E. van der Heide

Galling initiation due to frictional heating

The lifetime of sheet metal forming (SMF) tools is often limited by galling, a wear type that affects the surface quality of the products and the lifetime of SMF tools. Since SMF tools represent high economical value, it is clear that improvement and prediction of tool life is of high industrial importance. Therefore, models are required that can predict friction and wear related phenomena in SMF-processes, in particular galling. This paper demonstrates the application of a wear model, which is able to predict the initiation of galling in lubricated sheet metal forming processes, to laboratory results gained with the TNO slider-on-sheet tribometer. Experiments are conducted with different (coated) tool surfaces, in sliding contact with stainless steel sheet, using two lubricants. By comparing the critical temperature of the generated boundary layer with the flash temperature at the interface of the sheet and individual tool summits, it is possible to predict whether or not galling initiation will occur. It is shown that the laboratory results are in good agreement with the predicted results of the presented wear model. Galling initiation in lubricated sheet metal forming processes, can be avoided by the application of smooth tool surfaces with enhanced thermal conductivity and lubricants which form boundary layers with a high critical temperature.

The effect of lubricant selection on galling in a model wear test

Galling is a known failure mechanism in sheet metal forming (SMF) processes. As a result of this wear process, the amount of waste increases, the production process becomes hard to control and eventually expensive maintenance is required in order to continue production. Delaying or avoiding galling mechanisms by optimising the contacting materials and lubricant is therefore of high industrial importance. The presented work focuses on the effect of lubricant selection on galling, using a model wear test: the TNO slider-on-sheet tribometer. With this tribometer, galling mechanisms are studied using tool steel in sliding contact with lubricated deep draw steel DC 06. The wear data obtained with the test method agrees well with a general framework of lump initiation, lump growth and scoring (galling). Experimental data clearly demonstrates that a low coefficient of friction at the start of the experiment is not a reliable indicator for galling prevention. Hence, lubricant evaluation should be done based upon long-term testing instead of on short-term testing. Comparative results with a lubricant set shows that the TNO slider-on-sheet model wear test is suited for ranking forming lubricants with respect to their ability to avoid material transfer.

Single pulse and pulse train modulation of cutaneous electrical stimulation: a comparison of methods

Summary: Changing the amplitude of single rectangular pulse stimuli (SP) has the disadvantage of recruiting tactile and nociceptive fibers in a changing, unknown proportion. Keeping the amplitude constant, but applying a varying number of pulses in a train is another way of stimulus variation, keeping the proportion constant. So, pulse trains (PT) with a variable number of pulses but fixed amplitude might be more suitable to study nonperipheral aspects of processing of stimuli. In this study, we compared the effects of PT and SP stimulation on subjective Numeric Rating Scale scores of perceived stimulus strength and evoked potentials (EP). A total of 41 healthy subjects were electrically stimulated at the left forearm or left middle fingertip using SP and PT stimuli. Numeric Rating Scale scores and EPs were averaged from 105 randomized stimuli at 5 stimulus amplitudes or number of pulses for each subject. The relationships between stimulus amplitudes or number of pulses, EP components and Numeric Rating Scale scores differed depending on the stimulation method and stimulus location. Although the repeatedly reported Numeric Rating Scale-EP (N150-P200) correlation was reproduced for SP at the fingertip, no significant correlation was found for SP stimulation at the forearm. For PT this correlation was found for both stimulus locations. These findings demonstrate that SP and PT involve different ways of processing. The two methods result in different Numeric Rating Scale scores and EP components. Furthermore, PT stimulation is less dependent on stimulus location.

A new water absorbable mechanical Epidermal skin equivalent: The combination of hydrophobic PDMS and hydrophilic PVA hydrogel

Research on human skin interactions with healthcare and lifestyle products is a topic continuously attracting scientific studies over the past years. It is possible to evaluate skin mechanical properties based on human or animal experimentation, yet in addition to possible ethical issues, these samples are hard to obtain, expensive and give rise to highly variable results. Therefore, the design of a skin equivalent is essential. This paper describes the design and characterization of a new Epidermal Skin Equivalent (ESE). The material resembles the properties of epidermis and is a first approach to mimic the mechanical properties of the human skin structure, variable with the length scale. The ESE is based on a mixture of Polydimethyl Siloxane (PDMS) and Polyvinyl Alcohol (PVA) hydrogel cross-linked with Glutaraldehyde (GA). It was chemically characterized by XPS and FTIR measurements and its cross section was observed by macroscopy and cryoSEM. Confocal Microscope analysis on the surface of the ESE showed an arithmetic roughness (Ra) between 14-16 μm and contact angle (CA) values between 50-60°, both of which are close to the values of in vivo human skins reported in the literature. The Equilibrium Water Content (ECW) was around 33.8% and Thermo Gravimetric Analysis (TGA) confirmed the composition of the ESE samples. Moreover, the mechanical performance was determined by indentation tests and Dynamo Thermo Mechanical Analysis (DTMA) shear measurements. The indentation results were in good agreement with that of the target epidermis reported in the literature with an elastic modulus between 0.1-1.5 MPa and it showed dependency on the water content. According to the DTMA measurements, the ESE exhibits a viscoelastic behavior, with a shear modulus between 1-2.5MPa variable with temperature, frequency and the hydration of the samples.

Experimental Investigation of a Polymer Coating in Sliding Contact with Skin-Equivalent Silicone Rubber in an Aqueous Environment

A study on the effect of a brush coating of polyacrylic acid (PAA) grafted with poly(ethylene glycol) (PEG) (PAA-g-PEG) on friction was done for a sliding system that involves silicone skin L7350: a silicone rubber used by the Fédération Internationale de Football Association (FIFA) for the determination of skin–surface friction in combination with artificial grass. Friction experiments were carried out using a reciprocating flat-on-flat test setup with the selected brush coating and compared with PAA-coated and fluoroalkane-coated samples. The experiments were focused on the effect of water. Results for the coatings tested at dry conditions showed a coefficient of friction above 1. Effective lubrication by water was able to reduce friction to a coefficient of friction below 0.01 at low sliding velocities. The results are currently used to further develop low-friction products for sliding interactions with human skin; for example, artificial grass and possibly medical textiles.

A multivariable model for predicting the frictional behaviour and hydration of the human skin

The frictional characteristics of skin‐object interactions are important when handling objects, in the assessment of perception and comfort of products and materials and in the origins and prevention of skin injuries. In this study, based on statistical methods, a quantitative model is developed that describes the friction behaviour of human skin as a function of the subject characteristics, contact conditions, the properties of the counter material as well as environmental conditions.

On the Frictional Heating in Single Summit Contacts: Towards Failure at Asperity Level in Lubricated Systems

The influence of surface roughness and thermal conductivity on seizure in lubricated contacts is described in this work by quantifying the action of individual asperities in relation to local surface temperature rise. Application of the model to a contact situation in metal forming of stainless steel sheet material reveals the importance of a high quality finish in terms of the center line average roughness of the surface. Calculations show that seizure of single summit contacts can be avoided by surfaces with enhanced thermal conductivity.

The tribological behaviour and tribochemical study of B–N type borate esters in rapeseed oil—compound versus salt

Two novel borate ester additives, (2-(2-(bis(2-hydroxyethyl)amino)ethoxy)-1,3,2-dioxaborolan-4-yl)methyl oleate and a tris(2-hydroxyethyl)amine salt of (2-hydroxy-1,3,2-dioxaborolan-4-yl)methyl oleate were prepared and used as anti-wear and extreme pressure agents in rapeseed oil. The tribological performance was evaluated using a four-ball machine. The results show that the additives possess high anti-wear and extreme pressure properties. XANES, XPS and AFM were used to analyse the composition and structure of boundary films at the worn surfaces. The results for the compound and salt of borate esters are compared, and it is shown that the boundary films formed by compound or salt are similar and mainly composed of B2O3.

Characterization of a bimodal electrocutaneous stimulation device

In order to improve observability of the processing of nociceptive and tactile stimuli, a new electrocutaneous stimulation electrode was developed. The electrode aims at combining the exact timing of electric stimulation with preferential activation of nociceptive or tactile afferents. In order to determine the effect of stimuli applied through the bimodal electrode on subjects, a characterization study was perfomed. The needle electrode sensation thresholds were lower than the flat electrode thresholds. The subject averaged quality VAS score was significantly influenced by mode, the intensity score by NoP. The EPs were significantly influenced by both mode and NoP and suggest a difference in processing between the modes.

Intensity modulation of cutaneous electrical stimulation: EPs and subjective ratings

Chronic pain research is increasingly focused on the neuroplastic mechanisms underlying subjective pain experience. The latter is often measured using reported pain intensity, e.g. using a numeric rating scale (NRS). Evoked Potentials (EPs) reflect the cortical representation of applied stimuli and are experimentally used for observation of neural processing. Both sensation and cortical representation depend on central modulation of applied stimuli, as described by theories like Diffuse Noxious Inhibitory Control and the Gate Control Theory. In multiple mechanisms both tactile and nociceptive activations interact, indicating that the distribution of activated afferents (Aβ, Aδ-fibers) is relevant for correct interpretation of results. From this point of view, the traditional use of amplitude modulated single electrical stimulation pulses (SP) might not be optimal for studying central processing of pain as the amplitude changes this distribution (depending on the local fiber densities). Pulse trains (PT) with a variable number of fixed amplitude pulses might be more appropriate stimuli as they resemble the coding of stimulus intensity by skin receptors. In this study we therefore compared NRS scores and EP components (P90, N150, P200, P300) obtained with SP and PT modulated intensity (I). A total of 30 healthy female subjects were electrically stimulated at the tip of the left middle finger or forearm. NRS scores and EPs were averaged from 105 randomized stimuli at 5 intensity levels. Both the I-EP (components) and the I-NRS relationships differed depending on the modulation method and stimulation site. Although the repeatedly reported NRS-EP correlation was reproduced for SP at the finger, no significant correlation was found with stimulation at the forearm. For PT however, the NRS-EP correlation was found for both stimulation sites. These findings support the view that SP and PT methods offer a different excitation of neural mechanisms, which might be fruitful for future observation of central mechanisms underlying chronification of pain.