MSE BOOK NOTES-CHAP 6 Chap 6.2-Concepts of Stress and Strain 3 principle ways a load can be applied (1) tension; (2) compression; (3) shear TENSION TESTS one of most common stress strain tests output of tensile test recorded as load/force vs. elongation load deformations dependent on specimen size ex. requires twice the load to produce same elongation if cross sectional area of specimen doubled load & elongation normalized to parameters of engineering stress & engineering strain engineering stress () = F/A0 F ? instantaneous load applied perpendicular to specimen cross section; units = N or pounds force lbf A0 = original cross sectional area before any load applied (m2 or in2) Units are megapascals, MPa (SI) & pounds force per square ins (psi) Engineering strain () = (li - l0)/l0; l/l0 L0 ? original length before load is applied Li ? instantaneous length l ? deformation elongation or change in length at some instant unitless COMPRESSION TESTS can be conducted if in-service forces are of this type force is compressive & specimen contracts along direction of stress use same eqns as above but negative used when material?s behavior under large & permanent strains is desired or when brittle in tension SHEAR AND TORSIONAL TESTS = F/A0 F ? load or force imposed parallel to upper & lower faces A0 ? area - shear strain; defined as tangent of strain angle (theta) units same as tensile counterparts torsion is variation of pure shearstructural member twisted produce rotational motion about longitudinal axis of 1 end of member relative to other end GEOMETRIC CONSIDERATIONS OF THE STRESS STATE stresses computed from tensile, compressive, shear & torsional force states act parallel or perpendicular planar faces
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