Numerical study of scratch velocity effect on recovery of viscoelastic-viscoplastic solids

International audienceThe scratch test is a classical way to investigate the abrasive resistance of coatings and substrates. Because of the complex phenomena involved, the use of refined finite element analysis is often required to analyze the influence of specific parameters. In this paper, the influence of the tip velocity on the scratch recovery of polymer-like time-dependent solids is qualitatively investigated. More precisely the response of three constitutive models is analyzed: an elastic-viscoplastic model, a linear viscoelastic model and finally a viscoelastic-viscoplastic model. This last model is an original assembly based on the connection in series of the elastic-viscoplastic model and the linear viscoelastic model. For that, a new method allowing the connection in series of two different rheological models in a FE code is presented. To analyze the numerical results, the concept of representative stress and representative strain rate of a scratch test is introduced

Constant contact stiffness indentation relaxation test

Nanoindentation test is of great interest to characterize small scale mechanical behavior, thus a large literature exists on the field. Nevertheless, measurements of time dependent mechanical properties by this technique is still to be improved 1. It is proposed to investigate the indentation relaxation from a different point of view. Indentation relaxation tests are usually performed keeping a constant displacement over a prescribed time duration 2. This experimental procedure is consequently very sensitive to the system drift. Hence, determination of relaxation behavior is limited to few hundreds of seconds in the best cases. Weihs and Pethica 3 and Maier et al. 4, proposed to use the continuous contact stiffness measurement as a robust measure of the contact area. Based on these studies, a novel experimental procedure has been developed. Contact stiffness is kept constant after loading to a prescribed depth, for a define period, while displacement and load are monitored. As the contact stiffness measurement is not sensitive to drift, this method allowed to perform relaxation experiments with very long hold segment. Experiments on fused silica and polymers - i.e. PMMA, PC and PS - at room temperature have been performed with a constant contact stiffness maintained up to 10 hours. It has been shown that the dispersion on the force, F, was greatly reduced (see Figure 1). This could be understood as constant contact stiffness experiments were much less affected by the system drift than constant displacement ones. This new method opens the way to time dependent mechanical characterization in a wider range of conditions, especially long time experiments and high temperature indentation tests. Please click Additional Files below to see the full abstract

Two experimental set-ups designed for investigation of friction stir spot welding process

International audienceThe effects of positioning and clamping conditions of a specimen of friction stir spot welding are investigated in this paper in terms of axial force and torque generated during the process. For this purpose, two special designs of experimental set-ups embedding different positioning and clamping conditions are presented. A four-component mechanical sensor is used for the measurements. First, the effects of the rotational speed of the spindle and the plunge depth of the tool on the axial force and torque are studied. Second, the effects of positioning and clamping conditions are investigated through both set-ups designed, varying the spindle rotation speed. It is shown that the axial force and torque exhibit an important dependence with respect to the rotation speed of the tool and that their maxima depend on positioning and clamping conditions of the specimen

Indentation relaxation test: Opportunities and limitations

Small scale characterization of material’s mechanical behavior has been performed for fifty years using indentation tests. Many developments have been made in order to improve the reliability of both measurements and interpretations. However, determination of material’s time dependent mechanical properties by means of nanoindentation techniques is still to be enhanced 1. It is proposed to investigate the indentation relaxation – i.e. constant displacement – test as an alternative to the commonly used indentation creep – i.e. constant load – test. Effects of loading strain rate on the measured relaxation behavior are studied, analytically, from a linear viscoelastic model. It is shown that constant strain rate loading guarantees a depth-independent measure of the relaxation behavior. Moreover, indentation strain rate (ISR) affects the relaxation spectrum 2 up to a critical time constant 3 (see figure 1). These effects, highlighted analytically, are confirmed experimentally on PMMA. Limitations of the indentation relaxation test are also discussed. Two main difficulties arise from this kind of experiment. Acquisition of reliable measurements is limited, for long time characterization, by the system drift and, for short time, by the displacement control loop. A particular care has been taken in tuning the control feedback gains to limit displacement overshoot. Very low drift rate has been attained – under 0.015 nm.s-1 – This allowed for measurements at constant displacement up to 600 s. Please click Additional Files below to see the full abstract