Static Carrying Behaviour of a T-Beam
For a first investigation on the carrying behaviour of a glass-plastic hybrid element a T-beam of 1 m length was produced. The Figs. 5a and 5b show the position of the strain gauges (SG) on the beam which consisted of a 50 mm wide flange and a 71 mm high bridge. The glass and plastic sheets had a thickness of 3 mm.
In a 3-point bending test the stress change at the flange and at the bridge was investigated for a quasistatic stress. The load was increased in the interval 250 N, 500 N up to 1000 N and held 2 mins. A complete relief was carried out after each load. The concentrated load was brought to the centre of the beam by means of the construction shown in Fig. 6.
The load was brought in over a period of 50 sec. at 500 N and for 120 sec. at 1000 N to avoid effects of to fast stresses. The stress-time development is exemplarily shown for two steps. (see Figs. 7a and 7b). The load of 500 N produced a stress of 20 N/mm2 (tensile) in the glass at the underside of the web. The minimal difference between both graphs can be explained by the different positions of the strain gauges. During the time period 50 sec. – 170 sec. a slight increase of the stress occurred in the glass at 500 N. This seems to indicate a force rearrangement from the plastic to the glass. After the relief there was no residual stress in the glass to be found. The maximal deflection in this test was at 0.75 mm.
SG 1.2 biaxial
SG 7.8 uniaxial
Fig. 6: glass-plastic hybrid beam during the test
In Fig. 7b the max. carrying load of the T-beam is reached. At a stress of 38 N/mm2 (tensile) a breaking of both glass sheets in the web occurred after 75 sec. A typical picture of fracture for float glass in the web was the result. (see Fig. 8) A maximum deflection of 2.0 mm could be measured. The residual carrying capacity of the plastic was sufficient to avoid the complete destruction of the beam.