4) In a mould a wide and 3 mm thick plastic plate is cast. In his plate a centra

Question

4) In a mould a wide and 3 mm thick plastic plate is cast. In his plate a central crack is created with a length 2a = 50 mm. The plate is the mechanically tested in the length direction normal to the crack.
a) If the plate fails at a stress of 5 N/mm2, what then is the KC value of the material? Corrections for finite specimen size need no to be taken into account.
b) In a similar plate copper wires with a cross section of 2 mm2 are introduced in the length direction to act as reinforcements. These wires have a relative distance of 20 mm, while one of the wires crosses the central crack exactly through the middle (see figure). By testing the plate, tensile stresses develop in the parts of the 3 wires located between the crack flanks. These stresses are found to be equal to 24, 36 and 24 N/mm2 respectively at a test stress s = 5 N/mm2. How high is K1 at this moment if the stresses in the wires are assumed to load the crack flanks over the whole plate thickness?
c) To what level can the test stress ? be increased before failure occurs? Assume that the stresses in the wires are proportional to ??

Explanation / Answer

a) 44.3 N/mm ; b) 35.6 N/mm ; c) 6.2 N/mm

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