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ASTM D143 - Standard Test Methods for
Small Clear Specimens of Timber

Standard Test Methods for
Small Clear Specimens of Timber - PDF
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ASTM D143 — Standard Test Methods for Small Clear Specimens of Timber

Overview of ASTM D143

ASTM D143 defines a comprehensive set of standardized mechanical tests used to determine the fundamental strength and stiffness properties of small, clear wood specimens. These tests are intentionally performed on defect-free timber—free of knots, checks, and grain irregularities—to establish baseline material properties independent of natural variability found in structural lumber.

The results generated from ASTM D143 are widely used in:

  • Wood species characterization

  • Research and development

  • Comparative material studies

  • Validation of design values used in building codes

  • Academic and government laboratories

Rather than a single test, ASTM D143 is a family of test methods, each evaluating a different mechanical property of wood under controlled laboratory conditions.

Mechanical Properties Evaluated

ASTM D143 includes procedures for determining:

  • Static bending strength and stiffness

  • Compression parallel to grain

  • Compression perpendicular to grain

  • Tension parallel to grain

  • Shear parallel to grain

  • Cleavage

  • Hardness

  • Impact bending

Each test configuration requires a specific specimen geometry, loading direction, and fixture arrangement to ensure repeatable and comparable results across laboratories.

Test Specimens

Specimen Characteristics

ASTM D143 specimens are referred to as small clear specimens and are machined from solid wood stock with strict dimensional tolerances.

Key characteristics include:

  • Uniform cross-section

  • Straight grain orientation

  • No visible defects (knots, resin pockets, cracks)

  • Precisely machined surfaces

Typical specimen dimensions vary by test method but commonly range from:

  • Bending specimens: rectangular beams with defined span-to-depth ratios

  • Compression specimens: short prismatic blocks

  • Tension specimens: dog-bone or reduced-section profiles

  • Shear specimens: notched or slotted blocks to localize shear planes

Moisture content is carefully controlled and recorded, as it has a significant influence on measured mechanical properties.

Test Fixtures and Loading Apparatus

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Because ASTM D143 covers multiple mechanical tests, no single fixture applies to all methods. Instead, each test utilizes a purpose-designed fixture to ensure correct load application and stress distribution.

Common Fixture Types

  • Three-point and four-point bending fixtures for flexural testing

  • Compression platens for compression parallel and perpendicular to grain

  • Self-aligning grips or wedge grips for tension parallel to grain

  • Shear blocks and shear jigs designed to produce uniform shear stress

Fixtures must maintain precise alignment to prevent eccentric loading, which can significantly distort results—especially in brittle wood failure modes.

Universal Grip Co. supplies ASTM-compliant grips, compression platens, bending fixtures, and custom wood testing fixtures to support the full range of ASTM D143 test methods.

Testing Machine Requirements

ASTM D143 testing is typically performed on a servo-controlled universal testing machine (UTM) capable of maintaining stable loading rates and accurate force measurement.

Typical Machine Characteristics

  • Load capacity selected based on wood species and test type

  • High-resolution load cell for low-force sensitivity

  • Crosshead or actuator capable of constant rate of deformation

  • Rigid load frame to minimize system compliance

Displacement may be measured using:

  • Crosshead displacement (for strength tests)

  • External deflectometers or extensometers (for modulus determination)

Test Procedure Summary

While the specific procedure varies by test method, ASTM D143 generally follows this structure:

  1. Specimen conditioning to a specified moisture content

  2. Dimensional measurements (width, depth, gauge length, span)

  3. Fixture alignment and specimen placement

  4. Load application at a prescribed rate

  5. Continuous recording of force and displacement

  6. Failure observation and documentation

Loading rates are carefully defined to ensure quasi-static conditions and repeatable fracture behavior.

Calculations and Reported Results

ASTM D143 emphasizes both strength values and elastic properties, depending on the test performed.

Common Calculated Properties

Stress

σ=FA\sigma = \frac{F}{A}σ=AF​

Where:

  • FFF = applied force

  • AAA = original cross-sectional area

Used in compression, tension, and shear calculations.

Modulus of Rupture (Bending Strength)

MOR=3FL2bd2MOR = \frac{3FL}{2bd^2}MOR=2bd23FL​

Where:

  • FFF = maximum load

  • LLL = support span

  • bbb = specimen width

  • ddd = specimen depth

Modulus of Elasticity (Bending)

MOE=L3ΔF4bd3ΔδMOE = \frac{L^3 \Delta F}{4bd^3 \Delta \delta}MOE=4bd3ΔδL3ΔF​

Where:

  • ΔF\Delta FΔF = load increment in the elastic region

  • Δδ\Delta \deltaΔδ = corresponding deflection

Compression and Shear Strength

Calculated using maximum load divided by the appropriate stressed area.

Data Interpretation and Significance

Results from ASTM D143 are material property values, not structural design values. Because specimens are free of defects, the measured strengths are higher than those of commercial lumber.

These results are commonly used to:

  • Compare different wood species

  • Evaluate treatment effects (thermal, chemical, moisture)

  • Support research models and finite element simulations

  • Provide reference data for standards development

Careful attention to specimen preparation, alignment, and moisture control is essential for meaningful results.

Universal Grip Co. Testing Solutions

Universal Grip Co. provides complete ASTM D143 testing solutions, including:

  • Universal testing machines for wood and timber testing

  • ASTM-specific bending, compression, tension, and shear fixtures

  • Precision grips and platens designed for anisotropic materials like wood

  • Custom fixtures for non-standard specimen geometries or research applications

Our systems are designed to deliver repeatable, standards-compliant results for research laboratories, universities, and industrial testing facilities.

Need Help with ASTM D143 Testing?

If you have questions about fixtures, grips, or selecting the proper testing machine for ASTM D143 wood testing, contact Universal Grip Co. for expert guidance and complete testing solutions.

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