To put the question more accurately I consider the Saltwater Crocodile, which has been measured to have a bite force of 16.4 kN and estimates of up to 34 kN in 6.7-meters long individuals.
For turtles I consider the following species:
- Common land tortoise: Testudo graeca
- Common terrapin: Mauremys rivulata
- Green sea turtle: Chelonia mydas
- Leatherback sea turtle: Dermochelys coriacea
- Galapagos turtle: Chelonoidis nigra
Though not all species share the same geographical range as the saltwater crocodile they do represent different thicknesses and robustness of shells.
Note also that the giant extinct crocodylian Deinosuchus did crack open the shells of marine turtles, as it regularly preyed upon them. However, Deinosuchus' bite force is estimated up to 102 kN by Gregory Erickson.