Leather is a widely used component of many products such as shoes, car seats, garments and other leather goods. Because it is a natural material, a tanned hide will contain visual and hidden flaws. In addition, its mechanical properties vary over the hide. At present, hides are inspected and assessed by skilled operatives. Further, current objective leather testing requires removal of samples and is either destructive and/or incompatible with real time operation, and little or no information about the rest of the skin is provided. A novel mechanical scanning system was built for non-destructive leather testing. The investigation was focused on two of the most important physical leather properties, static compressibility across thickness and tensile properties for low strain regions. The results of static compression energy measurements for a compressive strain of 10 percent, showed a close agreement with the results of tests performed by a conventional compressibility tester. Further, the results of strain energy and stress measurements for a strain of 2 percent, revealed a very good correlation with the results of conventional tensile tests for a similar strain. The application of infrared thermography, a non destructive and contact less technique, to leather characterisation and inspection was investigated in this work. It was shown that this technique could be used for detecting defects in leather, as well as for estimating their size and deepness. However, defect visibility by infrared thermography is conditioned by the fact that a defective area has to cause different material properties or produce an internal thermal resistance. Further, the prohibitive cost of infrared thermography cameras for automation is a serious limitation for its application in current leather testing. It is recommended that the ideal testing system would be based on the combination of mechanical scanning, normal computer vision and infrared thermography. The normal computer vision part of this system would be responsible for measuring area and detecting defects that are visible in nature. The infrared thermography part of the system would be responsible for detecting the type of defects overlooked by the previous method, as well as some thermo-physical parameters. Finally, the Mechanical Scanning System would provide the physical properties of leather, like compressibility, tensile modulus, shear stress and softness that the vision based inspection systems are incapable of providing. In this way, every single skin could be completely characterised in terms of defects and physical properties.

This thesis mainly reports the dynamic mechanical behaviour of leather within the temperature region of -100°C and 300°C, where three major viscoelastic transition were identified, termed alpha, beta and gamma. The beta transition peak represents the glass transition temperature of the amorphous region of collagen molecules. It was also shown that tanning agents act as plasticisers and depress the glass transition temperature to a lower temperature. Thus the tanning process itself may be viewed as a plasticisation of the collagen molecule. In this event, tanning molecules interpose themselves between the collagen chains, thus reducing the forces holding the chain together. Different tanning agents show differing degrees of plasticisation. The effect of water on the viscoelastic transitions of leather was also investigated. It was shown that leather remains in a transitional viscoelastic region between -50 and 70°C regardless of the moisture content of a sample. This imparts unique properties to leather. Initially, the absorbed water molecules act as a plasticiser and depress viscoelastic transitions to a lower temperature region. Depending on the leather type, above a certain percentage of absorbed water splitting of the glass transition peak is observed. This may be due to a preferential hydration of certain hydrophilic amino acid residues leading to separation of the transitions due to hydrophobic and hydrophobic amino acid residues. It was demonstrated that the rate of stress relaxation is temperature dependent and the stress relaxation property of leather above and below the glass transition differs greatly. Two critical temperatures related to heat setting were identified, which may be termed the critical and the optimum temperature. The critical temperature is the temperature above which the set increases markedly and has been positively identified as the glass transition temperature. Finally, changes in the dynamic modulus during the drying of leather revealed information concerning the nature of the moisture-leather relationship at the critical stages accompanying drying. It was concluded that leather undergoes three different phases during drying where only the final phase is related to the final stiffness of the leather.