Contact Fatigue in Silicon Nitride

A study of contact fatigue in silicon nitride is reported. The contacts are made using WC spheres, principally in cyclic but also in static loading, and mainly in air but also in nitrogen and water. Damage patterns are examined in three silicon nitride microstructures: (i) fine ( F )-almost exclusively fully-developed cone cracks; (ii) medium ( M )-well developed but smaller cone cracks, plus modest subsurface quasi-plastic damage; (iii) coarse ( C )-intense quasi-plastic damage, with little or no cone cracking. The study focuses on the influence of these competing damage types on inert strength as a function of number of contacts. In the F and M microstructures strength degradation is attributable primarily to chemically assisted slow growth of cone cracks in the presence of moisture during contact, although the M material shows signs of enhanced failure from quasi-plastic zones at large number of cycles. The C microstructure, although relatively tolerant of single-cycle damage, shows strongly accelerated strength losses from mechanical degradation within the quasi-plastic damage zones in cyclic loading conditions, especially in water. Implications concerning the design of silicon nitride microstructures for long-lifetime applications, specifically in concentrated loading, are considered.     

Model for Cyclic Fatigue of Quasi-Plastic Ceramics in Contact with Spheres

A model of contact damage accumulation from cyclic loading with spheres and ensuing strength degradation in relatively tough, heterogeneous ceramics is developed. The damage takes the form of a quasi-plastic zone beneath the contact, consisting of an array of closed frictional shear faults with attendant "wing" microcracks at their ends. Contact fatigue takes place by attrition of the frictional resistance at the sliding fault interfaces, in accordance with an empirical degradation law, allowing the microcracks to extend. At large numbers of cycles or loads the microcracks coalesce, ultimately into radial cracks. Fracture mechanics relations for the strength degradation as a function of number of cycles and contact load are derived. Indentation–strength data from two well-studied coarse-grain quasi-plastic ceramics, a micaceous glass-ceramic and a silicon nitride, are used to evaluate the model. Comparative tests in static and cyclic contact loading confirm a dominant mechanical component in the fatigue. At the same time, the presence of water is shown to enhance the fatigue. The model accounts for the broader trends in the strength degradation data, and paves the way for consideration of key variables in microstructural design for optimum fatigue resistance.     

High-Temperature Strength of Melt-Infiltrated SiC-Mo(Al,Si)2 Composites

High-temperature fracture behavior of melt-infiltrated SiC-Mo(AI,Si)₂ composites was studied. The composites exhibited moderate strength up to ∼1600°C under a fast loading rate. The softening of the Mo(Al,Si)₂ phase determined dominantly the temperature over which the strength decreased. The composite with finer microstructure revealed a higher level of strength over the whole temperature range. At 1580°C, however, the composites revealed distinct strength degradation at a reduced loading rate.     

往复荷载下含孔洞缺陷衬砌混凝土的力学性能试验研究

通过添加不同含量的发泡聚苯乙烯(EPS)颗粒来预制目标孔隙率以模拟混凝土的不同孔洞缺陷,制备了不同孔隙率的C25和C30两种强度等级的混凝土试件,开展单调及往复荷载下含孔洞缺陷混凝土力学性能的试验研究,分析了混凝土试件破坏形态、强度、应变、弹性模量等随孔隙率的变化规律,探讨不同孔洞缺陷对混凝土力学性能的影响。试验结果表明:单调及往复荷载下,无预制孔洞缺陷的混凝土试件均表现为脆性破坏特征。但随着孔隙率的增加,混凝土试件的强度明显降低,应力-应变曲线逐渐趋于鱼肚状分布,试件由脆性向延性破坏转变,逐渐呈现多裂缝扩展特征,特别是对于往复加载情况。两种加载方式和两种强度等级条件下,试件主要力学参数随着预制孔隙率的增加均表现出一致的变化规律。峰值应力和弹性模量随孔隙率的上升呈指数下降,而峰值应变和应变极值呈线性增大。往复荷载下试件力学性能受孔隙率的影响程度均大于单调加载情况,而且,这种影响随混凝土强度的提高而减小。在往复加载过程中,孔隙率越大,峰值应力前试件的刚度比值越大,而峰后的刚度退化也愈严重。对于相同的孔隙率,混凝土强度等级越高,峰值应力前的刚度增长率及峰后的刚度退化程度越小。     

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。     

Durability and degradation mechanism of GFRP bars embedded in concrete beams with cracks

This paper reports an experimental study on the durability performance of glass fibre-reinforced polymer (GFRP) bars embedded in concrete beams with cracks. Accelerated tests were performed by immersing the beams in an alkaline solution at 60°C and in tap water at 23°C. Tensile tests were conducted to evaluate the residual tensile properties of the aged GFRP bars. Moreover, scanning electron microscopy, Fourier transform infra-red spectroscopy and differential scanning calorimetry were employed to observe and analyse potential deterioration of the fibres, matrix and fibre/matrix interface. The results showed that the tensile strength of the GFRP bars embedded in concrete beams decreased in all the different environments tested, and most of the tensile strength losses occurred during the initial stage. Actually, compared to the accelerated aging environment, the degradation of the tensile strength of GFRP bars embedded in concrete in the real-world environment is minor because the concrete, which has superior barrier properties, acts like a protective cover for the GFRP bar. Based on the test results, it can be concluded that a crack in concrete may increase the environment effect on the durability performance of the GFRP bar, but cannot change the mechanisms of mechanical degradation of the GFRP bar. In addition, the degradation rate of the GFRP bar was accelerated by sustained flexural loading. Consequently, it is important to restrain and alleviate the combined interaction of a harsh environment and sustained loading in practical engineering to enhance the durability of GFRP bars.     

造纸黑液干粉用量对PVC/CPE热塑性弹性体性能的影响

以造纸黑液经硫酸中和处理脱水后的黑液干粉为填料,采用熔融共混法制备了聚氯乙烯(PVC)/氯化聚乙烯(CPE)/黑液干粉热塑性弹性体复合材料。利用FTIR和TGA测试了黑液干粉的结构和热性能;利用微控电子万能试验机、TGA研究了黑液干粉含量对弹性体复合材料的力学性能、热降解性能和老化性能的影响。结果表明:黑液干粉中木质素等有机物含量为33%;添加黑液干粉能改善PVC/CPE弹性体复合材料的力学性能,当黑液干粉含量为30phr时,拉伸强度保持不变,断裂伸长率提高了8%,撕裂强度提高了5%;采用硬脂酸处理的黑液干粉,其用量为30phr时,复合材料综合性能较佳,其拉伸强度提高了7%,断裂伸长率提高了12%,撕裂强度提高了18%;黑液干粉含量30phr时,PVC/CPE弹性体复合材料热降解温度提高了5℃;添加黑液干粉的复合材料,在热氧老化后拉伸强度和邵尔A型硬度增加,断裂伸长率稍有下降。     

A theoretical model of the effect of distributed loading on the tensile strength of agglomerates as measured in the diametral compression test

The tensile strength of particle agglomerates is analyzed to indicate the effect of distributed loading through contact flattening during the diametral compression test. It is assumed that only the contact regions of the agglomerate are flattened and that the free boundary maintains its original position during loading. The increased packing density so produced is related to the total loading as a reaction force through an empirical relationship used to describe die compaction of powders. Agglomerate failure occurs when the maximum tensile stress caused by the platen loading exceeds the cohesive strength of the particle assemblage. Theoretical predictions of the effects of parameters such as bulk powder properties and the extent of load distribution on agglomerate strength are presented from the analysis.     

Strength Characteristics of Transformation-Toughened Zirconia

Mechanisms of failure in a transformation-toughened MgO-partially-stabiltzed ZrO₂ are identified using in situ observations. The observations are related to measured stress-strain curves, strengths under various loading conditions, strength degradation from surface damage, and comparative strengths and toughnesses of toughened and overaged materials. Both reversible and irreversible tetragonal-to-monoclinic transformations, as well as microcracking, are associated with nonlinear stress-strain curves. The influence of this nonlinear response on stresses produced by flexural loading is evaluated. Failure is preceded by stable growth of microcracks, resulting in R -curve behavior and damage-tolerant strength characteristics. The implications of R -curve response for strengthening and toughening characteristics are discussed.     

Strength Degradation of Glass Impacted with Sharp Particles: I, Annealed Surfaces

The strength characteristics of annealed brittle surfaces impacted with sharp particles were studied. A theory of the degradation process is constructed in three steps: (1) A sharp particle delivers an impulsive load to the target surface via a plastic contact; (2) the contact loading initiates and propagates median cracks in the surface; (3) the cracks thus induced reduce the strength of the material. Static indentation tests provide essential contact parameters for the degradation equations, thereby allowing for prediction of strengths under in-service conditions. Strength tests on soda-lime glass laths impacted with SIC grit confirm basic predictions of the theory. Higher toughness and lower hardness are the main material requirements for improved resistance to degradation. Initial flaw population in the target surface and projectile geometry are not important factors in the damage process. The study shows that impact energy is the important service variable in determining the extent of strength loss.     

Long‐term hygrothermal response of perforated GFRP plates with/without application of constant external loading

The use of glass fiber‐reinforced polymer (GFRP) composites is increasingly being considered in various applications where the composite is subjected to harsh hot and humid conditions. Although information on the performance of GFRP under hot and humid conditions is available, the characteristics and the response of perforated GFRP under such conditions have not been fully explored. In this article, the response of perforated GFRP plates subject to hot and humid environment is examined. The applicability and accuracy of Fick's model for establishing the amount of moisture absorption by such composites is examined, and an improved model is proposed. The article also demonstrates the influence of constant external loading on such perforated GFRP while undergoing conditioning in a hot and humid environment. Moreover, since the strength and stiffness of composites can be significantly affected by harsh environments, the degradation in the strength and stiffness of the perforated GFRP as a function of time is established. A new model is proposed wherein degradation of the strength of such perforated composites may be established as a function of time and geometric entities. The model can also account for the influence of the applied loading. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers     

复合材料胶接修补结构疲劳性能的数值和试验研究

为研究室温下复合材料胶接修补结构的疲劳性能,以三维渐进损伤理论为基础,创建了复合材料胶接修补模型,利用材料损伤判断子程序实现对修补结构的静拉伸失效载荷及剩余强度的预测分析,并进行了相关试验的对比分析。采用5种不同尺寸的圆形补片来评价修补效果,并利用超景深仪对修补试件的疲劳损伤扩展模式进行微观测量。结果表明:静载拉伸中,尺寸为3.5r的修补结构承载能力最好;疲劳循环中,尺寸为2.5r的修补结构剩余强度提升效果最好;疲劳载荷下,当循环次数较低时,修补结构的主要损伤为基体开裂,而随着循环次数的增大,主要损伤为纤维断裂。     

Development of adhesive bonds with reduced fracture strength as NDE benchmarks

A method for producing adhesively bonded aluminum joints with a predictable loss of fracture strength was developed and evaluated. The method uses an open-faced specimen geometry and a humid high-temperature environment to promote adhesive degradation. The rate of degradation was greatly increased over previous accelerated degradation schemes through the use of MgSO₄ as a contaminant. The contaminant was applied as an aerosol in a purpose-built duct having a controlled airflow. Specimens were prepared and subjected to accelerated aging under a variety of conditions and then fractured using a DCB loading jig. It was found that the contaminant surface concentration was a strong determinant of the fracture strength after hot-wet aging. Exposure to the hot-wet environment was shown to have little effect beyond an initial threshold. Standard ultrasonic imaging techniques were incapable of differentiating between fresh and hot-wet aged specimens, in spite of significant differences in the fracture strength. This is consistent with the hypothesis that the approach produced specimens that simulated the effects of environmental attack, since standard ultrasonic methods, such as those used in the present study, cannot detect such losses of fracture strength in the absence of any delamination between adhesive and adherend. FESEM and EDX analysis of the fracture surfaces showed residual aluminum, suggesting an intra-oxide locus of failure consistent with other accelerated degradation methods. The technique can be used to generate adhesive joint specimens to aid the development of ultrasonic methods capable of detecting the loss of fracture strength associated with environmental degradation.     

氧化铝、氮化硅和碳化硅的疲劳特性与寿命预测

分析了几种典型的结构陶瓷在长期载荷下的失效特性和差别。认为长期失效的本质是强度衰减,建立了一个疲劳失效的强度衰减模型和提出了寿命预测方法。分别研究了氧化铝、氮化硅和碳化硅几种常用工程陶瓷在常温和高温下的疲劳特性和差异。采用三点弯曲的受力方式测试了不同载荷水平下的断裂时间。结果表明：碳化硅的疲劳门榄值超过强度的80％,而且受温度影响最小;氧化铝的静疲劳受微小裂纹扩展控制;氮化硅的高温疲劳主要是蠕变机制导致强度衰减,疲劳门槛值不超过强度50％。由实验研究了氮化硅的高温静疲劳、动疲劳和循环疲劳三者在相同温度和相同应力峰值变化下的寿命关系,结果与计算一致。     

The effect of constant loading on the mechanical behavior of bulk adhesive

The aim of this research was to develop an experimental–numerical approach to characterize the effect of constant loading coupled with elevated temperature on epoxy bulk adhesive and to predict the stress degradation of bulk adhesive specimen under 15% and 25% tensile failure loads for the automotive industry. A power-law creep model was built to simulate the effect of temperature and loading on adhesive mechanical behavior, and the related strength degradation simulation has also been implemented using a creep strain-dependent ductile damage model. Experiments were conducted on bulk adhesive specimens under constant temperature coupled with mechanical load, and the corresponding experimental results provided creep parameters for the simulation procedure as well as effective validation with the numerical results in this study. The results obtained from experiments and numerical simulations were also in good agreement.     

Effects of low-energy impact and thermal cycling loadings on fatigue behavior of the quasi-isotropic carbon/epoxy composites

Effects of low-energy impact loading and thermal cycling on fatigue behavior of carbon fiber reinforced epoxy (carbon/epoxy) laminates are examined. A low-energy of 0.62 Joules was adopted to impact carbon/epoxy laminates prior to thermal cycling exposure and fatigue test. The temperature ranged between 60 and −60 °C for thermal cycling and the stress ratio of 0.1 with a frequency of 3 Hz for fatigue loading were used. Impact performances were tested on the virgin specimens and the thermal-cycling exposure specimens. Residual tensile strength and fatigue tests were performed on the laminate composites after being subjected to thermal cycling. The relationship between tensile strength reduction and fatigue performance after thermal cycling was investigated. Stiffness degradation during fatigue testing was monitored; the differences in stiffness for these three composites (virgin specimens, low-energy impacted specimens, low-energy impacted and thermal-cycling exposure specimens) were compared and the coupling effects of low-energy impact and thermal fatigue were studied. Furthermore, the S-N curves were also plotted and the variation was compared on the aforementioned three composites. SEM was used to examine the difference in fracture morphologies on the composites with and without suffering low-energy impact and thermal fatigue.     

Quantitative Analysis of Crack Shielding Degradation During Cyclic Fatigue of Alumina

Grain bridging degradation behind a crack tip is the main cyclic fatigue mechanism in nontransforming ceramics. In this work, a compliance function is used to quantify the shielding capacity of grain bridges during cyclic loading of alumina ceramics with different grain sizes. This allows to identify the different stages occurring during cyclic loading. Significant degradation is observed in the coarse grain material and a marked sensitivity to the loading level is outlined. At moderate loads, bridging degradation occurs prior to fatigue crack growth during an incubation period which can reach several million cycles. At low cyclic loads, the shielding capacity can be entirely degraded, leading to a cyclic fatigue threshold equivalent to that of the fine grain material.     

Tensile,Swelling, and Oxidative Degradation Properties of Crosslinked Polyvinyl Alcohol/Chitosan/Halloysite Nanotube Composites

Glutaraldehyde (GA) crosslinked polyvinyl alcohol (PVA)/chitosan (CS)/halloysite nanotube (HNT) composite films were prepared using a wet casting method. The tensile, morphology, thermal degradation, swelling, moisture, and oxidative degradation properties of crosslinked composite films were carried out. The presences of crosslinking in the composite films were confirmed by FTIR result. The tensile strength of the crosslinked composite films increased up to 0.5 wt% of HNTs loading. Increasing HNTs reduced the thermal degradation, swelling, and moisture properties of crosslinked composite films reduced with the increase of HNTs content. Results also indicated that the crosslinked composite films were degraded using Fenton reagent.     

Effect of Cleaning and Abrasion-Induced Damage on the Weibull Strength Distribution of Sapphire Fiber

Fractographic analysis revealed the presence of concurrent flaw populations in sapphire fibers which were tensile tested in the as-received condition (sized and unsized) and after various cleaning procedures. The following flaw populations were identified: surface flaws attributed to handling and abrasion damage (type A), volume or internal flaws attributed to shrinkage voids which form during the manufacturing process (type B), localized fiber surface reaction flaws introduced during the flame-cleaning procedure (type C), and self-abrasion surface flaws intentionally introduced on unsized fibers (type D). The strength distribution associated with each flaw type was characterized using a censored data Weibull analysis for both the least-squares and maximum-likelihood estimation methods. The strength distribution for type C (flame-cleaning) flaws exhibited an approximately 20% degradation in strength compared to the distribution for type A flaws. The strength distribution for type D (self-abrasion) flaws exhibited an approximately 35% degradation in strength compared to the strength distribution for type A flaws. This result underscores the need for fiber sizings to prevent damage during shipping and handling. However, higher purity sizings and/or improved procedures for sizing removal are required to mitigate cleaning-induced fiber strength degradation during composite fabrication.