Principal Stresses - Plane Stress

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PRINCIPAL STRESSES - PLANE STRESS The analysis of plane stresses is not done upon the isolation of an element only but it extends upon the analysis of all stresses existent in an isolated plane. A further analysis of the element is done to see all existent critical state and all probable stress combination. Consider the element as shown, [pic] but [pic] [pic] from the double angle formula [pic] then [pic] rearranging terms [pic] eqtn 1 [pic] using the double angle formula and rearranging [pic] eqtn 2 NOTE: (x, (y, (xy and ( are positive in the directions shown in the Plane Stress figure. To determine where the maximum value of (n occurs: [pic] [pic] [pic] solving for the angle (, [pic] eqtn 3 where (p locates the plane where (n is extremum. Equating eqtn 2 to zero, [pic] and solving for the angle (, [pic] eqtn 4 NOTE: 1. On planes of maximum or minimum normal stresses, shear stresses are zero. 2. Planes having zero shear stress are principal planes. 3. Normal stresses on principal planes are principal stresses. 4. Two values of principal stresses are 90( apart. From trigonometry, tan ( = tan (( + 180) tan 2(p1 = tan (2(p1 + 180) = tan (2(p2) Consider the triangle: [pic] [pic] substituting equations 5 & 6 to equation 1 [pic] [pic] where (p1 and (p2 are called principal stresses. Actual analysis would involve 3 principal stresses. A simplification reduces the third plane stress to zero, ((p3 = (z = 0). To determine where the maximum in-plane shearing stress occurs: [pic] [pic] [pic] where (( locates the

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