Fatigue crack propagation in mica-filled polyolefins

Edge notched samples of polypropylene (PP) and high-density polyethylene (HDPE) containing different mica concentrations were tested in mode I tensile loading. Crack growth was approximated by a non-linear regression of exponential form using statistical software (SAS). Characterization of fatigue crack propagation (FCP) was made using the Paris-Erdogan law. The crack front in PP was preceded by a wide plastic zone in which craze developed, leading to a discontinuous crack growth. Using spline functions, a margin between maximum and minimum FCP rates, recorded during the crack progression, is presented along with the average FCP rates. It is shown that mica-reinforced PP samples exhibit higher FCP rates than unfilled PP. In HDPE, mica reduces FCP rates resulting in a higher resistance to fatigue crack propagation. Effect of test frequency is presented for unfilled polymers and 10 percent mica concentration by weight in both matrices. An increase in the test frequency has no significant effect on FCP rates for both raw and mica-reinforced PP. Unfilled and mica-filled HDPE show noticeable decrease in FCP rates with increasing frequency.     

Fracture behavior of mica-reinforced polypropylene: Effects of coupling agent,flake orientation,and degradation

The effects of such factors as flake size, orientation, and surface treatment on the fracture mechanism of mica-reinforced polypropylene were investigated. Concepts of post-yield fracture mechanics were used to characterize the crack initiation and propagation resistances of the composite. It was found that better flake alignment slightly increases the yield strength but does not affect the crack growth resistance of the material. Flake size degradation during processing leads to an increased crack initiation resistance but reduces the tearing capacity and the maximum crack growth resistance of the composite, due to a reduction of the extent of matrix deformation. In the same manner, the use of a coupling agent diminishes the effect of matrix drawing and the total energy dissipated during the fracture process, even though the crack initiation resistance can be increased.     

Deformation and fracture behavior of polypropylene–ethylene vinyl alcohol blends compatibilized with ionomer Zn2+

This work investigated the deformation and fracture behavior of polypropylene–ethylene vinyl alcohol (PP/EVOH) blends compatibilized with ionomer Zn²⁺. Uniaxial tensile tests and quasistatic fracture experiments were performed for neat PP and for 10 and 20 wt % EVOH blends with different ionomer contents. The addition of EVOH copolymer to PP led to an increase in the Young's modulus whereas the yield strength was decreased with the EVOH content as a consequence of the higher stiffness of EVOH and the poor interfacial adhesion between PP and EVOH, respectively. Furthermore, the incorporation of EVOH into PP promoted stable crack growth. Neat PP displayed nonlinear load‐displacement behavior with some amount of slow crack growth preceding unstable brittle fracture, whereas most PP/EVOH blends exhibited “pseudostable” fracture characterized by slow crack growth that could not be externally controlled. All blends exhibited lower resistance to crack initiation than PP but the fracture propagation resistance was significantly improved. For 10 wt % EVOH blends, the resistance to crack initiation was roughly constant with the ionomer content up to 5%, then it increased with the further addition of compatibilizer. Conversely, for 20 wt % EVOH blends, the resistance to crack initiation appeared to be independent of the ionomer content. The better resistance to crack initiation exhibited by the 10 wt % EVOH blends could be attributed to a higher level of compatibilization in these blends. By contrast, 20 wt % EVOH blends with ≤2% ionomer content showed completely stable crack growth. In addition, J–R curves and valid plane strain fracture toughness values for these blends could also be determined. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1271–1279, 2005     

Initial slow crack growth behavior followed by rapid brittle fracture in a viscoelastic solid

Initial dynamic crack propagation behavior in a viscoelastic solid just after crack initiation was investigated by newly devised instrumentation at different temperatures. It was observed that initial slow crack growth precedes rapid brittle fracture. The very initial slow crack growth first appears as ductile fracture and successively as brittle crack propagation, and the latter only exists within very short crack passage. It is also recognized that this slow crack growth in a brittle manner greatly depends on the temperature.     

Fracture behaviour of magnesia refractory materials in tension with the Brazilian test

In this work, the tensile failure of magnesia, rebound magnesia-chrome and chrome-containing magnesia-spinel refractories under the Brazilian test were investigated. The digital image correlation and acoustic emission were applied simultaneously for ensuring the validity of Brazilian test and studying the fracture process. The brittle refractories fail abruptly while reaching their load peaks because of the unstable crack propagation. However, the chrome-containing magnesia-spinel refractory shows a reduced brittleness due to the pre-existing microcracks, which promotes quasi-stable crack propagation evidenced by the nonlinearity in the pre-peak region and the softening in the post-peak region. Besides, the thickness-to-diameter ratio has a great influence on the fracture behaviour, which also shows brittleness dependence. The fracture behaviour of rebound magnesia-chrome refractory varies from brittle to less brittle while the thickness increasing from 10 mm to 50 mm. The quasi-stable crack propagation favors the central crack initiation and ensures the tensile failure under the Brazilian test.     

Evaluation of crack-Growth resistance curve for a particulate ceramic–Metal composite

A technique is described to evaluate the crack growth resistance behaviour in brittle ceramic-base materials. In this method, the crack increment measurements during the stable crack propagation process are not required. The crack growth resistance curves are studied for a particulate ceramic–metal composite in the system lanthanium chromite–chromium. Experiments were performed with standard fracture mechanics single-edge notched beam specimens in a temperature range from room temperature up to 1100°C. Effect of temperature on crack growth resistance behaviour is discussed.     

Morphology–toughness correlation of polypropylene/ethylene–propylene rubber blends

A polypropylene homopolymer was blended with ethylene–propylene rubber in different mixing ratios. The influence of the ethylene–propylene rubber content on the toughness behavior was investigated. According to the results of instrumented impact tests, brittle‐to‐tough transition temperatures were found for different ethylene–propylene rubber contents. Critical transition temperatures could be determined not only in the region of predominantly unstable crack propagation but also in the region of stable crack initiation. In situ measurements provided information on the deformation processes on the crack tip. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3364–3371, 2006     

Effect of short chain branching on the viscoelastic behavior during fatigue fracture of medium density ethylene copolymers

A method to determine viscoelastic changes in medium density polyethylene (MDPE) pipe specimens associated with the crack tip during fatigue crack initiation (FCI) and propagation (FCP) experiments is described. The load-displacement curves are analyzed to obtain the phase angle, δ. Changes in δ are related to the number of cycles of crack initiation of three different MDPE copolymers: hexene (H), butene (B), and methyl pentene (MP) copolymers. These changes are related to craze formation and growth at the notch tip, leading to crack initiation and to the irreversible work, W_i, expended on them. Within a given material, step wise increments in δ distinguish the onset of crack initiation and the brittle-to-ductile transition in crack growth. The magnitudes of tan δ and W_i are noted to be in quantitative agreement with the resistance of the three copolymers to FCI and brittle propagation that rank in the order: isobutyl (MP) > ethyl (B) > butyl (H). Similar crystallinity of the three copolymers insinuates a hypothesis that variance in the nature of chain entanglements associated with the respective branch type might be accountable for the observed differences in viscoelastic character. The final stage of failure by ductile tearing is dominated by large scale plastic flow that seemingly overshadows the material differences governing time dependent brittle fracture.     

Effects of coupling agents on the mechanical and rheologica properties and mica-reinforced polypropylene

Dimethylaminoethyl methacrylate (DMAEMA) was investigated as a prospective coupling agent for mica-reinforced polypropylene. Composites prepared with the widely-used silane coupling agent, N-(4-vinylphenyl) methyl-N′-(3-trimethoxysilylpropyl)ethylenediamine monohydroehloride (Z-6032), were compared with the amino-coupled composite. Improvements in the flexural properties at room temperature were observed with the two coupling agents: a 22 percent increase in the strength with the silane compared to a. 16 percent increase with DMAEMA. Cone-and-plate viscometry at 220°C showed that the addition of coupling agents greatly, reduces the viscosity of the composite: a 50 percent decease was obtained with the silane-treated composite and a 20 percent decrease with DMAEMA.     

Delamination Cracking in a Laminated Ceramic-Matrix Composite

Delamination crack propagation has been investigated in a laminated fiber-reinforced ceramic-matrix composite. The crack growth initiation resistance has been shown to be dominated by the critical strain energy release rate for the matrix. However, the resistance increases with crack extension because of bridging effects associated with intact fibers and, in some cases, intact segments of matrix. The delamination cracks also assume a steady-state trajectory within a 0° layer close to the 0°/90° interface.     

Modeling creep behavior in ceramic matrix composites

In this work, a three-dimensional viscoplasticity formulation with progressive damage is developed and used to investigate the complex time-dependent constituent load transfer and progressive damage behavior in ceramic matrix composites (CMCs) subjected to creep. The viscoplasticity formulation is based on Hill's orthotropic plastic potential, an associative flow rule, and the Norton-Bailey creep power law with Arrhenius temperature dependence. A fracture mechanics-informed isotropic matrix damage model is used to account for CMC brittle matrix damage initiation and propagation, in which two scalar damage variables capture the effects of matrix porosity as well as matrix property degradation due to matrix crack initiation and propagation. The Curtin progressive fiber damage model is utilized to simulate progressive fiber failure. The creep-damage formulation is subsequently implemented as a constitutive model in the generalized method of cells (GMC) micromechanics formulation to simulate time-dependent deformation and material damage under creep loading conditions. The developed framework is used to simulate creep of single fiber SiC/SiC microcomposites. Simulation results are in excellent agreement with experimental and numerical data available in the literature.     

Mechanics of failure of plastics by thermal imaging examination of a thermoplastic polypropylene + EPDM blend

Thermal wave imaging combined with stress pattern analysis by measurement of thermal emission (SPATE) is used to study the failure behavior of a polypropylene (PP) + ethylene propylene diene (EPDM) polymer blend. Images corresponding to a propagating crack in a single edge-notched specimen were measured at three rates of testing: 4 mm/min, 8 mm/min, and 20 mm/min. Conversion to stress values is made through use of a thermoelastic function. It is found that in a 4 mm/min test the crack tip radii blunt rather than propagate with little initiation. Large-scale necking precedes fracture. At 8 mm/min, some blunting occurs, followed by fast crack propagation. At 20 mm/min, fast crack propagation occurs. The images are digitized to obtain the values of the temperatures at every point in the sample. Data corresponding to the plane of the propagating crack over the span of the test are presented.     

Crack propagation modes in fiber and particulate composites in the microstructure

The influence of the hard or soft inclusions and the mesophase layers in either a soft-hard-soft or hard-soft-hard combination of biphase plates submitted to dynamic tensile loads on the fracture mode and bifurcation process in both phases was investigated in this paper. It was assumed that the soft or hard matrix is infolding the hard or soft inclusion of the plate, so that the plate constitutes a meridional section of the representative volume element of a unidirectional fiber composite, or a principal section of a particulate. The influence of the mechanical properties of either phase on the crack propagation velocity and the initiation of crack bifurcation was studied by using high-speed photography and dynamic caustics. The results showed that the propagating crack tended to bifurcate either in the brittle or in the mesophase layer under certain conditions of propagation velocity. It was shown that bifurcation of a propagating crack depends on the elastic moduli and Poisson's ratios of the phases, as well as on the extent of the mesophase layer, which depended on the adhesion quality of phases.     

Morphological properties of impact fracture surfaces and essential work of fracture analysis of cellulose nanofibril‐filled polypropylene composites

Scanning electron microscopy (SEM) was employed to investigate crack initiation and propagation process in notched and unnotched Izod impact fracture surfaces of the cellulose nanofiber (CNF) and microfibrillated cellulose (MFC)‐filled polypropylene (PP) composites compared with microcrystalline cellulose (MCC)‐filled composites. CNF is in the form of short fibers 50–300 nm in diameter and 6–8 in aspect ratio, MFC is in the form of long fibrils 50–500 nm in diameter and 8,000–80,000 in aspect ratio, and MCC is in the form of particles 50 μm in average diameter and 1–2 in aspect ratio. The reinforcement material size of CNF and MFC are smaller than that of MCC which means that the larger interfacial area between filler and matrix leading to larger energy dissipation at the interface during the impact fracture. The reinforcement‐matrix debondings nearby MCC particles caused easy crack propagation which contributes smaller energy dissipation at the interface. A slip‐stick behavior and stress whitened area during the fracture were observed. Morphological investigation helps to explain impact fracture behavior. According to essential work of fracture (EWF) analysis of Izod impact results, EWF method is applicable to analyze impact fracture behavior and the energy consumed in crack initiation and propagation during the fracture process can be calculated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Use of processing aids and coupling agents in mica-reinforced polypropylene

Low molecular weight waxes prepared by the maleation of thermally degraded polypropylene are effective substitutes for silane coupling agents in filled polypropylene resins. The addition of 1 to 5 percent maleated polypropylene wax increased the tensile and flexural strength of mica-filled polypropylenes without significant changes in the modulus or fracture toughness. The waxes also aided dispersion, reduced melt viscosity, and permitted direct injection molding of powder blends without precompounding. The influence of maleated polypropylene wax coupling agents on the mechanical properties is similar to the published behavior of chlorinated paraffin waxes, but without the undesirable corrosiveness. The efficient utilization of untreated micas as reinforcing fillers for general purpose polypropylene homopolymers can have important cost savings in the fabrication of mica-reinforced automotive components.     

Disturbance and recovery in high speed (110) cleavage in single crystalline silicon

Stress perturbations and material defects can significantly affect the fracture initiation and propagation behaviors in brittle materials. In this work, we show that (110)[110] cleavage in silicon deflects onto (111) plane in the presence of contact stresses. The deflection is however not permanent as the crack returns to the (110) plane after a certain length of propagation, even in the case where the crack velocity is up to 78% of the Rayleigh wave speed. The recovery behavior indicates that the (110)[110] cleavage is invariably prevailing when perpendicular to the maximum stress. Following this indication, it can be concluded that the observed (110)[110]–(111) deflection in previous literature is more likely driven by the external disturbance rather than the crack velocity induced toughness evolution. We also highlight that the extra energy for the (110) recovery is minimized at the expense of a large propagation distance upon the plane switch.     

Fatigue failure mechanisms in polymers

Fatigue testing of polymers has revealed significant differences between the fatigue response of polymers and metals. Generally, fatigue failure in metals is a process of crack initiation, propagation, and failure. Also, fatigue damage in metals is cumulative and cycle dependent, but remains essentially independent of test frequency. Unlike that of metals, the fatigue behavior of polymers is influenced by viscoelastic effects. At high frequencies, softening and melting occur, and fatigue failure depends largely on the test frequency. At lower frequencies, fatigue failure becomes less sensitive to test frequency and results from crack initiation and propagation. These polymer characteristics arise from the production of hysteresis energy during fatigue. A portion of this energy is released as heat, some of which is dissipated, but most is absorbed in the sample, raising its temperature. This temperature rise leads to degradation of the material and a short fatigue life. Experiments were conducted to measure hysteresis energy and temperature rise for a talc-filled polypropylene. A mathematical model was developed to calculate the energy and temperature distribution during fatigue. Correlation of the temperature rise predicted by the model with that observed experimentally provided values for the various energy terms that quantitatively defined the thermomechanical fatigue response of this polymer.     

Influence of the elastic mismatch on the Hertzian cone crack path in ceramic bilayers

Contact damage is a key aspect in the structural integrity of ceramics, particularly ceramic coatings and multilayers that may have an elastic mismatch. An understanding of the critical load and trajectories of the crack produced by contact loads in such materials is valuable to characterize the damage tolerance and improve their reliability. In this work, the Hertzian cone crack initiation and propagation in brittle bilayers has been studied by FEM and verified by experimental observations. It was concluded that the elastic mismatch affects the crack initiation position and critical load for cone cracking. Critical loads are lower in bilayers than in monolithic materials. Cone crack trajectory and the corresponding fracture energy release rate are also affected by the elastic mismatch, which thus influences the damage tolerance of the system.     

Influence of molecular structure and reinforcement on fatigue behavior of tough polypropylene materials

Molecular structure and reinforcement heavily influence the crack growth resistance of polypropylene materials. Aim of this study is to investigate the fatigue behavior of different unreinforced and reinforced tough polypropylene materials used for piping applications. Due to high resistance against crack growth, these materials cannot be tested in the application relevant quasi‐brittle failure mode within feasible amounts of time. In this work, the new cyclic cracked round bar test, developed for tough polyethylene materials, has been examined as a possible method to characterize this important type of failure mode in homo‐, random‐, and reinforced polypropylene. Even though molecular mass distribution, which is often used to explain differences in crack growth resistance of polymers, was similar for unreinforced materials, fatigue lifetimes differed greatly. The mismatch of molecular mass and fatigue lifetime was mainly attributed to the different buildup and morphology of the base polymer. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43948.