Flat-Bottom Hole (FBH)
A flat-bottom hole is a reference reflector drilled into a calibration block to a precise depth with a flat bottom perpendicular to the beam axis, used as a known disc-shaped target for normal-beam sizing.
Definition
An FBH is a blind hole whose flat bottom presents a perfect disc reflector to a normal-beam probe. FBHs are used in DGS sizing and in aerospace forging acceptance, where flaws are specified in terms of equivalent FBH diameter (for example, '#3 FBH' is a 3/64-inch FBH).
What it is
A machined hole with a flat end, drilled to a known depth so that its flat bottom presents a reflector of known area perpendicular to the beam. Because area and depth are both known, it provides a reproducible amplitude reference.
Correct use
It underpins distance-amplitude correction and DGS/AVG evaluation, where flaw response is expressed as an equivalent flat-bottom-hole diameter. It is a reference, not a claim that real flaws look like drilled holes.
Governing standards
ASTM E127 and ASTM E428 cover reference block manufacture; ASME Section V Article 4 and its appendices define their use in calibration.
The usual mistake
Reading equivalent-reflector-size as physical flaw size. A rough, irregular, or tilted real flaw scatters energy that a machined flat surface returns, so equivalent size systematically understates it.
Frequently asked questions
Why are flat-bottom holes used instead of notches?
They serve different purposes. Flat-bottom holes give an area-based amplitude reference for volumetric evaluation and DGS sizing; notches simulate surface-breaking planar flaws for angle-beam calibration. Codes specify which is required for which technique.
Where Flat-Bottom Hole fits in an inspection programme
A term is only useful when it connects to a decision. Flat-Bottom Hole appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Side-Drilled Hole (SDH) — A side-drilled hole is a cylindrical reference reflector drilled perpendicular to the inspection surface, used as a length-independent angle-beam target for building DAC curves and verifying sensitivity.
- Distance-Gain-Size (DGS) — DGS is an ultrasonic sizing technique that compares the echo amplitude from an unknown reflector to a family of curves representing equivalent flat-bottom-hole sizes at varying distances, calculated from probe characteristics.
- Calibration Block — A calibration block is a reference specimen with known geometry and reference reflectors (side-drilled holes, flat-bottom holes, notches) used to set ultrasonic instrument sensitivity, range, and angle prior to inspection.
Further reading
flat bottom hole sensitivity in ut inspection
More equipment terms
Transducer · Dual-Element Probe · Angle-Beam Probe · Normal-Beam Probe · Couplant · Reference Reflector · Notch Reference · X-Ray Tube · Gamma Source · Iridium-192
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.