Influence of Temperature and Humidity on Dynamic Fatigue of Optical Fibers

The influence of temperature and humidity on dynamic fatigue characteristics of silicone resin-coated optical glass fibers is examined. Dynamic fatigue is shown to result from subcritical crack growth. Test results verify that subcritical crack growth during dynamic loading is a thermally activated process and that the effect of humidity on crack growth is a function of water vapor pressure.     

Effect of Humidity on Small-Crack Growth in Silica Optical Fibers

Subcritical small-crack growth in silica optical fibers was measured directly in air at 25°C for several humidity conditions from 30% to 90% relative humidity. The crack velocity data were shown to be closely fitted by a power function of the stress intensity factor. It was found that the power n is independent of humidity and takes a value of 21.7. It was also revealed that crack velocity varies with relative humidity to a power of 2.5, which agrees with estimates from the published dynamic fatigue data concerning the humidity dependence of the strength of pristine silica fibers.     

Dependency of Fatigue Predictions on the Form of the Crack Velocity Equation

A computer search technique was developed to analyze fatigue strength data using both exponential and power law forms of the subcritical crack velocity equation. All crack velocity equations would fit a given set of fatigue data equally well in the data range but failure predictions based on the different crack velocity equations diverge from each other outside the data range. The exponential form of the crack velocity equation best fit both the static and dynamic fatigue data of hot-pressed Si₃N₄ and optical glass fibers, whereas the power law form best fit the static and dynamic fatigue data of soda-lime glass and A1₂O₃. To determine the most appropriate crack velocity equation for a given material/environment system, it is recommended that fatigue data be obtained under different loading conditions and the data numerically regressed using the computer search technique with each of the possible crack velocity equations to find which best fits the data.     

Fatigue Mechanisms in High-Strength Silica-Glass Fibers

We use experimentally determined crack growth data for silica glass and a fracture mechanics model for delayed failure to predict the fatigue behavior for high-strength silica-glass fibers. The results of this model indicate that fracture mechanics methods can be used to adequately describe the fatigue behavior observed for high-strength silica-glass fibers at room temperature in humid conditions. The key feature to properly interpreting the fatigue of high-strength fibers is the use of a fracture-rate law in which the crack extension rate increases exponentially with applied stress. We show that a fracture mechanics approach to highstrength fiber fatigue can provide the basis for identifying additional fatigue mechanisms that may control failure in more aggressive fatigue environments.     

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.     

Oxidation of Silicon Carbide Fibers During Static Fatigue in Air at Intermediate Temperatures

The static fatigue of SiC-based fiber bundles and single fibers has been examined in previous papers, with emphasis placed on the analysis of the stress–rupture time data, and on the modelling of delayed failure from slow crack growth. The present paper investigates the oxidation of the fibers during static fatigue, at temperatures in the intermediate temperature range (500°–800°C). Two oxidation-induced phenomena have been evidenced: the formation of a thin silica film at the surface of fibers and the delayed failure of fiber bundles and single filaments. The stress–rupture time data are interpreted with respect to the chemical and structural characteristics of fibers, and to the oxide film growth rate. The structural analysis of the fibers was carried out using scanning electron microscopy and Auger electron spectroscopy. Delayed failure was found to result from slow crack propagation from surface defects, as a result of the consumption of the free carbon at grain boundaries and the local stresses induced by the SiC→SiO₂ transformation at the crack tip. The respective contributions of these phenomena to static fatigue are discussed.     

Fatigue crack growth model and mechanism of a random fiber SMC composite

The primary purpose of this study is to investigate the fatigue crack growth rate and mechanism in a random fiber sheet molding compound (SMC) composite. The fatigue crack growth rate in this study was measured using a compliance approach. The fatigue crack growth rate in the SMC studied here shows a good agreement with a new power law model proposed earlier. This new model takes into account not only the effect of the stress intensity factor range, but also the effect of crack growth at various stages of loading using a weight average stress intensity factor. It was observed that this new model can represent the fatigue crack growth rate of the SMC at three different load ratios in a single unifying curve.     

Fatigue fracture of long fiber reinforced nylon 66

The fatigue behavior of long fiber reinforced nylon 66 has been investigated by measuring fatigue crack propagation rates of injection molded samples. Plaques varying in thickness from 3 to 10 mm were employed for nylong 66 containing either glass, carbon or aramid fibers. Both conventional chopped, short fiber reinforcements and pultruded long fiber filled nylon 66 were examined. Long fiber reinforced nylon 66 exhibits improved fatigue resistance as shown by decreases in fatigue crack propagation rates compared to short fiber filled composites. Using a fracture mechanics analysis, it is shown that the improvements are due primarily to the higher moduli of the long fiber reinforced nylon 66, with only a slight increase in the calculated strain energy release rate associated with fatigue crack growth. For short or long glass fibers, and for short carbon fibers, the effects of fiber orientation on fatigue crack growth rates can be predicted from the fracture mechanics model. More significant effects of fiber length on fatigue fracture energies are noted for long aramid and long carbon reinforced nylon 66. It is also shown that thicker plaques can exhibit poorer fatigue fracture behavior owing to their inferior core sections.     

弯曲载荷作用下孔边角裂纹的应力强度因子与疲劳扩展规律

本文运用边界元方法提出了弯曲载荷作用下,平板中孔单边角裂纹裂尖的应力强度因子(SIF)的计算式。并通过实验研究了孔单边角裂纹在交变的弯矩作用下,裂纹的形貌变化规律及裂纹扩展速率,指出角裂纹沿长度方向的扩展可以用Paris公式来描述,而沿深度方向则不然,但两者之间具有幂函数的关系。     

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.     

A stochastic approach for continuous and discontinuous crack growth in polycarbonate under cyclic loading

This article presents a stochastic approach to predict fatigue crack propagation (FCP) diagrams of continuous crack growth (CCG) and discontinuous crack growth (DCG) in polycarbonate (PC) under cyclic loading. First, it is assumed that the macroscopic fatigue crack propagates stochastically. The transition probability is then expressed in conjunction with the craze fibril breakdown model for CCG. Second, the stochastic process is applied to DCG assuming that DCG occurs because of an unstable crack growth in the craze zone. A fracture criterion using a stress intensity factor is introduced for the unstable crack growth. As a result, we obtain an FCP diagram where the rate in CCG is lower than that in DCG. The stress intensity factor range for the DCG–CCG transition can be theoretically determined. Finally, to verify the present approach, the experimental data of DCG and CCG of PC are fitted to the Paris equation. In addition, the relationship between the DCG band size and the number of cycles required for DCG is predicted in order to compare it with the experiment data. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers     

Mixed-mode fatigue behavior of degraded toughened epoxy adhesive joints

Open-faced asymmetric double cantilever beam (ADCB) specimens of toughened epoxy-aluminum adhesive joints were aged either in a constant humidity environment or a cyclically changing environment to study the mixed-mode fatigue behavior. Under constant humidity environments, the fatigue threshold strain energy release rate initially decreased with aging time until it reached a constant minimum value for long times. In contrast, the crack growth rates continued to increase with aging time. It is hypothesized that at crack growth rates close to threshold the fatigue behavior is governed by the epoxy matrix, whereas at relatively high crack growth rates the fatigue behavior is governed by the loss of the rubber toughening mechanism. Increasing the aging temperature accelerated the degradation of the joints leading to a reduction in the time to reach the constant minimum value and increased the crack growth rates.Under a cyclic aging environment with intermittent salt spray, neither the threshold strain energy release rate nor the crack growth rates degraded until four weeks of aging. The superior fatigue performance of these joints compared to joints aged in constant humidity environments was due to the lower water concentrations in the adhesive while aging. This conclusion was supported by moisture uptake measurements of the adhesive in deionised and salt water environments that showed simple Fickian behavior at room temperature and dual-Fickian behavior at higher temperature. The salt water environment produced osmotic pressure that decreased the moisture concentration in the second stage of diffusion.     

Environmental Fatigue of Silicate Glasses in Humid Conditions

Failure strains of commercial silica, soda-lime-silicate, and E-glass fibers were measured using two-point bending in room temperature humid air. Humidity dependence and dynamic fatigue behavior were studied, and the fatigue reaction orders in terms of humidity were determined. In the humidity range tested (~0.1% to ~100%), the dynamic fatigue parameters for silica and E-glass are found to be greater in lower humidity (~0.1% to ~10%), whereas the fatigue parameter for soda-lime-silicate is independent of humidity. The humidity dependence of failure strains for all three glasses was more pronounced in high humidity (~10% to ~100%) than in low humidity (~0.1% to ~10%), indicating that the reaction order decreases with decreasing humidity. These observations were correlated with the different structures of the glasses and their corresponding fatigue mechanisms.     

氧化锆增韧陶瓷的室温动态疲劳

用动态疲劳试验法研究了３Ｙ－ＴＺＰ和３Ｙ－ＴＺＰ／Ａｌ_２Ｏ_３（２０ｗｔ％）陶瓷在空气中的室温动态疲劳，并讨论了疲劳慢裂纹扩展特性。另外利用动态疲劳数据对两种陶瓷的平均寿命进行了预测。两种陶瓷材料在室温下均存在着慢裂纹扩展，主要是由空气中水蒸汽的应力腐蚀所造成的，且裂纹是沿晶界玻璃相扩展的。相变诱发的表面压应力和裂纹尖端的正应变可提高疲劳抗力。在８００ＭＰａ应力作用下，３Ｙ－ＴＺＰ和３Ｙ－ＴＺＰ／Ａｌ_２Ｏ_３（２０ｗｔ％）的平均寿命分别为２４ｍｉｎ和７２ｈ，平均寿命随应力的增大而缩短．     

Structural changes in polyester fibers during fatigue

Structural changes occurring during the fatigue failure of polyester fibers have been identified, and a comparison has been made with untested fibers and fibers which were subjected to cyclic loading conditions which did not produce fatigue. Fatigue failure was seen to result in a distinctive fracture morphology. Infrared spectrometry and X-ray diffraction revealed a lowering of crystallinity under fatigue conditions but not under other loading conditions. Transmission electron microscopy and electron diffraction revealed the creation of amorphous zones which are supposed as coalescing to form an amorphous band seen along and ahead of the fatigue crack. The zone just ahead of the fatigue crack tip is shown to contain voids. Crack propagation involves, therefore, the joining up of these voids and development along the amorphous band.     

Simulation of Mixed-Mode I/II Fatigue Crack Propagation in Adhesive Joints with a Modified Cohesive Zone Model

A model to predict fatigue crack growth in bonded joints under mixed mode I/II conditions is developed in this work. The model is implemented in the finite element software ABAQUS using the related USDFLD subroutine. The present model is based on the cohesive zone (CZ) concept, where damage develops according to the value of the opening/sliding at the bondline under static loading, and according to a cyclic damage accumulation law under fatigue loading. The damage accumulation law is obtained by distributing the cyclic crack area increment over the process zone ahead of the crack tip, where the cyclic crack area increment is calculated according to a Paris-like law that relates the crack growth rate to the applied loading. In this way, the experimental crack growth rate is related directly to damage evolution in the cohesive zone, i.e., no additional parameters have to be tuned besides the quasi-static cohesive zone parameters.     

Delayed Failure of Hi-Nicalon and Hi-Nicalon S Multifilament Tows and Single Filaments at Intermediate Temperatures (500°–800°C)

Previous results have shown that tows of SiC Nicalon fibers are sensitive to the phenomenon of delayed failure, at temperatures below 700°C. The present paper examines the static fatigue of Hi-Nicalon and Hi-Nicalon S when subjected to constant load, at temperatures between 500° and 800°C in air. Multifilament tows and single filaments were tested. Experimental data show that the rupture times of tows depend on the applied stress according to the conventional power law t σ ⁿ = A . In contrast, the stress-rupture time data obtained on single filaments exhibit significant scatter. A model based on slow crack growth in single filaments shows that the stress-rupture of fiber tows follows the conventional time power law. The dependence on temperature was introduced. The model allowed sound calculations of tow lifetimes and characteristics of the slow crack growth phenomenon to be extracted from the tow stress-rupture time data.     

Predicted Static Fatigue Behavior of Specially Coated Optical Glass Fibers

Long-term static fatigue behavior of optical glass fibers is controlled by their strength, fatigue resistance, and zerostress aging behavior. The effectiveness of four special coatings in preventing the long-term static fatigue deterioration of optical glass fibers was evaluated by determining the dynamic fatigue behavior and the effects of zero-stress aging on strength of the four specially coated optical glass fibers in water from 25° to 85°C. The results clearly show that the strength, fatigue resistance, and aging behavior varied significantly between these specially coated fibers. By analysis of these experimental results in terms of fracture mechanics principles, the predicted static fatigue behaviors of the four fibers were compared. Ideally the optimum fiber is one that exhibits a high strength, low strength variability, high fatigue resistance, and high aging resistance. Each of these specially coated fibers had a deficiency in at least one of these properties.     

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.