On-line testing for mechanical stability of sheet material by applying twist

A module 10 is disclosed for in-line identification of weak items 14 of sheet material, such as silicon wafers suitable for use in photovoltaic cells, as they pass along a moving line of such items. The module includes provision to apply twist to the items as they pass along the line by applying bending in two generally diagonal directions. The applied degree of twist is such that it will not crack items with sufficient strength to provide acceptable products, so that after satisfactorily resisting the applied twist, acceptable wafers continue along the moving line, and after breakage or cracking unacceptable wafers can be removed from the line. The items may be carried by spaced apart belt conveyers 11, 12. The twist may be applied by rotatable discs (15-18, Fig 1c) mounted on rollers (TL, TR, BL, BR, Fig 1C) above and below the moving line of items. The discs may have tyres of suitable material not to damage the surfaces of the items.