Diffusion and deformation controlled creep crack growth

A mechanism for creep crack growth is proposed by which the crack grows by formation of grain boundary cavities ahead of the crack tip. Two cases are considered; firstly, when cavity growth is diffusion controlled and secondly, where growth is deformation controlled. The resultant crack growth rates predicted by these theoretical models are compared with experimental data.     

Subcritical crack growth at bimaterial interfaces: Part III. shear-enhanced fatigue crack growth resistance at polymer/metal interface

Fatigue crack growth along an Al/epoxy interface was examined under different combinations of mode-I and mode-II loadings using the flexural peel technique. Fatigue crack growth rates were obtained as a function of the total strain energy rate for G_II/G_I ratios of 0.3 to 1.4, achieved by varying the relative thickness of the outerlayers for the flexural peel specimen. Fatigue crack growth resistance of the interface was found to increase with increasing G_II/G_I ratio. Such a shear-enhanced crack growth resistance of the interface resulted in a gradual transition of crack growth mechanism from interfacial at the low G_II/G_I ratio to cohesive at the high G_II/G_I ratio. Under predominantly mode-I loading, the damage in the polymer took the form of crazing and cavitation. In contrast, laminar shear occurred under highly shear loading, resulting in a larger amount of plastic dissipation at the crack tip and improved fatigue crack growth resistance.     

SiC晶须增韧WC陶瓷刀具材料的研究

通过正交试验法对WC／SiCw陶瓷复合材料进行了成分和热压工艺参数优化，经优化制备的该类复合材料与纯WC材料相比，抗弯强度提高了约50％，断裂韧性提高了30％～40％，维氏硬度提高了10％～15％。切削试验数据证明，本试验制得的WC／SiCw复合陶瓷刀具材料的车削性能优于同类型硬质合金材料，表明通过SiCw替代金属粘结相来增韧补强WC陶瓷刀具材料的方法是可行的。     

A theory of diffusion controlled intergranular creep crack growth

A theory of intergranular creep crack growth in brittle materials has been developed. The mechanism of crack propagation is removal of atoms from the crack tip by stress induced grain boundary diffusion and their deposition at the grain boundary. The width of the crack is assumed to be constant during crack propagation. Any possible plastic deformation at the crack tip has been neglected. The stress relaxation ahead of the crack tip which arises due to the non-uniform deposition of material onto the grain boundary is calculated and the diffusion process governed by this relaxed stress studied. Thus the theory is analogous to those developed previously for the growth of r-type voids in grain boundaries. Both the steady state and the transient were considered. It was found that the rate of crack growth at any moment was determined by the nominal stress intensity factor K. A minimum stress intensity, K_min' exists below which no crack growth can take place. Conversely there is a maximum limiting rate of crack growth determined by the maximum surface diffusion rate. For K > K_min the steady state is always reached quickly and the length of the transient period t_tr is proportional to K⁻⁶. For K > K_min the rate of crack growth is proportional to K⁴. A comparison with experimental measurements on brittle ½Cr-½Mo-¼V steels show good agreement between the theory and experiment.     

Toughening Mechanisms of Alumina Matrix Ceramic Composite Toughened by Rare Earth Additive

Mixed rare earth elements were incorporated into alumina ceramic materials. Hot-pressing was used to fabricate alumina matrix composites with nitrogen atmosphere protection. Microstructures and mechanical properties of the composites were tested. It is indicates that the bending strength and fracture toughness of alumina matrix ceramic composites sintered at 1550℃ and 28 MPa for 30 min are improved evidently. Based on mixed rare earth elements acting as a toughening phase, AlTiC master alloys were also added in as sintering assistant, which could prompt the formation of transient liquid phase, and thus nitrides of rare earth elements were produced. All the above are beneficial to the improvement of mechanical properties of alumina matrix ceramic composites.     

Effects of interfacial strength on fatigue crack growth in a fiberreinforced Ti-alloy composite

The fatigue crack growth behavior of a Ti-6A1-4V composite with boron fibers was previously studied in the as-received and thermally exposed conditions. Fracture strengths of the composite, fiber, and interface were characterized together with fatigue crack growth rates and failure mechanisms. Utilizing the matrix and fiber properties as input, a recently proposed model was exercised to elucidate the effects of interfacial strength on crack growth rates in the composite. Comparison of experimental results with model calculations revealed that a weak fiber/matrix interface combined with a strong, high-modulus fiber led to interface debonding and crack deflection and produced the beneficial effects of increased threshold and reduced transverse crack growth rates. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Fatigue crack growth of SM-1240/TIMETAL-21S metal matrix composites at elevated temperatures

A series of high-temperature fatigue crack growth experiments was conducted on a continuous-fiberreinforced SM1240/TIMETAL-21S composite using three different temperatures, room temperature (24 °C), 500 °C, and 650 °C, and three loading frequencies, 10, 0.1, and 0.02 Hz. In all the tests, the cracking process concentrated along a single mode I crack for which the principal damage mechanism was crack bridging and fiber/matrix debonding. The matrix transgranular fracture mode was not significantly influenced by temperature or loading frequency. The fiber debonding length in the crack bridging region was estimated based on the knowledge of the fiber pullout lengths measured along the fracture surfaces of the test specimens. The average pullout length was correlated with both temperature and loading frequency. Furthermore, the increase in the temperature was found to lead to a decrease in the crack growth rate. The mechanism responsible for this behavior is discussed in relation to the interaction of a number of temperature-dependent factors acting along the bridged fiber/matrix debonded zone. These factors include the frictional stress, the radial stress, and the debonding length of the fiber/matrix interface. In addition, the crack growth speed was found to depend proportionally on the loading frequency. This relationship, particularly at low frequencies, is interpreted in terms of the development of a crack tip closure induced by the relaxation of the compressive residual stresses developed in the matrix phase in regions ahead of the crack tip during the time-dependent loading process.     

Strengthening and Toughening Effect of Yttrium on Al2O3/TiCN Ceramic Tool Material

Modern ceramic cutting tool materials with their excellent physical, mechanical properties and cutting performances promote greatly the development of metal cutting technology.Therefore, they are one of the most promising cutting tool materials in the coming Zlst century["'l. however, the intrinsic brittleness is a fatal weakness for ceramic tool materials. In order to reduce the brittleness and to increase the strength and the fracture toughness of the cutting tool materials, various research…     

The role of metal plasticity and interfacial strength in the cracking of metal/ceramic laminates

Two models based on elastic-plastic fracture mechanics and fiber bridging are developed to study the role of plastic yielding in metals and the interfacial strength of metal/ceramic laminates. There are two types of damage observed in metal/ceramic laminates: multiple cracking and macroscopic crack propagation. The former occurs around the macroscopic crack tip and thus distributes the damage and enhances the composite's toughness. The present models establish that there exists a critical metal/ceramic layer thickness ratio above which multiple cracking dominates and that this ratio decreases (hence increasing the possibility of multiple cracking) as the ratios of metal yield stress over ceramic strength, metal modulus over ceramic modulus, and metal/ceramic interfacial strength over ceramic strength increase. Good agreement between the present models and experimental results is observed for both damage modes, i.e. multiple cracking vs macroscopic crack propagation, and for critical stress intensity factors. The elastic-plastic fracture mechanics and fiber-bridging models predict that multiple cracking is ensured if the metal layer thickness is 2.5 times larger than the ceramic layer thickness, regardless of the metal/ceramic properties.     

Subcritical crack growth at bimaterial interfaces: Part II. microstructural effects on fracture resistance of metal/ceramic interfaces

The flexural peel technique was used to study the fracture resistance of two model A1₂O₃/A1 interfaces. The bimaterial interface was formed by bonding high-purity A1₂O₃ with molten Al-5 pct Cu alloy under pressure. The specimens were then heat treated so that the Al-Cu alloy reached peakaged and extended-overaged conditions. The fracture resistance curve was established for two interfaces with either the peak-aged or overaged Al alloy. The fracture resistance of the interface with the peak-aged Al-Cu alloy was higher in terms of both the initiation and peak toughness. While the peak toughness of the interface scaled with the yield strength of the metal, the initiation toughness differed by a factor of 8. The difference in the initiation toughness is discussed in terms of the disparity in the interfacial microstructure.     

Effects of oxidation on the fatigue crack growth resistance and the interfacial properties of a Ti-MMC laminate composite

Fatigue crack growth rates in a [0/90]_2s Ti-6Al-4V/SCS-6 cross-ply laminate, correlated with push-out tests, have been measured to assess the effects of varying test temperature, environment, load ratio (R), and initial stress intensity factor range (ΔK). The fatigue crack growth resistance is degraded in tests at 450 °C in air, but tests carried out at test temperatures of up to 450 °C under vacuum, both at R=0.1 and R=0.5, have shown crack arrest/catastrophic failure transitions (CA/CF), which are similar to those observed for specimens studied at room temperature and at 300 °C in air. Moreover, for such [0/90] composites, the critical role of intact 0 deg fibers bridging in the crack wake, in promoting fatigue crack growth resistance, has been confirmed. Sudden increases of fatigue crack growth rate can be attributed to individual fiber failure(s), which were detected by acoustic emission techniques. The effect of the experimental conditions (environment, test temperature, and duration) on the mechanical behavior (fatigue crack growth rate, push-out tests, and broken fibers pull-out lengths) of this laminate may be explained by the modification of the interfacial zone (decrease in the carbon layer thickness due to oxidation and formation of TiO₂).     

柔度法测定高温疲劳裂纹扩展速率da/dN的研究

介绍了利用MTS 810.13试验设备开展柔度法测定高温疲劳裂纹扩展速率da/dN的一些尝试,并得到了准确、有效的试验数据.实践证明,该试验方法具有良好的稳定性、较高自动化程度、较好的试验精度等技术优势.     

Cavitation damage and creep crack growth

The effect has been investigated of prior damage on the creep crack propagation characteristics of 0.5 pct Cr, 0.5 pct Mo, 0.25 pct V steel at 823 K. On a macroscopic basis, the parametersK ₁ andC ^* both appear to correlate withda/dt although the parameterC ^* is unable to distinguish between virgin and damaged specimens. Rupture lives in the predamaged specimens are reduced by up to 60 pct when compared to virgin samples. Microscopically, it is found that the nature of the cavitation damage suggests that surface and grain boundary diffusion processes may have a minimal part to play, crack growth being controlled by the growth of cavities which is in turn controlled by the deformation of the surrounding matrix. A number of microscopic models are compared with the experimental data and it is suggested that a model which gives the best correlation with results is one proposed on the basis of matrix deformation. Formerly of the Department of Metallurgy, Manchester University Formerly of the Department of Metallurgy, Manchester University     

Enhanced fatigue crack growth resistance at elevated temperature in TiC/Ti-6Al-4v composite: Microcrack-lnduced crack closure

The fatigue crack growth behavior of TiC/Ti-alloy composite was examined at 450 °C and compared to the room-temperature behavior. Contrary to the temperature-dependent fatigue crack growth behavior of the monolithic alloy, fatigue crack growth resistance of the composite was improved at the elevated temperature. At 450 °C, the fatigue threshold of the composite was found to be 50 Pct higher than at room temperature. Such an improved fatigue crack growth resistance is shown to result from extensive microcracking of reinforcing particles, which promotes fatigue crack closure at the elevated temperature.     

Near-threshold corrosion fatigue crack growth behavior of type 422 stainless steel at controlled maximum stress intensities

The K_max-controlled near-threshold fatigue crack growth behavior was investigated on 422 stainless steel in a boiling NaCl solution. During the test, there was a transition from corrosion fatigue to stress corrosion cracking. The transition occurred at very high load ratios (R=-0.91) and at very lowAK levels (≤2.1 MPa√m). The characteristics of stress corrosion cracking (SCC) were manifested by time-based crack growth rather than cycle-based crack growth, by crack extension under static loading, and by change in fracture mode. In corrosive environments, the small ripple loading imposed on structural materials should be recognized for engineering designs and failure analyses.     

A theory for creep by interfacial flaw growth in ceramics and ceramic composites

The nucleation and growth of flaws along grain boundaries and interfaces are known to cause significant reductions in elastic moduli and to play an important role in determining the deformation characteristics of ceramic materials at elevated temperatures. This paper presents an analysis of the creep behavior of deteriorating elastic solids where the principal mechanism of deformation is the growth of intergranular or interfacial flaws. The changes in elastic moduli induced by the growth of internal damage are used to derive the stress exponent in the power-law creep regime. When the flaws advance at a rate which is proportional to the local normal stress or normal strain, a power-law creep exponent of 2 is predicted for short time, steady-state creep for a population of aligned slit cracks and randomly oriented penny-shaped cracks. For long-time creep, the variation of nonsteady state creep strain rate as a function of the far-field stress and time is explicitly determined. General solutions for creep strain rates are also presented for situations where the microcrack growth rate has a power-law dependence on the local normal stress or stress intensity factor. The predicted dependence of creep strain rate on the far-field stress, the progression of damage and the consequent reduction in elastic moduli, overall creep ductility, and implications pertaining to microstructural and temperature effects on creep are found to be in accord with a wide variety of experimental observations for ceramics and ceramic composites. The temperature, stress and material conditions for which the proposed mechanism is applicable are discussed and a general theory of creep damage in progressively microfracturing elastic brittle solids is developed.     

Influence of interfacial reaction rates on the wetting driving force in metal/ceramic systems

The wetting of copper-silicon alloys of various compositions on vitreous carbon substrates at 1423 K was studied by the sessile drop method. The morphology and chemistry of products of interfacial reactions between silicon and carbon were characterized by scanning electron microscopy (SEM), electron probe microanalysis, and high-resolution optical profilometry. In addition to measurements of contact angles and spreading kinetics in the reactive Cu-Si/Cv system, similar measurements were performed for the nonreactive Cu-Si/SiC system. It was found that the reaction rate has no effect on the final contact angle, which is nearly equal to the thermodynamic contact angle of the alloy on the reaction product. These findings appear to be valid for a wide range of interfacial reaction rates and for different types of interfacial reactions.     

An analysis of stress corrosion crack growth by anodic dissolution

An analysis of the distribution of electrode potential within a stress corrosion crack which is growing by an anodic dissolution process has been used to define the electrode potential at the tip of the crack. This potential is used to predict the kinetics of crack growth. The influence of the applied stress intensity and the electrochemical properties of the crack tip and surface on the growth rate have been considered for low alloy steels in concentrated hydroxide solution and aluminum alloys in acidic chloride solution. Crack growth rates obtained in high concentration solutions are extrapolated to lower concentration solutions which may be expected in service environments. Predicted crack growth rates are in good agreement with published data.