Effects of defects on the in-plane dynamic crushing of metal honeycombs

The effects of defects and their distributions on the in-plane dynamic crushing of honeycomb panels were studied using explicit finite element modeling. The influence of defect locations and ratios is investigated on the deformation modes and the plateau stresses with respect to the impact velocity. Numerical results show that the dynamic performance of honeycomb displays a high sensitivity on the defect location, especially under low and moderate impact velocities. By introducing a defect correction factor β_m and using the one-dimensional shock wave theory, an empirical formula is given for the variation of honeycomb’s plateau stress with respect to the impact velocity and the defect ratio.     

The inhomogeneous deformation of polycarbonate circular honeycombs under in-plane compression

In-plane uniaxial and equi-biaxial compression tests were conducted quasi-statically on polycarbonate circular honeycombs. While the uniaxial compression tests were easy to implement, a special test rig was designed to carry out the in-plane (x–y) equi-biaxial compression tests in a conventional universal testing machine. The deformation characteristics of a honeycomb block under uniaxial compression are quantitatively described by tracking the variations of the cells’ parameters, such as the area strain and the angle of cells, during the deformation process of the honeycomb. As the distribution of the deformation within the honeycomb block under biaxial compression is much more complex, a series of color maps based on the area strain of each cell are produced to demonstrate the inhomogeneous deformation among the cells in the honeycomb block, by which the initiation of deformation inhomogeneity and its evolution in the honeycomb block are characterized. To identify the role of the friction between the honeycomb sample and the test rig, finite element analysis is conducted to simulate the collapse process of the circular honeycomb under equi-biaxial compression. Furthermore, an inhomogeneity index, I_inh, is defined as a function of the overall compression of a sample to quantify the severity of the deformation inhomogeneity of the honeycomb; this index facilitates quantitative comparisons among the results under various loading conditions. It is found that the value of I_inh of a sample is closely related to some internal factors, such as the localization band of deformation and the deformation pattern of cells.     

Effects of solid distribution on the elastic buckling of honeycombs

The elastic buckling strengths of honeycombs depend on their relative density, cell geometry and the elastic modulus of solid cell edges. In this study, we consider the effect of the distribution of solid between three cell edges and a vertex on elastic buckling using a semi-analytical integral-equation approach. At first, the geometry of three cell edges connected at a vertex with Plateau borders is analyzed and then employed to represent a repeating element for regular hexagonal honeycombs. The bending moments, rotational angle and the stiffness of a rotational spring corresponding to the constraint from inclined adjacent cell edges are derived for the vertical cell edge within the repeating element. Consequently, the elastic buckling strength of regular hexagonal honeycombs can be numerically obtained. Moreover, the effects of the distribution of the solid on the elastic buckling strengths of regular hexagonal honeycombs are presented and evaluated.     

Effects of concentrated rigid inclusions defects on the in-plane impact behavior of hexagonal honeycombs

Hexagonal honeycombs have exhibited significant advantages in energy absorption and they are increasingly used as absorbers under crush conditions. The in-plane crushing process of imperfect hexagonal honeycombs with concentrated rigid inclusions defects is simulated using finite element simulations. In each case, a constant velocity is applied to an impact plate which then crushed the honeycomb. Simulation results indicate that the defect location has a great influence on the deformation modes, especially at low and moderate velocity. After analyzing the apparent reflection about dynamic response at the impact end, the respective influences of local fraction of inclusions and foil thickness (relative density) on the crushing plateau stress on account of the crushing velocity are further discussed. Furthermore, the energy absorption capacity under constant velocity loading is studied. Due to the distribution of the concentrated rigid inclusions defects, the energy absorption can be controlled effectively.     

Finite element simulation of chip flow in metal machining

Finite element studies of machining are becoming ever more sophisticated. A basic approach which removes the need, in an elastic–plastic analysis, to follow the development of chip formation from initial contact between work and tool, is the iterative convergence method (ICM). It develops a steady-state chip formation from an initial state of a fully formed chip loaded against a tool. It relies for its accuracy on the assumption that its simplified loading path coincides with the real developed flow at the end of the simulation. This paper examines the robustness of this assumption by studying the sensitivity of the simulation to changes of detail, within the ICM method, of how the flow develops; and it compares the simulated results with experiments. The experiment involves the turning of three free cutting steels, for which experimental flow stress variations with strain, strain rate and temperature, as well as information about the friction interaction between chip and tool, are available. The changes to the simulation method considered here are the structure of the finite element mesh, the measures of judging when the flow is fully developed, how the chip separates from the work at the cutting edge and the friction laws used during the approach to fully developed flow. It is shown that these do affect the outcomes of the simulation but within the ranges studied only to a minor extent and good agreement with experiment is achieved.     

Effects of lubricant-additives on the kinetics and mechanisms of ZDDP adsorption on steel surfaces

The influence of temperature, co-additive type, and basestock on the rates of adsorption of zinc di-isopropyldithiophosphate on SKF-3 steel have been measured. Adsorption behaviour is more complex than previously observed on the similar EN-31 steel and Langmuir kinetics may apply only in the early stages of the process. All co-additives cause reductions in ZDDP adsorption as also does the change from hydrocarbon to diester basestock. Reasons for the observed behaviour are discussed.     

Chemi-emission of electrons from metal surfaces in the cutting process due to metal/gas interactions

The exo-electron emission behaviour from Ti, W, Mo, Fe, Al, Ni, Cd, Zn and Pb metals was investigated during and after being cut in O₂ and N₂ gases at a gas pressure of 3 × 10⁻² Pa and in a vacuum of 4 × 10⁻⁴ Pa. The results showed that the emission behaviour of electrons during and after cutting depends on the combination of metal and the surrounding gas species. The emission intensity of electrons while cutting various metals in O₂ gas increased sharply with an increase in the negative value of the heat cf formation, ΔH_f of the oxide. The emission intensity in N₂ was also higher at higher negative values of ΔH_f of the metal nitride. Electron emission intensity from the cut metal surface is concluded to be a function of the heat of formation of reaction products of the metal surface with the surrounding gas.     

Effects of lubricants on friction and wear of Ti(CN)/1045 steel sliding pairs

The friction and wear properties of Ti(CN)/1045 steel rubbing pairs were investigated under dry and lubricated conditions by using a pin-on-disk tribometer. The selected speed range was 0.8 to 3.2 m/s and the load range was 58.8 to 235.2 N. Distilled water and a mineral oil (no additives) were used for lubrication, respectively. The wear of Ti(CN) ceramic under dry conditions was caused mainly by adhesion between the rubbing surfaces and the microfracture of Ti(CN). With the load and speed increasing, the adhesion and diffusion between rubbing surfaces increased and resulted in wear increasement of Ti(CN). Because of the brittleness of ceramics, the microfracture wear of Ti(CN) increased rapidly when the load was raised to some high values. The lubricating and cooling effects of the lubricants could improve the frict on and wear. Compared with water, oil was much better in improving the tribological properties. The analysis results obtained from XPS and AES examinations showed that ferrous oxide was produced on the wear scars, which could reduce the adhesion between the rubbing surfaces to some extent. The lubricating effects of the oil under boundary lubrication conditions were attributed to the formation of carbon films on the rubbing surfaces.     

Influence of Material Properties Variation on Damage Evolution During Extrusion Processes

Traditional finite-element methods of the analysis and design of metal forming processes, generally neglect the physical intrinsic uncertainties of the various parameters involved, such as the material properties variation. This paper describes a finite-element model allowing for the calculation failure probability during a manufacturing process. The simulation can predict forming defects such as crack initiation and propagation. The main difficulty encountered in simulating the processes is describing the material degradation of the workpiece in a realistic way. In this case, choosing a behaviour law is crucial, since each successive step in the whole process has to be described accurately. The proposed model makes it possible, by application of a certain number of simulations, to optimise the production in order to avoid defects generated by crack initiation during the operation. ID="A1" Correspondence and offprint requests to: Dr R. Hambli, ISTIA-LASQUO, 62 Avenue Notre Dame du Lac, 49000 Angers, France. E-mail: rida.hambli@istia.univ-angers.fr     

The influence of lubricants on cam failure

This paper studies the effects of lubricants on the failure and reliability of cams. Experimental investigations of the influence of lubricants on cam failure have been carried out at a metalworking factory. This paper presents following:

– A classification of industrial oils for cams. To establish this classification, two essentials of parameters have been taken into account: the environment (ambient temperature) and the cam operating conditions (load and sliding velocity).

– A flowchart of lubricating oil selection procedure for cams as a function of all relevant influencing factors.

– An analysis of the symptoms and the causes of failure of machine tool cams.

– An analysis of the average life and curves of cam reliability of machine tools as a function of tribological properties of some industrial oils.

The obtained results have proved that the tribological properties of lubricants has a great influence on:

– reduction of failures,

– extension of the life and

– increase of the reliability of cams.

     

Three-dimensional FE analysis of ring rolling by employing thermal spokes method

This paper investigates the performance of the “Thermal Spokes Method” in modeling the effect of the guide rolls on the ring rolling process. If the guide rolls are not included FE, simulation encounters problems such as tilting of the ring and process instability. Thermal spokes are introduced to withstand any unwanted moment of the resultant rolling forces at the roll gap and to maintain the ring in a centralized location in the mill. By employing the thermal spokes method, much closer predictions for the lateral spread and flow patterns are calculated. This method is able to predict the tilting of the ring with respect to the stiffness of the elements of the adjustment mechanism. It is also possible by this method to predict the point at which the ring changes its support from one guide roll to another one and to study the deformation mechanism outside the roll gap. To verify the validity of the proposed method, the predicted results from this method are compared with some available references. Since the proposed method considers the effect of the guide rolls and consequently offers much closer predictions, it is a very suitable method for ring rolling simulations.     

Effect of material anisotropy on shear angle prediction in metal cutting—a mesoplasticity approach

Material anisotropy plays an important role in the formation of shear angle in metal cutting. Crystallographic textures contribute to an important source of material anisotropy. A simplified mesoplasticity model is proposed in this paper to predict the effect of crystallographic orientations on the shear angle formation in machining a polycrystalline work material. The most likely shear angle is the one at which the Taylor factor is minimum. A good agreement is found between the predicted shear angle in machining a polycrystalline OFHC copper and the experimental data reported in the published literature. The assumptions made in the model approximate well the cutting conditions commonly encountered in single point diamond turning process.     

Numerical experiments on the influence of material and other variables on plane strain continuous chip formation in metal machining

Finite element simulations of metal machining chip formation have been carried out with model materials that have been given a range of thermal softening and strain hardening behaviours. For materials that are approximately perfectly plastic, predictions of slip-line field theory regarding the dependence of chip/tool normal contact stress distribution on the combination of shear plane angle, friction angle and tool rake angle are reproduced. But it has not proved possible to generate the full range of non-unique fields predicted by slip-line theory. The introduction of strain hardening causes chips to thicken but with deviations at high hardening rates from the behaviour proposed by Oxley. These observations are generally in agreement with previously published physical test data. A study of the effect of increasing the cutting edge radius confirms the important effect of that, particularly on tool thrust forces. By continually comparing the results to expectations from more simple modelling, and asking the question ‘Is that expected?’, a general problem of creating a friction law applicable to both plastically flowing high stress conditions and to more lightly loaded elastic conditions has been recognised and is the subject of continuing work.     

Tribological properties of nickel-base nanocrystalline coatings sputter-deposited on austenitic stainless steel

500 nm-thick films are deposited on austenitic stainless steel by neutral (Ar⁺) or reactive (N⁺) ion beam sputtering of Ni or NiTi targets, with (or without) high energy 160 keV-Ar⁺ ion beam assistance. Most of the time the coatings are nanocrystalline and induce a large (excellent in some conditions) increase of the wear resistance. Only Ar⁺ ion beam sputtering of a NiTi target gives an amorphous deposit which does not improve the substrate tribological properties. The hardness and wear resistance of ion beam assisted films are larger than those obtained with non-ion beam assisted coatings. The presence of a hard TiN phase inside a ductile Ni phase, of grains with preferential orientation beneficial to slip, as well as film densification are the main factors which increase the wear resistance. The best results are obtained when the structure is composed of two phases, Ni and TiN. The TiN phase strengthens the already good tribological Ni properties and the Ni ductility induces mechanical accommodation during the friction process.     

The composition of lubricant-derived surface layers formed in a lubricated cam/tappet contact II. Effects of adding overbased detergent and dispersant to a simple ZDTP solution

The surface films formed by a set of oils of related additive chemistry, but with differing detergent and dispersant contents, have been chemically characterized using a combination of surface analysis techniques. The films were formed in a cam/tappet friction apparatus with a direct acting bucket tappet geometry. In the absence of overbased detergent, the amorphous films were composed essentially of inorganic zinc phosphates formed by the ZDTP anti-wear additive, and evidence of higher molecular weight phosphates (e.g. metaphosphates) was found. Adding overbased detergent and dispersant resulted in partial replacement of zinc by the detergent metal and loss of the higher molecular weight phosphates in favour of ortho- and pyro-phosphates.     

Environmental effects on the formation process of adhesive wear particles

Three kinds of experiments have been conducted to study the effect of environmental molecules on the formation process of adhesive wear particles. First, the growth process of transfer particles was continuously observed by scanning electron microscope. It was understood that the growth of the transfer particles was due to the continuous conversion of the disc surface material into transfer particles. Next, wear tests between a Sn pin and Sn disc were conducted in an oxygen environment from 7 × 10⁻³ Pa to 1.0 × 10⁵ Pa. The size and hardness of wear particles and the wear volume increased with increasing oxygen pressure. Wear tests between Sn and Sn were also conducted while Au metals were sputter-deposited onto the friction surface. The deposition of Au had the same effect on the size and hardness of the wear particles as the oxygen atmosphere. In general, the hardness of metals is increased by the small amount of impurity atoms in them. The inclusion of oxygen or gold atoms on the surface and in the transfer particle is supposed to have the same effect of enhancing the shear strength of the interface and the transfer particles. This is the possible mechanism of the continuous shear fracture within the bulk which results in the growth of transfer particles.     

无铅微互连焊点力学行为尺寸效应的试验及数值模拟

采用高精度微拉伸试验和有限元数值模拟方法研究不同微尺度的Sn-Ag-Cu无铅钎料模拟互连焊点力学行为和性能演变的尺寸效应。结果表明,当焊点高度恒定(225μm)而焊点直径逐渐减小(475～200μm)时,拉伸断裂强度显著提高且远高于体钎料的抗拉强度,断裂应变也逐渐增加;焊点的断裂位置及模式由较大直径时的界面低延性断裂转变为小直径时焊点中间部位的大变形颈缩断裂。模拟结果表明,由于焊点内力学拘束水平的不同,小直径焊点的界面应力较低且最大应力分布在焊点中间部分,易导致断裂发生在焊点中部,接头强度应较高;而大直径焊点中最大应力处于焊点界面,易导致界面金属间化合物层在较低外加应力下起裂,焊点断裂强度应较低。     

Effects of texture gradients and strain paths on localization phenomena in polycrystals

Applications of crystal plasticity theory to the numerical modelling of large strain plasticity phenomena are considered. In particular, instabilities and localized deformation phenomena for FCC polycrystals subjected to various deformation modes are investigated. In-house finite element analyses based on a rate-dependent crystal plasticity model have been developed to simulate the large strain behaviour for sheet specimens subjected to plane strain, plane stress, and simple shear deformation modes. In the formulation, the plastic deformation of an individual crystal is assumed to be due to crystallographic slip. In the simulations, polycrystalline aggregates are modelled at various scales. This formulation accounts for initial textures, as well as texture evolution during large plastic deformations. The numerical analyses incorporate parallel computing features. The results of simulations for the above-mentioned deformation modes are discussed, and the formation of localized deformation in the form of shear bands is investigated.     

The friction and wear properties of nanometre SiO2 filled polyetheretherketone

Nanometre SiO₂ filled-polyetheretherketone (PEEK) composite blocks with different filler proportions were prepared by compression moulding. Their friction and wear properties were investigated on a block-on-ring machine by running a plain carbon steel (AISI 1045 steel) ring against the composite block. The morphologies of the wear traces and the transfer film were observed by scanning electron microscopy (SEM). It was found that nanometre SiO₂ filled-PEEK exhibited considerably lower friction coefficient and wear rate in comparison with pure PEEK. The lowest wear rate was obtained with the composite containing 7.5 wt.% SiO₂. The SEM pictures of the wear traces indicated that with the frictional couple of carbon steel ring/composite block (fillec with 7.5 wt.% filler), a thin, uniform, and tenacious transfer film was formed on the ring surface. It was inferred that the transfer film contributed largely to the decreased friction coefficient and wear rate of the filled PEEK composites.     

Influence of surface roughness on normal-sliding lubrication

Contact between machine components can involve normal, sliding and rolling motion, either singly or in combination. Combined normal and sliding motion, which occur for example in the meshing of gear teeth and in heavily-loaded rolling elements, can present problems for lubrication. The purpose of the present experimental study was to investigate how surface roughness affects the lubricant film characteristics under conditions of combined normal and sliding motion. The experimental arrar gement consisted of a rotating roller which impacted a stationary ball in the presence of a lubricant. Under the same conditions of normal surface approach, increasing the surface roughness significantly decreased the level of roller sliding that could occur without breakdown of the lubricating film. This behaviour was similar to a step function. Of the several surface roughness parameters investigated, only those which involved the maximum peak-to-valley height correlated well with experimental results. In general, surface roughness had a greater effect on oil film breakdown than did either viscosity or load.