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Background: The contact force–deformation relation between two spheres is one fundamental property which determines the mechanical response of the colliding bodies. In general, collisions of solid spheres occur in a very short time, so a highly accurate and a non-contact measurement method is required. Objective: In this study, a concept for the experimental determination of contact force–deformation curves of two colliding spheres is presented. Thereby two identical steel spheres collide by using a pendular setup and the sphere's velocities are measured by Laser Doppler Vibrometers (LDVs). Methods: The displacements are obtained by integrating the velocity with time and the accelerations are obtained by differentiating the velocity with time. From these values, the deformation and the contact forces can be calculated. Then, the elastic results are compared with the Hertzian theory of impact and experiments are conducted with elasto-plastic spheres. Results: Although the deformations are in good agreement with these analytical values, the influence of numerical differentiation is observed in the contact force. However, it is shown that the elastic contact force–deformation curve of two colliding spheres can be obtained with reasonable accuracy by using LDVs. For elasto-plastic spheres the coefficient of restitution became smaller than for the purely elastic case, and the force–deformation curve with hysteresis are measured. Conclusions: The results of this study are considered to be reasonable as far as comparisons with those for elastic collisions. Therefore, more detailed verification by numerical analysis, such as finite element analysis, is desirable. [ABSTRACT FROM AUTHOR]

Friction joints are one of the fundamental means used for the assembly of structural components in engineering applications. The structural dynamics of these components becomes nonlinear, due to the nonlinear nature of the forces arising at the contact interface characterised by stick-slip phenomena and separation. Advanced numerical models have been proposed in the last decades which have shown some promising capabilities in capturing these local nonlinearities. However, despite the research efforts in producing more advanced models over the years, a lack of validation experiments made it difficult to have fully validated models. For this reason, experimental techniques which can provide insights into the local dynamics of joints can be of great interest for the refinement of such models and for the optimisation of the joint design and local wear predictions. In this paper, a preliminary study is presented where ultrasound waves are used to characterise the local dynamics of friction contacts by observing changes of the ultrasound reflection/transmission at the friction interface. The experimental technique is applied to a dynamic friction rig, where two steel specimens are rubbed against each other under a harmonic tangential excitation. Initial results show that, with a controlled experimental test procedure, this technique can identify microslip effects at the contact interface. [ABSTRACT FROM AUTHOR]

The effects of roughness and fractality on the normal contact stiffness of rough surfaces were investigated by considering samples of isotropically roughened aluminium. Surface features of samples were altered by polishing and by five surface mechanical treatments using different sized particles. Surface topology was characterised by interferometry-based profilometry and electron microscopy. Subsequently, the normal contact stiffness was evaluated through flat-tipped diamond nanoindentation tests employing the partial unloading method to isolate elastic deformation. Three indenter tips of various sizes were utilised in order to gain results across a wide range of stress levels. We focus on establishing relationships between interfacial stiffness and roughness descriptors, combined with the effects of the fractal dimension of surfaces over various length scales. The experimental results show that the observed contact stiffness is a power-law function of the normal force with the exponent of this relationship closely correlated to surfaces' values of fractal dimension, yielding corresponding correlation coefficients above 90 %. A relatively weak correlation coefficient of 60 % was found between the exponent and surfaces' RMS roughness values. The RMS roughness mainly contributes to the magnitude of the contact stiffness, when surfaces have similar fractal structures at a given loading, with a correlation coefficient of −95 %. These findings from this work can be served as the experimental basis for modelling contact stiffness on various rough surfaces. [ABSTRACT FROM AUTHOR]

The digital image correlation (DIC) and ultrasound techniques have both previously been employed to measure the contact stiffness of real engineering interfaces, but a comprehensive comparison of these techniques has not previously been carried out. Such a comparison is addressed in the present paper. The principal novelty in this work is that DIC and ultrasound are used to simultaneously measure contact stiffness in the same tests and on the same contact interface. The results show that ultrasound measures somewhat higher contact stiffness magnitudes than DIC: at an average normal contact pressure of 70 MPa, ultrasound was around three times stiffer. Given that the techniques are vastly different in their measurement approach (DIC measures micron-scale relative displacements from external side-on images of the interface, while ultrasound uses the reflection of an Ångstrom scale ultrasonic perturbation from the interior of the interface itself), this level of agreement is thought to be encouraging. The difference in results can partly be explained by consideration of inherent physical differences between the techniques which have previously received little attention. Ultrasound measurement will always give the local elastic 'unloading stiffness' (even at a plastically deforming contact); whereas, a load-deflection technique like DIC, will give the 'loading stiffness'. The reason for this difference is discussed in the paper and tests carried out under increasing tangential load in the pre-sliding regime illustrate this difference experimentally. Under normal loading, the increase in real contact area obscures the effect to some extent as both DIC and ultrasound stiffnesses increase with normal load. The results suggest that rough interfaces may be satisfactorily modelled as a variable stiffness spring whose stiffness increases with contact pressure as the smooth contact case is approached. [ABSTRACT FROM AUTHOR]

An experimental apparatus designed specifically for fretting experiments on mechanical lap joints is presented. A piezoactuator is used to impose fretting motion, and a tri-axial load cell is used to measure tangential force as well as possible misalignment forces. A laser nanosensor is employed to measure the relative motion between the joint halves. No post-processing and filtering of the data is needed to obtain the fretting response using this apparatus. Instead, raw data obtained from experiments with monolithic and 1-bolt aluminum and steel joints under various loading conditions suggest that noise, misalignment, stiffness and damping associated with the apparatus are minimal, and thus the fretting behavior of the mechanical lap joints is accurately captured. Analyses of typical fretting loops obtained by the proposed apparatus suggest that normal preload and maximum tangential displacement influence the critical joint parameters of stiffness and damping. Aluminum joints show a more compliant behavior with more energy dissipation compared to steel joints. [ABSTRACT FROM AUTHOR]

The paper describes experiments to investigate the frictional properties of a Titanium alloy (Ti-6Al-4V) and a Nickel alloy (Udimet 720) under representative engineering conditions. Flat fretting pads with rounded corners were clamped against a flat specimen and a servo-hydraulic tensile testing machine was used to apply cyclic displacement to the specimen. Slip displacement between the specimen and pad was measured remotely using an LVDT and locally using digital image correlation. The latter approach allowed accurate determination of the tangential contact stiffness from frictional hysteresis loops. The results obtained show that the contacts are significantly less stiff than would be predicted by a smooth elastic contact analysis. A finite element model of the experimental contact geometry was constructed and it was shown that good agreement with the experimental measurements of contact stiffness can be obtained with a suitable choice of elastic modulus for a compliant surface layer. [ABSTRACT FROM AUTHOR]