Abstract:
Eddy current testing has become a widely used nondestructive technique for
material testing and evaluation. This technique depends on the interaction between the
impedance of the test coil and the material as the probe scans the surface of a material
under investigation. By measuring the change in impedance of the test coil, the size of an
imperfection in the material can be determined. In this project, the first aim of the
research was to investigate and develop the use of the eddy current technique to detect
stress corrosion cracks (SCC) in the bore of fastener holes. Specimens were scanned with
the probe moving in and out the hole to produce a signal of the eddy current response. In
addition, due to the skin depth effect, the operating frequency was also taken into
consideration as an important parameter. A calibration standard test piece was prepared
to represent SCC around a fastener hole, with the cracks lying in a plane parallel to the
surface. Data from eddy current tests on real stress corrosion cracks has been used to
develop an eddy current calibration curve for predicting stress corrosion crack lengths in
larger components.
The second aim of the project was to use the eddy current technique to study SCC
crack growth in high strength aluminium alloy 7075-W under both tensile and
compressive loading conditions. The importance of parameters such as heat treatment,
grain shape, grain aspect ratio, component of stress, threshold stress intensity and
microstructure were all taken into consideration. The SCC development was found to
follow an intergranular path, which strongly depended on the microstructure of the
material. Tests were carried out using double cantilever beam specimens to measure the
threshold stress intensity, Kiscc, below which SCC would not occur. The specimens
showed evidence of exfoliation corrosion at the surface and corrosion product wedging
within the stress corrosion cracks, which caused further crack growth at the low applied
stresses.
Stress corrosion cracks were found to grow in 7075-W under high compressive
loading, whereas control tests without compressive test stress produced only exfoliation
corrosion and no crack growth. The mechanism of SCC in both tensile and compressive
loading was thought to be anodic dissolution of metal at the crack tip, with protective
films being disrupted by dislocation movement with the formation of active slip-steps.