique for fiber-reinforced toughened polymers is attributed to its potential to distinguish fiber-related toughness from fiber-induced matrix toughness. Strictly speaking, the essential fracture work is the specific energy required to create two new surfaces and that consumed in the fracture processes involved. The essential work was only proportional to the ligament area whereas the work dissipated outside the process zone was dependent on the volume of plastically deformed region. The latter is not a material property. Fiber-related fracture work, such as fiber bridging, breaking and pullout, can scale with the ligament length but cannot scale with the ligament squared. With fiber inclusions, supertough Nylon 6,6 exhibited concomitant strengthening and toughening.     

Fracture characterization of high-density polyethylene pipe materials using the J-integral and the essential work of fracture

In this paper, fracture mechanics concepts are reviewed and their relevance to examine the toughness of highly deformable materials such as high-density polyethylene (HDPE) pipe materials is discussed. Using two different specimen configurations (single edge notched bending and compact tension), it was found that the $J-R$ approach is unable to give pertinent indications on fracture toughness of HDPE. Alternatively, applying the essential work of fracture approach to double edge notched tension specimen, seems a more appropriate way to measure the fracture strength of HDPE and therefore to analyze the fracture process of such materials. Nevertheless, the severe necking occurring at the crack tip and in the plastic zone makes difficult the crack growth measurement, which clearly depends on the strain state and on the stress triaxiality level.     

Using local out-of-plane compression (LOPC) to study the effects of residual stress on apparent fracture toughness

To study and understand the effects of residual stresses on fracture behaviour, it is necessary to introduce well characterised and reproducible residual stresses into laboratory fracture specimens. One technique capable of providing such residual stresses is local compression, where the local compression is applied to the sides of a test specimen. In this paper, the technique is used to create a residual stress field in compact tension, C(T), specimens. The specimens are used subsequently to study the effects of residual stress on fracture. Finite element studies show that significant changes to the distribution of the residual stresses occur when the position of the compression tools is changed relative to the crack tip. It is also revealed that both a single and double pair of compression tools can generate both tensile and compressive residual stresses in the vicinity of the crack tip depending upon the location of the tools relative to the crack tip. The impact of local compression is illustrated by experimental results from room temperature fracture tests performed on two aluminium alloys, Al2650 and Al2024. Tensile residual stresses, created by the application of a single pair of compression tools, reduced the initiation fracture toughness of Al2650 by about one half. The ductile tearing resistance of Al2024 decreases when a double pair of tools introduces tensile residual stresses. Conversely, the tearing resistance increases when compressive residual stresses are created through local compression.     

Effects of constitutive parameters on the accuracy of measured fracture energy using the DCB-specimen

Several methods exist to estimate the fracture energy for adhesive joints using the double cantilever beam specimen and linear elastic fracture mechanics. Since the mechanical properties of all adhesives are non-linear, errors are generated. By use of an exact solution experiments are simulated. These are evaluated with eight different methods. The influence of the constitutive parameters is systematically studied. This influence is small for most methods. The error due to the choice of evaluation method is considerably larger. One of the commonly used methods gives accurate results; the error is less than 3%. However, most methods yield substantial errors.     

Fracture toughness analysis of O-POSS/PLA composites assessed by essential work of fracture method

Essential work of fracture (EWF) method was employed to investigate the effect of the octavinylisobutyl based polyhedral oligomeric silsesquioxane (O-POSS) addition in poly(lactic acid) (PLA) matrix on the fracture behavior of O-POSS/PLA composites. The 2 mm thick rectangular shaped PLA-matrix composites containing various weight ratios of O-POSS were injection molded after processing in a twin-screw extruder. Constant deformation rate tensile tests at room temperature were performed on double edge notched tensile (DENT) specimens with various ligament lengths. It was found that the addition of O-POSS to PLA improved the toughness. It was observed that a greater energy consumed after the maximum load reached on load–displacement curves for the composites. Optimum additive value was obtained at 7 wt% O-POSS.     

On the relation between the mode I fracture toughness of a composite laminate and that of a 0° ply: Analytical model and experimental validation

This paper proposes a simple model to predict the fracture toughness of multidirectional carbon–epoxy composite laminates using the fracture toughness of the 0° ply. The model is based on a combination of Linear-Elastic Fracture Mechanics and lamination theory, and uses as material properties the ply elastic properties and the fracture toughness of the 0° ply measured in compact tension test specimens. A good correlation is obtained by comparing the model predictions and experimental data obtained in center-cracked specimens manufactured using different lay-ups and materials.     

The fracture toughness of borides formed on boronized cold work tool steels

In this study, the fracture toughness of boride layers of two borided cold work tool steels have been investigated. Boriding was carried out in a salt bath consisting of borax, boric acid, ferro-silicon and aluminum. Boriding was performed at 850 and 950 °C for 2 to 7 h. The presence of boride phases were determined by X-ray diffraction (XRD) analysis. Hardness and fracture toughness of borides were measured via Vickers indenter. Increasing of boriding time and temperature leads to reduction of fracture toughness of borides. Metallographic examination showed that boride layer formed on cold work tool steels was compact and smooth.     

Estimation of fracture energy from the work of fracture and fracture surface area: I. Stable crack growth

Past attempts to determine fracture energy by the work of fracture (γ _WOF) technique, in most cases, have resulted in greater estimates due to the use of the cross-sectional area rather than the actual area of the fracture surface in calculations. The actual fracture surface area A _F of soda-lime-silica glass chevron-notch flexure specimens was estimated using atomic force microscopy. An equation for A _F was developed using the data from these tests. The use of A _F in the equation for γ _WOF resulted in γ _WOF values less than values reported from traditional fracture mechanics tests and from those obtained using the cross-sectional area. The implication is that the tortuosity of the fracture surface contributes to the energy expended during fracture and should be accounted for in the calculation of the fracture energy. These calculations provide an estimate for the minimum energy required to break bonds in the fracture process.     

Application of the essential work of fracture method in ranking the performance in service of high-density polyethylene resins employed in pressure pipes

High-density polyethylene resins have increasingly been used in the production of pipes for water- and gas-pressurized distribution systems and are expected to remain in service for several years, but they eventually fail prematurely by creep fracture. Usual standard methods used to rank resins in terms of their resistance to fracture are expensive and non-practical for quality control purposes, justifying the search for alternative methods. Essential work of fracture (EWF) method provides a relatively simple procedure to characterize the fracture behavior of ductile polymers, such as polyethylene resins. In the present work, six resins were analyzed using the EWF methodology. The results show that the plastic work dissipation factor, βw _p , is the most reliable parameter to evaluate the performance. Attention must be given to specimen preparation that might result in excessive dispersion in the results, especially for the essential work of fracture w _e .     

Effect of test conditions on the essential work of fracture in polyethylene terephthalate film

The tear resistance of polyethylene terephthalate film is characterized by the essential work of fracture method in mode I as a function of test speed and temperature. Attempts to extrapolate tearing resistance found by the method of essential work to commercial slitting processes are discussed. Limitations of the essential work of fracture method with regards to specimen size are evaluated. Based on the findings modifications to the test protocol are suggested.     

An integrated approach based on acoustic emission and mechanical information to evaluate the delamination fracture toughness at mode I in composite laminate

This paper addresses a new method based on the combination of mechanical behavior and acoustic emission (AE) information of composite materials during mode I delamination. The method is based on a special purpose function, called sentry function, which is defined as the logarithm of the ratio between mechanical energy and acoustic energy (f = Ln(E_s/E_a)). The sentry function is used to study the delamination process and to evaluate the delamination fracture toughness in mode I. The relationship between cumulative fracture toughness energy release rate (G_I) and the integral of the sentry function during crack propagation showed a transition point with two sensitive regions below and above it. This behavior can be followed to obtain the critical strain energy release rate value (G_Ic). Results obtained by means of the sentry function are compared with results obtained by a methodology proposed by other authors.     

Determination of the fracture toughness by applying a structural integrity approach to pre-cracked Small Punch Test specimens

The Failure Assessment Diagram (FAD) is a procedure for evaluating the structural integrity of cracked components. The component’s failure conditions (load and crack size) are based on the material fracture properties (K_mat) considering its plastic behavior. In this paper, a new methodology that combines the FAD approach and the load–displacement curve obtained from pre-cracked Small Punch Test (SPT) specimens is presented, in order to estimate the fracture toughness of 15.5PH stainless steel. This research is based on Finite Element Modeling (FEM) of the pre-cracked specimen to determine both the plastic collapse load and the stress intensity factors during the loading process. A set of interrupted tests on pre-cracked SPT specimens are also conducted in order to identify the initiation point of the crack propagation. This set of numerical and experimental values allows the toughness ratio (K_r) and load ratio (L_r), at the instant the cracked specimen fails, to be determined. The only unknown variable in this process is the value of the material toughness, K_mat. The parameters in question are then combined with the FAD of the material generated from the different options available on the ASME-API 579 procedure, thus yielding an estimate for the value K_mat. Finally, to evaluate the accuracy of this methodology, predicted toughness values are compared to those from normalized tests of the material being analyzed.     

Interpreting the stress ratio effect on delamination growth in composite laminates using the concept of fatigue fracture toughness

This paper provides a study on fatigue delamination growth in composite laminates using energy principles. Experimental data has been obtained from fatigue tests conducted on Double Cantilever Beam (DCB) specimens at various stress ratios. A concept of fatigue fracture toughness is proposed to interpret the stress ratio effect in crack growth. The fatigue fracture toughness is demonstrated to be interface configuration independent but significantly stress ratio dependent. An explanation for this phenomenon is given using SEM fractography. Fracture surface roughness is observed to be similar in different interfaces at the same stress ratio. But it is obviously more rough for high stress ratio in comparison with that for low stress ratio, causing the fatigue resistance increase. Therefore, the stress ratio effect in fatigue crack growth can be physically explained by a difference in resistance to crack growth.