Determination of Impact Toughness of Wood: Testing Principles, Methods, and Standards

Wood impact toughness determination measures the ability of timber to absorb energy under dynamic impact loading until fracture, providing a critical indicator of shock resistance for structural timber, flooring, tool handles, and sports equipment. This article explains the testing principle, specimen requirements, test procedures, and applicable standards such as GB/T 1940, ISO 3348, and ASTM D143, serving as a practical reference for manufacturers, researchers, and accredited third-party laboratories.

Technical Overview

Impact toughness (also known as impact bending strength) is a mechanical property that quantifies the capacity of wood to absorb energy under rapid, dynamic loading until complete fracture occurs. Unlike static bending strength, which is measured under slowly applied loads, impact toughness reflects the combined resistance of wood to both crack initiation and crack propagation under shock conditions. Because it is highly sensitive to grain deviations, knots, decay, and embrittlement caused by processing or treatment, impact toughness is widely regarded as one of the most discriminating indicators of overall wood quality.

The test is performed by fracturing a simply supported, rectangular beam specimen with a single swing of a pendulum hammer striking the specimen at mid-span. The energy absorbed during fracture is read from the machine scale and divided by the cross-sectional area of the specimen to yield the impact toughness value, typically expressed in kJ/m² (equivalently, in J/cm²). Higher values indicate tougher, more shock-resistant wood, while low values indicate brash, brittle behavior.

• GB/T 1940 — Test method for impact toughness of wood, specifying a 300 mm × 20 mm × 20 mm specimen tested over a 240 mm support span with a pendulum impact machine.
• ISO 3348 — Wood: determination of impact bending strength (withdrawn but still widely referenced in specifications and historical comparisons).
• ASTM D143 — Standard test methods for small clear specimens of timber, including an impact bending procedure using a drop hammer.
• JIS Z 2101 — Japanese standard for testing methods of wood, which includes an impact bending test for small clear specimens.
• GB/T 1931 / EN 13183-1 — Oven-dry methods for determining moisture content, which is measured concurrently to normalize test results.

Testing Samples

Small Clear Wood Specimens

The reference specimen for pendulum impact testing is a straight-grained, defect-free beam of 300 mm (longitudinal) × 20 mm (radial) × 20 mm (tangential) dimensions, with the ends cut perpendicular to the length. Specimens must be free of knots, decay, checks, and abnormal grain slope, and are conditioned to equilibrium at a temperature of approximately 20 ± 2 °C and 65 % relative humidity to reach a moisture content near 12 %. A statistically valid evaluation typically requires a minimum of six replicates per species or treatment condition.

Sawn Lumber and Structural Timber

For grading and quality verification of commercial timber, impact specimens may be machined from full-size members. When the full cross section cannot be tested directly, sub-sized clear specimens are cut from representative locations, and the sampling position within the board or log is documented so that results can be interpreted in context.

Engineered Wood Products

Glued laminated timber (glulam), laminated veneer lumber (LVL), plywood, and oriented strand board (OSB) can be evaluated for impact resistance using the same energy-per-area principle, with specimen orientation and dimensions adapted to the product standard. For layered products, the direction of the face layers relative to the impact direction must be reported, because it strongly influences the absorbed energy.

Bamboo and Other Lignocellulosic Materials

Bamboo strips, reconstituted bamboo, and other bio-based fibrous materials are increasingly tested for impact toughness using adapted Charpy-type configurations. The fundamental calculation — absorbed energy divided by cross-sectional area at the fracture plane — remains the same, allowing comparison across material classes.

Testing Items

• Impact toughness value, expressed in kJ/m² (or J/cm²), determined at the actual test moisture content.
• Absorbed energy at complete fracture, in joules (J), corrected for machine friction and air resistance.
• Fracture characterization, classified as fibrous/splintering (tough), brash/brittle, shear-dominated, or combined failure.
• Moisture content at the time of test, measured on fragments immediately after fracture using the oven-dry method.
• Air-dry density of the specimen, which is commonly reported alongside toughness because of the well-known correlation between density and energy absorption.
• Validity observations, such as incomplete fracture, specimens struck off-center, or absorbed energy outside the accurate working range of the machine.

Testing Methods

Specimen Preparation and Conditioning

Specimens are machined to the specified dimensions with smooth, parallel faces and accurate right angles. Before testing, they are conditioned in a controlled climate chamber until constant mass is achieved, which for most species corresponds to approximately 12 % moisture content. Immediately before the test, the width and height of each specimen are measured at mid-span to the nearest 0.1 mm, and the dimensions are recorded for the area calculation.

Pendulum Impact Test Procedure

The specimen is placed horizontally on the two anvils of a pendulum impact testing machine with a support span of 240 mm (for the 300 mm specimen). The specimen is oriented so that the pendulum strikes the designated face — radial or tangential — directly at mid-span, as required by the governing standard. Before testing, the machine is verified by performing free swings without a specimen to confirm the zero reading and friction losses. The pendulum is then raised to the release position and allowed to strike the specimen in a single swing, and the absorbed energy indicated on the scale is recorded. Any specimen that does not fracture completely, or that is struck at an incorrect position, is rejected and replaced.

Data Processing and Calculation

Impact toughness is calculated by dividing the corrected absorbed energy by the cross-sectional area of the specimen at the fracture plane: T = W / (b × h), where W is the absorbed energy in joules, and b and h are the specimen width and height in millimeters. The result is expressed in kJ/m² and reported to an appropriate precision, typically 1 kJ/m². The moisture content determined from the fractured pieces is reported together with the toughness value, or the value is adjusted to the 12 % reference moisture content where the applicable standard provides a correction procedure. Replicate results are summarized as a mean value with standard deviation and coefficient of variation.

Alternative Method: Drop-Hammer Impact Bending (ASTM D143)

For small clear timber specimens evaluated under ASTM D143, a 50 lb (approximately 22.7 kg) hammer is dropped onto the mid-span of a simply supported beam from successively increasing heights, in fixed increments, until complete failure occurs. The height of drop at failure is reported as the impact bending index of the specimen. This approach provides a relative measure of shock resistance that complements pendulum-type data and is still referenced in North American specifications.

Applications

• Structural timber assessment for applications exposed to shock and vibration, including bridge components, railway sleepers, crane mats, and scaffold boards.
• Sports equipment engineering, such as baseball bats, hockey sticks, racket frames, and ski cores, where energy absorption under repeated impact is essential.
• Tool handle manufacturing for axes, hammers, and picks, where shock resistance prevents sudden brittle failure in service.
• Wood flooring evaluation, particularly for sports halls and dance floors subjected to dynamic and impact loading.
• Transport packaging design, including pallets and crates that must withstand drop and impact events during handling.
• Vehicle and shipbuilding applications, such as truck flooring, deck planking, and dunnage timber.
• Scientific research, including species comparison, tree breeding selection, and studies on the effects of thermal modification, densification, chemical impregnation, and natural aging on wood brittleness.
• Factory quality control and incoming inspection, where impact toughness serves as a rapid screening indicator of material degradation or processing damage.

FAQ

What is wood impact toughness and in what units is it expressed?

Wood impact toughness is the energy absorbed per unit cross-sectional area when a specimen is fractured by a single impact. It is expressed in kJ/m² (or J/cm², where 1 J/cm² equals 10 kJ/m²), with higher values indicating greater shock resistance.

Which standards are used for impact toughness determination?

The most commonly applied standards are GB/T 1940 for the pendulum impact test on 300 mm × 20 mm × 20 mm clear specimens over a 240 mm span, ASTM D143 for drop-hammer impact bending on small clear timber, JIS Z 2101 for Japanese market requirements, and the historical ISO 3348 for impact bending strength.

How does moisture content affect the results?

Impact toughness generally increases with moisture content below the fiber saturation point, as wetter wood is more ductile, while very dry wood behaves in a more brash and brittle manner. For this reason, specimens are conditioned to approximately 12 % moisture content, and the actual moisture content is always measured and reported with the result.

How does impact toughness differ from static bending strength?

Static bending strength is determined under slow, steadily increasing load and reflects maximum stress resistance, whereas impact toughness measures energy absorption under dynamic loading and is far more sensitive to grain slope, knots, decay, and embrittlement. A wood species can have high static strength yet low impact toughness if it fails in a brash manner.

What are typical impact toughness values for common species?

Values vary widely with density and species: low-density softwoods commonly fall between roughly 20 and 50 kJ/m², while dense hardwoods such as oak, ash, and hickory may reach 70 to 150 kJ/m² or more. Comparison should always be made at equivalent moisture content and test conditions.

What factors cause abnormally low impact toughness results?

Common causes include cross grain and sloped grain, knots and their surrounding distorted fibers, incipient decay, high temperature drying or over-drying, excessive thermal modification, and testing at very low moisture content. Fracture surfaces showing flat, brittle failure rather than fibrous splintering are a visual indicator of such embrittlement.

How many specimens should be tested for a reliable result?

Because wood is a naturally variable material, a minimum of six to ten replicate specimens per condition is recommended for screening purposes, and larger sample sets are used when statistical comparison between species, treatments, or suppliers is required.