Evolution of real area of contact due to combined normal load and sub-surface straining in sheet metal

Understanding asperity flattening is vital for a reliable macro-scale modeling of friction and wear.In sheet metal forming processes,sheet surface asperities are deformed due to contact forces between the tools and the workpiece.In addition,as the sheet metal is strained while retaining the normal load,the asperity deformation increases significantly.Deformation of the asperities determines the real area of contact which influences the friction and wear at the tool-sheet metal contact.The real area of contact between two contacting rough surfaces depends on type of loading,material behavior,and topography of the contacting surfaces.In this study,an experimental setup is developed to investigate the effect of a combined normal load and sub-surface strain on real area of contact.Uncoated and zinc coated steel sheets(GI)with different coating thicknesses,surface topographies,and substrate materials are used in the experimental study.Finite element(FE)analyses are performed on measured surface profiles to further analyze the behavior observed in the experiments and to understand the effect of surface topography,and coating thickness on the evolution of the real area of contact.Finally,an analytical model is presented to determine the real area contact under combined normal load and sub-surface strain.The results show that accounting for combined normal load and sub-surface straining effects is necessary for accurate predictions of the real area of contact.

Skin tribology:Science friction?

The application of tribological knowledge is not just restricted to optimizing mechanical and chemical engineering problems.In fact,effective solutions to friction and wear related questions can be found in our everyday life.An important part is related to skin tribology,as the human skin is frequently one of the interacting surfaces in relative motion.People seem to solve these problems related to skin friction based upon a trial-and-error strategy and based upon on our sense for touch.The question of course rises whether or not a trained tribologist would make different choices based upon a science based strategy?In other words:Is skin friction part of the larger knowledge base that has been generated during the last decades by tribology research groups and which could be referred to as Science Friction?This paper discusses the specific nature of tribological systems that include the human skin and argues that the living nature of skin limits the use of conventional methods.Skin tribology requires in vivo,subject and anatomical location specific test methods.Current predictive friction models can only partially be applied to predict in vivo skin friction.The reason for this is found in limited understanding of the contact mechanics at the asperity level of product-skin interactions.A recently developed model gives the building blocks for enhanced understanding of friction at the micro scale.Only largely simplified power law based equations are currently available as general engineering tools.Finally,the need for friction control is illustrated by elaborating on the role of skin friction on discomfort and comfort.Surface texturing and polymer brush coatings are promising directions as they provide way and means to tailor friction in sliding contacts without the need of major changes to the product.

Dopamine hydrochloride and carboxymethyl chitosan coatings for multifilament surgical suture and their influence on friction during sliding contact with skin substitute

In order to reduce the damage to tissue and fill the interstices between fibers,multifilament sutures are frequently treated with certain coating materials.The objective of this study was to create and characterize dopamine hydrochloride(DA)and carboxymethyl chitosan(CMCS)coatings on surgical sutures and investigate their effects on the frictional performance of the surgical sutures during sliding through a skin substitute.The effects of the treatment on the physical and chemical characteristics of the surgical sutures were evaluated.The friction force of the surgical sutures during sliding through the skin substitute was experimentally determined using a penetration friction apparatus.The coefficient of friction(COF)was calculated using a linear elastic model and was used to estimate the frictional behavior of the surgical suture‐skin interactions.The results showed that the DA coating could evenly deposit on the surface of the etched multifilament surgical suture surfaces in a weakly alkaline buffer solution.The CMCS coating material could form a uniform film on the surface of the sutures.Minor changes in the surface roughness of the multifilament surgical sutures with different treatments occurred in this study.The friction force and the COF of the multifilament surgical sutures with DA and CMCS coating showed little change when compared with untreated multifilament surgical sutures.

Multiscale friction model for hot sheet metal forming

The accurate description of friction is critical in the finite element(FE)simulation of the sheet metal forming process.Usually,friction is oversimplified through the use of a constant Coulomb friction coefficient.In this study,the application of an existing multiscale friction model is extended to the hot stamping process.The model accounts for the effects of tool and sheet metal surface topography as well as the evolution of contact pressure,temperature,and bulk strain during hot stamping.Normal load flattening and strip drawing experiments are performed to calibrate the model.The results show that the model can relatively well predict friction in strip draw experiments when the tool surface evolution due to wear is incorporated.Finally,the application of the formulated multiscale friction model was demonstrated in the FE simulation of a hot-stamped part.

Design of bidirectional frictional behaviour for tactile contact using ellipsoidal asperity micro-textures

Tactile perception and friction can be modified by producing a deterministic surface topography.Change of surface feature arrangement and texture symmetry can produce an anisotropic frictional behaviour.It is generally achieved through skin hysteresis by promoting its deformation.This work investigates whether a bidirectional friction can be created with microscale ellipsoidal asperity textures,thus relying on the adhesive component of friction.For this purpose,four textured samples with various asperity dimensions were moulded with a silicone rubber having an elastic modulus comparable to that of the skin.Coefficient of friction measurements were conducted in-vivo in two sliding directions with a range of normal loads up to 4 N.Finite element method(FEM)was used to study elastic deformation effects,explain the observed friction difference,and predict surface material influence.Measurements performed perpendicular to the asperity major radii showed consistently higher friction coefficients than that during parallel sliding.For the larger asperity dimensions,a change of the sliding direction increased friction up to a factor of 2.The numerical analysis showed that this effect is mostly related to elastic asperity deflection.Bidirectional friction differences can be further controlled by asperity dimensions,spacing,and material properties.