Lezione 10: Fracture limited design

Question 1

Which are the 2 methods of fail-safe design?

Answer 1

• Yield before break: requires that the crack will not propagate even if the stress causes the part to yield
• Leak before break: required that a crack just large enough to penetrate both the inner and outer surface of the vessel is still stable

Question 2

What happens in rail cracking?

Answer 2

Crack in rails propagate even tough the wheels exert compressive forces on the surface - this is due to water entering the crack is and creating tensile stresses at the crack tip

Question 3

What is designing for fracture?

Answer 3

Product are often designed to fracture (perforated toilet paper, metal food can). Two common means of intentional fracture are tearing and peeling.

• Design for tearing is based on creating stress concentrations
• Design for peeling is based on balancing the pull force needed from the adhesive bond and the peel force required to break the bond