Effect of crystallinity and loading-rate on mode I fracture behavior of poly(lactic acid)

The aim of this study is to characterize the fracture behavior of biodegradable poly(lactic acid) (PLA). Especially, the effects of crystallinity and loading-rate on the fracture behavior are emphasized. Annealing was performed to control the crystallinity of the PLA samples prepared, and then their fracture toughness values were measured under quasi-static and impact loading conditions. The results showed that the quasi-static fracture toughness of PLA decreases with increase of crystallinity; on the other hand, the impact fracture toughness tends to increase with crystallinity. The crack growth behaviors of the PLA specimens having different crystallinity were also observed by polarizing and scanning electron microscopies. The microscopic results exhibited that under quasi-static loading, disappearance of multiple crazes in the crack-tip region results in the decrease of the fracture toughness with crystallinity. On the contrary, under impact loading, the increase of the fracture toughness with crystallinity is considered to be related to the increase of fibril formation.     

Quasi‐brittle to ductile transition in impact‐modified PVC

The toughness of impact‐modified poly(vinyl chloride) (PVC) compounds was examined by using a modified Charpy test. Increasing impact speed resulted in a quasi‐brittle to ductile transition in all PVC compounds. In the quasi‐brittle region, a PVC of 56,000 M_w fractured through a craze‐like damage zone that could be described by a modified Dugdale model. Furthermore, the same molecular‐weight PVC modified with either 10 pph (parts per hundred parts by weight) of chlorinated polyethylene (CPE) or 10 pph of methylmethacrylate‐butadiene‐styrene (MBS) impact modifier also conformed to the Dugdale model with the craze‐like damage zone. The CPE effectively improved the impact performance of PVC by shifting the quasi‐brittle to ductile transition to a higher loading rate. Compared to CPE, MBS was a better impact modifier, and its use resulted in a higher quasi‐brittle to ductile transition loading rate in the same PVC matrix. Fracture initiation toughness of all the materials was described by the Hayes‐Williams modification of the Dugdale model. The intrinsic brittle fracture energy obtained by extrapolation to zero craze length was determined only by the PVC matrix and was independent of the impact modifier. However, the kinetics of craze growth, and hence the response to rapid loading, depended on the impact modifier. Increasing the molecular weight of the PVC resin resulted in a more complex damage zone that was not amendable to the Dugdale analysis. J. Vinyl Addit. Technol. 10:11–16, 2004. © 2004 Society of Plastics Engineers.     

Load‐deflection behavior of fracture toughness testing of ceramics by SEVNB method

This paper aims to explore the load‐deflection behavior of fracture toughness testing of ceramics by single‐edge V‐notched beam (SEVNB) method. The fracture toughness of Si₃N₄, 3Y‐TZP, SiC, and 8Y‐FSZ ceramics were measured by SEVNB method and single‐edge notched beam (SENB) method, respectively. The load‐deflection behavior varies with R‐curve behavior of the ceramics, the test methods and the loading conditions. Through comparative analysis, the results show that the actual fracture toughness of ceramics by SEVNB method can be determined by maximum flexure load and the notch length at loading rate of 0.05 mm/min in air. The obtained actual fracture toughness values of Si₃N₄, 3Y‐TZP, SiC, and 8Y‐FSZ ceramics are 5.2 ± 0.21, 4.5 ± 0.12, 3.2 ± 0.15, and 1.6 ± 0.07 MPa ^. m^1/2, respectively.     

Accurate measurement of fracture toughness in structural ceramics

The measured values of fracture toughness for ceramics are closely correlated with the sharpness of notch tips, which in turn influences the accurate measurement of fracture toughness. Here, typical structural ceramics, i.e., 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), ZrB₂, ZrB₂-SiC and ZrB₂-SiC-Grapite, were used for the measurement of fracture toughness, and the effect of notch tip radius on the fracture toughness values of these typical structural ceramics was investigated. Ultra-sharp notches with a tip radius less than 1 μm can be fabricated by laser, lower than the critical notch tip radius in ceramics below which the fracture toughness value almost remains constant, and improved accuracy and consistency of fracture toughness measurement can be obtained by this method compared with traditional method.     

Craze growth and crack growth in poly(vinyl chloride) under monotonic and fatigue loading

Cracks in poly(vinyl chloride) sheet were loaded to known stress intensity factors and the craze length measured. These measurements, and the craze thickness profile, were compared with the Dugdale model of crack tip yielding. The fracture toughness of poly(vinyl chloride) was measured, and an analysis made of circular ‘advance fractures’ that occur on the fracture surface. Fatigue crack growth studies confirmed that crack growth occurs discontinuously once every few hundred cycles, and showed that the craze fracture mechanism is quite different from the monotonic loading failure mechanism.     

The effects of stress cracking on the fracture toughness of polycarbonate

The growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase in k_IC with time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.     

The effect of temperature and loading rate on the mode II interlaminar fracture properties of a carbon fiber reinforced phenolic

The combined effect of varying loading rate and test temperature on the mode II Interlaminar fracture properties of a carbon fiber reinforced phenolic resin has been investigated. End notch flexure tests at room temperature have shown that this composite offers a relatively modest value of G_IIc^NL at non‐linearity and that its interlaminar fracture toughness decreases with increasing loading rate. As the test temperature is increased, the quasistatic value of G_IIc^NL increases steadily and the reduction in G_IIc^NL with loading rate becomes less dramatic. At temperatures approaching the glass transition temperature of the phenolic matrix, the interlaminar fracture toughness of the composite begins to increase sharply with crosshead displacement rate. A more detailed understanding of the effect of varying the test conditions on the failure mechanisms occurring at the crack tip of these interlaminar fracture specimens has been achieved using the double end notch flexure (DENF) geometry.     

Time and temperature effects on the fracture toughness of rigid poly(vinylchloride) pipe materials

The fracture toughness of rigid poly(vinylchloride) pipe materials has been investigated over a range of temperatures and rates. Conditions are described for valid fracture toughness (K_IC) tests and notch insensitive (ductile) behavior; time-temperature effects on transitions in K_IC are defined. The modes of crack extension are characterized over a range of temperatures, and the mechanisms of crack resistance are discussed, including some quantitative data for the yielded zone at the crack tip.     

Fatigue behavior of medium-density polyethylene pipes

A round bar specimen and a square bar specimen cut out from medium-density polyethylene pipes with a notch were made and a fatigue test was conducted to cause a brittle fracture. The initiation and growth of a craze and crack at the tip of a notch was observed. In the range where loading cycles are few and displacement of the specimen does not increase, the craze prior to crack initiation occurs. Also, the effect of frequency was investigated. The pure creep failure and the fatigue failure at low frequency were compared. The lower the frequency, the smaller the reciprocal of the actual loading time T_f becomes. It is also found that this tensile fatigue test is a useful test method to assure the quality of pipes.     

Effect of loading-rate on fracture micromechanism of methylmethacrylate-butadiene-styrene polymer blend

Methylmethacrylate-butadiene-styrene (MBS) polymer blends having two different types of rubber particle distribution, monomodal and bimodal, were prepared, and their fracture properties and fracture mechanisms were investigated under quasi-static and impact loading. A fracture property, maximum J-integral J_max, was evaluated at both loading-rates, and it was shown that J_max values of the bimodal MBSs are much greater than that of the monomodal with small particles, and slightly better than that of the monomodal with large particles. Thick damage zones were observed in the crack-tip regions in the bimodal and monomodal with large particles, indicating larger energy dissipation during fracture initiation than in the monomodal with small particles in which damage zone is much thinner. TEM micrographs exhibit that extensive plastic deformation under quasi-static rate and multiple craze formation under impact loading rate are the primary toughening mechanisms in the bimodal MBS blends. By assessing both fracture properties and transparency, the bimodal blend with blend ratio: 2.5/7.5 (=140 nm/2.35 μm; total rubber particle content is 10 wt%) was proved to show the best performance as MBS polymer blend with satisfiable transparency and high fracture resistance.     

Mechanical properties of zirconia-based ceramics asfunctions of temperature

Commercially available ceramics, MgO–ZrO₂, CeO₂–ZrO₂, and an in-house fabricated zirconia-toughened mullite were examined in this study for use as a structural component in diesel engines. The fast fracture strengths of these materials were measured by loading ASTM C-1161-B specimens in four-point flexure at 30 MPa/s and at 20, 200, 400, 600, and 850 °C. The dynamic fatigue or slow crack growth susceptibility was assessed at 20 and 850 °C by combining the fast fracture strengths with strength data obtained by testing the same specimens in four-point flexure at 0.30 and 0.003 MPa/s stressing rates, as specified in the ASTM C 1368 standard. Fracture toughness was measured following the ASTM C-1421 standard and using chevron notch specimens in three-point flexure at room and elevated temperatures. The strength of the zirconia-toughened mullite was invariant to increases in the temperature and decreases in the loading rate, while the MgO–ZrO₂ and CeO₂–ZrO₂ materials exhibited strength degradation as temperatures increased and the loading rates decreased. Temperature was observed to have the greatest influence on facture toughness. As temperatures increased, the fracture toughness values dramatically decreased for all the materials examined in this study. Improvements in the fracture toughness are needed most for these ceramic materials in order to meet the structural requirements and to develop a more durable and reliable diesel engine component.     

Temperature and Loading Rate Effects on the Fracture Behaviour of Adhesively Bonded GFRP Nylon-6,6

The combined effect of varying test temperature and loading rate on the Mode II fracture toughness of plasma-treated GFRP Nylon-6,6 composites bonded using a silica-reinforced epoxy adhesive has been studied. End notch flexure tests have shown that the adhesive system used in this study offers a wide range of fracture energies that are extremely sensitive to changes in temperature and loading rate. Increasing the test temperature resulted in a substantial reduction in the Mode II fracture toughness of the adhesive, with the value of G_IIc at 60°C being approximately one-half of the room temperature value. In contrast, increasing the crosshead displacement rate at a given temperature has been shown to increase the value of G_IIc by up to 250%. Compression tests performed on bulk adhesive specimens revealed similar trends in the value of [sgrave]_y with temperature and loading rate. In addition, it was found that the plasma treatment employed in this study resulted in stable crack propagation through the adhesive layer under all testing conditions.

A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip was achieved using the double end notch flexure (DENF) geometry, which was considered in tandem with the fracture surface morphologies. Here, changes in the degree of matrix shear yielding and particle-matrix debonding were used to explain the trends in [sgrave]_y and G_IIc.     

Crack propagation in polycarbonate

Crack velocity measurements and fracture toughness tests have been carried out on extruded sheets of bisphenol-A polycarbonate. Slow bend tests provided quasi-static K_1c data while dynamic initiation values were obtained from instrumented Charpy impact tests. In both types of tests high-speed crack velocity measurements were made using conductive silver grids applied to the specimens. The variation of K_1c with strain rate and temperature was found to be small and in general agreement with expectations from the relaxation properties of polycarbonate. Notch acuity was found to have little effect in that values of K_1c and crack velocity from specimens having the standard Charpy notch were similar to values obtained from sharp-cracked specimens. Some anisotropy was present in the material and gave rise to a small variation in K_1c values with direction of crack propagation. Crack velocity and also the fracture mode transition temperature showed considerable dependence upon orientation. It was thought that dynamic toughness K_d, was influenced more than K_1c by partial orientation of amorphous regions in the material and that the relaxation time for re-orientation was probably significant here.     

Designing against fracture in brittle plastics

The fracture toughness of a variety of sharply notched tension, bending and rotating disc specimens of PMMA is examined using linear fracture mechanics. It is observed that rapid fracture with a brittle glassy appearance usually follows a period of slow crack growth, denoted by fan shaped markings of local ductility, though still brittle overall. In this near brittle regime the fracture toughness is sensitive to strain rate so that high values of effective surface energy are easily induced by rapid testing or notch bluntness. At impact rates the toughness increases again. For design purposes, in the absence of environmental effects, the onset of slow cracking and rapid (glassy) fracture, can be associated with fracture toughness K_1c of about 800 Ibf/in^3/2 (90 kg/cm^3/2) and 1600 Ibf/in^3/2 (180 kg/cm^3/2) respectively. Detailed studies have not been made on other materials but a guide to the levels of notch toughness and notch brittle temperatures are given for several plastics.     

Multiple crazing at crack tips in PVC under plane strain conditions

The nature of the yield zone at the crack tip of poly(vinyl chloride) (PVC) pipe materials has been investigated. Microscopy studies employing a plasma etching technique reveal the presence of multiple crazes ahead of the crack tip in the interior of specimens of pure PVC, CaCO₃ filled PVC, and PVC pipe compound. The craze zone and the fracture toughness of blade-notched specimens are compared with those of fatigue pre-cracked specimens. Both types of specimens have similar fracture toughness values and form multiple crazes upon loading, suggesting that multiple crazing Is an intrinsic property of the material. The kinetics of craze initiation and the development of the multiple craze zones have also been explored.     

Fracture toughness determination of Ti3Si(Al)C2 and Al2O3 using a single gradient notched beam (SGNB) method

In this work, we suggest a new and simple method named single gradient notched beam (SGNB) method for determining the fracture toughness of Ti₃Si(Al)C₂ and Al₂O₃ with four-point bending specimens. For the specimen with a gradient notch, a sharp natural crack will initiate and extends from the tip of the triangle under increasing load. Based on the straight through crack assumption or on the slice model, the stress intensity factor coefficient for this notched beam was derived. The fracture toughness can be calculated from the maximum load and the minimum of the stress intensity factor coefficient without knowing the crack length. To verify the feasibility and reliability of this suggested method, the SGNB method and two other conventional methods, e.g. the chevron notched beam (CNB) method and single edge notched beam (SENB) method, were performed to determine the fracture toughness of Ti₃Si(Al)C₂ and Al₂O₃. The measured fracture toughness values obtained from the SGNB method agreed well with those from conventional fracture toughness tests.     

Influence of a finite notch root radius on fracture toughness

The validity of fracture toughness data from tests with V-notched bending bars depends in the notch root radius and the presence of an R-curve behaviour. In a theoretical study it is shown how the notch radius affects the formally computed conventional toughnesses. These are computed under the assumption that the introduced notch with a small crack at the notch root acts as a long crack of the same total size and, in a stronger simplification, that the crack length is identical with the depth of the notch.     

Fracture toughness and crack growth mechanism for multiphase polymers

The fracture behavior of acrylonitrile-butadiene-styrene (ABS) was investigated using the J-integral method under monotonic loading. Two ways were used to monitor the onset of crack growth: the dyeing method and the length of craze region ahead of initial notch. The blunting at the crack tip and crack growth mechanism for ABS Was observed using a scanning electron microscope. Before the onset of crack growth, the energy put into material was dissipated to create crazes ahead of the initial notch and to deform the material at the crack tip. A part of the energy was released to create a new crack surface after the onset, of crack growth.     

Fracture toughness of zirconia from a shallow notch produced by ultra-short pulsed laser ablation

Fracture toughness of submicron grain size tetragonal zirconia polycrystals doped with 3 mol% yttria (3Y-TZP) is measured by the single edge V-notch beam (SEVNB) method from a shallow sharp notch produced by ultra-short pulsed femtolaser ablation (UPLA) on the surface of a bending bar. It is shown that the radius of the notch tip achieved is in the submicron range and the damaged volume in front of the notch tip is characterized by using focus ion beam milling and scanning electron microscopy. It consists of a narrow fully microcracked region less than ∼4 μm wide and ∼15 μm deep in front of the notch. If the extension of this region and the length of the notch are used in the determination of the fracture toughness (K_Ic) in the four bending test, the values obtained for submicron grain size 3Y-TZP are in agreement those obtained by using very sharp cracks. It is concluded that the SEVNB testing method with a sharp notch induced by UPLA may be used for K_Ic testing of submicron grain size ceramics.     

A method to estimate the influence of the notch-root radius on the fracture toughness measurement of ceramics

The fracture toughness measurement of ceramics is based on notched specimens. If the notch-root radius is too large, it leads to overestimate the actual fracture toughness of the material. It is then necessary to control the notch shape and to machine it carefully in order to have a root-radius small enough (<10 μm) to be below the sensitivity threshold of the material. Then, the notch confounds with a sharp crack. Alternatively, it is proposed in this work to bring a correction to the measured fracture toughness depending on the notch-root radius. No restriction is brought to this radius except that it must be small compared to the notch length.