Chapter 1: Specimen and defects The specimen used is the titanium grade 5 (Ti-5, also known as Ti-6Al-4V) block that’s cut into a custom shape. The dimension and the defects are indicated as below on Figure 1 & 2. Defect A Defect B 60mm 30mm 60mm 100mm 120mm Z 120mm X Y Isometric view of specimen with defects Figure 1: Dimensions of Titanium Grade 5 (Ti-5) specimen with Defects 12mm Transverse corner crack 10mm 12mm 8mm Jagged Transverse Subsurface Crack Jagged Transverse Surface Crack 12mm 14mm 8mm Jagged Transverse Subsurface Crack Side view of specimen with dimensions Jagged Longitudinal Subsurface Crack 3mm Dia. Spherical void Side view of specimen with defects Figure 2: Dimensions & location of defects There are two types of defects in the specimen, namely Defect A and Defect B. Defect A is a corner surface cracks and subsurface defects that appears on the XY and XZ plane, while Defect B is a surface and subsurface defect on the XY and XZ plane with a 3mm diameter of spherical void located inside the specimen. These defects consist of cracks with a width of 1mm. The suitability of different NonDestructive Testing (NDT) methods for detecting these defects will be discussed. The NDT methods under consideration are: 1. 2. 3. 4. 5. Ultrasonic Testing (UT) for sub surface defects Radiographic Testing (RT) for sub surface defects Liquid Penetrant Testing (LP) for surface defects Magnetic Particle Testing (MPT) for surface defects Eddy Current Testing (ECT) for surface defects Chapter 2: Defect A Defect A is a surface corner defects characterized by two jagged transverse cracks on the XY and XZ plane respectively. Two jagged subsurface cracks penetrate into the specimen to a maximum depth of 10mm in the form of a transverse crack, as illustrated in Figure 2. The lengths of the surface transverse cracks are measured at 12mm and 10mm, respectively. The first subsurface transverse crack extends downwards to a depth of 2mm. As the crack progresses in the Z direction, the depth increases to 6mm, and then reduces to 4mm further along. This crack with the depth of 10mm is now connected to the second subsurface jagged transverse crack that grow in the Y direction. The surface corner cracks in Defect A have a width of 1mm and a length of 10mm in Z-axis and 12mm in Y-axis, which makes them not visible to the naked eye due to their small size. Furthermore, the extent of penetration of these cracks cannot be observed visually. To detect Defect A, Liquid Penetrant (LP) testing can be performed. The titanium grade 5 (Ti-5) which is grey in colour would benefit from the use of a red penetrant to achieve good contrast. Prior to performing Fluorescent Penetrant (FP) testing, surface preparation is necessary to clean the surface. LP can be first applied on the top planes of the specimen. Once capillary action has occurred, a developer can be added. Excess developer can then be wiped away to reveal the longitudinal cracks. Deeper cracks would exhibit a higher volume of penetrant, allowing for comparison of crack depth. However, the exact depth of the crack may not be determined using this method. Another NDT to be considered is Magnetic Particle Testing (MPT). The defect surface should be thoroughly cleaned to remove any dirt, grease, oil, paint, or other contaminants that could interfere with the inspection process. The surface should be cleaned using suitable methods such as degreasing, wiping, sandblasting, or chemical cleaning, depending on the specific requirements and condition of the material. After cleaning, the surface should be prepared to ensure good contact between the magnetic particles and the surface. This typically involves roughening the surface to create a suitable profile for the magnetic particles to adhere to. Methods such as grinding, sanding, or shot blasting can be used to roughen the surface, taking care not to damage the material. Magnetic particles, which are usually a fine powder consisting of ferromagnetic materials, are applied to the prepared surface of specimen. This can be done using a dry method where the particles are dusted or sprayed onto the surface, or a wet method where the particles are suspended in a liquid carrier and applied to the surface by spraying or immersion. After the magnetic particles are applied, a magnetic field is applied to the component. This can be done using a permanent magnet, an electromagnet, or a magnetic yoke. The magnetic field induces magnetic flux into the material, and any magnetic particles that are attracted to areas of flux leakage caused by defects, such as cracks or discontinuities, will accumulate and form a visible indication on the surface. Any indications of magnetic particles that have accumulated at areas of flux leakage will be observed. The indications can be interpreted to identify the presence, location, size, and shape of defects. However, MPI could not detect the specific depth of the defects. On the other hand, Eddy Current Testing (ECT) can be performed to identify the defects. To perform ECT, it is preferable to have already located the defects, ideally using LP testing. EC testing should be conducted after LP testing. To calibrate the EC machine, a reference specimen with similar defects should be produced, allowing for calibration based on the skin depth formula to determine the required frequency for defect detection. The frequency selected should match the largest depth of the defects, which is 12mm, in order to detect all the cracks. ECT probes should be applied along the located surface cracks to obtain a response. Since Defect A is primarily a surface defect, the phase angle of the response is expected to be similar. However, the response curve of the ECT probe would vary in size due to the differences in total depth and size of each flaw being measured. An issue with using ECT for Defect A is that it originates from three planes and runs through an edge, which may affect the accuracy of ECT measurements due to the presence of open spaces and multiple planes that can impact the formation of eddy currents and, consequently, the results. Additional settings may be needed for the ECT machine to account for measuring edges and differences in specimen height. Another limitation of EC is that it can only accurately measure straight vertical cracks and may not accurately measure the total length of a jagged crack, potentially only capturing the vertical length, leading to inaccurate measurements. Among the three NDT methods mentioned above, namely LP, MPT, and ECT, LP and ECT are more suitable for detecting defect A. Both methods require preparation and time, as well as post-processes after conducting the inspection. For instance, ECT necessitates demagnetization of the specimen, while LP requires surface cleaning to remove the dyes. Furthermore, only ECT has the capability to measure and obtain the depth and size of Defect A, whereas LP can only detect the surface portion of Defect A. In fact, a combination of both LP and ECT would be favourable, with LP being used initially for crack detection, which would expedite the process, followed by ECT for relative measurements of crack depth and size. Chapter 3: Defect B Defect B has the surface and subsurface defect on the XY and XZ planes with a 3mm diameter of spherical void located inside the specimen. It is located 60mm from the YZ plane in X direction and 60mm from XZ plane in Y direction. The cracks with 1mm width and propagate in the X, Y and Z direction. To identify and quantify the defect, both Ultrasonic Testing (UT) and Radiographic Testing (RT) were taken into consideration, while thermography was ruled out due to the material being a titanium, which would result in rapid heat dissipation to the surroundings. There is no specific constraint on the direction in which UT and radiography can be applied. In this section, we will explore the application of both UT and radiography from the top XY plane and on the YZ plane from the size. UT involves utilizing soundwaves generated by piezoelectric transducers to identify flaws. UT equipment typically consists of a pulser, receiver, signal capture, and waveform display. It is essential to verify if the UT equipment is capable of measuring or detecting the specific defect. Firstly, the near field length is determined using the formula 𝑧𝑔= 𝐷2/4𝛌. The measured 𝑧𝑔 must be at least 20mm, as that is the depth of the nearest flaw. Additionally, the beam divergence must be wide enough to capture the entire crack and the back surface. UT can be applied from either the top or the side plane, as it only requires access from one side. However, it is important that the attenuation caused by the specimen is low enough to ensure accurate results. A couplant, which is a substance that improves the transmission of ultrasound, should be applied to the surface where the transducer is placed, as there is a significant impedance mismatch between air, specimen, and transducer. A specific gate can be used to collect the crack displayed at the required surface. For instance, either a singular transducer or an ultrasonic array can be applied for the measurement. When applying UT from the top surface of the specimen, an angular beam inspection would be preferable. Depending on the specific measurement requirements, we could use a single transducer in either pulse mode or a pitch catch mode. Pulse mode would allow UT to detect cracks based on the reflection of soundwaves from the defects which provide a result of variations in amplitude. However, for this method to be effective, the cracks would need to be perpendicular to the UT wave produced. In the case of Defect B, which has cracks in multiple orientations, the angles of these flaws would need to be determined beforehand in order to detect them accurately. By adjusting the angle of Perspex or the material to match the angle of the individual cracks, it is possible to generate a UT wave that is perpendicular to the flaws. This can be achieved using Snell's law, 𝑠𝑖𝑛𝜃1/𝑐1= 𝑠𝑖𝑛𝜃2/𝑐2 which states that the sine of the angle of incidence in one medium is equal to the sine of the angle of incidence in another medium, multiplied by the respective velocities of the two media. By considering the material's velocity (c) and the angle of the Perspex (𝜃1), it is possible to calculate 𝜃2, which would result in a UT wave that is perpendicular to the angled cracks. To ensure detection of all the cracks, soundwaves would need to be applied in multiple directions. The cracks could be visualized in different settings, such as A-scan, B-scan, or C-scan, which may show the raw data or converted images. However, these methods may not fully capture the defects as they are composed of multiple cracks with varying orientations. Another UT technique that can be used is Time-of-Flight Diffraction (TOFD), which operates in pitch catch mode and is independent of flaw orientation. This method produces a D-scan, which is an image captured from a top view, obtained from an A-scan that indicates the upper and lower tips of reflections. However, UT may not be able to detect defects that are aligned with each other. For instance, the longitudinal crack in Defect B would likely be detected as it is located below multiple cracks coming from different directions. UT would only detect cracks that are located above the longitudinal crack. Furthermore, jagged cracks may not be accurately represented in the UT images. While soundwaves can still reflect from these jagged cracks, the reflected soundwave may not directly reach the receiver, resulting in inaccurate readings. These voids can cause the directed soundwave to reflect in multiple directions, which may prevent the soundwave from being reflected back to the receiver and thus not displaying a reading. We can apply UT from a different direction which is from the right side of the YZ plane, producing waves in the X direction. As 2 of the defects do not propagate in X direction, those defects would be oriented perpendicular to the waves propagating in the X direction. In order to ensure proper transmission of ultrasound, a coupler would still need to be applied to the side plane. Alternatively, a contact transducer that produces waves in the X direction could be used. To account for the differing thickness of the specimen in X direction, it is preferable to set a gate width of 60mm. This gate would encompass the location of Defect B, while also mitigating potential confusion caused by varying specimen thickness. A C-scan which is displaying the length in both Y and Z directions would produce the entire flaw of defect B. Another approach Radiographic Testing (RT) can be applied to examine Defect B. Access is required from both sides of the material for Radiography, as X-rays need to be applied from one side and the film needs to be applied on the other side. If the X-rays are applied from the top plane, a film must be put on the bottom plane. X-rays are directed towards the block along the line of the suspected defects. The X-ray source is positioned at the appropriate distance and angle, and the exposure settings. Defects would appear as dark images on the film, while the rest of the specimen would produce lighter images. In the case of Defect B, it is necessary for it to have a contrast that is greater than 2% of the total depth in the X and Z direction. This means that cracks or voids with a depth of 1.4mm or greater in the X and Z direction would be detectable. A low kilovolt (kV) X-ray would be selected to obtain a contrasting and accurate image of Defect B. However, the selected energy must be sufficient to penetrate the full depth of the specimen, which is 60mm in Z direction and 120mm in X direction. Using appropriate exposure factors, such as lower kilovolt (kV) and milliampere (mA) settings, can help optimize image quality and sharpness and resulting in a contrasting image. Overexposure or underexposure can affect image sharpness. The distance between the X-ray source and the film can enhance the geometric sharpness. However, the SFD should still be within the recommended range to ensure proper exposure and avoid image distortion. The X-ray beam should be directed at an angle parallel to the cracks in defect B in order to obtain an accurate portrayal. This would necessitate multiple X-ray shots to effectively capture the defects. Initially, a shot would be taken vertically from the top plane to measure the depth of the defects, which would include spherical voids and potentially some of the angled cracks. Additionally, six additional shots would be required at different angles to capture all the remaining cracks. When observed from a top view, the results would reveal a single long crack in the X and Y direction because the defects are situated in the same XY plane, and radiography in this direction would not be able to distinguish between individual cracks. Additionally, cracks located at different depths but on the same XY plane would also remain undetected. The X-ray applied directly would only be able to detect the void with 3mm diameter and the longitudinal cracks that is propagated in X direction. As a result, only the cracks on the topmost layer would be shown and the radiography method would not yield precise measurements of the depth of each defect. Instead, it would only provide an estimation based on the coloration displayed on the film, with deeper cracks appearing darker. Radiography testing can be performed from the right side of the specimen which is YZ plane and the film to be placed on the left side of the specimen to fully detect the Defect B. The voids with diameter of 3mm and 2 of the transverse jagged cracks can be detected. Comparing the UT and RT, UT would be preferred NDT because it has the capability to fully capture Defect B when measured from the side view. Both methods necessitate training and in-depth knowledge for proper operation. UT may be considered a more cost-effective technique compared to radiography. Besides, UT would fully display Defect B, including voids when applied directly from the side view. However, UT may require more time compared to radiography as UT requires additional calibration and prerequisites, such as accurate orientation of flaws in Defect B, and higher energy levels than radiography. On the other hand, radiography would provide a quicker indication of the presence of defects, albeit with less accuracy. Despite the X-rays not being parallel to the defects, radiography would still be able to detect the defects, albeit with diminished contrast. As a result, radiography would be a more straightforward and expedient choice, although it may have limitations in terms of accuracy. Chapter 4: Conclusion The LP and MPI methods used together are the best NDTs to measure surface defects. UT is preferred, but radiography is necessary to fully detect and measure Defect B for subsurface defects. In fact, these methods rely on the availability of information about defect dimensions and locations, which may not always be provided in real-world scenarios, NDT would be required to detect and characterize each defect, which can be time-consuming and costly. Combining different NDT techniques may result in more precise defect detection, but it could also increase costs, which may not always be preferred. Additionally, real-world defects are often more complex than the defects discussed above, making their detection even more challenging.
