Experimental Estimation of ΔKth by K-Decreasing Method for 4340 Steel

Design criterion of some parts in aerospace structures is based on no crack growth philosophy. The threshold value of cyclic stress intensity factor (ΔKth) is an important parameter in this criterion. On the other hand, when the value of ΔK is smaller than ΔKth the crack will not grow. The methods of estimation of ΔKth in laboratory are described in the ASTME647 standard. Advanced equipments and very precise techniques are required in order to conduct this test These requirements make this test to be known as one of the most difficult specific fracture mechanics tests. In this paper, ΔKth is estimated by conducting tests on C(T) specimens made of steel 4340 with the help of advanced test equipments available in the fracture mechanics research laboratory of K.N. Toosi University of Technology. Decreasing cyclic stress intensity factor method has been used in these tests. In order to overcome the crack tip blunting in the pre-cracking stage, the test is started with a specific ΔK and followed with smaller values of ΔK. For this purpose the load will be decreased for obtaining at least five data points of fatigue crack growth rate in region 10^-9