Increase In Friction Stress Research Articles

Nitrogen‐Enhanced Temperature Dependence of Grain Refinement Strengthening in Austenitic Stainless Steel

Hall–Petch relationships at room temperature and lower temperatures are investigated for 316L steel (low‐N steel) and 0.43% nitrogen‐added 316L steel (high‐N steel) produced by solution nitriding. The yield stress is separated into thermal and athermal components, and the effects of nitrogen on the former are clarified. In the case of low‐N steel, the yield stress increases as the temperature decreases, and the degree of the increase is greater for fine‐grained steels. This is not only due to the increase in friction stress (including solid solution strengthening) but also results from the increase in grain refinement strengthening caused by the increase in the Hall–Petch coefficient at low temperatures. By contrast, in high‐N steel, the temperature dependence of the yield stress is more remarkable than in low‐N steel. However, the amount of increase in friction stress at low temperatures is not very different between the low‐N and high‐N steels. Instead, the increase in grain refinement strengthening caused by nitrogen is greatly enhanced at lower temperatures. This result suggests that the combination of nitrogen addition and grain refining is very effective at strengthening austenitic steels under low temperatures.

Течение Куэтта горячего вязкого газа

Найдено новое точное решение уравнений движения вязкого газа для плоского стационарного сдвигового течения горячего (800-1500 K) газа между движущимися с разными скоростями параллельными пластинами (аналог несжимаемого течения Куэтта). Одна из пластин считалась теплоизолированной. Для зависимости коэффициента вязкости от температуры принята формула Сазерленда. В отличие от других известных точных решений, вместо аналогии Рейнольдса (предположение о линейной связи между коэффициентами вязкости и теплопроводности) для вычисления коэффициента теплопроводности использована более точная формула, имеющая в рассматриваемом диапазоне температур ту же точность, что и формула Сазерленда (2 %). С использованием полученного точного решения исследовано качественное влияние сжимаемости на напряжение трения и на профили температуры и скорости. Показано, что (если одна из пластин теплоизолирована) сжимаемость газа приводит к увеличению напряжения трения. Проведено сравнение нового точного решения с известным точным решением (V. N. Golubkin, G. B. Sizykh, 2018), полученным с использованием формулы Сазерленда для коэффициента вязкости и аналогии Рейнольдса для коэффициента теплопроводности. Обнаружено, что оба решения приводят к одинаковым выводам о качественном влиянии сжимаемости на напряжение трения и на профили температуры и скорости. Однако прирост напряжения трения, вызванный сжимаемостью, при использовании аналогии Рейнольдса оказался недооцененным в два раза. Это показывает, что предположение о линейной связи между коэффициентами вязкости и теплопроводности может приводить к заметным количественным ошибкам.

Tribology behavior on scratch tests: Effects of yield strength

A three-dimensional (3D) scratch model is proposed to investigate the effects of yield strength of both coatings and substrates. With the help of combined Coulomb and plastic friction, the obtained results comprehensively interpret the experimental phenomena in most metals that with the growth of hardness after heat treatment the scratch friction coefficient (SFC) increases. This interpretation could not be done before. Scratch tests on the surface with or without the coating are discussed. Without the coating the SFC increases due to the decrease of the area with plastic slippage and/or the increase of friction stress during the increase of the yield strength in the material. With a softer substrate the friction stress decreases but the SFC increases, which is caused by the growth of the entire contact area and surface deformation. Conversely, with a stronger substrate the SFC decreases due to an intensified plastic slippage. The obtained results pave a new way to understanding the effects of yield strength on scratch tests, interpret experimental phenomena, and should be helpful for an optimum design in experiments.

Effect of viscosity on material behavior in friction stir welding process

Temperature-dependent elastic viscoplastic material model was used for the numerical simulation of the friction stir welding process. The non-elastic response of the rate-dependent material in the large deformation problems was calculated by using the closest point algorithm. The numerical results show that the shape of the equivalent plastic strain looks like onion rings and the spacing of the rings is approximately equal to the forward movement of the tool in one rotation. The equivalent plastic strain is increased with the increase of viscosity coefficient due to the increase of friction stress in the pin-plate interface. The region which is influenced by the rotating tool is decreased with the decrease of viscosity coefficient. The radial and circumferential stresses in front of the pin are greater than the ones behind the pin. This difference can be reduced with the decrease of viscosity.

An experimental method to study high speed sliding characteristics during forward and reverse slip

This paper introduces an experimental technique to investigate dynamic friction characteristics of sliding interfaces at normal pressures up to 125 MPa, slip speeds up to 15 m/s and slip distances of approximately 10 mm. This new apparatus involves a novel modification of the conventional torsional Kolsky bar apparatus employed extensively in the past to investigate high-strain-rate behavior of engineering materials. The experimental configuration allows critical frictional parameters such as the friction-stress, slip speed and slip displacement to be resolved on a micro-second time scale without the use of transducers at the frictional interface. Moreover, the experiment provides information on the evolution of dynamic friction stress during forward as well as reverse slip. Using this experimental configuration dynamic friction experiments were conducted on 6061-Al (T6)/1018 steel and 7075-Al (T6)/tool-steel (D3) tribo-pairs. The results of experiments on 6061-Al (T6)/1018 steel indicate that steady-state kinetic friction is obtained within the rise time of the torsional loading pulse. The coefficient of kinetic friction is observed to increase with the roughness of the tribo-pair surfaces. For a soft/hard tribo-pair interface (7075 Al/tool-steel (D3)) the surface roughness of the harder material (tool-steel (D3)) constituting the tribo-pair is observed to control the frictional force. The measured coefficient of kinetic friction is approximately independent of the interfacial slip velocity for sliding velocities in the range of 2–10 m/s and normal pressures of approximately 100 MPa. Moreover, it is observed that the dynamic friction stress during reverse slip is almost twice as large when compared to friction stress in the forward direction. This increase in friction stress is understood to be due to frictional contact between fresh metallic surfaces formed by the breakdown of oxide and other surface films and anisotropy in frictional surfaces developed during the forward slip.