On the sharpness of straight edge blades in cutting soft solids: Part I - indentation experiments

The sharpness of a blade is a key parameter in cutting soft solids, such as biological tissues, foodstuffs or elastomeric materials. It has a first order effect on the effort, and hence energy needed to cut, the quality of the cut surface and the life of the cutting instrument. To date, there is no standard definition, measurement or protocol to quantify blade sharpness. This paper derives a quantitative index of blade sharpness via indentation experiments in which elastomeric materials are cut using both sharp and blunt straight edge blades. It is found that the depth of blade indentation required to initiate a cut or crack in the target material is a function of the condition or sharpness of the blade’s cutting edge, and this property is used to formulate a so-called “blade sharpness index” (BSI). It is shown theoretically that this index is zero for an infinitely sharp blade and increases in a quadratic manner for increasing bluntness. For the blades tested herein, the sharpness index was found to vary between 0.2 for sharp blades and 0.5 for blunt blades, respectively. To examine the suitability of the index in other cutting configurations, experiments are performed using different blade types, target materials and cutting rates and it is found that the index is independent of the target material and cutting rate and thus pertains to the blade only. In the companion Part II to this paper a finite element model is developed to examine the effect of blade geometry on the sharpness index derived herein.     

300M超高强度钢车削加工性能仿真模拟分析研究

目的研究不同切削参数对300M超高强度钢切削性能的影响。方法通过单因素试验法,采用Advant Edge切削仿真软件,建立300M钢三维有限元模型,对不同切削参数下车削300M钢的切削力、刀片温度、刀片应力及切屑形状进行分析。结果在300M钢车削过程中,刀片温度随着切削速度增大而增大,但切削力和刀片应力反之;背吃刀量和进给量越小,切削力、刀片应力及刀片温度越小;切削刃半径越小,切削力越小,但小的切削刃半径使得刀片应力变大,容易导致刀片磨损。车削300M钢的切屑呈锯齿螺旋状,切屑温度为带状分布,切削速度越高,进给量、背吃刀量越大,切削刃半径越小,切屑温度越高。结论在300M钢车削加工中,应选用较高的切削速度,适中的切削刃半径,较小的进给量和背吃刀量。     

较浅压入下针尖曲率半径对纳米压入硬度影响的比较研究

基于3种不同曲率半径压头针尖对熔融石英进行纳米压入,用原子力显微镜（AFM）直接法测得压头针尖的面积函数及针尖曲率半径。研究表明,在极浅压入条件下,压头曲率半径的变化会导致硬度值的测量误差,曲率半径越小的压头针尖随接触深度的变化会更快得到真实值;相同的压入深度,曲率半径小的压头针尖测得的压入硬度值比曲率半径大的测得的压入硬度值更接近其真实值。     

A crystal plasticity study of cyclic constitutive behaviour, crack-tip deformation and crack-growth path for a polycrystalline nickel-based superalloy

Crystal plasticity has been applied to model the cyclic constitutive behaviour of a polycrystalline nickel-based superalloy at elevated temperature using finite element analyses. A representative volume element, consisting of randomly oriented grains, was considered for the finite element analyses under periodic boundary constraints. Strain-controlled cyclic test data at 650 °C were used to determine the model parameters from a fitting process, where three loading rates were considered. Model simulations are in good agreement with the experimental results for stress–strain loops, cyclic hardening behaviour and stress relaxation behaviour. Stress and strain distributions within the representative volume element are of heterogeneous nature due to the orientation mismatch between neighbouring grains. Stress concentrations tend to occur within “hard” grains while strain concentrations tend to locate within “soft” grains, depending on the orientation of grains with respect to the loading direction. The model was further applied to study the near-tip deformation of a transgranular crack in a compact tension specimen using a submodelling technique. Grain microstructure is shown to have an influence on the von Mises stress distribution near the crack tip, and the gain texture heterogeneity disturbs the well-known butterfly shape obtained from the viscoplasticity analysis at continuum level. The stress–strain response near the crack tip, as well as the accumulated shear deformation along slip system, is influenced by the orientation of the grain at the crack tip, which might dictate the subsequent crack growth through grains. Individual slip systems near the crack tip tend to have different amounts of accumulated shear deformation, which was utilised as a criterion to predict the crack growth path.     

The brittle to ductile transition of single crystal materials—An indentation model

A model for the brittle to ductile transition of brittle single crystal materials under indentation has been investigated. Continuous dislocation pile-ups against the wedge tip are used to explain the plastic deformation. The indentation depth is attributed to the dislocation pile-ups. The critical indentation depth p _cof brittle to ductile transition is proposed. Thus, the single crystal material is in brittle mode during the indentation loading if the indentation depth is greater than p _c. Otherwise, it is ductile. Micrographs support this modeling. Indentation on the surfaces of (100) or (110) in fcc and bcc single crystal materials is compared. The parameter Sis proportional to the number of dislocations and to the reciprocal of wedge angle. The value of Sis smaller for (100) than for (110) in fcc structure, but the trend of bcc structure is opposite. The shape of indenter is similar to that of grinding particles consisting in cutting tools. In order to maintain cutting efficiency in ductile mode, the cutting tool must be replaced if the grinding particles are blunt.     

Evaluation of fracture toughness of cartilage by micropenetration

Failure properties of cartilage are important in injury repair and disease, but few methods exist for measuring these properties, especially in small animals. To meet this need, a new indentation/penetration method for measuring fracture toughness of cartilage is proposed. During indentation, a conical tip is displaced into the surface of the cartilage, causing first a non-penetrating indentation, and then a penetration into the tissue. The method assumes that tissue penetration occurs during periods of rapid work, which are identified from a curve of work rate vs. time. Total penetration depth is determined by summing the displacement during these periods. Fracture work is the work that occurs during rapid work, or penetration, and fracture toughness defined as the fracture work divided by one-half the penetrated surface area of the indenting tip. The method was validated by indentation testing of bovine cartilage. Penetrating indentations with a conical tip were performed in bovine patellar cartilage and depth of penetration and fracture toughness predicted. For comparison with the indentation data, depth of penetration was measured in histological sections. These measurements agreed well with the predicted depth. Predicted fracture toughness also agreed with values measured via a macroscopic test. This newly described method has promise as a general method for measuring fracture toughness in cartilage, particularly in small animals, since penetrating tips with small tip radius can be manufactured and penetration may be accomplished in cartilage of minimal thickness.     

Fracture mechanics model for subthreshold indentation flaws

A fracture mechanics model for subthreshold indentation flaws is. described. The model describes the initiation and extension of a microcrack from a discrete deformation-induced shear fault (shear crack) within the contact zone. A stress-intensity factor analysis for the microcrack extension in residual-contact and applied-stress fields is used in conjunction with appropriate fracture conditions, equilibrium in Part I and non-equilibrium in Part II, to determine critical instability configurations.In Part I, the K-field relations are used in conjunction with the Griffith requirements for crack equilibrium in essentially inert environments to determine: (i) the critical indentation size (or load) for spontaneous radial crack pop-in from a critical shear fault under the action of residual stresses alone; (ii) the inert strengths of surfaces with subthreshold or postthreshold flaws. The theory is fitted to literature data for silicate glasses. These fits are used to calibrate dimensionless parameters in the fracture mechanics expressions, for later use in Part II. The universality of the analysis in its facility to predict the main features of crack initiation and propagation in residual and applied fields will be demonstrated. Special emphasis is placed on the capacity to account for the significant increase in strength (and associated scatter) observed on passing from the postthreshold to the subthreshold domain.     

A theoretical study of steady-state wedge cutting through metal plates

The process of steady-state wedge cutting through steel plates is investigated by analyzing experimental data and developing a simple and realistic model of the problem. Kinematics of the cutting process is discussed and three characteristic deformation zones are identified: (i) plastic zone at wedge tip, (ii) transient bent flap, and (iii) transition zone. Deformation energies are calculated for each zone, and the upper bound theorem in plasticity is applied to derive a closed-form solution for the cutting force. Frictional effects were considered including machining friction near the wedge tip and sliding friction between the transient flap and the wedge surface. Prediction of the steady-state cutting model is compared with experiments conducted at Cambridge University, MIT, and Det norske Veritas. Good correlation with the test data is reported. A parametric study is performed and on that basis a simple approximate solution is developed. An interesting association was discovered with previous empirical formulas proposed by Minorsky, Vaughan, and Kuroiwa.     

The influence of indenter bluntness on the apparent contact stiffness of thin coatings

In the present paper, the influence of punch tip sharpness on the interpretation of indentation measurements is considered.Firstly, in order to obtain analytical insight into the nature of the problem, closed form solutions are presented for the indentation of a homogeneous elastic half-space by an axisymmetric indenter of arbitrary shape, including Hertzian, conical, and conical indenter with a rounded tip.Next, a fast and efficient numerical implementation of a semi-analytical approach to the solution of problems about frictionless axisymmetric indentation of an elastic layer on a dissimilar substrate is described. The approach allows rapid determination of the load-displacement curves for an arbitrary punch shape.The aim of the study was to explore the implications of a finite indenter tip radius for the problem of property identification of thin films. The variation of the apparent substrate stiffness with indentation depth was established for several generic indenter shapes, namely for flat, conical and parabolic punches, and for a conical punch with a rounded tip. It is demonstrated that in each of these cases the depth variation can be described by a simple curve belonging to a family of two-parametric functions. On the basis of these findings we assess the efficiency of using different punch shapes for property determination. A procedure is proposed for this purpose which relies on the use of the depth variation profiles established here. Finally, the influence of imperfect punch shape on the accuracy of analysis is considered.     

管材滚压剪切有限元分析

基于DEFORM 3D有限元软件建立了管材滚压剪切的三维有限元模型,并对不同工艺参数下的管材滚压剪切过程进行了解析计算。选用Johnson-cook模型作为材料的断裂损伤模型,经过试算和比较得到合适的材料损伤参数。分析了进刀率、刀具楔角以及管材壁厚等参数对刀具的载荷、管材实际切断深度的影响规律。研究结果将为管材实际切割...     

陶瓷的断裂韧性与缺口半径 Ⅰ.断裂韧性测试技术

本文用一种新的简单方法制备了不同半径的尖缺口，用单边切口梁试件测试了不同缺口半么氧化铝陶瓷的断裂韧性，地七咱不同测试方法测得的，同一种材料的断裂韧性进行了对比，讨论陶瓷材料断裂韧性的最佳测试方法和测试中对缺口半径的要求。     

Low velocity impact modeling in composite laminates capturing permanent indentation

This paper deals with impact damage and permanent indentation modeling. A numerical model has been elaborated in order to simulate the different impact damage types developing during low velocity/low energy impact. The three current damage types: matrix cracking, fiber failure and delamination, are simulated. Inter-laminar damage, i.e. interface delamination, is conventionally simulated using interface elements based on fracture mechanics. Intra-laminar damage, i.e. matrix cracks, is simulated using interface elements based on failure criterion. Fiber failure is simulated using degradation in the volume elements. The originality of this model is to simulate permanent indentation after impact with a “plastic-like” model introduced in the matrix cracking elements. This model type is based on experimental observations showing matrix cracking debris which block crack closure. Lastly, experimental validation is performed, which demonstrates the model’s satisfactory relevance in simulating impact damage. This acceptable match between experiment and modeling confirms the interest of the novel approach proposed in this paper to describe the physics behind permanent indentation.     

The role of knife sharpness in the slitting of plastic films

An experimental apparatus has been built that provides information on forces associated with slitting plastic films. The apparatus uses commercially available scissors as an analogue to the counter-rotating knife blades used in industry. Cutting forces were measured using sharp and worn blades at a range of slitting speeds from 0.05–2.5 m s^–1. Two important film-base materials were studied; the first was polyethylene terephthalate (PET) and the second, acetal butylate. The influences of speed and knife-edge radius were measured for each base material. For the PET-based film, the forces increased as the blade became dull and decreased with cutting speed. At low speeds, the force required to cut the acetal butylate film was independent of speed, but a sharp decrease in force was observed at a speed that depended upon the blade sharpness. An empirical model for the slitting process has been developed that can estimate the forces measured.     

Material characterization by instrumented spherical indentation

It was illustrated by the author in the previous work that combinations between material properties and indentation parameters can be used as mixed parameters in dimensionless functions to capture the sharp indentation response of materials. These issues are further extended for spherical indentation in the present study. Instrumented spherical indentation was performed by a parametric finite element analysis for a wide range of materials with maximum indentation depth-indenter radius ratios rising from 0.01 to 0.3 to investigate several fundamental features within the frame work of limit analysis. Frictional effects are taken into account. Regarding dimensional analyses and using a Taylor series expansion, a new set of dimensionless functions is constructed for spherical indentation parameters and hardness associated to a 70.3° conical indenter. Based on formulated functions, a reverse analysis procedure is suggested to extract material properties and hardness from spherical indentation force-depth curves with respect to two different indentation depth-indenter radius ratios. Effects of indenter compliance on indentation parameters and reverse results are considered. The accuracy of the proposed method is studied and discussed by carrying out reverse and sensitivity analyses for 22 representative materials with rigid and deformable indenters.     

极浅压入下纳米压痕仪针尖面积函数校准方法的研究

采用原子力显微镜直接扫描纳米压痕仪针尖法、球面拟合法和熔融石英标准样块的间接测量法对极浅压入下纳米压痕仪的针尖面积函数进行比较分析。实验表明,在极浅压入下,原子力显微镜直接法由于真实地反映了针尖尖端的几何形貌因而获得的面积函数更为准确可靠。建立了相应的数学模型,对于直接法测量中主要的误差,即由于原子力显微镜针尖曲率半径带来的误差进行了分析,结果表明在极小压入深度下压入深度越小,原子力显微镜针尖曲率半径带来的压痕仪针尖面积函数相对误差越大。     

Reinforcement delamination of metallic beams strengthened by FRP strips: Fracture mechanics based approach

The delamination failure of metallic beams reinforced by externally bonded fibres reinforced polymers (FRP) is addressed in this paper and a simplified fracture mechanics based approach for the edge delamination of the reinforcement strips is illustrated. The criterion is based on the evaluation of the energy release rate (ERR) using both analytical and numerical models. The analytical models consist of a simplified version of a “two parameters elastic foundation” and “transformed section” model while the numerical analyses refer to the modified virtual crack closure technique (MVCCT). The main aim of the paper is to establish a fracture mechanics failure criterion based on the ERR and the specific fracture energy of the bonded strips. The criterion is an alternative approach to the well known stress based method to asses the load carrying capacity of the adhesive joint. The accuracy of the simplified approaches is shown through a numerical example which refers to a steel beam strengthened by carbon fibres reinforced polymers (CFRP).     

T-stress and its implications for crack growth

Some of the “irregular” crack growth behaviour observed in different specimen geometries may not be unrelated. Discrepancies in fatigue crack growth rate have been observed in different specimen geometries of the same material; crack front “tunnelling” and out-of-plane crack growth have been found in mode I tension at elevated temperature. The results presented in this paper seem to indicate the relevance of a crack tip constraint parameter, the elastic T-stress, to the irregular crack growth behaviour that conventional LEFM fails to explain.     

Derivation of effective stress intensity factors from measured crack face displacements

When a crack is subjected to cyclic shear-mode loading, crack faces interference wedge the crack open and reduce the effective ΔK_II. The methods proposed in the literature to prevent it or to derive the effective ΔK_I and ΔK_II are discussed. It is shown that when crack tip plasticity becomes important it tends to make displacements larger than those predicted by LEFM and to “hide” friction effects. Finite element simulations combining friction and plasticity can separate these two effects, but the analysis of force-sliding displacement loops derived from displacement field measurements based on image correlation is a more straightforward and efficient method.     

Fatigue analysis of brittle materials using indentation flaws

The results of an experimental dynamic fatigue study on glass-ceramic specimens containing indentation flaws are analysed in terms of the theory developed in Part 1. A Vickers indenter is used to introduce the flaws, and a conventional four-point bend apparatus to break the specimens. Base-line data for testing the essential theoretical predictions and for evaluating key material/environment parameters are obtained from polished surfaces, i.e. surfaces prepared to a sufficient finish to ensure removal of any pre-existing spurious stresses. The fatigue tests are carried out in water. Inert strength tests in dry nitrogen are used to calibrate appropriate equilibrium fracture parameters, with dummy indentations on selected control specimens providing a convenient measure of the critical crack dimensions at failure. Regression analysis of the dynamic fatigue data yields values for apparent kinetic parameters, which are converted to true kinetic parameters via the transformation equations of Part I. Regeneration of the fatigue function from the theory using the parameters thus determined gives a curve which passes closely through the experimental data points, thereby providing a self-consistent check of the formalism. The implications of the results in relation to the use of macroscopic fracture parameters in the prediction of strength properties for materials with small-scale flaws is an important adjunct to this work. Finally, a recommended procedure for the general testing of dynamic fatigue properties of ceramics using indentation flaws is described.