Characterisation of injected EPBC plaques using the essential work of fracture (EWF) method

The influence of processing induced morphology, thickness and ethylene content (EC) of different ethylene-propylene block copolymers on fracture properties has been studied using the essential work of fracture (EWF) method. To analyse the influence of EC, four different materials were chosen with 0, 5.5, 7.8, and 8.4% in weight EC. Each material was injected in three different thicknesses (1, 2 and 3 mm). The resulting plaques were tested using the EWF method in both main orientations; the melt flow direction and transverse to melt flow direction. Different fracture behaviours have been observed, some of them preventing the applicability of the EWF method. Polarised light microscopy observations have revealed the existence of a skin/core structure, which is reduced with an increase in thickness and EC.     

Effect of the specimen dimensions and the test speed on the fracture toughness of iPP by the essential work of fracture (EWF) method

The fracture parameters of an isotactic polypropylene are studied by the essential work of fracture method. The influence of the specimen height, width and thickness and the effect of the test speed are investigated. Results show that this method is very useful for studying the plane‐stress fracture of this kind of materials in form of films and sheets. Varying the width (30 to 60 mm) and the test speed (2 to 100 mm/min) has no relevant influence, whereas the results are only length independent in a range from 40 to 100 mm. The influence of the thickness is very high, obtaining an important decrease of the specific essential work as the thickness is increased in a range from 38 to 2500 μm. This result is justified with the fracture surfaces obtained, observed by SEM, in which an evolution of the fracture behavior is seen as a function of thickness (38, 100, 500, 1000, 2500 μm). © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 177–187, 1999     

An essential work of fracture study of the toughness of thermoset polyester coatings

The fracture toughness of a range of thermoset polyester paints with different cross-link densities has been studied, using the essential work of fracture (EWF) method. The glass transition temperature, T_g, of each of the materials was measured using differential scanning calorimetry, and found to lie between 8 and 46 °C. EWF tests were performed on the paint films at a range of temperatures around the measured glass transition temperature of each material. The essential work of fracture, w_e, at T_g was found to decrease with increasing cross-link density from around 20 kJ/m² at a cross-link density of 0.4 × 10⁻³ mol/cm³ to around 5 kJ/m² for cross-link densities of approximately 1 × 10⁻³ mol/cm³ or higher. A maximum in the essential work of fracture was observed at around T_g when w_e was plotted versus temperature, which could be attributed to the effect of an α-relaxation at a molecular level. The polyesters were found to be visco-elastic, and the applicability of the EWF test to the study of these visco-elastic thermoset materials is discussed.     

Fracture toughness assessment of poly(ethylene terephthalate) blends with glycidyl methacrylate modified polyolefin elastomer using essential work of fracture method

The static fracture toughness of poly(ethylene terephthalate) (PET) melt blended with a modifier containing glycidyl methacrylate (GMA)‐grafted ethylene‐propylene rubber and homopolymerized GMA was studied on injection molded specimens by adopting the essential work of fracture (EWF) method. It was found that the essential and nonessential or plastic work both decrease with an increasing amount of modifier (up to 20 wt %) if the PET matrix is amorphous and nonaged. The scatter in the EWF data for the blend with 10 wt % modifier was found by presuming concurrent mechanisms between microcrystallization and morphology‐dependent cavitation and fibrillation processes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 842–852, 2001     

Toughness assessment of elastomeric polypropylene (ELPP) by the essential work of the fracture method

The essential work of the fracture (EWF) method was employed to determine the fracture performance of thermoplastic polypropylene (PP) elastomers. Three types of elastomeric polypropylene (ELPP) of homo- and copolymer nature based on different catalysts were involved in this study. Tests were carried out in both I and III fracture modes to check the applicability of the EWF approach for such elastomers. It was found that the trouser tearing test (mode III) overestimates both the specific essential (w_e) and plastic work (w_p) terms when the tearing resistance of the ELPP is higher than the resistance to tensile loading. With decreasing crystallinity (i.e., by decreasing length of the stereoregular chain segments of the block copolymers or increased content of comonomer) w_e increased, whereas in respect to w_p,, an opposite tendency was found. This was interpreted by possible changes in the thermoreversible network structure of the ELPP in which crystalline domains act as network knot points in the amorphous PP matrix. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 873–881, 1998     

Evaluation of the tear properties of polyethylene blown films using the essential work of fracture concept

In the case of very thin materials such as blown films, the applied stress state in front of the crack tip is normally a plane stress condition, and the deformation around the crack tip due to the remote stress is very large. However, current standard test methods for quantifying the fracture toughness of thin films, such as the Elmendorf tear test, cannot explain or represent the tear characteristics accurately. The common way of interpreting the test results from the Elmendorf tear test is to develop an empirical correlation and then compare the average values. In this paper, essential work of fracture (EWF) tests for five commercial polyethylene (PE) blown films have been conducted, and the fundamentals of their tear properties based on fracture mechanics have been studied. The results from the EWF test are interpreted based on two important parameters, i.e., the essential work of fracture (W_e) and the non-essential work of fracture (W_p). Further, the relationship between these parameters and the current standard Elmendorf tear test is shown.     

Comparison of fracture behavior of nylon 6 versus an amorphous polyamide toughened with maleated poly(ethylene-1-octene) elastomers

The fracture behavior of an amorphous polyamide (Zytel 330 from DuPont), a-PA, and nylon 6 toughened by maleated poly(ethylene-1-octene) elastomers are reported. The deformation mechanisms during fracture were verified by examining an arrested crack tip and the surrounding regions using transmission electron microscopy analysis. a-PA blends show higher levels of impact strength and lower ductile-brittle transition temperatures than nylon 6 blends. Fracture toughness, characterized by both linear elastic fracture mechanics techniques in terms of the critical strain energy release rate, G_IC, and the essential work of fracture methodology, i.e. the limiting specific fracture energy, u_o, and the dissipative energy density, u_d, using thick (6.35 mm) samples with sharp notches, depends on ligament length, rubber content, rubber particle size and test temperature. In general, a-PA blends show larger values of u_d than do nylon 6 blends while the opposite is seen for u_o. The amorphous polyamide shows a similar critical upper limit on rubber particle size, or interparticle distance, for toughening as the semi-crystalline nylon 6; thus, it is clear that the crystal morphology around the rubber particles must not be the dominant cause of this critical size scale. The deformation mechanisms involved include cavitation of rubber particles followed by some crazing and then massive shear yielding of the matrix.     

Effects of molecular structure on the essential work of fracture of amorphous copolyesters at various deformation rates

The fracture behavior of amorphous copolyesters with different molecular structure was studied with double edge notched tensile loaded specimens (DENT) using the essential work of fracture (EWF) approach. Various deformation rates ranging from 1 to 1000 mm/min were employed. Amorphous poly(ethylene terephthalate) (aPET) exhibited considerably higher specific essential and non-essential work of fracture than the copolyesters containing either cyclohexylenedimethylene (aPET-C) or neopentyl glycols (aPET-N). At high deformation rates, ductile/brittle fracture transition was observed with aPET-C and aPET-N, while aPET always fractured in ductile mode within the entire deformation rate range. These phenomena were ascribed to the different molecular flexibility and entanglement density of the copolyesters. The specific EWF of the aPET as a function of deformation rate went through a minimum. The initial decrease in toughness was caused by the hampered segmental mobility due to the increased deformation rate. The subsequent increase in toughness was attributed to the adiabatic heating induced temperature rise in the process and plastic zones. Strain-induced crystallization of the aPET was observed at ν=500 and 1000 mm/min, which may also contribute to the increase of the specific EWF.     

The application of the essential work of fracture methodology to the plane strain fracture of ABS 3-point bend specimens

The applicability of the EWF methodology to 3-point bend (SEB) specimens under conditions other than plane stress has been assessed experimentally. Different fracture conditions, pure plane strain and plane strain/plane stress transition, were obtained by varying the specimen thickness and testing temperature (20 and 80 °C). Post-mortem fracture surfaces appeared always completely stress-whitened, indicating ductile fracture. The load-line displacement plots are similar over a well-defined range of ligament lengths for which the application of the EWF methodology was in principle possible. Nevertheless, in experiments conducted at room temperature, crack growth was observed to initiate before maximum load and complete ligament yielding. This behaviour was confirmed through plastic collapse analyses. A critical ligament length was found, over which the total specific work of fracture was dominated by edge effects. Below this critical ligament length, EWF methodology was still applicable and it was possible to extrapolate reliable w_Ie values.     

Effect of strain rate on the fracture toughness of some ductile polymers using the essential work of fracture (EWF) approach

The plane strain fracture toughness of two ductile polymers, ultra high molecular weight polyethylene (UHMWPE) and acrylonitrile‐butadiene‐styrene (ABS), was measured by using the essential work of fracture approach. Truly plane strain fracture toughness (w_Ie) was measured for ABS at quasi‐static and impact rates of loading. For UHMWPE, the measured values were only “near” plane strain values (w_Ie*). It was confirmed both w_Ie* and w_Ie were independent of specimen type but dependent on strain rate. For UHMWPE, there was a negative strain rate effect, i.e., w_Ie* decreased with increasing loading rate. At low quasi‐static loading rate (v = 10 mm/min), w_Ie* was constant at 55 kJ/m². It then decreased to 15 KJ/m² when the loading rate was increased to 100 mm/min, and remained at that value even up to impact rate of loading (v = 3.7 m/s). For ABS, a mild positive strain rate effect was observed. w_Ie increased from 13 kJ/m² at v = 10 mm/min to 17 kJ/m² at v = 3.7 m/s.     

Understanding the protective role of a gradient titanium oxide ceramic layer on Ti6Al4V against corrosion via analyses of Mott-Schottky curve and electron work function (EWF)

Thermal oxidation (TO) process was employed to generate a gradient titanium oxide ceramic layer for improving corrosion performance and service safety of Ti6Al4V alloy. The semiconductor characteristic of the TO layer was evaluated in CO₂-saturated simulated oilfield brine. The generated TO layer with a thickness of about 20 μm was dense and continuous without cracks or spalling characteristics. The TO layer mainly comprised of an oxide ceramic layer (rutile TiO₂ ceramic phase, minor anatase one, and Al₂O₃) and an oxygen diffusion layer. The conducted electrochemical analysis suggested that the corrosion resistance of Ti6Al4V alloy was improved using TO surface strengthening process. It was demonstrated that the TO layer with semiconductor characteristics showed a transition from n-type (donor) to p-type (acceptor) with the increasing applied electric potential. The electron work function of the TO layer was higher than that of Ti6Al4V alloy with a naturally formed passive film. The improvement in corrosion properties was attributed to the excellent chemical stability and semiconductor properties of the metal oxide ceramic phases (TiO₂, Al₂O₃) in the TO layer.     

Influence of annealing treatments on the essential work of fracture of biaxially drawn poly(ethylene terephthalate)

The biaxial sequential stretching process of poly(ethylene terephthalate) produces films with a fibrillar microstructure in which fibrils are parallel to the transverse extrusion direction. The mechanical properties of such films are strongly anisotropic due to both the orientation of crystallites and the properties of the intrafibrillar and interfibrillar amorphous phases. The idea is to modulate the properties of the amorphous phase without altering the fibrillar structure by annealing treatments. The morphology (crystallinity and orientation of the crystalline phase) of the annealed films is characterized and their mechanical properties (tensile tests and essential work of fracture) are tested in the longitudinal direction (parallel to the micro fibrils) and in the transverse direction (perpendicular to the micro fibrils). The crystalline phase orientation is the key parameter governing modulus anisotropy. Concerning crack propagation, annealing treatments lead to opposite evolution of the specific essential work of fracture parameter (w_e) in the longitudinal and transverse directions. Thus, it is possible to erase fracture propagation anisotropy through an adequate annealing treatment. Copyright © 2012 Society of Chemical Industry     

Plane‐stress fracture behavior of syndiotactic polypropylenes of various crystallinity as assessed by the essential work of fracture method

The fracture behavior of 1‐mm‐thick syndiotactic polypropylene (sPP) sheets of various crystallinity (owing the various stereo‐ and regioregularity) was studied by the essential work of fracture (EWF) concept. The specific work of fracture parameters were determined on tensile‐loaded deeply double‐edge notched (DDEN‐T) specimens at various deformation rates (v = 1, 10 and 100 mm/min) at ambient temperature. As the DDEN‐T specimens showed full ligament yielding prior to subsequent necking, the energy partitioning method was adopted for data reduction. The specific essential work of fracture slightly increased as a function of crystallinity. Its yielding‐related constituent increased with both crystallinity and deformation rate similarly to that of the yield strength and E‐modulus. The specific plastic work parameters were less sensitive to crystallinity and deformation rate. The strain‐induced crystalline phase transition from helical toward all‐trans conformation was accompanied by voiding and crazing.     

Effects of gauge length and strain rate on fracture toughness of polyethylene terephthalate glycol (PETG) film using the essential work of fracture analysis

Essential Work of Fracture (EWF) analysis was used to study the fracture toughness of a PETG film. In the study of the gauge length (Z) effect on the specific essential work (w_e) using Z = 50, 100, 150, 200 and 250 mm, it is observed that w_e is Independent of gauge length, except that a slightly lower w_e value was measured for Z = 50 mm. Interestingly, for specimens with long gauge lengths (Z ≥ 150 mm in this study), brittle fracture occurred. The minimum ligament length at which ductile/brittle transition took place was observed to decrease with increasing gauge length. There is a small strain rate effect on w_e with loading rates less than 1 mm/min. But with higher loading rates, w_e showeld no strain rate sensitivity.     

Effect of annealing on fracture behavior of poly(propylene‐block‐ethylene) using essential work of fracture analysis

The influence of annealing conditions on the fracture behavior of poly(propylene‐block‐ethylene) sheets was investigated by means of the essential work of fracture method, and was complemented by the study of the effect of annealing on crystal structure, using differential scanning calorimetry. It was shown that both the crystal perfection degree and crystallinity could be improved substantially as annealing temperature (Ta) increased, while the prolonged annealing time at 80°C mainly resulted in the improvement of crystallinity. The reasons for an increase in the specific essential work of fracture and a decrease in the specific plastic work item as crystal perfection degree and crystallinity grew are discussed. The displacement to failure of double edge notched tension specimens decreased gradually with increasing T_a, and the double‐plastic zone could be observed in all specimens. In addition, a novel method to aid the accurate measurement of intense stress‐whitening outer plastic zone height by adjustment of illumination conditions is proposed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3438–3446, 2007     

Fracture toughness evaluation of K resin® grafted with maleic anhydride‐ compatibilized polyamide 6/K resin® blends

The mechanical behavior and fracture toughness of polyamide 6 (PA6)/K resin^® (K) blends, with and without maleic anhydride‐grafted K resin^® (K‐g‐MAH) incorporated, have been investigated. The results showed that the tensile strength, elongation at break and impact strength of PA6/K blends were improved considerably on incorporating K‐g‐MAH. This results from the improvement of compatibility between the PA6 and K phases. The essential work of fracture (EWF) method was employed to determine the fracture toughness of PA6/K blends with and without K‐g‐MAH incorporated. The effect of composition on the EWF parameters of the blends was particularly investigated. The results showed that a significant improvement in the specific EWF (w_e) of PA6/K blends occurred when K‐g‐MAH was incorporated. The effect of K‐g‐MAH content on the fracture toughness of the PA6/K/K‐g‐MAH blends was mainly achieved through its influence on the specific essential and nonessential work of fracture in the yielding process. Copyright © 2007 Society of Chemical Industry     

Fracture characterization of low‐density polyethylenes by the essential work of fracture: Changes induced by thermal treatments and testing temperature

The tensile deformation and fracture behavior of commercially available low‐density polyethylene (LDPE) films, having different molecular characteristics, was studied. Submitting samples to specific thermal histories controlled the morphological structure of these semicrystalline polymers. Phase‐structure analysis of the resulting materials was performed by DMA and DSC analyses. The plane‐stress essential work of fracture methodology was chosen because the materials used had failed after complete necking of the remaining ligament. Significant differences in behavior, induced by thermal treatments, were found for the tensile yield stress and the specific nonessential work of fracture, but not in the specific essential work of fracture. The results show that the mechanical properties and fracture behavior depend not only on the crystallinity levels and molecular weight characteristics of the samples, but also upon the degree of structural continuity. The β‐relaxation process, associated with the crystal‐amorphous interphase, strongly influences the fracture behavior at testing temperatures chosen below the β‐relaxation temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 781–796, 1999     

Classification of ceramics and glass (edge chipping and fracture toughness)

The standard classification of advanced ceramics is based on their strength, although in many cases the performance of products made of such materials is controlled by their deformation behavior and fracture resistance. In this article, ceramics and glass are classified according to their edge chipping resistance (EF-method). Such a classification is based on the idea of a baseline (direct proportionality between edge chipping resistance and fracture toughness of ceramics that are similar to the model material of linear elastic fracture mechanics). Use was made of various elastic and inelastic, oxide and non-oxide ordinary ceramics, composite ceramics capable and incapable of retarding cracks and intended for engineering and biomedical applications. Attention is also given to silicate glass.     

Determination of the fracture toughness of polybutylene terephthalate (PBT) film by the essential work method: Effect of specimen size and geometry

The fracture behavior of PBT films of thicknesses 0.125, 0.175, 0.275, 0.375 and 0.5 mm was investigated according to Essential Work of Fracture (EWF) method. Single edge and double edge notched specimens of varying ligament lengths were tested in tension producing load‐displacement traces that were typical of ductile failure. A linear relationship was obtained between the total specific work of fracture (W_f) and ligament length (L). This linearity was maintained as ligament length exceeded the plastic zone size or one third of sample width. It was found that while the specific essential work of fracture (W_e) was independent of thickness, the specific non‐essential work of fracture (βW_p) decreased with increasing thickness. It was found also, that work of fracture parameters were independent of the specimen width for the range of thicknesses used in the present study. Nevertheless, for a sample width of 20 mm, a lower w_e and a higher βw_p value was obtained. Good agreement was found between values of W_e obtained from SENT and those obtained from DENT specimens; the value of βw_p was consistently higher for SENT specimens.     

Characterization of the fracture properties of aragonite‐ and calcite‐filled poly(ε‐caprolactone) by the essential work of fracture method

Biodegradable polymers, blends, and composites are often investigated during tissue engineering studies, but their fracture properties, which are important mechanical engineering characteristics, are often disregarded or wrongly treated. In this study, essential work of fracture tests were performed on calcium carbonate filled poly(ε‐caprolactone), a very ductile polymer, to determine the effects of different filler shapes (calcite spheres and aragonite whiskers), sizes, and contents on the fracture parameters. Increasing the filler content caused stability problems during crack propagation, and this influenced the self‐similarity of the load–displacement response and resulted in the yielding point being missed. Moreover, the yielding‐related essential work of fracture and the energy dissipating during yielding were found to be almost independent of the filler content and thus could be indicators of matrix–filler adhesion. A shape effect of aragonite whiskers appeared during stable crack propagation; the motion of the particles and the friction on their surface slightly increased the dissipated energy quantum and resulted in a more oriented molecular structure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011