Stress intensity factor in silica alcogels and aerogels

The stress intensity factor (K-IC) of alcogel and aerogel sets is measured by the single edge notched beam (SENB) technique. Sets of gels are made at various tetramethoxysilane (TMOS) content and gelation catalyst (neutral and basic catalyst). It was found that the toughness increases with the TMOS content and is strongly enhanced by the supercritical drying (SD) step for the neutral set. The K-IC increase is associated to a higher load bearing and to the formation of new links during SD. The critical flaw size is calculated from K-IC and fracture stress. It is shown that the critical flaw size changes with the parameters TMOS%, catalyst and SD. This change can be correlated to the macropore volume evolution which suggests that macropores can be considered as responsible for the failure. (C) 2000 Elsevier Science B.V. All rights reserved.