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.
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.
A flat bottom hole is a machined reference reflector — a flat-ended hole drilled to a controlled depth so its base presents a disc perpendicular to the beam. Because its size and distance are known exactly, it produces a reproducible amplitude that ultrasonic sensitivity can be set against, which is what makes results comparable between instruments and operators.
The flat base is the whole point. A disc normal to the beam reflects predictably, so amplitude becomes a function of diameter and metal path alone, and reference blocks can be manufactured to give a defined response. Two families are used together: area-amplitude blocks hold metal path constant while varying hole diameter, showing how amplitude responds to reflector size, and distance-amplitude blocks hold diameter constant while varying metal path, mapping how amplitude falls with distance through beam spread and attenuation. Diameters are conventionally quoted in sixty-fourths of an inch, so a number three flat bottom hole is three sixty-fourths across, a convention that persists in aerospace and forging specifications regardless of the units used elsewhere in the document. Flaws are then reported as an equivalent flat bottom hole size — the diameter of the reference reflector that would return the same amplitude at that depth.
Source: ASTM E127 for aluminium alloy ultrasonic reference blocks; ASTM E428 for the fabrication and control of steel reference blocks; ASTM E317 for evaluating performance characteristics of ultrasonic examination instruments; ASME Boiler and Pressure Vessel Code Section V Article 5 for ultrasonic examination of materials; AMS-STD-2154 for ultrasonic inspection of wrought metals.
Reference reflector types and what each is used to establish
Reflector
Geometry presented to the beam
Primary use
Flat bottom hole
Flat disc normal to the beam
Sensitivity setting and size equivalence for straight-beam material inspection
Side drilled hole
Cylinder transverse to the beam
Distance amplitude correction for angle-beam weld inspection
Notch
Corner reflector at a surface
Simulating surface-breaking flaws for angle-beam calibration
Area-amplitude block set
Constant metal path, varying hole diameter
Relating amplitude to reflector size
Distance-amplitude block set
Constant hole diameter, varying metal path
Relating amplitude to depth, capturing spread and attenuation
Angle-beam weld inspection generally calibrates on side drilled holes and notches; flat bottom holes belong to straight-beam material inspection.
What equivalent flat bottom hole size does not mean
Reporting a flaw as equivalent to a number five flat bottom hole says that it returned the amplitude a five sixty-fourths inch disc would return at that depth. It does not say the flaw is that size, and treating the two as the same is the most consequential misunderstanding attached to the term.
Real flaws are rarely flat, rarely normal to the beam, and rarely smooth. A tilted planar flaw reflects much of the energy away from the probe and reads far smaller than it is. A rough or branched flaw scatters. A volumetric inclusion cluster can read larger than any single constituent. In each case the equivalent size is a comparison of amplitude, not a measurement of extent.
This is exactly why fracture mechanics based assessment under API 579-1 / ASME FFS-1 asks for flaw dimensions from a sizing technique rather than for an amplitude equivalence. Amplitude-based reporting is a screening and acceptance tool; it is not an input to a criticality assessment.
Why the reference block itself has to be controlled
A reference block only produces a reproducible amplitude if it is made and maintained to specification. The relevant ASTM standards control the hole diameter and bottom flatness, the surface finish of the entry surface, and the material — because attenuation and velocity differ enough between heats and product forms to shift the response.
Acoustic similarity between the block and the part is the requirement people most often overlook. A block of nominally the same alloy but a different product form or heat treatment can attenuate differently, and calibrating on it introduces a systematic error into every reading taken afterwards. Specifications for critical work therefore call for blocks made from material representative of the product being examined.
Blocks also degrade in service. Corrosion in the hole, damage to the entry surface, and wear from repeated coupling all change the response, which is why blocks carry identification and periodic verification rather than being treated as permanent artefacts.
Where flat bottom holes still dominate
Wrought product inspection is the natural home of the flat bottom hole. Aerospace forgings, billets and plate are commonly inspected to specifications that state acceptance directly in flat bottom hole equivalence, so the reflector is embedded in the acceptance standard rather than merely used to set gain.
The convention survives in those industries because it is workable for the flaw population that matters there — inclusions and porosity in a relatively uniform product, examined by straight beam at known depth, where amplitude equivalence is a reasonable proxy for severity.
It transfers poorly to in-service weld inspection, where flaws are planar, oriented and surface-connected. That work calibrates on side drilled holes and notches and, where sizing is needed, uses tip diffraction, time of flight diffraction or phased array sectorial imaging rather than amplitude comparison.
What is a flat bottom hole used for in ultrasonic testing?
Setting and verifying sensitivity. Because its diameter and metal path are known and its flat base presents a predictable reflector normal to the beam, it produces a reproducible amplitude that instrument gain can be set against, making results comparable between instruments, probes and operators.
Why are flat bottom hole sizes given in sixty-fourths of an inch?
It is a manufacturing convention carried forward from the reference block standards and the aerospace specifications that adopted them. A number three flat bottom hole is three sixty-fourths of an inch in diameter, and the numbering persists in forging and wrought product specifications regardless of the units used elsewhere in the same document.
What is the difference between area-amplitude and distance-amplitude blocks?
Area-amplitude blocks hold the metal path constant and vary hole diameter, showing how amplitude responds to reflector size. Distance-amplitude blocks hold diameter constant and vary metal path, mapping how amplitude falls with depth through beam spread and material attenuation. Both are needed to interpret an amplitude meaningfully.
Does an equivalent flat bottom hole size tell you the flaw size?
No. It states only that the flaw returned the amplitude a reference disc of that diameter would return at that depth. Tilted planar flaws read small, rough flaws scatter, and inclusion clusters can read large, so equivalence is a comparison of response rather than a measurement of extent.
When are side drilled holes used instead?
For angle-beam weld inspection. A side drilled hole presents a cylinder transverse to the beam and returns a usable response across a range of angles, which suits shear-wave calibration where a flat disc normal to the beam is not achievable. Flat bottom holes belong to straight-beam material inspection.
Does the reference block have to match the material being inspected?
Acoustically, yes. Velocity and attenuation vary with alloy, product form and heat treatment enough to shift the response, so calibrating on a block that is not acoustically representative introduces a systematic error into every subsequent reading. Critical specifications require blocks made from representative material.
Frequently asked
Can phased array equipment be calibrated on flat bottom holes?
For straight-beam material inspection, yes, and the same equivalence logic applies. For sectorial angle-beam work the calibration reflectors are normally side drilled holes and notches, because the response has to hold across the swept angular range.
How often should reference blocks be verified?
On the interval set by the governing specification and the employer's written practice, and whenever damage is suspected. Corrosion inside the hole, entry surface wear and coupling damage all alter the response, so blocks are controlled and identified rather than treated as permanent.