Fatigue failure mechanisms in polymer composites

Axial fatigue of two, commercial, talc-filled polypropylenes was studied. A significant result of our investigation was the identification and characterization of the failure mechanisms and the effects of frequency and hysteretic heating. Frequency was found to be important to the fatigue response of the polymers. At low frequency, fatigue failure appeared to be a process of crack initiation and growth with very little dissipative heating. At high frequency, the fatigue process was dominated by dissipative heating resulting in significant creep and modulus loss, and failure was much accelerated and due mainly to material softening and melting. Correlation of temperature rise with on-line energy loss in selected experiments enabled us to extract information from the fatigue process. The results provided some quantitative understanding of the different fatigue failure mechanisms at low and high frequencies.     

CRACK GROWTH IN ADHESIVELY BONDED JOINTS SUBJECTED TO VARIABLE FREQUENCY FATIGUE LOADING

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

Crack growth in adhesively bonded joints subjected to variable frequency fatigue loading

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

Failure mechanisms in polyolefines: The role of crazing, shear yielding and the entanglement network

Macroscopic deformation and failure modes of polyolefines are reviewed in terms of deformation and failure models based on the craze initiation and propagation model of Kramer-Berger and the craze-crack transition model of Kramer-Brown. Although these models were formulated for amorphous polymers they are also valid for semi-crystalline polymers. The important role of the underlying molecular entanglement network in this approach is reflected by the strain hardening behaviour which is shown to be a robust measure for predicting slow crack growth performance. The polymer network response explains the experimentally observed presence of two Brittle-Ductile transitions, one at low temperature or high strain rates, linked with chain scission which dominates crazing, the other at elevated temperatures or low strain rates which involves disentanglement crazing. The relation between these two Brittle-Ductile transitions and the major transition temperatures for molecular mobility such as the glass transition and the crystal α relaxation temperature are discussed. Valid strategies for increasing the crack propagation resistance in polyolefines are reviewed. Finally an outlook for further research to complement the present knowledge base is formulated.     

Crack initiation and propagation in circular rubber bearings subjected to cyclic compression

The fatigue failure mechanism of rubber bearings under cyclic compression is important in evaluating their fatigue lives and thus is analyzed theoretically and numerically here. At first, the stress distributions in a bonded rubber cylinder derived from three different existing models were utilized to calculate the cracking energy densities within it. Next, the location of fatigue crack initiation and the direction of subsequent crack propagation in circular rubber bearings were consecutively determined. Furthermore, finite element numerical results were compared to those obtained theoretically from the three models to check their validity in predicting the fatigue crack initiation and propagation in circular rubber bearings. Based on the quasi‐statically theoretical and numerical results, it is found that the fatigue cracks initiate first at the outermost boundary between rubber and steel plates and propagate later inwards to the center of circular rubber bearings. The corresponding fatigue failure mechanism obtained theoretically and numerically is consistent with experimental findings reported previously. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Changes in mechanical behavior during fatigue of semicrystalline thermoplastics

The tensile fatigue behavior of two engineering thermoplastics (polyacetal and nylon_6,6) were studied by measuring changes in the dynamic viscoelastic response together with changes in potential energy density, strain energy density, and irreversible work. The results show that both stress softening and hardening can occur in controlled load cyclic conditions. At high stress levels and/or frequencies, both the polyacetal and nylon_6,6 show evidence of thermal softening as characterized by changes in their dynamic viscoelastic properties and decrease in storage modulus with corresponding increases in loss modulus and loss tangent. This effect is supported by observed decreases in the overall crystallinity as measured in DSC experiments. At lower stress levels (the mechanically dominated region), all results indicate that, although fatigue crack propagation (FCP) is one of the mechanisms governing the fatigue life, its contribution is minor and crack initiation time constitutes the majority of the fatigue life. Also, during the initiation stage, both materials become less viscoelastic and more elastic. This phenomenon is evidenced by overall reductions in the loss modulus, loss tangent, and irreversible work densities while the storage modulus is maintained. © 1995 John Wiley & Sons, Inc.     

Frequency sensitivity of fatigue processes in polymeric solids

One is faced with an interesting challenge when trying to explain the effect of test frequency on polymer fatigue performance. While hysteretic heating arguments appear sufficient to explain a diminution of fatigue resistance with increasing cyclic frequency in unnotched test samples, the enhancement of fatigue resistance in many polymers with increasing cyclic frequency in notched samples is still not clearly understood. In large measure, this is due to contradictory trends in fre-quency-sensitive material properties which affect the fatigue process. In this paper, a number of proposed fatigue models dealing with the time and strain rate dependence of elastic modulus, yield strength, creep and localized crack tip heating are examined and confronted with available data from the literature. Additional fatigue crack propagation data for poly(methyl methacrylate), poly (vinyl chloride), polystyrene, poly-carbonate, nylon 66, poly(vinylidene fluoride) and poly(2,6-dimethylphenylene oxide) were obtained and are reported herein. These data were obtained over a maximum frequency range of 0.1 to 100 Hz and, for selected polymers, with various waveforms. Frequency sensitivity is shown to be greatest in those polymers that show a high tendency for crazing. Relative fatigue behavior is found to reflect a competition between strain rate and creep effects. Where creep effects dominate, the total crack growth rate may be viewed as consisting of the summation of pure fatigue and creep components, respectively. Finally, the β transition appears to have a role, with frequency sensitivity being at a maximum for polymers where the β transition at room temperature occurs in the range of the experimental test frequency.     

Mechanisms of fatigue damage and fracture in semi-crystalline polymers

Fatigue crack profiles and fracture surfaces of poly(vinylidene fluoride) (PVDF), nylon-6,6 (N66), and poly(acetal) (PA) were studied to ascertain the mechanisms of cyclic damage and fatigue crack propagation in semicrystalline polymers. Crack tip damage is believed to begin as small trans-spherulitic and inter-spherulitic tensile crazes. However, compressive yielding within the reverse plastic zone at the crack tip crushes and elongates the spherulites in the direction of crack growth. Consequently, the microstructure of the polymer in advance of the crack front is different from the original morphology of the spherulitic bulk material as evidenced by the resulting fracture surface appearance. When the test temperature is below the glass transition temperature, however, plastic deformation is limited, and fatigue fracture occurs before significant disruption of the spherulitic structure. In this case, the fracture surface morphology reflects the original microstructure of the bulk polymer.     

Fatigue crack growth and fracture in glass sphere-filled nylon 6

The common degrading effect of glass beads on the static fracture energy and the fatigue crack propagation response in nylon 6 materials is examined by conducting fracture mechanics tests and by considering the progress of cracks through the composites. The scanning electron micrographs indicate that the cracks travel through regions of polymer matrix and also along the interfaces between polymer and glass beads. It is demonstrated that, although fracture of the polymer regions requires considerable energy, cracking of the interfaces usually absorbs very little. Thus, the crack propagation is preferably concentrated on these microstructural regions, which is the cause of the decrease in fracture energy and increase in fatigue crack growth rate with increasing amount of glass spheres in the composite. Partial properties of the matrix and the interface are introduced in order to describe the fracture behavior and to improve the understanding of the gross fracture processes. The combination of these partial properties with the volume fraction of filler and certain geometrical factors by a modified rule of mixture leads to critical values for the failure of the composites, which are in reasonable accord with the measured fatigue and fracture data.     

Kinetics of fatigue failure of polystyrene

To study the mechanism of crack initiation in the fatigue of amorphous polymers, flexural fatigue tests have been performed on injection molded samples (ISO 1 standard 4 mm thick 10 mm large) of polystyrene homopolymer (PS), at a frequency of 10 Hz. Two types of fatigue tests were performed: (i) Monosequential Fatigue Test (MFT) in which cycling loading is not interrupted: (ii) Polysequential Fatigue Test (PFT) during which fatigue periods alternate with rest periods. The number of cycles to rupture has been determined for various values of the strain amplitude, temperature and duration of rest periods. Two kinetics regimes were observed: (i) at temperature higher than 75°C, the thermal regime is quasi adiabatic and failure results from polymer softening. In this regime, rest periods have a strong stabilizing effect, since they limit self heating; (ii) At temperatures lower than 75°C, the thermal regime is quasi isothermal and fracture results from crazing followed by brittle rupture. Rest periods (on the order of half fatigue lifetime) have no effect on the number of cycles to rupture. The influence of temperature here is considerably lower than in the “adiabatic” regime. The endurance limit values ?_∞ are close to the critical strains for crazing ?_C in static conditions. A kinetic model based on the above observations is proposed for strain amplitudes close to the endurance limit (2?_∞ ≥ ? ≥ ?_∞). This model reasonably fits the experimental data.     

Fatigue Crack Propagation at Polymer Adhesive Interfaces

Fatigue (slow) crack growth in epoxy/glass, epoxy acrylate/glass and epoxy/PMMA interfaces was studied under constant and cyclic loading at both high and low humidities using the interfacial, four-point flexure test. Finite element analysis was used to determine the energy release rate and phase angle appropriate for the different crack geometries observed. The experimental results show that for the polymer/glass interfaces, the primary driving force for fatigue crack growth is the applied energy release rate at the crack tip and that increasing test humidity enhances crack growth under constant loading but has an insignificant effect under cyclic loading. At low humidity the crack growth rates under cyclic loading are significantly greater than under constant loading. For epoxy/PMMA interfaces the crack growth results were independent of the applied energy release rate, relative humidity, and cyclic vs. constant loading, within experimental scatter. In addition, for polymer/glass interfaces the effect of phase angle (13 to 54°) on crack growth rates is not significant. However, for epoxy/PMMA interfaces the applied energy release rate for the initiation of crack growth is considerably greater for a phase angle of 66° than for 5°, indicating that increasing shear at the crack tip makes the initiation of crack growth more difficult. These results are discussed in terms of possible mechanisms of fatigue crack growth at polymer adhesive interfaces.     

Craze initiation,crack growth,and lifetime trends in fatigue of poly(vinylchloride)

The fatigue behavior of pure poly (vinylchloride) (PVC) and a PVC pipe compound has been investigated. Unnotched S-N lifetime, fatigue-crack growth, and craze/crack-initiation data are presented. The data trends, coupled with direct-microscopic observation, suggest that the unnotched-specimen lifetime in fatigue is dominated by the craze/crack-initiation process. This differs from the observed consistency of crack propagation and specimen-lifetime trends in several other polymers, whose failure can be traced to the initiation and growth to instability of a single dominant craze/crack.     

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.     

Cyclic fatigue of silica refractories – effect of test method on failure process

Silica refractories serving in high temperature industrial installations fail due to thermo-mechanical cyclic loads. The failure process was investigated by performing cyclic fatigue tests of several methods. In uni-axial compression the samples were tested either with constant force or displacement amplitude or with fixed upper displacement limit. In bending constant displacement amplitude tests were done. The investigation was supported by monotonic loading tests and microstructural analysis. It was determined that the fatigue failure occurs due to the degradation of interlocking in the crack wake. Cracking of larger grains is important for the crack initiation. The test set-up and the loading procedures significantly influence the potential to resist the crack propagation. Cyclic loading produces less brittle failure than monotonic loading. The displacement controlled method allows more gradual, less brittle, failure than the force controlled method. The potential of the crack arrest is less developed in bending than in compression.     

Fatigue-fracture mechanism of slowly notched poly(ethylene terephthalate) polymers

Summary Fatigue fracture behavior of slowly notched polyethylene terephathalate (PET) polymers were investigated at temperatures close to their transition temperatures up to well above their glass transition temperatures. Detailed characterization on the morphology of the notched roots showed that the crack tip during crack propagation became more dull with increasing testing temperature. The failure cycle (N_f) of these samples increased with increasing temperatures until it reached the transition temperatures of PET polymers, and most of the increase in N_f is due to the increased time consumed in the initiation period. On the other hand, the initial crack growth rate increased significantly and N_f of these samples decreased dramatically as the temperature increased well above the glass transition temperature. This interesting temperature dependence of fatigue behavior is explained due to the change of molecular motion of PET polymers at this temperature range.     

Strain rate dependence of failure processes in polycarbonate and nylon

A study has been made of two types of failure, namely, monotonic fractures using Charpy-type specimens and fatigue crack propagation using rectangular plates containing an initial central notch. The work was conducted on an amorphous polymer (polycarbonate) and a semicrystalline polymer (nylon N 6.6). Monotonic tests were performed in an Instron testing machine between 0.002 and 20 in./min, and in a Charpy testing machine between 2000 and 11800 in./min. The cyclic tests (under constant K conditions) were carried out at frequencies that ranged from 0.1 to 20 Hz. A model for the relationship between the cyclic rate of crack growth and appropriate LEFM parameters, previously described, has now been converted into cyclic strain energy transformations. In computing the strain energy, the value of the time-dependent modulus of the material was used; and under cyclic loading conditions the glassy (short time) value was employed. The authors have proposed that the modulus measurements obtained at very low temperatures, where the viscous response of the material is highly restricted, will approximate the glassy value, E_g, found by conducting relaxation measurement tests at different temperatures down to ?197°C. Within the range of tests conducted, the fracture toughness values of both PC and N 6.6 apparently decrease with increase in loading rate. Fatigue crack growth in both materials is influenced by loading frequency and cyclic waveform. These variations may be related to the magnitude of the viscous energy loss and hence to the available energy for crack extension per cycle.     

Crystalline phase transformation of polytetrafluoroethylene in a fatigue test

In this study, we aimed to characterize the mechanical response of polytetrafluoroethylene (PTFE) laminates under a tension–tension load‐control fatigue test (frequency = 5 Hz, load ratio = 0) and provided an analysis of the failure patterns of the PTFE material with consideration of crystalline phase transformation. In the final results, the evolution of the cyclic creep strain and stress–number of cycles to failure (S–N) curves presented duplex properties accompanying the fatigue life increasing to high cycles (cycle fatigue > 10⁵). A simple phenomenological damage index was defined in this study to describe the cyclic creep process. Additionally, the scanning electronic machine investigation suggested that local fibrosis caused by crystalline phase transformation to phase I led to the initiation of fatigue crack, and the fiber formation and orientation was found to be beneficial to a higher tensile strength and better resistance to crack propagation. The aspect of cyclic‐load‐induced crystallization was observed by the microfocus hard X‐ray diffraction beamline from a new insight. The crystalline phase transformation led to a gradient distribution of crystallinity and lateral crystallite size along the crack propagation direction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41113.     

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.     

Failure of Glass with Subthreshold Flaws

Conventional postthreshold crack analysis cannot be used to predict the strength and fatigue behavior of glass with subthreshold flaws. Therefore, a fracture mechanics model for failure of glass with subthreshold indentation flaws was developed. This model accounts for both the near- and farfield residual stresses associated with the indentation impression. It is shown that these stresses play a major role in the initiation and subsequent propagation of cracks that eventually cause failure. The model predicts "pop-in" of a well-developed crack and failure under continuous and discontinuous crack growth in both inert and fatigue conditions. The results of experiments with bare fused silica fibers with indentation subthreshold flaws in inert and fatigue (water) environments were in good agreement with the predictions by the model.     

The influence of temperature and absorbed water on the fatigue crack propagation in nylon-6,6

The effect of absorbed water on the fatigue crack propagation (FCP) of nylon-6,6 was investigated over a range of test temperatures and is correlated with dynamic mechanical properties. Both the storage modulus, a measure of specimen stiffness, and the loss compliance, a measure of energy dissipation and hysteretic heating, influence FCP response. At a given temperature, fatigue resistance is greatest for a given water content corresponding to an optimum combination of storage modulus, E′, and loss compliance, D″. The use of an empirical shift parameter to normalize the temperature dependence of the FCP behaviour of nylon with various water contents is discussed.