A defect in soft materials expands rapidly under a critical hydrostatic tension, causing cavitation or fracture. The mechanical properties of the material and the size of the defect have decisive influence on the failure mechanism. As different modes of material failure, the transition between cavitation and fracture is still elusive. Combining the Griffith's fracture criterion and finite deformation theory, we hereby obtained the critical initial defect size of cavitation/fracture transition. In the experiments, a cavitation rheology device was built to measure the failure mode of polyacrylamide hydrogels by inserting needles with various sizes. By observing the surface roughness and measuring critical expansion pressure of the bubble, we verified that the transition size is controlled by fractocohesive length Γ / W f , which can be orders of magnitude smaller than elasto-adhesive length Γ / E . The present work reveals the transition mechanism between cavitation and fracture, quantifies the transition size, and provides a promising route to predict the failure behaviors of highly stretchable materials.
In this paper, a systematic study on the effect of flaws on flexural strength of ultra-high temperature ceramics (UHTCs) was carried out by using three typical materials (i.e., ZrB 2 , ZrB 2 -SiC and ZrB 2 -SiC-G ceramics) containing micro-sized flaws with controllable sizes, shapes as well as orientations. Depending on the flaw sizes and orientations, different fracture modes were analyzed by SEM observation and theoretical calculation. Based on Emmerich’s hole/strength model and the maximum strain-energy release-rate mixed-mode fracture criterion, a universal flaw/strength model was developed to describe the flexural strength of flawed UHTCs after taking both material properties and flaw’s geometries into account, and compared well with the experimental data in both mode I and mixed-mode conditions. Prediction results of the proposed model have further verified that substantially consistent with the flexural strength reports of UHTCs in the literature as well as the theoretical strengths calculated from the elastic modulus, which means that the proposed model has a great potential in guiding the design of high-strength ceramics in full-scale (from atomic-scale to macro-scale).
Critical flaw size of structural ceramics is generally less than 100 μm and is therefore too small to be reliably evaluated. In this study, micro-sized flaws (as small as 10 µm) were introduced via femtosecond laser for the first time, and the critical flaw sizes of ZrB 2 and ZrB 2 -SiC-Graphite ceramics (12.9 µm and 36.1 µm, respectively) were accurately evaluated by linear extrapolation in double logarithmic coordinates. The critical flaw sizes calculated from the indentation fracture toughness (14.3 µm and 34.3 µm for ZrB 2 and ZrB 2 -SiC-Graphite ceramics, respectively) agreed well with the experimental results, while significant overestimation would occur by using fracture toughness values measured by means of single-edge notched beam, single-edge V-notched beam and single-edge precracked beam methods, which was mainly due to the fact that the notches in these three tests had the depth scale nearly two orders of magnitude higher than those of the inherent flaws.
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