Tensile Strength Degradation of Ceramics from Prior Compression

Both theory and studies on models predict that cracks extend stably form flaws in brittle materials subjected to compression. Once cracks have originated and extended, the tensile strength transverse to the cracks should be reduced. Tests on siliconized carbide verify this prediction; however, tests on hot-pressed silicon nitride refute it. Possible morphologies of the inherent flaws are speculated.     

Temperature Dependence of Tensile Strength for a Woven Boron-Nitride-Coated Hi-Nicalon™ SiC Fiber-Reinforced Silicon-Carbide-Matrix Composite

The temperature dependence of tensile fracture behavior and tensile strength of a two-dimensional woven BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composite fabricated by polymer infiltration pyrolysis (PIP) were studied. A tensile test of the composite was conducted in air at temperatures of 298 (room temperature), 1200, 1400, and 1600 K. The composite showed a nonlinear behavior for all the test temperatures; however, a large decrease in tensile strength was observed above 1200 K. Young's modulus was estimated from the initial linear regime of the tensile stress–strain curves at room and elevated temperatures, and a decrease in Young's modulus became significant above 1200 K. The multiple transverse cracking that occurred was independent of temperature, and the transverse crack density was measured from fractographic observations of the tested specimens at room and elevated temperatures. The temperature dependence of the effective interfacial shear stress was estimated from the measurements of the transverse crack density. The temperature dependence of in situ fiber strength properties was determined from fracture mirror size on the fracture surfaces of fibers. The decrease in the tensile strength of the composite up to 1400 K was attributed to the degradation in the strength properties of in situ fibers, and to the damage behavior exception of the fiber properties for 1600 K.     

Tensile Testing of Ceramics

A simple, self-aligning, uniaxial test firtiire has been developed for testing ceramics. The primary emphasis is placed on a simple specimen geometry and test procedure. The strain gage measurements on α-Sic and steel specimens indicate very good uniformity in the tensile stresses (with variation less than 1%) across the cross section. The tensile testing of SiC and SiC/TiB₂ composites exhibits 55% and 79% failures, respectively, in the gage section with possible further improvements. Average tensile strength values of 160 and 202 MPa were obtained for α-SiC and SiC/TiB₂ respectively.     

Thermal Expansion Coefficients of Unidirectional Fiber-Reinforced Ceramics

The influence of internal stresses, due to the thermomechanical mismatch between the fiber and the matrix, on the thermal expansion behavior of unidirectional fiber-reinforced ceramics is considered. Using the composite cylinder model, the effective thermal expansion coefficients of the composite are calculated from the total strains, which consist of the strains due to temperature changes and the strains induced by the presence of internal stresses. The results reveal that when the fiber and the matrix have the same elastic constants, the rule of mixtures approach can be used to obtain the thermal expansion coefficients of the composite, as observed in previous analytical solutions. Also, for the case of low volume fractions of fibers with Young's moduli much larger than those of matrices, and the thermal expansion coefficients lower than those of matrices, the transverse thermal expansion coefficient of the composite is higher than that of either the fiber or the matrix. However, unlike previous studies, the present analysis provides a physical basis for this phenomenon in terms of the internal thermal stress state within the composite.     

Tensile Properties of Arenga pinnata Fiber-Reinforced Epoxy Composites

The aim of this study is to determine the tensile properties of Arenga pinnata fiber as a natural fiber and epoxy resin as a matrix. The Arenga pinnata fibers were mixed with epoxy resin at the various fiber weight percentages of 10%, 15%, and 20% Arenga pinnata fiber and with different fiber orientations such as long random, chopped random, and woven roving. Hand lay-up processes in this experiments were to produce specimen test with the curing time for the composite plates is in the room temperature (25–30°C). Results from the tensile tests of Arenga pinnata fiber reinforced epoxy composite are that the 10 wt.% woven roving Arenga pinnata fiber showed the highest value for maximum tensile properties. The tensile strength and Young's modulus values for 10 wt.% of woven roving Arenga pinnata fiber composite are 51.725 MPa and 1255.825 MPa, respectively. The results above indicate that the woven roving Arenga pinnata fiber has a better bonding between its fiber and matrix compared to long random Arenga pinnata fiber and chopped random Arenga pinnata fiber. Scanning electron microscopy (SEM) tests were carried out after tensile tests to observe the interface of fiber and matrix adhesion.     

Micromechanical Analysis of Short Cracks in Fiber-Reinforced Ceramics

The finite-element method is used to perform a micromechanical analysis of a crack in a fiber-reinforced ceramic, when the crack length is the same order of magnitude as the fiber spacing. The stress intensity factor is calculated under mode-I loading for various crack lengths and fiber volume fractions. Cracks with and without fibers bridging the faces of the crack are analyzed.     

The Effect of Environmental Treatments on Fiber Surface Properties and Tensile Strength of Sugar Palm Fiber-Reinforced Epoxy Composites

Fiber glass has been used widely in manufacturing industries, especially marine industries, because of low cost and high strength. However, glass fiber can cause acute irritation to the skin, eyes, and upper respiratory tract. This study looked at the possibility of substituting glass fiber with natural fiber in composite materials. The surface properties of sugar palm fiber (Arenga pinnata) were modified using seawater and freshwater as treatment substances. This led to biological, chemical, and water degradation of the sugar palm fiber. Morphological and structural changes in the fibers were investigated using a scanning electron microscope (SEM). A series of tensile tests based on ASTM D638-99 was carried out on epoxy composites with 15% sugar palm fiber by volume. It was found that seawater and freshwater treatments improved the surface properties of the sugar palm fiber and thus resulted in better adhesion quality as compared to untreated fiber. An improvement in tensile strength also supported this finding. Treatment with seawater for 30 days proved to be the best, with 67.26% increase in tensile strength.     

Experimental Observations of Frictional Heating in Fiber-Reinforced Ceramics

The influence of fatigue loading history and microstructural damage on the magnitude of frictional heating and interfacial shear stress in a unidirectional SiC fiber/calcium aluminosilicate matrix composite was investigated. The extent of frictional heating was found to depend upon loading frequency, stress range, and average matrix crack spacing. The temperature rise attained during fatigue can be significant. For example, the temperature rise exceeded 100 K during fatigue at 75 Hz between stress limits of 220 and 10 MPa. Analysis of the frictional heating data indicates that the interfacial shear stress undergoes an initially rapid decrease during the initial stages of fatigue loading: from an initial value over 20 MPa, to approximately 5 MPa after 25 000 cycles. Over the range of 5 to 25 Hz, the interfacial shear stress was not significantly influenced by loading frequency. The implications of frictional heating in fiber-reinforced ceramics are also discussed.     

Effect of Interfaces on the Properties of Fiber-Reinforced Ceramics

The mechanical properties of a series of six fiber-reinforced ceramics and glasses have been evaluated with the objective of critically assessing present understanding. A major parallel theme has been the characterization of the interface and an assessment of the thermomechanical properties of the interfaces as they relate to composite behavior. The results establish that the available mechanical property models correlate well with experiments, provided that independent measurements are made of the residual stress, the interface sliding stress, and the in situ strength properties of the fibers. In addition, trends in the sliding stress are found to be qualitatively consistent with those expected for sliding along debonded surfaces.     

Tensile and Fatigue Behavior of Silicon Carbide Fiber-Reinforced Aluminosilicate Glass

The onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross-ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.     

Creep Analysis of Bend Specimens Subject to Tensile Cracking

Expressions are derived which permit the analysis of creep data, obtained in bending, for poly crystalline ceramics subject to tensile cracking. A numerical example based on literature data indicates that by considering the effect of cracks much better agreement is obtained between diffusion coefficients inferred from the creep data and those obtained by direct diffusion measurements.     

Tensile Fracture Toughness Measurements in Ceramics

A novel procedure is outlined whereby fracture toughness values for ceramics can be measured under uniaxial tension (mode I) in specimens containing a fatigue crack. Circumferentially notched rods of rapolycrystalline aluminum oxide were precracked in cyclic compression to introduce a fatigue crack at room temperature, following the technique proposed by Suresh and co-workers.^7,10,11 Subsequently, the precracked rods were fractured in pure tension. Highly reproducible values of fracture toughness were obtained using this method.     

Tensile Strength of SiC Fibers

An experimental investigation has been conducted on the effects of gauge length on the tensile strength of SiC fibers. The results show that the overall strength distribution cannot be described solely in terms of the two-parameter Weibull function. The overall distribution is found to be consistent with two concurrent flaw populations, one of them being characteristic of the pristine fibers, and the other characteristic of the additional flaws introduced into the fiber during processing of the composite.     

Residual Stresses in Fiber-Reinforced Ceramics due to Thermal Expansion Mismatch

Principles for the calculation of residual stresses due to thermal expansion mismatch and previous solutions are reviewed. A general model for this subject is proposed. Considering the effective thermoelastic properties of a unidirectional composite in axial and transverse directions allows a more comprehensive treatment of the residual stress situation in fiber-reinforced composites. Parameter variations show the important influence of the thermomechanical properties of an interfacial layer, which is disregarded in most of the conventional calculations.     

Application of Sol-Gel Processing Techniques for the Manufacture of Fiber-Reinforced Ceramics

This paper discusses the application of freeze gelation of sil-ica sols for the manufacture of low-cost, net-shape components in ceramic-matrix composites (CMCs). A brief analysis is made of the central features which determine the stiffness, strength, and toughness of composite materials insofar as they may be applied to this particular class of CMCs. The critical processing parameters, relating mainly to the matrix phase, are reviewed and the results of the program so far obtained are interpreted in the context of this analysis. It is clear that interactions between processing parameters make optimization a complex process. In short-fiber composites, matrix porosity dominates composite properties, while in continuous-fiber systems, it is fiber degradation that controls the composite performance.     

复合材料圆管弯曲强度研究

对两组不同支距(100,200,300,400,500mm),在管芯加与不加塞子情况下的纤维增强复合材料圆管进行了弯曲试验。结果显示加与不加塞子,对圆管的弯曲强度的试验结果影响很大。对于所测试的玻璃纤维增强复合材料圆管,管芯不加塞子情况下的纤维增强复合材料圆管的弯曲强度约为加塞时的42%～60%,试验结果离散性较加塞时高出2%～4%。为探索一种合理的复合材料圆管弯曲性能的评价方法,本文对此现象进行了分析,建立了复合材料圆管弯曲强度模型,并采用修正的Hill平面应力准则给出了加与不加塞子情况下两者弯曲强度之间的关系。理论计算结果与测试值符合得很好,表明可以采用管芯加塞弯曲实验方法,再利用本文所建立的关系获得复合材料圆管较真实的弯曲强度。     

Long-Term Tensile Creep Testing for Advanced Ceramics

A tensile creep test technique for long-term (10 000 h or more) measurement is described. A specimen is gripped by ceramic grips with a simple design in the hot zone of a furnace. By optimizing the grip geometry, the ratio of the parasitic bending strain to the average tensile strain can be suppressed to less than 1%. The careful hand polishing to remove machining flaws has been found essential for reliable creep tests. The long-term test is demonstrated for silicon carbide at 1400°C under a stress of 200 MPa, by routinely replacing the heat elements, thermocouples, etc., during the test. The stability of the creep curve, which has been obtained by an optical extensometry system, is normally within ± 2 μm.