24 grade200steelcantolerate muchlargerflaws grade200 y

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Unformatted text preview: . 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 ~ ( ) 0.284 150 – 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 : – Charpy – 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 • Examples: – Aircrafts – 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.

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