Crack-tip opening displacement (CTOD) measurements were made 100 üm behind the crack tip on standard three-point bend specimens using a laser-based interferometric technique. Four materials (SAE-01 steel, 4340 steel, HY-100 steel, and tungsten) were tested at loading rates ranging from quasi-static (K ≈ 1 MPa∙ m½/s) to dynamic (K ≈ 106 MPa ∙ m½/s).
The quasi-static and intermediate rate tests and analyses were conducted according to the guidelines specified in ASTM Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399). In these lower rate tests, load was applied by a servohydraulic testing machine and a clip gage was used to record the crack-mouth opening displacement (CMOD). From these data, the stress intensity factor, K, and the fracture toughness can easily be determined from the standard methodology.
In the high rate tests, inertial effects are prevalent and K is not as easily measured. The initial measurements of CTOD, however, can be used to provide a calibration between the static and dynamic tests if K is assumed to increase linearly with time during the 10 to 15 μs before the crack starts to propagate. If this assumption holds, dynamic time-CTOD results can be converted to K-CTOD curves using a companion specimen technique. Further, the dynamic fracture toughness can then be found by superimposing static and dynamic K-CTOD results and employing a reduced slope technique to locate the onset of crack propagation.
This dynamic technique has been modeled after existing practices defined in ASTM Test E 399 and leads to experimental scatter comparable to that found in static fracture-toughness tests. The results show that there is no dramatic change in the fracture toughness over this range of loading rates for any of the materials studied.