The effect of water immersion on fatigue crack growth of two engineering rubbers

Fatigue properties of two engineering rubbers have been measured in air and water. The fatigue crack growth rate, dc/dN, where c is the crack length and N the number of cycles, was measured as a function of tear energy for chloroprene rubber (CR) and natural rubber (NR). In general, the effect of water immersion on crack growth rates was relatively small. For NR, little effect of water immersion was seen and the fatigue threshold, which is the limit below which no mechano-oxidative fatigue growth will occur, was measured as 25 J/m² in both environments. For CR, a factor of two to three times lower crack growth rates was obtained in water compared to air, probably due to less influence of oxygen in water. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 941–946, 1998     

Characterisation of subcritical crack growth in ceramics using indentation cracks

Abstract

The introduction of indentation cracks into brittle materials has proved to be a useful tool in characterising subcritical crack growth, notably in efficiently measuring the kinetic growth parameters and in defining whether the material exhibits a fatigue threshold. The accuracy of the subcritical crack growth parameters obtained using indentation mechanics can be excellent, provided the stress intensity factor associated with the indentation cracks is well characterised. Indentation cracks can also be used to measure crack velocity as a function of the applied stress intensity factor by direct observation. In such testing, it is critical that the changes in crack shape as the crack extends are known or accurately predicted. Numerical simulations suggest that the shape changes can be influenced not only by the testing geometry but also the growth kinetics. Finally, it is shown that the fatigue threshold can be determined by allowing median cracks to extend subcritically during indentation.     

The effect of a prefabricated crack on the crack growth in ceramics during quenching

The influence of a prefabricated crack on thermal-shock cracking during quenching is studied in real-time. The results show that after the thermal-shock crack extends to the prefabricated crack, the secondary crack may appear at the lower end of the prefabricated crack. The total vertical length of the crack and the probability of the secondary crack occurrence will gradually increase with the prefabricated crack angle. Besides, the influence of the prefabricated crack distance from the edge on thermal-shock crack growth is also considered. The simulation results of meso-damage mechanics are consistent with experimental observation. This article quantitatively investigates the effect of the prefabricated crack on the thermal-shock crack propagation in ceramics, expanding the research on the mechanism of thermal-shock failure.     

Modelling damage and creep crack growth in structural ceramics at ultra-high temperatures

A continuum damage model based on multiaxial ductility exhaustion of accumulated creep strains is proposed to predict creep crack growth (CCG) in structural ceramics at ultra-high temperatures where it is known that power law creep operates. The paper focuses on monolithic ZrB₂ ultra-high temperature ceramic (UHTC), for which a reasonable set of material creep data is available. The predominant deformation mechanism shown by ZrB₂ at temperatures greater than 1800 K and at stresses above 200 MPa is power law creep. Using the creep constitutive properties that have been found for this material, the proposed methodology is applied to a representative three-point bend geometry, which is planned to be tested. Relevant Fracture Mechanics parameters such as stress intensity factor, K, and steady state creep parameter, C^*, are evaluated and compared with available models. In this way the essential properties required to develop predictive damage simulations are investigated, underlining the importance of having accurate material test data.     

Failure by slow crack growth of SiC fibers: Comparing the lifetime scatter for single filaments and multifilament tows

SiC-based fibers are subjected to slow crack growth, a crack growth mechanism activated by stresses and the chemical environment, as identified by static fatigue testing. Such tests can be performed on filaments or bundles. Although both types of specimens show a similar lifetime variation, testing of bundles is often preferred. This work compares the scatter of lifetimes predicted for filaments and for tows using a dedicated Monte Carlo simulation tool relying on the following hypotheses: The stress applied to a single filament can be defined, whereas the size of its most critical flaw cannot; on a bundle, neither the applied stress nor the strength of the critical filament (which triggers the cascade failure and the tow failure) can be defined. Depending on the parallelism of fibers inside the bundle and their strength dispersion, the lifetime scatter can be narrower for filaments compared to tows or vice versa.     

Effect of ceramic matrix compositions on pore structure and properties of lightweight foamed ceramics

Foamed ceramics were prepared with polishing slag and shale as raw materials. According to the phase diagram of CaO-MgO-Al₂O₃-SiO₂, a reasonable composition system was designed to obtain foamed ceramics with good property. The related properties of foamed ceramics were characterized by X-ray fluorescence, X-ray diffraction, thermogravimetric analysis/differential scanning calorimetry, field emission scanning electron microscopy, energy disperse spectroscopy, etc. The results indicate that, when the amount of polishing slag and shale are 30% and 60% respectively, foamed ceramic with bulk density of 0.225 g·cm⁻³ has good compressive strength and thermal conductivity, which are 1.95 MPa and 0.065 W/m·K, respectively. The element compositions of the matrix materials affect the melt viscosity, resulting in glass phase separation and the crystallization. The main phases of the foamed ceramics are anorthite CaSi₂Al₂O₈, spinel MgAl₂O₄ and spinel Mg(Al, Fe)₂O₄. The internal relationship among melt, pores, and crystal precipitates was discussed.     

High-temperature fatigue crack propagation mechanism of orthogonal 3-D woven amorphous SiC Fiber/SiC/YSi2-Si matrix composites

To elucidate degradation mechanisms attributable to high-temperature fatigue crack propagation, a study was conducted of 3-D woven SiC_f/SiC CMC in which amorphous SiC fiber was used as a reinforcement material and in which a matrix was formed through low-temperature melt infiltration. From a high-temperature fatigue test conducted at 1373 K in the atmosphere with stress of 142 MPa or more, the fracture lifetime of newly developed SiC_f/SiC CMC was found to be longer than that of SiC_f/SiC CMC, which uses crystalline SiC fiber. Furthermore, repeatedly applying high temperatures during high-temperature fatigue tests and using X-ray computed tomography, fatigue cracks were found to propagate in a direction across 0-degree fiber bundles that undergo stress. Electron mapping of regions with crack propagation revealed that oxidation eliminates boron nitride (BN), which has a crack deflection effect. The SiC fibers and matrix are fixed through the formation of oxides. Cracks propagate because of the consequent decrease in toughness of the SiC_f/SiC CMC. In regions without crack propagation, fracture surfaces were not covered with oxides. These regions underwent forcible fracture in the final stage of the high-temperature fatigue tests. From the test results presented above, SiC_f/SiC CMC is considered to undergo fracture when the effective cross-sectional area is reduced because of crack propagation accompanying oxidation and when the test load exceeds the tensile strength of the residual cross-sectional area. However, some cracks in the matrix produced by a low-temperature melt infiltration process were closed by oxides derived from YSi₂. Because of crack closing, crack propagation is presumed to be avoided. Also, LMI-CMC showed excellent high-temperature fatigue properties at pressures higher than 150 MPa, which exceeds the proportional limit.     

Predicting the failure by slow crack growth of SiC-based multifilament tows: Batch-to-batch discrepancies as linked to fiber slack

This work investigates the scale effect observed on slow crack growth parameters for SiC-based fibers (Nicalon® and Tyranno® ZMI) and how it can be affected by the variability from a batch of tow to another one. A 10% difference on the stress exponents (n_t) was numerically estimated using a dedicated Monte Carlo simulation. This effect is however marginal when compared to the lifetime scatter itself. This partly originates from the fiber slack in used tow specimen, assessed by tensile test completing the experimental work. The number of tensile tests to be performed is discussed. Specimen gauge length is an additional and significant source of discrepancies affecting the fiber alignment and subsequently the tow stress exponent.     

Simulating the variability and scale effect for slow crack growth in Hi-Nicalon SiC-based tows: A parametric study

The delayed failure time of SiC-based multifilament tows under static fatigue condition is as broadly scattered as for individual filaments, despite it is commonly used as strong scatter reduction. Moreover, the stress exponent (n) is hierarchy-dependent as revealed by a Monte-Carlo algorithm: decreasing from filament (micro) scale to tow (meso) scale. This is demonstrated to originate from the mismatch between the stress applied to the critical filament (affecting the growth kinetics), variable because of fiber misalignment in the tow, and its tow-averaged value used for endurance diagram construction. In the context of this algorithm, it is shown that n would evolve with fiber parallelism, tow stress range or the critical filament rank whereas filament strength distribution plays a secondary role. The tow structure shall therefore be considered as a major parameter for composite part design aiming long service life.     

Effect of Atmospheric Humidity on the Fatigue Crack Propagation Behavior of Short Cracks in Silicon Nitride

The effect of the environment on crack-growth processes in silicon nitride was studied by investigating the static and fatigue crack-growth behavior of small surface cracks, as influenced by testing (i) in the ambient environment, (ii) in distilled water, (iii) under vacuum, and (iv) in toluene. A principal finding was that testing under cyclic conditions led to crack-growth rates that were much higher in air than in toluene, whereas testing under static conditions in air or toluene led to minor differences in the rate of static fatigue crack growth. This difference in sensitivity to the environment under static and cyclic loading conditions was attributed, in part, to a much-greater extent of microcracking at the surface ahead of the main crack in air under cyclic conditions, in comparison to that in other environments. This propensity for microcracking at the surface in air under cyclic conditions also was reflected in the aspect ratios of the crack shapes that developed.     

Systematic investigations of fatigue crack growth behavior of a PE‐HD pipe grade in through‐thickness direction

Fatigue crack growth (FCG) experiments were performed on a commercial high‐density polyethylene (PE‐HD) pipe grade. To investigate the influence of different specimen types on FCG results, tests were conducted using compact tension (CT) specimens and cracked round bars (CRB). The effects of frequency and R‐ratio on FCG behavior were also studied. Furthermore, FCG tests were interrupted in the region of stable crack propagation. The crack front and the front of the process zone ahead of the crack were systematically characterized via microscopic methods in the thickness direction of the specimen. The experimental data are employed to study the mechanisms of process zone development and to determine the effective crack length by compliance relationships. This detailed information allows modeling FCG in PE‐HD at various positions in the thickness direction of the specimen. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103:1745–1758, 2007     

Effect of water and other dielectrics on subcritical crack growth in portland cement paste

In studying the effect of water and a series of aliphatic alcohols on subcritical crack growth in cement paste, log crack velocity-stress intensity factor curves were obtained by means of a double-torsion technique. The relative position of the curves was found to be dependent on the dielectric constant of the test media. Flexural strength of cement paste saturated in organic fluids is also dependent on the dielectric constant. Stress corrosion processes involving chemical attack of Si---O bonds in cement paste appear to be operative.     

亚临界水中盐的析出规律研究

在亚临界条件下,研究氯化钠和硫酸钠在亚临界水中的析出规律,主要考察了温度、压力对盐的去除率的影响。结果表明:氯化钠在350℃,5 MPa的条件下去除率达到最大;硫酸钠在350℃,4 MPa的条件下去除率达到最大。通过研究两种盐的析出规律,可以为超临界水氧化装置的预热反应器中预先去除废水中的无机盐(主要是氯盐和硫酸盐)提供科学依据,从而减缓和降低高盐对超临界设备的腐蚀和阻塞问题。     

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.     

Theory of slow,steady state crack growth in polymer glasses

Cracks in polymer glasses can grow slowly preceded by a craze, a narrow zone of plastic cavitation. The craze widens by drawing more polymer from its surfaces into its fibrils but the fibrils themselves fail by local creep. When the crack tip moves at velocity v the loading at the crack tip can be described by a local stress intensity factor K which is the sum of the ‘apparent’ stress intensity factor K_A and a plastic contribution K_p (usually negative). K_p is found to be $\text{?KP(K)}\text{v}$ where P(K) is an integral over the craze boundary displacement law. Fibril failure by local creep leads to a power law, v ∞ K^m. From these relations K and v can be determined as a function of K_A. The plot of K vs. K_A is multiple-valued with a stable branch (at high K) and an unstable branch (at low K) separated by a minimum value of K_A which represents a threshold for stable, steady state crack growth. There is also a v threshold, below which cracks will not grow steadily. These predictions, the form of the v?K_A curve and implications for slip-stick crack growth are compared with recent experiments.     

有机酸对甜高粱渣亚临界水解产物的影响

半纤维素的高效转化是提高甜高粱渣原料全组分利用的关键技术之一。采用亚临界水热预处理方法,并将强度因子R₀引入研究过程,考察了不同温度（160~200℃）和反应时间（10~60 min）对甜高粱渣水解反应的影响。在这基础上,进一步考察了多种有机酸（乳酸、醋酸及乳酸+醋酸）对亚临界水解效果的影响。实验表明,当强度因子lgR₀=3.96（180℃,40 min）时,采用不外加酸的亚临界水热预处理工艺得到的最大木糖浓度为4.79 g·L^-1;有机酸的加入可强化水解反应,提高木糖浓度;与单一乳酸或醋酸处理方法相比,加入同浓度的乳酸+醋酸既可以促进半纤维素水解,又可以抑制副产物生成;在温度180℃,时间40 min,乳酸+醋酸（乳酸：醋酸=6：4）的浓度1%（质量）的条件下,木糖浓度为7.92 g·L^-1。     

Vertical crack distribution effect on the TBC delamination induced by crack growth from the ceramic surface and near the interface

The propagation of vertical crack on the surface of thermal barrier coatings (TBCs) may affect the interface cracking and local spallation. This research aims to establish a TBC model incorporating multiple cracks to comprehensively understand the effects of vertical crack distribution on the coating failure. The continuous TGO growth and ceramic sintering are together introduced in this model. The influence of the vertical crack spacing and non-uniform distribution on the stress state, crack driving force, and dynamic propagation is examined. Moreover, the influence of coating thickness on the crack growth driving is also explored. The results show that large spacing will lead to early crack propagation. The uniform distribution of vertical cracks can delay the spallation. When the spacing is less than 4 times ceramic coat thickness, the cracking driving force will come in a steady-state stage with the increase of vertical crack length. Prefabrication of vertical cracks with spacing less than 0.72 mm on the coating surface can greatly decrease the strain energy. The results in this study will contribute to the construction of an advanced TBC system with long lifetime.     

Slow crack growth and hydrothermal aging stability of an alumina-toughened zirconia composite made from La2O3-doped 2Y-TZP

A very tough zirconia matrix is interesting to fabricate alumina-toughened zirconia (ATZ) and composites generally processed from 3Y-TZP do not exhibit very high toughness. The strategy of lowering the yttria content to increase toughness however is normally associated with an increased hydrothermal aging susceptibility. In this work, a 0.4 mol% La₂O₃ doped 2Y-TZP matrix was investigated to realize a 20 wt.% alumina toughened zirconia composite with a substantially high aging resistance. The higher transformation toughening in the composite shifted the V-K_I towards higher K_I values, while preserving the slope of the curve, resulting in a threshold K_I0 of 4.0 MPa m^1/2 and fracture toughness (K_IC) of 7.1 MPa m^1/2. These composites can offer a better compromise between aging and crack resistance than traditional 3Y-TZPs and plain ATZ composites without La₂O₃ doping.     

Origin of the Creep-Crack Growth Threshold in a Glass-Ceramic

The growth threshold, K _th, of large creep cracks in a glass-ceramic has been investigated to determine the causes of the arrest and nonpropagating behaviors below a characteristic stress intensity, K , value. Using replication techniques, the dominant creep demage mechanisms during creep-crack growth, which included grain-boundary cavity and microcrack formation, were identified as a function of K levels and crack extension rates. Quantitative measurements of cavity density, D ( r ), and microcrack density, Ψ( r ), revealed that the values of D ( r ) and Ψ( r ) both decrease with the K level and with increasing distance, r , from the crack tip at a given K level. At K values below K _th, the cavity density became zero as preexisting grain-boundary cavities were sintered, suggesting that the growth threshold originated from sintering of creep cavities. At K levels above K _th, microcracks located in the wake of the main crack were observed to be sintered also. Possible mechanisms responsible for the apparent self-healing of microcracks and creep cavities in the glass-ceramic are provided, together with theoretical and experimental support.