The following discussion on the fracture test procedure is partly adapted from the book by Dieter [2]. Different types of specimens are used for measuring KIc (Figure 5.24). The compact test specimen (CT) is quite popular. After the notch is machined into the specimen, the sharpest possible crack is produced at the notch root by fatiguing specimen in low cycle, high-strain mode (typically 1000 cycles). Plane strain fracture toughness is quite unusual in that there is no advance assur­ance that a valid KIc can be measured in a particular test. Equation (5.33) is used with an estimate of the expected KIc to determine the specimen thickness required for plane strain loading condition.

>r

P

Figure5.24 Three specimen designs (CT, three-bend, and notched round specimen) for Kc measurement. From Ref. [2].

The test should be carried out in a testing machine that provides for a con­tinuous autographic record of load (P) and relative displacement across the open end of the notch (proportional to crack displacement). The three types (type-1, type-II, and type-III) of load-crack displacement curves that can be obtained depending on the type of material are shown in Figure 5.25. The ASTM procedure requires to first draw a secant line OPs from the origin with a slope that is 5% less than tangent OA. This determines the Ps value. Now draw a horizontal line at a load equal to 80% of Ps and measure the distance along this line from the tangent OA to the actual curve. If the value of x1 exceeds one-fourth of the corresponding distance xs at Ps, the material is too ductile to obtain a valid KIc. If the material is not too ductile, the load Ps is then designated as Pq and used in the calculation.

The value of PQ determined from the load-displacement curve is used to calcu­late a conditional value of fracture toughness denoted by Kq using the following equation (for CTspecimen):

The crack length (a) used in this equation is measured after fracture. Then, calculate the factor 2.5(Kq/ct0)2. If this quantity is less than both the thickness and the crack length of the specimen, then Kq is actually KIC. Otherwise, it is necessary to use a thicker specimen to determine KIc. The measured value of Kq can be used to estimate the new specimen thickness through Eq. (5.33). Table 5.2 shows fracture toughness and tensile strength values of three engineering alloys. It demonstrates that maraging steel that has the highest plain strain fracture toughness would be able to tolerate crack for a given applied stress or with an existing crack of known length and will withstand the highest stress.

Example Problem

A tensile stress of 300 MPa acts on a sheet sample of an alloy (FCC: E = 200 GPa, oy = 350 MPa, KIC = 40 MPa m1/2) with a central crack of length 5 mm. Is the sam­ple going to fail in a brittle manner?

Solution

We find KI for the case and compare it with KIC.

KI = oppa = 300/p x 2.5 x 10~3 = 26.59 MPam1/2 taking Y = 1 and this is less than KIC = 40 MPa m1/2, meaning the sample does not fail in a brittle manner.

5.1.6