LINEARLY INCREASING STRESS TEST (LIST) FOR SCC RESEARCH

This paper presents a new testing apparatus tor stress corrosion research. In this linearly increasing stress test (LIST) method, plain un-notched specimens are simultaneously exposed to an environment and subjected to an applied stress increasing linearly at a controlled rate. Experiments are repeated at various applied stress rates and the parameters measured include SCC initiation stress (the stress at which stress corrosion cracks start to propagate), fracture stress and average crack velocity. The LIST test is similar to the constant extension rate test (CERT) test, with the essential difference that the LIST test is load-controlled whereas the CERT test is displacement-controlled. The LIST test is particularly aimed at service conditions of loaded parts and structures; that is, service conditions described as load-controlled. This paper describes the design, construction and operation of the test apparatus and some typical results are given to illustrate its performance. Experiments have been conducted using as-quenched 3.5Ni-Cr-Mo-V turbine rotor steel specimens in aerated distilled water at 90-degrees-C at stress rates varying from 0.00034 MPa s-1 to 2.08 MPa s-1. The crack velocity increased with increasing applied stress rate. The fracture surfaces produced by scc were predominantly intergranular. The scc initiated at stresses significantly below yield stress. Up to initiation stress, the CERT and LIST test are essentially identical. Thereafter, once a stress corrosion crack has initiated, there is every advantage in quickly finishing the test. This is seen as an advantage of the LIST test. In the LIST test, once stress corrosion cracks have initiated, the time to failure is controlled only by the crack propagation rate and when the crack reaches a critical length, rapid failure occurs, whereas there can be quite a long period in a CERT test after crack initiation while the stress corrosion cracks yaw open. Thus the LIST method may be quicker than the CERT tests. An electronic servocontrol system was incorporated into the LIST apparatus to remove small load and strain transients.