Enhanced fatigue life of carbon nanotube‐reinforced epoxy composites

The effects of carbon nanotube (CNT) inclusion on cyclic fatigue behavior and the residual mechanical properties of epoxy composites after different degrees of fatigue have been studied. Tension–tension cyclic fatigue tests were conducted at various load levels (25–50 MPa) to establish the relationship between stress and the number of cycles to failure (S–N curves). The residual strength and modulus were measured after loading at 30 MPa for 5000, 15,000, and 25,000 cycles. The incorporation of a small amount of CNTs increased the fatigue life of epoxy in the high‐cycle, low‐stress‐amplitude regime by 1550%. Micrographs indicate the key mechanisms for enhancement in fatigue life such as CNT crack‐bridging and pullout. POLYM. ENG. SCI., 52:1882–1887, 2012. © 2012 Society of Plastics Engineers     

Microstructure‐based fatigue modeling of an acrylonitrile butadiene styrene (ABS) copolymer

In this article, we experimentally investigate the structure–property relationships of an acrylonitrile butadiene styrene (ABS) copolymer for fatigue and use a microstructure‐based multistage fatigue (MSF) model to predict material failure. The MSF model comprises three stages of fatigue damage (crack incubation, small crack growth, and long crack growth) that was originally used for metal alloys. This study shows for the first time that the MSF theory is general enough to apply to polymer systems like ABS. The experimental study included monotonic testing (compression and tension) and fully reversed uniaxial cyclic tests at two frequencies (1 Hz and 10 Hz) with a range of strain amplitudes of 0.006 to 0.04. Cyclical softening was observed in the ABS copolymer. Fractography studies of failed specimens revealed that particles were responsible for crack incubation. Although polymeric materials can be argued to be more complex in terms of failure modes and thermo‐mechano‐chemical sensitivity when compared with most metal alloys, results showed that the MSF model could be extended successfully to capture microstructural effects to polymeric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40882.     

Fracture mechanical analysis of two commercial polyoxymethylene homopolymer resins

This research addresses the fracture mechanical analysis of two commercially available polyoxymethylene homopolymer resins. Two types of experiments are used: monotonic fracture toughness tests and cyclic fatigue crack growth tests. The resulting total lifetimes in the fatigue crack growth tests are split up into the appropriate parts of crack growth initiation and fatigue crack propagation. Fracture surfaces of monotonic and cyclic tests are analyzed using light microscopy and scanning electron microscopy. Besides the mechanical tests, the morphology within the used compact tension specimens is examined in detail by using differential scanning calorimetry, small‐ and wide‐angle X‐ray scattering, and polarized light microscopy. The results are analyzed and discussed, considering observations in the previous studies published in the literature. It is shown that both materials can be well analyzed using linear elastic fracture mechanics and their fracture mechanical properties are presented in conjunction with a detailed documentation of the microstructure. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40831.     

Kinetics of fatigue and creep crack propagation in PVC pipe

The kinetics of creep and fatigue crack growth in PVC pipe were studied in order to develop a methodology for predicting long‐term creep fracture from short‐term fatigue tests. Fatigue and creep crack propagation followed the conventional Paris law formulations with the same power 2.7: da/dt = A_fΔK^2.7_I and da/dt = BK^2.7_I, respectively. The activation energy for creep crack propagation, obtained from the temperature dependence of the Paris law prefactor, allowed extrapolation of high temperature creep fracture to low temperature creep crack growth rates. The activation energy for fatigue crack propagation was much lower than that for creep. Therefore, fatigue and creep could not be directly correlated by using the prefactor in the covenentional Paris law formulations. Furthermore, a unique value of the Paris law prefactor did not describe frequency and R‐ratio (amplitude) effects in fatigue crack propagation. Nevertheless, conformity of crack growth rates measured under all conditions to the same Paris law power suggested that correlation should be sought in alternative formulations of the crack growth rate.     

Crack-tip Degradation Processes Observed during in situ Cyclic Fatigue of Partially Stabilized Zirconia

It is proposed that reduced transformation zone widths in Mg-PSZ in cyclically versus critically propagated cracks are due to reductions in the crack-tip toughness, consistent with an intrinsic cyclic fatigue mechanism. Cyclic fatigue crack growth in Mg-PSZ was observed in situ in a SEM. Following cyclic fatigue, the samples were critically broken and the fracture surfaces observed. Extensive crack bridging by the precipitate phase was observed near the crack tip, and it is proposed that this crack bridging significantly affects the material's intrinsic toughness. Frictional degradation of the precipitate bridges occurs during cyclic loading and hence reduces the critical crack-tip stress intensity factor for crack propagation. Reductions in the critical crack-tip stress intensity factor also lead to reductions in the transformation zone widths during cyclic loading and hence the level of crack-tip shielding caused by phase transformation. This appears to be the mechanism of cyclic fatigue. A degree of uncracked ligament bridging was also observed and is linked with the frequency of random large precipitates. However, analysis shows that its effect upon crack growth rates under cyclic load is limited.     

Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.     

The role of crystalline phase on fracture and microstructure evolution of polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) is a semi-crystalline polymer, which has been employed in a range of engineering applications due to its extremely low coefficient of friction, resistance to corrosion, and excellent electrical insulation properties. Despite failure-sensitive applications such as surgical implants, aerospace components, motor seals, and barriers for hazardous chemicals, the mechanisms of crack propagation in PTFE have received limited coverage in the literature. Moreover, PTFE exhibits complex crystalline phase behavior that includes four well-characterized phases with both local and long range order. Three crystalline structures (phases II, IV, and I) are observed at atmospheric pressure with transitions between them occurring at 19 and 30 °C. This observation provides a unique opportunity for investigation of the effects of a polymers crystalline phase on fracture and microstructure evolution. Moreover, due to the presence of three unique ambient pressure phases near room temperature, it is essential to develop an understanding of the effects of temperature-induced phase transitions on fracture mechanisms of PTFE to prevent failure over the normal range of operating temperatures. In this work, we present values for the J-integral fracture toughness of PTFE for a range of temperatures and loading rates employing the single specimen normalization technique. Crack propagation in PTFE is found to be strongly phase dependent with a brittle-to-ductile transition in the crack propagation behavior associated with the two room temperature phase transitions. Increases in fracture toughness are shown to result from the onset of stable fibril formation bridging the crack plane and increased plastic deformation. The stability of drawing fibrils is primarily determined by temperature and crystalline phase with additional dependence on loading rate and microstructure anisotropy. [LAUR-05-0004]     

Crack Shielding in Ce-TZP/Al2O3 Composites: Comparison of Fatigue and Sustained Load Crack Growth Specimens

Crack shielding stress intensities in in situ loaded compact tension specimens of two types of ceria-partially-stabilized zirconia/alumina (Ce-TZP/Al₂O₃) composites with prior histories of subcritical crack growth in sustained and tension-tension fatigue loading were directly assessed using laser Raman spectroscopy. Crack-tip stress fields within the transformation zones were measured by measuring a stress-induced frequency shift of a peak corresponding to the tetragonal phase. The peak shift as a function of the applied stress was separately calibrated using a ball-on-ring flexure test. Total crack shielding stress intensity was estimated from the far-field applied stress intensity and the local crack-tip stress intensity assessed from the measured near-crack-tip stresses. The shielding stress intensities were consistently lower in the fatigue specimens than in the sustained load crack growth specimens. The reduced crack shielding developed in the fatigue specimens was independently confirmed by measurements of larger crack-opening displacement under far-field applied load as compared to the sustained load crack growth specimens. Thus, diminished crack shielding was a major factor contributing to the higher subcritical crack growth rates exhibited by the Ce-TZP/Al₂O₃ composites in tension–tension cyclic fatigue. Calculations of zone shielding considering only the dilatational strains in the transformation zones accounted for 81% and 86% of the measured values in the sustained load crack growth specimens, but significantly overestimated the shielding in the fatigue specimens. Possible reasons for the diminished crack shielding in the fatigue specimens are discussed.     

Peak stress intensity dictates fatigue crack propagation in UHMWPE

The majority of total joint replacements employs ultra-high molecular weight polyethylene (UHMWPE) for one of the bearing components. These bearings may fail due to the stresses generated in the joint during use, and fatigue failure of the device may occur due to extended or repeated loading of the implant. One method of analysis for fatigue failure is the application of fracture mechanics to predict the growth of cracks in the component. Traditional analyses use the linear elastic stress intensity factor K to describe the stresses near a loaded crack. For many materials, such as metals, it is the range of stress intensity, ΔK, that determines the rate of crack propagation for fatigue analysis. This work shows that crack propagation in UHMWPE correlates to the maximum stress intensity, K_max, experienced during cyclic loading. This K_max dependence is expected due to the viscoelastic nature of the material and the absence of crazing or other cyclic load dependent crack tip phenomena. Such a dependence on a non-cyclic component of the stress allows cracks to propagate under load with little or no fluctuating stresses. Consequently, traditional fatigue analyses, which depend on the range of the stress to predict failure, are not always accurate for this material. For example, significant static stresses that develop near stress concentrations in the component locking mechanisms of orthopedic implants make such locations likely candidates for premature failure due the inherent underestimate of crack growth obtained from conventional fatigue analyses.     

Fatigue Crack Propagation in Ceria-Partially-Stabilized Zirconia (Ce-TZP)-Alumina Composites

Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO₂-12 mol% CeO₂-10 wt% Al₂O₃ ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs of different transformation yield stresses obtained by sintering for 2 h at temperatures of 1500°C (type A), 1475°C (type B), 1450°C (type C), and 1425°C (type D). The threshold stress-intensity range, ΔK_th, for initiation of fatigue crack propagation increased systematically with decreasing transformation yield stress obtained with increasing sintering temperature. However, the critical stress-intensity range for fast fracture, ΔK_c, as well as the stress-intensity exponent in a power-law correlation (log (da/d N ) vs log ΔK) were relatively insensitive to the transformation yield stress. The fatigue crack growth behavior was also strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. In particular, the threshold stess-intensity range, Δ K _th, increased with increasing size of the transformation zone formed in prior quasi-static loading. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates. Calculations of crack shielding from the transformation zones indicated that the enhanced crack growth susceptibility of Ce-TZP ceramics in fatigue is not due to reduced zone shielding. Alternate mechanisms that can lead to reduced crack shielding in tension-tension cyclic loading and result in higher crack-growth rates are explored.     

Fatigue crack propagation behavior of polyether ether ketone under biaxial loading

Polyether ether ketone (PEEK) has become a promising material in total joint replacement. However, it still faces the risk of fatigue fracture during service. In this paper, the effects of biaxial stress ratio λ, cyclic stress ratio R, and load phase difference θ on fatigue crack propagation (FCG) behavior of PEEK are investigated. In the case of vertical cracks, results show that the FCG rate of PEEK increases with the R value, while decreases with the increase of λ value. Furthermore, the effective stress intensity factor range ΔK_eff can uniformly describe the biaxial FCG behavior at different cyclic stress ratios. In the case of 45° slant cracks, compared with mode-I intensity factor range ΔK_I, the energy release rate range ΔG is more accurate for describing the FCG behavior under various load phase differences. In addition, the investigation on the 45° crack propagation path shows that a bifurcated Y-shaped crack appears under 180° load phase difference, while no bifurcated crack appears under 90° load phase difference and uniaxial loading. Three different methods are used to predict the crack propagation path. The comparison results show that the maximum circumferential stress (MTS) criterion can well predict the crack propagation path under out-of-phase biaxial loading and uniaxial loading.     

Empirical models for matrix cracking in a CFRP cross‐ply laminate under static‐ and cyclic‐fatigue loadings

This paper presents empirical models for predicting matrix crack density in a carbon fiber reinforced plastic (CFRP) cross‐ply laminate under static‐fatigue and cyclic‐fatigue loadings. First, a modified slow crack growth (SCG) law, that covers the whole range of stress ratio R of tension‐tension fatigue (0 ≤ R ≤ 1), was proposed. The modified SCG law and three conventional SCG laws were then combined with Weibull's probabilistic failure concept for predicting fatigue matrix crack density in a cross‐ply laminate. Matrix crack density was expressed as a function of R, the maximum stress in the transverse ply and the number of cycles. Next, fatigue tests were performed for R of 0, 0.2, 0.4, 0.6, and 1 to determine the applicability of these four models. Finally, constant fatigue life (CFL) diagrams were investigated based on the modified model. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Fatigue properties of vibration‐welded postindustrial waste nylon with glass fibers at room and elevated temperatures

The effect of temperature on the tensile and fatigue strength of vibration‐welded and unwelded postindustrial waste nylon 6 reinforced with 30 wt% glass fiber (PIWGF) was experimentally examined, and the results were compared to those obtained from a 30 wt% glass fiber reinforced prime nylon 6 compound (PAGF) from a previous study. Fatigue tests were performed under sinusoidal constant amplitude tension‐tension load at a stress ratio of R = 0.1 and within the frequency range of 2–10 Hz at temperatures from 24 to 120°C. Stress levels from just under the tensile strength down to the run‐out point at 5 million cycles were used. It was found that increasing temperature led to a significant decrease in both tensile strength and fatigue life. For PIWGF, there was ～20% strength reduction under both static tensile and cyclic loading as compared to PAGF. For both welded and unwelded PIWGF, the endurance ratio; i.e., the ratio of fatigue strength to static tensile strength, was ～45% regardless of the temperature. The fatigue notch factor (K_f) was between 1.4 and 1.8 for all test temperatures examined. POLYM. ENG. SCI., 55:799–806, 2015. © 2014 Society of Plastics Engineers     

Cyclic interaction between normal and shear stresses of an epoxy polymer—Experiments and model predictions

This investigation focuses on the axial‐torsional loading interaction of an epoxy polymer, Epon 826/Epi‐Cure Curing Agent 9551. Thin‐walled tubular specimens were subjected to combined constant tensile (or shear) stress and cyclic shear (or tension) loading schemes. Pure tensile creep and shear creep tests were also performed to compare the creep deformation to that with superimposed cyclic shear or cyclic tension. Test data clearly showed that cyclic shear (or cyclic tension) have a readily discernible effect on the tensile (or shear) creep deformation. Similarly, a superimposed constant tensile (or shear) load affects the hysteresis responses in cyclic shear (or cyclic tension). A nonlinear constitutive model developed by the authors was used to simulate the observed normal‐shear stress interaction. Due to the inclusion of an effective stress parameter in its nonlinear function, this model was able to account for the normal‐shear coupling effect. However, the incorporation of a general loading/unloading rule led to inaccurate simulation of the observed oscillatory creep response. A modification of the general rule was proposed and better predictions on both the cyclic and the creep responses could be obtained. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Relationship between the material properties and fatigue crack‐growth characteristics of natural rubber filled with different carbon blacks

An investigation of the influence of different types of carbon black on fatigue crack‐growth behavior was undertaken. Fatigue tests were carried out on edge‐notched specimens under cyclic tension loading. A power‐law dependency between the crack‐growth rate and tearing energy was obtained. Natural rubber (NR) filled with N330 (the mean diameter is 30 nm) carbon black possessed the lowest values of exponent b and constant B (the two crack growth parameters determined from cyclic crack growth testing), which denoted the strongest resistance to crack growth at a given tearing energy. Strain‐induced crystallization was investigated by the modified Mooney–Rivlin equation and showed the earliest appearance and strongest ability of the crystallization of the NR/N330 composite at the largest amount of bound rubber. The study on the viscoelastic properties by dynamic mechanical analysis confirmed that the NR/N330 composite had the largest viscoelastic contribution, which was attributed to the viscoelastic dissipation in the viscoelastic region in front of the crack tip. All of these results confirm the best crack‐propagation resistance of NR filled with N330. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Fatigue behavior and life prediction of PI/SiO2 nanocomposite films

In this report, the fatigue behavior and lifetime of Polyimide/silica (PI/SiO₂) hybrid films are investigated. To evaluate the fatigue property of this class of hybrid films, the stress‐life cyclic experiments under tension–tension fatigue loading with 10 Hz of the frequency are performed, and the stress ratio is 0.1. Dynamic creep and cyclic softening/hardening are analyzed based on the change of hysteresis loops during the fatigue process. The structure‐property relations are discussed to further understand their phenomenon and deformation mechanisms. To predict the fatigue life of this class of hybrid films, a semiempirical model is proposed based on fatigue modulus concept. The simulated results are well agreeable with the testing values. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Fatigue behavior and modeling of thermoplastics including temperature and mean stress effects

An experimental investigation was conducted to evaluate fatigue behaviors of two thermoplastics. The effects considered include mold flow direction, thickness, mean stress, temperature, and frequency. Tension‐compression as well as tension?tension load‐controlled fatigue tests were performed at room temperature, ?40°C and 85°C. Incremental step cyclic deformation tests were also performed to generate cyclic stress?strain curves to determine strain‐life fatigue properties. The effect of mean stress was modeled using various parameters. The Walker mean stress model and a simple model with a mean stress sensitivity factor proved to be the most effective models to correlate the wide range of experimental data generated. POLYM. ENG. SCI., 54:725–738, 2014. © 2013 Society of Plastics Engineers     

Fatigue behavior of polyamide 66/glass fiber under various kinds of applied load

In this study, the fatigue behavior of polyamide 66 reinforced with short glass fibers and especially the role of glass fibers has been investigated under two kinds of cyclic loading. tension–tension fatigue tests with stress controlled and alternative flexural fatigue test with strain controlled were carried out. The main topics include microscope damage observation, described by fiber/matrix debonding and interfacial failure, endurance limit with Wohler curves, effect of self‐heating temperature. For both tests, the surface temperature increases with an increasing applied load. The results show that the self‐heating has an important effect in the failure point where the Wohler curves join each other. The fracture surface was analyzed by scanning electron microscope for both applied loads. The stress ratio is −1 for alternative flexural fatigue test and 0.1 and 0.3 for tension–tension fatigue test ones at frequencies ranging 2–60 Hz. POLYM. COMPOS.,, 2012. © 2012 Society of Plastics Engineers     

Effects of residual stress and orientation on the fatigue of injection molded polysulfone

The tensile fatigue behavior of unnotched injection molded polysulfone specimens has been investigated. The effects of orientation and residual stress were studied by comparing asmolded specimens with annealed or annealed and quenched specimens with a known residual stress pattern. The treatments are shown to have differing effects at high stresses, where failure is by shear yielding and necking, and at intermediate stresses, where failure is by fatigue crack propagation. The geometries of fatigue cracks are described for each case. An attempt is made to separate the effects of crack and craze initiation from crack propagation, and cyclic loading from cumulative time under load.     

Compressive fatigue behaviour of refractories with carbonaceous binders

Furnace linings of magnesia-carbon and micro-porous carbon bricks experience cyclic compressive loads. An experimental programme has been carried out to assess the sensitivity of these materials to compressive fatigue failure. Next to room temperature tests, a number of high temperature tests have been performed. Results of the fatigue tests have been analysed together with the data of the monotonic stress–strain loading and creep tests. Compressive fatigue failure has been seen in both the materials. The less brittle material has shown lower fatigue life. The curve relating the fatigue strains with the amount of cycles has been of classical sigmoid shape with three phases. The strain rates of the secondary (linear) phase have shown good correlation with the number of cycles to failure. The grain–matrix interface has been found to play the critical role in the initiation and propagation of the fatigue cracks.