Fracture Toughness of 6.4 Mm (0.25 Inch) Arc-Cast Molobdenum and Molybdenum-Tzm Plate at Room Temperature and 300 Degrees C Page: 4 of 11
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consistently between 177 and 254 m (7 and 10 mils) in width and 10 mm (0.400 inches) past the load
line of the sample. The surface finish was according to ASTM E-399.
Cyclic Pre-cracking: A cyclic compressive load was used for initiating the pre-crack (3). Benefits of
this method include pre-crack stability, symmetry and low risk of fracture for brittle specimens.
Specimens were cycled 2500 times and visually inspected before the maximum load was increased.
A typical loading sequence was 2500 cycles (20 to 40Hz, R=0.1) at 455 kg (1000 Ibs) maximum load,
followed by 2500 cycles at 909 kg (2000 Ibs) maximum load etc. Cycling was continued in this
manner until a crack was observed from some point at the notch root. If a crack was visible and
growing, no further increase in load was applied until crack advance stopped. Using this method,
specimens were pre-cracked at maximum loads less than 4545 kg (10,000 Ibs) and total cycle count
below 40,000.
Test Method: Room temperature toughness testing followed ASTM E-399 (7) guidelines wherever
possible. Even though the pre-cracking method described above does not meet E-399 requirements,
the testing yielded meaningful results by following the other guidelines as highlighted below:
* Loading rate 0.55 to 2.75 MPa m'/sec (30 to 150 ksi in'/min).
* Pre-crack length must be a minimum of 1.2 mm (50 mils) past the notch root.
* Pre-crack must be within 100 of the notch plane.
* Surface crack lengths between 85% and 115% of the internal average.
* Maximum load at failure no more than 110% of that used for K1c calculation.
From a sample thickness standpoint, E-399 requires the specimen thickness to be greater than 2.5
times (K/ ay)2. Only one of the room temperature specimens, as noted in the results, showed enough
plasticity that the required specimen thickness should have exceeded the 6.4mm (0.25 inch) plate
thickness. As temperature and toughness increase, and yield strength decreases, it would not be
unusual for certain materials to require very thick (75 mm / 3 inch) samples for valid K tests using E-
399. Because this is impractical for most cases of testing, other toughness test methods have been
developed. The ASTM E-1820 (7) procedure applies to materials tested under these conditions. A
quantity known as J is defined based on crack length, sample geometry and plastic energy absorbed
during testing. Like the quantity K described above, J is also a measure of the energy required to
create an additional unit of crack advance.
The J test induces either unstable crack extension or stable crack extension (tearing) in the sample.
Instability results in a single-point value of toughness, while stable tearing results in a continuous
fracture toughness versus crack-extension relationship (R-curve) from which significant point values
may be determined. 71 his method requires continuous measurement of load versus load-line
displacement crack mouth opening as tearing occurs. In the work presented here the load line
opening is not measured; rather crack opening is measured on the edge of the sample as in ASTM E-
399. The J method can be used to determine toughness with a single sample if high-resolution crack
opening measurements can be made using a compliance unloading technique to determine in-situ
crack lengths. An example of the load versus crack opening displacement measurements and derived
R curve that were used to calculate the J values is shown in Figure 3. The values of K1c are then
calculated from the measured J values for comparison to the room temperature values. A full
description of these test methods and surrounding calculations is determined in ASTM E-1829 (7).
Although not all ASTM validity requirements were strictly met, the values obtained in this investigation
are a good first approximation of the fracture toughness of molybdenum alloys.
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Shields, J. A., Jr.; Lipetzky, P. & Mueller, A. J. Fracture Toughness of 6.4 Mm (0.25 Inch) Arc-Cast Molobdenum and Molybdenum-Tzm Plate at Room Temperature and 300 Degrees C, article, April 11, 2001; West Mifflin, Pennsylvania. (https://digital.library.unt.edu/ark:/67531/metadc743116/m1/4/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.