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The effect of normal load and rotational rate on the coefficient of friction (COF) of CoCr sliding against alumina was evaluated using a pin on disk tribometer. Tribological tests were carried out by using CoCr disks of 38 mm diameter and alumina balls of 6 mm diameter as counterpart (pin). An aqueous solution containing 3g/L of HA, concentration equivalent of that found in the synovial fluid, was used as lubricant and it was in continuous recirculation throughout the test. The COF values for two normal loads, 1N and 5N, and two rotational rates, 60 rpm and 120 rpm, have been obtained for a constant sliding distance of 500 m. The tribological tests were performed in duplicate in order to verify the reproducibility of the results. The results show that the COF varies with the load and with the rotational rate, so that, for the same load, the COF increases as the rotational rate decreases. This effect is more significant at lower loads. Meanwhile, the COF values are higher for loads of 1N than for 5N regardless of the rotational rate. The wear tracks were observed by optical microscopy and the images revealed the widest tracks at 5N and 60 rpm.

Applying accurate normal load to a specimen in direct shear tests under constant normal stiffness (CNS) is of importance for the quality of the resulting data, which in turn influences the conclusions. However, deficiencies in the test system give rise to a normal stiffness, here designated as system normal stiffness, which results in deviations between the intended and actual applied normal loads. Aiming to reduce these deviations, this paper presents the effective normal stiffness approach applicable to closed-loop control systems. Validation through direct shear tests indicates a clear influence of the system normal stiffness on the applied normal load (13% for the test system used in this work). The ability of the approach to compensate for this influence is confirmed herein. Moreover, it is demonstrated that the differences between the measured and the nominal normal displacements are established by the normal load increment divided by the system normal stiffness. This further demonstrates the existence of the system normal stiffness. To employ the effective normal stiffness approach, the intended normal stiffness (user defined) and the system normal stiffness must be known. The latter is determined from a calibration curve based on normal loading tests using a stiff test dummy. Finally, a procedure is presented to estimate errors originating from the application of an approximate representation of the system normal stiffness. The approach is shown to effectively reduce the deviations between intended normal loads and the actual applied normal loads., QC 20200701

Applying accurate normal load to a specimen in direct shear tests under constant normal stiffness (CNS) is of importance for the quality of the resulting data, which in turn influences the conclusions. However, deficiencies in the test system give rise to a normal stiffness, here designated as system normal stiffness, which results in deviations between the intended and actual applied normal loads. Aiming to reduce these deviations, this paper presents the effective normal stiffness approach applicable to closed-loop control systems. Validation through direct shear tests indicates a clear influence of the system normal stiffness on the applied normal load (13% for the test system used in this work). The ability of the approach to compensate for this influence is confirmed herein. Moreover, it is demonstrated that the differences between the measured and the nominal normal displacements are established by the normal load increment divided by the system normal stiffness. This further demonstrates the existence of the system normal stiffness. To employ the effective normal stiffness approach, the intended normal stiffness (user defined) and the system normal stiffness must be known. The latter is determined from a calibration curve based on normal loading tests using a stiff test dummy. Finally, a procedure is presented to estimate errors originating from the application of an approximate representation of the system normal stiffness. The approach is shown to effectively reduce the deviations between intended normal loads and the actual applied normal loads., Funding details: BeFo 391; Funding details: Svensk Kärnbränslehantering, SKB; Funding details: Nuclear Waste Management Organization, NWMO; Funding text 1: Open access funding provided by RISE Research Institutes of Sweden. The authors would like to acknowledge the research funding granted by BeFo Rock Engineering Research Foundation (grant proposal BeFo 391) and SKB, Swedish Nuclear Fuel and Waste Management Co, Solna, Sweden. The authors would also like to thank Adjunct Professor Diego Mas Ivars at SKB, Swedish Nuclear Fuel and Waste Management Co; Associate Professor Fredrik Johansson at KTH Royal Institute of Technology; and Adjunct Professor Erland Johnson at RISE Research Institutes of Sweden AB for their valuable contribution to this work.

Applying accurate normal load to a specimen in direct shear tests under constant normal stiffness (CNS) is of importance for the quality of the resulting data, which in turn influences the conclusions. However, deficiencies in the test system give rise to a normal stiffness, here designated as system normal stiffness, which results in deviations between the intended and actual applied normal loads. Aiming to reduce these deviations, this paper presents the effective normal stiffness approach applicable to closed-loop control systems. Validation through direct shear tests indicates a clear influence of the system normal stiffness on the applied normal load (13% for the test system used in this work). The ability of the approach to compensate for this influence is confirmed herein. Moreover, it is demonstrated that the differences between the measured and the nominal normal displacements are established by the normal load increment divided by the system normal stiffness. This further demonstrates the existence of the system normal stiffness. To employ the effective normal stiffness approach, the intended normal stiffness (user defined) and the system normal stiffness must be known. The latter is determined from a calibration curve based on normal loading tests using a stiff test dummy. Finally, a procedure is presented to estimate errors originating from the application of an approximate representation of the system normal stiffness. The approach is shown to effectively reduce the deviations between intended normal loads and the actual applied normal loads., Funding details: BeFo 391; Funding details: Svensk Kärnbränslehantering, SKB; Funding details: Nuclear Waste Management Organization, NWMO; Funding text 1: Open access funding provided by RISE Research Institutes of Sweden. The authors would like to acknowledge the research funding granted by BeFo Rock Engineering Research Foundation (grant proposal BeFo 391) and SKB, Swedish Nuclear Fuel and Waste Management Co, Solna, Sweden. The authors would also like to thank Adjunct Professor Diego Mas Ivars at SKB, Swedish Nuclear Fuel and Waste Management Co; Associate Professor Fredrik Johansson at KTH Royal Institute of Technology; and Adjunct Professor Erland Johnson at RISE Research Institutes of Sweden AB for their valuable contribution to this work.

The vertical load on a moving tire was estimated by using accelerometers attached to the inner liner of a tire. The acceleration signal was processed to obtain the contact patch length created by the tire on the road surface. Then an appropriate equation relating the patch length to the vertical load is used to calculate the load. In order to obtain the needed data, tests were performed on a flat-track test machine at the Goodyear Innovation Center in Akron, Ohio; tests were also conducted on the road using a trailer setup at the Intelligent Transportation Laboratory in Danville, Virginia. During the tests, a number of different loads were applied; the tire-wheel setup was run at different speeds with the tire inflated to two different pressures. Tests were also conducted with a camber applied to the wheel. An algorithm was developed to estimate load using the collected data. It was then shown how the estimated load could be used in a control algorithm that applies a suitable control input to maintain the yaw stability of a moving vehicle. A two degree of freedom bicycle model was used for developing the control strategy. A linear quadratic regulator (LQR) was designed for the purpose of controlling the yaw rate and maintaining vehicle stability.