FRACTURE TEST DEVELOPMENTS TO ADDRESS TESTING CONCERNS

As more refractories development included the fracture testing of refractories, it became evident that there were problems with obtaining representative results for the usual laboratory-scale test specimens. This was quite obvious with work-of-fracture specimens, as the scatter of individual results from multiple tests on the same refractory body yielded widely ranging results with unacceptably large con-fidence intervals. It was not difficult to understand the reason for this problem, as a simple visual examination of the fracture surfaces related the test results to the character of the fracture surfaces. Coarse aggregates were observed to be very important in the fracture process, often causing very large work-of-fracture values when the aggregate was near to the apex of the chevron, or remaining tri-angle ligament. It became evident that the coarse aggregate microstructure of

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refractories would not allow for a representative sampling of the refractory for the small laboratory-scale specimens. Undoubtedly, one of the reasons was that it did not allow for complete development of the crack process zone region. Neither the microcracking and crack branching in front of the advancing crack, nor the brid-ging phenomena in the wake of the crack system, reached a steady-state configur-ation. A much larger test specimen was clearly in order to properly assess the coarse aggregate refractory structures. Of course, entire bricks, some a meter in dimension, are available as potential test specimens for many types of refractories.

Researchers at VRD in Leoben, Austria, have promoted the wedge-split-ting fracture test where much larger specimens are possible and better specimen averaging and a much greater degree of crack process zone development are possible (23, 24). Although this test has not presently received widespread acceptance, nor has it been exploited to its fullest, it has the promise to provide considerable additional insight into the refractory fracture process. Unfortu-nately, several researchers using the wedge-splitting specimen test have chosen to redefine some of the standard fracture terminology and have created con-fusion for those without familiarity with the field. Systematic, orderly tests with this technique for different refractory microstructure variations are in order, and as a function of temperature to further elucidate the important fracture mechanisms.

The wedge-splitting test has some features in common with the previously noted J-integral compliance method. It merits further description for those who may wish to employ it for future refractory fracture testing. Figure 2 illustrates a schematic of the test specimen after Buchebner and Hartmut (24). It is evident that the name of the test, wedge-splitting, is quite a representative description. It has an elaborate fixture arrangement within a starter notch configuration and two side notches to restrict crack wandering during the actual test. It is loaded in compression and is able to generate fracture parameters from analysis of the load-displacement curve. Because of the specimen size and the large fracture sur-face area, there is generally no difficulty in obtaining fully stable fractures for refractories, and thus a reliable record of the total energy for crack propagation through the specimen is easily obtained with a stiff testing machine.

Figure 3 illustrates typical load-displacement curves for wedge-splitting refractory specimens. Depicted is a refractory of the “brittle” variety (perhaps a high fired superduty fireclay) and a “tough,” or more energy-consuming, variety (such as a highly microcracked 70% alumina). When the load-displacement curve is integrated, analogous to Eq. (8), it is possible to obtain the total work-of-fracture. Some researchers using this technique have defined it as the specific fracture surface energy and used a G_Finstead of the total work-of-fracture, 2gwof. This may be a convenient nomenclature if it is intended to later relate this result to the strain energy release rate, G_C, or perhaps even a form of some variant of J_C,

but the experimental measurement is clearly that of the work-of-fracture as speci-fied originally by Nakayama (7).

As the load-displacement curves are typically stable ones for the wedge-splitting specimens, they exhibit an initial linear elastic region and a moderate nonlinear region, which is followed by a maximum load value, similar to the first of the diagrams in Figure 1 for the J-integral, then a long tail of decreasing load with increasing displacement. It is also possible to use the maximum of this load-displacement curve for additional information, although the value of this Figure 2 The wedge-splitting test specimen and configuration for fracture measure-ments. (From Ref. 24.)

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information remains to be fundamentally demonstrated. Some researchers have suggested that the integration of the load-displacement curve to the peak as a measure of the strain energy release rate, G_C, is

GC¼

ðP maxP du

2A , (9)

where the integral is taken to the maximum load, P_max, and A is the remaining ligament or the eventual fracture surface area. This approach would have more merit if all test specimens in the universe were the same size. However, it should be recalled that a similar type approach is utilized in the J-integral compliance technique where the stored elastic strain energy in the test specimen at the maxi-mum load is subtracted to specifically identify the crack process zone energy. As the stored elastic strain energy will vary with the test specimen size and the par-ticular refractory material, it must be accounted for independently from the observed area under the load-displacement curve. This suggests that simply attri-buting the maximum load to a G_Cquantity is not fundamentally correct. How-Figure 3 Schematic load-displacement curves for refractories in the wedge-splitting test.

ever, the G_C-value determined in this way may be a satisfactory approximation for development purposes within a single class of refractories such as mag-chrome or alumina-magnesia-carbon.

A particularly interesting and quite practical approach to utilize the above results from the wedge-splitting test is to assess the ratio of the area under the stable fracture curve from the initial loading point to the maximum, referred to as G_Cabove, and the total area of the stable load-displacement curve that yields the quantity referred to above as G_F. The ratio of the two quantities may be used to define a toughness, ductility, or flexibility ratio as

ratio ¼ G_F=GC: (10)

This ratio will have a large value, .10, for those refractories with high energy consumption during crack propagation relative to the energy required for crack initiation. This ratio is really nothing more, actually less on a fundamental basis because of the problems with GC, than the well-known ratio relating the work-of-fracture and the single-edge notched-beam fracture surface energy:

ratio ¼g_wof=g_nbt¼2Eg_wof=K_Ic², (11)

where the K_Ic-value is that determined by the single-edge cracked notched-beam test as previously noted and described by Eq. (3). The E is the elastic modulus of the refractory. When the separate issues of energy for propagation and initiation are expressed as above, it becomes quite obvious as to just why industrial experi-mentalists have attempted to obtain both K_Icandgwoffrom a single test specimen, such as the single-edge cracked notched-beam test. The use of the wedge-splitting test specimen also may eventually accomplish this goal.

It is appropriate to note that either of the two ratios expressed by Eqs. (10) and (11) is a measure of the energy requirement for crack propagation divided by a quantity related to the energy required for crack initiation. The higher this ratio, the better the thermal shock damage resistance of the refractory and the greater the ability of the refractory to adapt to thermal and mechanical strains in service. The latter is frequently because the microstructure of the refractory is able to accomodate extensive internal cracking processes and still retain its mechanical integrity. The latter indirectly relates to the extent of the long tail por-tion of the load-displacement curve, either in the original work-of-fracture test or the wedge-splitting test. As pointed out by Buchebner and Hartmut (24), this is desirable for applications such as rotary cement kilns where the original perfect roundness of the steel kiln shell is changed throughout its lifetime from natural degradation processes.

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