A new cyclic loading method for measuring sheet fracture toughness

This paper reports on a new method for the measurement of sheet fracture toughness. In this method, samples cut in DENT geometry are cyclically loaded. The maximum load for each cycle is incremented after each cycle until the sample fractures. The work in the final cycle is then the work required to fracture the sample. The fracture toughness determined using the new cyclic technique is compared with that determined using the Essential Work of Fracture (EWF) technique, for samples with a wide range of fracture toughness, and it is shown that cyclic technique gives a fracture toughness around 8% lower than the EWF technique. For 3 samples, cyclic fracture toughness values were measured for ligament lengths from 3.3 to 14.0 mm and compared with the EWF fracture toughness in each case. The cyclic fracture toughness was found to be independent of the ligament length, except for very weak samples, and to be 5–10% lower than the EWF fracture toughness. The differences between the EWF and cyclic fracture toughnesses were probably due to the development of damage zones around the crack tips before the fracture. The advantages of the new cyclic technique over the EWF technique are that it requires less sample area as only one ligament length is required, is quicker and can be readily automated.     

Fracture behavior and environmental stress cracking resistance (ESCR) of HIPS/PE blends and the effect of compatibilization on their properties

Attempts have been made to study the fracture behavior and environmental stress cracking resistance (ESCR) of HIPS/PE blends. The effect of compatibilization on their properties was also studied. EWF tests were conducted to measure the essential specific work of fracture (w_e) and non-essential specific work of fracture (βw_p). The ESCR of the samples was investigated using a special modified tensile creep test under an aggressive environment (sunflower oil). It was found that EWF methodology could be applied to uncompatibilized and compatibilized HIPS/PE blends as well as HIPS. The essential specific work of fracture of compatibilized HIPS/PE blends was higher than uncompatibilized HIPS/PE blends and pure HIPS, while its non-essential work of fracture was higher than uncompatibilized blends and lower than pure HIPS. The results also showed that the ESCR of HIPS decreases with incorporation of PE, but an effective compatibilization of this blend increases its ESCR even higher than pure HIPS. The different properties of compatibilized and uncompatibilized blends and HIPS, in EWF and ESCR tests, were attributed to the different mechanisms of fracture in these materials. The different mechanisms of fracture were justified using morphological studies performed on fracture surfaces of each sample. SEM images showed that there is a reasonable correlation between mechanisms of fracture and microstructure of the samples.     

Fracture strength and adhesive strength of hydroxyapatite-filled polycaprolactone

Fracture toughness and tear strength of hydroxyapatite (HAP)-filled poly(ε-caprolactone) (PCL) with increasing HAP concentration were studied. The toughness was assessed in terms of essential work of fracture (EWF). Adhesive strength between HAP and PCL interfaces was evaluated using T-peel testing. The adhesion between the two components was found to be relatively strong. Double edge notched tension (DENT) and trousers test specimens were used for the EWF tests. The effect of HAP phase in PCL on the fracture and tearing toughness was investigated. The results obtained from the EWF tests for the HAP-filled PCL complied with the validity criteria of the EWF concept, namely, (1) geometric similarity for all ligament lengths; (2) fully yielded ligament and (3) plane-stress fracture condition. Values for specific essential work of fracture (w _e ) and specific plastic work of fracture (βw _p ) were found to decrease with increase in HAP concentration. The testing procedure showed promise in quantifying the tearing resistance and rising R-curve behavior common in natural materials and it can be extended to other biomaterials that exhibit post-yield deformation. A quantitative assessment based on fracture mechanics of the adhesive strength between the bioactive interfaces plays an important role for continued development of tissue replacement and tissue regeneration materials.     

Effect of composition on fracture behavior of polypropylene–wollastonite–polyolefin elastomer system

The fracture behavior of polypropylene (PP)–wollastonite–polyolefin elastomer (POE) in the mixed mode region was studied using the essential work of fracture (EWF) method. The relationship between the microstructure and the fracture parameters was analyzed. The effect of wollastonite content on the essential work of fracture and the work of plastic deformation was discussed. The energy dissipation during a double-edge-notched tension (DENT) test was calculated with the EWF method. It was found in the mixed mode region that σ_n increases with shortening of the ligament length region as plastic constraint effect rises and variation of the specific total work of fracture with ligament length was still reasonably linear within the mixed mode region. With increasing wollastonite content, w _e (specific essential work of fracture) increases, while the βw _p (specific non-essential work of fracture) decreases. The measurements of energy dissipation show that improvement in the fracture toughness of PP–wollastonite–POE is mainly due to the increase in crack propagation resistance during the necking and tearing processes after yielding, while the plastic deformation capability of the material depends mainly on the properties of fracture behavior before yielding. It is also found that the impact strength of the material decreases with increasing wollastonite content. However, the composition with high impact strength has lower specific essential energy of fracture and lower long-term fracture resistance, indicating that EWF is a better indicator of long-term fracture properties than the impact strength. DSC results show that the presence of wollastonite hinders crystallization of the PP.     

New energy partitioning approach to the measurement of plane-strain fracture toughness of high-density polyethylene based on the concept of essential work of fracture

This study explores three approaches to determining plane-strain fracture toughness of high-density polyethylene (HDPE), based on the concept of essential work of fracture (EWF). The first approach adopted the traditional method that uses total energy consumed in the testing to determine the plane-strain fracture toughness. The second and the third approaches used energy-partitioning principles to exclude the energy consumption in a later stage of the testing from the calculation of the EWF value. The 2nd approach used a new energy partitioning scheme that excluded the energy consumed at the last stage of plastic deformation; and the 3rd approach was based on an existing energy partitioning scheme proposed by another group, to exclude the energy consumption after the maximum load was reached. The results show that only the 2nd approach generates the EWF value that is independent of the specimen thickness. The paper recommends that the 2nd approach be the most suitable method for determining the plane-strain fracture toughness for ductile polymers like HDPE.     

Impact essential work of fracture toughness of ABS/polyamide-6 blends compatibilized with olefin based copolymers

The impact fracture toughness of acrylonitrile-styrene-butadiene/polyamide-6 (ABS/PA6) blends compatibilized with 5% by weight carbon monoxide modified ethylene-n butyl acrylate-maleic anhydride (EnBACO-MAH) or ethylene-methyl acrylate-glycidyl methacrylate (EMA-GMA) copolymers were examined as a function of blend ratio by standard Charpy tests, Essential Work of Fracture (EWF) Methodology and fracture surface morphologies. The samples were first processed in twin-screw extruder and they were subsequently injection moulded. The incompatibilized blends and neat-PA6 fractured in brittle manner, whereas compatibilized blends fractured in ductile manner. The EWF values yielded a maximum when weight percentages of ABS and PA6 were equal to each other. The values obtained in the case of EnBACO-MAH were higher than that of EMA-GMA regardless of blend composition in EWF tests. The trend of impact strengths observed in standard notched Charpy impact tests was in accordance with that of EWF values of blends. The morphology of the ABS/PA6 blends exhibited differences as a function of the component ratio and compatibilizer type. These differences in topology of the fracture surfaces of the blends were utilized to understand the deformation mechanism, and to correlate the fracture toughness values of the blends.     

High rate toughness of ductile polymers

Impact toughness of two highly ductile polymers: acrylonitrile-butadiene-styrene (ABS) terpolymer and polypropylene block copolymer (PPBC) - was evaluated using the essential work of fracture (EWF) - and a J-R resistance single specimen curve - S_pb techniques. The EWF has proved to be capable of determining toughness from the total fracture energy of several samples differing in initial ligament length and the linear regression of the data. On the other hand, the S_pb method, which is based on the load separation principle, is able of constructing J-R curves by inferring instantaneous crack growth length from the sole comparison between one sharp and one blunt-notched load-displacement traces. Results show that both methodologies can be used under impact conditions when evaluating ABS polymers. However, ABS impact fracture toughness value yielded by the EWF method, w_Ie, was larger than the J_0.2 value obtained from the S_pb method. This difference was imputed to the more progressive development of the necking zone in front of the crack tip under plane strain conditions. On the contrary, for very ductile fracture behavior like that demonstrated by PPBC in which J-controlled conditions were not achieved and hence J-R curves could not be built the EWF appeared as a valuable alternative to characterize impact toughness.     

In-plane and Out-of-plane Fracture Toughness of Physically Aged Polyesters as Assessed by the Essential Work of Fracture (EWF) Method

The in-plane (mode I) and out-of-plane (mode III) fracture behavior of amorphous (co)polyester sheets are studied as a function of physical aging using the essential work of fracture (EWF) concept. Physical aging was followed by measuring the yield stress and enthalpy relaxation of the (co)polyesters. The specific essential work of fracture term did not reflect the physical aging either in mode I or III loading. On the other hand, the corresponding values were closely matched indicating the mode III type loading turns into mode I for thin films. Aging was best manifested in the mode I – related EWF parameters, viz. specific yielding-related essential work and non-essential necking/tearing related work.     

Effect of annealing process in water on the essential work of fracture response of ultra high molecular weight polyethylene

The toughness of ultra high molecular weight polyethylene (UHMWPE) before and after annealing process in water was investigated by the essential work of fracture (EWF) method. Annealing in water at 80 °C for various aging periods of the 5 mm thickness with various ligament lengths single edge notched tension (SENT) specimens was performed in hot water. Tensile tests were performed at 2 mm/min constant deformation rate at room temperature in order to determine EWF parameters. After tensile tests, the fracture cross-sections of SENT specimens were investigated by scanning electron microscope (SEM). From the 60th day of annealing process in water, it was seen that the fracture toughness of the material decreased while water absorption in samples increased.     

The Essential Work of Fracture (EWF) method – Analyzing the Post-Yielding Fracture Mechanics of polymers

The Post-Yielding Fracture Mechanics describe the fracture behaviour of pre-cracked films and thin sheets that show yielding phenomenon at the crack tip during fracture. The Essential Work of Fracture method (EWF) has been used for this type of fracture characterization, determining two parameters: the specific work of fracture, w_e related with the real fracture process area, and the specific non-essential work of fracture, w_p that corresponds with the work done in the outer region of the crack tip.The EWF technique has been successfully employed especially with polymers, allowing the study of the influence of many variables in fracture properties, unavailable using other techniques such us K_IC or J_IC determination. In this work, the fundamentals of the technique and examples of application are reviewed, presenting a brief summary of the most relevant contributions of our group to the EWF method.     

Essential work of fracture and failure mechanisms of polypropylene-clay nanocomposites

The fracture behavior of polymer nanocomposites (PNCs) based on a polypropylene with organo-modified clays (2 wt.%) and different coupling agents was studied by means of essential work of fracture (EWF). The PNC microstructure was characterized by clay particle dispersion at the micron scale (>1 μm) and sub-micron scale (200 nm to 1 μm), with good intercalation and partial exfoliation (<100 nm). Tensile testing showed significant improvements (+25-50%) corresponding to nanoparticle reinforcement effects. Fracture surfaces revealed that fracture occurred by void initiation at larger clay particles, followed by void growth and coalescence as the surrounding matrix stretched into ligaments. EWF improvements (+20%) were noted for PNCs that had fewer micron scale particles and showed higher tensile improvements. Toughness improvements were attributed to higher voiding stresses and improved matrix resistance attributed to finer, more oriented clay nanoparticles.     

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.     

Influence of surface-modified TiO2 nanoparticles on fracture behavior of injection molded polypropylene

We prepared surface-modified TiO₂ nanoparticle (21 nm)/polypropylene nanocomposites using a twinscrew extruder and an injection molding machine. The TEM (transmission electron microscopy) and SEM (scanning electron microscopy) images showed homogeneous dispersion of nano-TiO₂ at 1 vol.% filler content and weak nanoparticle matrix interfacial adhesion. It was found that the essential work of fracture (EWF) approach, usually characterizing fracture toughness of ductile materials, was no longer applicable to the nanocomposite samples because of the extreme crack blunting and tearing processes observed in the EWF tests. As an alternative approach, the specific essential work-related yield was used for assessment of the plane-strain toughness, as suggested in the literature. The results indicated that the addition of 1 vol.% nano-TiO₂ did not toughen the polypropylene (PP) matrix at all. On the other hand, it was observed from the EWF tensile curves that the nanoparticles enhanced the ductility of the PP matrix greatly, the reason of which was probably ascribed to the high level of molecular orientation of the injection molded samples, as revealed by the polarized optical microscopy (POM). Because of the highly ductile behavior induced by the nanoparticles, the fracture energy achieved two-to three-fold increase, depending on the ligament lengths of the samples. The difference between the toughness and ductility of nanocomposites was discussed.     

Ductile Fracture Study of Stainless Steel AISI 304L Thin Sheets Using the EWF Method and Cohesive Zone Modeling

The purpose of this study is to model the ductile fracture phenomenon using experimental and numerical methods. The stainless steel AISI 304L thin sheets are studied including two thicknesses (0.8 and 1.5 mm). A mechanical characterization is firstly done in order to obtain the main mechanical properties useful for the numerical modeling. In order to determine the essential work of fracture (EWF), DENT specimens are used involving the two thicknesses. The results obtained in terms of EWF for the two thicknesses are close; we find that the essential work of fracture can be considered as an intrinsic criterion for thin sheets. A cohesive zone modeling (CZM) is used in the present study; the model is represented by a traction–separation law (TS). The cohesive elements are implemented in the finite element model, and the material parameters of the model are determined by the mechanical and fracture characterizations. A satisfactory reproduction of the experimental tests is obtained. A good correlation is also obtained between the essential work of fracture determined experimentally and the work of separation used as cohesive zone model parameter.     

On the essential work of fracture of linear low-density-polyethylene. I. Precision of the testing method

The precision (i.e. the repeatability) of the essential work of fracture (EWF) method in determining the fracture parameters of a highly extendible linear low-density-polyethylene film is investigated. In order to minimize any interference from external variables, a random data collection procedure is adopted to extract, from a large data set, various EWF samples with sizes ranging from 11 to 150 data points. Two different notching procedures have been considered, involving different tools (scalpel or razor blade) and cutting methodologies.The notching procedure has only a marginal influence in terms of the correlation coefficient of the linear regression and standard error on the specific essential work of fracture (w_e). However, the mean of w_e values is markedly affected by the notching procedure, being its influence on the specific non-essential work of fracture (βw_p) parameter relatively lower. The dispersion of the w_e and βw_p data around their mean values decreases as the sample size increases, with a trend clearly affected by the notching procedure.     

Comments on “Fracture study of the composite using essential work of fracture method: PP-SEBS-g-MA/E1 clay” [Materials and Design 53 (2014) 741–748]

In the present paper a few comments on the paper “Fracture study of the composite using essential work of fracture method: PP-SEBS-g-MA/E1 clay” Materials and Design 53 (2014) 741–748, have been discussed. The comments are mainly focused on the methodology employed for evaluation of fracture parameters. The EWF method has been used for characterization of the fracture parameters of all the samples, while it seems that some of the samples did not satisfy the main prerequisites of the EWF approach. With this respect the obtained data and discussed results are questionable.     

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).     

Influence of test parameters on the measurement of the essential work of fracture of zinc sheets

The Essential Work of Fracture (EWF) concept is used to characterize the fracture of thin plates of a zinc alloy. The consistency and applicability of the EWF approach are discussed. The successive stages of the fracture process were studied: the evolution of the shape of the crack tip was observed with a scanning electron microscope, the shape of the plastic zone and the evolution of necking in the ligament ahead of the crack tip were measured using a laser profilometer and the onset of cracking in the ligament was detected by means of a TV camera coupled with acoustic emission recording. The influence of both test parameters and material parameters on the Essential Work of Fracture (W _e ) and on the CTOD (_c) are elucidated. The effects of specimen geometry, deformation rate, texture and grain size are especially investigated. The main advantages and drawbacks of the EWF method are highlighted.     

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 .     

Mechanical and thermal behaviour of polyamide 6/silicon carbide nanocomposites toughened with maleated styrene–ethylene–butylene–styrene elastomer

Polyamide 6 (PA6) based nanocomposites reinforced with 1–7 wt% silicon carbide nanoparticles (SiC_p) and toughened with 20 wt% maleated styrene–ethylene–butylene–
stryrene (SEBS‐g‐MA) were fabricated by melt blending followed by injection moulding. The effects of SiC_p addition on thermal and mechanical properties of such nanocomposites were investigated. Differential scanning calorimetry tests showed that SiC_p act as effective nucleators for the crystallization of PA6 and enhance the degree of crystallinity. Mechanical measurements revealed that SiC_p addition improves the Young's modulus and yield strength of PA6/SEBS‐g‐MA 80/20 blend at the expenses of tensile ductility and impact strength. The essential work of fracture (EWF) approach under tensile mode was employed to characterize the fracture toughness of PA6/SEBS‐g‐MA/SiC_p nanocomposites. EWF results indicated that SiC_p addition reduces both the specific EWF and specific non‐essential plastic work of fracture. Thus SiC_p additions were detrimental to the fracture toughness of PA6/SEBS‐g‐MA 80/20 blends.