Fracture and R-curve behavior of an intermetallic β-stabilized TiAl alloy with different nearly lamellar microstructures

Four different microstructures of an engineering multi-phase Ti–43.5Al–4Nb–1Mo–0.1B alloy (in at.%) were fracture mechanically tested under monotonic loading conditions from room temperature up to 700 °C. Monotonic loading crack resistance curves were measured using the potential drop technique and the calculated crack lengths were compared to optically measured crack lengths during in-situ experiments under the light-optical microscope. By examining the fracture surfaces and the polished side faces of the samples in the scanning electron microscope the fracture mechanisms were evaluated. It was found that the fracture toughness mainly depends on structure and phase arrangement at the boundaries and can be enhanced up to 30% by having thick boundary areas with a few micrometers in width, consisting of unconnected β_O and γ grains. In contrast, a connected β_O-phase at the colony boundaries as well as a texture which provide lamellae parallel to the subsequent fracture surface reduce the fracture toughness.     

Aging and fracture of human cortical bone and tooth dentin

Mineralized tissues, such as bone and tooth dentin, serve as structural materials in the human body and, as such, have evolved to resist fracture. In assessing their quantitative fracture resistance or toughness, it is important to distinguish between intrinsic toughening mechanisms, which function ahead of the crack tip, such as plasticity in metals, and extrinsic mechanisms, which function primarily behind the tip, such as crack bridging in ceramics. Bone and dentin derive their resistance to fracture principally from extrinsic toughening mechanisms, which have their origins in the hierarchical microstructure of these mineralized tissues. Experimentally, quantification of these toughening mechanisms requires a crack-growth resistance approach, which can be achieved by measuring the crack-driving force (e.g., the stress intensity) as a function of crack extension (“R-curve approach”). Here this methodology is used to study the effect of aging on the fracture properties of human cortical bone and human dentin in order to discern the microstructural origins of toughness in these materials.     

Dependency of fracture toughness of plasma sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coatings on lamellar structure

The fracture toughness of plasma-sprayed Al₂O₃ coatings in terms of critical strain energy release rate G _Ic was investigated using a tapered double cantilever beam (TDCB) approach. This approach makes the fracture toughness be measured only using the critical fracture load disregarding crack length during test. The Al₂O₃ coatings were deposited under different spray distances and plasma powers to clarify the effect of spray parameters on the G _Ic of the coatings. The fracture surfaces were examined using scanning electron microscope. On the basis of an idealized layer microstructure model for thermal sprayed coatings, the theoretical relationship between the cohesive fracture toughness and microstructure is proposed. The correlation between the calculated fracture toughness and observed value is examined. It was found that the fracture toughness of plasma sprayed Al₂O₃ coatings is not significantly influenced by spray distance up to 110 mm, and further increase in spray distance to 130 mm resulted in large decrease in the fracture toughness of the coatings. The G _Ic value predicted based on the proposed model using lamellar interface mean bonding ratio and the effective surface energy of bulk ceramics agreed well with the observed G _Ic data. Such agreement evidently shows that the fracture toughness of thermally sprayed ceramic coatings at the direction along coating surface is determined by lamellar interface bonding.     

Phase assemblage effects on the fracture and fatigue characteristics of magnesia-partially stabilized zirconia

A detailed investigation on the relationships between phase assemblage and fracture and fatigue characteristics of Mg-PSZ has been conducted. In doing so, three completely different microstructural conditions were first attained through different thermal treatments and then their flexural strength, fracture toughness and crack growth resistance and fatigue crack growth (FCG) behaviour were evaluated. The obtained results are discussed considering the interplay between microstructural features and dominant crack-microstructure interaction and its influence on the operation of given toughening and mechanical fatigue mechanisms for each phase assemblage studied. FCG resistance, under both sustained and cyclic loading, is found to be closely related to the corresponding fracture toughness of each phase assemblage. However, real mechanical fatigue effects are estimated to be, once they are rationalized with respect to particular environmental-assisted cracking behaviours, an exclusive function of crack path type. Finally, different cyclic fatigue mechanisms for Mg-PSZ are pinpointed depending upon the prevalent transgranular or intergranular FCG morphology.     

Effects of rate on crack growth in a rubber-modified epoxy

The rate-dependent fracture behavior of a 10-phr rubber-modified epoxy was investigated using double-cantilever-beam tests at various crosshead speeds. Dramatic rate effects were observed in the R-curve behavior and in the relationship between the applied energy-release rate and the crack velocity. Furthermore, a transition between fracture with toughening mechanisms operating (kinetic crack growth) and brittle behavior (dynamic crack growth) was observed. This transition depended on the crack velocity and applied energy-release rate. Such behavior is expected to depend on how the intrinsic toughness and/or the extrinsic toughening mechanisms are influenced by strain rate. It was shown that the size of the process zone was only weakly dependent on the crack velocity until the onset of dynamic fracture. Furthermore, the extent of void growth was virtually independent of the crack velocity in the kinetic regime. These results appear to rule out the notion that crack-tip shielding is significantly affected by rate effects in this rubber-modified epoxy. Rather, the rate effects may arise from a rate-dependent intrinsic toughness. It was observed that the intrinsic toughness decreased significantly with increasing crack velocity. The crack instability was shown to be associated with an abrupt cessation of the development of the process zone, with both cavitation and void growth being totally suppressed.     

脆性材料断裂韧性测试及表现断裂韧性的估算

用２种测试技术测试了脆性材料的断裂韧韧性,一种是利用楔入法在悬臂梁试件预裂出自然裂纹;另一种是用研磨法在三点弯曲试件上作出微米级的尖缺口裂纹。分别测试了３种材料的断裂韧性,并用提出的模型分别评估了多晶和非晶脆性材料的断裂韧性。     

EFFECT OF CREEP FRACTURE TOUGHNESS ON CRACK INITIATION AND GROWTH

本文研究了蠕变断裂韧性对二种低合金耐热钢蠕变裂纹开裂和扩展的影响。试验表明:随着蠕变断裂韧性提高,抗蠕变裂纹开裂和扩展能力增加。材料呈韧性或脆性状态时,蠕变裂纹萌生和扩展过程不同。韧性状态时,裂纹为穿晶和晶界二种混合形式:穿晶裂纹可在晶内碳化物处发生,或在晶界上形核后向晶内扩展,晶界裂纹仍是由晶界上空洞形成和相互连接而成,裂纹可沿晶界和晶内扩展,但不连续。脆性状态时,裂纹沿晶界发生,它是由晶界形成空洞和相互连接而成,扩展仅沿晶界发生。     

Role of the grain-boundary phase on the elevated-temperature strength,toughness, fatigue and creep resistance of silicon carbide sintered with Al,B and C

The high-temperature mechanical properties, specifically strength, fracture toughness, cyclic fatigue-crack growth and creep behavior, of an in situ toughened silicon carbide, with Al, B and C sintering additives (ABC-SiC), have been examined at temperatures from ambient to 1500°C with the objective of characterizing the role of the grain-boundary film/phase. It was found that the high strength, cyclic fatigue resistance and particularly the fracture toughness displayed by ABC-SiC at ambient temperatures was not severely compromised at elevated temperatures; indeed, the fatigue-crack growth properties up to 1300°C were essentially identical to those at 25°C, whereas resistance to creep deformation was superior to published results on silicon nitride ceramics. Mechanistically, the damage and shielding mechanisms governing cyclic fatigue-crack advance were essentially unchanged between ∼25°C and 1300°C, involving a mutual competition between intergranular cracking ahead of the crack tip and interlocking grain bridging in the crack wake. Moreover, creep deformation was not apparent below ∼1400°C, and involved grain-boundary sliding accommodated by diffusion along the interfaces between the grain-boundary film and SiC grains, with little evidence of cavitation. Such unusually good high-temperature properties in ABC-SiC are attributed to crystallization of the grain-boundary amorphous phase, which can occur either in situ, due to the prolonged thermal exposure associated with high-temperature fatigue and creep tests, or by prior heat treatment. Moreover, the presence of the crystallized grain-boundary phase did not degrade subsequent ambient-temperature mechanical properties; in fact, the strength, toughness and fatigue properties at 25°C were increased slightly.     

Crack propagation behavior in TiAl–Nb single and Bi-PST crystals

The fracture toughness of directional solidified Ti–(45,47)Al–3Nb, Ti–(45,47)Al–3Nb–0.2Si–0.1C, Ti–(45,47)Al–3Nb–0.3Si–0.2C type I alloys and their contribution to crack growth resistance of TiAl–Nb alloys were studied using PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation: an in-plane kink angle and a through-thickness twist angle. Therefore, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. Crack growth resistance in colony boundary was identified by three-point bend test and crack advance was monitored by interrupted in situ test. From three-point bend test, it was found that the colony boundary could offer significant resistance to crack growth under large twist angle difference. Fracture toughness of type I specimens (in which crack propagates against lamellae boundaries) of the alloys decreased slightly with increasing Si and C contents and increased rapidly with decreasing Al content. The toughness for type I specimens was controlled by α₂–α₂ spacing in which the delamination-type separation occurred. Compared to 47Al alloys, α₂–α₂ spacing in 45Al alloys increased by decreasing Al content, therefore, fracture toughness increased rapidly. These results are discussed and the ability to improve toughness by changing Al content, Si and C addition in TiAl–Nb alloys produced by directional solidification is suggested.     

Microindentation fracture behavior of surface modified alumina ceramic using glass infiltration

This study investigates the effect of a low expansion glass (Mg₃Al₂Si₆O₁₈) treatment on the surface fracture toughness of sintered alumina. The surface fracture toughness was determined by direct indentation method (Vickers indentations), carried out at different loads ranging from 9.8 to 196 N. The crack lengths on the surface at each load were found to be decreased (8–12%) by glass treatment and the corresponding crack resistance values increased by about 17–20%. Both sintered and glass treated specimens showed rising trend in crack resistance values as the indentation load was increased. There was also a significant increase in the Weibull modulus value of crack resistance. Improved properties of glass treated sample were attributed to the formation of a relatively larger process zone surrounding the crack, crack arrest behavior due to the compressive stresses and the crack bridging phenomena. The compressive stresses were generated from the thermo-elastic properties mismatch: (a) between the glass and the ceramic in the glass infiltrated zone, and (b) the glass–ceramic composite layer and the ceramic substrate.     

Effect of carbide distribution on the fracture toughness in the transition temperature region of an SA 508 steel

An investigation was conducted into the effect of carbide distribution on fracture toughness in the ductile–brittle transition temperature region of an SA 508 steel used for nuclear reactor pressure vessels. Tensile properties and elastic–plastic cleavage fracture toughness were measured in the transition temperature region, and the fracture toughness data were interpreted by using a simple fracture model containing carbide size distribution. This modeling study indicated that the critical nearest-neighbor distance between coarse carbides was an important microstructural factor affecting elastic–plastic fracture toughness, since it satisfied a linear relationship with the critical distance between a crack tip to a cleavage initiation site. These findings suggested that reducing the total number of carbides, particularly the number of M₃C carbides larger than the critical size, and homogeneously distributing fine M₂C carbides, were useful ways to improve fracture toughness in the transition temperature region.     

Fracture Toughness of Functionally Graded Steels

In this study, fracture toughness of functionally graded steels in both crack divider and crack arrester configurations has been studied. Spot-welded plain carbon steel and austenitic stainless steel with different thicknesses and arrangements were used as electrodes of electroslag remelting to produce functionally graded steels. Fracture toughness of the specimens in crack divider configuration was found to depend on the arrangements of the primary electrodes’ pieces together with the type of the containing phases. In crack arrester configuration, the fracture toughness was found to depend on the crack tip position and the distance of the crack tip with respect to the bainitic or martensitic intermediate layers.     

X70管线钢不同温度下断裂韧性实验研究

以三维弹塑性断裂理论为基础，对有限厚度板裂纹端部应力场、三维应力约束进行了分析，通过对不同厚度、不同初始裂纹长度在不同温度下三点弯曲试件的断裂韧性测试断口观测和理论分析获得如下结果：离面约束对裂尖应力场及断裂韧性有强烈的影响；断口均产生分层裂纹，其位置、大小和数量与试样厚度、温度和裂纹初始长度有关；温度较低时，分层裂纹距主裂纹根部一定距离，分层裂纹宽度较小，对厚度效应影响较小；温度较高时，分层裂纹首先出现在主裂纹根部，分层裂纹宽度较大且充分张开，降低了试样的有效厚度，对X70管线钢进行性能评价时必须考虑管道壁厚、层裂和环境温度的耦合作用。     

The influence of chromium on mechanical properties of austempered ductile cast iron

An investigation was carried out to examine the influence of microstructure and chromium on the tensile properties and plane strain fracture toughness of austempered ductile cast iron (ADI). The investigation also examined the growth kinetics of ferrite in these alloys. Compact tension and round cylindrical tensile specimens were prepared from ductile cast iron with Cr as well as without Cr. These specimens were then given four different heat treatments to produce four different microstructures. Tensile tests and fracture toughness tests were carried out as per ASTM standards E-8 and E-399. The crack growth mechanism during fracture toughness tests was also determined. The test results indicate that yield strength, tensile strength, and fracture toughness of ADI increases with an increase in the volume fractions of ferrite, and the fracture toughness reaches a peak when the volume fractions of the ferrite are approximately 60% in these alloys. The Cr addition was found to reduce the fracture toughness of ADI at lower hardness levels (<40 HRC); at higher hardness levels (≥40 HRC), the effect of chromium on the fracture toughness was negligible. The crack growth mechanism was found to be a combination of quasi-cleavage and microvoid coalescences, and the crack trajectories connect the graphite nodules along the way.     

Laminated particulate-reinforced aluminum composites with improved toughness

An architectural approach for toughening discontinuously reinforced aluminum (DRA) alloys is described. It is based upon exploiting the higher apparent toughness of thin DRA lamina to obtain a laminate of higher thickness and toughness. The laminated composite consisted of alternate layers of a 7093/SiC/15p DRA, and an unreinforced aluminum–manganese alloy, 3003. Fracture toughness tests in the crack divider configuration showed a toughness improvement of 79% in an underaged condition and an improvement of 53% in the peak-aged condition compared to the monolithic DRA. Fractographic observations of the primary void size, and its close correspondence with the fracture surface of thin specimens, provided evidence that the individual DRA lamina indeed experienced sufficient loss of constraint in the thickness direction. This was further confirmed by observations of delamination in the fast fracture domain. However, the bond strength was quite good, as evidenced by very little delamination in the fatigue crack growth region, and by a lack of such damage in tension specimens. In essence, the laminate behaved as a smart structure, being resistant to failure under normal conditions, but allowing full loss of constraint in the severe stress–strain field ahead of a loaded crack. Modeling efforts were consistent with a reduction of hydrostatic stress, through the loss of out-of-plane constraint for the laminas, although the predicted level of toughness improvement was lower than observed. Overall, this study clearly demonstrates that the fracture toughness of laminated composites can be engineered based on the understanding of constraint effects associated with specimen thickness in the DRA composite.     

Microstructural model of intergranular fracture during tensile tests

A microstructural model of intergranular fracture in textured materials is presented. In this model, the material is represented by a two-dimensional microstructure with non-regular polygonal grains which represents material's texture and grain shape measured in experiments or calculated from Monte Carlo simulations. The grain boundary character, grain boundary energy, and fracture stress are assigned to each grain boundary according the grain boundary character distribution. Intergranular fracture susceptibility is analyzed by defining the probability of finding a continuous path along the grain boundaries which are intrinsically susceptible to fracture. In this analysis the orientations of the grain boundary with respect to the applied or residual tensile stress axis is considered. The probability of intergranular fracture for each grain boundary depends on the intergranular fracture resistance, the interface orientation relative to the stress axis, and a value of the tensile stress acting on the grain boundary. The crack arrest distance and the fracture toughness are calculated in terms of the frequency of low-energy grain boundaries, fracture stress of low-energy grain boundary, angle distribution of grain boundary interfaces, and anisotropy of grain shape. The results indicate that the fracture toughness increases and the crack arrest distance decreases dramatically with increasing the frequency of the low-energy grain boundaries. Lowering the grain boundary energy can improve the fracture toughness and decrease the crack arrest distance. The angle distribution of grain boundary interfaces and the grain shape factor are also very effective in controlling the fracture toughness. High fracture toughness of polycrystalline materials is related to the presence of a high frequency of low-energy boundaries which are resistant to fracture. The best fracture toughness for brittle materials can be achieved by controlling the frequencies of the low-energy grain boundaries, the grain boundary character, and the boundary inclination.     

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen

Fatigue crack growth test was performed to evaluate fatigue behavior of 304 stainless steel specimens with or without laser processing (welding and surface treatment) in air and gaseous hydrogen. As the crack propagation normal to the laser welding or scan direction, the laser-processed specimens exhibited a higher resistance to crack growth in the low stress intensity factor range (ΔK) than the as-received steel plates regardless of testing environments. However, the marked retardation of crack growth behavior vanished for welded specimens subjected to a 850 °C/h stress relief treatment or with a shorter distance from notch tip to the weld centerline in the test.Fatigue-fractured appearance of the steel plate tested in air was composed of mainly transgranular fatigue fracture and some flat facets, along with a small amount of intergranular fracture. While quasi-cleavage fracture and few twin boundary separations were observed for the same specimen in hydrogen. On the other hand, the lower crack growth rate of laser-processed specimens in both air and hydrogen was accompanied with rubbed areas on the fracture surfaces. It was found that the extent of quasi-cleavage fracture was related to the formation of strain-induced martensite, which would contribute to an increased fatigue crack growth rate of all specimens in gaseous hydrogen.     

Investigations on the microstructure and room temperature fracture toughness of directionally solidified NiAl–Cr(Mo) eutectic alloy

The microstructures and room temperature fracture toughness of directionally solidified NiAl-xCr-6Mo (x = 28, 32 and 36 at%) alloys were investigated. Fully eutectic microstructure could be obtained in the alloys over a wide composition range. High temperature gradient could increase the planar/cellular transition rate and expand the eutectic coupled growth zone. The volume fraction of Cr(Mo) strengthening phase increased with the increasing content of Cr, accordingly, the fracture toughness of NiAl–Cr(Mo) alloys also gradually increased. The fracture toughness of 26.15 MPa m^1/2 was obtained in the NiAl-36Cr-6Mo hypereutectic alloy solidified at withdrawal rate of 10 μm/s and temperature gradient of 600 K/cm, which is the highest value in the NiAl–Cr–Mo alloy system until now. Well-aligned microstructure was beneficial to the enhancement of the fracture toughness, while the existence of primary phase seriously deteriorated the toughness. All the directionally solidified NiAl–Cr(Mo) alloy failed as brittle quasi-cleavage fracture. Some toughening mechanisms, such as crack bridging, crack nucleation, crack blunting, crack deflection, interface debonding and shear ligament toughening as well as linkage of microcracks were observed. In addition, mobile dislocation generated from the interface also had significant influence on the toughness.