Patent ID: 7487051

Claim:
A method of evaluating the fracture toughness of a material using a continuous indentation technique, said continuous indentation technique continuously measuring an indentation load and an indentation depth while a continuous indentation test with an indenter repeatedly applies the indentation load to and removes the indentation load from the material and then evaluates properties of the material using a relationship between the indentation load and the indentation depth, the method comprising the steps of: measuring a stress coefficient, a strain hardening modulus and a yield stress using the continuous indentation technique and calculating a reduced elastic modulus (E r ) of the material; calculating an effective elastic modulus (E eff ) from the reduced elastic modulus; determining an initial elastic modulus (E 0 ) from an elastic modulus in a first range of the effective elastic modulus, which was determined using a plurality of unloading curves obtained using the continuous indentation technique; setting a void volume fraction (f) to a specific value within a range from 0.13 to 0.17 when a fracture type of the material is a I-type fracture type, and setting the void volume fraction (f) to a specific value within a range from 0.23 to 0.27 when the fracture type of the material is a II-type fracture type; calculating a damage parameter (D) from the void volume fraction using an equation D = π ( 4 3 ⁢ π ) 2 3 ⁢ f 2 3 ; calculating a critical elastic modulus (E*) using the damage parameter; calculating a characteristic fracture initiation point (h*) of the indentation depth using the critical elastic modulus; and calculating the fracture toughness (K JC ) of the material using the characteristic fracture initiation point of the indentation depth using an equation K JC = 2 ⁢ ⁢ E 0 ⁢ ω f = E 0 ⁢ lim h -> h * ⁢ ∫ 0 h ⁢ F A C ⁢ ⅆ h = E 0 π ⁢ k ′ ⁢ ln ⁡ ( 2 ⁢ ⁢ R 2 ⁢ R - h * ) , wherein 2ω f is a critical indentation energy, A C is a contact surface area between the material and the indenter, F is the indentation load, R is a radius of the indenter of the continuous indentation tester, and k′ represents a slope (L max /h max ) between a maximum indentation load and a maximum indentation depth in an unloading curve.