Dynamic tensile characteristics of TRIP-type and DP-type steel sheets for an auto-body

This paper is concerned with the dynamic tensile characteristics of transformation-induced plasticity (TRIP)-type and dual phase (DP)-type steel sheets at intermediate strain rates ranging from 0.003 to 200 s⁻¹. The dynamic responses of TRIP600, TRIP800, DP600 and DP800 steel sheets are investigated with the evaluation of stress–strain curves, the strain rate sensitivity, the fracture elongation and the effect of pre-strain. The dynamic responses were acquired from dynamic tensile tests at the intermediate strain rates with a high-speed material testing machine developed. Experiments were carried out with specimens whose dimensions were carefully determined by finite element analyses and experiments to induce uniform deformation in the gauge section at the intermediate strain rates. The tensile tests provide stress–strain curves and the strain rate sensitivity. Experimental results show two important aspects for TRIP-type and DP-type steel sheets quantitatively: The flow stress increases as the strain rate increases and the fracture elongation and the formability of TRIP-type sheets are better than those of DP-type sheets at the intermediate strain rates. The pre-strain effect was also investigated for two types of metals at the intermediate strain rates. TRIP600 and DP600 steel specimens pre-stained by 5% and 10% were elongated at the strain rate of 0.003 s⁻¹ for quasi-static loading, and then tested at strain rates of 0.003, 1, 10 and 100 s⁻¹. The results demonstrate that the mechanical properties of TRIP600 and DP600 steels are noticeably influenced by the pre-strain when the strain rate is over 1 s⁻¹. The ultimate tensile strength as well as the yield stress increases due to the pre-strain.     

High strain rate compression of titanium and some titanium alloys

A modified Hopkinson bar was used to compress specimens of commercially-pure titanium, IMI 125 and titanium alloys, AMS 4911B and AMS 4916B, at natural strain rates of between 3 × 10³ s⁻¹ and 3 × 10⁴ s⁻¹. All three materials deformed in a viscous manner with a linear increase of flow stress with strain rate and a macroscopic viscosity of 4·5 kPa s, 5·7 kPa s and 9·2 kPa s, respectively. At strain rates above about 1 × 10⁴ s⁻¹ there was a decrease in macroscopic viscosities to 0·9 kPa s and 2·5 kPa s for the titanium and 4911B alloy respectively, which is attributed to changes in the dislocation drag mechanisms. The results for the 4916B alloy at the higher strain rates were too scattered to give a definite trend. It is suggested that the increase in the macroscopic viscosity with alloying is due to an increase in the dislocation drag coefficient by solution hardening and to a reduction in the density of mobile dislocations by dislocation pinning.

It is also proposed that the susceptibility of these materials to catastrophic shear failure which occurs without prior linear work-softening is due to the higher propensity for titanium and titanium alloys to shear failure as a consequence of their thermo-mechanical properties. The decrease of strain at which shear occurred with increasing strength is in accord with this suggestion.     

The high strain rate compression of 1023 and 4130 steels

Steel specimens of type 1023 and heat treated 4130 steel of hardnesses, between 257 and 450 HV10, were compressed at strain rates between 3 × 10³ and 3 × 10⁴ s⁻¹ using a modified Hopkinson bar. Both steels showed viscous type flow behaviour with a linear increase in flow stress with strain rate. For the 1023 steel there were two linear regions, one from 3 to 12 × 10³ s⁻¹ with a macroscopic viscosity of 4·7 kPa s and two from 14 to 27 × 10³ s⁻¹ with a viscosity of 0·7 kPa s. These results indicate some change in the controlling mechanism. For the 4130 steel the macroscopic viscosity decreased linearly with increasing hardness from 7·4 kPa s at 257 HV 10 to 2·2 kPa s at 450 HV 10. This was probably due to a decrease in the density of mobile dislocations with increased tempering temperatures. At lower hardness values (257 and 300 HV 10) and higher strain rates, a levelling off of the flow stress occurred but the data were too scattered to give a definite trend. The heat generated due to deformation of the 1023 steel was sufficient to produce steady state flow at strains above about 0·3. In the 4130 steel the thermo-mechanical properties were such that linear work softening was observed at a rate inversely proportional to the square root of the strain rate and independent of the hardness. This suggests that the work softening was a function of properties of the common ferrite matrix. The strain at which the work softening commenced decreased with increasing hardness, because of the more rapid heat generation at higher flow stresses. At higher hardness and higher strain rates 4130 steel failed by shear. Shear failure occurred at lower strain rates with increasing hardness and, for a given hardness, at lower strains with increasing strain rate. This behaviour was consistent with Recht's analysis of catastrophic shear failure.     

Dry friction and wear of toughened zirconias and toughened aluminas against steel

The unlubricated friction and wear behavior of toughened zirconias and toughened aluminas against hardened steel was studied using the Falex ring-and-block technique. Three experimental ZrO₂-Y₂O₃ ceramics, two commercial ZrO₂-MgO ceramics and two commercial Al₂O₃-ZrO₂ ceramics were investigated. Each ceramic was tested at 500 rev min⁻¹ (92 cm s⁻¹) and 2000 rev min⁻¹ (367 cm s⁻¹) at normal loads in the range 2.3–40.8 kgf. The materials characteristics of the ceramics before test, the features of the tested samples, and the friction and wear data are presented and related. Under mild wear conditions, all the ceramics exhibited low wear, with the ZrO₂-Y₂O₃ samples having the lowest. The wear of the toughened zirconias exhibited a strong sensitivity to sliding speed, while the toughened aluminas did not. Also, the lower toughness ceramics were susceptible to macroscale structural damage (cracking and chipping) even when the overall wear was low. Micro structural examination of a tested friction pair (ring, ceramic block and wear debris) has shown that the wear process is very complex, encompassing many mechanisms which are described. A generalized wear equation relating wear to load, sliding speed and sliding time is proposed.     

粉煤灰漂珠/聚氨酯复合泡沫铝材料压缩性能与本构关系研究

采用压力渗透法制备出了铝基复合泡沫材料,填充材料是以粉煤灰漂珠为主要组分、硬质聚氨酯泡沫为粘结剂的复合泡沫材料.通过准静态实验和分离式霍普金森压杆(Split Hopkinson pressure bar,SHPB)动态压缩的方法研究了复合泡沫铝的压缩力学响应,然后建立了动态本构关系.研究表明,复合泡沫铝的压缩应力-应变曲线与其它泡沫材料的应力-应变曲线类似,文中的两种铝基复合泡沫具有应变率效应,复合泡沫铝较密度相近未填充前的泡沫铝基具有更高的压缩强度与能量吸收能力.但由于漂珠尺寸的不同,导致两种复合泡沫铝的动态压缩结果不尽相同,且小颗粒复合泡沫铝在动态冲击下吸能效果最好.在本研究实验的应变率和密度范围内,本文建立的本构模型曲线与实验曲线吻合较好.     

粉煤灰漂珠/聚氨酯复合泡沫铝材料压缩性能与本构关系研究

采用压力渗透法制备出了铝基复合泡沫材料,填充材料是以粉煤灰漂珠为主要组分、硬质聚氨酯泡沫为粘结剂的复合泡沫材料.通过准静态实验和分离式霍普金森压杆(Split Hopkinson pressure bar,SHPB)动态压缩的方法研究了复合泡沫铝的压缩力学响应,然后建立了动态本构关系.研究表明,复合泡沫铝的压缩应力-应变曲线与其它泡沫材料的应力-应变曲线类似,文中的两种铝基复合泡沫具有应变率效应,复合泡沫铝较密度相近未填充前的泡沫铝基具有更高的压缩强度与能量吸收能力.但由于漂珠尺寸的不同,导致两种复合泡沫铝的动态压缩结果不尽相同,且小颗粒复合泡沫铝在动态冲击下吸能效果最好.在本研究实验的应变率和密度范围内,本文建立的本构模型曲线与实验曲线吻合较好.     

Study of lubricated impact using optical interferometry

An experimental study of lubricated impacts between a steel ball and a flat glass surface has been performed. The experimental set-up consists of a Ø70 mm ball mounted on a pendulum which impacts onto a lubricated glass disc. The contact region is studied by means of optical interferometry using a monochromatic light source, a microscope and a high-speed video recording equipment.

The lubricants are of PAO type and the viscosity ranges from 27 to 2600 mm² s⁻¹ at the test temperature. The impact velocity is varied between 0.08 and 0.29 m s⁻¹.

A dimple occurs at the centre of the contact where the lubricant is trapped. The influence of viscosity and impact velocity on the dimple's depth and diameter is studied.     

Sliding speed-temperature wear transition maps for Si3N4/iron alloy couples

The superior high temperature resistance of silicon nitride (Si₃N₄) based ceramics makes them suitable for tribological applications above room temperature or in high speed unlubricated sliding. There are some published works on the wear behaviour of Si₃N₄/metal alloys. However, experimental data are shown in a form that is not of direct use for engineers involved in materials selection. In the present work, Si₃N₄ pins were tested against tool steel and grey cast iron on a pin-on-disc tribometer. Ceramics were produced by hot-pressing and tested without lubrication at variable temperature and sliding speed. SEM/EDS and XRD analysis were used for chemical and microstructural characterisation of worn surfaces and wear debris. At low speeds (0.05–0.5 m s⁻¹) and room temperature, Si₃N₄ surfaces are polished-like due to a combination of humidity-assisted tribo-oxidation and abrasive action of very fine wear debris. At high sliding speeds (2–3.5 m s⁻¹), as well as for temperatures in the range 400–600°C, an extensive coherent tribolayer mainly composed by iron oxides spreads over the ceramic surfaces. Polishing and protection by adherent tribolayers are the mechanisms responsible for observed severe and mild wear regimes, respectively. Wear maps are constructed showing the transition of wear regimes in Si₃N₄/iron alloys contacts determined by constant flash temperature curves. Equations for calculation of bulk and flash contact temperatures in tribocontacts between dissimilar materials are deduced.     

Mechanical behavior of erosion-corrosion scales on steels as characterized by single-particle impacts

Samples of 2.25Cr-1Mo (less than 0.5 Si) and 2.5Cr-0.55Mo-1.4Si steels were eroded-corroded at 450 and 650 °C using fluidized bed combustor bed particles at velocities of 10 and 20 m s⁻¹. The steel with higher silicon content showed significantly lower metal loss rates under all conditions.

The samples were subsequently subjected to single-particle impacts using spherical WC particles at velocities around 50 m s⁻. The impact response of the scales could be explained in terms of a combination of substrate hardness and scale morphology effects but could not be consistently related to the superior erosion-corrosion resistance of the steel with higher silicon content. All scales were composed of oxidation product and deposited bed material erodent. Samples eroded-corroded at 450 °C had denser, more mechanically stable scales which could be associated with the generally lower erosion-corrosion rates at this temperature. At 650 °C the scales were more loosely packed, especially at the lower erosion-corrosion velocity, which resulted in apparent ductility by permitting them to densify under impact. Scales were either segmented or continuous in appearance. Thick continuous scales maintained their integrity under the lower velocity conditions of the erosion-corrosion tests, thus leading to low metal losses, but spalled catastrophicaliy under the single impacts. Segmented scales spalled in smaller pieces under single impacts. It is proposed that the segmented scales would exhibit significant failure under low velocity conditions, thus providing less protection to the steels than continuous scales under similar conditions.     

A comparison of the erosion rate of casing steels by sand-oil suspensions

The erosion rates (expressed as grams per square millimeter per minute) of three casing steels (P110, N80 and K55) and hot-rolled 1020 steel in quartz sand-oil suspensions, GFN 140, solids loading 0.5–4 wt.%, at speeds from 9.35 to 18.7 m s⁻¹ have been measured in a slurry pot tester. The velocity exponent was determined as 2.4 and the relationship of erosion rate to solids loading and particle size (for the particle size range 53–180 μm) determined. The erosion rates for all the steels were found to lie within a scatter band of ±15% and could not be related predictively with the measured tensile properties. Results are discussed in terms of the rate of particle impact on the eroding target and the need to control flow conditions through appropriate design to minimize erosion.     

The effects of iron particles on the friction and wear mechanisms of ceramics in ethanol

For the combinations of an Si₃N₄ pin and five kinds of ceramic disk (SiC, Si₃N₄, Al₂O₃, ZrO₂, TiC), a friction and wear test was carried out in ethanol and in ethanol containing iron particles (1 wt.%, average diameter d = 200 nm, D = 12 μm under cohered condition) under a load in the range 5.88–11.50 N, at a sliding velocity of 0.138–0.196 m s⁻¹. A topographical analysis was also performed on the microasperities of the wear surfaces to estimate the behavior of the iron particles, and the degree of surface damage. As a result, the following facts were found. (1) The addition of iron particles in ethanol decreased both the wear rates of SiC and TiC disks and the mating pins, and also decreased the wear rate of the Al₂O₃ disk but increased that of the mating pin. The addition increased the wear rates of both ZrO₂ and Si₃N₄ disks and the mating pins. (2) The average coefficients of friction with the addition of iron particles were greater than those without iron particles. (3) The wear rates of pin and disk depended on the topographies of wear surfaces and the wear index Γ.     

Identification of material hardening parameters by three-point bending of metal sheets

Calibration of hardening and partly viscoplastic parameters of the previously published material model was the primary concern of this paper. The method used for identification of the material hardening parameters for metals, the three-point cyclic bending of sheets, constitutes a basis for this work. Plastic hardening parameters were determined by comparing load–displacement curves from FE simulations with those from the tests. Since viscoplasticity is assumed, stress–strain curves from uniaxial tension tests at selected strain rates for strain-rate sensitive materials were employed to calibrate corresponding viscoplastic parameters. The optimization problems are solved by means of a commercial optimization code, LS-OPT, using a response surface methodology. The objective is to minimize (by the least-squares method) the sum of the differences between measured and simulated loads. The material parameters were identified for two high-strength steel alloys (ZSte340 and DP600, strain-rate sensitive materials), and one aluminium alloy (AA5182).     

Effect of sliding speed on friction and wear behaviour of nanocrystalline nickel tested in an argon atmosphere

The sliding speed dependence of the coefficient of friction (COF) and wear rate (W) of a nanocrystalline (nc) Ni with a grain size of 15 ± 3 nm and a hardness of 5.09 ± 0.11 GPa was compared to that of a microcrystalline (mc) Ni with a grain size of 20 ± 5 μm and a hardness of 1.20 ± 0.05 GPa. The sliding wear tests were performed in an argon environment under a constant normal load of 2 N using three different sliding speeds of 0.2 × 10⁻², 0.8 × 10⁻² and 3.0 × 10⁻² m/s. The lesser wear damage in the nc Ni at any given speed was attributed to its higher hardness and its greater elastic depth recovery ratio compared to the mc Ni. The mc Ni's COFs and wear rates were independent of the sliding speed over the relatively small range used. However, the same small increase in sliding speed caused an 86% reduction in the nc Ni's wear rate, from 3.44 × 10⁻³ to 0.47 × 10⁻³ mm³/m, and a 31% increase in its COF, from 0.49 ± 0.05 to 0.64 ± 0.06. A modified Archard equation was proposed to predict wear rates of Ni as a function of grain size and sliding speed. Increasing the sliding speed made it increasingly difficult for surface damage by plastic deformation to occur in the nc Ni, because the grain-boundary-induced deformation mechanisms are more difficult to operate at higher strain rates. At the highest speed, the smallest amount of debris was generated, which was not sufficient to form protective tribolayers leading to a high COF value.     

Constitutive modeling for Ti-6Al-4V alloy machining based on the SHPB tests and simulation

A constitutive model is critical for the prediction accuracy of a metal cutting simulation. The highest strain rate involved in the cutting process can be in the range of 10~4–10~6 s~(–1). Flow stresses at high strain rates are close to that of cutting are difficult to test via experiments. Split Hopkinson compression bar(SHPB) technology is used to study the deformation behavior of Ti-6Al-4V alloy at strain rates of 10~(–4)–10~4s~(–1). The Johnson Cook(JC) model was applied to characterize the flow stresses of the SHPB tests at various conditions. The parameters of the JC model are optimized by using a genetic algorithm technology. The JC plastic model and the energy density-based ductile failure criteria are adopted in the proposed SHPB finite element simulation model. The simulated flow stresses and the failure characteristics, such as the cracks along the adiabatic shear bands agree well with the experimental results. Afterwards, the SHPB simulation is used to simulate higher strain rate(approximately 3×10~4 s~(–1)) conditions by minimizing the size of the specimen. The JC model parameters covering higher strain rate conditions which are close to the deformation condition in cutting were calculated based on the flow stresses obtained by using the SHPB tests(10~(–4)–10~4 s~(–1)) and simulation(up to 3×10~4 s~(–1)). The cutting simulation using the constitutive parameters is validated by the measured forces and chip morphology. The constitutive model and parameters for high strain rate conditions that are identical to those of cutting were obtained based on the SHPB tests and simulation.     

Analysis of wear behaviour of titanium carbonitride coatings

The kinetics of chemical reactions occurring during the coating process and the wear behaviour of Ti(C,N) coatings, including variations in wear displacement with the temperature of the lower specimen, were investigated in this work. The variations in the wear displacement and the temperature of the lower specimen are regressed by an eight-order polynomial function. The lower specimens were coated by a titanium film as underlayer and three kinds of coating material including TiN, Ti(C,N) or TiC were deposited as the top layer. The Ti(C,N) coatings were prepared by varying the gas flow rates of nitrogen and acetylene to form eight kinds of specimen. The tribological behaviour demonstrated by these eight specimens is discussed. The experimental data for the atomic ratios of [C] and [N] can be well expressed using the theory of diffusion rate and the theory of reaction rate for the deposition of ceramic coatings. The variations in the wear displacement gradient with the temperature of the lower specimen can give information on the adhesive behaviour arising before and after three-body wear. The wear rates of the upper and lower specimens due to adhesive wear are dependent on the operating conditions. The specimen with a higher final wear displacement was likely to produce on the upper specimen a higher wear rate when operating at 0.705 m s⁻¹. The thicker the adhesive layer, the lower the wear rate of the lower specimen produced. When the sliding speed was elevated to 1.41 m s⁻¹, the specimen with a higher final wear displacement often produced a lower wear rate on the upper specimen, and also caused higher wear rates on the lower specimens.     

Tribological behavior of select MAX phases against Al2O3 at elevated temperatures

The layered M_n+1AC_n ternary carbides – MAX phases – Ta₂AlC, Ti₂AlC, Cr₂AlC and Ti₃SiC₂ were tested under dry sliding conditions against alumina at 550 °C and 3 N load (for a stress of ≈0.08 MPa) using a pin-on-disk tribometer. Ta₂AlC and Ti₂AlC exhibited low specific wear rates, SWRs, (≤1 × 10⁻⁶ mm³/N m), while the coefficients of friction, μ, were 0.9 and 0.6, respectively. At 0.4, μ of Ti₃SiC₂ was the lowest measured, but the SWR, at ≈2 × 10⁻⁴ mm³/N m, was high. With a μ of 0.44 and a SWR of 6 × 10⁻⁵ mm³/N m the Cr₂AlC sample was in between. No visible wear of Al₂O₃ counterparts was observed in all the tribocouples. Tribofilms, which were mainly comprised of X-ray amorphous oxides of the M and A elements and, in some cases, unoxidized grains of the corresponding MAX phases, were formed on the contact surfaces. The correlations between observed tribological properties and tribofilm characteristics are discussed.     

Improvement in tribological properties of plasma-sprayed Cr3C2–NiCr coating followed by electroless Ni-based alloy plating

Electroless-plated Ni-based alloy coatings, Ni, Ni–Co and Ni–Mo coatings with thickness less than 5 μm were deposited on surfaces of plasma-sprayed Cr₃C₂–NiCr coating. The tribological properties of these electroless-plated coatings against the as-sprayed Cr₃C₂–NiCr coating as sliding pairs were investigated with a block-on-ring arrangement in air at room temperature. It was found that all the Ni-based alloy coatings effectively improved the tribological properties of the Cr₃C₂–NiCr coating. Especially when the Cr₃C₂–NiCr coatings plated with Ni–Co and Ni–Mo coatings were against the as-sprayed Cr₃C₂–NiCr coating as sliding pairs, friction coefficients of 0.10 to 0.13 and coefficients wear coefficients less than 10⁻⁶ mm³·N⁻¹·m⁻¹ were achieved. Through examination and analysis of the worn surfaces employing scanning electron microscopy and X-ray photoelectron spectrometer, the improvement in tribological properties of the Cr₃C₂–NiCr coating may be attributed to the transformation of wear mechanism and the formation of CrO₃ on the worn surfaces.     

Liquid impact erosion of Al-Mg and Al-Cu alloys

The response of Al-Mg and Al-Cu alloys to multiple liquid impact in the velocity range 90 –148 m s⁻¹ has been investigated. The variation in erosion rate with Al-Mg composition is discussed in terms of a change in the erosion mechanism from ductile rupture of small particles to the removal of whole grains by intercrystalline fracture. This transition occurs with both increased magnesium content and increased velocity of impact. In contrast, Al-4%Cu fails by a mixture of ductile rupture and transgranular fracture The latter mode exhibits river patterns typical of cleavage but occasionally fatigue-like striations are also visible.     

Slurry wear properties of pump lining materials

In order to study the slurry wear properties of elastomeric lining materials for sand, sludge and sewage pumps, several tests were carried out under various test conditions with a slurry jet device and prototype pump. Preliminary tests were done with 13 materials including polymers, metals and ceramic coatings. Polyurethane, fluid elastomer and rubber showed good wear resistance compared with the hard metals, and were examined in detail. The test conditions were as follows: jet velocity v (8–25 m s⁻¹), impingement angle (10°−90°), mean diameter of silica sand d (42–415 μm), and sand concentration by weight c (0.1–7 wt.%). The slurry wear rates of the polyurethane and the fluid elastomer were maximum at the impingement angle of approximately 30°. The rubber showed almost the same wear rate regardless of impingement angle. The slurry were rate increased according to a power law of the jet velocity above a critical velocity and sand concentration. As the particle size increased above a critical size at the same weight concentration, the wear rate first increased but then decreased. The influence of impingement variables and particle variables observed using both test methods shows a similar tendency for each lining material, so the slurry wear loss can be described with a single empirical equation. Comparing the slurry wear resistance of three lining materials, the polyurethane showed the lowest resistance because it contained many pores which were produced during the lining process. The rubber showed the highest slurry wear resistance which is probably related to its high tear strength.     

The friction and wear properties of nylon

The effects of temperature on the friction of nylon are examined at various sliding velocities. Maximum friction values are obtained, but it is found that when specimens slide continuously at various velocities, the magnitude of the maximum friction varies with the velocity, while when specimens slide alternately at 10 and 0.1 mm s⁻¹, nearly the same maximum value is obtained. In the latter case, the displacement of the friction-velocity curves at various temperatures along the velocity axis can give a master curve while in the former case this is impossible.

The effects of temperature, velocity and load on the wear of nylon are also examined. Minimum values of the wear are obtained in all cases, whereas maximum values of the friction are obtained. The mechanism of such behaviour is discussed.