From Tensile Strength to Dart Impact: Why Traditional Tests Miss Real Packaging Failures

From Tensile Strength to Dart Impact: Why Traditional Tests Miss Real Packaging Failures

Many packaging engineers are comfortable with tensile, elongation, and tear tests, but still struggle with unexpected field failures. Bags may pass all traditional tests yet still crack, burst, or puncture in real logistics. The reason is that tensile tests and similar “quasi-static” measurements do not fully capture the dynamic, localized nature of impacts and drops. This is where dart impact testing fills a critical gap.

Tensile testing applies a steadily increasing load along a defined direction, typically measuring stress–strain behavior, yield strength, and elongation at break. These properties are essential for material design but do not replicate the sudden, localized load introduced by a falling object or sharp corner. Similarly, tear tests measure how a pre-cut notch propagates under controlled loading, and coefficient-of-friction tests address sliding behavior, but neither alone can quantify how a random, concentrated impact will interact with a multi-layer film.

In contrast, the falling dart impact test introduces a single, well-defined impact event. The film is clamped around a circular opening, and a rounded dart is dropped from a fixed height. The deformation is multi-axial and localized, combining tension, bending, and stretching near the point of impact. This type of loading is much closer to what happens when a pallet corner hits a bag, or a box edge presses into a film during transport. As a result, dart impact results, especially the characteristic failure mass M50, often correlate better with real-world breakage than tensile strength alone.

Relying only on tensile data can lead to misleading conclusions. For example, a film may show high tensile strength but still be brittle under impact, especially at low temperatures. Another film might have moderate tensile properties but excellent energy absorption under a dart impact due to its structure, orientation, or additives. Without impact data, these differences remain hidden, and packaging may be optimized for the wrong performance indicators.

The most robust approach is not to choose between tensile and dart impact, but to use them together. Tensile and tear tests describe the material’s baseline mechanical behavior, while dart impact tests provide a practical measure of how the film withstands sudden, localized loads. Combining these datasets allows for more informed decisions on downgauging, resin selection, and laminate design. Over time, correlating dart impact results with field performance and complaint data helps build specifications that truly protect product integrity.