Determination of Wood Cleavage Strength: Standards, Test Methods, and Applications

Wood cleavage strength testing quantifies a timber specimen's resistance to splitting along the grain, a critical parameter for fastener design, traditional joinery, and product quality control. This article explains the testing principles, specimen requirements, procedures, and calculation methods defined by recognized standards such as GB/T 1943 and ASTM D143. Read on to learn how accredited third-party laboratories determine radial and tangential cleavage strength in newtons per millimeter.

Technical Overview

Wood cleavage strength, also known as splitting or riving resistance, describes the capacity of wood to withstand forces that tend to split it apart along the longitudinal grain direction. Because wood is a natural anisotropic material, its resistance to crack propagation differs dramatically depending on whether the load acts parallel or perpendicular to the fiber arrangement, and cleavage failure typically occurs along the grain planes where inter-fiber bonds are weakest.

The test is performed on small, defect-free specimens into which a steel wedge or cleavage shoe is inserted. A tensile load is applied through the wedge until the specimen splits completely into two parts. The result is expressed as the maximum force divided by the specimen width, in newtons per millimeter (N/mm), which distinguishes cleavage strength from tensile strength perpendicular to grain (expressed in MPa). Principal reference standards include GB/T 1943 for the determination of wood cleavage strength and ASTM D143, which covers cleavage testing within its suite of small clear specimen test methods for timber, with sampling requirements governed by ISO 3129.

Key factors influencing measured values include wood species, density, moisture content, ray volume, grain angle, and growth characteristics. Accurate determination therefore requires strict control of specimen quality, conditioning, and loading conditions as specified in the relevant standards.

Testing Samples

Small Clear Solid Wood Specimens

Testing is conducted on straight-grained, defect-free material free of knots, decay, checks, and abnormal grain deviation. Under GB/T 1943, the standard specimen measures 20 mm tangentially by 20 mm radially by 50 mm longitudinally, with a central groove machined at one end to receive the cleavage wedge. Specimens are sampled from representative trees or boards following statistically sound sampling plans in accordance with ISO 3129 or equivalent national procedures.

Radial and Tangential Orientation Specimens

Two distinct specimen sets are prepared: one in which splitting occurs along the radial plane (radial cleavage) and one along the tangential plane (tangential cleavage). Because ray tissue is distributed differently across these planes, the two orientations typically yield different strength values, and both are usually characterized to fully describe splitting behavior of a species or product.

Engineered and Modified Wood Products

Glued laminated timber laminations, cross-laminated timber lamellae, thermally modified timber, chemically impregnated wood, and other modified materials can also be evaluated. Specimens are taken from the finished product where feasible, with grain direction, adhesive layers, and treatment zones documented, since processing history can significantly alter splitting resistance.

Testing Items

The cleavage test program typically includes the following measured and derived parameters:

• Maximum cleavage (failure) load, in newtons (N)
• Radial-plane cleavage strength, in N/mm
• Tangential-plane cleavage strength, in N/mm
• Specimen dimensions, in particular the width over which splitting occurs
• Moisture content at the time of test, with values adjusted to a 12% reference condition where required
• Supporting properties such as air-dry density or basic density
• Failure mode and fracture surface characteristics, confirming valid splitting along the intended plane

Testing Methods

Specimen Preparation and Conditioning

Specimens are machined to the specified dimensions within standard tolerances, taking care to avoid overheating or surface burning during cutting. Before testing, all specimens are conditioned in a standard atmosphere of 20 ± 2 °C and 65 ± 5% relative humidity until constant mass is reached, which corresponds to a moisture content of approximately 12%. Moisture content is verified on representative pieces to document the test condition.

Apparatus

Testing employs a calibrated universal testing machine with force measurement accuracy of ±1% or better, fitted with a wedge-type cleavage fixture consisting of matched steel wedges or cleavage shoes attached to self-aligning tension grips. The fixture ensures that the load is applied symmetrically along the center plane of the specimen, promoting a straight fracture path.

Test Procedure

• Measure the specimen width at the mid-length of the intended fracture plane to the nearest 0.1 mm.
• Insert the steel wedge or cleavage shoe fully into the central groove of the specimen.
• Mount the assembly in the testing machine so that the specimen axis coincides with the load line.
• Apply the load at the uniform rate prescribed by the governing standard until the specimen splits completely.
• Record the maximum load sustained and examine the fracture surface to confirm a valid failure along the intended plane.

Calculation and Expression of Results

Cleavage strength is calculated by dividing the maximum load by the specimen width over which splitting occurred. The result is reported in newtons per millimeter for GB/T 1943, or as load per unit width in pound-force per inch under ASTM D143 conventions. When the moisture content at test deviates from 12%, results are adjusted using the correction factors specified in the applicable standard. Statistical summaries, including the mean, standard deviation, and coefficient of variation of valid replicate specimens, are reported for each orientation.

Applications

Determination of wood cleavage strength supports a wide range of engineering, manufacturing, and research activities:

• Fastening design: assessing the risk of splitting when nails, screws, bolts, and dowels are driven near board edges or ends
• Traditional joinery: evaluating the integrity of mortise-and-tenon, dovetail, and other interlocking joints loaded in splitting
• Drying and processing control: understanding susceptibility to end checks and splits during kiln drying, re-sawing, and machining
• Species comparison: building material property databases for selection of timbers in structural and decorative applications
• Modified wood development: quantifying how thermal treatment, densification, or chemical impregnation alters splitting resistance
• Engineered product quality control: verifying lamella suitability in glued laminated and cross-laminated timber production
• Specialty products: validating performance of tool handles, sports equipment, flooring, and furniture components subject to splitting loads

FAQ

What is the difference between cleavage strength and tensile strength perpendicular to grain?

Cleavage strength measures resistance to splitting under a concentrated line-type load applied through a wedge, and is expressed as force per unit width (N/mm). Tensile strength perpendicular to grain measures uniform resistance across a stressed cross section and is expressed as stress (MPa). The two properties are related but not interchangeable.

Which standards govern wood cleavage strength testing?

The primary references are GB/T 1943, which specifies the cleavage test method for small clear wood specimens, and ASTM D143, which includes a cleavage procedure within its small clear timber test methods. Sampling and general requirements may follow ISO 3129, and testing is commonly performed by accredited third-party laboratories.

Why do radial and tangential cleavage strengths differ?

Ray tissue runs in the radial direction and bridges tangential fracture planes, so these rays must be ruptured when splitting occurs along the tangential surface. As a result, tangential cleavage values are often higher than radial ones, although the exact relationship depends on species and ray volume.

What are typical cleavage strength values for common woods?

Values vary widely with density and species. Common softwoods generally fall in the range of roughly 5 to 12 N/mm, while dense hardwoods may reach 20 N/mm or more. Moisture content also has a significant influence, with wetter wood generally splitting more easily.

How many specimens are required for a valid determination?

Standards call for multiple replicate specimens for each orientation and material source so that mean values and variability can be reported reliably. The exact number depends on the purpose of the test, whether for research, material characterization, or quality control, and on the sampling plan defined by the applicable standard.

How is moisture content handled in the test?

Specimens are conditioned to approximately 12% moisture content before testing, and the actual moisture content is measured and reported alongside the results. If the measured moisture content differs from the 12% reference, standardized correction factors are applied so that reported values are comparable across laboratories and test programs.