Brittle fracture in austenitic steel

The fracture mode of austenitic steel may feature a ductile to brittle transition (DBT), depending on alloy composition and temperature. The DBT variation can be explained in terms of the actual deformation structure evolving during cold work and the correlated internal stresses. The crucial features of microstructure causing brittle fracture are found to be the intersections of deformation twins and the total density of free dislocations. To avoid brittle fracture, the stresses of intersecting twins must be screened by dislocations. Manganese and nitrogen promote brittle fracture since they lower the stacking fault energy and thus shift the onset of twinning to low strain levels where the total dislocation density is low. Nickel additions oppose this trend. The results of the microstructural fracture model agree well with experimental results and the model is confirmed by continuum-mechanical considerations.     

Brittle to ductile transition in cleavage fracture

In many cleavable solids, cleavage cracks can propagate at steady state by laying down a trail of dislocations emanating from the crack tip and lying on planes inclined to the crack front. These crack-tip initiated dislocations produce shielding at the crack tip that reduces both the crack tip tensile stresses and the shear stresses on the inclined planes. They also blunt the crack. Cleavage cracks, can nevertheless, still propagate under appropriately increased stress intensity conditions to keep the crack tip tensile stress constant. A condition is reached in the propagation of such slightly blunted cracks where a small increment in temperature or a decrement in the crack velocity permits the nucleation of a new set of dislocations that produce additional shielding and blunting which tip the balance against the crack-tip tensile stresses. This results in a transition from brittle cleavage to ductile behavior. The steady state specific plastic work that can just be tolerated by a propagating cleavage crack before it catastrophically blunts is calculated to be only of the order of 10% of the specific surface energy. Although most geometrical details of the dislocation emission process are adequately modeled, the calculated brittle to ductile transition temperatures are found to be more than an order of magnitude higher than those that have been experimentally measured. This discrepancy is a result of the present inadequate methods of modeling activation configurations by considering the dislocation loop radius as the only activation parameter, while proper modeling of such configurations must consider also the Burgers shear displacement of the loop as an activation parameter. Such two parameter analyses, however, require accurate information on interlayer atomic shear resistance profiles for specific crystals which are presently not available. The analysis furnishes ready explanations of the toughening effects of so-called “ductilization” treatments and embrittling effect of aging and dislocation locking, as well as the relatively large difference between the lowest levels of toughness between fracture in polycrystals and in single crystals.     

Brittle fracture in polycrystalline Ir-0.3 pct W

Polycrystalline iridium fails by brittle intergranular fracture at room temperature. We have previously shown that this fracture is intrinsic because segregation of impurity elements to the grain boundaries was not found. We examined the structure of grain boundaries in Ir-0.3 pct W by transmission electron microscopy and found numerous ledges in the boundaries. In most boundaries the ledges aligned with the {111} crystallographic planes in one of the grains. In addition to ledges we observed numerous microscopic cracks near the edges of the thin foils. Most cracks occurred along twin boundaries. However, a substantial number of cracks that were not associated with twins also aligned with {111} planes in the grain matrix, establishing the {111} planes as secondary cleavage planes; {100} are the primary planes. The ease of cleavage along {111} planes makes the predominantly {111} oriented ledges ideal sites for crack initiation and propagation along grain boundaries, thereby explaining the intrinsic nature of intergranular fracture in iridium.     

硼铬微合金钢的脆性断裂失效分析

为了研究硼铬微合金钢的脆性断裂失效机理,以工厂履带拖拉机支重轮的中心轴脆性断裂为研究背景,通过对中心轴断口的观察以及不同位置的组织和晶粒度的对比,得出了硼铬微合金钢的断裂失效机理。利用FactSage和Thermo- Calc软件对TiN夹杂物的析出机理和控制进行理论计算。结果表明,钛合金的不完全溶解和钛、氮元素质量分数的不合理控制,导致大量的大尺寸硬质TiN夹杂物在凝固过程中析出,使得钛元素的加入没有起到很好地细化晶粒的效果,中心区域的铁素体＋奥氏体组织的晶粒度过于粗大。大尺寸TiN夹杂物作为裂纹源和心部粗大的晶粒导致了硼铬微合金钢的脆性断裂失效。热力学计算表明,TiN夹杂物在凝固过程中形成,低钛低氮和钛、氮质量分数的合理搭配可以有效推迟TiN夹杂物的析出,降低其尺寸。较小的夹杂物尺寸和细小的晶粒均可以有效增强材料抵抗裂纹扩展的能力。     

Brittle fracture of an Au/Ag alloy induced by a surface film

The film-induced cleavage model of stress-corrosion cracking (SCC) has been tested using an Ag-20 at. pct Au alloy in 1 M HClO₄ solution. Brittle cracks, both intergranular (IG) and transgranular (TG) in nature, were formed by high-speed loading of a thin foil covered with a dealloyed (nanoporous gold) layer. These cracks were found to propagate through the dealloyed layer and into the uncorroded bulk face-centered cubic (fcc) material for a distance of many microns. Hydrogen embrittlement (HE) can be excluded on thermodynamic grounds; thus, only film-induced cleavage can explain the observed decoupling of stress and corrosion in the fracture process.     

Influence of cerium additions on high-temperature-impact ductility and fracture behavior of iridium alloys

High-temperature tensile impact testing was carried out on Ir + 0.3 wt pct W alloys doped with cerium and thorium individually, and with cerium and thorium together. Impact ductility was evaluated as a function of grain size and test temperature. Cerium by itself was not as effective as thorium in improving the grain boundary cohesion, even though it segregated more strongly than thorium to the grain boundaries. This lower grain boundary cohesion was responsible for lower impact ductility and higher brittle-to-ductile transition temperature of cerium-doped alloys compared to those of the thorium- or thorium plus cerium-doped alloys. Reduction in thorium content by a factor of 5 (from 50 to 10 appm) in the bulk did not result in any significant reduction in hightemperature impact ductility or an increase in the brittle-to-ductile transition temperature as long as sufficient cerium was added to provide grain refinement. Grain boundary strengths of thorium- and thorium plus cerium-doped alloys were almost identical.     

Brittle fracture initiation associated with the strain localization in a heat-affected zone of a low carbon steel

Brittle fracture initiation in the ductile-brittle fracture transition region in the heat-affected zone (HAZ) of weldments of a low carbon steel has been investigated. Consistent with the previous results from blunt notch Charpy tests, brittle fracture initiation was observed in the case of J-integral tests to take place at the intersection of small bainitic ferrite grains of different orientations within a mixed area of bainitic ferrite and quasipolygonal ferrite in proximity to the boundary between a coarse bainitic ferrite. Partial load drop during loading, pop-in phenomena, in fracture mechanics tests in the low-temperature region is caused by essentially the same mechanism as for unstable brittle fracture initiation. Inhomogeneous microstructure in the HAZ gives rise to intense strain localizations in the mixed area of bainitic ferrite and quasipolygonal ferrite due to the constraint of plastic deformation therein and may produce accumulated defects that form an incipient crack for the brittle fracture. Partial load drop proceeds in association with repetitive initiations of brittle facets and their ductile linking. The strong temperature dependence of the magnitude of partial load drop is likely to show that the temperature dependence of the brittle fracture initiation is controlled by the first initiation of a brittle facet and the ductile linking with the following induced facets. Existence of coarse bainitic ferrite grains is a prerequisite for the extension of an incipient crack.     

火焰原子吸收光谱法测定铂铱合金中铱

样品经王水在烘箱中于180℃消解12h后,加入10mL100g/L氯化镧溶液和10mL100g/L硫酸铜溶液为释放剂,采用火焰原子吸收光谱法(FAAS)测定铂铱合金样品中铱。研究表明,铱浓度在15~100μg/mL范围内与其吸光度呈良好的线性关系,线性回归方程为A=0.00083ρ+0.0015,相关系数为0.9995,方法的检出限为0.0028μg/mL。采用方法对Pt25Ir和Pt10Ir样品中铱进行测定,并加入铱标准溶液进行加标回收试验,测得结果的相对标准偏差(n=11)为1.6%和2.3%,加标回收率为95%~101%。     

Oxidation of iridium

Iridium wires self-reistance-heated to temperatures in the range of 1675 to 2260°C (1948 to 2533 K) were oxidized in naturally convected oxygen at pressures in the range of 0.00132 to 1.32 atmospheres (134 to 1.34× 10⁵ Pa). The experimental results were closely corre-lated by a theoretical rate equation based upon control of the oxidation rates by diffusion of Ir(g), IrO₂(g) and IrO₃(g) through the gaseous boundary layer. Values were obtained for the standard-state free-energies of formation of IrO₂(g) and IrO₃(g), and their temperature dependencies were described by empirical equations.     

Validating Theories for Brittle Damage

Validating simulated predictions of internal damage within armor ceramics is preferable to simply assessing a model’s ability to predict penetration depth, especially if one hopes to perform subsequent “second strike” analyses. We present the results of a study in which crack networks are seeded by using a statistically perturbed strength, the median of which is inherited from a deterministic “smeared damage” model, with adjustments to reflect experimentally established size effects. This minor alteration of an otherwise conventional damage model noticeably mitigates mesh dependencies and, at virtually no computational cost, produces far more realistic cracking patterns that are well suited for validation against X-ray computed tomography (XCT) images of internal damage patterns. For Brazilian, spall, and indentation tests, simulations share qualitative features with externally visible damage. However, the need for more stringent quantitative validation, software quality testing, and subsurface XCT validation, is emphasized.     

Micromechanical Analysis of Anisotropic Damage in Brittle Materials

A general three-dimensional micromechanical approach to modeling anisotropic damage of brittle materials such as concrete, rocks, or certain ceramics is presented. Damage is analyzed as a direct consequence of microcracks growth. Following a rigorous scale change methodology, the macroscopic free energy of the microcracked medium is built considering either open and closed microcracks. Moreover, the microcracks opening/closure criterion as well as the moduli recovery conditions (unilateral effects) are addressed in stress-based and strain-based formulations. An alternative derivation of the homogenized properties, based on the well-known Eshelby method, is also presented and extended here to closed cracks. From the micromechanical analysis, an energy-based yield condition is formulated and illustrated in various stress subspaces. Assuming that the normality rule applies, we then present the damage evolution law and the rate form of the constitutive model. The main capabilities and advantages of the micromechanical model are illustrated through various examples in which material microstructure evolutions are presented.     

Effects of Preloading on Brittle Solids

A general theory that addresses the preloading effects on the strength of brittle solids as a consequence of the progressive cracking is developed within the framework of continuum damage mechanics theories. To accommodate the effects of rotation of the loading paths in the stress-strain behavior of the material, an effective damage parameter is defined. The rate of the damage parameter is obtained from the consistency condition of the loading (damage) surface. Based on the directionality of the loading paths, damage is stored in appropriate directions through the components of material compliance. The compliance and inelastic strain tensors are separated into tensile and compressive components to address the material behavior in damage modes I and II, respectively. Due to the lack of experimental data that assess the effects of orthogonal preloading, the model is compared with failure simulations published by the University of Colorado. Stiffness recovery upon load reversal, an important feature of brittle solids, is modeled by introducing an effective compliance tensor in mode I. Finally, to demonstrate the model's ability to replicate both proportional and nonproportional stress paths, it is compared against the experimental data.     

船用锚链钢脆性断裂的研究

研究了30Mn2V三级锚链钢在调度下的脆性裂及其性转变温度,实验证明：30Mn2vV三级锚钢的脆性转变温度范围为－70℃－－80℃,为锚链钢的低温脆性断裂提供了有利的科学依据。     

New Biaxial Failure Criterion for Brittle Materials in Compression

This paper focuses on the relationship of the stress intensity factor (SIF) between biaxial compression KII and uniaxial compression KII0. This relationship is very important for the study of biaxial failure criterion because if this relationship can be found, the biaxial failure criterion can be easily set up by use of the uniaxial failure criterion, which is very simple (KII0 ≤ KIIC or σ ≤ σC). For this purpose, a new model for a brittle material plate containing an inclined crack in compression, based on the theoretical analysis and the calculated results of the boundary collocation method (BCM), is proposed. By using this model and the BCM results, the following topics are first discussed: (1) The orientation of the most unfavorable crack; and (2) the stress condition for a crack with zero SIF value. Second, the relationship between KII and KII0 is found and described in a simple formula. Finally, a new biaxial failure criterion, which is expressed in terms of the principal stresses, is developed, and several conclusions are presented.     

Shear Analysis of Concrete with Brittle Reinforcement

The design of steel-reinforced concrete relies on lower-bound plasticity theory, which allows an equilibrium-state to be postulated without considering compatibility. This is of particular benefit in shear design, due to the complexity of shear-transfer, where simplified models such as the truss analogy are used. Lower-bound plasticity theory, however, relies on stress-redistribution. If brittle reinforcement [such as fiber-reinforced-plastic (FRP)] is used in concrete, lower-bound plasticity theory cannot be applied. This paper studies how compatibility, equilibrium, and the material constitutive laws can be combined to establish the actual conditions within an FRP-reinforced beam subjected to shear. A crack-based analysis is proposed to model shear failure in a beam with brittle reinforcement. The analysis is used to illustrate the importance of satisfying compatibility requirements, and the results are contrasted with the current shear design proposals for FRP-reinforced concrete.