Racquet strings are at a certain tension before impact. Impact causes the string to elongate and to increase in tension as a result. Peak tension is the original tension plus the increase in tension.
Peak tensions depend on the starting tension (not strung tension, but actual tension at time of play), but given a string strung around 62 pounds, peak tensions will range from about 75 pounds for gut, 80–90 pounds for nylon, 90–100 pounds for polyester, and 100–160 pounds for aramid.
Stiffer strings tend to have higher tension increases and thus have higher peak tensions.