Investigations of yield stress,fracture toughness,and energy distribution in high speed orthogonal cutting

This paper presents a novel prediction method of the yield stress and fracture toughness for ductile metal materials through the metal cutting process based on Williams' Model [38]. The fracture toughness of the separation between the segments in serrated chips in high speed machining is then deduced. In addition, an energy conservation equation for high speed machining process, which considers the energy of new created workpiece surfaces, is established. The fracture energy of serrated chips is taken into the developed energy conservation equation. Five groups of experiments are carried out under the cutting speeds of 100, 200, 400, 800 and 1500 m/min. The cutting forces are measured using three-dimensional dynamometer and the relevant geometrical parameters of chips are measured with the aid of optical microscope. The experiment results show that the yield stress of machined ductile metal material presents an obviously increasing trend with the cutting speed increasing from 100 to 800 m/min while it decreases when the cutting speed increases to 1500 m/min further. Meanwhile, the fracture toughness between the chip and bulk material displays a slightly increasing tendency. In high speed machining, the fracture toughness of the separation between the segments in serrated chips also presents increasing trend with the increasing cutting speed, whose value is much greater than that between the chip and bulk material. In the end, the distribution of energy spent in cutting process is analyzed which mainly includes such four portions as plastic deformation, friction on the tool–chip interface, new generated surface and chip fracture. The results show that the proportion of plastic deformation is the largest one while it decreases with the cutting speed increasing. However, the proportions of energy spent on new created surface and chip fracture increase due to the increasing of both the chip's fracture area and the fracture toughness.     

A7N01P-T5铝合金断裂韧度的厚度效应

采用国家标准GB4161-2007对高速列车承载底架焊接结构材料A7N01P-T5铝合金型材的断裂韧度KC进行了测试和分析,测试了3种厚度母材的KC,基于能量理论和线弹性力学提出A7N01P-T5铝合金断裂韧度随试样厚度变化关系KC(B)的解析公式,并进一步采用国家标准GB21143-2007的CTOD试验对其进行验证.结果表明,该公式具有较高的精度和普适性,可用于确定不同板厚材料的KC,将公式应用于不同壁厚条件下的焊接结构的完整性评定,以此节省人力物力,并大幅度缩短焊接结构完整性评定及结构剩余寿命的计算周期.同时为各种金属材料的KC获取提供通用性的方法和参考准则.     

Cohesive interface simulations of indentation cracking as a fracture toughness measurement method for brittle materials

Cracks in brittle solids induced by pyramidal indenters are ideal for toughness evaluation since the indentation stress fields decay rapidly from the contact center and any cracks will be eventually arrested. Thus, if the applied energy release rate can be determined analytically, the material toughness can be deduced by measuring the crack length. However, such a driving force calculation is a nontrivial task because of the complex stress fields; only a number of limit cases can be solved, such as the long half-penny cracks (at least two times larger than the contact size) in the classic Lawn–Evans–Marshall (LEM) model. Important questions such as the evolution from short cracks to median/radial and then to half-penny cracks, the form of the scaling relationship that relates fracture toughness to material hardness and indenter angles, the threshold load for indentation cracking, etc., cannot easily be answered without a detailed knowledge of the co-evolution history of the stress fields and crack morphology. To this end, a finite element model of four-sided pyramidal indentation adopting cohesive interface elements is developed to study the effects of indenter geometry, load, cohesive interface parameters, and material properties on the initiation and propagation of the median/radial/half-penny crack systems. The validity and artifacts of the cohesive interface model are carefully examined, and the crack morphologies under various indentation and material parameters are systematically studied. Numerical predictions lead to a quantitative evaluation of the threshold load for indentation fracture, and an improved method for the evaluation of material toughness from the indentation load, crack size, hardness, elastic constants, and indenter geometry, which compare favorably to a large set of experiments in the literature. It is also found that the toughness evaluation method is very sensitive to Poisson’s ratio – an observation that has previously received little attentions. An approximate analysis for short cracks is developed based on the fracture mechanics of annular cracks and the embedded-center-of-dilatation model for indentation-induced residual stress fields.     

A study on the influence of workpiece properties in ultrasonic machining

In ultrasonic machining (USM), brittle materials are machined by repeated impacts on the workpiece through a medium of abrasive slurry. Material removal rates are influenced by the various process parameters including the properties of the workpiece material. In this investigation, the influence of fracture toughness and hardness of workpiece materials are studied and reported. Fracture toughness is found to be an important parameter. Experiments are conducted with both conventional and rotary USM modes. Machining performance in the rotary mode is found to be much superior to the conventional mode. The results of static, sliding and rolling indentation tests reported by other investigators are used for explaining the importance of the fracture toughness.     

用裂纹密度参数评价PVD硬质涂层断裂韧性的研究

本文根据裂纹体自洽模型,推导出裂纹密度参数关系式,并采用压痕法,通过统计压痕域的裂纹几何,计算裂纹密度参数,以此来评价涂层的断裂韧性.涂层的裂纹密度参数与涂层的膜/基结合力有良好的对应关系,裂纹密度参数的设置对研究涂层的断裂韧性,揭示涂层的磨损与剥落过程,在理论上和工程上都颇有意义.     

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.     

断裂韧度与钢组织性能的关系

本文阐述了断裂韧度与材料本征因素和基本力学性能的关系.合金成分、微量元素、夹杂物和第二相、显微组织与晶粒度是控制断裂韧度的关键因素,提出了改善断裂韧度的一些思路和方法,如改善晶界状态、细化晶粒尺寸、控制夹杂物的含量、变性变质夹杂物、改善材料组织结构都能改善材料的断裂韧.断裂韧度既是强度、塑性、冲击韧性的综合反映,同时具有独立的力学意义,断裂韧度与材料力学性能之间的关系使经济、有效地预测断裂韧度成为可能.     

复相添加剂／3Y-TZP复合增韧氧化铝陶瓷的研究

利用复相添加剂和3Y-TZP的叠加作用，对氧化铝陶瓷的增韧机制和途径进行了研究。结果表明：复相添加剂有助于Al2O3／3Y-TZP复合材料中氧化铝晶粒的异向生长，其增韧效果与3Y-TZP产生了叠加效应。实验材料密度接近理论密度；抗弯强度556．35MPa；断裂韧性6．73MPa．m^1/2；综合力学性能明显提高。     

RELATIONSHIP BETWEEN J-INTEGRAL AND FRACTURE SURFACE AVERAGE PROFILE

To investigate the causes that led to the formation of cracks in materials, a novel method that only considered the fracture surfaces for determining the fracture toughness parameters of J-integral for plain strain was proposed The principle of the fracture-surface topography analysis （FRASTA） was used. In FRASTA, the fracture surfaces were scanned by laser microscope and the elevation data was recorded for analysis. The relationship between J-integral and fracture surface average profile for plain strain was deduced. It was also verified that the J-integral determined by the novel method and by the compliance method matches each other well.     

Toughening strategies for MoSi2-based high temperature structural silicides

The key materials issue associated with the future of high temperature structural silicides is the improvement in the low temperature fracture toughness of these materials. Many industrial applications require materials with fracture toughness levels in the vicinity of 10 MPa m^1/2, while turbine applications require toughness levels of 15 MPa m^1/2 or higher. Two important routes currently being pursued to increase silicide fracture toughness are the composite approach and the alloying approach. Composites based on the reinforcement of MoSi₂ with ceramics are discussed. Alloying approaches for MoSi₂ based on limited theoretical guidance are outlined.     

精细陶瓷材料断裂韧性的测定

通过论述单刃缺口染三点弯曲法、拉伸法、显微压痕法等同定材料断裂韧性Klc值方法的基本原理,综合考虑各种测量方法对精细陶瓷材料测量断裂韧性精确度的影响．以及测定过程的可操作性,结合实际测定试验结果分析,最终确定显微压痕法作为测定精细陶瓷材料断裂韧性KIC值常规检测方法．     

Adhesion and acoustic emission analysis of failures in nitride films with a metal interlayer

Interfacial fracture has been induced between a tantalum nitride film with an aluminum interlayer on a sapphire substrate using nanoindentation. To identify failures for which a model calculation is valid a commercial acoustic emission sensor has been used to study the details of the failure event. The interfacial fracture energy of the system with an aluminum interlayer under the loading conditions at the crack tip is approximately 8 J/m². Within narrow bounds, this toughness value is reproducible using three different theoretical approaches. The acoustic emission signal is used to determine a lower bound interfacial crack velocity of 5 m/s. The majority of the failure occurs at the aluminum-sapphire interface, suggesting that the fracture energy and crack velocity determined are related to the toughness of this interface and not the nitride-aluminum interface.     

高钢级套管韧性指标适用性计算方法研究(上)

利用CEGB—R6安全评价方法,基于材料真应力一应：变行为建立了高钢级套管失效评估曲线,对高钢级套管材料的断裂韧性K1与常温冲击功Akv的关系进行了统计研究,根据失效评估技术获得了含缺陷套管韧性指标的适用性计算方法。通过人工预制裂纹、有限元数值模拟和内压爆破全尺寸实验,对韧性指标计算方法进行了有效验证。将所得的计算结果与API Spec 5CT标准规定的韧性指标进行分类对比,指出API标准的韧性计算方法是以防止脆性启裂为设计依据的,由于仅考虑了屈服强度和套管壁厚,而对套管外径、服役中产生的缺陷几何形貌等因素缺乏考虑,因此在实际服役中存在较大的安全隐患。     

氧化铝基陶瓷材料断裂韧性的测量与评价

用压痕法和单边切口梁法测量了氧化铝基陶瓷材料的断裂韧性,对这两种方法所测得的结果进行了分析对比,确定了最适用的压痕法计算公式。研究发现,原料粉末的起始粒度对用压痕法计算材料KIC的最优公式选择并无影响,但对材料的断裂韧性及抗弯强度有不同程度的影响,亚微米第二相增强微米基体的材料综合力学性能最佳;TiN的加入使Al2O3-TiC基体材料的断裂韧性有所增加;在较小的压痕载荷下计算出的材料的断裂韧性值更接近实际;在用不同的公式计算同一材料的断裂韧性值时,断裂韧性值与压痕载荷之间有相似的关系曲线。     

高钢级套管韧性指标适用性计算方法研究(下)

利用CEGB—R6安全评价方法,基于材料真应力-应变行为建立了高钢级套管失效评估曲线,对高钢级套管材料的断裂韧性K1与常温冲击功Akv的关系进行了统计研究,根据失效评估技术获得了含缺陷套管韧性指标的适用性计算方法。通过人工预制裂纹、有限元数值模拟和内压爆破全尺寸实验。对韧性指标计算方法进行了有效验证。将所得的计算结果与API Spec 5CT标准规定的韧性指标进行分类对比,指出API标准的韧性计算方法是以防止脆性启裂为设计依据的。由于仅考虑了屈服强度和套管壁厚。而对套管外径、服役中产生的缺陷几何形貌等因素缺乏考虑。因此在实际服役中存在较大的安全隐患。     

陶瓷涂层断裂韧性的表征

由于陶瓷本身的脆性,陶瓷涂层的应用受到了极大的制约。断裂韧性是反映材料抵抗裂纹失稳扩展能力的力学性能指标,陶瓷涂层因为脆性和低维特点,对其断裂韧性的表征存在较大困难,目前主要有临界应力强度因子KⅠC、临界裂纹扩展能量释放率GⅠC和裂纹密度β三种表征方法。本文对上述方法进行了总结、分析。     

EFFECT OF GRAPHITE NODULES ON FRACTURE TOUGHNESS OF NODULAR CAST IRON

本文在球铁三点弯曲试件的裂纹尖端区模拟球铁材料的微观结构,建立了宏观与微观相结合的局部复相离散计算模型和复相材料中J积分基本关系式,编制了适合计算多相材料弹塑性大应变及J积分的有限元程序,对石墨形状、大小及体积百分比等对球铁应力强度因子及断裂韧性的影响作了比较全面的分析,和相应实验结果符合得较好。     

Prediction of the fracture toughness of a ceramic multilayer composite – Modeling and experiments

A procedure to predict the fracture toughness of a ceramic multilayer composite made of different phases is presented. The procedure requires experiments to measure the intrinsic fracture toughness of the phases and to determine the required material data. The numerical modeling includes a conventional finite element stress analysis and the calculation of the crack driving force based on the concept of configurational (material) forces. The procedure yields the fracture toughness of the composite as a function of the crack length. A bending bar consisting of layers made of alumina and an alumina–zirconia composite is investigated. The bar has a crack perpendicular to the interfaces. The spatial variations of both the thermal residual stresses and the elastic modulus induce shielding and anti-shielding effects on the crack, which are quantified. The numerically predicted fracture toughness is compared with the experimental values.     

Rapid determination of the fracture toughness of metallic materials using circumferentially notched bars

Three types of material whose fracture toughness tests were previously performed by using circumferentially notched bars, namely (1) a dual-phase steel with three different morphologies; (2) an Al-Zn-Mg-Cu-wrought alloy; and (3) Al-Si-cast alloys with three different Si contents, were investigated in terms of accuracy and reliability of the testing method. Also, the advantages of using circumferentially notched bars for fracture toughness determination of metallic materials were discussed. With the help of stress concentration factors, which are associated with the bluntness of the notch, correction factors for the fracture toughness calculations are derived. The corrected fracture toughness values are found to be close to the uncorrected ones, implying that the testing procedure is reliable.     

烧结NdFeB永磁合金的断裂韧性

用Zwick万能实验机测量了烧结NdFeB永磁合金样品的三点弯曲强度、断裂韧性KIC及弹性模量E。在三点弯曲试验中测得塑变功为0，而断裂功与最大消耗功相等，表明烧结NdFeB合金具有极大的缺口敏感性。缺口敏感的脆性材料用单边切口粱法（SENB）测量KIC时要求较高的抛光精度，以消除表面划痕的影响，比较而言，压痕法测量KIC简便迅速。但由于所用NdFeB合金为烧结复相材料，其微观结构的不均匀性使实验数据较为分散。本研究借助Palmqvist裂纹系统半椭圆裂纹模型HV-KIC的计算公式，采用开微缺口的单边缺口粱试样的KIC和压痕硬度及系列压痕裂纹参数，经回归分析确定了该公式中的经验常数，使之能够较为准确地表征烧结NdFeB永磁合金的断裂韧性。