The critical stress intensity factor K1C describes the resistance of a material to crack extension. The stress intensity factor is also referred to as fracture strength. ASTM E399 describes the determination of the material characteristic value of fracture mechanics under cyclic loading with constant amplitude.
The crack growth of a material is described in the crack growth curve. This curve is divided into three regions:
- Region I: low crack growth rate; threshold value ΔKth value at which crack growth is just beginning
- Region II: constant crack growth rate; is mathematically described with the Paris curve, fatigue crack growth da/dN
- Region III: high crack growth rate; ends with fracture, critical stress intensity factor K1C
ASTM E399 for determination of the critical stress intensity factor K1C addresses region III of the crack growth curve.
K1C determination is normally performed on brittle materials. A defined crack is first created in the specimen by pre-cracking it in accordance with ASTM E399. At 2.5% before reaching the defined crack length, the stress intensity is reduced.
In the next step, the specimen is pulled at a constant rate until it breaks and the KQ value is reached. After the test, the determined KQ value is set in relation to the specimen width, crack length and the offset yield of the material. If this ratio meets the minimum validity criterion specified in the standard, KQ is declared as a valid K1C value.
The crack growth is determined with a suitable crack opening displacement extensometer and the calculated compliance method.
In addition to the very common compact tension (CT) specimens, bending specimens (SEB) can also be used to determine the critical stress intensity K1C.