MATERIAL PROPERTIES I. ELASTIC (OR HOOKIAN) BEHAVIOR: A. Material acts like a spring B. Any kind of strain is completely and instantaneously recoverable C. Hookian behavior: sd = Ee E = constant, Young’s modulus E = sd /e D. E describes the elastic properties of a material 1. E defines the slope on s verses e plot 2. E is a way to quantify how stiff a rock is in terms of its elastic behavior 3. Large E = stiff rocks, requires large amounts of stress to obtain a certain amount of strain 4. Small E = soft rocks, more strain for less stress 5. Elastic behavior has no dependence on time a. t0 - apply stress, material strains as long as stress is held b. t1 - remove stress, strain is instantaneously removed II. VISCOUS (NEWTONIAN) BEHAVIOR: FLUIDS A. Mechanical analog: dashpot shock absorber filled with oil B. Time dependent - whatever happens depends on how stress is applied C. Strain increases linearly when stress is held constant D. Viscous behavior: e = de/dt (strain rate) - rate of change of strain through time 1. Time dependent 2. Results in flow - after the material is stressed it retains some degree of strain sd = he (stress = constant x rate of strain) a. If h is constant = Newtonian behavior * Straight line on sd vs e plot b. h may be a function of (s, e, t, T,...), then non-Newtonian * Non-linear in sd vs e space * Viscosity does depend on t, T, etc... ex: honey, warm and cold * Most materials are non-Newtonian III. PLASTIC (St. VENANT) BEHAVIOR: A. Material has some inherent strength (yield strength) such that below a certain value of stress (yield stress) there is no permanent strain B. Above that certain value of stress (yield stress) there is permanent strain C. When a plastic substance is stressed above a certain critical (yield) stress it deforms continuously stress must overcome g (gravity) and m coefficientt of friction) 1. Mechanical analog: St. Venant Body (can’t stress a plastic material above this value, it flows plastically above) D. Elastic plastic: most materials that approach plastic behavior exhibit elastic characteristics below the yield point E. Strain beyond the yield point is permanent F. Once plastic flow begins, it continues as long as the stress is applied G. Nothing in the real world is perfectly plastic (elastic and viscous materials do exist) IV. WORK HARDENING: as strain increases, the amount of stress required to produce the next increment of strain is larger A. This means that the line of sd and e is not horizontal, i.e., not perfectly plastic behavior B. Materials usually work-harden at low temperatures and pressures 1. Buildings - as they are built, the lower parts of steel reinforcements increase strength by work hardening 2. Knife blades -are made harder-stronger by pounding because they are work hardened and will take more stress to produce the same strain V. STRESS AND STRAIN FOR ROCKS: assuming constant e A. sd vs e curves are highly dependent on the experimental conditions: 1. Pressure - Griggs et al. (1951, GSAB) studied the Yule Marble and found the Yule Marble became stronger with increasing Pcf(confining pressure) a. Ultimate strength (highest point on sd vs e plot) increases with increasing Pcf b. Ductility increases with increasing Pcf