A new method is introduced for measuring fracture toughness, K_IC, of materials ranging from metallic alloys to brittle ceramics. A valid value of K_IC is determined using a round-rod specimen having a grooved spiral line with a 45° pitch. When this uniquely configured specimen is subjected to pure torsion, an equibiaxial tensile/compressive stress-strain state is created to effectively simulate that of conventional test methods using a compact-type specimen with a thickness equivalent to the full length of the spiral line. K_IC values are estimated from the fracture load and crack length with the aid of a 3-D finite element analysis. K_IC of a mullite ceramic material yields 2.205 MPa√m, which is 0.2% higher than the vendor's data. K_IC of A302B steel is estimated to be 55.8 MPa √m, which shows higher than compact tension (CT) test value by~2%. 7475-T7351 aluminum yields 51.3 MPa√m, which is higher than vendor's value in the TL orientation by -0.8% and higher than 0.5T-CT value by 6%. Good agreement between the K_IC values obtained by different methods indicates the proposed method is theoretically sound and experimental results are reliable.