introuduced on certain condition to increase the toughness.From Table,it cad see that the microcracks do have toughening effect on MDF cement.This result not only shows the method of improving the toughness of MDF cement,but also predicts the fracture concepts in ceramic may be used to describe the fracture behaviors of MDF cement,and the Griffith's theory is invalid to explain MDF cement. 19 012 19009 (a) (b) Fig.1 BEI of polished surface of two group specimens (a)group A (b)group B Table Group A Gaoup B Flextural strength (MPa) 70.8 73.5 Young's modulus (GPa) 35.3 32.8 Un-notched fracture energy (J/m2) 647 736 Strain (% 0.200 0.224 Critical stress intensity factor (a/d=0.3)(MN/m2) 3.2 4.1 Notched fracture energy (a/d=0.3) (J|m2) 176 261 The characteristics of crack propagation path were also studied by SEM. Fig.2 shows the crack propagation path of a notched specimen which have been bent to failure.From Fig.2 we can see the crack propagate almost entirely in the matrix,the crack propagation path is tortuous and the crack is always aro- und the clinker.In addition there were large branches along the main crack (Fig.2-c),there were a series of microcracks coalescence with the main fracture crack (Fig.2-b). Fig.3 shows SEM examination of fracture surface of MDF cement.There exist a great amount of microcracks on the fracture surface.This also shows that there are a lot of branches along the fracture path. 587川 。 五 · 血 宁 ， 。 · ， ， ’ 。 。 ’ 一 。 ” 。 “ 。 。 。 。 。 。 。 。 · 一 · · ， 一 一 ， 。 一 。 一