Jan 21 class notes

Jan 21 class notes - Yield stress and ultimate stress of a...

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Not to scale, because 1. strain from B to C maybe more than 10 times the strain occurring from O to A. 2. strain from C to E are many times greater than those from B to C. Yield stress and ultimate stress of a material are also called the yield strength and ultimate strength. Strength is a general term refers to the capacity of a structure resist loads.
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FIG. 1-12 Stress-strain diagram for a typical structural steel in tension (drawn to scale). The strains from zero to point A are so small as compared to the strains from point A to E and can not be seen (OA is a vertical line…) • Metals, such as structural steel, that undergo permanent large strains before failure are ductile A desirable failure of ductile material: visible distortion occur if the loads become too large. • Ductile materials: aluminium, copper, magnesium, lead, molybdenum, nickel, brass, nylon, teflon Stress ( σ ) – strain ( ε ) diagrams
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FIG. 1-13 Typical stress-strain diagram for an aluminum alloy. Aluminium alloys • Although ductile… aluminium alloys typically do not have a clearly definable yield point… • However, they have an initial linear region with a recognizable proportional limit Structural alloys have proportional limits in the range of 70-410 MPa and ultimate stresses in the range of 140- 550 MPa
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Copyright 2005 by Nelson, a division of Thomson Canada Limited FIG 1-14 Arbitrary yield stress determined by the offset method Offset method • When the yield point is not obvious, like in the previous case, and undergoes large strains, an arbitrary yield stress can be determined by the offset method. A straight line is drawn on the stress-strain diagram parallel to the initial part of the curve but offset by the standard strain such as 0.002. The intersection of the offset line and the stress-strain curve (point A) defines the yield stress.
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FIG. 1-16 Typical stress-strain diagram for a brittle material showing the proportional limit (point A ) and fracture stress (point B ) Brittle materials • Brittle materials fail at relatively low strains and little elongation after the proportional limit. • Brittle materials: concrete, marble, glass (ideal brittle material because it exhibits almost no ductility), ceramics and variety of metallic alloys. • The reduction in the cross-sectional area until fracture (point B) is insignificant and the fracture stress (point B) is the same as the ultimate stress. • Metals, such as structural steel, that undergo permanent large strains before failure are ductile. • Materials that fail in tension at relatively low values of strain are classified as Brittle.
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1.4: Elasticity • What happens when the load is removed (i.e. the material is unloaded)? • Tensile load is applied from O to A,
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This note was uploaded on 04/03/2009 for the course MAE 243 taught by Professor Liu during the Spring '08 term at WVU.

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Jan 21 class notes - Yield stress and ultimate stress of a...

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