the point, at which this occurs is known as the **proportional limit**. The corresponding stress in the extreme fibers of the beam is known as "**fiber stress at proportional limit**." Fiber stress at proportional limit, is computed by
(insert equation)
2.1202. **Modulus of rupture (fbu)**. Modulus of rupture is computed by the same formula as was used in computing, fiber stress at proportional limit except, that maximum load ls used in place of load at proportional limit. Since the formula used is based upon an assumption of linear variation of stress across th cross section of the beam, modulus of rupture is not truly a stress existing at time of rupture, but is useful in finding the load-carrying capacity of a beam.
2.1203. **Work to maximum load**. The energy absorbed by the specimen up to the maximum load is represented by the area under the. load-deflection curve from the origin to a vertical line through the abscissa representing the maximum deflection at which the maximum load is sustained. It is expressed, in tables 2-6 and 2-7, in inch-pounds per cubic inch of specimen. Work to maximum load is computed by
Work to Pmax = area under curve to Pmax/bxdxL (2:7)
2.121. **Compression parallel to grain**. In the compression-parallel-to-grain test, a 2- by 2- by 5-inch block is compressed in the of its length at a constant rate (0.024 inch per minute). The load is applied through a spherical bearing block, preferably of the suspended self-aligning type, to insure uniform distribution stress. On some of the specimens, the load and the deformation in a 6-inch central gage length are read simultaneously until the proportional limit is passed. The test, is discontinued when the maximum load is passed and the failure appears.
Figure, 2-4 (b) shows it test set-up, and typical load deflection curves for Sitka spruce and yellow birch. Data on a number of properties are obtained from this test. These are discussed as follows:
2.1210. **Modulus of elasticity**. The modulus of elasticity is determined from the slope of the straight-line portion of the graph, the steeper the line the higher the resolution. The modulus of elasticity is computed by
(insert equation)