valid for elastic-plastic fracture mechanics (EPFM) Determination of plane strain fracture toughness When a material behaves in a linear elastic way prior to failure, such that the plastic zone is small compared to the specimen dimension, a critical value of Mode-I stress intensity factor can be an appropriate fracture parameter. This method provides a quantitative measure of fracture toughness in terms of the critical plane strain stress intensity factor. The test must be validated once complete to ensure the results are meaningful. The specimen size is fixed, and must be large enough to ensure plane strain conditions at the crack tip. The specimen thickness affects the degree of constraint at the crack tip which in turn affects the fracture toughness value Fracture toughness decreases with increasing specimen size until a plateau is reached. Specimen size requirements in ASTM E 399 are intended to ensure that measurements correspond to the plane strain plateau by ensuring that the specimen fractures under nominally linear elastic conditions. That is, the plastic zone must be small compared to the specimen cross section. Four specimen configurations are permitted by the current version of E 399: the compact, SE(B), arc-shaped, and disk-shaped specimens. Specimens for tests are usually fabricated with the width equal to twice the thickness . They are fatigue pre-cracked so that the crack length/width ratio () lies between 0.45 and 0.55. Thus, the specimen design is such that all of the key dimensions, , , and −, are approximately equal. This design results in the efficient use of material, since the standard requires that each of these dimensions must be large compared to the plastic zone. Plane-strain fracture toughness testing When performing a fracture toughness test, the most common test specimen configurations are the single edge notch bend (SENB or three-point bend), and the compact tension (CT) specimens. Testing has shown that plane-strain conditions generally prevail when: where is the minimum necessary thickness, the fracture toughness of the material and is the material yield strength. The test is performed by loading steadily at a rate such that KI increases from 0.55 to 2.75 (MPa)/s. During the test, the load and the crack mouth opening displacement (CMOD) is recorded and the test is continued till the maximum load is reached. The critical load <PQ is calculated through from the load vs CMOD plot. A provisional toughness KQ is given as . The geometry factor is