An investigation of the edge-sliding mode in fracture mechanics

A boundary collocation procedure has been applied to the Williams stress function to determine the elastic stress distribution for the crack tip region of a finite, edge-cracked plate subjected to mode II loading at the crack tips. The asymmetric specimen selected was particularly suitable for the determination of plane strain fracture toughness for mode II loading. Numerical solutions for stress intensity factors for the edge-sliding mode obtained by the boundary collocation method were in close agreement with values obtained from photoelastic experiments.Fracture tests of several compact shear specimens of 2024-T4 aluminum were conducted in order to experimentally investigate the behavior of the edge-sliding mode. In each case a brittle shear failure was observed and mode II fracture toughness values were obtained. The average value for K_IIc obtained from two tests was 39.5 ksi(in)^{$\text{1}\text{2}$}. No K_Ic. data for 2024-T4 were available for comparison purposes; however, K_Ic values for a similar alloy, 2024-T351, have been reported as 34ksi(in)^{$\text{1}\text{2}$} which is only about 15 per cent below the corresponding K_IIc value.     

On the mechanics of wire cutting of cheese

The wire cutting process is used in the food industry during the manufacture and testing of products. The cutting process involves fracture as well as large strain deformation and surface friction. This paper investigates the mechanics of the wire cutting process of cheese through a combination of experiments, theory and finite element simulations. The experiments revealed that there was secondary damage on the cut surface, thus a higher fracture energy would be consumed than the common assumption of a single crack propagation. The numerical simulations showed that there was a six-fold change in the strain rate when wire diameters of 0.25  to 2 mm are used. This strain rate effect was modelled through a modification of a previous theoretical analysis of the wire cutting process. The numerical models were also used to predict the cutting forces using two failure criteria: critical strain, which was applied to the initiation of cracking, and a cohesive zone model to simulate crack propagation. Both criteria showed reasonable success in predicting the cutting forces, particularly for cuts made with small wire diameters.     

An investigation on cutting quality by adding polymer in abrasive water jet

ABSTRACT

An experimental investigation is presented to improve the cutting quality in abrasive water jet (AWJ) cutting of marble by an addition of polyacrylamide (PAM). Considering experimental data, the kerf widths have a remarkable change when the PAM concentration approaches to 400 ppm. The deviation between top and bottom kerf width reaches the minimal value when PAM concentration is equal to about 600 ppm. In addition, the surface topography analyses illustrate that an addition of PAM can broaden the cutting wear zone and make the cutting quality better. Furthermore, the effects of PAM on the surface roughness are assessed by a profilometer. It is eventually found that the surface roughness decreases initially and then increases greatly with the increase of the depth of cut. Additionally, the minimum surface roughness occurs when the PAM concentration is 600 ppm, which agrees well with the experimental result of kerf width. An increasing stand-off distance or traverse speed produces a higher surface roughness.     

A fracture mechanics analysis of cutting and machining

The process of cutting and machining is analysed using concepts developed in the fracture analysis of beam specimens. Increasing cutting forces and decreasing tool rake angles lead to a sequence of deformation processes from elastic bending to elastic-plastic bending and finally to shear yielding in the chip. The conditions for each mode of deformation are identified. Fracture toughness is included in the analysis as is, in addition, the notion of root rotation at the crack tip. Under some circumstances this gives rise to the condition of the tool tip touching the crack tip during which energy is transferred directly to the fracture process. The tool-chip interface is characterised by Coulomb friction and by the inclusion of an adhesion toughness to model the effects of hot polymeric chips sticking to the rake face of the tool. The combined effects of bending and shearing leading to chip curling and coiling are also analysed.     

Contact mechanics at nanometric scale using nanoindentation technique for brittle and ductile materials

In the last years, Nanoindentation or Instrumented Indentation Technique has become a powerful tool to study the mechanical properties at micro/nanometric scale (commonly known as hardness, elastic modulus and the stress-strain curve). In this review, the different contact mechanisms (elastic and elasto-plastic) are discussed, the recent patents for each mechanism (elastic and elasto-plastic) are summarized in detail, and the basic equations employed to know the mechanical behaviour for brittle and ductile materials are described.     

An investigation of yield surfaces based on dislocation mechanics—II: Application

The yield criterion and hardening law proposed in Part I are used to calculate the first and second subsequent yield surfaces of aluminium. The obtained yield surfaces exhibit the following features: They shrink in the direction of prestressing, they show a rounded vertex near the prestress point, they become inflated on the side of prestress and deflated on the opposite side, and finally they may exclude the origin.     

Contribution to the mechanics of free oblique continuous cutting

The following analysis determines the direction of the chip flow in free oblique cutting, based on the fact that the friction force on the tool face takes the direction of the relative motion of the chip along this face. This direction is shown to be one of minimum deviation of the material as it shears.

The condition of minimum work yields a general expression for the shear angle. The dimensions of the chip action and the cutting forces are predicted by theory. An energy balance for oblique cutting is made. This general solution yields the well-known solutions of Merchant's model for orthogonal cutting as an obvious special case.     

基于虚拟仪器技术的车辆综合试验台测试系统

以机械工程测试理论为基础,采用虚拟仪器软件技术,辅以多参量数据采集模块化硬件,在VC 6.0环境下设计开发了基于虚拟仪器技术的车辆综合试验台数据采集与监控系统,对车辆综合试验台面临的技术老化状况进行了改进,并详细地介绍了该系统的硬软件结构、功能和设计思想及理论依据.经实验验证,其性能符合要求,完成了对试验台常用信号的采集以及对试验台的开关控制.     

An investigation of yield surfaces based on dislocation mechanics—I: Basic theory

With the help of dislocation theory a new hardening law and a yield criterion are introduced and the initial and subsequent yield surfaces for both single crystals and polycrystalline aggregates are studied.     

数控机床切削稳定性分析及实验研究

针对因机床结构的复杂性及零件加工轨迹不确定性,刀尖频响函数切削中难以保持恒定易造成稳定域图变化,导致机床切削参数选择不确定性问题,结合模态实验分别研究数控机床沿不同进给方向及机床主轴处于不同位置的颤振稳定域图预测;对同型号两台数控机床的切削稳定性进行对比分析。由实验采集切削中声音信号验证机床沿不同进给方向的切削稳定性存在差别。研究结果对实际加工中避免切削颤振及工艺参数选择具有指导意义。     

An experimental investigation as to the effect of cutting parameters on roundness error and surface roughness in cylindrical grinding

This paper presents the results of an experimental investigation as to the effects of grinding parameters on roundness error and surface roughness in cylindrical grinding. Many variables including the wheel materials, wheel loading and dressing, workpiece metallurgy, work drive mechanisms, work holding methods, coolant types, feeds and speeds, machine stiffness and age, surface conditions, centre conditions, floor vibrations all influence the quality of ground parts. However, the composite sum of these grinding parameters creates static and dynamic forces. It is obvious that the roundness error and surface roughness are created by many parameters, but in this study, only the effects of the depth of cut, work speed and feed rate which create the grinding forces in cylindrical grinding are investigated. The grinding experiments were planned according to the principles of orthogonal arrays (OAs), developed by Taguchi, and were performed so as to understand the effects of these parameters on roundness error and surface roughness. The experimental data was analysed by using statistical tools: the percent contribution from an analysis of variance (ANOVA) and the correlation between machining parameters with roundness error (R) and also surface roughness (Ra). Roundness was found to be the most related with the cutting speed, grinding force and depth of cut, while surface roughness is related to feed rate and work speed.     

An estimate of the metrological correctness of test-bed checks of a roller reducing gear

A system of differential equations describing the operation of a test bench for checking roller reducing gear is considered. Experimental and calculation-theoretical data are compared. It is concluded that it is metrologically incorrect to make such checks. Translated from Izmeritel’naya Tekhnika, No. 1, pp. 32–33, January, 2009.     

Experimental investigation of the contact mechanics of rough fractal surfaces

The nonstationary character of roughness is a widely recognized property of surface morphology and suggests modeling several solid surfaces by fractal geometry. In the field of contact mechanics, this demands novel investigations attempting to clarify the role of multiscale roughness during physical contact. Here we review the results we recently obtained in the characterization of the contact mechanics of fractal surfaces by depth-sensing indentation. One class of experiments was conducted on organic thin films, load-displacement curves being acquired by atomic force microscopy using custom-designed tips. Another class of experiments focused on well-defined crystalline and mechanically polished ceramic substrates probed by a traditional nanoindenter. We observed the first-loading cycle to be considerably affected by surface roughness. Plastic failure was found to dominate incipient contact while contact stiffness increased on decreasing fractal dimension and roughness. Our findings suggest fractal parameters to drive contact mechanics whenever the penetration depth is kept below the interface width.     

An investigation into the damage development and residual strengths of open-hole specimens in fatigue

An extensive experimental program was carried out to investigate and understand the sequence of damage development throughout the life of open-hole composite laminates loaded in tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/−45₂/0₂]_S, [45/90/−45/0]_2S and [45/90/−45/0]_4S were examined. These were selected on the basis that under quasi-static loading the [45₂/90₂/−45₂/0₂]_S configuration exhibited a delamination dominated mode of failure whilst the [45/90/−45/0]_2S and [45/90/−45/0]_4S configurations showed a fibre dominated failure mode, previously described as “pull-out” and “brittle” respectively. Specimens were fatigue loaded to 1 × 10⁶ cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles, which were inspected using X-ray computed tomography (CT) scanning. A static residual strength program was carried out for run-out specimens of each configuration.     

振动裁切的机理与应用研究

从运动学方面对比分析了普通裁切与振动裁切过程中裁刀与纸叠间的相对运动关系。通过分析知,实现振动裁切效果的前提条件是裁切速度小于裁刀的最大振动速度,二者的相对关系决定了一个振动裁切周期内的裁切时间。裁切速度越小,振动频率越高,每一振动裁切周期内的裁切时间越短。振动裁切过程中产生的脉冲裁切力与纸张剪应力、裁刀振动周期、裁切时间以及裁切厚度有关。振动裁切优于普通裁切效果的原因是,脉冲裁切力的细微分割效果减小了一次性裁切面积,从而降低了裁切抗力,提高了裁切质量。     

3D fracture mechanics investigation on surface fatigue crack propagation

Surface fatigue crack propagation is the typical failure mode of engineering structures. In this study, the experiment on surface fatigue crack propagation in 15MnVN steel plate is carried out, and the crack shape and propagation life are obtained. With the concept of ‘equivalent thickness’ brought into the latest three‐dimensional (3D) fracture mechanics theory, one closure model applicable to 3D fatigue crack is put forward. By using the above 3D crack‐closure model, the shape and propagation life of surface fatigue crack in 15MnVN plates are predicted. The simulative results show that the 3D fracture mechanics‐based closure model for 3D fatigue crack is effective.     

An investigation of the features of development of high-velocity localized shear in metal

An experimental method of shear tests of metals with loading rates up to 10 m/sec has been developed. The results of shear tests of AMg6 alloy with a loading rate of up to 5 m/sec and a deformation rate of up to 10⁵ sec^–1 are presented. Deformation curves and values of critical shear deformations are given. Features of the kinetics of development of high-velocity localized shear in AMg6 alloy are investigated.Translated from Problemy Prochnosti, No. 3, pp. 41–45, March, 1995.