M.B. de Rooij

Analysis of material transfer from a soft workpiece to a hard tool: Part I: Lump growth model

An important mechanism reducing the lifetime of the deepdrawing tools is material transfer from the workpiece to the tool, resulting in the buildup of lumps on the tool surface. In this study, a lump growth model, based on transfer mechanisms observed in experiments, is developed. The development of the summit height distribution of the tool surface is calculated from the growth behavior of the individual lumps. Important parameters influencing material transfer are determined from the calculated summit height distributions. It follows that lump growth can be decreased and, under certain conditions prevented, by a higher (surface) hardness of the workpiece, a lower roughness of the tool surface, a lower nominal contact pressure, and a lower shear strength of the interface. These results are in general agreement with results obtained from experiments. Experimental results are presented in an accompanying paper (de Rooij and Schipper, 2000).

Tool‐Ply Friction In Composite Forming

Friction between fibre composite laminates and rigid tool materials is an important phenomenon in composite forming processes and the resulting product geometry, fibre orientations and fibre stresses. Pull‐out experiments are presented and compared with a novel meso mechanical model based on Reynolds’ equation for thin film lubrication. The film thickness is derived from this analysis, rather than postulated as in earlier publications. Good agreement is found between the experiments, the meso mechanical model results and earlier empirically determined master curves.

Plastic Deterministic Contact of Rough Surfaces

In this paper, a theoretical and experimental investigation is presented to study the contact behavior of the plastic contact of deterministic rough surfaces. Analyses exclude bulk deformation of the rough surface and concentrate to the contact on asperity level. Surface asperities are modeled by an array of elliptic paraboloids where the unit event of a single contact is analyzed using an elastic-plastic elliptical contact model. A new method to determine the surface topography change due to plastic deformation is presented. Results show that the theoretical model developed predicts the contact area and the deformed geometry of the rough surface very well.

On the Transition from Bulk to Ordered Form of Water: A Theoretical Model to Calculate Adhesion Force Due to Capillary and van der Waals Interaction

The adhesion force due to capillary interaction between two hydrophilic surfaces is strongly dependent on the partial pressure of water and is often calculated using the Kelvin equation. The validity of the Kelvin equation is questionable at low relative humidity (RH) of water, like in high vacuum and dry nitrogen environments, where water is only present as layers of several molecules thick at the surfaces. A model from ordered to bulk form of water has been developed using the Brunauer, Emmett, and Teller adsorption model. The results show that the adhesion force calculated using the Young–Laplace and Kelvin equations at low (5–30 %) RH is underestimated. The total adhesion force shows changes when the RH is changed from 0 to 100 %. In dry conditions, at RH below 10 %, the total adhesion force is contributed by the van der Waals interaction due to solid–solid contact. The total adhesion force then increases and remains constant being equal to the superposition of van der Waals interaction due to solid–solid contact and van der Waals interaction due to adsorbed water layers on the surfaces. The total adhesion force further increases slowly with the increase in RH incorporating capillary forces and then decreases at very high RH due to screening of van der Waals forces. This change in adhesion force occurs from solid–solid interaction to ordered form of water at low RH and from ordered form to bulk form of water at high RH along with the screening effect of van der Waals interaction. The results have been compared with the experiments and it has been seen that at small length scales, the model is in agreement with the existing experimental data. However, at large length scales roughness of the surfaces should be taken into account.

Adhesion of RFL-coated aramid fibres to elastomers: the role of elastomer-latex compatibility

The performance of fibre-reinforced composites is strongly dependent on the nature and the strength of the fibre–matrix interface. Good interfacial bonding is required to ensure load transfer from matrix to reinforcing fibres. For rubber-reinforced composites, resorcinol formaldehyde latex (RFL) is known as a fibre surface coating which is able to provide good adhesion between rubber and fibres. In the present study, aramid fibres are investigated, because of their significantly higher modulus and strength, compared to other commercial fibres. Their adhesion after being coated with RFL, in compounds based on natural rubber (NR) and in NR blended with a small amount of styrene butadiene rubber (SBR) is investigated. It is shown that though having very similar tensile properties, the latter compound has much better adhesion to RFL which is also less sensitive to RFL ageing, compared to the pure NR compound. It is argued that an interphase region is formed between RFL and the elastomer, which is stronger for the compound containing SBR due to its enhanced compatibility with the latex part of the RFL.

Transient Thermal Effects and Heat Partition in Sliding Contacts

In tribological applications, calculating the contact temperature between contacting surfaces makes it possible to estimate lubricant failure and effectiveness, material failure, and other phenomena. The contact temperature can be divided into two scales: the macroscopic and the microscopic scales. In this article, a semi-analytical transient temperature model is presented, which can be used at both scales. The general theory is presented here and used to calculate the contact temperatures of single micro- and macrocontacts. For the steady state situation, the results obtained are in good agreement with those found in literature. Further, it is shown that the simplification of modeling a microcontact as an equivalent square uniform heat source to simplify the calculation of the maximum temperature is justified in the fully plastic regime. The partition is calculated by setting a continuity condition on the temperature field over the contact. From the results, it can be concluded that at low sliding velocities the steady state assumption, which is often used for microcontacts, is correct. However, at higher sliding velocities, the microcontact is not in the steady state and transient calculation methods are advised.

The influence of laser line hardening of carbon steel AISI 1045 on the lubricated wear against steel AISI 52100

To diminish wear in tribological systems it is not always necessary to provide the entire surface with a wear resistant layer. Depending on the application it is sufficient to harden locally the load carrying areas which are subjected to wear. Such areas can be treated properly by a laser, either totally or partially. This paper describes the effects of laser line hardening on the wear behaviour of carbon steel AISI 1045 against ball bearing steel AISI 52100. It is shown that the wear resistance of carbon steel AISI 1045 can be improved considerably by line hardening the surface. The wear resistance of the laser line hardened surfaces is comparable with that of carburised steel AISI 1045. However, the properties of the laser line hardened areas determine the wear behaviour of the entire system. Furthermore, the experimental work indicates that the type of heat treatment, carried out prior to line hardening in order to improve the microstructure of the steel, has no significant effect on the wear behaviour of the tribological system.

A wear measurement method based on the comparison of local surface heights

A new wear measurement method capable of measuring changes of the microgeometry due to wear by image processing techniques is presented. An advantage of this method over other methods is that it gives information on local height changes on the surface caused by material removal by wear and material addition by material transfer. Using this method, it is possible to measure and characterise wear and material transfer at the surface microgeometry level. The method will be explained in detail and some examples illustrating the method will be shown.

Analysis of Material Transfer From a Soft Workpiece to a Hard Tool: Part II—Experimental Verification of the Proposed Lump Growth Model

In this study, the lump growth model, described in an accompanying paper (de Rooij and Schipper, 2000) is validated by means of experiments performed on a deepdrawing simulator. In the experiments, the influence of material and roughness properties of both sheet and tool on the galling behavior is determined. For these experiments, a deepdrawing simulator and a selection of aluminum and zinc coated sheets with several (coated) deepdrawing tools are used. Good agreement is found between results of the lump growth model and the sheet metal forming experiments.

Friction in Forming of UD Composites

Inter‐ply and tool/ply friction play a dominant role in hot stamp forming of UD fiber‐reinforced thermoplastic laminates. This research treats friction measurements of a PEEK‐AS4 composite system. To this end, an in‐house developed friction tester is utilized to pull a laminate through two heat controlled clamping platens. The friction coefficient is determined by relating the clamp force to the pull force. The geometry of the gap between the clamping platens is monitored with micrometer accuracy. A first approach to describe the relation between the geometry and frictional behavior is undertaken by applying a standard thin‐film theory for hydrodynamic lubrication. Experimental measurements showed that the thin‐film theory does not entirely cover the underlying physics. Thus a second model is utilized, which employs a Leonov‐model to describe the shear deformation of the matrix material, while its viscosity is described with a multi‐mode Maxwell model. The combination of both models shows the potential to...