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Unformatted text preview: Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi CH 5 Page 1 of 10 CH 5: Failures Resulting from Static Loading A static load is a stationary ( constant magnitude , location and direction ) force or couple (moment or torque) applied to a member. Failure in machine design means that a part become permanently distorted ( i.e. yielded ) thus its function is compromised. Therefore, the failure strength we refer to is the yield strength (or if the material does not yield such as some brittle materials failure will mean fracture “ultimate strength”). • When a material is subjected to uniaxial stress , it will fail when the stress reaches the failure strength. - What about if it is subjected to biaxial or triaxial stress , when would it fail? - Is there a difference in the failure mechanisms of ductile and brittle materials? - Should stress concentrations be considered? Stress concentration As seen previously in chapter 3, stress concentration is a highly localized effect. Would stress concentrations cause failure of a part? • If the material is ductile ( and the load is static ),the design load might cause local yielding in locations having high stress concentrations and the stress is redistributed in the surrounding area thus the load can be carried without causing failure of the part. Thus, for ductile materials , stress concentrations are not considered ( ܭ ௧ ൌ ͳ ). • Brittle materials, in general, do not yield before fracture thus with the presence of stress concentration, the stress will continue to increase until it cause failure of the part. Therefore, for brittle materials stress concentrations must be considered . s An exception to this rule for brittle materials is for materials containing micro discontinuities (bubbles, graphite, flakes, etc.) such as cast iron where those Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi CH 5 Page 2 of 10 act as cracks thus causing higher stress concentrations than those caused by holes or filets. Therefore, stress concentrations should not be considered for materials such as cast iron. Static Strength When designing a machine element, the best way to determine its strength is to test the part under exactly the same loading conditions the part will experience in service. ( of course many tests are needed to account for variability ). However, performing such testing is very costly and can not be justified unless the part is used for a critical application or if the part is produced in large quantities. Thus, in most cases it is necessary to design using the published material properties ( yield strength, ultimate strength, percentage elongation, etc. ) which is obtained from testing under uniaxial stress ....
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This note was uploaded on 10/25/2010 for the course MECHINCAL 2010 taught by Professor علاءحجازي during the Spring '10 term at Hashemite University.
- Spring '10