Abstract
This paper introduces GB／T 12969.2-91 Eddy Current Method for inspecting titanium and titanium alloy pipes. Eddy current method is an non destructive testing (NDT) technique for performing the inspection of titanium and titanium alloy pipes. The technique is an electromagnetic test which can detect surface and subsurface discontinuities in the material such as cracks and other defects. GB／T 12969.2-91 test standards defines the requirements and specifications for the eddy current method. Furthermore, the paper explains eddy current principles, eddy current instrument system setup and operation, quality assurance procedures and data interpretation in detail.
1. Introduction
Titanium and its alloys possess various advantages such as high strength, light weight, and corrosion resistance, making them suitable for uses in aerospace and other highly demanding industries. However, due to their inherent physical and chemical properties, they are susceptible to certain defects such as cracks that can affect the service life of components. In order to ensure their structural integrity, it is therefore necessary to inspect the components.
One of the most commonly used techniques for inspecting titanium and titanium alloy pipes is the GB／T 12969.2-91 Eddy Current method. This inspection technique is an electromagnetic method that is used to detect surface and subsurface discontinuities in the material that can affect the structural integrity of the component such as cracks and other types of defects.
2. GB／T 12969.2-91 Eddy Current Method
GB／T 12969.2-91 specifies the requirements and specifications for the eddy current testing of titanium and titanium alloy pipes. According to the standards outlined in this document, the eddy current method has two variants, EMAT for inspecting ferromagnetic materials, and HFET for inspecting non-ferromagnetic materials. The EMAT variant is applicable to titanium and titanium alloy pipes with diameters between 10mm and 102mm while the HFET variant is used to inspect titanium and titanium alloy pipes with diameters between 38mm and 153mm.
2.1 Eddy Current Principle
At its most basic, eddy current testing relies on the induction of an electrical current within the material to be tested. This current generates a secondary, or eddy current, that is then measured to detect surface and subsurface discontinuities in the material.
To generate the eddy current, an alternating electrical current is passed through a conductor. When the conductor reaches its target material, the material becomes a secondary conductor, also known as an eddy current field. The alternating electrical current in the eddy current field produces a secondary current that is known as the eddy current.
The eddy current generates a magnetic field that interacts with the subsurface discontinuities of the material. These interactions create repulsive forces between the eddy currents and the defects which can then be detected and measured.
2.2 Eddy Current Instrument System
An eddy current system consists of two main components: an eddy current probe and an eddy current instrument. The eddy current probe is composed of an exciter coil, a pick-up coil, and a probe body that houses the two coils. The exciter coil generates the alternating electrical current during the test. The pick-up coil measures the induced currents that are generated by the eddy current field.
The eddy current instrument is the device used to measure and interpret the signals generated by the eddy current probe. The instrument typically consists of an amplifier, a demodulator, and a display. The amplifier takes the signal generated by the eddy current probe and amplifies it to a level that can be measured. The demodulator is then used to decode the signal. Finally, the display displays the decoded signal in a readable format.
2.3 Quality Assurance
In order to ensure the accuracy of the eddy current test, two quality assurance procedures should be followed. The first is a calibration check of the eddy current instrument that should be done on a regular basis to ensure the accuracy of the instrument. The second is a verification procedure which involves comparing the results of the eddy current test with results from other nondestructive testing methods such as visual testing and radiography.
2.4 Data Interpretation
The eddy current signal is typically displayed as a waveform made up of a series of concentric circles representing signal amplitude. The signal amplitudes measured at each point indicate the presence of subsurface discontinuities. A high signal amplitude indicates a larger discontinuity while a lower signal amplitude indicates a smaller discontinuity.
3. Conclusion
GB／T 12969.2-91 Eddy Current Method is a widely used technique for performing nondestructive testing of titanium and titanium alloy pipes. The paper discussed the general principles of the technique, components of the eddy current system, and quality assurance procedures. Additionally, the paper provided an overview of data interpretation for eddy current testing. Following the specifications as outlined in GB／T 12969.2-91, eddy current testing can be used to ensure the structural integrity of titanium and titanium alloy components.
