Compressive yield strength is a fundamental mechanical property that defines the stress at which wood begins to deform plastically under compressive loading, making it essential for the safe design of columns, studs, truss members, and engineered wood products. This article provides a comprehensive technical guide to the determination of compressive yield strength of wood, covering applicable standards such as ASTM D143, ISO 3787, EN 408, and GB/T 1935, specimen preparation, testing procedures, yield point determination, and typical applications in quality control and product certification.
Technical Overview
Wood is a natural, orthotropic material whose mechanical response depends strongly on the direction of loading relative to the grain. Compressive yield strength refers to the stress level at which wood transitions from elastic (recoverable) deformation to plastic (permanent) deformation under compressive loading. Because wood does not exhibit the pronounced yield plateau characteristic of mild steel, the yield strength is typically identified either as the proportional limit stress or by applying an offset method (commonly a 0.2 percent strain offset) to the recorded stress-strain curve.
Compression parallel to the grain governs the behavior of columns, wall studs, and compression chords of trusses, while compression perpendicular to the grain controls bearing conditions at supports, notches, and bolted connections. Typical clear-wood compressive strengths parallel to the grain range from approximately 25 MPa for low-density softwoods to more than 60 MPa for dense hardwoods, whereas perpendicular-to-grain values are usually only one-tenth to one-sixth of the parallel-to-grain values.
The determination is controlled by internationally recognized test methods, principally ASTM D143 for small clear specimens of timber, ISO 3787 for ultimate stress in compression parallel to grain, EN 408 for structural timber and glued laminated timber, and GB/T 1935 for compressive strength parallel to wood grain. These tests are routinely performed by accredited third-party laboratories, research institutions, and in-house quality control facilities.
Testing Samples
Small Clear Wood Specimens
Small clear specimens are free of knots, decay, checks, resin pockets, and excessive grain deviation, so that the intrinsic material properties of the species can be measured. Typical dimensions include 50 mm × 50 mm × 200 mm per ASTM D143, 20 mm × 20 mm × 60 mm per ISO 3787, and 20 mm × 20 mm × 30 mm per GB/T 1935, with the longest dimension oriented along the grain. End-grain surfaces must be machined flat, parallel to each other, and perpendicular to the longitudinal axis to ensure uniform load distribution.
Structural-Size Timber and Glued Laminated Timber
For grading and design-value verification, full cross-section members are tested in accordance with EN 408. The test piece is cut to a length of approximately six times the larger cross-sectional dimension and retains the manufacturing surface unless otherwise specified, so that the result reflects the performance of commercial material including natural characteristics permitted by the grade.
Engineered Wood Products
Glulam, laminated veneer lumber (LVL), cross-laminated timber (CLT), parallel strand lumber, I-joist webs, and laminated bamboo products are sampled so that layup, adhesive type, and panel or beam orientation are representative of production. Specimen orientation relative to the loading direction must be documented because engineered products often exhibit directional properties that differ from solid sawn wood.
Conditioning Requirements
Before testing, specimens are conditioned to equilibrium at 20 ± 2 °C and 65 ± 5 percent relative humidity, which produces a moisture content of approximately 12 percent. Because compressive strength is highly sensitive to moisture, the actual moisture content of each specimen must be determined immediately after testing and reported together with the strength value.
Testing Items
Typical measurands included in a compressive yield strength determination program are listed below.
- Compressive yield strength parallel to grain, determined at the proportional limit or by the offset method
- Proportional limit stress in compression parallel and perpendicular to grain
- Ultimate compressive strength parallel to grain
- Compressive yield strength and bearing strength perpendicular to grain
- Compressive modulus of elasticity along and perpendicular to the grain
- Yield strain and strain at maximum compressive load
- Density and moisture content at the time of test
- Strength values adjusted to the reference moisture content of 12 percent
Testing Methods
Specimen Preparation and Conditioning
Specimens are machined to the tolerances of the governing standard, inspected for grain alignment and defects, weighed, and measured at multiple positions to establish the cross-sectional area. They are then conditioned in a climate chamber until mass stability is achieved, ensuring that all comparative results correspond to a common moisture baseline.
Apparatus
- A calibrated universal testing machine of suitable capacity, commonly 100 kN to 300 kN for small clear specimens, verified in accordance with ISO 7500-1 or an equivalent accuracy class
- Hardened, parallel compression platens, with a spherical bearing block to minimize eccentric loading
- Extensometers, compressometers, or LVDT displacement transducers for measuring deformation over a defined gauge length
- Calipers and a calibrated balance for dimensional and mass measurement
- A drying oven at 103 ± 2 °C for post-test moisture content determination
Test Procedure
- Measure and record the cross-sectional dimensions of each specimen and compute the area of the loaded face.
- Center the specimen between the platens and apply a small seating preload to settle the loading train.
- Attach the deformation measurement device over the gauge length defined by the standard.
- Apply the load at a constant crosshead rate, typically in the range of 0.3 mm/min to 0.6 mm/min for small clear specimens, so that the yield or maximum load is reached within approximately one to two minutes.
- Record the load-deformation curve continuously until the defined yield condition, the specified deformation limit, or the maximum load is reached.
- Determine the yield load from the recorded curve using the applicable yield criterion.
- Determine the moisture content and density from sections cut from the tested specimen.
- Calculate the compressive stress values and adjust them to the 12 percent reference moisture content where required.
Determination of the Yield Point
Since wood lacks a distinct yield point, one of the following criteria is applied according to the governing standard. The proportional limit method identifies the stress at which the load-deformation curve first departs from linearity, evaluated visually or by a defined deviation criterion such as a small strain offset from the initial elastic slope. The offset method constructs a line parallel to the elastic loading slope, shifted by a specified offset strain, commonly 0.2 percent, and takes the intersection with the stress-strain curve as the compressive yield strength. For compression perpendicular to the grain, several standards define yield at a prescribed deformation or strain limit because the material continues to carry increasing load through progressive cell crushing and densification.
Calculation and Moisture Correction
The compressive yield strength is calculated as the yield load divided by the original cross-sectional area, expressed in megapascals. Where the test moisture content differs from the 12 percent reference, results are adjusted using species-dependent correction relationships, generally indicating an increase of roughly 3 to 5 percent in compressive strength for each percentage point reduction in moisture content below the fiber saturation point. All corrected values, together with raw data, moisture content, and density, are reported to enable traceable comparison between laboratories and material batches.
Applications
- Derivation of characteristic strength values and allowable design stresses for structural timber codes and design standards
- Verification of visual and machine strength grading output for sawn timber production
- Factory production control and incoming inspection of glulam, LVL, CLT, and other engineered wood products
- Compliance testing of timber products for import, export, and certification against EN, ASTM, GB/T, and JIS requirements
- Research and development on wood modification technologies, including thermal treatment, densification, and chemical impregnation
- Comparative evaluation of new or lesser-known timber species for structural utilization
- Forensic and failure analysis of timber structures, scaffolding, formwork, and packaging components
- Benchmarking of bamboo-based and wood-plastic composite materials against solid wood performance baselines
FAQ
What is the compressive yield strength of wood?
It is the compressive stress at which wood begins to exhibit significant permanent deformation. Because the transition is gradual, the value is conventionally reported as the proportional limit stress or as the stress determined by a defined offset criterion on the compression stress-strain curve.
Why does wood not show a distinct yield point like steel?
Wood is a polymeric, cellular composite. Under longitudinal compression, failure develops progressively through microbuckling and kink-band formation in the cell walls rather than through sudden dislocation-based yielding, producing a smooth transition from elastic to plastic behavior without a pronounced yield plateau.
Which standards govern the determination?
The most widely applied methods are ASTM D143 for small clear timber specimens, ISO 3787 for ultimate stress in compression parallel to grain, EN 408 for structural-size timber and glulam, and GB/T 1935 for compression parallel to grain. Related properties are covered by the ISO 13061 series of test methods for small clear wood specimens.
How does moisture content affect the results?
Below the fiber saturation point, compressive strength increases as moisture content decreases, typically by about 3 to 5 percent per percentage point of moisture change. For this reason, specimens are conditioned to approximately 12 percent moisture content, and results are corrected and reported at this reference level.
What is the difference between compression parallel and perpendicular to the grain?
Parallel-to-grain compression involves the direct buckling and crushing of long wood fibers and yields relatively high strength values. Perpendicular-to-grain compression flattens the cells across their width, producing much lower strength and a ductile densification response, which is why bearing design at supports is governed by separate perpendicular-to-grain limits.
What specimen sizes and equipment are required?
Small clear specimens typically measure 50 mm × 50 mm × 200 mm per ASTM D143, 20 mm × 20 mm × 60 mm per ISO 3787, or 20 mm × 20 mm × 30 mm per GB/T 1935, while structural-size members are tested full scale per EN 408. A calibrated universal testing machine with parallel platens, a spherical seat, and displacement measurement instrumentation is required in all cases.
How does yield strength differ from ultimate compressive strength?
Yield strength marks the onset of plastic deformation and is used to define serviceability and long-term performance limits, whereas ultimate compressive strength corresponds to the maximum load sustained by the specimen. For member design, both values are considered alongside stability criteria such as slenderness-dependent buckling of columns.