An experimental method for determining ductile tearing energy of thin metal sheets

This paper presents a few typical experimental methods for determining the value of tearing energy under a simple plane stress condition and proposes a novel one for it. In this method, several unixaial tensile tests were conducted with test specimens of various lengths. It was assumed that, for each test, the total energy required to tear the specimen can be decomposed into two parts: one associated with tearing (including the plastic deformation in the vicinity of torn cross section) and the other with the plastic deformation over the whole working volume of the specimen. Consequently, when plotting the total energy divided by the specimen width and thickness against the working length of the specimen, a straight line was obtained. The intercept of this line gives the value of tearing energy, corresponding to a zero length specimen. For mild steel, this value was found to be 1520 kJ/m². Comparisons were made between the present result and those by other test methods.     

An experimental investigation of circular saw vibration via a thin plate model

This paper deals with important practical problems related to vibration reduction in a circular saw used for wood cut-offs. An investigation is made of the static and dynamic characteristics of thin plate structures as a model of a circular saw, to obtain fundamental knowledge to improve circular saw performance. The circular saw is represented experimentally by a thin rectangular plate clamped by two shims. The bending rigidity and damping capacity of a structure are measured for six different clamping methods. The results are evaluated by a coefficient, which is introduced through Merritt's regenerative chatter vibration theory. The most effective thin plate structure is found to improve vibration-proof capacity. This structure is easily applicable to an actual circular saw.     

Three-dimensional quantitative in situ study of crack initiation and propagation in AA6061 aluminum alloy sheets via synchrotron laminography and finite-element simulations

Ductile crack initiation and propagation in AA6061 aluminum alloy for a fatigue precrack have been studied in situ via synchrotron radiation computed laminography, a technique specifically developed for three-dimensional imaging of laterally extended sheet specimens with micrometer resolution. The influence of the microstructure, i.e. due to the presence of coarse Mg₂Si precipitates and iron-rich intermetallics, on the void nucleation process is investigated. Coarse Mg₂Si precipitates are found to play a preponderant role in the void nucleation and ductile fracture process. Void growth and void coalescence are then observed and quantified by three-dimensional image analysis during crack initiation and propagation. Parameters for a Gurson–Tvergaard–Needleman micromechanical damage model are identified experimentally and validated by finite-element simulations.     

On laser welding of thin steel sheets

Abstract

This paper presents a process–structure–property relationship study of laser welds as a continuous consolidation method for joining thin monophased steel foils, thereby providing a more effective, less costly method to construct automotive catalytic converters. A body centred cubic (bcc) iron–chromium–aluminium alloy doped with Mischmetal was utilised in this study. Both pulsed and continuous wave modes were used to establish the limit welding diagrams for lap joint configuration. Actual laser welding parameters were selected using several testing conditions. The laser welds behaved substantially different from the base material under creep and high temperature oxidation. The difference was mainly attributed to the changes in grain morphology, precipitation of aluminium nitrides and carbides, and relocalisation of the reactive elements during liquid metal flow upon keyhole formation, solidification and cooling.     

加厚油管镦锻实验研究

通过工艺试验,得出了管坯在大长径比条件下的变形规律及失稳情况,为加厚油管局部锻粗模具设计提供了必要的数据。     

An investigation of phase transformation behavior in sputter-deposited PtMn thin films

Sputter-deposited, equiatomic PtMn thin films have application in giant magnetoresistive spin valves, tunneling magnetoresistive spin valves, and magnetic random access memory. However, the as-deposited films are found to be a disordered A1 phase in a paramagnetic state rather than an antiferromagnetic phase with L1₀ structure, which is needed for device operation. Therefore, a postannealing step is required to induce the phase transformation from the asdeposited A1 face-centered-cubic phase to the antiferromagnetic L1₀ phase. The A1 to L1₀ metastable transformation was studied by x-ray diffraction and differential-scanning calorimetry. An exothermic transformation enthalpy of −12.1 kJ/mol of atoms was determined. The transformation kinetics were simulated using the Johnson-Mehl-Avrami analysis.     

Stress-induced martensitic phase transformation in thin sheets of Nitinol

Using the in situ optical technique known as digital image correlation (DIC), full-field quantitative strain maps of localization have been experimentally obtained for the first time in thin sheets of Nitinol. The use of DIC provides new information connecting previous observations on the micro- and macro-scales. It shows that the transformation initiates before the formation of localized bands, and the strain inside the bands does not saturate when they nucleate. The effect of rolling texture on the macroscopic stress–strain behavior was observed and it is shown that the resolved stress criterion or Clausius–Clapeyron relation does not hold for polycrystalline Nitinol. Finally, the effect of geometric defects on localization behavior was observed.     

Laser pulsed welding in thin sheets of Zircaloy-4

A simulation technique for the welding process design to obtain the high reliability of nuclear fuel spacer grids is presented. A finite element model of pulsed laser welding for Zircaloy-4 plate is developed, which incorporates the phase transitions and the dynamic material properties with transient temperature. The effects of pulsed laser parameters on the pool shape and welding-induced distortion are analyzed by simulations and verified by experiments. The parametric study presents that the welding induced distortions are highly dependent on the geometry of the molten zone and the heat affected zone.     

An experimental investigation of rotary ultrasonic face milling

Reliable and cost-effective machining of advanced ceramics is crucially important for them to be widely used in a number of critical engineering applications. The potential of Rotary Ultrasonic Machining (RUM) process has been recognized as one of the reliable and cost-effective machining methods for advanced ceramics and commercial machinery is available for the process. One limitation of the commercial RUM machines is that only circular holes can be efficiently machined. An approach to extend the RUM process to face milling of ceramics was proposed and the development of the experimental apparatus as well as the preliminary experimental results were published earlier in this journal. As a follow-up, this paper will present the results of an experimental investigation of the newly-developed Rotary Ultrasonic Face Milling (RUFM) process. In this investigation, a five-variable two-level fractional factorial design is used to conduct the experiments. The purpose of these experiments is to reveal the main effects as well as the interaction effects of the process parameters on the process outputs such as Material Removal Rate (MRR), cutting force, material removal mode and surface roughness.     

Resistance welding of thin stainless steel sandwich sheets with fibrous metallic cores: experimental and numerical studies

Abstract

This paper concerns resistance spot welding (RSW) of two types of thin stainless steel sandwich sheet. The cores of these materials, made of stainless steel fibres, are highly porous (> around 85 vol.-%) and have low thermal and electrical conductivities. However, these conductivities change during the compression and heating associated with RSW. A sequentially coupled finite element model has been developed, in which the crushed core is treated as a continuum, with properties which vary throughout the process. It is shown that a constitutive relationship of the type commonly used for crushable foams can be successfully employed to simulate the deformation of the sandwich sheets. The thermoelectrical part of the model incorporates the effects of the associated phase transformations and changes in interfacial conductance. It is shown that the predictions are broadly consistent with data obtained during welding experiments. The model is used to explore the effects of welding parameters on weld characteristics (weld pool formation and weld nugget shape).     

扭转构件的疲劳裂纹形成寿命预估

利用损伤力学推导了大范围损伤下的预估圆轴扭转的疲劳裂纹形成寿命的封闭解答公式,给出了确定损伤演化方程中材料常数的方法及算例。     

An experimental investigation of the Fe-Ni-Sb ternary phase diagram

The ternary system Fe-Ni-Sb has been investigated in the region between 42 and 100 at. % Sb using conventional methods including DTA, X-ray techniques, metallography, and electron probe mi-croanalysis (EPMA). A continuous solid solution was found to exist between the NiAs(B8)-type phases Fe _1+x Sb and Ni _1±x Sb, whereas the marcasite-type phase Fe _x Ni _1-xS b ₂ is only stable in the ranges 0.50 ≤x≤ 1.00 and 0.00 ≤x ≤ 0.03, respectively. A skutterudite-type compound FeNiSb ₆ —isotypic with CoSb ₃ —was found to decompose peritectically at 642 ‡C. An isothermal sec-tion of the phase diagram was studied at 600 ‡C by X-ray powder diffraction using the Guinier tech-nique, and the lattice parameters of Fe _x Ni _1-x Sb ₂ and (Fe _y Ni _1-y ) _1±x Sb were determined as a function of composition. Several samples were investigated by microprobe analysis to fix the phase bounda-ries of the (Fe _y Ni _1-y Sb-phase. Four invariant ternary reactions were found in the Sb-rich part of the phase diagram, all of them within the narrow temperature range between 615 and 642 ‡C. The results of the DTA measure-ments were also used to construct the liquidus surface in the entire composition range investigated. All experimental information was combined with literature data to yield a complete ternary phase diagram in the composition range between 42 and 100 at. % Sb.     

Possibility of detection of crack initiation in high temperature water

The initiation of stress corrosion cracks in sensitised austenitic stainless steel under high temperature water conditions (simulated boiling water reactor environment) was detected, applying electrochemical noise measurement technique. The stress corrosion cracks were generated on pre-oxidised, unnotched standard tensile round bar specimens under constant load conditions. The elementary transients associated with stress corrosion crack initiation were found to be embedded in the basic noise pattern coming from the oxide growth reaction on the tensile specimen. Thus a simple calculation of standard deviations to identify localised corrosion phenomena under high temperature water conditions cannot be used. The changes in the track of power density spectra proved to be much more suitable to indicate stress corrosion crack initiation under high temperature water conditions.     

Subsurface crack initiation and propagation mechanisms in gigacycle fatigue

In the very high cycle regime (N_f > 10⁷ cycles) cracks can nucleate on inclusions, “supergrains” and pores, which leads to fish-eye propagation around the defect. The initiation from an inclusion or other defect is almost equal to the total crack growth lifetime, perhaps much more than 99% of this lifetime in many cases. Integration of the Paris law allows one to predict the number of cycles to crack initiation. A cyclic plastic zone around the crack exists, and recording the surface temperature of the sample during the test may allow one to follow crack propagation and determine the number of cycles to crack initiation. A thermo-mechanical model has been developed. In this study several fish-eyes from various materials have been observed by scanning electron microscopy, and the fractographic results analyzed as they related to the mechanical and thermo-mechanical models.     

High-cycle fatigue crack initiation and growth in TIMETAL LCB

This investigation has examined the influence of grain boundary α contiguity on the high-cycle fatigue behavior of aged TIMETAL LCB, with the fatigue performance being evaluated under tension-tension loading conditions atR=0.1 in laboratory air and 25 Hz. Fractographic analysis indicated that fatigue initiation, independent of processing history, involved subsurface crack formation. Serial section studies also indicated that crack initiation occurred at the interface between the triple-point α and the aged β matrix. Further back-scattered electron microscopy examination of the aged microstructures indicates that the observed differences in high-cycle fatigue behavior can be understood by considering the effect of processing history on the connectivity of grain boundary α, with decreased connectivity being associated with enhanced fatigue performance. This paper was presented at the Beta Titanium Alloys of the 00’s Symposium sponsored by the Titanium Committee of TMS, held during the 2005 TMS Annual Meeting & Exhibition, February 13–16, 2005 in San Francisco, CA.     

板状构件的疲劳裂纹形成及疲劳寿命预估

采用虚功原理分析了大范围损伤条件下平面应力板状构件的疲劳裂纹形成及疲劳寿命预估问题。导出了平面应力板状构件在大范围损伤条件下的疲劳裂纹形成及疲劳寿命预估的封闭解答。该方法力学概念清晰、明确,可广泛用于分析各类板状构件在大范围损伤条件下的疲劳裂纹形成及疲劳寿命预估问题。为预估板状构件的疲劳裂纹形成及疲劳寿命提供了新思路和新方法。     

An experimental investigation into soft-pad grinding of wire-sawn silicon wafers

Silicon is the primary semiconductor material used to fabricate microchips. A series of processes are required to manufacture high-quality silicon wafers. Surface grinding is one of the processes used to flatten wire-sawn wafers. A major issue in grinding of wire-sawn wafers is reduction and elimination of wire-sawing induced waviness. Results of finite element analysis have shown that soft-pad grinding is very effective in reducing the waviness. This paper presents an experimental investigation into soft-pad grinding of wire-sawn silicon wafers. Wire-sawn wafers from a same silicon ingot were used for the study to ensure that these wafers have similar waviness. These wafers were ground using two different soft pads. As a comparison, some wafers were also ground on a rigid chuck. Effectiveness of soft-pad grinding in removing waviness has been clearly demonstrated.     

Regularization of the damage mechanics for predicting crack initiation

Several damage models/theories were postulated in the past decades to overcome the actual deficiencies of the traditional fracture theory in predicting crack initiation. Recent reserarch indicates the reasonability of damage theories in predicting crack initiation as well as the degradation of material properties in varying degrees. However, in spite of the achievement of the novel developed damage theory on its way to predict crack initiation, there remains the intrinsic drawback of the response dependence of material point on its infinitesimal neighborhood invoked in classic continuum mechanics. In the present paper, the origin of the intrinsic drawback of the specialized ductility dissipation based damage theory in predicting crack initiation of pre-cracked specimens is outlined first; then, two regularizations, i.e., intrinsic mesh size and post non-localization methodologies, are employed to eliminate the drawback of the damage theory within the framework of classic continuum mechanics. The essence of the damage theory is that unrecoverable deformation inevitably results in the degradation of the deformation-carrying capacity. In turn this irreversible dissipation can be reasonably taken as the damage variable to characterize the deterioration of the deformation-carrying capacity of material. Surely, the bases of both regularizations are that the movements of material elements and the evolutions of microstructures are highly correlated and synergetic; they lead to the formation of slip-bands and stretched zones. Furthermore, pre-cracked specimens are taken as examples to show the rationality of the improvements.