Modulus of Rupture and Elasticity

MOR is the clear-wood failure stress from standard small specimens tested at 12% moisture. Lumber strength ratings (like 2x4 grade designation) are derived from MOR but include a safety factor and apply to full-sized lumber with defects. Actual structural capacity of a beam is much lower than MOR because real wood has knots and checking.

12% is standard equilibrium moisture representing moderate interior conditions. Wood is much stronger when dry (low moisture) and weaker when wet (high moisture). Testing at a standard moisture enables consistent comparison across samples and species. If you test at different moistures, apply moisture adjustment factors.

MOR is stress (load per unit area). Maximum load = MOR × (b × h / (3L/2)) = 2MOR × b × h / (3L), where b and h are width and height, L is span. For a 2 cm × 2 cm × 30 cm specimen, a typical MOR of 50 MPa corresponds to a failure load of about 400 N.

MOE directly predicts elastic deflection: deflection = (5 × load × span³) / (384 × MOE × I), where I is the moment of inertia. However, this formula assumes purely elastic behavior; wood creeps under sustained load, so actual long-term deflection is higher. Use a deflection multiplier (typically 1.5–2) to account for creep.