Kansei or affective engineering is the discipline of designing products to be psychophysically more appealing to the human mind and senses. Touch-feel perception of the materials used in consumer products ranging from portable electronics, furniture to automotive interiors plays an important role in the attractiveness of a product. Touch-feel perception is a qualitative measure and is an extrinsic property of the material. To better assist designers and material scientists to optimise aspects of a material for touch-feel perception, it is important to find a link between the qualitative touch-feel attributes with quantitative intrinsic properties of the materials. There is ongoing research in trying to decipher the links between touch-feel perception expressed through semantic psychophysical descriptor words, to physical parameters of the material sample such as the surface topographical, mechanical and tribological attributes. The objective of this work is to fill the current knowledge gap between micro-surface physical properties and customer’s perceptual response to surface tactile sensory information as well as their affective preference through theory, correlation models and experimentation.

A conceptual framework of surface tactile evaluation system can be divided into three parts: measurement of the surface physical characteristics, sensory evaluation and correlation analysis. To this end, the thesis documents the development of a friction measurement apparatus including an artificial finger to estimate the friction of a material against human skin in an accurate and repeatable manner. Secondly, correlation analyses were performed on the skin-against-material friction and the tribological factors, including the material surface parameters (e.g. roughness) and physical characteristics (e.g. hardness) of various metal and thermoplastic materials. Finally, the human touch-feel perception was assessed through a questionnaire and the results were modelled to obtain a link between the tribological factors and touch-feel perception.

Generally, human beings feel a surface by stroking or sliding one’s finger, which experiences friction. It is challenging to objectively describe the friction experienced by a human finger with respect to surfaces being stroked, as different surfaces and different working conditions can all influence the results. In order to understand the interaction between different surfaces and the friction experienced by a human finger, one has to minimise the variation due to human fingertips and touch conditions across experiments, such as fingertip humidity, temperature and elastic properties. To achieve this, a friction measurement apparatus incorporating an artificial fingertip has been developed. The artificial fingertip is made of multi-layered materials to mimic the structure, shape, softness and friction properties of a real human fingertip. The friction test apparatus consists of the artificial fingertip, a linear flexure mechanism and a reciprocal linear stage. It is capable of measuring the contact force and friction force simultaneously to give an estimate of the friction coefficient of the material-under-test. Twelve aluminium samples and five steel samples of different surface finishes were tested under different contact forces and stroking speeds. Comparisons were made between the friction results measured in vivo by a human fingertip and those by the artificial fingertip. The results have shown that for the material samples investigated, measurements from the artificial finger achieved a high correlation with results from real human fingers (r2 = 0:8 ~ 0:98) for surface ground steel and milled aluminium. Therefore the artificial finger can be used to mimic the friction characteristics of a real human fingertip and more importantly to measure the skin-against-material friction accurately and in a repeatable way.

In addition, in order to better understand the contact mechanism between the artificial finger and the surface, a suitable theoretical model which incorporates how the contact force relates to the contact area is essential. To enable the modelling of the contact mechanism, the Young’s modulus of the artificial fingertip has to be identified, as it is an essential input parameter for all contact theory models as well as FEM. The artificial finger was measured by using micro- and nano-indentation with Berkovich/spherical-tipped indenters. The contact area measurement was conducted by loading a custom-built glass plate on the artificial fingertip and observing the contact area under an optical microscope. Hertz theory was used to model the fingertip and predictions were compared against finite element analysis. The results support the fact that the Hertz contact theory is valid for modelling the contact mechanism of the artificial finger. Thermoplastic elastomers (TPE) and copolymers of elastomer are commonly used in manufacturing car interiors to give the surface a less harsh and more pleasing feel. Ongoing research has been trying to decipher the links between touch-feel perception expressed through semantic psychophysical descriptor words, to physical parameters of the material sample such as the surface topographical, mechanical and tribological properties. A series of five patterned and five coated TPE surfaces provided by an automotive manufacturer were characterized-topographical parameters of the samples by a surface profiler and mechanical/tribological parameters by a nanoindenter. The friction characteristics of these specimens were measured by the friction test apparatus and the artificial finger. The results showed that the artificial finger is representative of a human finger in its friction-sensing capability.

In the second part of the thesis, the relationship between the skin-against-material friction coefficient and the surface topography parameters Rq and Sm were deduced according to Hertz contact theory. The theory gives good agreement with experimental results. In addition, the relationship between the friction coefficient and the other mechanical parameters such as the Young’s modulus, skewness, kurtosis, surface slope were investigated through correlation analysis. Finally, 54 people of different age and gender were asked to rank the specimens in terms of 5 pairs of psychophysical descriptors, such as `rough/smooth’, `cold/warm’, `slippery/sticky’, `soft/hard’ and `like/dislike’. A rank-ordered logit model was deployed to correlate the human touch feel perception rankings and the thermoplastic samples, and the results were compared with correlation methods used in previous work. The results indicated the specific parameters which are correlated with human touch-feel perception and also their relative contributions. The results form a good guideline for material scientists and designers to, for example, build more touch-desirable car interior materials and consumer packaging.