Essential work of fracture: An approach to study the fracture behavior of acrylic bone cements modified with comonomers containing amine groups

The fracture behavior of acrylic bone cements modified with comonomers containing amine groups was studied using the essential work of fracture approach. The cements were prepared with either 2-(diethylamino)ethyl-acrylate (DEAEA), 2-(dimethylamino)ethyl-methacrylate (DMAEM) or 2-(diethylamino)ethyl-methacrylate (DEAEM) as comonomer in the liquid phase. Double-Edge-Notched Tensión (DENT) specimens were tested in a universal testing machine at 5 mm/min. The results showed that the essential work (w_e) and nonspecific value of fracture (βw_p) of bone cements modified with all percentages of comonomer were notably increased compared with unmodified bone cement. From Scanning Electron Microscopy (SEM) micrographs, ductile behavior was observed for modified bone cements, i.e. the crack propagation is stable, whereas the unmodified cement exhibited brittle behavior indicating unstable crack propagation. The use of the essential work of fracture approach is suggested to determine the fracture behavior of cements that do not exhibit a linear stress-strain relationship.     

The effects of annealing on fatigue crack propagation in polyethylene

Fatigue crack propagation tests on annealed and quenched medium-density polyethylene showed the annealed specimens to have much lower resistance to crack initiation and subsequent propagation. Although the same fracture mechanism, in which the brittle crack gradually becomes more ductile, prevailed in both cases, the voided and fibrillated crack tip root craze in the annealed material was much weaker that the nonfibrillated quenched root craze. Microstructural analyses indicate that the annealed material had separate crystallite populations, whereas the quenched material had a more homogeneous morphology. The highest melting fraction of the annealed material was composed of lamellae that were about 270 Å thick, and the quenched lamellae were estimated to be 160 Å thick. The reduced fatigue crack propagation resistance of the annealed material was suggested to be a result of a lower concentration of tie molecules and its reduced damping capability, compared to the quenched material. © 1995 John Wiley & Sons, Inc.     

Minimum energy pathways of brittle and ductile deformation/fracture processes

An alternative approach to exploring deformation and fracture processes from molecular dynamics simulations is presented. The underlying concept is based on the transition path sampling scheme which was proven powerful for unprejudiced mechanistic analyses of rare reaction events and phase transitions. Transferring this method to deformation and fracture is particularly rewarding for the study of brittle materials. Similar to unbiased investigations of nucleation events in first order phase transitions, our simulations do not require the preparation of cavities to induce fracture processes. The mechanisms of cavitation and fracture propagation are instead obtained as simulation results. As two case studies, transition path sampling related to brittle fracture of ZnO nanorods and to ductile deformation of Al nanoblocks is compared to conventional simulations imposing constant-strain rates.     

Full notch creep test (FNCT) of PE-HD – Characterization and differentiation of brittle and ductile fracture behavior during environmental stress cracking (ESC)

The damage mechanisms slow crack growth (SCG) and environmental stress cracking (ESC), relevant for PE-HD materials are characterized based on improved full notch creep testing (FNCT) of two selected typical PE-HD materials for container applications. In this context, a distinction of the failure mechanisms as well as a categorization of involved media is suggested. Employing a novel FNCT device, elongation data were obtained in addition to conventional time-to-failure results of stress-dependent as well as temperature-dependent measurements. Changes in failure behavior, as determined by fracture surface analysis based on light microscopy (LM) and laser scanning microscopy (LSM), are correlated with FNCT results and used to introduce an additional possibility for the identification of brittle/ductile fracture behavior.     

Mechanical and fracture behavior of polypropylene/poly(ethylene-co-vinyl alcohol) blends compatibilized with ionomer Na

In this work the deformation and fracture behavior of PP/EVOH blends compatibilized with ionomer Na⁺ at room and low temperature was studied. Uniaxial tensile tests on dumb-bell samples and fracture tests on single-edge notched bending (SENB) specimens were performed for 10 wt.% and 20 wt.% EVOH blends with different ionomer content at 23 °C and −20 °C. The incorporation of EVOH to PP led to less ductile materials in tension as judged by the lower values of the ultimate tensile strain displayed by all PP/EVOH blends in comparison to neat PP. In contrast, the ionomer Na⁺ addition partially counteracted this effect. The compatibilizing effect of ionomer Na⁺ was also evident in fracture results since higher values of the fracture parameter were obtained for the ternary blends. SEM observations also confirmed this effect. On the other hand, PP/EVOH blends exhibited different fracture behavior with test temperature. All blends showed “pseudo stable” behavior at room temperature characterized by apparently stable crack growth that could not be externally controlled. On the contrary, blends behaved as semi-brittle at −20 °C with some amount of stable crack growth preceding unstable brittle fracture. Finally, irrespectively of the temperature or the ionomer content all PP/EVOH blends exhibited more ductile fracture behavior with a higher tendency to stable crack propagation than neat polypropylene.     

Molecular weight–mechanical property interrelationships. V. Poly(vinyl chloride) fracture surfaces: The effect of molecular weight and vapor environment

The fracture and craze surfaces of four PVC fractions (M?_w = 51000 to 228000) and two bimodal blends were examined with a scanning electron microscope. The fraction with the lowest molecular weight gave brittle fractures when fatigued in nitrogen and ethanol vapor. Walls of crazed ductile matter formed at the surface of higher molecular weight samples. Thickness of this ductile material increased with molecular weight. There appeared to be a balance between craze propagation into the sample and brittle fracture due to dilitational and tensile stresses in the interior regions of the test films.     

Fracture resistance measurement of fused deposition modeling 3D printed polymers

A method is presented to characterize the fracture resistance and interlayer adhesion of fused deposition modeling (FDM) 3D printed materials. Double cantilever beam (DCB) specimens of acrylonitrile butadiene styrene (ABS) were designed and printed with a precrack at the layers' interface. The DCBs were loaded in an opening mode and the load-displacement curves were synchronized with the optical visualization of the crack tip to detect the critical load at the crack initiation. A finite element model, coupled with J-integral method and fracture surface analysis was then developed to obtain the apparent fracture resistance (J_cr,a) and the interlayer fracture resistance (J_cr,i), as a measure of the interlayer adhesion. The maximum J_cr,i was measured to be 4017 J/m², a value close to the fracture resistance of bulk ABS. Both J_cr,a and J_cr,i increased with the printing temperature. This method can find a great importance in the structural applications of printed materials.     

Interpretation of fracture surface features in polycarbonate

The fracture surface of notched impact specimens of polycarbonate tested in the range 130°C to ?196°C have been studied by use of optical microscopy and scanning electron microscopy. The morphological features associated with the initiation and propagation of fracture have been determined and interpreted in terms of the processes which occur in association with fracture, notably crazing. The fracture processes are similar to those observed in poly(methyl methacrylate) and polystyrene. The transition from ductile to brittle fracture is attributed to the case of crazing relative to shear yielding.     

Ductile‐to‐Semiductile Transition in PP‐MWNT Nanocomposites

A ductile‐to‐semiductile transition in the crack resistance behaviour of PP/MWNT composites is discussed, using an essential work of fracture approach based on a post yield fracture mechanics concept and its possible interrelation to the structural attributes studied by TEM, SEM, and WAXD. A maximum in the non‐essential work of fracture is observed at 0.5 wt.‐% MWNT content, which demonstrates the enhanced resistance to crack propagation compared to pure PP, followed by a sharp decline with the increase in MWNT content to 1.5 wt.‐%, which reveals a ductile‐to‐semiductile transition. Fracture kinetic studies present a qualitative picture of the nature of such a transition in terms of a) switch over from non‐steady (in pure PP) to steady‐state crack tip opening displacement rate (in nanocomposites), and b) a ductile‐to‐semiductile transition; largely as a result of delayed‐yielding of the nanocomposites.

     

Direct fracture toughness determination of a ductile epoxy polymer from digital image correlation measurements on a single edge notched bending sample

This work presents a combined experimental and numerical study on the fracture toughness behaviour of a ductile epoxy resin system. Quasi-static fracture tests using single edge notched bending (SENB) specimens were conducted under room temperature conditions. In addition, the digital image correlation technique was employed to experimentally map the full-field displacements and strains around the notch and crack tip, allowing direct calculation of the J-integral fracture toughness. The magnitude of fracture toughness was found to be 1.52 ± 0.03 kJ/m², showing good consistency with the results measured according to the standard analytical formulations. A numerical model of the single edge notch bending specimen was built to compute the local strain field around the crack tip, together with the fracture toughness parameter. Good agreement was confirmed for both the experimental J-integral fracture toughness and the local surface strains around the crack-tip from the digital image correlation based optical technique, compared to the results obtained by numerical simulation. The fracture surfaces of the samples were examined using an optical microscope to analyze the failed surface morphology and the corresponding failure mechanisms.     

Fatigue crack propagation in high-density polyethylene

An investigation of the influence of crystalline microstructure on fatigue crack propagation (FCP) in high-density polyethylene (HDPE) is reported. Various thermal histories were used to generate samples with the same crystallinity and supermolecular structure for three different molecular weight HDPEs. Estimation of tie chain densities were obtained from measurements of brittle fracture stress and predicted from the estimated chain dimensions of the polymers using the modified version of the approach originally taken by Huang and Brown. A significant decrease in FCP resistance and a clear transition to a more brittle fracture surface was observed with decreasing molecular weight. Detailed studies of damaged zones preceding the growing crack show a transition to a more highly branched crack structure for those samples associated with a higher FCP resistance. These results strongly suggest that the branched damaged zone structure improves the FCP resistance by enlarging and blunting the crack tip and, therefore, consuming more energy during the fatigue crack propagation. Additional efforts were made to prepare samples with the same crystallinity and tie chain density, but different supermolecular structure. However, in contrast to reports in the literature, no significant difference in FCP resistance was observed for specimens with different average spherulite sizes. This is probably because the propagating crack front is preceded by a significant zone of plastic deformation and is not expected to directly encounter the spherulites.     

Thermomechanical properties of glassy cereal foods

The main objective of this paper is to discuss the relationship between physical state, fracture mechanism, and texture for low moisture cereal-based foods. Experiments were also carried out to get a better understanding of the role of water. At room temperature, extruded bread and white bread (previously) dehydrated, then rehydrated in atmospheres with controlled humidities) exhibited a brittle behavior up to around 9% moisture. At 13.7% moisture, they were ductile. A significant loss in the crispness of extruded bread was observed between 8.5 and 10% moisture. The glass transition temperature (T _g) was measured, using dynamic mechanical thermal analysis (DMTA), for samples with up to 40% moisture. The resultingT _g curve showed that the important changes in fracture mechanisms and crispness occurred while the samples were still in the glassy state. The viscoelastic behavior of both extruded and white breads suggested that a secondary relaxation occurred around 10?C. Another event was observed around 70?C for low moisture sample, using DMTA. This event was attributed to disruption of low energy interactions.     

Slow-brittle-fracture transition in polymethylmethacrylate

A slow crack growth was achieved in initially edge-cracked specimens made of a high-molecular weight PMMA by regulating the cross-head speed of loading by a computer-driven testing machine. The strain rate \(\dot \varepsilon \) used during the tests varied between \(\dot \varepsilon \) =1× l0^?6 s^?1 and 1×10^?4 s^?1. It was shown that, in this zone of slow quasi-static loading of brittle polymethylmethacrylate specimens under conditions of plane stress, the crack initiated for a critical value of loading, at some characteristic zone of strain-rate variation at the crack tip. It was established that for strain rate between \(\dot \varepsilon \) =0.18×10^?5 s^?1 and \(\dot \varepsilon \) =0.45×10^?4 s^?1 brittle cracks were propagating always slowly with velocities in the range ofc=3 to 5×10^?2 m/s. For values ofv _s outside this transition zone fracture was typically brittle with high crack-propagation velocities. As the strain rate was varying beyond the stable low-velocity region, a two-step crack velocity pattern was operative, where the one step took always low values, and the other step corresponded to crack-propagation velocities significantly higher than these limits, tending to typical brittle-fracture velocities of the material. Oscillations of the velocityc at the transition zones, or, in many cases all over the zone of slow propagation of the crack, indicated the unstable character of crack propagation, influenced by different stress raisers and especially by the opposite longitudinal boundary of the specimen. Stress intensity factor values during crack propagation, evaluated from the front (cuspoid) and the rear (external) caustic, which remained alwaysk _g-dominant, were following similar trends as the variation of the crack propagation velocity.     

Effect of blend composition and related morphology on the quasi-static fracture performance of LLDPE/PP blends

In the present work, the effect of composition and related morphology on the fracture behavior of LLDPE/PP blends was thoroughly investigated. Fracture behaviors evaluated under quasi-static loading conditions and different fracture mechanics methodologies were applied to assess fracture toughness depending on the materials behavior. For pure PP and 2575 blend, J at instability was chosen whereas for blends which exhibited completely ductile behavior (such as LLDPE, 7525 and 5050), the EWF methodology was used. Fracture mechanisms were elucidated with the aid of scanning electron microscopy, and results correlated with blends morphology. It was observed that fracture properties are mostly dominated by the majority component properties. In addition, for the 5050 blend, the presence of a co-continuous morphology is responsible for the high scatter of experimental data obtained.     

XPS study of Cr segregation in a martensitic stainless steel

Cr martensitic steels are promising materials for structural applications in future nuclear fusion reactors. Because the embrittlement after tempering treatments can be a serious problem, the fracture mode of a steel with 10.5 wt% of Cr treated at 700°C for 18 h has been investigated through Charpy tests in the temperature range from −100°C to +150°C. X-ray photoelectron spectroscopy (XPS) analyses carried out on the fracture surfaces evidenced the segregation of Cr in both ductile and brittle (quasicleavage) fields. The unexpected result indicates that Cr segregation weakens the atomic bonds; thus, the fracture path in both the cases corresponds to the zones with higher Cr content.     

Impact fracture behavior of nylon-6/ABS blends

Tensile and impact properties of uncompatibilized nylon-6/ABS blends have been studied over the entire range of compositions. The blends were prepared by extrusion and, subsequently, injection molded into tensile specimens and rectangular plaques. The impact fracture performance was characterized using recently proposed models based on fracture mechanics, for various fracture behaviors. The results showed that nylon-6 breaks in a brittle manner. With the addition of ABS, the blend exhibits the same behavior with a slightly lower impact resistance up to about 60 wt %. A sudden jump in the value of impact fracture energy is observed around 70 wt % ABS with a brittle—ductile transition in the mechanism of fracture. The transition in fracture mechanisms is confirmed through observation of the fracture surfaces by scanning electron microscopy (SEM). Tensile tests showed that the elongation at break increases only slightly between 0 and 50% ABS content, but a significant jump occurs around 70% ABS, reaching a 6-fold increase in comparison to that of the pure components. SEM observation of etched samples shows that a cocontinuous morphology occurs around 70 wt % ABS. The peak observed for the elongation at break and the jump in impact performance, as well as the onset of brittle–ductile transition, are attributed to this morphological effect. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2583–2592, 1997     

Crack propagation behavior in rubber materials

Within the linear viscoelastic theory, crack tip fields are calculated at various crack tip velocities. A transition from rubbery to glassy material behavior in the vicinity of the crack tip can be observed. Shear and bulk behavior is analyzed separately. Whereas the increase of tearing energy at higher crack tip velocities can be ascribed to the shear behavior, bulk behavior influences the fracture mechanism. The results support experimental investigations that the instability separating stable from unstable crack propagation is related to a change in the fracture mechanism. At low crack tip velocities, material separation is the result of formation, growth, and coalescence of cavities. At high crack tip velocities, cavitation is suppressed and fracture is driven by a rather brittle mechanism resulting in a decreased amount of energy to propagate the fracture process zone. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 5, pp. 882–891. This article was submitted by the authors in English.     

Fracture toughness of nylon‐6 blends with maleated rubbers

The fracture toughness of blends of nylon‐6 with maleated ethylene–propylene rubber and maleated styrene/hydrogenated butadiene/styrene triblock copolymer was investigated with a single‐edge‐notched three‐point‐bending instrumented Dynatup test. The blends for which the rubber particle size was less than 0.7 μm fractured in a ductile manner over the whole range of ligament lengths, whereas the blends with particles larger than 0.7 μm showed a ductile‐to‐brittle transition with the ligament length. In this regime, ductile fracture was observed for specimens with short ligaments, whereas brittle fracture was seen for those with long ligaments. The ductile fracture behavior was analyzed with the essential‐work‐of‐fracture model, whereas linear elastic fracture mechanics techniques were used to analyze the brittle fracture behavior. The fact that the ductile fracture energy was larger for the blends with the styrene/hydrogenated butadiene/styrene triblock copolymer than for those with ethylene–propylene rubber was due to the larger dissipative energy density of the blends based on the styrene/hydrogenated butadiene/styrene triblock copolymer. Both the critical strain energy release rate (G_IC) and the plane‐strain critical stress intensity factor (K_IC) increased as the rubber particle size decreased for both blend systems. The G_IC and K_IC parameters had similar values, regardless of the rubber type, when the rubber particle size was fixed. The transition ligament length was near the size criterion for plane‐strain conditions for both blend systems. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1739–1758, 2004     

Fatigue crack propagation mechanisms in poly(methyl methacrylate) by in situ observation with a scanning laser microscope

Crack propagation tests were performed on an amorphous polymer, poly(methyl methacrylate), to investigate fatigue crack propagation mechanisms. A scanning laser microscope with a newly developed tensile testing machine was used to observe in situ crack propagation in compact‐type specimens. A crack usually propagated within the craze located at the crack tip under both static and cyclic loading conditions. When a crack stably propagated into the craze under static loading conditions, bright bands composed of the broken craze were observed at the edges along the crack wakes. However, there were successive ridges and valleys in place of bright bands along the crack wakes under cyclic loading conditions. When stable fatigue cracks were propagated at the loading half‐cycle in each cycle, new craze fragments appeared that were similar to the bright bands under static loading. However, the thickness of these fragments decreased in the following loading cycle, and a new valley was formed. This suggested that the valleys were formed by the contact between the fracture surfaces near the crack tip during unloading. Fatigue crack propagation is thought to be due to fibrils weakened by crack closure between fracture surfaces. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3103–3113, 2001     

Dynamic fracture testing of polymethyl-methacrylate (PMMA) single-edge notched beam

Dynamic fracture in single-edge notched polymethyl-methacrylate (PMMA) beams have been investigated by three-point-bending impact testing with a drop-weight machine. A high-speed camera combined with the digital image correlation (DIC) method is used to capture the impact-induced crack initiation and propagation, as well as the beam deformation fields and the open mode strain at the original notch tip. The crack propagation length is recorded and the instantaneous crack velocity is calculated. Furthermore, the dynamic fracture toughness K_Id is quantified from the loading-displacement relations at different impact velocities. The effects of the impact velocity and impact energy on dynamic fracture toughness, fracture initiation strain, as well as the corresponding influences on the fracture propagation velocity, are discussed.