9-6 Elasticity; Stress and Strain Most objects change shape under force or stress. If objects are stressed enough, they will fracture. Hooke’s Law F=kDL …where DL = change in length. See page 253 chart of F –v- DL . Notice the terms Elastic region, proportional limit , Plastic limit, Elastic limit and Breaking point.

9-6 Continued The maximum force that can be applied without breaking, is called the ultimate strength. DL =(1/E)(F/A) L0 Where E is known as the elastic modulus or Young’s modulus. (p254) See Example 9-12 p255.

9-6 continued When a rod or material is being stretched by a force on both ends, it is under tension or tensile stress. When a material or rod has compressive force on both ends, we say it is under compressive stress. When a lateral force acts on a rod, we say it is under shear stress. For shear strain, use… DL =(1/G)(F/A) L0 Where G is the shear modulus

9-6 Continued If an object is subjected to pressure or force from all directions, DV/V0 =-(1/B)(DP) Where B is the bulk modulus.

9-7 Fracture If forces are too much, an object will break, or fracture. See chart on p 258. See Example 9-13. Pre stressed concrete has stress placed on the reinforcement during curing of the concrete.

Spanning Spaces: Arches and Domes The post and beam was the original form of doorway, the series of arches followed. See p 261 3-dimensionally, the arch gave rise to the dome. See Example 9-15

HW P270 44-47, 55, 62