computed-tomography-ndt

name	computed-tomography-ndt
description	CT scanning for NDT — industrial CT, voxel resolution, flaw detection threshold, porosity analysis, GD&T verification, ASTM E1570/E1814, aerospace and AM applications.
metadata	{"priority":7,"promptSignals":{"phrases":["CT scanning NDT","industrial CT","computed tomography NDT","CT scan inspection","X-ray CT","CT porosity analysis"],"minScore":3}}

Computed Tomography (CT) for NDT — Complete Skill

CT Scanning Principles

X-ray CT: rotate part through X-ray beam; collect 2D projections at many angles; reconstruct 3D volume via filtered backprojection or iterative algorithms

Main components:

• X-ray source: microfocus (focal spot < 20 μm) for high resolution; conventional (> 100 μm) for large parts
• Detector: flat panel detector; typical 1024×1024 to 4096×4096 pixels
• Rotary table: part rotates through 360°; 1000–3000 projections
• Reconstruction: Feldkamp-Davis-Kress (FDK) cone beam or iterative (SART/TV)

Voxel: 3D volume pixel; minimum detectable feature ≈ 2–3× voxel size

Resolution vs. part size: Voxel size = (detector pixel size × magnification)⁻¹ × SDD / SOD SOD = source-to-object distance; SDD = source-to-detector distance

Practical voxel sizes:

Part size	Achievable voxel [μm]
< 5 mm	1–5
5–50 mm	5–50
50–200 mm	20–100
200–500 mm	50–200
> 500 mm (steel)	Limited by penetration (100–500 μm or CT not feasible)

X-Ray Penetration and kV Selection

Lambert-Beer law: I = I₀ × exp(-μ × t) [μ = linear attenuation coefficient [m⁻¹]; t = material thickness [m]] μ = function of material and X-ray energy (keV)

kV selection by material and thickness:

Material	Thickness [mm]	kV recommended
Aluminum	10–50	60–150 kV
Aluminum	50–200	150–250 kV
Steel	5–20	200–350 kV
Steel	20–100	350–450 kV
Inconel	5–30	250–450 kV
Titanium	10–50	200–350 kV

Beam hardening: polychromatic X-ray → soft X-rays absorbed preferentially → cupping artifact → correction filter or iterative reconstruction

Flaw Detection

Detectability limit: Detectable flaw size ≈ 2–3 × voxel size For 50 μm voxel: detect pores ≥ 100–150 μm

ASTM E1570 (aerospace CT acceptance): Compares CT image to reference CT of known defects Sensitivity classification by ability to detect standard reference flaws Acceptance: define class of defects not acceptable (e.g., no pore > 0.5 mm, no crack > 5 mm)

Porosity analysis: Threshold scan volume → identify void voxels Report: pore volume fraction [%], max pore size [mm], pore distribution (histogram) ASTM E1814: CT for AM porosity characterization

Crack detection: Cracks < 1 voxel wide → not detectable as open crack; detectable only as density artifact Effective minimum crack width detection: ~3 voxels

Dimensional Verification

GD&T from CT: Extract surface (isosurface at threshold density) → point cloud → align to CAD → compare Dimension deviation: typical ± 2–5 voxel accuracy for dimensional measurements

Accuracy qualification: Calibrate with NIST-traceable artifact; measure gauge ball or step wedge at various orientations VDI/VDE 2630: metrological CT calibration guideline

Tolerance capability: For 50 μm voxel CT: dimensional accuracy ≈ ±0.1 mm (3σ); better with calibration Not suitable for ISO IT5 or finer tolerances without extreme care

CT for Additive Manufacturing

Key inspections:

• Internal porosity (LOF pores, keyhole, gas pores): 3D map by size and location
• Internal channels: verify flow passage geometry (no partial blockage from un-melted powder)
• Wall thickness measurement: verify compliance to design
• AM lattice verification: check strut diameter and connectivity

Acceptance criteria (aerospace AM example): No LOF pore > 0.3 mm in critical stress zones No interconnected porosity chains Total porosity < 0.1% in critical zones

CT for Assemblies (Non-Destructive)

Assembled CT: scan complete assembly without disassembly Observe: joint gaps, interference fits, internal component alignment, trapped voids

Multi-material challenges: High-density metals + low-density polymer: large dynamic range → ring artifacts, beam hardening Reduce by: dual kV, filtered reconstruction, iterative algorithms

Industry Applications

Industry	CT application
Aerospace	Turbine blade internal cooling channels; casting porosity; composite delamination
Medical devices	Implant geometry verification; coating thickness
Automotive	Cylinder head casting porosity; engine assemblies
Electronics	PCB solder joint inspection; BGA void analysis
Nuclear	Fuel rod pellet density; cladding thickness

Limitations

Penetration: steel > 100 mm → requires high energy (> 400 kV or 1–9 MeV linac); long scan times; poor resolution Scan time: 30 min – 8 hr (depends on resolution, projections, averaging) Cost: high (capital $250K–$5M+; scan $500–$5000+ per part) Artifacts: beam hardening; ring artifacts; scatter (reduce with scatter correction hardware/software)

Standards

Standard	Scope
ASTM E1570	CT examination in aerospace
ASTM E1814	CT examination of AM parts
ASTM E2339	Digital detector array radiography
VDI/VDE 2630	CT in dimensional metrology
EN 13068	Radioscopic examination

Output

Provide: X-ray voltage kV, tube current [mA], focal spot size [μm], SOD and SDD [mm], voxel size [μm], number of projections, minimum detectable flaw size [mm] (≈ 3× voxel), dimensional accuracy [mm] (3σ), porosity report (total [%], max pore [mm], distribution), specific flaw type detected (pore/crack/inclusion/channel deviation), comparison to accept/reject criteria, artifacts present (beam hardening/ring), and applicable standard (ASTM E1570, ASTM E1814, VDI/VDE 2630).