Knit Testing
Burst force alone says a knit is strong. The distension tells you whether it got there by being tough or just by stretching a long way first.
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Multi-directional extension recovered from the dome geometry
The dome is treated as a spherical cap, which is the standard idealisation; a real knit thins unevenly and the ball contact area grows as it penetrates, so the strain figure is a construction comparison rather than a material property. Stretch fabrics with high elastane will show large distension at modest force — read both numbers together, never one alone.
Ball Burst Strength & Deformation Modeler (ASTM D6797) — free, with the formula and a worked example, at Textile School.