Threshold Stress for Crack-Healing of Si3N4/SiC and Resultant Cyclic Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were hot-pressed in order to investigate the crack-healing behavior under stress. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under cyclic or constant bending stress, and the resultant bending strength and cyclic fatigue strength were studied. The threshold stress for crack-healing was investigated at healing temperatures of 1000° and 1200°C. The cyclic fatigue strengths of crack-healed specimens were also investigated at healing temperatures of 900° and 1000°C. The main conclusions are as follows: (1) The threshold cyclic and constant stresses for crack-healing, below which pre-cracked specimens recovered their bending strength, were 300 MPa, which was 75% of the bending strength of the pre-cracked specimens and (2) the crack-healed specimens exhibited quite high cyclic fatigue strength at crack-healing temperatures of 900° and 1000°C.     

Mechanical Behavior and High-Temperature Performance of a Woven Nicalon™/Si-N-C Ceramic-Matrix Composite

A modern ceramic-matrix composite (CMC) has been extensively characterized for a high-temperature aerospace turbine-engine application. The CMC system has a silicon-nitrogen-carbon (Si-N-C) matrix reinforced with Nicalon fibers woven in a balanced eight-harness satin weave fabric. Tensile tests have demonstrated that this CMC exhibits excellent strength retention up to 1100°C. The room-temperature fatigue limit was 160 MPa, ∼80% of the room-temperature tensile strength. The composite reached run-out conditions under cyclic (10⁵ cycles at 1 Hz) and sustained tension (100 h) conditions at a stress of 110 MPa, which was ∼35 MPa above the proportional limits at temperatures up to 1100°C in air. At stress levels >110 MPa, cyclic loading at 1000°C caused a more severe reduction in life, based on time, compared with sustained tension. Further life degradation was observed in the 1000°C fatigue specimens that were exposed to a salt-fog environment. This degradation decreased the fatigue life ∼85% at the stress levels that were tested.     

Crack-Healing Behavior of Si3N4/SiC Ceramics under Cyclic Stress and Resultant Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm surface length was made on each specimen. The crack-healing behavior under cyclic stress of 5 Hz, and resultant cyclic fatigue strengths at healing temperatures of 1100° and 1200°C, were systematically investigated. The main conclusions are as follows: (1) Si₃N₄/SiC composite ceramics have an excellent ability to heal a crack at 1100° and 1200°C. (2) This sample could heal a crack even under cyclic stress at a frequency of 5 Hz. (3) The crack-healed sample exhibited quite high cyclic fatigue strength at each crack-healing temperature, 1100° and 1200°C.     

Static and Cyclic Fatigue of Alumina at High Temperatures

Fatigue behavior of alumina at 1200°C was investigated. Uniaxial tensile tests were conducted in both static and cyclic loading. A variety of loading wave forms were applied during the cyclic tests. Cyclic lifetime is found to be cycle shape dependent and controlled by the duration of the hold time at the maximum tensile stress in a cycle. Cyclic loading with a higher strain rate and a short duration of maximum stress during each cycle provides a beneficial effect on lifetime in comparison to static loading at the same maximum stress. The time to failure for cyclic loading with a longer hold time at maximum stress is very close to the static loading lifetime. Viscous boundary phase may be the primary contributor to the improved cyclic fatigue resistance for cyclic loading with a short duration of maximum stress.     

Static and Cyclic Fatigue of Alumina at High Temperatures: II, Failure Analysis

Failure mechanisms of an alumina, tested at 1200°C under static and various cyclic loading conditions, were examined. Slow crack growth of a single crack is the dominant mechanism for the failure in specimens under cyclic loading with a short duration of maximum stress at all applied stress levels, as well as at high applied loads for static loading and cyclic loading with a longer hold time at maximum stress. At low stress levels, failure of static loading and cyclic loading with a longer hold time at maximum stress might occur by formation and/or growth of multiple macrocracks. More importantly, for all the given loading conditions. The viscous glassy phase behind the crack tip could have a bridging effect on the crack surfaces. A simplified model for calculating effective stress intensity factor at the crack tip under static and various cyclic loading demonstrated a trend consistent with the stress–life data.     

Influence of Loading Frequency on the Room-Temperature Fatigue of a Carbon-Fiber/SiC-Matrix Composite

The influence of cyclic loading frequency on the tensile fatigue life of a woven-carbon-fiber/SiC-matrix composite was examined at room temperature. Tension-tension fatigue experiments were conducted under load control, at sinusoidal frequencies of 1, 10, and 50 Hz. Using a stress ratio (σ_min/σ_max) of 0.1, specimens were subjected to maximum fatigue stresses of 310 to 405 MPa. There were two key findings: (1) the fatigue life and extent of modulus decay were influenced by loading frequency and (2) the postfatigue monotonic tensile strength increased after fatigue loading. For loading frequencies of 1 and 10 Hz, the fatigue limit (defined at 1 × 10⁶ cycles) was approximately 335 MPa, which is over 80% of the initial monotonic strength of the composite; at 50 Hz, the fatigue limit was below 310 MPa. During 1- and 10-Hz fatigue at a maximum stress of 335 MPa, the modulus exhibited an initially rapid decrease, followed by a partial recovery; at 50 Hz, and the same stress limits, the modulus continually decayed. The residual strength of the composite increased by approximately 20% after 1 × 10⁶ fatigue cycles at 1 or 10 Hz under a peak stress of 335 MPa. The increase in strength is attributed in part to a decrease in the stress concentrations present near the crossover points of the 0° and 90° fiber bundles.     

The hygrothermal effect on the fatigue behavior of carbon fiber/ peek laminate composites (I)

The hygrothermal effects on the fatigue behavior of the Carbon/PEEK laminated composites before and after impact damage were examined in this study. The [0/45/90/-45]_2s AS-4/PEEK laminated composites were immersed in 80°C hot water for 45, 90 and 200 days,and subjected to falling weight impact with an energy of 8.58 J and then immersed in 80°C hot water for 45 days. It was found that the tensile strength of AS-4/PEEK laminated composites decreased with the increase of exposure period. The injured AS-4/PEEK composites were subjected to a static load and a tensiontension fatigue load at various levels of stress amplitudes. The effect of stress amplitude on the fatigue life was studied. The experimental fatigue life under different stress amplitude tests were estimated by the median rank order statistic cumulative distribution function. Then,the fitting curves for estimated data were analyzed by the Weibull distribution function. The S-N curves for a series of cyclic loads at various survival probabilities were presented. The damage behaviors of composites after fatigue load test were also investigated by scanning electron microscope(SEM). Results indicated the fatigue lives of immersed specimens were shorter than those without hygrothermal effect, the impact damage affects the fatigue life of composite significantly.     

Fatigue behavior and damage evolution of SiC/SiC composites under high-temperature anaerobic cyclic loading

In the present study, the fatigue behavior and damage evolution of SiC/SiC minicomposites at elevated temperatures in oxygen-free environment are investigated which are important for their application and are still unclear. The high-temperature fatigue test platform is developed and the fatigue stress-life curve and the stress-strain response are obtained. The test result shows that the life of the material at elevated temperature is shorter than that at room temperature under the same stress level. Moreover, the hysteresis loop width and the residual strain increase with the increasing of the cycles while the hysteresis modulus decreases during the fatigue cycling. The evolution process of matrix cracks is observed using the real-time remote detection system. It is found that matrix cracking is insensitive to the cyclic loading which is similar to room temperature and is due to that the degeneration of the interfacial shear stress reduces the area of high stress in matrix. The fiber/matrix interfacial shear stress under different cycles is determined based on the fatigue modulus of each hysteresis loop. The result shows a fatigue enhancement phenomenon for the interface which is not observed at room temperature.     

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     

Influence of Stress Ratio on the Elevated-Temperature Fatigue of a Silicon Carbide Fiber-Reinforced Silicon Nitride Composite

The influence of stress ratio on the tensile fatigue behavior of a unidirectional SiC-fiber/Si₃N₄-matrix composite was investigated at 1200°C. Tensile stress ratios of 0.1, 0.3, and 0.5 were examined. Fatigue testing was conducted in air, at a sinusoidal loading frequency of 10 Hz. For peak fatigue stresses below the proportional limit of the composite (approximately 195 MPa at 1200°C) specimens survived 5 × 10⁶ cycles, independent of stress ratio. At peak stresses above the proportional limit, fatigue failures were observed; fatigue life decreased significantly as the stress ratio was lowered from 0.5 to 0.1. Creep appears to be the predominant damage mechanism which occurs during fatigue below the proportional limit. Both mechanical cycle-by-cycle fatigue damage and creep contribute to specimen failure at peak stresses above the proportional limit.     

Effect of Heat Treatments on the Crack-Healing and Static Fatigue Behavior of Silicon Carbide Sintered with Sc2O3 and AlN

Crack-healing behavior of silicon carbide ceramics sintered with AlN and Sc₂O₃ has been studied as a function of heat-treatment temperature and applied stress. Results showed that heat treatment in air could significantly increase the indentation strength whether a stress is applied or not. After heat treatment with no applied stress at 1300°C for 1 h in air, the indentation strength of the specimen with an indentation crack of ∼100 μm (≈2c) recovered its strength fully at room temperature. In addition, a simple heat treatment at 1200°C for 5 h under an applied stress of 200 MPa in air resulted in a complete recovery of the unindented strength at the healing temperature. However, higher applied stress led to fracture of the specimens during heat treatment. The static fatigue limit of the specimens crack healed at 1200°C for 5 h under 200 MPa was ∼450 MPa at the healing temperature. The ratio of the static fatigue limit of the crack-healed specimen to the unindented strength was ∼80%.     

Effect of crack-healing and proof-testing procedures on fatigue strength and reliability of Si3N4/SiC composites

A Si₃N₄/SiC composite was hot-pressed. Using this material, fatigue tests on crack-healed and proof-tested specimens were conducted at 1000–1400 °C. A surface elliptical-crack of about 110 μm in diameter was introduced on the specimens using a Vickers hardness indenter. The crack-healing was performed at 1300 °C for 1 h in air, mainly. The fatigue limit of the crack-healed and proof-tested specimen (C.P specimen) decreased slightly with increasing test temperature. However, the crack-healed specimen is not sensitive to low-cycle fatigue up to 1400 °C, and the fatigue limit is almost equal to the minimum bending strength at each temperature. To investigate the reason, the crack-healing behavior under cyclic stress was carried out systematically at 1200 °C in air. A 110 μm surface crack could be healed perfectly at 1200 °C in air under cyclic stress with a frequency of 0.001–5 Hz. From this, it can be concluded that [crack-healing+proof test] and crack-healing during service are useful techniques for maintaining structural integrity of these ceramic components.     

反复荷载下GFRP筋与混凝土黏结性能

对直径为16 mm,埋深分别为4d、5d、8d的玻璃纤维增强(GFRP)筋标准立方体拉拔试件进行静载和反复荷载作用下的拉拔试验,研究了2种不同应力水平（60 %Fm、80 %Fm）的反复荷载作用下GFRP筋与混凝土之间的黏结滑移关系,埋深与黏结强度关系,黏结刚度、加载端滑移量随循环次数的演变规律,得到了反复荷载下黏结滑移滞回曲线变化规律。结果表明,反复荷载下较少的循环次数对黏结强度和滑移量的影响不大;当反复荷载应力水平不高、循环次数较少时,黏结强度没有显著的退化,反而在一定程度上有所增加;较高应力水平反复荷载下,GFRP筋与混凝土之间的黏结强度退化较显著。     

Cyclic Fatigue of Hot-Pressed Si3N4

The cyclic fatigue behavior of two grades of hot-pressed silicon nitride was investigated. Flat, cantilever-type specimens were tested at temperatures up to 1300°C, in air, where the load was applied by an eccentric driver rotating at 1800 rpm, with a zero mean stress. The lifetime of the lower purity material at temperatures up to 1200°C was controlled by a stress corrosion mechanism. Above 1200°, and for both grades of material, plastic deformation, probably by grain boundary sliding, was rate controlling.     

Accelerated fatigue properties of unidirectional carbon/epoxy composite materials

It has been confirmed that polymer matrix composites possess viscoelastic behavior. This means that one could accelerate the fatigue testing by changing the stress amplitude, frequency, or temperature. This study is to investigate the accelerated fatigue properties, which are resulted from the viscoelastic behavior, of carbon/epoxy composites and to predict their fatigue life. For this purpose, a series of fatigue tests of unidirectional specimens are conducted at room temperature under different stress ratios and stress frequencies. A group of sigmoid S‐N curves, which are suitable for the whole fatigue life, and the corresponding parameters are developed for different cyclic loading conditions. A transformation method, which can transform a reference S‐N curve to the corresponding S‐N curve of the assigned fatigue conditions, is established by the parameters. And this S‐N curve can be utilized to predict the fatigue life of the composite at the assigned stress ratio or stress frequency. The comparison between the linear and sigmoid S‐N curves is also carried out to show the advantages of the latter model in the whole fatigue life. POLYM. COMPOS., 27:138–146, 2006. © 2006 Society of Plastics Engineers     

Statistical fatigue strength evaluation and inelastic deformation generated during static and cyclic loading in Ce-TZP/alumina nanocomposite: Part 1—in air environment

The statistical fatigue strength evaluations of an intragranular type Ce-TZP/Al₂O₃ nanocomposite (Ce-TZP/A-N), such as its initial strength, static and cyclic fatigue lives, and its dispersion, were investigated in comparison to 3Y-TZP. The strength degradations during static and cyclic loading of Ce-TZP/A-N were fairly small, and the dispersions of the fatigue life were also quite small compared to those of 3Y-TZP, especially for the case of cyclic loading. In addition, fairly large inelastic deformations (converted strain ≈0.1–0.3%) were observed in the non-failure fatigued specimens after both static and cyclic loading. The amount of inelastic deformations was generally higher under the static loading than under cyclic loading, and increased with increasing the applied stress. In contrast, no inelastic deformation was identified for 3Y-TZP. By means of X-ray diffraction analysis, a good correlation between the amount of inelastic strain and the transformed monoclinic content was recognized for both static and cyclic loading.     

Apparent Enhanced Fatigue Resistance under Cyclic Tensile Loading for a HIPed Silicon Nitride

Cyclic tensile loading tests of a commercial HIPed silicon nitride at elevated temperatures have indicated apparent "enhanced" fatigue resistance compared to static tensile loading tests under similar test conditions. At 1150°C, stress rupture results plotted as maximum stress versus time to failure did not show significant differences in failure behavior between static, dynamic, or cyclic loading conditions, with all failures originating from preexisting defects (slow crack growth failures). At 1260°C, the stress rupture results showed pronounced differences between static, dynamic, and cyclic loading conditions. Failures at low static stresses (<175 MPa) originated from environmentally assisted (oxidation) and generalized creep damage, while failures at similar times but much greater (up to 2 x) cyclic stresses originated from preexisting defects (slow crack growth failures). At 1370°C, stress rupture results did not show as pronounced differences between static, dynamic, and cyclic loading conditions, with most failures originating from environmentally assisted (oxidation) and generalized creep damage.     

Dependence on mean stress and stress amplitude of fatigue life of EPDM elastomers

Abstract

This paper describes the dependence on test parameters of the fatigue resistance of EPDM. Fatigue was investigated using dumbbell specimens under load control at 1 Hz until failure. Tests were made in order to create a common Wöhler (S–N) curve while increasing the stress amplitude and also to show the influence of increasing minimum stress at constant stress amplitude on fatigue properties. The results of these tests confirmed the well known amplitude dependence of fatigue life in filled rubbers. An additional significant influence on fatigue life is seen to be the minimum stress applied during each cycle to these materials. Fatigue life is not dependent on strain crystallisation in EPDM as it is for natural rubber. The results of this research give component designers the opportunity to increase the fatigue lives of components made from this material.     

Cyclic Fatigue of Ce-TZP/AI2O3 Composites: Role of the Degradation of Transformation Zone Shielding

The origin of cyclic fatigue in two Ce-TZP/Al₂O₃ composites was investigated by (a) measurements of residual stresses in the transformation zones and crack-tip stress intensities in in situ loaded compact specimens using microprobe Raman spectroscopy, (b) examination of the crack-tip transformation zones by transmission electron microscopy, and (c) measurements of crack-growth rates in cyclic fatigue and in sustained loading at 400°C, a temperature at which stress-induced transformation of the tetragonal zirconia to the monoclinic polymorph was suppressed. Transformation zones formed during cyclic fatigue consistently showed lower compressive residual stresses and higher crack-tip stress intensities than the zones formed in sustained loading. Transmission electron microscopy revealed monoclinic laths of smaller average twin spacing and of multiple types of lattice correspondence in the transformation zones of the fatigue specimens as compared to the sustained-load specimens. Crack-growth measurements at 400°C indicated a significant suppression of the cyclic fatigue effect in the absence of transformation plasticity. These results in combination pointed to degradation of transformation-zone shielding as an important contributing cause of cyclic fatigue in Ce-TZP/Al₂O, composites. A more efficient accommodation of the transformation strains within the zones appears to be the underlying mechanism of the degradation of zone shielding in cyclic fatigue.     

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.