Fully Noncontact Hybrid NDT for 3D Defect Reconstruction Using SAFT Algorithm and 2D Apodization Window

Abstract

Nondestructive testing of metallic objects that may contain embedded defects of different sizes is an important application in many industrial branches for quality control. Most of these techniques allow defect detection and its approximate localization, but few methods give enough information for its 3D reconstruction. Here we present a hybrid laser-transducer system that combines remote, laser-generated ultrasound excitation and noncontact ultrasonic transducer detection. This fully noncontact method allows access to scan areas on different object's faces and defect details from different angles/perspectives. This hybrid system can analyze the object's volume data and allows a 3D reconstruction image of the embedded defects. As a novelty for signal processing improvement, we use a 2D apodization window filtering technique, applied along with the synthetic aperture focusing algorithm, to remove the undesired effects due to side lobes and wide-angle reflections of propagating ultrasound waves, thus enhancing the resulting 3D image of the defect. Finally, we provide both qualitative and quantitative volumetric results that yield valuable information about defect location and size.

Keywords: 3D reconstruction; NDT; SAFT; apodization; defects; laser ultrasonics; noncontact transducers; synthetic aperture; weighting function.

Figure 1

Beamwidth angle Δθ for a point scatterer at depth Z; when the point scatterer is deeper Z2 > Z1, the synthetic aperture size (ΔX) changes accordingly. The apodization window size should be adapted to the synthetic aperture window size.

Figure 2

(a) Experimental setup used for the excitation and detection of ultrasound waves; (b) Schematic representation of the object under study with the three-face scan areas. Different Cartesian axes are represented at each scan face; (c) Representation of the 3D object under investigation using the synthetic aperture focusing technique (SAFT) algorithm with the scanning area at one of the faces with the Exciter (T), Receiver (R), and scatterer point in volume.

Figure 2

(a) Experimental setup used for the excitation and detection of ultrasound waves; (b) Schematic representation of the object under study with the three-face scan areas. Different Cartesian axes are represented at each scan face; (c) Representation of the 3D object under investigation using the synthetic aperture focusing technique (SAFT) algorithm with the scanning area at one of the faces with the Exciter (T), Receiver (R), and scatterer point in volume.

Figure 3

The SAFT algorithm results of the three-face experiment: (a) X₁Y₁ plane slice for face 1 at Z₁ = 99 mm; (b) X₁Z₁ plane slice for face 1 at Y₁ = 40 mm; (c) X₂Y₂ plane slice for face 2 at Z₂ = 95 mm; (d) X₂Z₂ plane slice for face 2 at Y₂ = 51 mm; and (e) X₃Y₃ plane slice for face 3 at Z₃ = 70 mm f) X₃Z₃ plane slice for face 3 at Y₃ = 36 mm. (color map at bottom right applies to all subfigures).

Figure 3

The SAFT algorithm results of the three-face experiment: (a) X₁Y₁ plane slice for face 1 at Z₁ = 99 mm; (b) X₁Z₁ plane slice for face 1 at Y₁ = 40 mm; (c) X₂Y₂ plane slice for face 2 at Z₂ = 95 mm; (d) X₂Z₂ plane slice for face 2 at Y₂ = 51 mm; and (e) X₃Y₃ plane slice for face 3 at Z₃ = 70 mm f) X₃Z₃ plane slice for face 3 at Y₃ = 36 mm. (color map at bottom right applies to all subfigures).

Figure 4

3D reconstruction of the defect by combining the resulting SAFT images from the three faces and superimposing the actual cylindrical shape of the defect. (a) Isometric view resulting from face 1 (front view/X₁Y₁) scan area inspection by applying a reject threshold for data with an intensity below 65%; (b) Front view resulting from face 1 scan area inspection by applying a reject threshold for data with intensity below 65%; (c) Isometric view resulting from face 2 (Side view/X₂Y₂) scan area inspection by applying a reject threshold for data with an intensity below 65%; (d) Side view resulting from face 2 scan area inspection by applying a reject threshold for data with an intensity below 65%; (e) Isometric view resulting from face 3 (Top view/X₃Y₃) scan area inspection by applying a reject threshold for data with an intensity below 65%; (f) Top view resulting from face 3 scan area inspection by applying a reject threshold for data with an intensity below 65%; (g) Isometric view resulting from the 3d reconstruction by applying a reject threshold for data with an intensity below 65%; (h) isometric view by applying a stricter reject threshold for data with an intensity below 85%. (Color map at bottom right applies to all subfigures.)

Figure 4

3D reconstruction of the defect by combining the resulting SAFT images from the three faces and superimposing the actual cylindrical shape of the defect. (a) Isometric view resulting from face 1 (front view/X₁Y₁) scan area inspection by applying a reject threshold for data with an intensity below 65%; (b) Front view resulting from face 1 scan area inspection by applying a reject threshold for data with intensity below 65%; (c) Isometric view resulting from face 2 (Side view/X₂Y₂) scan area inspection by applying a reject threshold for data with an intensity below 65%; (d) Side view resulting from face 2 scan area inspection by applying a reject threshold for data with an intensity below 65%; (e) Isometric view resulting from face 3 (Top view/X₃Y₃) scan area inspection by applying a reject threshold for data with an intensity below 65%; (f) Top view resulting from face 3 scan area inspection by applying a reject threshold for data with an intensity below 65%; (g) Isometric view resulting from the 3d reconstruction by applying a reject threshold for data with an intensity below 65%; (h) isometric view by applying a stricter reject threshold for data with an intensity below 85%. (Color map at bottom right applies to all subfigures.)