Effect of Heat & Stress on String Tension

While much have been said about how heat causes tension loss in synthetic gut, I have not read nor heard much about how heat affects poly strings.

So I did a simple experiment comparing poly with synthetic gut...

Objectives
(1) To test effect of heat on string tension, and
(2) To test effect of impact stress on string tension.

Methodology
- A piece each of new synthetic gut and poly string was manually pre-stretched for 120 seconds, then tensioned thrice at 50 lbs in the 6th and 7th main in a tennis racket, then tied-off.
- The pre-stretching and triple tensioning was to reduce tension loss through string relaxation.
- Tie-off tension loss would be very significant but not of interest in this study.
- Each impact stress was simulated by using a ruler to measure and depress each string about 1 cm deep, thrice. 
- Note that when each piece (ie 6th main or 7th main) of the same string type is depressed, sometimes the tension differential would change between these two string. So the average is taken for greater accuracy.
- For heat stress, a hairdryer was held 1 cm away from the strings and moved slowly along the string for a total of 1 minute each time. The temperature was constant at about 40 degrees Celsius, which was well below the temperature in a parked car in the sun. (link1)(link2)(link3)
- All tension loss measurements were done through string frequency in Hertz (Hz).

Findings

Remember, the strings were both heavily pre-stretched and tensioned thrice. Despite that, what stood out was, from the total tension loss:
(1) poly lost less through string relaxation compared to syn gut,
(2) syn gut lost almost nothing through impact stress whereas poly suffered badly,
(3) heat affects syn gut twice as badly as poly, and
(4) as long as kept away from heat, syn gut's tension holding is impeccable whereas poly must be restrung very often for hard-hitters.

From my previous 10 full syn gut and 10 full poly string jobs, I felt the strings became unplayable (dead, harsh, no spin, poor control, etc), when the tension loss (in hz) reached about 9% for syn gut and about 6% for poly.

Based on the above, poly became unplayable to me after the 1st impact stress. Syn gut "died" after 2 mere minutes of heat stress.

Keep strung rackets away from heat if possible and restring poly very very often.