Fracture behavior of laminated metal-metallic glass composites

The crack growth behavior of metallic glass in laminated metal-metallic glass composites was investigated and compared to the crack growth characteristics of monolithic metallic glass. The composite arrangement significantly increases the crack growth resistance of the glass. Growth in the monolithic glass is catastrophic, whereas in the composite, it is stable. The behavior is described in terms of crack growth resistance(R) curves and discussed in terms of extrinsic and intrinsic contributions to toughness. It is found that an extrinsic factor,i.e., matrix bridging, makes the major contribution to increased crack growth resistance and that a limiting crack opening displacement model interprets the experimental data quite well. Enhanced glass deformation in the crack tip region, manifested by multiple shear band formation, is responsible for the intrinsic toughening observed. Physical models are developed to estimate the level of intrinsic toughening due to this effect.     

Water Penetration—Its Effect on the Strength and Toughness of Silica Glass

When a crack forms in silica glass, the surrounding environment flows into the crack opening, and water from the environment reacts with the glass to promote crack growth. A chemical reaction between water and the strained crack-tip bonds is commonly regarded as the cause of subcritical crack growth in glass. In silica glass, water can also have a secondary effect on crack growth. By penetrating into the glass, water generates a zone of swelling and, hence, creates a compression zone around the crack tip and on the newly formed fracture surfaces. This zone of compression acts as a fracture mechanics shield to the stresses at the crack tip, modifying both the strength and subcritical crack growth resistance of the glass. Water penetration is especially apparent in silica glass because of its low density and the fact that it contains no modifier ions. Using diffusion data from the literature, we show that the diffusion of water into silica glass can explain several significant experimental observations that have been reported on silica glass, including (1) the strengthening of silica glass by soaking the glass in water at elevated temperatures, (2) the observation of permanent crack face displacements near the crack tip of a silica specimen that had been soaked in water under load, and (3) the observation of high concentrations of water close to the fracture surfaces that had been formed in water. These effects are consistent with a model suggesting that crack growth in silica glass is modified by a physical swelling of the glass around the crack tip. An implication of water-induced swelling during fracture is that silica glass is more resistant to crack growth than it would be if swelling did not occur.     

Direct Observation on the Evolution of Shear Banding and Buckling in Tungsten Fiber Reinforced Zr-Based Bulk Metallic Glass Composite

The evolution of micro-damage and deformation of each phase in the composite plays a pivotal role in the clarification of deformation mechanism of composite. However, limited model and mechanical experiments were conducted to reveal the evolution of the deformation of the two phases in the tungsten fiber reinforced Zr-based bulk metallic glass composite. In this study, quasi-static compressive tests were performed on this composite. For the first time, the evolution of micro-damage and deformation of the two phases in this composite, i.e., shear banding of the metallic glass matrix and buckling deformation of the tungsten fiber, were investigated systematically by controlling the loading process at different degrees of deformation. It is found that under uniaxial compression, buckling of the tungsten fiber occurs first, while the metallic glass matrix deforms homogeneously. Upon further loading, shear bands initiate from the fiber/matrix interface and propagate in the metallic glass matrix. Finally, the composite fractures in a mixed mode, with splitting in the tungsten fiber, along with shear fracture in the metallic glass matrix. Through the analysis on the stress state in the composite and resistance to shear banding of the two phases during compressive deformation, the possible deformation mechanism of the composite is unveiled. The deformation map of the composite, which covers from elastic deformation to final fracture, is obtained as well.     

粗轧过程轧件表面裂纹演变行为热力耦合有限元分析

 连铸坯可能出现表面裂纹,其在轧制过程中的演变行为严重影响着轧制产品的质量。本文采用热力耦合有限元方法对轧制过程轧件表面裂纹演变行为进行了分析。获取了轧制过程中轧制变形区内轧件表面裂纹形状变化规律、裂纹附近区域应力场和温度场分布场分布情况。计算结果表明在轧制入口区域,裂纹逐渐闭合,然而,在轧制出口区域,裂纹又重新扩展开,裂纹尖端处呈现拉应力状态。同时,发现变形区内裂纹尖端发生温度成双峰变化规律。     

Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel

The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.     

The Use of Atomic Force Microscopy to Study Crack Tips in Glass

This article presents a review of the application of atomic force microscopy (AFM) to crack-tip corrosion during subcritical crack growth in glass. The two principal experimental techniques used in this type of study are (1) the direct observation of crack motion by scanning the tip of a crack during crack growth and (2) the examination of fracture surfaces once the specimen has been fractured in two. The first technique has been used to demonstrate and quantify water condensation at crack tips during subcritical crack growth and is particularly useful at low crack velocities. The second technique has been used to quantify the crack-tip corrosion process and the shape of the crack tip during crack growth. In this article, we discuss experimental results showing that the environment that develops at the tips of freshly fractured glass surfaces in soda lime glass can corrode the glass surfaces near the crack tip. Soda lime silicate glass contains mobile alkali ions that will exchange with hydronium ions in solution at the crack tip, forming a highly basic solution that is corrosive to glass. Experimental evidence for such corrosion has been obtained by the atomic force microscope, which demonstrates a displacement of the two fracture surfaces near the crack tip that can be as much as 20 nm, depending on how long the crack is held open at the fatigue limit. Despite the corrosion and displacement of the crack surfaces, the crack tip itself appears to remain sharp, suggesting that the fatigue limit in soda lime silicate glass is not due to crack-tip blunting. Most likely, the fatigue limit is a consequence of ion exchange at the crack tip, in which hydronium ions in the crack-tip solution exchange with sodium ions in the glass. As hydronium ions are larger than sodium ions, this exchange process leaves a compressive stress within the fresh fracture surface of the glass that resists crack motion and results in a stress-corrosion fatigue limit, as first proposed by Bunker and Michalske. In agreement with this mechanism, no fatigue limit is observed for silica glass, which also exhibits no ion exchange. As the crack-tip solution in silica glass is only mildly acidic, pH ≈ 5, corrosion does not occur at crack tips of this glass as supported by the observation that no crack-tip displacements are observed in silica glass by AFM. As the proposed ion exchange mechanism used to explain the stress corrosion limit in glass is at variance with the belief that the fatigue limit in glass is the result of crack-tip blunting, we discuss the possibility of plastic deformation at crack tips in glass and conclude that the available experimental data does not support such a model. At the present time, chemical reaction based crack growth theories are most consistent with the body of crack growth data that is available on glass and are probably the best explanation for the phenomenon.     

Fracture Analysis of Shear Deformable Bi-Material Interface

Based on a novel split bi-layer shear deformable beam model capable of capturing the local deformation at the crack tip, the explicit closed-form solutions of bi-material interface fracture are presented in this paper. A recently developed novel shear deformable bi-layer beam theory is briefly reviewed, from which the deformation at the crack tip is explicitly derived. A new expression for the energy release rate is then obtained using the J integral, in which several new terms associated with the transverse shear force are present; this represents an improved solution compared to the one from the classical beam model. By exploiting the two concentrated crack tip forces, the general loadings acting at the crack tip are decomposed into two groups which produce only the mode I and mode II energy release rates, respectively; the total energy release rate is thus decomposed into the mode I and II components in a global sense. The stress intensity factor referred to as local decomposition is also obtained including the transverse shear effect. The difference between the global and local mode decompositions is clarified, and a simple relationship between them is provided. The effect of the existence of a thin layer of adhesive on the stress intensity factor is further studied by an asymptotic analysis. A simple and improved expression for the T stress, the nonsingular term of stress at the crack tip, is also given. The fracture parameters of several commonly used interface fracture specimens are summarized. The present fracture analysis including the transverse shear effect is in better agreement with finite element analyses and shows advantages and improved accuracy over the available classical solutions.     

Fracture Behavior of Multidirectional DCB Specimen: Higher-Order Beam Theories

Mathematical models, for the stress analysis of symmetric multidirectional double cantilever beam (DCB) specimen using classical beam theory, first and higher-order shear deformation beam theories, have been developed to determine the Mode I strain energy release rate (SERR) for symmetric multidirectional composites. The SERR has been calculated using the compliance approach. In the present study, both variationally and nonvariationally derived matching conditions have been applied at the crack tip of DCB specimen. For the unidirectional and cross-ply composite DCB specimens, beam models under both plane stress and plane strain conditions in the width direction are applicable with good performance where as for the multidirectional composite DCB specimen, only the beam model under plane strain condition in the width direction appears to be applicable with moderate performance. Among the shear deformation beam theories considered, the performance of higher-order shear deformation beam theory, having quadratic variation for transverse displacement over the thickness, is superior in determining the SERR for multidirectional DCB specimen.     

The noncontinuum crack tip deformation behavior of surface microcracks

The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.     

Effects of tungsten fiber on failure mode of zr-based bulk metallic glassy composite

The authors systematically investigated the effects of tungsten fiber on failure mode as well as deformation and fracture mechanisms in tungsten fiber-reinforced Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 bulk metallic glassy composite under uniaxial compression at room and high temperatures. At room temperature, the failure mode of the composite changes from shear fracture to longitudinal splitting failure with increasing fiber volume fraction. Similar to the observations in monolithic metallic glasses, the shear fracture angle of the composite is approximately equal to 39∼40 deg, indicating that the Mohr-Coulomb criterion is suitable to give the critical shear fracture condition of the composite. When the compression tests were performed below the glass transition temperature of Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 metallic glassT _g, the deformation behavior of the composite strongly depends on the strain rates and the test temperature, which is quite similar to the deformation behavior of monolithic metallic glasses in the supercooled liquid region. The corresponding failure mode of the composite changes from shear or splitting fracture to bending failure with decreasing strain rate or increasing test temperature. The failure modes at the temperature nearT _g are mainly controlled by the metallic glass matrix due to the decrease in its viscosity at high temperature. Based on these multiple failure modes, the effects of test temperature and tungsten fiber volume fraction on deformation and fracture mechanisms are summarized.     

Observation of fatigue damage process in SiC fiber-reinforced Ti-15-3 composite at high temperature

Unnotched SiC (SCS-6) fiber-reinforced Ti-15-3 alloy composite is subjected to a tension-tension fatigue test in a vacuum of 2×10⁻³ Pa at 293 and 823 K with a frequency of 2 Hz and R=0.1. Direct observation of the damage evolution process during the test is carried out by scanning electron microscopy (SEM). Test temperature dependent and independent fatigue damage behaviors are observed. The early stage fiber fractures observed at the polished surface are not influenced by the test temperature; however, matrix crack initiation and propagation behaviors differ greatly with temperature. The evolution of interface wear damage also differs with temperature, becoming more severe at 823 K, and the interface wear damage zone increases with the increase of the number of fatigue cycles. The macroscopic fatigue damage appears as a modulus reduction associated with interface sliding, matrix crack propagation, and plastic deformation of the matrix. The deformation zone of the composite tested at 823 K spreads more than that at 293 K. The fatigue life of the composite tested at 823 K is longer than that at 293 K. This behavior is related to the difference in spread of the damage zone in the matrix.     

Localized vs homogeneous deformation in Fe82B15Si3 amorphous alloy

Amorphous alloys are known to deform plastically either by a localized shear mode or in a homogeneous manner. Conditions that favor homogeneous deformation in an amorphous alloy consisting of 82Fe, 15B and 3Si in at. pct have been established by a free-bend test over a range of temperatures. These results are in turn related to the stress-strain rate-temperature relationship obtained by a load relaxation testing method. The intense, localized shear deformation is shown to occur at relatively high strain rate and low temperature regions where the rate sensitivity of flow stress,m, is small. With increasingm, plastic flow tends to take place homogeneously provided that the mode of deformation remains stable. The strain rate sensitivity of flow stress increases with decreasing strain rate and increasing temperature; the computed diffuse shear transformation zone also changes in the same manner. However, the general trend changes above the glass transition temperature, i.e., the diffuse shear transformation zone decreases and the flow strength increases with increasing temperature.     

The brittle-to-ductile transition in doped silicon as a model substance

In order to investigate the importance of the dislocation velocity for the brittle-to-ductile transition temperature, fracture experiments were performed on precleaved, dislocation-free silicon single crystals at elevated temperatures. The well known doping dependence of the dislocation velocity in silicon is used to obtain detailed information about the conditions in the plastic zone. At high temperatures, dislocations are generated at an applied stress intensity distinctly lower than the critical stress intensity for inducing cleavage in the dislocation-free samples at room temperature. As a consequence of this, the plastic zone which develops around the crack tip is saturated before the tensile stress at the crack tip reaches the cohesive stress. Because of saturation of the plastic zone, the brittle-to-ductile transition temperature is determined by the velocity at which dislocations move outward from the crack tip at low shear stresses existing in the vicinity of the shielded crack.     

An investigation of toughening in NiAl composites reinforced with yttria-partially stabilized zirconia particles

The results of an investigation of toughening mechanisms in NiAl composites reinforced with yttria-partially stabilized zirconia polycrystals are presented. Different yttria stabilization levels in the zirconia, between 0 and 6 mole pct are employed. It is shown that substantial improvements in fracture toughness are obtained in all the composites reinforced with partially stabilized zirconia particles. The phase contents and microstructures of the composite systems are characterized by X-ray diffraction and transmission electron microscopy (TEM) techniques. Crack tip deformation was also studied using crack tip TEM analysis, and laser Raman spectroscopy was used to estimate the size of the transformation zone. The results show that transformation toughening is significant only in the 2 mole pct yttria-stabilized zirconia composite. Toughening is also shown to occur via slip phenomena within the NiAl grains in the near-tip regions of the composites reinforced with 2, 4, or 6 mole pct yttria-stabilized zirconia particles.     

Elastic-plastic fracture by homogeneous microvoid coalescence tearing along alternating shear planes

Mating fracture surfaces developed by slow cracking in salt water and rapid overload fracture were studied by replication and scanning electron microscopy to determine the formation mechanism of the periodic ridges and valleys (“zigzag” fracture) characteristic of surfaces in materials which separate by homogeneous microvoid coalescence. The most significant findings are: 1) The apex of each ridge, which consists of a rim of tear dimples, is sharply defined; while by contrast the bottom of each mating valley is gently rounded, as the mating tear dimples have been severely elongated to form an extensive stretch zone, the back edge of which mates with the peak of the ridge. Thus it appears that this form of fracture is a continuous process in that a moving crack front is maintained, even though the plane of the crack changes orientation with crack length. The crack advances along the first plane inclined to the macroscopic Mode I fracture plane, is then interrupted by a forward stretching step, which is followed by cracking along the second inclined plane. A rationale for this behavior is presented in terms of a simple tension-compression model within a constrained plastic zone. 2) Elongated dimples on each incline are of two types, tear and shear, with the former clearly dominating. This observation implies a strain gradient in the process zone near the crack tip, with the point of maximum strain located at the crack tip. The photomicrographs support a model involving tearing along alternating shear planes.     

Effects of crack tip stress states and hydride-matrix interaction stresses on delayed hydride cracking

A model of slow crack propagation based on the delayed hydride cracking (DHC) mechanism in hydride-forming alloys has been critically examined and evaluated to take account of recent experimental and theoretical advances in the understanding of hydride fracture and terminal solid solubility (TSS). The model predicts that the DHC velocity is a sensitive function of the hydrogen concentration induced in the bulk of the material as a result of the direction of approach to test temperature. For test temperatures approached from below, factors such as the hydridematrix accommodation energies, the stress state at the crack tip, and the value of the yield stress have a strong effect on the DHC arrest temperature in the technologically interesting temperature range of 400 to 600 K. A fracture criterion is explored based on the need to achieve a critical hydride length in the plastic zone at the crack tip. A necessary condition for DHC is that the crack tip hydride must grow to this critical length. An approximate estimate is made for the steady-state growth limit of the crack tip hydride as a function of the direction of approach to temperature and the crack tip stress state. For temperatures approached from below, growth of the crack tip hydride is limited to just outside the plastic zone boundary at low temperature, gradually receding toward and inside the plastic zone boundary with increasing temperature. At lowK _I values, this limits the crack tip hydride lengths to below their critical values for fracture. This could be one condition forK _IH . For test temperatures approaches from above, the growth limit is significantly increased, and the sensitivities to the above parameters become less evident.     

Frequency interactions in high-temperature fatigue crack growth in superalloys

The influence of high frequency loading on the subsequent low frequency crack growth behavior in nickel-based alloy 718 in laboratory air environment at 923 K has been investigated through the use of a sequential high/low frequency load waveform. The parameters that have been examined include the crack growth rate, fracture surface morphology, and slip line density at and below the fracture surface. Results of this study indicate that prior application of high frequency loading results in reduction of the subsequent low frequency crack growth rate. An attempt is made to interpret this type of modification as being a result of the crack tip condi- tioning through the increase in the slip line density during the high frequency part of the loading cycle. Furthermore, by linking the type of selective oxide formed at the crack tip to the degree of deformation in the crack tip zone, a correlation has been made between the increase in the slip line density in the crack tip zone during the preceding high frequency loading and the increase of the crack resistance to environment degradation effects during the subsequent low frequency loading.     

Calculation models for average stress and plastic deformation zone size of bonding area in dentine bonding systems

Average stress during shear bond testing, and deformation behaviour during nano-indentation testing were calculated for the bonding area as a bonding site for copolymerization with resin composite in dentine bonding systems. First average stress was calculated in the bonding area between bovine dentine and composite resin. Secondly, the plastic deformation zone size was calculated using an elastic/plastic deformation zone model after a nano-indentation test. The result clearly showed that average stress depended upon the elasticity of the bonding area, the elasticity value ratio of the composite resin-to-bonding area, and interfacial stress between the dentine and the adhesive resin. In this bonding area, the elasticity/hardness ratio changed depending on the thickness of the bonding area as well as the plastic deformation zone size (b), expressed as a (b/2a)-value (indented triangular length (2a) at nano-indentation test) expanded with increasing the elasticity value.     

Steady state vacancy concentration around a plastic crack

The vacancy chemical potential associated with the crack region and with the lattice dislocations representing the plastic zone are identified in terms of the energy of the dislocation configuration. In order to determine the steady state vacancy concentration, the configuration is considered as made up of several internal sources of stress. The diffusion equation under steady state is solved for a crystal containing each source of stress. Further, superposition of the concentration of vacancies around each source is used to determine the total concentration of vacancies for small scale deformation at the crack tip. On the other hand, for large scale deformation at the tip, matching boundary conditions are applied to determine the concentration in each region containing an internal source. Both the discrete dislocation and the single lattice dislocation representations of the crack are employed to determine the crack growth rate. The results are used to emphasize the influence of the plastic zone on the crack growth rate by vacancy diffusion mechanism. Formerly Assistant Professor, Department of Engineering Science and Mechanics, Tennessee Technological University. Cookeville, TN 38501.