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Item type:Publication, Access status: with files , Influence of material and geometrical parameters on the adhesive performance of vibration-modulated soft contactsVibroadhesion, the modulation of interfacial adhesion by superimposing small-amplitude, high-frequency vibrations on an adhesive contact, offers a simple and fast strategy to rapidly tune contact forces. A systematic understanding of how key geometrical and material parameters govern this behavior is still lacking. Here, we present a comprehensive experimental investigation of vibroadhesion in viscoelastic PDMS–glass Hertzian contacts, examining the influence of substrate thickness 𝑡, radius of the indenter 𝑅, material properties like modulus and thermodynamic surface energy, unloading rate 𝘳 and preload 𝘍cₒ on the stickiness performance of vibration-modulated interfaces such as: pull-off force 𝘍ₚₒ contact radius 𝒶, mean stress at pull-off σₚₒ, maximum enhancement with respect to the static adhesion test, range of vibration amplitudes that sustain adhesion enhancement. We find that across all tests, vibration activation triggers an abrupt expansion of the contact area, which upon unloading, results in a substantial increase of the pull-off force up to a saturation level always coinciding with the vibration level causing interfacial instabilities at the interface (wrinkles). Although the absolute value of the pull-off force may be strongly influenced by geometrical and material parameters, the maximum adhesion enhancement with respect to the reference static test remained consistently close to a 13-fold increase. We obtained the maximum mean stress at pull-off of 186 kPa when indenting with the smallest sphere. The unloading rate r weakly affects the adhesive performance and the preload Fcₒ is nearly irrelevant in determining the pull-off force 𝘍ₚₒ. Taken together, these results provide both physical insights and practical guidelines for designing vibration-modulated adhesive interfaces for rapid, reversible, and tunable adhesion.Publicationtype: Journal Article17 17 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A graded elastic modulus concept to eliminate stress or strain energy density singularity at sharp notches and cracks, with consequent elimination of size-scale effect on strengthIt has been recently suggested by the author that in the classical problem of a sharp wedge or crack loaded in plane (mode I and/or mode II), the stress singularity can be removed by grading the elastic properties of the underlying material from the notch tip by using a power law, E∼rβ. While the treatment is extended to the case of mode III (antiplane shear) which permits closed form results, we also discuss two ways to deal with the likely effect of material's grading on strength. In one, already explored in the previous paper, the strength is a power law of the modulus, and we suggest an “optimal” design by keeping the dominant stress constantly equal to the strength. In a second method, we propose to cancel the singularity in the strain energy density, which requires a much stronger grading, and we also possibly take into account that the critical strain energy density is a power law of the modulus. Noticing that only in the presence of a singularity a length scale can be defined experimentally by testing a very large notch and a very small one, according to the Theory of Critical Distances (TCD), the effect of cancelling singularity also implies independence on size/scale and constant strength. It is concluded that the technique is much more powerful than drilling a hole or rounding the tip of the notch/crack. Moreover, if a “smart” material could be designed to damage itself as to reduce its modulus when near a high stress concentration according to our prescriptions, it would naturally self-heal, opening up interesting applications.Publicationtype: Journal Article35 16 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Optimal grading of elastic modulus at contact corners with friction with application in fibrillar adhesivesRecent studies have reported that micropillars composed of graded materials exhibit high adhesion performance in fibrillar adhesive systems. To uncover the origin of this enhanced performance and further promote the application of graded materials, we investigate the general contact mechanics of wedges composed of power-law graded materials. Our findings show that the stress singularity at the apex of the wedge can be eliminated by increasing the exponent of the power-law modulus. For cases with perfectly bonded interfaces, frictionless interfaces, and interfaces with Coulomb friction, the critical conditions for eliminating stress singularities are presented, thereby suggesting how to obtain optimal designs.Publicationtype: Journal Article19 71 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Transient wave propagation in a 1-D gradient model with material nonlinearityA novel nonlinear 1-D gradient model has been previously proposed by the authors, combining (i) the higher-order gradient terms that capture the influence of material micro-structure and (ii) a nonlinear softening material behavior through the use of a hyperbolic constitutive model. While the previous study focused on the existence and properties of solitary-type waves, the current study focuses on the characteristics of the transient wave propagation in the proposed model. Findings show that as nonlinearity increases, the bulk of the wave slows down, and its shape becomes more distorted in comparison to the response of the linear system. The energy analysis reveals that, unlike the linear system, the nonlinear one continuously exchanges energy, in which the kinetic energy decreases over time while the potential one increases. Furthermore, the spectral (wavenumber) energy density of the nonlinear-elastic system presents peaks at large wavenumbers. However, these are eliminated when a small amount of linear viscous damping is added indicating that they are not physically relevant. A notable feature that persists despite the presence of damping is the formation of small-amplitude waves traveling in the opposite direction to the main wave. Generalized continua, like gradient elasticity models, miss the small energy scatter by the micro-structure. This study shows that adding material nonlinearity to a homogeneous generalized continuum can capture reverse energy propagation, though at much smaller magnitudes than the main wave. These findings shed light on the characteristics of the transient wave propagation predicted by the proposed nonlinear 1-D gradient model and its applicability in, for example, predicting the seismic site response.Publicationtype: Journal Article15 31 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Experimental characterization and mechanical behavior of 3D printed CFRP(2022-07-01); ; ; Additive manufacturing technology is pointed as the next generation production process. It not just allows the manufacturing of high complex design but also reduces the required production time from model to part. The main obstacle that holds 3D printing from growing up is the weak rigidity of the deliver part. To address this issue, continuous carbon fiber is nowadays merged and included in the printing process. The printing technology of reinforcement carbon fiber is not yet mature and needs to be explored more. Understanding the mechanical behavior of the printed carbon fiber composites can help improving this process. The present study is a contribution enhancing the understanding of the behavior of 3D printing parts based on carbon fiber reinforcement. Four types of standardization tests including tensile, interlaminar shear (ILSS) and dynamic mechanical thermal analysis (DMTA) have been performed for the mentioned purpose. Furthermore, first tensile-tensile fatigue results are presented. The mechanical properties and their corresponding microstructure as well as the effect of the fiber volume fraction have been analyzed. While the amount of pores found must still be reduced, the printed samples already show a very promising mechanical behavior. The work also includes different numerical models for the validation purpose.Publicationtype: Journal Article64 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A continuum mechanical model for the description of solvent induced swelling in polymeric glasses: Thermomechanics coupled with diffusionThe problem of interest is the numerical modelling of Case II diffusion in polymeric glasses. Case II diffusion is characterised by a strong coupling between diffusion and deformation and is highly temperature dependent. In this work, a general continuum mechanical framework describing a three-way coupling of thermomechanics and diffusion is adapted to the description of Case II diffusion. Numerical studies are carried out to examine the capability of the model and the nature of the coupling. It is shown that the model is well suited for the description of solvent induced swelling as it predicts all characteristic properties of Case II diffusion.Publicationtype: Journal ArticleScopus© Citations 14 237 262 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Friction induced dynamics of ball joints : instability and post bifurcation behaviorIt is well known that ball joints may be subject to friction induced oscillations. In the present analysis the stability behavior of a ball joint model with Coulomb and Stribeck-type friction is studied and the post bifurcation behavior of the resulting limit cycles is determined. The bifurcating limit cycles can be followed for both types of friction, and velocity dependent friction is not necessary to yield vibrations, as was proposed in the literature before. The linear instability is found to be of the whirl or flutter type. The results suggest that, depending on the ball joint and friction parameters, a complicated variety of nonlinear periodic solutions exists. It is shown that the previously often used approach of computing parameter values where stable sliding turns unstable can not even serve as a first rough indicator for the existence of friction induced oscillations. Instead, existence areas for stable/unstable sliding and co-existing stable limit cycles give a far better impression of the system characteristics, and can even give hints on measures that can help to eliminate friction whirl vibrations from ball joints. The analysis yields that a low friction coefficient, low loads and high system or interface damping seem promising to calm squeaking joints, as is physically plausible. Furthermore, the combination of the momentary speed of rotation and the relative component kinematics has a huge impact on the existence of limit cycles - hence the human gait is likely to influence the occurrence of squeaking in total hip arthroplasty.Publicationtype: Journal Article6