Intimate Apparel
The channel does not fail on the first wearing. It fails on the four hundredth, which is why the static puncture figure is the wrong one to design against.
— N
What the channel has left against repeated wear
A negative fatigue margin means poke-through is a matter of time rather than chance. The fatigue fraction depends on the channel construction, the wire tip finish and how the garment is laundered — fit it from returned garments rather than assuming, and remember a sharp or burred wire end concentrates stress far beyond what the tip radius suggests.
Bra Underwire Channel Tension & Puncture Margin Calculator — free, with the formula and a worked example, at Textile School.