All materials testing machines have a finite stiffness- the inherent stretchability of the testing frame, loadcell and grips under the testing load. Manufacturers aim to minimize this effect, in order to ensure it is the sample which deforms rather than the machine. Some use special pre-tensioning of the bearings, leading in turn to higher rates of wear and inevitably causing the quoted stiffness to fall away over the lifetime of the machine. Sometimes the stiffness is quoted without including the loadcell and grips, even though they are generally the most compliant part of the system.
The effects can be overcome by closed loop control of the sample extension, as is often done with servo hydraulic machines. Lloyd Instruments have just introduced a system to apply the same correction to screw driven machines, enabling the machine extension to be measured or programmed, removing the effect of machine compliance. The machine stiffness becomes virtually infinite, regardless of machine capacity and column configuration, and constant strain rate tests become exactly that, even on very stiff samples. The recorded values of extension are also corrected, even on cycling tests. The system can be retrofitted to existing Lloyd Instruments machines of the LR Plus, or EZ Test Series.
Lloyd Instruments and Davenport are supported in Australia by TSE Technical and Scientific Equipment.
