Metal structures and components can exhibit surface defects while in service [4] or during manufacturing [6]. The exposure to harsh conditions during service, such as cyclic loading of stress and strain in a corrosive environment can initiate the growth of defects near to the surface. Additional factors, such as corrosive chemicals or even water can contribute to the defect growth into the sample [3]. In manufacturing, surface defects can occur as a result of chemical, processs, and engineering parameters [5]. Figure 1.3 shows examples of surface defects in metal slab and bloom/billet, defined and categorised by the International Iron and Steel Institute (IISI) [6]. Transverse and longitudinal cracks can be present on the top surface and at the corner (marked as 1, 2, 3, and 4). Star cracks, deep oscillation marks, pinholes and macro inclusions can be found on the top surface. The causes of these defects include non-uniform heat transfer due to chemical composition, faults in the mould, uneven cooling, and also the presence of impurities. For some metals that are put under processes such as annealing or cold work, the material properties can change [45]. The metal can become more brittle and easier to break, if a crack is initiated on the surface. A test for this sortof cracking at high temperature can be used to feedback information to the operator to improve the casting process during operation.
Generally, surface defects may not be harmful if they are relatively small in size [3]. However, if left undetected they can continue to grow into the material, which has various consequences; for example bridges can collapse, underground or overground pipelines carrying chemicals or oils can leak, causing damage and pollution to surround-
(a) (b)
Figure 1.4: A section of rail containing a type of rolling contact fatigue crack, known as head checks (after Railtrack PLC [3]).)
ings, on top of the financial loss which occrus. The final product when manufacturing e.g billets may not meet market specifications and will need to be reprocessed, with ad- ditional cost added to the manufacturing process. Two specific types of surface defects, rolling contact ratigue (RCF) on rail tracks and stress corrosion cracking (SCC), which are of most interest for this thesis, are described in Sections 1.5.1 and 1.5.2.
1.5.1 Rail defects
A real life example of the severe impact which can be caused by surface defects in rail is given by the train derailment near Hatfield in the UK, in October 2000, where four people died and over seventy people suffered injuries [46]. The investigation into the incident revealed that the derailment was caused by surface defects in the rails known as rolling contact fatigue (RCF) cracks [3, 4, 46, 47]. An example of this type of crack is shown in the photgraph in Figure 1.4(a) (taken from [3]). The photograph shows a form of RCF crack, known as “head check”, with the section of rail cut to reveal the geometry of head checks below the surface. The cracks initiate on the surface, and continue growing at an angle of about 20° to the surface, in the direction of the train traffic [47]. As the cracks propagate deeper than about 5 mm vertically into the rail, they can grow branches, as depicted in the simplified diagram in Figure 1.4(b). At this point, the branch can either can grow back to the surface, causing a spallation of the surface, or continue growing into the rail, causing a breakage [3].
tion [3,47]. The former relates the crack depth to the crack length on the surface based on an establised relationship, while the later utilises reflections of ultrasound waves from the crack. The visual inspection method is considerably slow and time consuming, especially if hundreds of miles of rail need to be inspected. Despite being adapted for standard practice, the accuracy of this method is also questionable as the crack length- depth relationship is not fully understood, as it is based on common observation. It is also not convenient to shut down train services for a long period of time in order to conduct the inspection. The ultrasonic inspection can give a more accurate estimation of the crack depth compared to the visual inspection in a shorter time.
Figure 1.5 illustrates how an ultrasonic probe (contact piezoelectric transducer) is used in pulse-echo mode to detect and characterise RCF cracks. The probe generates a specific mode in a particular incident angle as indicated in the figure, and detects any reflections coming from the cracks. The incident angle is decided based on the knowledge of the expected crack angles, and maximum reflection can be achieved if the ultrasonic beam is perpendicular to the crack [20]. One of the drawbacks of this method is that a deeper crack present behind the crack (shown to Figure 1.5) can be masked. The part of the crack on the right hand side that lies within the shadowed region behind the crack cannot be detected, and this can cause underestimation of the depth of the crack on the right hand side, and missing serious defects.
The remedial action following inspection depends on the estimated crack depth. A remedial grinding is carried out in a region where RCF is expected or known to occur, as a preventive measure [3]. The top layer of the rail containing the crack is ground off, in order to remove the part where the crack initiated. If the crack is measured to be deeper than this, normally the crack is classified as severe and would require the section of rail to be replaced. Each of these actions is associated with a different cost and time required. Rerailing is more expensive and therefore is not cost efficient. The accuracy, consistency and speed of the inspection is very important in making the right decision at the right time. Ideally, an inspection of the rail must be able to identify a defect as an RCF crack from any other type of crack such as weld joint failure. This can be done by determining if the crack is inclined to the surface at certain angle, and finding its orientation, i.e. if it is either following the direction of traffic or not. The remedial action required would be different for non-RCF type cracks. Another important feature for the rail inspection is that it must give an accurate measure of the crack depth. With this, the decision for action to be taken can be made with confidence.
Figure 1.5: Traditional ultrasonic inspection of rolling contact fatigue cracks. The narrow crack on the left-hand side shields the ultrasonic beam from reaching the deeper crack on the right-hand side.
1.5.2 Stress corrosion cracking
Another type of surface crack that is of similar interest to the research reported in this thesis is stress corrosion cracking (SCC) [48, 49]. This is normally found in boilers, pipelines and weld joints, where it is known to cause breakage, putting the intregrity of these structures at risk. SCC is particularly dangerous because fast mechanical fracture can be triggered, which leads to catastrophic failure of the structures. Typically, SCC initiates on the surface before starting to branch into the bulk of the material, as depicted in Figure 1.6. The exact mechanism of SCC is still an active area of research [45, 50], but generally it is known to be due to the combination of stress and corrosive environment. The need to detect and characterise SCC is clear, based on the structural risk it could impose [51]. Therefore, a fast, efficient and reliable technique is required to achieve this.
The two main examples given here, RCF and SCC, are the motivation for this study, in seeking the most suitable method for detecting and characterising surface cracks, compared to the available methods reviewed in Section 1.1. The ultrasonic methods, particularly the non-contact approach have been chosen, mainly due to their high accuracy and potential for fast inspection, in addition to other advantages. This will be discussed further in Section 1.6.
Figure 1.6: Stress corrosion cracking, reproduced from [49].