4.1 Eddy Current Array for Crack Detection and Sizing in Carbon Steel Welds—Eddy current array allows for a fast examination of carbon steel welds for surface-breaking cracks located on the surface closest to the sensor. However, not all ECA probe designs allow for accurate depth sizing of such discontinuities over a significant range (several millimeters, for example). To achieve proper crack depth sizing, the system shall exhibit certain characteristics, such as: 1) a lift-off signal that allows monitoring that lift-off over an adequate range of values, 2) suitable phase separation between the lift-off signal and the defect signal, 3) the capability to make use of the lift-off monitoring for crack depth determination, 4) the capability to take into account material properties variations for crack depth determination along and across the weld and, 5) a uniform sensitivity across the sensing elements of the array in order to provide an effective single-pass examination, as it is expected when using an array sensor.

4.2 Array Sensors and Single Sensing Element Sensors—Depending on the weld geometry, it may be possible to use either a sensor array or a sensor with a single sensing element. The sensor array would provide a better spatial representation of the weld and an improved probability of detection. The size of the array, as well as the size and number of individual sensing elements within the array depend on the weld geometry and other factors such as target discontinuities. When a single-sensing element sensor is used, it shall produce signals that exhibit the characteristics listed in subsection 4.1 and the maximum distance from the scan line to a target discontinuity, potentially detectable at a specified probability of detection, is typically 5 mm.

4.3 Conformable Sensors—Examining welds that are not ground flush typically requires a conformable array sensor, minimally along one axis. A conformable sensor is key to allow the individual sensing elements to follow the profile of the weld cap, and to provide a uniform response over the region of interest during the examination when the array is oriented transverse to the weld and scanned along the length of the weld.

4.4 Crack Depth Range—The crack depth sizing range over which the array sensor can provide accurate measurement depends on the sensor geometry, such as individual sensing element size and configuration. For example, larger sensing elements may provide the ability to size deeper cracks, but offer limited detection capability for shallow cracks. Appropriate array sensor selection and operating frequency is critical to ensure adequate performance for a given application. Typical operating frequencies range between 10 kHz and 500 kHz.

4.5 Coating Thickness Range—The coating thickness range over which the array sensor can reliably detect and size cracks depends on the individual sensing element size and overall probe geometry, among other parameters. For any coated weld examination, a verification that the coating thickness is within the probe specification range is critical to ensure adequate results.

4.6 Crack Length Range—The crack length range over which the array sensor performs best depends on the individual sensing element size and on any data processing performed on the data. The size of the individual sensing element mainly affects the minimum crack length detectable, while data processing (a high pass filter, for example) may have a critical impact on the maximum measurable crack length.

4.7 Sensitivity Uniformity—In order to maximize the probability of detection and allow accurate length and depth sizing, it is critical that the sensitivity across the sense elements of the array be uniform. The array sensor shall exhibit variations in sensitivity no greater than 15%. The sensitivity across the array depends on the size and configuration of single sensing elements and shall be considered to determine the overall array accuracy. Overlapping individual sensing elements may be required to achieve the adequate level of sensitivity uniformity (e.g. multiple staggered rows of single sensing elements is typical).

4.8 Sizing and Accuracy—Depending on the material properties and weld surface condition, there is an optimal measurement performance range for the system. The instrument and sensor array probe, the air reference measurement and known material reference measurement, along with the operation procedure typically allow depth sizing within ±30 % of its true depth. Depth sizing accuracy is reduced when the system is operated outside its optimal range.

1.1 This practice covers the use of eddy current array sensors for nondestructive examination of carbon steel welds. It includes the detection and sizing of surface-breaking cracks in such joints, accommodating for nonmagnetic and nonconductive coating up to 5 mm thick (typical) between the sensor and the joint. The practice covers a variety of cracking defects, such as fatigue cracks and other types of planar discontinuities, at various locations in the weld (heat-affected zone, toe area, and weld cap, for example). It covers the length and depth sizing of such surface-breaking discontinuities. This practice can be used for flush-ground and not flush-ground welds. For specific ferrous alloys or specific welded parts, the user may need a more specific procedure.

1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.