Ductile to brittle fracture transition of a plain carbon steel and its thermal activation model

An investigation has been made on the ductile to brittle fracture transition of a C-Mn steel due to variation of loading rates at constant temperatures. The transition takes place with increasing loading rate, which is similar to that caused by decreasing temperature. An equation has been derived from the thermal activation analysis which correlates the fracture toughness with temperature and loading rate. A model of fracture transition has been proposed from the thermal activation movement of atoms.     

Influence of austempering temperature on the mechanical properties of a low carbon low alloy steel

In this investigation, a new low alloy and low carbon steel with exceptionally high strength and high fracture toughness has been developed. The effect of austempering temperature on the microstructure and mechanical properties of this new steel was examined. The influence of the microstructure on the mechanical properties and the fracture toughness of this steel was also studied.Test results show that the austempering produces a unique microstructure consisting of bainitic ferrite and austenite in this steel. There were significant improvement in mechanical properties and fracture toughness as a result of austempering heat treatments. The mechanical properties as well as the fracture toughness were found to decrease as the austempering temperature increases. On the other hand, the strain hardening rate of steel increases at higher austempering temperature. A linear relationship was observed between strain hardening exponent and the austenitic carbon content.     

Study of the fracture toughness and fracture morphology of polybenzimidazole

Polybenzimidazole (PBI) is a relatively new polymeric material exhibiting unusual properties that are attributable to its aromatic-heterocyclic monomer structure. Owing to its high strength, stiffness and excellent stability in hostile chemical and thermal environments, PBI is being used increasingly in critical applications. As a result, understanding the failure mechanisms of the material is vital. This paper presents the results of a study of the fracture toughness and fracture morphology of polybenzimidazole. The standard compact tension specimen was used as the basic experimental specimen in this study. The fracture tests were performed in an Instron tensile testing machine. The effects of varying the loading rate, and the ratio of the initial crack length,a, to the ligament length,W, were investigated. The fracture surface morphology was examined using optical and scanning electron microscopy. The results of this study indicate that the precracking technique significantly affects the measured fracture toughness. Also, an increase in the loading rate causes a significant decrease in fracture toughness. Examination of the fracture morphology reveals that PBI fracture surfaces exhibit many of the characteristics expected of a tough engineering plastic.     

Nb-Ti-Cr-Si基超高温合金的室温断裂韧性研究进展

Nb-Ti-Cr-Si基超高温合金具有良好的高温强度和高温抗氧化性能,但室温断裂韧性较差。为了提高该合金的室温断裂韧性,分析了该合金室温断裂韧性较低的原因,对有坩埚整体定向凝固技术、整体定向凝固+高温均匀化处理和热压烧结法等工艺对Nb-Ti-Cr-Si基超高温合金的室温断裂韧性的影响做了评述,指出了提高该合金室温断裂韧性将来努力的方向。     

Effects of crystalline particles on mechanical properties of strip-cast Zr-base bulk amorphous alloy

Effects of crystalline phase particles formed in a strip-cast Zr-base bulk amorphous alloy on strength, ductility, and fracture toughness were investigated by directly observing microfracture processes using an in situ loading stage installed inside a scanning electron microscope chamber. The compressive and fracture toughness test results indicated that strength, ductility, and fracture toughness of the strip-cast amorphous alloy were higher than those of the as-cast monolithic amorphous alloy, although the strip-cast alloy contained a considerable amount (4.5 vol.%) of hard, brittle crystalline particles. According to the in situ microfracture observation, crystalline particles were easily cracked under low stress levels, acted as blocking sites of shear band or crack propagation, and provided initiation sites of multiple shear bands. Thus, the improvement of mechanical properties in the strip-cast alloy could be explained by mechanisms of (1) blocking of crack propagation, (2) formation of multiple shear bands, and (3) crack deflection by crystalline particles.     

Influence of loading rate on shear fracture toughness for failure mode transition

When subjected to shear loading of sufficiently high rate, many materials do not fail by cracks, propagating at an angle of 70° with respect to the ligament, but by adiabatic shear bands, which extend nearly straight in the direction of the ligament. Work is reported on investigations for determining the dependence of the impact shear fracture toughness as a function of loading rate, in particular in the regime of failure mode transition from cracks to adiabatic shear bands. For achieving high rate shear conditions of loading, edge cracked specimens are asymmetrically impacted at the cracked edge by a projectile accelerated by an air gun. The resulting mode-II stress intensity factors and the times of onset of failure are determined by a specially developed strain gauge measuring technique. Results on shear fracture toughnesses with increasing loading rate are reported for two structural materials, a 1% chromium steel and a high strength aluminum alloy. Whereas decreasing fracture toughnesses are observed with increasing loading rate when failure occurs by tensile cracks, the fracture toughness increases with loading rate when failure occurs by adiabatic shear bands.     

DYNAMIC FRACTURE TOUGHNESS MEASUREMENTS AND LOCAL APPROACH MODELLING OF TITANIUM ALLOYS

Two titanium alloys TA6V and TD5AC were tested. Tensile tests were performed, under static and dynamic loadings, on cylindrical notched and fatigue precracked specimens. The visco-plastic constitutive equations of the alloy were found by fitting finite element computations with the experimental results. The dynamic fracture toughness was obtained by applying the convolution method of Bui and Maigre. Results for the TA6V alloy did not display significant variations of fracture toughness with loading rate, whereas for the TD5AC alloy an increase was measured. The critical void growth was found to be independent of the strain rate. Fair predictions of the fracture toughness under static as well as under dynamic conditions could be achieved by finite element computations using these experimental critical void growth values.     

Viscoelasticity of ceramics at high temperatures

The dependence of the fracture toughness, K _IC, on the loading rate has been calculated. On the basis of linear elastic fracture mechanics (LEFM) a strong dependence of the fracture toughness on the loading rate is obtained if subcritical crack growth is taken into account. If the subcritical crack growth parameters n and B are sufficiently small, which correspond to a high velocity of crack extension, the fracture toughness should decrease at lower loading rates. This behaviour is similar to the well-known decrease of bending strength. The experimental results for alumina containing glassy phase as a model material, however, show a maximum in a certain regime of loading rates. A model is established, which combines LEFM and the viscoelasticity, and leads to a maximum of K _IC at a certain loading rate dependent on the viscosity of the glassy phase.     

Direct metal deposition (DMD) of H13 tool steel on copper alloy substrate: Evaluation of mechanical properties

H13 tool steel powder was clad on copper alloy substrate both directly and using 41C stainless steel (high Ni steel) powder as a buffer layer by direct metal deposition (DMD). Cu-steel bimetallic die casting and injection molding tools are of high interest for reduction of cycle time by efficient heat extraction due to high thermal conductivity of copper. The mechanical properties of these bimetallic structures were investigated in terms of bond strength, impact energy and fracture toughness. The bond interfaces of these claddings showed porous and crack free transition regions. The bond strength was higher in the directly clad H13 tool steel compared to the H13 tool steel clad with 41C stainless steel as buffer layer. The fracture morphology in tensile test specimens showed ductile dimple fracture. Presence of necking just below the interface depicted the softening of substrate in heat affected zone (HAZ) during cladding. The Charpy impact energy is little higher in the 41C stainless steel buffered specimens compared to the directly clad H13 tool steel specimens but the fracture toughness results showed reduction of fracture toughness in the 41C stainless steel buffered specimens due to the low strength in the tensile test. However the fracture toughness value was in the ductile region for both deposits.     

Enhancement of fracture toughness in high strength steel by microstructural control

The development of new alloys with improved mechanical properties has been seriously hampered in the past by the inability of a metallurgist to relate quantitatively the variables of microstructure and fracture toughness. The emergence of a unified theory of fracture toughness in the past decade has done much to alleviate this difficulty. As a consequence of a recent interdisciplinary research effort involving both the disciplines of physical metallurgy and experimental fracture mechanics, we have been able to develop alloys with engineering properties superior to those of commercially available materials. This research has required the creation of new and unusual microstructures, utilizing a variety of thermal and thermomechanical processes. The quantitative relationships of mechanical properties (strength, ductility, work hardening, and fracture toughness) with composition and microstructure are discussed in detail for the newly developed TRIP steels. In the report of another development, it is shown how the fracture toughness of low alloy quenched and tempered steels with yield strengths over 200,000 psi can be improved by as much as 70 per cent by microstructural control. Lastly, the initial results of research on alloys intended for cryogenic service are described. The composition, heat treatment, microstructure and properties of an alloy having more than three times the toughness of the presently used alloys are discussed.     

TA15钛合金两类组织对疲劳性能和断裂韧度的影响

研究了TA15钛合金片状和双态两种典型组织对疲劳性能和断裂韧度的影响,结果表明:在S-N曲线的高应力区,双态组织的疲劳强度高于片状组织;在低应力区,情况则相反,且片状组织的疲劳极限(656MPa)高于双态组织(565MPa).片状组织的疲劳裂纹扩展速率低于双态组织,且断裂韧度K1C高于双态组织,即片状组织的损伤容限性能优于双态组织.     

激光增材制造镍基复合材料界面连接机制与断裂行为

基于颗粒增强镍基复合材料优异的结构/功能特性，在航空航天、核电军工和电子电工等领域有着广泛的应用前景。本文选用机械球磨混粉+激光选区熔化方法 (SLM)制备了碳化钨(WC)颗粒增强IN718复合材料(WC/IN718)，对复合材料内部异质界面连接机制、强化机制和断裂行为进行了分析。研究结果表明：随着WC颗粒含量的增加(0wt%～20wt%)，试件成形良好，WC颗粒均匀分布在基体内部，异质界面处无缺陷产生，界面处产生了贫碳的W₂C层和碳化物层，基体合金主要呈柱状晶生长。由于熔池内部能量密度分布不同，低温位置WC颗粒的断裂方式为先形成界面反应层后由热应力引起断裂，高温位置WC颗粒优先发生断裂，断裂成小尺寸颗粒，后与熔化的基体合金形成界面反应层，弥散分布在基体内部。随着WC颗粒含量的增加，复合材料的强度呈现升高的趋势，而断裂韧性降低，抗拉强度最高可达1 280 MPa，强化机制主要为载荷传递强化，断裂机制为WC颗粒的脆性断裂和基体合金的韧性断裂。     

390MPa级低合金高强钢的低温韧性

采用系列冲击试验研究了控轧控冷技术生产的390MPa级低合金高强钢的低温韧性,并分析了其低温韧性与组织特征的关系。结果表明：该钢具有良好的低温韧性,在-40℃时的冲击功为127J,远大于相关标准的技术要求,按照能量法确定的韧脆转变温度为-56℃;由于该钢晶粒十分细小,裂纹在扩展过程中频繁改变断裂路径,提高了其抵抗解理断裂的能力,从而使其具有良好的低温韧性。     

复相Al3Ti基合金的高温强化

报导了对Ａｌ３Ｔｉ结合合金进行复相强化的研究,利用适量的Ｎｂ合金化,在Ｌｌ２Ａｌ３Ｔｉ基体中形成分散的第二相,其室温和高温强度显著提高,韧性也有改善。改变制备工艺,使合金发生重有序和析出过程,形成具有高度弥散微粒的复相细晶组织,合金的室温和高温强度进一步提高。并探讨了其强韧化作用机理。     

Modelling ductile fracture behaviour from deformation parameters in HSLA steels

In this work, an attempt is made to model the ductile fracture behaviour of two Cu‐strengthened high strength low alloy (HSLA) steels through the understanding of their deformation behaviour. The variations in deformation behaviour are imparted by prior deformation of steels to various predetermined strains. The variations in parameters such as yield strength and true uniform elongation with prior deformation is studied and was found to be analogous to that of initiation fracture toughness determined by independent method. A unique method is used to measure the crack tip deformation characterized by stretch zone depth that also depicted a similar trend. Fracture toughness values derived from the stretch zone depth measurements were found to vary in the same fashion as the experimental values. A semiempirical relationship for obtaining ductile fracture toughness from basic deformation parameters is derived and model is demonstrated to estimate initiation ductile fracture toughness accurately.     

退火热处理对TA15钛合金组织性能的影响

研究了不同的退火热处理制度对TA15钛合金显微组织、室温拉伸性能、高温拉伸性能、室温冲击韧性及硬度的影响。结果表明：在相变点以上温度退火,合金具有较高的室温、高温强度,但室温塑性、高温塑性、室温冲击韧性较低;在相变点以下温度退火,合金的室温、高温断裂强度在860℃退火时出现峰值,而室温塑性、高温断面收缩率和室温冲击韧性则随着退火温度的升高而提高;同单重退火相比,双重退火、三重退火对提高合金性能的作用不大。     

Influence of notch severity on the impact fracture behavior of aluminum alloy 7055

In this paper, the conjoint influence of notch severity and test temperature on the impact behavior of an Al-Zn-Mg-Cu alloy 7055 in the T7751 microstructural condition is presented and discussed. Notch angles of 45°, 75° and 90° were chosen for a standard charpy impact test specimen containing two notches. For a given angle of the notch the increase in dynamic fracture toughness, with test temperature, is most significant for the least severe of the notches, i.e. 45°. At a given test temperature, the impact toughness of the T7751 microstructure decreased with an increase in notch severity. An increase in notch severity resulted in essentially Mode I dominated fracture at all test temperatures. The influence of localized mixed-mode loading is minimal for the alloy has low dynamic toughness. The impact fracture behavior of the alloy is discussed in light of alloy microstructure, mechanisms governing fracture and the deformation field ahead of a propagating crack.     

预加载下岩石的动态力学性能研究

动态和静态载荷共同作用下的岩石力学特性是深部地下岩石工程的关键问题。设计了用于测试静态预加载下岩石动态力学性能的分离式霍普金森压杆系统,并详细介绍了具有预加载装置的分离式霍普金森压杆系统的原理、数据分析和应力波的传播过程。通过具有预加载装置的分离式霍普金森压杆系统研究了岩石在不同预拉伸应力下的拉伸强度。结果表明:动态拉伸强度和总拉伸强度随着加载率的增加而增加,同时,在相同加载率下,动态拉伸强度随着预拉伸载荷的增加而减小,而总拉伸应力与预拉伸载荷的大小无关。此外,对不同预加载条件下岩石的动态断裂韧度也进行了研究,实验结果说明岩石的动态断裂韧度和总断裂韧度随着加载率的增加而增加。在相同加载率下,动态断裂韧度随着预加载荷的增加而减小,而总断裂韧度随着加载率的增加而增加。     

含孔玻璃纤维/环氧树脂复合材料-铝合金层板的拉伸损伤行为与热暴露响应

采用试验和数值方法研究了含孔玻璃纤维/环氧树脂(GF/EP)复合材料-铝合金层板在不同热暴露温度下的拉伸剩余强度和损伤失效模式,揭示了层间损伤、纤维损伤及基体损伤的演化过程。结果表明:随着热暴露温度升高,含孔GF/EP复合材料-铝合金层板剩余强度不断下降,拉伸破坏呈现出明显的纤维断裂与层间分层混合失效模式。热暴露温度越高或开孔直径越大,GF/EP复合材料-铝合金层板的层间分层损伤区域越小。随着载荷的增大,沿加载方向的0°纤维和基体的损伤分别呈现出类似“漏斗”形和“花瓣”状的损伤演化形式,而层间损伤区域呈现出一对相对开孔对称的三角形损伤演化形式。基于GF/EP复合材料-铝合金层板的剩余强度和损伤失效模式的数值仿真与试验结果吻合较好。      

Formulation and implementation of strain rate-dependent fracture toughness in context of the phase-field method

The phase-field approach is a promising technique for the realistic simulation of brittle fracture processes, both in quasi-static and transient analysis. Considering fast loading, experimental evidence indicates a strong relationship between the rate of strain and the material's resistance against fracture, which can be considered by a dynamic increase factor for the strength of the material. The paper at hand presents a novel approach within the framework of phase-field models for brittle fracture. A rate-dependent fracture toughness is formulated as a function of the rate of crack driving strain components, which results in higher strength for faster loading. Beside the increased amount of energy necessary to evolve a crack at a high strain rate loading situation, the model incorporates quasi-viscous stress-type quantities that are not directly related to the formation of the crack and exist only in the phase-field transition zone between broken and sound material. The governing strong form equations for a transient simulation are derived and the relevant information for an implementation of the model into a finite element code is outlined in detail. The performance of the model is demonstrated for static and dynamic benchmark simulations and for a comparison to experimental findings.