(a) Four Methods:
- Visual Inspection: While seemingly simple, trained inspectors can often identify surface cracks and other defects under proper lighting and magnification. This method is quick and accessible but limited to surface flaws.
- Dye Penetrant Testing (PT): A coloured liquid is applied to the surface and allowed to seep into cracks. After wiping the surface, a developer draws the dye back out, highlighting crack locations. This is effective for surface and near-surface cracks but requires careful cleaning and interpretation.
- Ultrasonic Testing (UT): High-frequency sound waves are transmitted through the material and reflected back. Cracks disrupt the wave path, generating signals detected by the receiver, indicating their presence and location. This works for internal and surface cracks but requires skilled operators and specialised equipment.
- Eddy Current Testing (ET): Magnetic fields induced by a coil interact with the material, and cracks affect the resulting electrical currents. This method works best for surface and near-surface cracks in conductive materials like metals and can be automated for rapid inspection.
(b) Two Procedure Descriptions:
1. Ultrasonic Testing (UT):
- Process: A transducer sends short bursts of ultrasound waves into the material at specific angles. These waves propagate and reflect back from interfaces within the material, including cracks. The received signals are analysed to determine the time and intensity of reflections.
- Strengths: Can detect internal and surface cracks, pinpoint crack location and depth, and evaluate crack severity. Works on various materials, including metals, composites, and ceramics.
- Limitations: Requires skilled operators and specialised equipment, interpretation of signals can be complex, and access to both sides of the material may be needed.
2. Eddy Current Testing (ET):
- Process: A coil generates an alternating magnetic field that induces eddy currents in the material. Cracks disrupt the flow of these currents, causing changes in the electromagnetic field detected by the coil. These changes are analysed to identify crack locations and characteristics.
- Strengths: Fast and automated, suitable for surface and near-surface crack detection in conductive materials, can be used for continuous monitoring during operation.
- Limitations: Only works on conductive materials, less effective for deep cracks, and requires careful calibration and interpretation of readings.
These are just two examples, and the choice of method depends on the specific material, crack type, application, and desired level of detail.