To accurately attain K c , the value of c must be precisely measured. This is done by taking the test piece with its fabricated notch of length c ′ ′ and sharpening this notch to better emulate a crack tip found in real-world materials. Cyclical prestressing the sample can then induce a fatigue crack which extends the crack from the fabricated notch length of c ′ ′ to c . This value c is used in the above equations for determining K c .