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6. (20vol%) F.D. Gace et al., Ceram. Eng.
Sci. Proc., Vol. 7 (1986) pp. 978-82. • This large NASA rocket casing…
– … failed during proof testing
– … failed at a weld between two plates
– … was made from grade 250 maraging steel
• σy 1650 MPa, KIc 80 MPa√m 23 7 m diameter rocket casing • Grade 250: σy 1650 MPa
KIc 80 MPa√m
weld metal KIc 60 MPa√m • Compare the critical flaw size:
ac ~ ( K Ic design 2
) 2 ~ K Ic 80 2
ac1 / ac 2 ~ (
150 – The critical flaw size is about 4x larger for grade 200 steel.
– It’s also better because it yields more before failure. •
24 Grade 200 steel can tolerate much larger flaws. • Grade 200: σy 1500 MPa
KIc 150 MPa√m • 2 standardized tests used to measure impact energy notch toughness :
– Izod Charpy • The difference between the tests lies in the way the specimen is supported
Izod http://met‐tech.com/mechanical_testing.html •
• A283 Steel Photograph of fracture surfaces of A36 steel Charpy V‐notch specimens tested at the indicated temperatures • Many materials become brittle at low temperatures.
– BCC materials like steel fail by cleavage along the 100 plane • Material exhibiting a ductile to brittle transition should only be used above the transition temperature so as to avoid brittle failure Impact energy vs. Temperature Plot 3 general behaviours Influence of carbon content on the Charpy energy‐temperature behaviour for steel • A form of failure at relatively low stress levels
• Structures that are subjected to fluctuating and cyclic stresses or loading
– Machine components • Can occur even if stress level is considerably lower than the tensile or yield strength max y Fatigue failure of a crank shaft after one year use – 20 ksi •
– • •
– Reversed Stress Cycle: stress alternates from a maximum tensile stress to a maximum compressive stress of equal magnitude Repeated stress cycle: maximum and minimum stresses are asymmetrical relative to the zero‐stress level Random stress cycle: random in amplitude and frequency Mean stress: Stress ratio: 1 m max min 2 min
R max Range of stress: r max min
Stress amplitude: r 1 a ( max min )
22 You can test fatigue str...
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This note was uploaded on 10/14/2011 for the course ENGINEER CHEM ENG 3 taught by Professor Ghosh during the Spring '11 term at McMaster University.
- Spring '11