Connecting the macro and microstrain responses in technical porous ceramics. Part II: microcracking

Impact of the non-linear character of the compressive stress–strain curves on thermal and mechanical properties of porous microcracked ceramics

“…However, upon stress hold we observe a relaxation towards tensile strains. Upon unloading, the behavior is linear (closed microcracks do not slide back immediately, and stiffen the material [20,21]), but upon application of a second cycle, the axial modulus slightly decreases, indicating that some permanent damage has been introduced (please note that the unloading branch of cycle 2 could not be recorded). Further cycling decreases the axial modulus, but it basically remains within a factor of 0.9 lower than the stiffness of the baseline sample.…”

“…Recently, the non-linear stress-strain behavior of porous microcracked monolithic ceramics has also been investigated in the case of honeycomb porous ceramics [19][20][21] for filtration applications, dealt with in the present work. In particular, Pozdnyakova et al 19 have found that different materials display different dependences of the axial modulus on applied compressive stress E(σ).…”

“…Bruno and Kachanov 21 have modeled the uniaxial compressive test on the same kind of ceramics in terms of microcrack densities of open and closed sliding microcracks. The abovementioned effects (closure, sliding, propagation) have been shown not to be systematic for every composition and porosity value, 20 but the most important outcome of these studies is that the values of Young's modulus generated by RUS 22 and sonic resonance 23 are valid only at low applied loads. This statement has direct consequences if we think that an increase of the Young's modulus is commonly associated to that of the strength.…”

Temperature dependence of Young׳s modulus of silica refractories

“…It should be mentioned that "hysteresis loops" of Young's moduli have been reported for Al-and Mg-silicates several times [28][29][30] and have usually been explained by microcrack closure and opening. It is commonly acknowledged that the precondition for microcracking to occur is a certain minimum grain size [31] and anisotropic thermal expansion of the grains (crystallites).…”

“…However, since there is no indication of irreversible shrinkage or swelling, and the porosity is the same after cooling as before heating, these microstructural changes can only consist in additional microcracking, i.e. microcracking that goes beyond the reopening of pre-existing microcracks during cooling, which is the commonly accepted explanation for the "hysteresis loop" between the heating and cooling branches [28][29][30]. In fact, it is well known that the cooling of kilns lined with silica bricks to below the high-low transition temperature of cristobalite induces tensile stresses that cause cracking and spalling of the silica refractories [3].…”