Flat crush strength testing measures the ability of corrugated fiberboard, honeycomb paperboard, and other cellular materials to resist compressive forces applied perpendicular to their surface. This critical performance test evaluates flute and core structure integrity, helping manufacturers ensure that packaging, panels, and cushioning materials can withstand stacking, handling, and transport loads. This article explains the technical principles, applicable standards, sample requirements, test methods, and typical applications of flat crush strength testing.
Technical Overview
Flat crush strength testing (also known as flat crush resistance testing or FCT) is a fundamental mechanical test used to evaluate the compressive performance of corrugated fiberboard, corrugating medium, honeycomb paperboard, and rigid cellular materials. During the test, a compressive load is applied perpendicular to the plane of the specimen between two parallel platens, and the maximum force the material sustains before the flute or cellular structure collapses is recorded.
Unlike edgewise crush testing, which evaluates the vertical stacking strength of a board loaded on its edge, flat crush strength testing focuses on the structural integrity of the flutes or core cells themselves. A high flat crush value indicates that the corrugated structure can maintain its shape and thickness under applied pressure, which is essential for printing quality, cushioning performance, and long-term load retention. The result is typically expressed in kilopascals (kPa) for board and panel materials, or in newtons (N) for corrugating medium tested by the Concora method.
Flat crush strength testing is widely performed by accredited third-party laboratories, manufacturers' in-house quality control departments, and research institutions. The most commonly applied international standard is ISO 3035 for corrugated fiberboard, along with related methods in the ISO 7263 series for corrugating medium and ASTM D1621 for rigid cellular plastics, with corresponding national standards published by GB and JIS standards organizations.
Testing Samples
Corrugated Fiberboard
Single-wall, double-wall, and triple-wall corrugated fiberboard are the most frequently tested materials. Specimens are cut from representative board samples free of defects such as delamination, crush lines, or moisture damage. Testing evaluates whether the flute profile has been properly formed and bonded during the corrugating process.
Corrugating Medium (Fluting Paper)
Corrugating medium is tested in its fluted form using the Concora medium test (CMT). The paper is fluted in a laboratory fluting device, adhered to carrier strips, and compressed flatwise to assess how well the flutes will perform within finished board.
Honeycomb Paperboard and Core Materials
Honeycomb panels used in pallets, furniture panels, doors, and heavy-duty packaging are tested flatwise to verify the compressive rigidity of the hexagonal cell structure. The test is equally applicable to other core materials, including mineral wool and composite sandwich cores.
Rigid Cellular and Foamed Materials
Rigid foams such as polyurethane, polystyrene, and polyethylene used in thermal insulation and protective packaging are tested for flatwise compressive strength and compressive modulus in accordance with ASTM D1621.
Testing Items
Typical test items covered under flat crush strength testing services include the following:
- Flat crush resistance (FCT) of corrugated fiberboard, expressed in kPa
- Concora medium crush (CMT) of corrugating medium, expressed in N
- Flatwise compressive strength of honeycomb paperboard and panels
- Flatwise compressive strength and compressive modulus of rigid cellular plastics
- Comparative evaluation of different flute profiles (A, B, C, E, F flute) and adhesive systems
- Effect of moisture content and humidity conditioning on flat crush performance
- Verification of supplier declarations and incoming material quality control data
Testing Methods
Specimen Preparation and Conditioning
Specimens are die-cut to the dimensions specified by the applicable standard, commonly a defined test area in the range of 50 to 100 square centimeters, depending on the method. Edges must be clean, perpendicular, and free from burrs or pre-crushed flutes. Before testing, specimens are conditioned in a standard atmosphere of 23 °C ± 1 °C and 50% ± 2% relative humidity in accordance with ISO 187, because moisture content has a significant influence on compressive strength results.
Compression Test Procedure
In the standard procedure for corrugated fiberboard, the specimen is supported at its edges — in the wax-supported method, the edges are immersed in molten wax to form a rigid supporting frame — and then placed between the parallel platens of a universal compression testing machine. The platens move toward each other at a constant rate, typically 12.5 ± 2.5 mm/min, until the flute structure collapses. The maximum force recorded at failure is divided by the specimen test area to calculate the flat crush strength in kPa.
For corrugating medium, the Concora method specified in the ISO 7263 series is applied: a conditioned strip is fluted in a laboratory fluter, secured to holding tapes, and compressed flatwise after a defined rest period. For rigid cellular plastics, square or rectangular blocks are compressed in the flatwise direction at a constant rate, and both compressive strength at yield or at 10% deformation, and the compressive modulus, are determined as required by ASTM D1621.
Data Processing and Reporting
A statistically valid number of specimens is tested, and the mean value, standard deviation, maximum and minimum values are calculated and reported. A complete test report includes the test standard applied, specimen dimensions and test area, conditioning atmosphere, compression rate, individual and mean results with units, failure mode observations, and the identification of the testing laboratory and test date.
Applications
Flat crush strength testing supports quality assurance and product development across many industries:
- Corrugated packaging production: verifying flute formation, adhesive bonding quality, and resistance to crushing during printing, die-cutting, and converting operations
- Transport packaging design: confirming that boxes, partitions, and pads retain their cushioning and structural functions throughout the distribution cycle
- Honeycomb panel manufacturing: qualifying core materials for pallets, furniture, interior panels, and structural sandwich constructions
- Thermal insulation and construction: demonstrating the flatwise compressive performance of rigid foam boards used in building envelopes
- Supplier qualification and incoming inspection: comparing materials from different sources against specification limits
- Research and development: optimizing paper grades, flute geometry, adhesive systems, and process parameters
- Regulatory and customer compliance: providing documented test evidence for export packaging, e-commerce logistics, and industry certification requirements
FAQ
- What is flat crush strength and how is it expressed? Flat crush strength is the maximum compressive stress, in kPa, that a corrugated or cellular material withstands when loaded perpendicular to its surface before the flute or cell structure collapses. For corrugating medium tested by the Concora method, the result is reported in newtons.
- How does flat crush testing differ from edge crush testing? Flat crush testing loads the board on its face and evaluates flute integrity and resistance to surface pressure, while edge crush testing loads the board on its edge and evaluates vertical stacking or box compression performance. Both tests are complementary in a complete board evaluation program.
- Why is wax sometimes used in the test? Wax edge support is specified in the wax-supported method to provide a rigid frame around the specimen, prevent premature edge failure, and ensure that the measured force reflects the true flat crush resistance of the flute structure.
- Which standards govern flat crush strength testing? Corrugated fiberboard flat crush is tested according to ISO 3035, corrugating medium according to the ISO 7263 series, and rigid cellular plastics according to ASTM D1621, with corresponding national methods published by GB and JIS standards organizations. Conditioning follows ISO 187.
- What factors influence the test results? Flute profile and flute height, paper grammage and quality, adhesive bond strength, moisture content, testing speed, and specimen preparation accuracy all significantly affect flat crush results, which is why standardized conditioning and controlled procedures are essential.
- Can flat crush testing be performed by third-party laboratories? Yes, accredited third-party laboratories routinely perform flat crush strength testing and issue test reports that are widely accepted for supplier qualification, customer compliance, and export documentation.
