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.     

Dynamic 4 ENF test for a strain rate dependent mode II interlaminar fracture toughness characterization of unidirectional carbon fibre epoxy composites

A dynamic 4ENF testing procedure has been proposed to characterize the influence of loading rate on mode II fracture interlaminar toughness of a unidirectional composite material. The stable crack growth accomplished by the proposed dynamic 4ENF procedure allows achievement of the R-curves and the in-situ compliance calibration of each specimen. This enables performance of a monotonic dynamic test and presents a great advantage for the dynamic interlaminar characterization of the composite material. The dynamic G_IIC-s values obtained are similar to those determined by quasi-static loading conditions, which suggests that the proposed testing procedure is able to determine accurately the dynamic G_IIC of a composite material. However, for the tested material and within the analysed loading rate range, both the determined R-curves and G_IIC values do not show any clear sign of loading rate dependence.     

Interlaminar fracture toughness of unidirectional DCB specimens: A novel theoretical approach

A novel theoretical approach is presented to calculate the mode I interlaminar fracture toughness (G_Ic) of double cantilever beam (DCB) specimens with low ratio of initial crack length-to-thickness (a₀/2h). This method is based on a sixth-order beam theory, namely Reddy-Bickford beam (RB), on Winkler elastic foundation (WEF) to account for both transverse shear deformation of the beam and local effects at the delamination front (root rotation). RB with only two generalized displacements w and ?; and three boundary conditions at ends and loading points of a shear deformable beam gives more accurate results than the fourth-order Timoshenko beam theory. The accuracy of the proposed method in prediction of initiation G_Ic values is evaluated together with other available models considering the experimental fracture toughness for moderately thick unidirectional E-glass/epoxy DCB specimens with small initial delamination lengths.     

The time and temperature dependence of fracture toughness for thermoplastics

A method for obtaining comparative and intrinsic toughness of thermoplastics involves deriving two apparent material properties, yield stress and a critical value of stress field intensity factor (fracture toughness). These are obtained at various times under load and temperatures. A ductility factor can then be derived and is the ratio of the square of fracture toughness to yield stress. Ductility factors can create a league table of toughness for thermoplastics which correlates reasonably with impact toughness energies.Individual values of fracture toughness and yield stress provide an intrinsic measure of toughness. Consequently, the relative toughness of a range of thermoplastics are explained in these terms. Materials discussed include homopolymers and copolymers of propylene, uPVC, PMMA (and rubber toughened versions) and PES.There is an importance in obtaining geometry independent fracture toughness measurements, otherwise likely artefacts are introduced. Such considerations are discussed for uPVC and polypropylene materials.     

Fracture toughness of gamma irradiated polycarbonate sheet using the essential work of fracture

Polycarbonate (PC), a ductile polymer, has been found by both linear elastic fracture mechanics and impact tests to present a ductile-brittle transition, which depends on notched specimen thickness, test speed and gamma irradiation. Owing to large amounts of plastic deformation, fracture toughness measurements by these test methods are not precise. In the present communication, a better method, the Essential Work of Fracture (EWF), to assess the fracture characteristics in plane state of stress was for the first time used to evaluate the fracture toughness of PC sheets subjected to gamma irradiation dose. Three-points bend tests of sharp pre-cracked specimens with different ligament lengths were 340 kGy gamma irradiated. EWF results showed that the total fracture work increased linearly with length for both non-irradiated and gamma irradiated conditions. A significant decrease in EWF fracture toughness was associated with brittleness promoted by gamma irradiation. This brittleness was also confirmed by macro and microscopy (SEM) evidence.     

Effect of nitrile butadiene rubber/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/YSZ fillers for PMMA denture base on the thermal and mechanical properties

This study is to investigate the effect of nitrile butadiene rubber (NBR as impact modifier) together with Al₂O₃/YSZ (toughening) as filler loading in PMMA denture base on the thermal and mechanical properties. PMMA matrix without fillers was mixed between PMMA powder and 0.5 mass% of BPO, and it is used as the control group. The liquid components consist of 90% of methyl methacrylate (MMA) and 10% as the cross-linking agent of ethylene glycol dimethacrylate. The denture base composites were fabricated by incorporating PMMA powder and BPO and fixed at 7.5 mass% NBR particles and filler loading (1, 3, 5, 7 and 10 mass%) of Al₂O₃/YSZ mixture filler by (1:1 ratio) as the powder components. The ceramic fillers were treated with silane (γ-MPS) and the powder/liquid ratio (P/L) according to dental laboratory practice. The TGA data obtained show that the PMMA composites have better thermal stability compared to unreinforced PMMA, while DSC curves show slightly similar Tg values. DSC results also indicated the presence of unreacted monomer content for both reinforced and unreinforced PMMA composites. The fracture toughness, Vickers hardness and flexural modulus values were statistically increased compared to the unreinforced PMMA matrix (P?<?0.05).     

Tensile,fracture and impact behavior of transparent Interpenetrating Polymer Networks with polyurethane-poly(methyl methacrylate)

Transparent Interpenetrating Polymer Networks (IPNs) with poly(methyl methacrylate) (PMMA) as the stiff phase and polyurethane (PU) as the ductile phase with varying PMMA:PU ratios in the range of 90:10 to 70:30 were formulated. Static tensile and fracture tests indicate significant failure strain and crack initiation toughness enhancements with a loss of stiffness relative to PMMA. Dynamic fracture tests were conducted using a long-bar impact loading apparatus in conjunction with an optical method and high-speed photography. Low-velocity impact tests were also performed using a drop-tower. Dynamic fracture and low-velocity impact responses show that an optimum range of PMMA:PU ratios in the IPNs can produce enhanced fracture toughness and impact energy absorption capability when compared to PMMA. Fractographic examination supports macro-measurements by showing a distinct change in surface morphology associated with improved macroscale fracture toughness.     

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.     

Special fracture mechanics specimens for multilayer plastic pipes testing

Pipes consisting of layers of different materials (multilayer pipes) are considered. The fracture toughness value of the main pipe is taken into account as a parameter relevant to fracture assessment connected with the resistance of pipe material against slow crack growth. With the aim of simplifying estimation of main pipe material fracture toughness, non-homogeneous test specimens cut directly from multi-layer pipes are suggested and numerically analysed. The values of the corresponding stress intensity factor K_I and biaxiality factors B are calculated for the case of two and three layer test specimens. Based on the results obtained, the transferability of fracture toughness values measured on laboratory specimens to pipe systems is discussed. It is shown that in most cases of multi-layer commercial pipes and routine fracture toughness measurements the values of the stress intensity factor calculated on the basis of homogeneous specimens can be used.     

Effect of defect size on fracture strength of dental low fusion porcelain

Various grinding defects were produced on the surface of specimen dental low fusion porcelain in an attempt to establish the relationship between defect size and fracture strength. In addition, the applicability of the process zone size-fracture criterion in assessing the material properties of dental low fusion porcelain was examined. Super porcelain AAA E3 (Noritake Co., Japan) was used as dental low fusion porcelain. The bending strength and fracture toughness value were estimated by the three-point bending test. After glazing, grinding flaws were introduced by grinding the specimen with abrasive papers of various mesh sizes. In order to calculate the fracture toughness value of dental low fusion porcelain, we introduced a surface crack using a Vickers indenter. The results were discussed based on the process zone size-fracture criterion. The size of cracks caused by grinding was estimated with the process zone size-fracture criterion and Newman-Raju formula. As the defect size decreased, the fracture stress approached the strength for smooth specimen without defect. The K(c) value showed a tendency to approach the K(lc) value when the defect size increased. The relationship between the fracture stress, sigma(F), and the equivalent crack length, a(e), was in good agreement with the theoretical relations deduced from the criterion in dental low fusion porcelain.     

Experimental studies on dynamic fracture behavior of thin plates with parallel single edge cracks

Dynamic fracture behavior of polymer PMMA thin plates with three- and four-parallel edge cracks was studied by means of the method of caustics in combination with a high-speed Schardin camera. A series of dynamic caustic patterns surrounding the crack tip and fracture path of the specimen were recorded simultaneously by two types of focused images. Some dynamic fracture parameters such as the dynamic stress intensity factor, crack velocity and acceleration were determined. The evolution of dynamic stress intensity factors on the parallel edge cracks, due to the dynamic unloading effect, was analyzed from the viewpoint of the release of elastic strain energy.     

Deformation mechanisms of nanoclay‐reinforced maleic anhydride‐modified polypropylene

Fracture properties and deformation mechanisms of nanoclay‐reinforced maleic anhydride‐modified polypropylene (MAPP) were investigated. Elastic–plastic fracture mechanics was employed to characterize the toughness in light of substantial postyield deformation for the reinforced MAPP. Upon introduction of 2.5 wt % clay loading in maleated MAPP, it was observed that tensile strength, modulus, and fracture initiation toughness concomitantly increased substantially. Continued increase in clay loading thereafter only led to stiffening and strengthening effects to the detriment of fracture toughness. A plot of the J‐integral initiation fracture toughness versus the plastic zone size demonstrated that toughening arose from plastic deformation in the reinforced matrix. Careful examination of deformed tensile specimens using small angle X‐ray scattering (SAXS) showed 2.5 wt % clay gave rise to the highest equatorial scattering, which indicates the presence of microvoids in the matrix. The SAXS results were consistent with that shown in subcritically loaded crack‐tip deformation zone using transmission electron microscopy. Thus, both macroscale three‐point bend fracture data and SAXS results led us to consistent findings and conclusions. Further increase in clay loading above 2.5 wt % reduced the scattering the matrix plasticity and thus the fracture toughness. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2759–2768, 2004     

Numerical Assessment of PE 80 and PE 100 Pipe Lifetime Based on Paris-Erdogan Equation

Summary: A novel accelerated fracture mechanics extrapolation procedure based on cyclic test with cracked round bar (CRB) specimens was verified by a correlation of real pipe failure time to simulated failure times at a temperature of 60 °C. The procedure was applied to predict the long-term failure of modern PE 80 and PE 100 pipes 23 °C. Moreover, the used stress intensity factor concept also allows to consider the impact of arbitrary additional loading situations like soil loads or point loads and to assess pipe lifetime under complex loading situations.     

Interlaminar Fracture Toughness Characterization of Laminated Composites: A Review

Abstract

Interlaminar fracture toughness had been the subject of great interest for several years and is still interesting to the research community. In this article, a comprehensive analysis of fracture toughness in FRP laminates is presented. Primarily, toughness studies are undertaken on glass and carbon fiber reinforced composites under mode-I and mode-II loading conditions. The fracture behavior and its failure pattern depend on a number of parameters: fiber sizing/coating, matrix modification, insert film, fiber volume fraction, stacking sequence, specimen geometry, loading rate and temperature change. In fact, a state-of-the-art process enables increasing fracture resistance with “matrix toughening by carbon nanotubes (CNT) inclusion”. It enables production of materials having ultra-high strength and low weight. The present study has highlighted the available techniques of CNT incorporation: mechanical mixing, grafting and interleaving. Other aspects, such as the dispersion level, matrix viscosity, fiber surface roughness, loading weight %, bonding strength with epoxy, height and density of grown CNT, energy absorption mechanism during delamination, etc., have been examined as well. Although a clear correlation of all these parameters with fracture toughness is hard to establish, there is growing understanding of the surface-grown CNTs and interleaving processes as they ensure significant increase in fracture toughness.     

Shear band formation and mode II fracture of polymeric glasses

Mode I and II fracture studies were performed from quasistatic to low velocity impact rates on polymethyl methacrylate (PMMA) and polycarbonate (PC). Mode II tests used an angled double‐edge notched specimen loaded in compression. The shear banding response of PMMA is shown to be highly sensitive to rate, with diffuse shear bands forming at low rates and sharp distinct shear bands forming at high rates. As the rate increases, shear deformation becomes more localized to the point where Mode II fracture occurs. PC is much less rate dependent and stable shear band propagation is observed over the range of rates studied with lesser amounts of localization. A new theory is formulated relating orientation in a shear band to intrinsic material properties obtained from true‐stress true‐strain tests. In a qualitative sense the theory predicts the high rate sensitivity of PMMA. A kinematic limit for orientation within a shear band is also derived based on entanglement network parameters. Mode II fracture in PMMA is shown to occur at this kinematic limit. For the case of PC, the maximum impact rates were not high enough to reach the kinematic limit. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Refined fixture design for effective essential work of fracture toughness characterization of m-LLDPE thin films

Characterization of Mode-I fracture toughness of ductile polymeric thin films is nontrivial. Proper specimen preparation and experimental procedures are required to ensure in-plane tensile loading. In this study, a custom-built double-edge notched tensile test fixture was employed to characterize the Mode-I essential work of fracture (EWF) toughness of metallocene linear low-density polyethylene (m-LLDPE) films. Effects of specimen geometry, strain rate and film orientation on the specific essential work of fracture, w_e, and the specific non-essential work of fracture, w_p, were investigated. Results indicate both EWF parameters are independent of the crosshead speed, gauge length (distance between upper and lower clamps) and specimen width within the ranges tested. w_e is significantly higher for thinner films and for crack propagation perpendicular to the blown film machine direction (MD). The usefulness of EWF for evaluating m-LLDPE fracture toughness is discussed.     

Effect of molecular orientation on the fracture behavior of carbon black-filled natural rubber compounds

The fracture behavior of carbon black-filled natural rubber compounds, differing in filler content, was studied performing tensile tests in biaxial loading conditions, using a central notched cross-shaped specimen. The test consisted of two steps: a drawing step was initially performed loading the specimen in the direction parallel to the notch plane, up to different draw ratios, and then the specimen was loaded in the direction normal to the notch plane up to fracture. Using a fracture mechanics approach, the fracture toughness was evaluated as a function of the draw ratio applied in the drawing step. A correlation between the fracture phenomenology observed and molecular orientability and orientation was attempted. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1509–1515, 2010     

Experimental fracture study of MWCNT/epoxy nanocomposites under the combined out-of-plane shear and tensile loading

In order to explore the role of multi-walled carbon nanotubes (MWCNTs) on the fracture behavior of epoxy-based nanocomposites, fracture tests were conducted under the combined out-of-plane shear and tensile loading. Epoxy resin LY-5052 together with MWCNT contents of 0.1, 0.5 and 1.0 wt% were used to produce nanocomposite specimens. The results showed that increasing the contribution of out-of-plane shear from pure mode I towards pure mode III enhanced fracture toughness for both pure epoxy and nanocomposites. Additionally, it was found that in both loading conditions of pure mode III and mixed mode I/III, increasing MWCNT content up to 1.0 wt% enhanced fracture toughness with an ascending trend. The mechanisms involved in the fracture behavior of polymer-based nanocomposites were also studied in detail using the photographs taken from the fracture surfaces by scanning electron microscopy.     

Fracture process and molecular kinetics of PMMA

The critical value of the stress intensity factor, K_1c, (fracture toughness) has been measured for poly(methyl methacrylate) (PMMA) over a wide range of testing speed (K ? 4 to K? ? 50 × 10⁶ lb/in^3/2-sec) and temperature (from ?197°C to +21°C) in air and inert gas, by use of single-edge notch, double cantilever beam (normal and compact types) and instrumented impact tests. Some features of the toughness curves were found to be subject to time–temperature shifts and were explained in terms of relaxation motions of parts of the polymer molecule, (second-order thermodynamic transitions). Correlation with published data on mechanical energy losses in vibration experiments, dielectric losses and NMR results provided an identification of the operative relaxations. The coincidence of the fracture mode transition in the glassy state with the peak of the β relaxation was observed and the trend of K_1c values in a transition region was attributed to the close relationship of K_1c to the complex modulus for the hard glassy state. Impact and propagating crack values of K_1c have been discussed in general terms, and the limitations of fracture mechanics in studying a time-dependent material property have been considered in the light of recent rheological studies on the fracture of polymers.