Mechanical tests and behaviour of membranes

In the design stage of a new membrane roof, several properties of the material from which the roof will be built need to be determined experimentally. For dimensioning, the Young s modulus has to be known, and for the cutting pattern the compensation values have to be defined. Both of these values can be measured in a biaxial test using the loads of the respective project. 

Labor Blum has developed a biaxial machine (Fig. 5.1) which in our opinion is of the right size to obtain a homogeneous stress field that is large enough for an exact measurement. The overall dimensions of this biaxial machine are 2.44 x 2.44 m, and the planar two-dimensional test sample measures about 1.10 x 1.10 ml 

7 Biaxial machine of Labor Blum with climatic chamber from -20°C up to 70°C. 

The force is induced to the sample via clamps and straps. On every side of the square sample, seven straps are fixed in clamps that are attached via force measurement rings to low-friction ball screws which are driven by altogether 28 servo motors. Each servo motor is controlled independently. The servo motors are able to move perpendicularly to the force direction. The force is applied independently and uniaxially into every strap. To control the servo motors it is necessary to provide a control output variable for the force in the spindle. This must then be measured with a resolution better than 1%. Therefore the force measuring rings we use have been developed by ourselves. The biaxial measurement field has dimensions of 0.70 X 0.70 m, and loads of up to 200 kN/m can be applied. Strains are measured with potentiometric sensors. The external control circuit is set up digitally by means of a PC. 

This machine has been developed in an EC project by Labor Blum (Stuttgart, Germany) in cooperation with IF (Constance, Germany), Ferrari (La Tour du Pin, France) and Audra (Friedberg, Germany). 

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