Testing standard
Standard Test Methods for Small Clear Specimens of Timber
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D143 is the American battery of mechanical tests for wood measured on small clear specimens — pieces free of knots, checks and other defects, so the result describes the species rather than the individual piece. It covers static bending, compression parallel and perpendicular to the grain, hardness, shear parallel, cleavage, tension parallel and perpendicular, impact bending, toughness and nail withdrawal, at a primary 2 by 2 in. (50 by 50 mm) cross section or, for two of those tests only, a secondary 1 by 1 in. (25 by 25 mm) section.
D143 is a battery rather than a single test. On small pieces of wood free of knots, checks and other defects it runs static bending, compression parallel and perpendicular to the grain, hardness, shear parallel, cleavage, tension parallel and perpendicular, impact bending, toughness and nail withdrawal.
The point of a small clear specimen is that it isolates the material from the piece. A structural timber fails where its worst knot is; a clear specimen measures what the wood itself can do, which is what species comparison and allowable-stress work need. D143 and D198, its structural-size counterpart, are described in D143 as together affording a basis for establishing allowable stresses.
Two specimen sizes run through the standard and they are not interchangeable. The primary method uses a 2 by 2 in. (50 by 50 mm) cross section, and the standard advises using it wherever possible because that section embraces a number of growth rings and is less influenced by the difference between earlywood and latewood.
The secondary method uses 1 by 1 in. (25 by 25 mm) and exists only for static bending and compression parallel, for when clear material of the larger section cannot be found. Six tests stay at 2 by 2 in. regardless, and toughness and tension parallel use their own smaller sections. The standard cautions that results between two different specimen sizes are not necessarily directly comparable, and pooling them is the classic error with this method.
Two sizes run through the standard and they are not interchangeable. The standard says so itself, and pooling them is the classic error with this method.
There is no single D143 rate. Each method sets its own, and compression parallel sets a strain rate rather than a crosshead speed.
MOR = 3PL / (2bd²)
The standard centre-loading flexure relationship, applied to the 28 in. or 14 in. span. D143 fixes the span so that this stays valid at a span-to-depth ratio of at least 14.
MOE = PL³ / (48 Δ I)
Δ must be the neutral-plane deflection taken with the yoke, referenced to points above the supports. Crosshead travel includes machine compliance and inflates the result.
σc = P / A
Deflection must come from the yoke, not the crosshead. Machine compliance inside a crosshead reading is indistinguishable from specimen deflection and drives modulus of elasticity up.
Static bending is centre loading on a fixed span with laterally adjustable knife edges, so a stick with slight twist still beds down on both supports. The bearing plates and rollers sit close enough to the central plane that the distance from support point to that plane does not exceed the specimen depth.
Specifications
Compression parallel to the grain needs at least one platen on a spherical bearing, so load distributes over end grain that is never perfectly square. Without it the specimen crushes a corner first and fails at the end rather than in the body.
Specifications
Shear parallel to the grain runs in its own notched-block rig rather than on platens. Its geometry sits in a clause that is not published openly and is not reproduced here.
SpecificationsSpeed is set per test and it is not one number. Static bending runs at 0.10 in. (2.5 mm) per minute for the primary specimen and 0.05 in. (1.3 mm) per minute for the secondary. Compression parallel is given as a strain rate instead — 0.003 in. per in. of nominal length per minute — so the absolute crosshead speed differs between the two specimen lengths.
Displacement measurement carries stated classes, verified to Practice E2309: Class A for compression parallel, Class B within the proportional limit in bending, Class C acceptable beyond it. Force verification points at Practice E4, and the required accuracy percentage sits in the calibration clauses, which are not published openly.
The fixtures are specific. Bending needs laterally adjustable knife-edge supports with bearing plates and rollers, so a slightly twisted specimen still seats. Compression parallel needs at least one platen on a spherical bearing. Bending deflection is taken on the neutral plane with a yoke, referenced to points above the supports.
Deflection read at the crosshead rather than on the neutral axis is the most damaging habit: machine compliance enters the reading and modulus of elasticity comes out high. The yoke exists to remove it.
Compression parallel specimens that crush at the ends rather than in the body are the second recurring problem. The standard's own remedy is a spherical bearing platen and ends held at a very slightly lower moisture content than the body.
Moisture drift across a test day undoes everything else, which is why the conditioning clause is written as firmly as it is. Inch-pound units are normative here and SI is informative, so reporting SI as though it governed is a documentation error.
The choice is not a convenience. It changes which tests are available and it makes the results a separate population.
| Primary method | Secondary method | |
|---|---|---|
| Cross section | 2 × 2 in. (50 × 50 mm) | 1 × 1 in. (25 × 25 mm) |
| Available for | Every test in the standard | Static bending and compression parallel only |
| Static bending specimen | 2 × 2 × 30 in. (50 × 50 × 760 mm) | 1 × 1 × 16 in. (25 × 25 × 410 mm) |
| Bending span | 28 in. (710 mm) | 14 in. (360 mm) |
| Bending rate | 0.10 in. (2.5 mm)/min | 0.05 in. (1.3 mm)/min |
| Bearing block radius | 3 in. (76 mm) | 1 1/2 in. (38 mm) |
| Compression specimen | 2 × 2 × 8 in. (50 × 50 × 200 mm) | 1 × 1 × 4 in. (25 × 25 × 100 mm) |
| Compression gauge length | ≤6 in. (150 mm) | 2 in. (50 mm) |
The standard cautions in its own Introduction that test results between two different sizes of specimen are not necessarily directly comparable, and advises the primary method wherever possible. A dataset mixing the two is not one dataset.
| ASTM D143 | ASTM D198 | |
|---|---|---|
| Specimen | Small, clear, defect-free | Lumber in structural sizes, defects included |
| What it measures | The material | The piece, as it would be used |
| Typical use | Species comparison, clear wood strength values | Grading, allowable stress verification, connections |
| Bending span | Fixed at 28 in. or 14 in. | Set by the member |
D143 itself says the two together afford a basis for establishing allowable stresses. A clear-wood figure is an upper bound on what a knotty structural member will do, never a substitute for testing one.
It is the American set of test methods for determining the mechanical properties of wood using small clear specimens — pieces selected free of knots, checks and other defects. It covers static bending, compression parallel and perpendicular to the grain, hardness, shear parallel, cleavage, tension parallel and perpendicular, impact bending, toughness and nail withdrawal. The methods were originally approved in 1922 and the current edition is D143-25.
Because the two questions are different. A structural member fails wherever its worst defect is, so testing one tells you about that piece. A clear specimen measures what the species itself can do, which is what you need to compare species, to establish clear wood strength values and to derive allowable stresses. D143 says explicitly that it and D198, the structural-size method, together afford a basis for establishing those stresses.
The cross section. Primary is 2 by 2 in. (50 by 50 mm) and secondary is 1 by 1 in. (25 by 25 mm). Primary is advised wherever possible, because that section takes in several growth rings and is less swayed by the difference between earlywood and latewood. Secondary exists only for static bending and compression parallel to the grain, for material from which a clear 2 by 2 in. piece of the required length cannot be cut.
No. The standard cautions in its own Introduction that results between two different specimen sizes are not necessarily directly comparable. They are separate populations and mixing them into one dataset is the commonest error made with this method. Say which method produced each figure, every time.
There is no single rate — each method sets its own. Static bending runs at 0.10 in. (2.5 mm) per minute on the primary specimen and 0.05 in. (1.3 mm) per minute on the secondary. Compression parallel to the grain is specified as a strain rate instead, 0.003 in. per in. of nominal specimen length per minute, which works out to different crosshead speeds for the 8 in. and 4 in. specimens.
Almost always because deflection was taken from crosshead travel. That reading contains the machine's own compliance and the seating of the specimen on the supports, neither of which is the wood bending. D143 requires the deflection of the neutral plane at mid-length, measured with a yoke referenced to points in the neutral plane above the supports, using a device capable of at least a Class B rating to Practice E2309 within the proportional limit.
Either the end grain is not square and parallel, or there is no spherical bearing on the platen, or the ends are wetter than the body. The standard addresses all three: it asks for special care in preparing the end surfaces, requires at least one platen on a spherical bearing, and notes that failures develop in the body most reliably when the ends are at a very slightly lower moisture content than the body.
A temperature only. Specimens must be at 68 ± 6 °F (20 ± 3 °C) when tested, and the temperature is recorded as a specific part of the test record. Air-dry specimens are dried to approximately constant weight, and anything whose moisture changed during preparation is reconditioned. There is no numeric humidity requirement — the standard says only that humidity control is desirable, so do not import a 23 °C and 50 percent atmosphere from a plastics method.
ASTM D198 is the structural-size counterpart and the two are designed to be read together. D5536 covers sampling forest trees to obtain clear specimens, D2555 establishing clear wood strength values, D2395 density, D4442 moisture content, D4761 lumber and wood-based structural materials, D3043 structural panels in flexure. Practice E4 verifies force and Practice E2309 the displacement devices.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Not stated by the standard; it follows from species and specimen. A dense hardwood in compression parallel on the 2 by 2 in. section reaches the tens of kilonewtons, and static bending on a 28 in. span far less. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | unknown — D143 points at ASTM E4 for force verification and the required percentage sits in its calibration clauses, which were not retrieved | ISO 7500-1 Class 0.5 — the method sets no class of its own |
| Strain measurement | An extensometer to ASTM E2309 Class A for compression parallel; Class B within the proportional limit in static bending; Class C beyond it, gauge length compression parallel: not exceeding 150 mm (6 in.) for the primary specimen, 50 mm (2 in.) for the secondary | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Laterally adjustable knife-edge supports with bearing plates and rollers and a fixed radiused bearing block for static bending; a platen on a spherical bearing for compression parallel; a yoke for neutral-plane deflection; separate rigs for shear parallel, cleavage and nail withdrawal. | Our shear fixtures or compression anvils, built to the specimen |
| Environment | Specimens at 68 plus or minus 6 degrees Fahrenheit (20 plus or minus 3 degrees Celsius) at the time of test, with the temperature recorded. Air-dry specimens dried to approximately constant weight. No numeric humidity is specified — the standard says only that humidity control is desirable. | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
