1. 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.

2. 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.

3. 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

4. 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.

5. 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.

6. 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

7. HW
P , 55, 62