Development of a Screening Frequency Scanning Test Method

Abstract

Fatigue crack growth rates (FCGR) in corrosive environment depends on loading frequency. Frequency scanning testing is often used to determine this effect. However, it is well known that the effect of loading frequency also depends on the magnitude of stress intensity factor range, ΔK. It is generally found that, with decreasing loading frequency, FCGR decreases in the low ΔK regime, increases and then decreases after reaching the saturating loading frequency in the intermediate ΔK regime, and keeps increasing in the high ΔK regime. To accurately characterise the effect of loading frequency on FCGR, several frequency scanning tests are required for a particular application (corrosive environment, material, welding procedure etc), each at a different ΔK level. These are time consuming and expensive tests. A novel screening frequency scanning test method has thus been developed. The method is similar to the step load fracture toughness test method often used to make a quicker estimate of fracture toughness of material in corrosive environment. In the screening frequency scanning test, both loading frequency and ΔK are changed in steps. At a relatively low and constant ΔK level, loading frequency is reduced in steps, after a certain amount of crack growth. Once the FCGR exhibits decreasing or has achieved a saturating loading frequency with decreasing loading frequency, ΔK is then increased to another higher level and the above process is repeated; the above procedures are repeated until the target maximum ΔK and the lowest loading frequency have been achieved. This method allows an estimate of the effect of loading frequency on FCGR in a large ΔK range using a single specimen. The results of the screening frequency scanning tests demonstrated that this method was feasible and provided a good and quick estimate of the effect of loading frequency on FCGR.