i measured the thickness of the copper filament at 0.101mm, and the width at 1.97mm. This cross sectional area comes to that of about a number 24 AWG copper wire, which has a fusing current of about 29 Amps. This can be seen on the datasheet for the 20A rated fuse.
But on the fuse drawing it shows that the filament is actually a perforated copper strip and not completely solid--there are punched out sections to thin the filament. So the resistance is given as 4.2mOhms, when i calculated it would be ~2mOhms for a solid strip.
i saw evidence of the perforations when i first dismantled the fuse--there were 2 parallel sand tubes at one of the break joints. But it fell apart before i could get a photo.
The soldered filament into the end caps can be viewed as a fixed-fixed beam clamped at both ends for mechanical vibration analysis.
This leads me to believe even more that vibration can result in fatigue cracks in the filament, especially in the thinned out sections.
So i think that the fuse mechanical lifetime in our cars is about 5-6 years or so, although a couple of folks reported really early failures which could be infant mortality, a manufacturing tolerance factor.
Maybe this should be a maintenance item--just replace it after 3 years to be safe.
i'm gonna buy some 40 Amp replacement fuses. If someone wants to swap out their fuse before it breaks, i would like to dissect a used older fuse to see if there are any fatigue cracks beginning to grow.