Whether it’s furniture foam, shoe foam, automotive foam, electronic foam, cleaning foam, or bra foam, various physical properties such as hardness, resilience, tensile strength, elongation, and tear strength are commonly tested. Below is a summary of the definitions and standard testing methods for these properties for reference.
Hardness
Hardness is generally categorized into surface hardness and indentation hardness:
Surface Hardness: For foam materials, Shore C or Shore F durometers are commonly used to measure surface hardness directly on the foam sample.
Indentation Hardness: Following national standards, a foam sample with a uniform thickness of typically 50mm (without surface skin) is placed on an indentation hardness testing machine. The foam sample is usually circular or rectangular, with a surface area smaller than the pressure plate. A 200mm-diameter circular plate is often used. The force required to compress the foam to a specified percentage of its original thickness is recorded as the indentation hardness (measured in N). Additionally, compression strength is calculated by dividing the indentation stress by the area of the pressure plate.
Tensile Strength
Tensile strength refers to the maximum tensile stress a material can withstand per unit area before breaking.
For testing, a dumbbell-shaped foam sample is used. The sample must be intact and undamaged. The maximum force applied at break (Fb) divided by the original cross-sectional area of the sample (S) yields the tensile strength, with the result expressed in N/(MPa).
Elongation
Elongation is defined as the ratio of the increase in the distance between gauge marks on the sample at break to the initial gauge length.
This is relatively straightforward: for instance, a foam sample with an initial undamaged length of 1 meter is stretched until it breaks. The length increase is then divided by the original length (1 meter) and multiplied by 100% to calculate the elongation.
Tear Strength
Tear strength measures the force required to propagate a tear in a sample of specified dimensions. A notched sample is clamped in the testing machine and stretched at a constant speed. The maximum tearing force recorded after the sample tears over 50mm is divided by the sample thickness to calculate the tear strength.
Resilience
Resilience typically refers to ball rebound resilience. A steel ball of a specified weight is dropped from a predetermined height onto a foam sample. The rebound height is measured and expressed as a percentage of the drop height. For example, if the ball drops from 1 meter and bounces back to 0.6 meters, the rebound resilience is 60%.
