Weak links work harden with use, which results in a well used one having a higher breaking strain than a fresh one. Its not unusual to have a weak link last months, only to then have two or three break replacements in quick succession, until one survives long enough to get enough launches to also be work hardened.
Something lacking in the design department here. If they survive & become work hardened they are no longer the "weak" link they are supposed to be. How could one be re-calibrated to the next strength up? Prob impossible without breaking it to find out.
If weak links are designed like this then they are worthless, dangerous, chance of ripping the tow hook out of the glider. I would suggest some thought be put into them.
I believe they are just a short length of steel plate of varying thickness to suit the weight required. Bad idea.