Transcript Document
MAE 343 - Intermediate Mechanics of Materials Tuesday, Sep. 28, 2004 Textbook Sections 2.1-2.4 Modes of Mechanical Failure Elastic Deformations Yielding and Ductile Fracture Failure Prevention Analysis • ROLE: basis for successful mechanical design • FAILURE: defined as IMPROPER FUNCTIONING of a MACHINE or a PART • MODES of FAILURE: defined as physical processes that take place or combine their effects to produce failure of a machine or machine part • READING ASSIGNMENT – Section 2.3 – Be able to define 23 different modes of failure Elastic Deformation for Different Types of Loadings (Stress Patterns) • Straight uniform elastic bar loaded by centered axial force (Figs. 2.1, 2.2, 2.3) – Similar to linear spring in elastic range F ky, y f l f l0 – “Force-induced elastic deformation”, f must not exceed “design allowable”: failure is predicted to occur if (FIPTOI): f-max>f-allow – “Spring Constant (Rate)” for elastic bar k (lb / in) F / y F / f – Normalize “Force-deflection” curve to obtain “Engineering Stress-Strain Diagram” f F , f , so that : A0 l0 k ax F f A0 A0 E , where : E f ( Hooke' s Law) l0 f l0 Temperature-Induced Elastic Deformations • Temperature fluctuations lead to stress-free elastic deformations if: – Slow temperature changes and small gradients – No external constraints imposed on the part – Thermal Stress arises when displacement is constrained fi ti 0 E fi Ei • Temperature-Induced Elastic Strains l0 t l0 l0 – Directional dependent in anisotropic materials – Normal, recoverable strain t ti i YIELDING and DUCTILE FRACTURE • YIELDING – occurs when applied uniaxial stress exceeds YIELD STRENGTH of the material, and PLASTIC STRAINS arise, i.e. Syp • DUCTILE RUPTURE – when plastic deformation proceeds to separate a part into different pieces, i.e. – FIPTOI: Su • MULTIAXIAL Stress States require a Combined Stress Theory of Failure – Distortion Energy Theory (Octahedral Shear Stress Theory) – Maximum Shearing Stress Theory