Buckling Stress and Wrinkle Testing in Materials, Especially Cardboard and Corrugated Cardboard

Buckling stress refers to the critical compressive stress at which a structural element (such as a column, plate, shell, or panel) suddenly loses its stability and deforms (buckles) laterally instead of continuing to compress axially. This failure mode frequently occurs at stress levels well below the material's yield or compressive strength, particularly in thin or thin-walled structures.

In thin sheets or panels (commonly used in packaging materials), this involves out-of-plane deflection under buckling, compression, bending, or combined loads.

The crease test evaluates a material's (especially cardboard or corrugated cardboard) ability to form a controlled, durable fold line without cracking or excessive damage. Crease creates a deliberate line of weakness for folding in packaging (e.g., boxes, cartons). The main tests are:

• Measuring wrinkle stiffness or recovery force (e.g., the force at which a 90-degree wrinkle or a 15-degree bend is held).
• “BS 6965-1 Folding properties of cardboard sheets - Method for determining the fold recovery of a 90-degree fold”.

Wrinkles create localized weaknesses, geometric defects, or reduced stiffness in compressed panels or boxes that can trigger or contribute to buckling.

In corrugated cardboard packaging, wrinkles form on the edges and lids of boxes (e.g., regular corrugated containers). These wrinkles act as initial defects or hinge lines, reducing the critical buckling load of the side panels under top-down compression.

Studies conducted on corrugated cardboard panels have shown the following:

• The panels buckle in the post-elastic range due to orthotropic properties (different stiffness in the mechanical and transverse directions).
• Wrinkles create uncertainties in load-carrying capacity by reducing local bending stiffness and promoting earlier buckling or post-buckling deflection.
• Particularly on the concave side during bending or compression, local buckling of the linings (surfaces) usually occurs before global failure.

Experimental panel compression tests reveal load-displacement curves exhibiting explicit buckling behavior: initially linear response, followed by nonlinear post-buckling behavior up to material failure.

Analog (traditional die-based) and digital folding methods affect the results:

• Digital folding can alter edge crush strength, shear rigidity, bending rigidity, and overall box compression strength.
• Additional or poorly constructed folds promote buckling, significantly reducing compressive strength.

In a detailed study conducted on corrugated panels:

• Analytical models estimate the critical buckling load at approximately 958 N, while experiments show approximately 814 N (due to defects such as folding and nonlinear paper behavior).
• The behavior after buckling is adapted to the curves, and failure usually results from the inner lining stress reaching its limits.

Folding quality is also linked to layer separation during folding, allowing for layered buckling-like behavior in the folding region.

Common tests that link creases and wrinkles in packaging include:

• Edge crush test: Measures compressive strength perpendicular to the grooves; it is affected by wrinkles.
• Box compression test: Evaluates full box buckling under load; creases reduce capacity.
• Panel flexion tests: These tests compress simply supported, grooved panels to examine the effects of sprains and wrinkles.
• Wrinkle recovery/stiffness tests: Measure folding quality and indirectly affect buckling resistance.

Buckling/wrinkle interactions are also important in automotive coatings. For example, buckling stress/wrinkle testing according to some standards such as the “EN 4214 Aviation series - Aluminum alloy AL-P7010-T651“ standard variants for adhesion under deformation.

In general, understanding buckling stress helps predict packaging performance, while wrinkle testing ensures reliable folding. Poor wrinkling accelerates buckling failure in real-world stacking/handling and reduces box strength by promoting panel deflection or local liner imbalance.

Buckling stress testing is extremely important because buckling is a very dangerous and insidious type of injury.

• It happens suddenly.
• This occurs at a stress much lower than the material's normal strength (yield strength / crushing strength).
• Once buckling begins, the structure loses almost all of its load-bearing capacity very quickly.
• Even a very small additional load after buckling can lead to large, unpredictable deformation.

The most important reasons for performing sprain tests are as follows:

• Safety prevents catastrophic sudden collapses. If ignored, the real-world consequences are buildings, bridges, towers, cranes, and shelving collapsing. Typical areas where it is critical are civil and structural engineering.
• Failure occurs much earlier than expected. If ignored, the real-world consequences are that the column breaks at 20-40% of its material strength. Typical areas where it is critical are columns, supports, thin walls, and panels.
• Normal stress analysis cannot detect buckling. If ignored, the real-world consequences—linear static analysis showing very little deviation—will still occur. Typical areas where it is critical are virtually any thin compression element.
• There are very few warning signs before failure. If ignored, the real-world consequences are that there is no major permanent deformation before collapse. Typical areas where it is critical are aerospace, automotive chassis, and storage racks.
• It is extremely important for thin, long, and tall structures. If ignored, the real-world consequences are that the longer, thinner, and taller the structure, the more dangerous it becomes. Typical areas where it is critical include towers, masts, long booms, thin shells, and plates.
• It is crucial in packaging (especially boxes). If ignored, the real-world consequences are that boxes collapse during stacking and products are damaged. Typical areas where it is critical include corrugated cardboard boxes, pallet stacking, and shelf-ready packaging.
• It saves a significant amount of money. Ignoring it would have real-world consequences, preventing both over-design and under-design. Typical areas where it is critical are all sectors (aviation, automotive, packaging, construction).

For many years, our organization, with its competent and experienced team, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide buckling stress and bending testing services to businesses.