Application of image analysis techniques to determine strain distribution in leather

The optimum cutting of various parts of a shoe, prior to shoe manufacture requires knowledge of the topographical variation of what are termed “lines of tightness”. Currently the cutting operation for shoe parts is guided by a general assumption about the pattern of the lines of tightness. There is a need to have available a system which can determine, in a non-destructive way, the lines of tightness in an indvidual piece of leather. Initially an image analysis system was developed to investigate the uni-axial deformation behaviour of leather. This technique provided more information about the stress-strain behaviour of a leather sample along the gauge length than a conventional mechanical test and it was possible to accurately measure the strain distribution along the gauge length. A system was developed which could determine the relative displacement of marked spots along the gauge length of the sample using images captured during a uni-axial, bi-axial or multi-axial tensile test. The separation of the marked spots along the direction of applied stroke allowed the determination of longitudinal strain while contraction across the width was also measured in some cases, which was useful in calculating the Poisson’s ratio of leather for which a great variation was observed between different locations (Butt, Belly, Neck etc). Various approaches were investigated to determine the lines of tightness. Firstly, the local Poisson’s ratio was observed since a higher value of this parameter was associated with these lines of tightness. Secondly, biaxial stretching of leather by a series of actuators for each axis indicated the lines of tightness along the actuator with lower strain values. Thirdly, the strain was measured when the leather was stretched along number of known axes. This latter technique appeared to be the best approach and mathematical modeling was investigated to provide further refinement. A mechatronics-based device by industrial application of the third approach was also proposed. The software was written using a graphical programming system (LabVJ EW)