Testing Machines and Systems for Metals - Zwick - #9

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9 Fig. 1: CT specimen in Vibrophore (HFP) Fig. 2: SENB specimen in Vibrophore (HFP) Fracture Mechanics KIc Determination Fracture toughness KIc is an important material property for metallic materials in safety-related applications such as aircraft construction, power-station building and automobile manufacturing and is determined using a specimen into which an artificial crack has been introduced. The specimen is loaded until failure and the fracture toughness KIc is determined from the load-deformation curve and the crack length. Details of the test procedure are contained in the relevant standard (ASTM E 399) and the two-stage test can be performed very efficiently using Zwick Vibrophores (HFP). Crack formation in the specimen is instigated by the mechanically produced notch followed by cyclic loading. The high frequency used allows rapid generation of a defined crack (‘precracking’) and the process is highly reproducible, thanks to the high sensitivity of the resonant frequency to crack formation. The specimen geometry most frequently used is illustrated in Fig. 1; the specimen is known as a CT (Compact Tension) specimen. The load is applied through pins inserted into holes in the specimen, giving a mixed tensile and flexure loading. Flexure specimens (known as SENB specimens, Fig. 2) are also used. While the testing method is simpler for the flexure specimen than the CT specimen, the required specimen volume is significantly greater. This is clearly shown in the illustrations.