Effect of morphology on the crack propagation of the semicrystalline polyester poly(1,4-dimethylene-trans-cyclohexyl suberate)

Crack propagation of the semicrystalline polymer poly(1,4-dimethylene-trans-cyclohexyl suberate) (MCS) was studied as a function of polymer morphology. MCS was characterized in terms of degree of crystallinity and crystal growth kinetics. Spherulitic band size and radius show similar temperature dependencies. The energy to propagate a crack was correlated with spheruliticr adius for a low-molecular-weight material (M_n = 24 500). Brittle fracture occurs in this material with little large-scale plastic deformation. What plastic deformation there is, however, correlated with spherulitic band orientation. A higher-molecular-weight sample (M_n = 38 000) shows plastic deformation over the entire temperature range studied. Energy to fracture agrees with a modified Griffith criterion in which the characteristic dimension is spherulitic radius. Annealing experiments show that energy to fracture is controlled by lamellar thickness, decreasing with increasing thickness. Fracture morphology shows little interspherulitic failure, with intraspherulitic failure (low-molecular-weight material) or plastic deformation (high-molecular-weight material) being the prevalent modes.     

Failure analysis of carbon black filled styrene butadiene rubber under fatigue loading conditions

Abstract

The present paper deals with the fatigue crack growth in a carbon black-filled styrene butadiene rubber (CB-SBR) under fully relaxing loading conditions. More precisely, it is devoted to the determination of the scenario of crack growth. For that purpose, an original ‘microcutting’ technique, previously applied by the authors on natural rubber (NR), is used to observe microscopic phenomena involved in fatigue crack growth thanks to scanning electron microscopy (SEM). Results show that the crack tip grows following a tearing line by generating ligaments; it explains the differences between fatigue responses of crystallisable and non-crystallisable rubbers during crack propagation. So, contrary to crystallisable elastomers such as NR, the microstructure of SBR is similar at crack tip and in the bulk material, and the crack tip does not resist crack propagation. Moreover, the morphology of fracture surfaces only depends on particles encountered by the fatigue crack during its propagation.     

Temperature dependence of crack-propagation parameters and crack morphology of the semicrystalline polyester poly(1,4-dimethylene-rans-cyclohexyl suberate)

Temperature-dependent crack propagation in the semicrystalline polymer poly(1,4-dimethylene-trans-cyclohexyl suberate) was studied as a function of forming temperature (spherulitic morphology) and molecular weight. All experiments were conducted between the glass transition temperature and the crystal melting point. Higher forming temperature (larger spherulitic size) produced lower energy to propagate (G_p). For a given morphology (single forming temperature), increasing the propagation temperature decreased the propagation energy. An equation relating G_p to the microscopic viscosity of the amorphous polymer fraction was derived. Experimental data were fitted to the equation, and Arrhenius activation energies for microscopic flow were obtained. The results show a decrease in activation energy with increased forming temperature and molecular weight, and are discussed in terms of lamellar tie-molecule population. Fracture morphology is related to thermal parameters and chain entanglements. The results indicate that much of G_p is due to plastic deformation.     

Fatigue of viscoelastic polymers: 2. Fractography

The results of a fractographic study of fatigue failure in viscoelastic polymers is presented. Tests were conducted on a spherulitic low-density polyethylene in reversed loading using non-symmetrical deformation conditions. The microfractographic features have been found to depend on the deformation programme, the temperature of the test and the position on the fracture surface. The latter has been related to the different stages of growth revealed by plotting crack-growth propagation data. As well as interspherulitic fracture, other mechanisms that are sometimes prevalent involve cold-drawing, (fibrillation), and/or lamellar reorientation. The appearance of fatigue fracture surfaces of a non-spherulitic low-density polyethylene, a high-density polyethylene and a plasticized poly(vinyl chloride) corroborate the structural interpretations proposed to account for the observations with the spherulitic low-density polyethylene.     

Fatigue fracture of reaction injection molded (RIM) nylon composites

This study was undertaken to determine how milled glass fibers affect the fatigue resistance of reaction injection molded (RIM) nylon 6. Specifically the effects of glass content, fiber length, orientation, and surface treatment were investigated. The fatigue crack growth rates for unfilled and glass-filled samples were observed to follow the well-known Paris equation in terms of dependence on cyclic stress intensity factor. For the unfilled nylon a line shaped zone was observed in advance of the crack tip. Fractography results suggest that the zone was the projection of the actual crack tip profile through the thickness of the sample rather than a distinct plastic or deformation zone. The fatigue fracture surface exhibited a patchy type structure with features 50–150 μm in size, suggesting a void coalescence type of mechanism as has been reported for injection molded nylons. A diffuse damage zone, several millimeters in size, was observed at the crack tip for the glass-filled RIM nylon 6. The zone was observed to pulsate with the applied oscillating load. The growth of the damage zone volume with increasing crack length (and thus increasing stress intensity factor range) followed the Paris law, as did the crack growth rate data. The damage mechanism is attributed to void formation and microcracking at the fiber–matrix interface. The results of this study show that, for milled glass-reinforced RIM nylon 6, the crack growth rates were much more rapid than observed for injection-molded nylon 6 containing chopped glass fibers. This difference is attributed to the greatly reduced glass fiber lengths for the milled glasses.     

Crack growth mechanism in some polyamides

Fracture toughness was measured with single-edge notched three-point bend specimens. The materials used were semicrystalline polymers, polyamide-66, polyamide-1010 and polyamide-610. Their fracture surface was investigated with the help of scanning electron microscopy. The crack growth mechanism is revealed and a model suggested. The stable crack growth feature of the fracture surface is the dimples. The stable crack growth includes: the crack tip blunting, the voids initiating, coalescing and extending, the fibrillated polymer rupturing and contracting, then the formation of dimples on the fracture surface. The unstable crack growth formed a fan cleavage zone. The crack growth passed along the boundaries of spherulitic crystals in which many subcracks have occurred. The mechanism of discontinuous subcrack propagation in ductile polyamide-610 was discovered. In the compressive zone of the bend specimen, the fracture mechanism is similar to the cleavage zone and exhibits shear failure response.     

Fatigue fracture behaviour of PEEK: 2. Effects of thickness and temperature

Experimental results on the effects of specimen thickness and environmental temperatures on fatigue fracture behaviour of poly(ether ether ketone) (PEEK) are reported. Low cycle fatigue experiments are conducted on injection moulded single-edge notched specimens of 1.57, 2.70 and 5.42 mm in thickness at ambient temperatures, and on specimens 2.70 mm thick at environmental temperatures of 39, 50, 63, 75 and 100°C. In all the thickness experiments and in the experiments with temperatures of 39 and 50°C, the crack tip profile is initially round. At long crack lengths the crack tip profile changes to a triangular shape. When the test temperature is 63, 75 and 100°C, the crack tip remains round throughout the fracture process. The crack tip angle is primarily dependent upon the test temperature. Examinations of the fracture surfaces and transverse sections indicate that in the thickest specimen, relatively rough fracture surfaces are observed and a few discontinuities (crazes or cracks) underneath the main crack path. Thus, crack propagates in a ‘brittle’ manner. In all other experiments both ‘brittle’ and ‘ductile’ modes of fracture are observed. The point of transition from ‘brittle’ to ‘ductile’ fracture is dependent upon the specimen thickness and test temperature. Fatigue striations are seen throughout the fracture surfaces. Correlation of the striations and the number of cycles indicates a one-cycle crack growth mode. Hysteretic losses during fatigue crack growth are negligible until a few cycles prior to unstable fracture. Crack opening displacements are independent of the specimen thickness and increase with rise in temperature. When crack growth rates are correlated with the elastic energy release rate, they are independent of specimen thickness and increase with increase in temperature.     

An In-Situ Failure Model for Adhesive Joints

In this paper, a theoretical model based on the fracture mechanics principle is built to describe the in-situ failure process of adhesive joints. The central concept of the model is that the adhesive fracture is controlled by the plastic zone developed at the crack tip. On the basis of an approximate crack tip stress distribution, a quantitative representation is found to relate the adhesive fracture energy G_1c(joint) to certain bulk resin properties: fracture toughness G_1c(bulk), yield stress σ_y, and Young's modulus E. It is found that the factor σ_y ²/E is sometimes more important than G_1c(bulk) in controlling G_1c(joint). The in-situ failure model interprets well the temperature and loading rate dependent phenomena of adhesive joint fracture reported in the literature. A correlation between the resin material variables and the adhesive fracture is thus established.     

Toughness of nanoscale and multiscale polyamide‐6,6 composites

Fracture initiation and fracture propagation toughening (R‐curve behavior) of polyamide 6,6 (PA‐66) polymers with different types of layered silicate clay having nanoscale (fully dispersed) or multiscale (mixed nanoscale/microscale) structure were studied. These results were compared to fracture data for conventional kaolin clay particulate reinforcements and a PA‐66 polyblend containing rubber and rigid poly(styrene‐co‐acrylonitrile) particulates. The stiffness increase due to the intercalated clay was the same as would be predicted by classical models for conventional elongated reinforcements at the same volume fraction level. The special benefit of the nanoscale reinforcement derived from their high surface area of contact with the matrix. Toughness in layered silicate clay composites was enhanced by better dispersion of the clay, by exfoliation of the clay layers, and by a stronger clay/matrix interface. A multiscale microstructure was found to be the more desirable microstructure, combining toughness from the nanodispersed clay with resistance curve behavior from the micrometer‐sized particulates. Fracture toughness was proportional to the crack‐tip plastic zone size at fracture, indicating that the clay reinforcements, by influencing shear deformation in the crack tip region, played an important role in promoting toughness. There was indirect evidence for the formation of a zone of damage within the crack‐tip plastic zone that could explain why toughness was not optimal.     

The brittle fracture of amorphous thermoplastic polymers

The brittle fracture properties of polyphenylene oxide, polysulfone, polycarbonate, and poly(methyl methacrylate) thermoplastic polymers were investigated over a wide range of temperatures. Fracture energy measurements were made using double edge-notched tensile samples. Tensile strength, tensile strain, and initial elastic modulus were measured for calculation of the fracture energy and further analysis of the polymer behavior. It was found that mechanical transitions in the tensile properties corresponded reasonably well with transitions in the fracture energy in the temperature range investigated. Fracture surface photographs permitted visual analysis of the fracture process. It was found that the roughest fracture surface corresponded to the maximum in the fracture energy for a given polymer. A theory for prediction of polymer tensile yield strain is presented, based on the volume dilation concept. The implications of this theory are discussed in terms of the crack tip flow process leading to brittle fracture.     

Role of crack tip shielding mechanisms in fatigue of hybrid epoxy composites containing rubber and solid glass spheres

The fatigue crack propagation (FCP) resistance of epoxy-based composites containing various concentrations of solid glass spheres (SGS) and/or reactive liquid rubber (CTBN) was examined. The FCP results show that the simultaneous use of rubber and solid glass spheres (hybrid composites) results in synergistic improvement in FCP resistance of composites through the entire crack growth regime. The nature of synergistic interactions was elucidated by careful examination of the fatigue fracture surfaces and the subfatigue fracture surfaces of fatigue samples. It was shown that when rubber particles cavitate in the vicinity of the glass spheres, regardless of the nature of the interface, glass particle debonding from the matrix is suppressed due to a change in the crack tip localized stress state. This, in turn, results in improved pinning/bridging efficiency of the glass spheres. Furthermore, it was shown that crack tip plastic zone-rubber particle interactions induce a transition in FCP behavior of rubber-modified epoxies. Consequently, crack tip shielding mechanisms become active when the size of the plastic zone at the crack tip becomes large compared to the size of the rubber particles. © 1995 John Wiley & Sons, Inc.     

Fracture behavior of polyetherimide (PEI) and interlaminar fracture of CF/PEI laminates at elevated temperatures

To investigate the effects of environmental temperature on fracture behavior of a polyetherimide (PEI) thermoplastic polymer and its carbon fiber (CF/PEI) composite, experimental and numerical studies were performed on compact tension (CT) and double cantilever beam (DCB) specimens under mode‐I loading. The numerical analyses were based on 2‐D large deformation finite element analyses (FEA). Elevated temperatures greatly released the crack tip triaxiality (constraint) and promoted matrix deformation due to low yield strength and enhanced ductility of the PEI matrix, which resulted in the greater plane‐strain fracture toughness of the bulk PEI polymer and the interlaminar fracture toughness of its composite during delamination propagation with increasing temperature. Furthermore, the high triaxiality was developed around the delamination front tip in the DCB specimen, which accounted for the poor translation of matrix toughness to the interlaminar fracture toughness by suppressing the matrix deformation and reducing the plastic energy dissipated in the plastic zone. Especially, at delamination initiation, the weakened fiber/matrix adhesion at elevated temperatures led to premature failure of fiber/matrix interface, suppressing matrix deformation and preventing the full utilization of matrix toughness. Consequently, low interlaminar fracture toughness was obtained at elevated temperatures. POLYM. COMPOS., 26:20–28, 2005. © 2004 Society of Plastics Engineers.     

Polymer fatigue fracture diagrams: BPA polycarbonate

A fatigue fracture diagram for BPA polycarbonate has been created from fatigue lifetime data obtained from knit line notched samples. This fatigue fracture diagram maps out stress-temperature zones where fatigue fracture is dominated by crack growth through leading crazes and zones where fatigue fracture occurs through shear fracture at 45 degrees to the load direction. Both craze and shear planes coexist in the fatigue crack tip plastic zone, and both compete to determine the ultimate crack growth behavior. The shear planes preferentially develop (and fracture) at higher temperatures and stresses, but this fracture process is quite slow. Consequently, an inversion in the fatigue lifetime curve is observed, with longer lifetimes at higher stresses. This inversion is easily understood as a transition between a craze branch and a shear branch on the fatigue lifetime plot. When the fatigue lifetime curve is plotted for data at different temperatures, with the stresses normalized to the yield stress at the respective test temperatures, the craze branch data from different temperatures overlap. This overlap can be explained by the N = 2 power law dependence of crack growth in the discontinuous crack growth regime.     

The fracture behaviour of a PXE/HIPS polyblend

As part of an overall examination of the fatigue crack propagation (FCP) behaviour of impact-modified polymers, a study of the fracture morphology of a PXE/HIPS polyblend polymer subjected to monotonic and cyclic loading conditions is reported. The HIPS rubbery-phase particles are found to fail by particle rupture in both fatigue and fast fracture. Another impact modifying addition, PE, is found to fail by a combination of interfacial rupture and tearing, the balance depending on the prevailing stress intensity value and the strain rate. Matrix failure is via multiple crazing at low fatigue crack growth rates, but shear yielding is believed to become a major fracture mechanism with increasing K. The degree of plastic deformation of the matrix increases with increasing strain rate. This fact is manifested by the increasing void size associated with the interfacial separation of the PE particles.     

Influence of thickness and processing history on fatigue fracture of nylon 66. Part II: Crack tip morphology

Fatigue crack propagation rates in injection molded nylon 66 were previously shown to be strongly affected by prior processing history. To provide a physical basis for the observed acceleration in crack growth rates, microtomed sections were cut through the tips of stable fatigue cracks and examined by optical microscopy. A reduction in spherulite size occurs with reprocessing along with an accompanying decrease in the amount of deformation at the crack tip. For the initially processed nylon 66 this deformation consists of a vast array of independently initiated craze-like zones. Patchy type regions observed on the fatigue fracture surface are similar in size to the initially formed crazed zones. Crack advance occurs by the breakdown and coalescence of the crazed regions via matrix shearing. The extensive damage zone is believed to result in a reduction in stress intensity at the crack tip thereby reducing the crack propagation rates. For the reprocessed nylon 66, one observes fewer crazes and a sharper fatigue crack tip with a consequent acceleration in crack propagation rates and a smoother fracture surface.     

Crack initiation in PVC for subsequent linear elastic fracture mechanics analysis

Reproducible starter-cracks for subsequent linear elastic fracture mechanics analysis have been grown in PVC by fatigue cycling at 80 Hz. The crack growth rate has been related to the fracture surface markings and to the opening mode stress intensity factor (K_I) of the fatigue cycle. Termination of the fatigue crack growth when crack growth rate is constant ensures a smooth mirror fracture surface and a sharp crack tip.     

Electroplating on crystalline polypropylene. I. Compression molding and adhesion

A direct relationship between polymer processing and metal adhesion is evident from studies of compression-molded isotactic polypropylene (PP). Cooling rate, nature of the mold surface, and after-plated annealing are shown to affect the peel adhesion of the plated components. This report described (1) the relation of compression molding variables to polymer surface morphology, (2) the oxidative cracking behavior of the surface due to pretreatment with chromic–sulfuric acid in terms of crystallite orientation and crystallinity, and (3) the effect of surface crack patterns on adhesion. The nature of the mold surface is the single most important variable for controlling the surface morphology of PP. Compression molding PP against oxidized aluminum or copper produces a spherulitic surface, whereas molding the polymer against Mylar or Teflon produces a transcrystalline surface. Surface etching of PP homopolymer produces sponge-like crack patterns characteristic of the morphology. Radial patterns are observed on spherulitic surfaces and random patterns, on transcrystalline ones. The various surface patterns are developed in the oxidative process by swelling of amorphous material followed by oxidative stress cracking and dissolution. Metal-to-polymer adhesion, as measured by the peel test, may involve failure at the interface or within the polymer. Three factors are shown to be important: (a) the geometry of the interface, (b) the diminished strength of the polymer surface arising from attack by the oxidizing acid, and (c) the crystallinity of the fissured polymer surface. The highest peel values are associated with conditions that lead to deep and frequent fissuring of the polymer surface and minimum oxidative damage.     

Fracture toughness and crack instability in tough polymers under plane strain conditions

The plane strain fracture toughness and fracture mechanisms of several tough engineering plastics have been studied and compared with poly(methyl methacrylate) (PMMA), a relatively brittle polymer. The tough polymers all are observed to form a multiple craze zone at the crack tip, which is shown to be the primary source of plane strain fracture toughness in these materials. The multiple craze zone is retained during slow crack growth but is metastable, and at a critical stress intensity and associated crack velocity, the system passes through a transition to a greatly accelerated single craze mode of unstable propagation.     

Fracture of fiber reinforced composites

Characteristics of the fracture of fiber reinforced plastic composites are described in terms of the elastic stress distribution at the crack tip, the mechanism of crack tip damage, and the modes and conditions of final fracture. The three-dimensional, stress field at the tip of a sharp crack in a laminate is presented and contrasted to traditional two-dimensional models. The response of the material in the form of inter- and intraply damage formation and growth under increasing load is characterized, and its effect in blunting the main crack is examined. The final fracture conditions, which may range from quasi-brittle to notch insensitive, are discussed and related to the damage zone extension. Observed and anticipated effects of various material and geometric parameters are also discussed.     

Plastic zone in front of mode I crack in phenolphthalein polyether ketone

The plastic zone size and crack opening displacement of phenolphthalein polyether ketone (PEK-C) at various conditions were investigated. Both of them increase with increasing temperature (decreasing strain rate), i.e. yield stress steadily falls. Thus, the mechanism increasing the yield stress leads to increased constraint in the crack tip and a corresponding reduction in the crack opening displacement and the plastic deformation zone. The effect of the plastic deformation on the fracture toughness is also discussed.