ISO 6238 Wood-to-wood adhesive bonds compressive shear strength test

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ISO 6238 Wood-to-Wood Adhesive Shear Strength Tester | UnitedTest

ISO 6238 determines shear strength of wood-to-wood adhesive bonds via compressive loading test method for adhesive qualification and quality control. UnitedTest manufactures ISO 6238 compliant testing machines for wood adhesive bond performance evaluation.

ISO 6238 is an international adhesive standard specifying the test procedure to measure the shear strength of wood-to-wood adhesive joints by compressive loading on block shear specimens. This test targets adhesive qualification and batch quality control for wood adhesives, and it is not designed for testing end-use finished wood items including furniture, wood panels, or engineered timber assemblies.

The compressive block shear test setup creates controlled shear stress across the bonded wood interface, delivering consistent shear strength data to evaluate adhesive formulation performance, bonding process parameters and raw adhesive batch consistency. This standard is widely used by adhesive manufacturers, wood material laboratories and third-party testing institutes in timber, joinery and wood composite industries.

UnitedTest designs and manufactures precision ISO 6238 wood-to-wood adhesive shear strength testing machines. Our test equipment provides stable compressive loading and accurate force measurement to generate repeatable shear strength results for adhesive qualification and laboratory quality verification.

Test Principle

Two prepared wood blocks are bonded with the adhesive under study, conditioned, cut into standard lap/shear joints, and then placed in a compressive shearing tool. A continuously increasing compressive force is applied so the bond fails in shear parallel to the bondline and to the wood grain direction. The maximum force at failure is converted to shear stress using the bonded area, and the failure mode is assessed as percentage wood failure versus adhesive/cohesive failure.

The underlying idea is that, for a good wood adhesive, failure should often occur partly or largely in the wood rather than at the interface; therefore both strength and wood-failure percentage are reported.

Test Equipment required for ISO 6238 Adhesives Wood-to-wood adhesive bonds compressive shear strength test

Universal testing machine

Compression/tensile testing machine in compression mode with constant crosshead rate, maximum-load capability, force accuracy ±1%, loaded result between 15% and 85% of machine capacity.

Self-aligning shearing tool to give uniform lateral force distribution; autographic load–displacement recording recommended.

ISO 6238 Adhesives Wood-to-wood adhesive bonds compressive shear strength test

Adhesive preparation

Balance and dosing equipment: mix proportions within ±1%.

Mixer: homogeneous mixing with minimal aeration, except foamed adhesives.

Spreader/coater: wire-wound bar, roller, curtain coater, or hand applicator; spread uniformity within ±5% of target.

Press/clamps: uniform pressure over the whole bonded area within ±5% of target; heated platens where needed, temperature control ±2°C.

Test Specimen Information:

Two specimen geometries are available; Specimen A (40 mm × 50 mm bonded area, thickness 20 mm) shall be used in case of dispute.

Specimen A: Bond area 40 mm × 50 mm, thickness T = 20 mm

Specimen B: Bond area 25 mm × 25 mm, thickness T = 10 mm

Specimen quantity:

Routine quality‑control test: minimum 3 test joints cut from each of three separate bonded blocks.

Higher‑precision test: minimum 5 test joints from each of four separate bonded blocks.

Each bonded block is assembled from matched‑density wood pairs; multiple specimens are sawn from one large bonded assembly block with standardized saw kerf offset. Grain direction runs parallel to the longest dimension of wood blocks.

Block and joint preparation

Blocks are paired by similar relative density.

From each bonded block, multiple joints are cut; the document illustrates up to five joints per block.

Final bonded width is reduced to L ± 0.5 mm and overlap to W ± 0.5 mm; saw cuts stop at the bondline and are perpendicular to the main axis.

Routine QC: minimum 3 test joints from each of 3 bonded blocks.

Higher precision: minimum 5 test joints from each of 4 bonded blocks

Test Parameters & Stipulations

1. Failure time: specimen must fracture within 60 ± 20 seconds under continuous cross‑head motion.

2. Force recording: report breaking force in kN rounded to nearest 100 N.

3. Wood‑failure rating: evaluate both fracture surfaces jointly; estimate to nearest 10 % of fractured area.

Test Procedures for ISO 6238 Adhesives Wood-to-wood adhesive bonds compressive shear strength test

1. Collect pre‑test variables from adhesive manufacturer (Annex A normative): recommended wood moisture, surface preparation, mixing instruction, spread rate, open/closed assembly time, pressing pressure‑time‑temperature, post‑bond conditioning conditions.

2. Prepare wood blocks: select qualified timber; knife‑plane bonding faces; pair blocks of similar density.

3. Mix adhesive strictly per manufacturer instructions; weigh blocks before and after spreading to verify adhesive mass. Apply adhesive and assemble wood pairs, apply bonding pressure as recommended, mark each bonded block with unique identifier.

4. Release pressure and condition bonded blocks in standard atmosphere as Clause 7 specifies.

5. Machine conditioned bonded blocks: trim both ends, plane/saw to adjust overlap dimensions to tolerances; cut individual test‑joint specimens, assign serial numbers. Store cut specimens under the same conditioning environment until testing.

6. Mount specimen inside self‑aligning shear fixture of compression testing machine. Run compression test so that failure occurs in 60 ± 20 s. Record maximum breaking load.

7. Examine fractured surfaces under oblique light, estimate percentage wood failure for each joint.

8. Compute mean breaking force/stress, standard deviation and mean wood‑failure percentage.

9. Compile complete test report covering standard reference, adhesive information, timber specification, bonding parameters, conditioning, test‑room climate, cross‑head rate, block/specimen counts, individual results, statistics and deviations from standard procedure plus test date.

Industry Application Fields

Adhesive manufacturers: development and batch QC of PVAc, urea-formaldehyde, phenol-resorcinol, epoxy, polyurethane, hot-melt, etc. for wood bonding.

Wood products and joinery: furniture, cabinets, carpentry, flooring adhesives.

Engineered wood and timber engineering laboratories: screening adhesives before structural-grade qualification, though structural performance often requires additional standards.

Research institutes and third-party testing labs: comparative benchmarking, failure analysis, supplier qualification.

Standards/regulatory work: generating reproducible shear data for specifications and acceptance.

Related Stadard:

ISO 6237	Adhesives - Wood-to-wood adhesive bonds - Determination of shear strength by tensile loading
EN 205	Adhesives - Wood adhesives for non-structural applications - Determination of tensile shear strength of lap joints
ASTM D906	Standard Test Method for Strength Properties of Adhesives in Plywood Type Construction in Shear by Tension Loading
JIS K6802	Phenolic resin adhesives for wood
BS 5350-C5	Methods of test for adhesives - Determination of bond strength in longitudinal shear for rigid adherends
ASTM D143 / BS 373	wood shear/physical property testing; Instron lists ISO 6238 with ASTM D143, JIS K6802, JIS Z2117, BS 373, JAS 601 for wood shear.
EN 205	non-structural wood adhesives, tensile shear of lap joints.
EN 302	Series adhesives for load-bearing timber structures, including shear test methods and shrinkage effects.
EN 17224	Determination of compressive shear strength of wood adhesives at elevated temperatures
ISO 13445	Block shear for rigid substrates; broader adhesive shear, not wood-specific.
GB/T 33333	Determination of shear strength by tensile loading in wood adhesives
ISO 6238	Adhesives - Wood-to-wood adhesive bonds - Determination of shear strength by compressive loading
ASTM D905	Standard Test Method for Strength Properties of Adhesive Bonds in Shear by Compression Loading

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A single column tensile tester is a sophisticated material testing instrument designed for measuring mechanical properties of various materials under tension, compression, bending, shear, and other loading conditions. As the name suggests, it features a single vertical column supporting a moving crosshead that applies force to test specimens.

UnitedTest manufactures and exports compressive shear test fixtures, ISO 6238 wood adhesive shear jigs, ASTM D905 block shear fixtures, ASTM D4501 rigid‑substrate shear tools, and compatible universal testing machines. Buy ISO 6238 and ASTM D905 compressive shear test fixtures from UnitedTest.

Vice Tensile test fixture for plastics is based on the test standatd ASTM D638, determination of the tensile properties of unreinforced and reinforced plastics in the form of standard dumbbell-shaped test specimens when tested under defined conditions of pretreatment, temperature, speed.

ISO 6238 Wood Adhesive Shear Test Machine | UnitedTest Compressive Shear Tester

Q1: What is ISO 6238 test used for?

A1: ISO 6238:2018 is a laboratory test standard to measure compressive‑loaded shear strength of wood‑to‑wood adhesive bonds. It evaluates wood‑bonding adhesives in controlled lab conditions, not for inspecting finished commercial wood products. It supports adhesive R&D, formulation comparison, quality control and certification baseline data.

Q2: Why is ISO 6238 test important for wood adhesive materials?

A2: It delivers repeatable, globally‑comparable bond‑strength data. Two core outputs are measured: shear strength (maximum load the glue joint can bear) and percentage wood failure. High wood‑failure percentage means bond strength exceeds wood substrate strength, proving good adhesion instead of weak adhesive‑interface delamination. Manufacturers rely on it to screen glue formulas, optimize spread rate/pressing parameters and avoid mass‑production bonding failures.

Q3: What is the difference between Specimen A and Specimen B in ISO 6238?

A3: Specimen A: bonded area 40 mm × 50 mm, thickness 20 mm (mandatory for dispute resolution)

Specimen B: bonded area 25 mm × 25 mm, thickness 10 mm (optional for routine QC) Specimen A is legally preferred for official reports or when parties disagree on test results.

Q4: How many specimens do I need for ISO 6238 testing?

A4: ‑ Routine quality control: minimum 3 test joints from each of 3 separate bonded blocks‑ Higher‑precision testing: minimum 5 test joints from each of 4 separate bonded blocksMultiple bonded blocks are required rather than cutting all samples from one single glued block, to account for wood‑material variation.

Q5: What is the key difference between ISO 6238 and ISO 6237?

A5: Both measure wood‑to‑wood bond shear strength.

ISO 6238: uses compressive loading (push force to create shear across bond‑line)

ISO 6237:2017: uses tensile loading (pull force for shear). Compressive test fixtures are often mechanically more robust and avoid tensile‑grip slipping risks.

Q6. What is “percentage wood failure” and why does it matter?

A6: After shear fracture, the bonded area is inspected to estimate how much failure occurred in the wood rather than at the adhesive interface or within the adhesive. High wood failure generally indicates the bond is stronger than, or comparable to, the wood near the bondline. Low wood failure may indicate poor wetting, contamination, incorrect pressure/time/temperature, or adhesive incompatibility.

Q7: What is the required failure time for ISO 6238 compression‑shear test?

A7: The specimen must fail within 60 ± 20 seconds from test start. Operators adjust cross‑head speed to hit this failure‑time window, not set a fixed mm/min speed value directly.

Q8: What is “wood failure percentage” and how do we calculate it in ISO 6238?

A8: Wood‑failure percentage estimates how much fracture area breaks inside wood substrate instead of at adhesive interface. Inspect fracture surfaces with oblique 10°‑15° light source. Evaluate both fractured sides together, round result to nearest 10 %. If fracture does not cover full bonded area, calculate percentage relative only to actual fractured area

Q9: Is ISO 6238 suitable for finished furniture or laminated timber products?

A9: No. ISO 6238 applies only to lab‑prepared standard test blocks and specimens, not finished manufactured goods. Real‑product inspection should use separate product‑level standards.

Q10: Why do my ISO 6238 test results show high standard deviation?

A10: Common root causes: ‑ Poor wood surface flatness or inconsistent wood moisture content before gluing ‑ Non‑uniform adhesive spread rate ‑ Eccentric loading (missing or worn self‑aligning shear fixture) ‑ Specimen cutting errors, saw cuts penetrating bond‑line ‑ Insufficient number of bonded blocks (too few source blocks for specimens) ‑ Not following standard conditioning time and humidity rules.

Q11: Why Choose ISO 6238 Wood-to-Wood Adhesive Compressive Shear Testing Machines by UnitedTest?

A11: UnitedTest manufactures ISO 6238:2018 compressive shear testing machine & self‑aligning shearing fixture for wood‑to‑wood adhesive bond shear strength test. Full‑compliance lab equipment for wood glue R&D, QC and certification testing. Get your custom quotation today.

UnitedTest is a professional global manufacturer for material mechanical testing equipment, delivering complete ISO 6238:2018‑compliant testing solutions for wood‑bonding adhesive laboratories, adhesive manufacturers, furniture R&D institutes and third‑party test houses.

ISO 6238 specifies compressive‑loading shear‑strength measurement for laboratory‑made wood‑to‑wood bonded joints, widely used to evaluate PVAc, polyurethane, thermosetting and bio‑based wood adhesives. Obtaining accurate, repeatable test data strictly depends on compliant universal testing hardware plus the dedicated self‑aligning shearing tool fixture required by the standard.

UnitedTest’s ISO 6238 solution package includes:

‑ Universal compression‑capable testing machine with ±1 % force measurement accuracy, calibrated to ISO 7500‑1 requirements; software supports automatic calculation of shear stress, mean value, standard deviation and editable ISO 6238 test‑report templates.

‑ Custom‑made ISO 6238 dedicated self‑alignment compressive‑shear fixture (shearing tool), matching specimen A (40 × 50 mm) and specimen B (25 × 25 mm) geometries, eliminating eccentric‑loading errors.

‑ Optional auxiliary lab accessories: specimen measuring kits, oblique light assembly for wood‑failure percentage assessment.

Our testing system helps labs execute full workflow: specimen break‑load capture, wood‑failure recording, statistical computation and standardized test‑report export, fully following ISO 6238 clauses and normative annex requirements.

Why UnitedTest for your ISO 6238 testing setup?

✅ Full standard compliance for ISO 6238:2018, also compatible with related standards ISO 6237, ASTM D905.

✅ Self‑aligning shear‑fixture design ensures even force distribution, reduces high‑variance test results.

✅ Powerful test software outputs shear‑strength data, wood‑failure statistics and ready‑to‑use lab reports.

✅ One‑stop service: equipment configuration guidance, fixture installation, on‑site training and regular calibration support.

✅ Tailored solution for adhesive R&D batch testing or quality‑control routine inspection workflows.

If you are sourcing ISO 6238 compressive shear test machine or replacement shearing‑tool fixture for wood‑adhesive bond testing, contact UnitedTest to receive technical datasheet and custom quotation.

We offer customization to meet your specific needs. Our expert team will collaborate with you to develop the perfect product for you