A strain-based fracture model for stress corrosion cracking of low-alloy steels

The stress corrosion cracking (SCC) behavior of 4135 steel under different heat treatments is analyzed in an attempt to relate microstructural characteristics with macroscopic measurements of SCC resis-tance, especially the very impressive improvements associated with changes from intergranular (IG) to transgranular (TG) fracture paths. Considering that local hydrogen embrittlement at the crack tip causes SCC processes, a local cracking criterion, based on a critical strain depending on hydrogen concentration, is assumed to control the process. Stress corrosion cracking is viewed as a discontin-uous series of unstable crack extensions through the locally embrittled regions. The model developed on this basis explains the macroscopic behavior observed at the threshold situation and partially at stage II propagation and clarifies the role of the metallurgical variables in each of the types of fracture detected.     

Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

To establish correlations between microstructure and mechanical properties for the Ti-ll alloy, twelve different combinations of hot die forging and heat treatment, in the a + 8 and Β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566‡C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior. Formerly a National Research Council Associate, Air Force Materials Laboratory Formerly with AFML     

Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

To establish correlations between microstructure and mechanical properties for the Till alloy, twelve different combinations of hot die forging and heat treatment, in the α+β and β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566°C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior.     

锌锅用热轧钢板解理断裂行为分析简

 通过断裂试样断口的宏观和显微分析、显微组织表征、拉伸和冲击试验以及解理断裂应力条件,讨论分析了锌锅用低强度级别钢板弯曲成形断裂的微观解理断裂行为。结果表明,钢板发生解理断裂的微观机制与冲击试样断裂相同,即晶粒尺寸控制的穿过晶界的裂纹扩展是解理断裂的临界事件。粗大的铁素体晶粒的面积分数过高显著降低了裂纹扩展阶段所需的局部解理断裂应力σf。断口宏观分析判断在钢板边部应存在导致应力集中的初始裂纹源,这极大降低了启动解理断裂的断裂应力并同时提高裂纹源前端的正应力σyy,扩大了解理断裂活跃区至初始裂纹前端,从而不可避免地发生脆性解理断裂。     

Thermal stress fracture of refractory lining components: Part III. Analysis of Fracture

The fracture behavior of refractory components heated from one end is simulated using a twodimensional constant heating rate thermoelastic model and the maximum principal tensile stress fracture criterion. Dimensionless graphical relationships that can be used to predict location of fracture and orientation of cracking are presented. Dimensional analysis and the finite element numerical method are used to develop a general solution for the total strain energy. Based on the premise that extent of crack propagation is directly related to available strain energy at fracture and inversely related to the surface energy per unit area, the solution for total strain energy is used to derive a damage resistance parameter useful for the design and selection of refractory components that accounts for material properties, geometry, and heating and cooling rate. Model predictions of location of fracture, orientation of cracking, and extent of crack propagation are in general agreement with experimental results previously reported in the literature. Limitations of the two-dimensional thermoelastic model are discussed.     

Dynamic fracture toughness of 4340 VAR steel under conditions of plane strain

Plate impact experiments are conducted to study the dynamic fracture processes in 4340 VAR steel which occur on submicrosecond timescales. These experiments involve the plane strain loading of a planar crack by a plane tensile pulse with a duration of approximately 1 μs. The loading is achieved by impacting a precracked, disk-shaped specimen by a thin flyer plate. Motion of the rear surface of the specimen, caused by waves diffracted from the stationary crack and by waves emitted from the running crack, is monitored at four points ahead of the crack tip using a laser interferometric system. The measured rear surface motion is compared with the calculated motion using the finite element method to gain understanding of the dynamic fields that occur near the crack tip during crack initiation and propagation. For low temperature experiments, the measured rear surface particle velocity fields are in good agreement with the computed profiles obtained for a constant velocity crack propagation model. For the room temperature experiments, the experimental free surface particle velocityvs time profiles show a sharp spike, with a duration of less than 100 ns at the moment of crack initiation. The spike, which is not predicted by the inverse square root singular stress fields of linear elastic fracture mechanics, is understood to be related to the onset of crack growth. Critical values of the fracture toughness are estimated from the crack initiation times determined both from the velocity time profiles and the elastodynamic modeling of crack advance. The toughness values obtained increase with increasing impact velocity and are as large as 170 MPa√m at the highest impact velocity. Such relatively high values appear to be consistent with the ductile mode of crack initiation observed at all impact velocities used in the present study. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

Observations on the effect of cementite particles on the fracture toughness of spheroidized carbon steels

The results of experimental studies of the influence of cementite particles on the fracture toughness of a number of spheroidized carbon steels at low temperatures were analyzed in terms of current theories of crack-tip behavior. The fracture toughness parameterK _IC was evaluated by using circumferentially notched and fatigue-cracked cylindrical specimens. The conclusions are summarized as follows: 1) In general,K _IC decreases with increasing volume fraction and increasing size of the carbide particles. 2) Crack initiation occurs at the carbide particles. 3) Crack propagation occurs by cleavage if the stress conditions satisfy the Ritchie, Knott and Rice criterion that a critical cleavage stress is achieved over a minimum microstructural size scale. The critical stress is that required to propagate a crack from a particle and the minimum size scale is of the order of 1 to 2 grain sizes. 4) Crack propagation occurs initially by fibrous rupture if the stress intensification is insufficient to attain the critical cleavage stress. P. Rawal was formerly affiliated.     

A study of the mechanism of stress-corrosion crack propagation in AISI 4340 steel in aqueous 3.5 wt.% NaCl solution

Stress-corrosion (SC) crack propagation in AISI 4340 steel has been studied with 2 mm thick single edge-notched (SEN) specimens under constant load conditions as a function of applied potential and tempering conditions in an aqueous 3.5 wt.% NaCI solution at 30°C. The SC crack lengths were estimated by using the electrical potential method. As the amount of cathodic polarization increased, the SC crack propagation rate increased. Anodic polarization yielded opposite results. These polarization effects on the SC crack propagation are discussed in terms of absorbed hydrogen resulting from a cathodic reaction on the specimen surface. SC cracks propagated by intergranular fracture through most of the inner region, but shear lips were formed at the near subsurface, irrespective of applied potential and tempering temperature. This is explained in terms of the stress state dependency of hydrogen behaviour. Above experimental evidence well supports the theory that SC crack propagation is controlled by the hydrogen embrittlement (HE) process. The SC crack propagation rate decreased in the sequence of 300, 200, and 400°C-tempered specimens. This is discussed as being related to the microstructural and yield stress effects.     

The effect of environment on the mechanism of Stage I fatigue fracture

Fatigue crack propagation in nickel-base superalloys at low and intermediate temperatures occurs predominantly in the Stage I mode, along {111} slip planes. Cracking normally starts at an external surface and the Stage I fracture surface has a cleavage appearance. Both of these factors indicate that the environment may play an important role in this mode of propagation. To assess the role of environment in Stage I fracture and to determine the mechanism of failure, fatigue tests were run in air and vacuum on single crystals of low-carbon MAR-M200. The fatigue life at room temperature is significantly greater in vacuum than in air, and the improvement in life increases as the stress range is reduced. Fatigue crack propagation in specimens tested in air and in vacuum is entirely in the Stage I mode, but only the specimens tested at low stress ranges in air have a cleavage appearance. In vacuum and at high applied stress levels in air, fracture surfaces have a matte appearance with fewer fracture steps and river lines. At high magnifications, a dimpled structure is observed on these fracture surfaces. The fatigue life in air can be attributed to a faster rate of crack growth resulting from oxygen adsorption at the crack tip. A model for Stage I fatigue crack propagation in planar slip materials is presented which is an extension of the Griffith-Orowan criterion to cases where localized cleavage occurs at a crack tip in fatigue.     

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.     

Grindability of Ti alloys

The grindability of Ti-based alloys was analyzed by considering the fracture behavior of individual alloys in response to the stress field of a grinding wheel. First, the stress field under a grinding wheel was computed by treating the grinding wheel as a cylindrical disk with a flat region acting on a flat substrate. The initiation and propagation of microcracks in the substrate was then examined on the basis of the contact stress field and one of two fracture criteria: (1) a critical stress criterion for the onset of cleavage crack initiation, and (2) a critical stress intensity factor criterion for the initiation and propagation of shear cracks. Grindability was computed as a function of grinding speed and microstructure for several Ti-based cast alloys containing α, α + β, or β microstructure with or without the intermetallic precipitates. Model predictions indicated that the grindability of Ti alloys increases with decreasing fracture toughness or tensile ductility. The theoretical results are compared against experimental data in the literature to elucidate the roles of microstructure in grindability. The comparison revealed that alloying addition that leads to the formation of brittle intermetallics enhances grindability by reducing fracture toughness, tensile ductility, and the resistance to crack initiation and propagation.     

冲击荷载下岩石裂纹扩展研究进展

为了给深部资源开采和大型地下空间工程中围岩体的变形机理及稳定性控制提供理论基础,通过查阅大量关于表征岩石裂纹扩展的裂纹扩展模型、应力强度因子和断裂韧性的国内外文献,总结了前人的研究成果。依据现有研究,提出了动荷载作用下岩石裂纹扩展的几点建议：（1）综合考虑弹性力学、断裂力学和损伤力学建立岩石材料从微观断裂到宏观破坏这一演变过程的理论模型,使理论模型更加适应岩石材料的非线性特征;（2）采用分形、自组织和混沌等非线性理论表征动荷载作用下岩石内部以及表面裂纹的扩展演化特征;（3）采用颗粒离散元和有限差分模拟岩石材料裂纹扩展演化特征。     

Segregation effects in the fracture of brittle materials: CaAl2O3

The fracture properties of the model ceramic system CaAl₂O₃ have been studied by strength measurements at controlled flaw sizes and direct observations of crack propagation, with a view to determining the effect of grain-boundary Ca segregation. In contrast to the assumptions of previous works the fracture properties are not controlled by the grain-boundary properties alone, but by an increasing toughness with crack extension, a T-curve, observed in all the microstructural variations examined (changing grain-size and grain-boundary Ca concentration). The origin of the T-curve is identified as the formation and subsequent rupture of ligamenting bridges of material, acting as restraining elements behind the crack tip. Models of the bridging process are developed to quantify the T-curves underlying the observed strength behavior, highlighting the equal roles played by the intrinsic interfacial (grain-boundary) properties, and the toughening mechanisms, in determining the overall fracture response of these and similar polycrystalline materials.     

The effects of microstructure,strength level,and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.     

The relation between microstructure and toughness in 7075 aluminum alloy

Electron microscopy, fractography, and notched tear tests have been used to investigate the effects of heat treatment upon the fracture behavior of aged 7075 aluminum alloy sheet. Toughness, as measured by crack propagation energy, decreases as the yield stress increases; the toughness of an overaged structure is inferior to that of an underaged structure at the same yield stress. The decrease of toughness with increased aging time is accompanied by a change in fracture mode from predominantly transgranular to intergranular. Transgranular fracture proceeds by dimple rupture and is facilitated by chromium-rich particles which are dispersed throughout the microstructure. Intergranular fracture proceeds by the fracture of grain boundary precipitate particles. The variation of fracture mode with aging time is attributed to a steady decrease of the intergranular fracture stress relative to the transgranular fracture stress, due to increasing grain boundary particle size. A possible explanation of this effect is discussed using the stress concentration due to colinear crack arrays as an analogy. The effects of quenching variations and two-step aging are discussed. It is shown that, in aged 7075, microstructural variables such as the width of precipitate-free zones and the nature of the matrix precipitate do not have a controlling effect on toughness.     

Rapid crack propagation in a high strength steel

The relation between fracture velocity and the energy dissipated by unstable fractures in high strength 12.7 mm-thick plates of SAE 4340 steel has been measured using the wedgeloaded double-cantilever-beam (DCB) specimen. The experiments are analyzed using the dynamic beam-on-elastic-foundation model. In agreement with the model, steady-state crack velocities are attained. In addition, the theoretical velocity-arrest length relation is closely obeyed. Statically calculated values of the stress intensity at arrest,K _a, are relatively invariant, but in view of the kinetic energy contribution, are not regarded as a materials property. Increases in crack velocity up to ∼860 ms^-1 are accompanied by a 2-fold increase in dynamic toughness (a 4-fold increase in the dynamic fracture energy) and by corresponding increases in the size of the shear lips. Measurements of the plastic work associated with the shear lips show that the per-unit-volume shear lip fracture energy, φ_SL = 0.21 J/mm³, is essentially constant over this range of velocity. These agreements imply that kinetic energy imparted to the test piece during propagation is substantially recovered and makes a significant contribution to the crack driving force.     

Analytical Prediction of Fatigue Crack Propagation in Metals

An analytical model for fatigue crack propagation of long cracks in metals and metal alloys is presented. The key features of the model are an extension of Griffith’s theory of fracture to include fatigue, a dislocation model for the crack tip opening displacement, and cyclic plasticity-induced closure. The net cyclic stretch of the process zone at the crack tip plays a major role in the fatigue crack propagation under cyclic loading. Only constant amplitude loading is considered in this paper. The model predictions utilize only the readily available material properties, such as Young’s modulus, yield strength, threshold stress intensity factor, and the fracture toughness. There are no empirical fitting constants. The model predictions are validated by an extensive amount of published fatigue crack growth studies. The agreement between the model predictions and the experimental data is good.     

A Quest for a New Hot Tearing Criterion

Hot tearing remains a major problem of casting technology despite decades-long efforts to develop working hot tearing criteria and to implement those into casting process computer simulation. Existing models allow one to calculate the stress-strain and temperature situation in a casting (ingot, billet) and to compare those with the chosen hot tearing criterion. In most successful cases, the simulation shows the relative probability of hot tearing and the sensitivity of this probability to such process parameters as casting speed, casting dimensions, and casting recipe. None of the existing criteria, however, can give the answer on whether the hot crack will appear or not and what will be the extent of hot cracking (position, length, shape). This article outlines the requirements for a modern hot tearing model and a criterion based on this model as well as the future development of hot tearing research in terms of mechanisms of hot crack nucleation and propagation. It is suggested that the new model and criterion should take into account different mechanisms of hot tearing that are operational at different stages of solidification and be based on fracture mechanics, i.e., include the mechanisms of nucleation and propagation of a crack. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunt,” which occurred March 13–15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

Dislocation mechanism based model for stage II fatigue crack propagation rate

Repeated plastic deformation, which of course depends on dislocation mechanism, at or near the crack tip leads to the fatigue crack propagation. By involving the theory of thermally activated flow and the cumulative plastic strain criterion, an effort is made here to model the stage II fatigue crack propagation rate in terms of the dislocation mechanism. The model, therefore, provides capability to ascertain

• 1.(a) the dislocation mechanism (and hence the near crack tip microstructures) assisting the crack growth,
• 2.(b) the relative resistance of dislocation mechanisms to the crack growth and,
• 3.(c) the fracture surface characteristics and its interpretation in terms of the dislocation mechanism.

The local microstructure predicted for the room temperature crack growth in copper by this model is in good agreement with the experimental result taken from the literature. With regard to the relative stability of such dislocation mechanisms as the cross-slip and the dislocation intersection, the model suggests an enhancement of crack growth rate with an ease of cross-slip which in general promotes dislocation cell formation and is common in material which has high stacking fault energy (produces wavy slips). Cross-slip apparently enhances crack growth rate by promoting slip irreversibility and fracture surface brittleness to a greater degree.     

残余应力对55SiMnVB弹簧钢疲劳性能的影响

研究了５５ＳｉＭｎＶＢ弹簧钢板疲劳断口特征及裂扩展过程。结果表明，喷丸强化在表层产生的残余压应力可明显提高疲劳抗力，增强裂纹闭合效应，降低裂纹扩展速度。