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Stress corrosion cracking (SCC) is prevalent in many different industrial ap- plications, producing a significant loss of mechanical strength with only a small loss of material with defects forming complicated partially-closed branched defects, making it very difficult to detect on casual inspection[17]. The nucleation of a SCC defect can lead to fast fracture failure, with a very swift defect growth rate, or fa- tigue cracking which is a slower process in which the defect size increases at a slower

rate due to the action of cyclic stresses on a component and is therefore detectable prior to component failure[17]. Defects from either mechanism have devastating con- sequences if they occur in sensitive components, such as in pipelines[108]and nuclear reactors[109]_.

SCC requires three factors; a susceptible material, a corrosive agent appropri- ate for the specific material and sufficient tensile stress[17]. There are three different mechanisms by which SCC can occur; active path dissolution, hydrogen embrittle- ment and film-induced cleavage[17]. All of the SCC mechanisms involve the action of a corrosive agent on the material of the structure, producing a position of weakness within the structure, usually located at a grain boundary or a pre-existing surface imperfection[17].

Active path dissolution occurs along a path of higher corrosion susceptibility, such as a grain boundary, at which corrosive agents can gather, meaning it is easier for corrosion to occur at these positions when compared to the rest of the structure. The action of stress on this form of corrosion acts to open the defect, exposing the internal part of the defect to the corrosive agent, thereby accelerating the corrosion growth[17].

Hydrogen embrittlement is caused by the dissolution of hydrogen atoms into the metal, which is possible as they are far smaller than the atoms of the metal. The hydrogen atoms are drawn to regions of higher stress, such as crack nucleation sites, and the presence of hydrogen impurities at the crack tip acts to embrittle the metal, making cleavage easier and accelerating crack growth[17].

The final mechanism, film-induced cleavage, is common in structures in which a metal is coated with a brittle film, such as a painted pipeline, where a crack that originates in the film layer can continue to propagate into the metal underneath. If the original defect in the film was caused by corrosion then the corrosive process will continue within the metal[17]_.

The corrosion process can only take place if the correct corrosive agent for the chosen material is present. This can be as simple as the accidental transport of aerated water (introducing CO and CO2) in a carbon steel pipeline that is only

designed to transport pure water. The defect grows from the corrosion site when the material is placed under tensile stress, which can be caused by something as mundane as the vibrations caused by the passage of a fluid through a pipe. This leads to rapid crack growth and component failure, unless the defect can be located early and the part monitored or replaced[17], an example of this is the rapid growth of defects in steam generator tubes carry pressurised steam at high temperatures in pressurised water nuclear reactors[109].

500 µm

Figure 1.10: Micrograph of SCC defect showing complicated branching nature. Im- age courtesy of BP.

The methods commonly used to detect SCC defects include acoustic emission (in which a transient ultrasonic wave is locally generated on cracking and can be used to identify the cracking event)[110], eddy current testing (which is only sensi- tive to near surface defects in conductors)[76]_{, dye penetrant testing (with resulting}

plant shutdown)[111], radiographic testing (with its associated cost)[51] and ultra- sonic testing[112]. Currently the ultrasonic techniques are limited to recording the time of flight diffraction that occurs at the defect[112]_{, however, this is relatively}

insensitive to defects on the sample surface, driving the need to develop a more re- liable ultrasonic technique such as the near-field technique presented in this thesis. SCC defects form complicated defect structures with many branches of dif- ferent depths and orientations to the surface, as shown in figure 1.10, which makes the interaction of ultrasound with the defect difficult to interpret. SCC defects have a complicated reflection and transmission character due to the partially-closed na- ture of the defect and ultrasound will be transmitted through sections of the defect that are closed and reflected by those that are open, this makes it difficult to reliably determine the variation of either coefficient with defect depth, making it challenging to characterise the defect by conventional means[113,114].

With the aim of this thesis being to develop a defect characterisation method using near-field signal enhancements in materials, such as steel and aluminium, that are susceptable to SCC, for applications such as pipelines and storage contain- ers[15,16], it was decided that the interactions should first be studied in the simplest sample geometry, that of a thin plate. Open mouthed v-shaped defects of different depths were machined into thin sheets (thickness between 0.5 mm and 1.5 mm) so

as to simulate an open SCC defect in a storage container (see chapters 5 and 6). Artifical calibration defects have been sucessfully used in many applications as a means of understanding the physics of a simplified system prior to testing it on real defects[22,95,104,107,115,116]_{. The artificial defects used here enabled an understanding}

of the near-field interactions to be developed, which was then validated for a vari- ety of real defects (chapter 7), using the understanding obtained from the artificial defects.