Deformation rate dependence of work of fracture parameters in polybutylene terephthalate (PBT)

Essential work of fracture (EWF) analysis was used to study the effect of loading rate (v = 2, 5, 25 and 50 mm/min) on fracture toughness of PBT film of thicknesses 0.175, 0.275, 0.375 and 0.5 mm. Using single edge notched tension (SENT) specimens, it was found that the specific essential work of fracture, w_e, increases slightly with increasing loading rate but Within the specified range of loading rate, it showed no significant variation with respect to thickness. The specific non-essential work of fracture, βw_p, decreased with increasing rate and increasing thickness.     

Fracture behaviour of polyethylene naphthalate (PEN)

Fracture toughness of a semi-crystalline polyethylene naphthalate (PEN) film of thicknesses 0.050, 0.075 and 0.125 mm was measured as a function of temperature and loading rate using both double edge notched tension (DENT) and single edge notched tension (SENT) specimens. The specific essential work of fracture (EWF) and the multi-specimen J-integral methods were used to evaluate fracture toughness. The variation of the specific total work of fracture (w_f) with ligament length (L) was linear for ligament lengths between 5 and 15 mm. Within this range, w_f versus L was independent of thickness at all temperatures but was dependent on both temperature and loading rate. The specific EWF (w_e) was found to be independent of thickness and loading rate but showed three regions of varying temperature dependence. Between 23 and 80°C (region I) w_e was essentially independent of temperature but increased with temperature between 80 and 120°C (region II) and decreased with temperature thereafter (region III). At glass transition temperature (i.e. 120°C), w_e reached a maximum value of 75 kJ/m². The specific non-EWF (βw_p) increased with both loading rate and temperature. The greatest change in βw_p value with respect to temperature was obtained in region II.The plot of J-integral versus crack extension (Δa) was independent of thickness but was dependent upon temperature. w_e was found to be equivalent to both J_0.2 and J₀.     

Essential work of fracture (EWF) analysis for compression molded alternating poly(propylene carbonate)

In this investigation, the main objective was to study the mechanical properties of alternating poly(propylene carbonate) copolymer (PPC). The PPC used in this study was derived from propylene oxide and carbon dioxide using zinc glutarate as catalyst. The molecular weight of the PPC copolymer used in this study has M?_n～33,000. The synthesized PPC was compression molded into sheets of thickness ～1mm. The fracture toughness of the PPC films was determined using the essential work of fracture (EWF) technique, at a laboratory temperature of 20°C, and a loading rate of 1 mm/min. During the EWF measurement, a significant amount of plastic deformation has taken place around the initial ligament region. The measured specific total fracture work (w_f) was observed to vary in a linear fashion with the specimen ligament (l), and hence satisfied the basic requirement for EWF analysis. The specific essential fracture work (w_e) for the PPC film was measured to be 11.0 kJ/m². The PPC showed a prominent recovery behavior. The severely deformed region surrounding the fracture ligament was observed to recover completely 8 days after fracture testing. Polym. Eng. Sci. 44:580–587, 2004. © 2004 Society of Plastics Engineers.     

On the essential work of fracture method: Energy partitioning of the fracture process in iPP films

Summary The fracture properties of an iPP are investigated by the EWF method. A separation between crack initiation and propagation fracture parameters is done by splitting the total energy of the load-displacement curves in two. The influence of the DDENT specimen height and the test rate on these different parameters is studied, obtaining that varying the height has no influence in the range 40 to 80mm, but changing the crosshead speed (2 to 100mm/min) has an effect on the fracture parameters. It is interesting to note that the “Initiation Specific Essential Work” (w_e ^I) seems not to be sensible to the stress-state transition. Received: 20 October 1998/Revised version: 1 December 1998/Accepted: 7 December 1998     

Fracture of polybutylene terephthalate (PBT) film

Combined effects of thickness and temperature on essential work of fracture (EWF) of polybutylene terephthalate (PBT) film were studied using single edge notched tension (SENT) and double edge notched tension specimens. It is found that specific essential work of fracture (w_e) for PBT is independent of temperature below T_g (≈80 °C), but decreases above T_g. Between temperatures 25 and 100 °C, w_e was independent of film thickness in the range 0.125-0.375 mm. The specific non-essential work of fracture (βw_p) was temperature and thickness dependent, being greater for the SENT type specimens. Specimen orientation had no influence on w_e but strongly affected βw_p. It was found that βw_p is greater for cracks propagating normal to the extrusion direction as compared to the parallel direction.     

Impact specific essential work of fracture of compatibilized polyamide‐6 (PA6)/poly(phenylene ether) (PPE) blends

The essential work of fracture (EWF) method has aroused great interest and has been used to characterize the fracture toughness for a range of ductile metals, polymers and composites. In the plastics industry, for purposes of practical design and ranking of candidate materials, it is important to evaluate the impact essential work of fracture at high‐rate testing of polymers and polymer blends. In this paper, the EWF method has been utilized to determine the high‐rate specific essential fracture work, w_e, for elastomer‐modified PA6/PPE/SMA (50/50/5) blends by notched Charpy tests. It is found that w_e increases with testing temperature and elastomer content for a given specimen thickness. Morphologically, there are two failure mechanisms: shear yielding and pullout of second phase dispersed particles. Shear yielding is dominant in ductile fracture, whereas particle pullout is predominant in brittle fracture.     

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.     

Effects of injection molding–induced morphology on the work of fracture parameters in rubber-toughened polypropylenes

The effects of the injection-molding induced skin-core morphology on the fracture behavior of rubber-toughened polypropylene (RTPP) systems were studied by employing the essential work of fracture (EWF) method. RTPP with 31 wt% ethylene/propylene rubber (EPR) showed no skin-core structure after molding and the EWF approach worked well in this case. In contrast, RTPP with 10 wt% EPR exhibited a pronounced skin-core morphology: EPR deplection and enrichment was observed in the skin and core region, respectively. This morphology caused necking instead of crack growth in deeply double edge-notched (DDENT) specimens under tensile loading along the filling direction (MFD). The necking process not only was accompanied by a large scatter but also yielded highly unrealistic specific essential work of fracture (w_e) values. This skin-core structure was also the reason for an anistropic EWF response of this system observed by loading the specimens both in longitudinal (L) and transverse (T) directions to the MFD. The failure sequence and its characteristics were studied by light microscopy (LM) and infared thermography (IT). It was concluded that the EWF approach cannot be applied for RTPP with a prominent skin-core structure. Since yielding preceded the limited crack growth prior to necking in the loading directin for the DDEN-T specimen of RTPP with 10 wt% EPR, the yielding-related specific essential work (w_e,y) was used for toughness comparison. In case of RTPP with 31 wt% EPR, where yielding was less pronounced prior to the crack growth, the work of fracture until the maximum load was assigned to the yielding-related work of fracture (w_f,y) used for computing w_e,y. The latter value seems to be closely matched to the plane-strain essential work of fracture value.     

Determination of fracture toughness of poly (ethylene terephthalate) film by essential work of fracture and J integral measurements

Abstract

The plane stress fracture toughness of a semicrystalline poly (ethylene terephthalate) (PET) film of thickness 0·125 mm has been measured as a function of specimen size, specimen geometry, loading rate, and temperature using the essential work of fracture (EWF) approach. It was found that the specific essential work of fracture w _e was independent of specimen width, specimen gauge length, and loading rate, but was dependent upon specimen geometry and test temperature. Below the glass transition temperature (93°C), w _e for double edge notched tension (DENT) type specimens was temperature insensitive, but increased with temperature for single edge notched tension (SENT) type specimens. The w _e value for SENT specimens was consistently higher than for DENT specimens. Estimation of w _e via crack opening displacement was reasonable using the relationship w _e = σ_n e _0,y; estimations made via similar type equations were either too high or too low and were generally unsatisfactory. It was found that values of J integral obtained by power law regression and linear extrapolation of the J–R curves to zero crack growth were lower than w _e. The power law regression of the J–R curves with ?a taken as half the crack opening displacement value at maximum load gave J _c values which agreed reasonably well with w _e.     

Combined effect of temperature and thickness on work of fracture parameters of unplasticized PVC film

The combined effect of temperature and thickness on the essential work of fracture (EWF) parameters for an unplasticized poly(vinyl chloride) (uPVC) film was investigated using double edge notched tension specimens. It was found that for the range of temperatures (23°C to 60°C) and thicknesses (0.15 mm to 0.40 mm) studied here, specific essential work of fracture (w_e) was independent of temperature at each thickness but increased with thickness at each temperature. It was found that at each temperature, w_e and its yielding (w_e,y) and necking/tearing components (w_e,nt), all increased linearly with increasing thickness. However, whilet w_e showed no significant variation with respect to temperature, its yielding component (w_e,y) decreased and its necking/tearing component increased (w_e,nt) with increasing temperature. It was found that estimated values of w_e and its components w_e,y and w_e,nt via crack opening displacement values were by and large unsatisfactory, being either much higher or lower than the directly measured values.     

Essential work of fracture analysis of short glass fiber and/or calcite reinforced ABS/PA6 composites

The effect of reinforcing agent type and composition on the fracture behavior of short glass fiber (SGF), CaCO₃ particle, and glass fiber/CaCO₃ hybrid reinforced ABS/PA6 blend based composites have been studied by using the essential work of fracture (EWF) method. Two millimeter thick rectangular shaped samples were first processed in twin‐screw extruder and they were subsequently injection molded. Double edge notched tensile (DENT) specimens with various ligament lengths were subjected to tensile tests at 2 mm/min constant deformation rate at room temperature in order to determine EWF parameters. For the neat matrix and 10 wt% calcite reinforced materials fractured in ductile manner, that is, the ligament fully yielded and the crack stably propagated unlike the other compositions. For the neat matrix, both the specific EWF, w_e, and the nonessential work of fracture, βw_p, values dramatically decreased with increasing reinforcement weight ratio regardless of the agent type. The analyzing of yielding and necking/tearing components of essential and nonessential parameters showed that for the samples reinforced with SGF w_e,nt > w_e,y and β_ntw_p,nt > β_yw_p,y, indicating that a majority of fracture energy was dissipated in the necking and tearing stages of fracture process. POLYM. ENG. SCI., 54:540–550, 2014. © 2013 Society of Plastics Engineers     

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     

Influence of processing on ethylene propylene block copolymers (II): Fracture behavior

The present work investigates the relationships between the microstructural state and fracture properties in commercial polypropylene‐based materials. In this case an isopolypropylene homopolymer and three ethylene propylene block copolymers (EPBC) with different ethylene content (EC) have been studied. A variety of morphologies were obtained by a combination of several processing methods (injection molding, injection molding‐annealing, and compression molding) and thickness. Fracture behavior of deeply double‐edged notched specimens was evaluated by scanning electron microscopy (SEM) and by the essential work of fracture (EWF) method, analyzing the influence of processing, thickness (t), EC, and orientation respect to melt flow direction (MD and TD). The testing direction and EC are the most relevant variables that affect the ability of the crack tip to deform plastically during the crack propagation, determining the final fracture behavior. The fracture parameters obtained with the EWF method, specific EWF, w_e, and plastic item, βw_p, have proved to be very sensitive to the processing induced morphology, finding interesting relationships between such morphologies (characterized by crystallinity index, orientation level, and skin/core ratio) and the fracture parameters of the plaques. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2714–2724, 2006     

On the measurement of fracture toughness of soft biogel

In this study, the rate dependent energy dissipation process and the fracture toughness of physical gels were investigated using agarose as a sample material. Both the J‐integral and Essential work of Fracture (EWF) methods were examined. To assess the quasi‐static fracture toughness of gels, linear regression was performed on critical J (J_c) values at different loading rates resulting in a quasi‐static J_c value of 6.5 J/m². This is close to the quasi‐static EWF value of 5.3 J/m² obtained by performing EWF tests at a quasi‐static loading rate (crosshead speed of less than 2 mm/min). Nearly constant crack propagation rates at low loading rates, regardless of crack length, suggest viscoplastic chain pull‐out is the fracture mechanism. At high loading rates failure was highly brittle, which is attributed to sufficient elastic energy accumulation to precipitate failure by chain scission. We conclude that in physical gels quasi‐static fracture toughness can be evaluated by both the J‐integral and EWF methods provided the effects of loading rate are investigated and accounted for. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Effects of 2-ethyl-4-methylimidazole and tetraethylammonium bromide on the fracture properties of epoxidized soybean oil based thermoset

In this paper, we aim to report the effects of catalyst (types and concentrations) on the fracture mechanics of epoxidized soybean oil (ESO) based thermosets. ESO resin was thermally cured using methylhexahydrophthalic anhydride curing agent in the presence of two types of catalysts, i.e., tetraethylammonium bromide and 2-ethyl-4-methylimidazole (EMI). The loading of the catalysts varied from 0.3 to 0.8?phr. The fracture behaviour of ESO thermoset was examined on the basis of the principle of linear elastic fracture mechanics (LEFM) and essential work of fracture (EWF). LEFM measurements were performed using single-edge notched tensile and double-edge notched tensile (DENT) tests, while, EWF measurements were carried out using DENT tests. The fracture morphologies of the ESO thermosets were characterized via field emission scanning electron microscopy. It was determined that the plane-strain fracture toughness (K _IC), the specific EWF (w _e), and the specific plastic fracture work (??w _p) of ESO thermosets were significantly influenced by the types and loading of catalysts. In addition, the fracture toughness properties were associated with the crosslink density of the ESO thermosets. In addition, it was found that the brittle?Cductile transition of EMI-catalyzed ESO thermosets can be assessed by the combination of LEFM and EWF in the fracture toughness measurement.     

Essential work of fracture evaluation of fracture behavior of glass bead filled linear low‐density polyethylene

The effect of the glass bead (GB) size and bead content on the fracture behavior of GB‐filled linear low‐density polyethylene (LLDPE) composites was evaluated by means of the essential work of fracture (EWF). The results indicated the specific EWF (w_e) is lower for the composites than that of pure LLDPE and the obtained w_e values do not show significant differences for the filled samples with different GB diameters. The non‐EWF or plastic work (βw_p) also decreased with the addition of GBs, indicating that less energy is absorbed during the fracture process for the composites filled with different diameter GBs. For the composites filled with GBs of different contents, the w_e decreased with increasing GB contents and the βw_p that was higher than that of pure LLDPE at relatively low contents also decreased with the content of GBs. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1781–1787, 2006     

Influence of temperature on plane stress ductile fracture of poly(ethylene terephthalate) film

Abstract

Double edge notched poly(ethylene terephthalate) (PET) specimens of varying ligament lengths and 0·125 mm thickness have been pulled to complete fracture between 23 and 160°C. Within this temperature range, propagation of the crack was always stable, producing load–displacement curves at various ligament lengths that were geometrically similar to one another. Essential work of fracture (EWF) analysis was used to study the effect of temperature on fracture toughness. A linear relationship was obtained between specific total work of fracture W _f and ligament length over the entire temperature range under consideration. The slope of the line, which is termed specific non-essential work of fracture βw _p , showed a maximum near the glass transition temperature of the material (T _g ≈ 93°C). Beyond this point, βw _p decreased sharply with increasing temperature. The intercept of the line at zero ligament length, which is referred to as specific essential work of fracture w _e , showed three types of variation with respect to temperature. Below T _g , w _e was found to be more or less independent of temperature; above T _g it increased with temperature and reached a maximum value at the end of the leathery region (～120°C); beyond which it decreased steadily.     

Fracture characterization of recycled high density polyethylene/nanoclay composites using the essential work of fracture concept

The effect of nanoclay on the plane‐strain fracture behavior of pristine High density polyethylene (HDPE) and recycled HDPE blends was studied using the essential work of fracture (EWF) concept. The failure mode of EWF tested specimens was found to be associated with the specific non‐EWF (β_Bw_p,B). Adding 6‐wt% of nanoclay to pristine HDPE and 2‐wt% to recycle‐blends greatly decreased the β_Bw_p,B values and led to a transition from ductile to brittle failure mode. A fractographic study revealed that the difference in failure modes was caused by the changes in micro and macro morphologies, which could be related with the specific EWF (w_e,B). In the ductile failure, w_e,B is governed by the fibril size; adding nanoclay and recycled HDPE to pristine HDPE decreased the fibril size and subsequently lowered the w_e,B value. In the brittle failure, the w_e,B value was enhanced by creating a rough fracture surface. Adding nanoclay to pristine HDPE, a steadily decrease in w_e,B was measured until 4‐wt% after which the change was insignificant. Conversely, nanoclay content more than 2‐wt% in recycle‐blends greatly decreased the w_e,B value. A transition map was constructed to illustrate the potential failure mode and the associated fracture morphology based on the tested material compositions. POLYM. ENG. SCI., 56:222–232, 2016. © 2015 Society of Plastics Engineers     

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.     

Fracture behaviour of poly[ethylene–(vinyl alcohol)]/organo‐clay composites

BACKGROUND: Ethylene–(vinyl alcohol) (EVOH) copolymer/organo‐modified montmorillonite (OMMT) composites were investigated. Composites with two different percentages by weight of OMMT were prepared using a melt‐extrusion procedure in a twin‐screw extruder, using EVOH as matrix. Films made of EVOH and EVOH/OMMT composites were prepared in a cast‐film extrusion line. RESULTS: The mechanical properties were evaluated by tensile tests and the fracture behaviour was analysed using the essential‐work‐of‐fracture (EWF) method. Fracture characterization was carried out for the two main processing directions: melt flow direction and transverse direction. Fractographic observations were made using scanning electron microscopy. CONCLUSION: The tensile test results indicated good compatibility between EVOH and OMMT. In addition, the fracture tests showed the influence of the clay particle arrangement on the fracture behaviour, showing an increase in the specific essential work of fracture, w_e, which was attributed to the EVOH–OMMT interaction. The plastic term, βw_p, showed different trends depending on the test direction, explained by the size of the plastic zone and the restrictions to the EVOH plastic flow promoted by the clay particles. In this sense, the EWF method is shown to be a very useful tool for the analysis of structure–property relationships in polymer–organo‐clay composites. Copyright © 2009 Society of Chemical Industry