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Materials Lab Report

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MECH3362: Materials II
Laboratory Report

Brittle Rupture of Glass Plates

Guirguis Rezk SID 199991603 Objectives:
  To understand mechanisms of the fracture of brittle materials (in this case, glass) To recognize the variations in deformation and failure mechanisms which occur with a difference in flaw size, (i.e. between scratched and unscratched samples) Defining flaw sizes in glass plates by employing linear elastic fracture mechanics (LEFM) Utilizing the Weibull statistics approach in predicting probable failure in brittle materials.

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Testing Procedure:
1. The thickness (t) and width (w) of the rectangular glass sample is measured. The first glass slide is placed in the Instron 3-point bending apparatus with the …show more content…

This tension is produced by the bending moment of the load. For this reason the plates were placed with scratches facing downwards so that the procedure could test the effect of flaw size on fracture stress. Otherwise the experiment would have probably shown little difference between scratched and unscratched samples. The scratched samples also showed consistent directions in that they were nearly always perpendicular, this was due to linkage of cracks. Whereas for unscratched glass, the crack will propagate according to the local shape and direction of the defect. The three point bending test is used for brittle materials only for two reasons. The first is because brittle materials cannot be tested with conventional uniaxial tension tests because of early failure at the grips. The second reason is that LEFM can only be applied in bending tests to brittle materials because yielding occurs around the same point as failure, so failure stresses can be used to calculate normal tensions in the beam using σ = Mc/I. That is the stress versus strain diagrams remain nearly linear up until failure. Fracture toughness is the ability of a material to withstand brittle fracture in the presence of a crack or flaw. The equation as mentioned earlier is derived from linear elastic fracture mechanics and stipulates that crack length is inversely proportional to fracture stress. The

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