Constitutive Relationship between Crack Propagation and Specific Damping Capacity in Steel.

摘要

An expression stating crack propagation as a function of specific damping capacity was developed. Development was based on the premise that fatigue damage is due to plastic work. Plastic work is calculated from changes in damping between fatigue cycles where the damping changes are assumed to result from an increase in size of the plastic zone at the tip of the crack. A series of fatigue tests, using 4340 and 1018 steel specimens, were monitored for damping changes during fatigue crack growth. Each notched and uncracked beam specimen was mounted in a knife-edge fixture and fatigued by a four-point load. The fatigue had a stress ratio of zero. Between fatigue load cycles, a vibration generator would resonate the specimen mounted in the knife-edge fixture. Strain measurements from the specimen, and force measurements from the vibration generator, were used to calculate internal damping. Test results were compared to a dynamic mathematical model of the cracked specimen to verify the crack growth-specific damping relationship. Two types of damping were noted in the region of the fatigue crack. First, dislocation damping at the crack tip was observed in 1018 steel for large plastic zones. Second, damping proportional to the beam slope at the crack was observed in 4340 steel for small plastic zones and attributed to rubbing of the crack faces. Experimental results of both types of damping are duplicated by a computer program of the mathematical model.