V.-T. Kuokkala

Dynamic Tensile Behaviour of TRIP and DP Steels at Different Temperatures

The stress-strain response of TRIP 700 and DP 600 steels was studied at a wide range of strain rates and temperatures using a special high/low temperature tensile Hopkinson Split Bar (THSB) device. The mechanical properties of the studied steels, especially of the TRIP steel, were found to be strongly affected by both temperature and strain rate. The beneficial TRIP effect in the studied steel reached its maximum at temperatures between 75-150 °C. The transformation behaviour of the retained austenite in the TRIP steel was studied by XRD, revealing that the phase transformation rate increases with decreasing temperature and decreases with increasing strain rate. A phenomenological numerical model was also presented to describe the behaviour of the TRIP and DP steels at different temperatures and strain rates.

Effect of strain rate, moisture and temperature on the deformation behavior of polymer roll covers

Soft polymer roll covers, which are used in certain positions of paper manufacturing machines, have a vital role in the dynamics of two mating rotating rolls (i.e., nip dynamics). The polymer covers are often used in moist conditions where the loading rates are rather high and temperatures may vary from 45 to 60°C. In this paper, we study the dynamic mechanical behavior of two soft polyurethane composite roll covers under different conditions of temperature, moisture, and loading rate. For the tests in compression, both servohydraulic materials testing machines and the split Hopkinson pressure bar technique were used in the strain rate range of 0.001–1500 s−1. The specimens, which were to be tested under moist conditions, were immersed in paper machine water (pH 4.5) until saturated moisture content was reached. The materials showed remarkable softening as well as decrease in the strain rate sensitivity in moist conditions.

The effect of microstructure and lead content on the tribological properties of bearing alloys

Owing to the presence of lead in the alloy, leaded tin bronzes have excellent seizure resistance in bearing applications where lubrication may become absent or impaired. However, ecological concerns over the toxicity of lead have caused a growing demand for a feasible substitute. This requires a thorough understanding of the role of microstructure and lead content on the tribological properties of leaded tin bronze bearings. In this study, the effect of these qualities was tested with a specialised device simulating thrust bearings in boundary lubrication conditions, as well as with conventional unlubricated pin-on-disc testing. Ball-on-disc testing was also tried to see the effect of reversing the pin-on-disc material pair on tribological behaviour in dry sliding. The presence of lead in the tested bronze alloys was found to increase the capability of the alloy to sustain a lubricant film. The effect of the actual amount of lead was small beyond 4 wt%. In pin-on-disc testing, the microstructure of the alloy was found to have a significantly higher effect on tribological behaviour than lead content. Fine microstructure and small lead globule size were found to lead to a tendency to microcracking at the lead/copper matrix interfacial boundaries, resulting in rapid wear rates and low friction coefficients in dry sliding. The capability of lead to smear between sliding surfaces was verified in ball-on-disc testing.

Modelling and testing of elastomer impact deformation under high strain rates

Abstract Elastomers are frequently used in applications involving repeated impacts of hard abrasive particles on surfaces at high or moderate strain rates. Operational conditions of components experiencing erosive and impact type wear are sometimes difficult to mimic in laboratory conditions, and as such, it is common to approach behaviour and lifetime predictions by meticulously studying different types of single impacts. This is particularly true for characterising and modelling of high strain rate impact events between hard particles and a wearing elastomer surface. This work presents and applies such a methodology for two specific elastomer materials: a natural rubber (NR) and styrene–butadiene rubber (SBR) compounds. The elastomer materials are subjected to quasistatic and dynamic testing conditions for determination of hyper- and viscoelastic material properties. The results are used in an iterative calibration procedure for establishing related constitutive models by applying the Ogden and Prony series models.

Effect of quartzite and granite in wear surfaces on dry sliding

Abstract The wear surfaces abraded with quartzite and granite were subjected to scratch tests. Sharp and blunt indenters were used with various constant loads to produce controlled abrasive wear tracks. The characteristics of deformation mechanisms and material removal were further studied using a scanning electron microscope to determine the differences in the tribological behaviour between the quartzite and granite wear surfaces. The results indicate that quartzite residues are more uniformly distributed as individual particles on the wear surfaces and therefore provide more stable frictional forces. In the case of granite the abrasive residues are rather non-uniformly collected into piles of abrasives.

Dynamic Thermo-Mechanical Response of Austenite Containing Steels

Austenite can be made thermally stable at room temperature by alloying the steel for example with nickel or manganese, which brings the martensite start temperature of the alloy below RT. Also low alloy steels can contain relatively high amounts of (retained) austenite brought about by appropriate heat treatments, which increase the carbon content of some of the austenite grains to such high levels that thermal martensite transformation does not take place in them. Well known steels containing stable or metastable austenite at room temperature are austenitic and duplex stainless steels, Hadfield manganese steels, low alloy TRIP steels, and high manganese TRIP and TWIP steels. Depending on the deformation temperature and strain rate, deformation induced martensite transformation and/or twinning, or the lack of them, can lead to quite extraordinary behavior and strength and elongation combinations. In this paper, the strain hardening behavior and strain rate sensitivity of several fully or partially austenitic steels are discussed in view of their microstructural development during deformation. The discussion is based on the experiments conducted on these materials in wide ranges of strain rate and temperature with conventional materials testing machines and various Hopkinson Split Bar techniques.