Initiation and Propagation of Bond Cracks as Detected by Laser Holography and Acoustic Emission
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INITIATION AND PROPAGATION OF BOND CRACKS AS DETECTED BY
LASER HOLOGRAPHY AND ACOUSTIC EMISSION ARUP MAJI* and S. P.SHAH** *Department of Civil Engineering, Northwestern University, Evanston, Illinois 60201. "**Department of Civil Engineering, Northwestern University, Evanston, Illinois 60201.
ABSTRACT Interfaces between hardened portland cement matrix and aggregates are considered relatively weak and a source of microcrack initiation. Based on optical and scanning electron microscopic studies of fractured concrete specimens, researchers have studied the role of interface in the fracture mechanism of cement composites. Acoustic Emission source location technique and laser holography provide noninvasive and nondestructive tools to study microcracking as well as the pertinent displacement field associated with cracking. In this paper, some results obtained using laser holography, speckle methods and acoustic emission source loction measurements for model concrete specimens containing circular aggregate inclusions are described. Effects of diameter of circular inclusions on mode of cracking are reported.
SCOPE The load deformation behavior of concrete is influenced by the quality of aggregate. It is well known that interface cracking is the primary cause of nonlinearity in Concrete [Ref. 1]. Initiation of bond cracks and subsequent propagation and joining of mortar cracks causing ultimate failure was studied in Reference 2. Progressive microcracking was studied by Shah and Sankar [Ref. 3] using petrographic techniques. The techniques hithereto used for such studies involved loading specimens to specific load levels and subsequent sectioning procedures. The present study offers a continuous nondestructive and noninvasive alternative means to study the nature of microcracking. The influence of pore size and the size of aggregate on the mortar were studied using three complementary tools: 1) 2) 3)
Holographic interfero metry to study surface cracking. Speckle photography to study nature of bond crack growth. Acoustic Emission to study internal microcracking
Two important aspects of the tests conducted were: 1) Use of "model concrete" with prearranged inclusions and voids. 2) Use of closed loop testing system to study crack growth through strain softening in unlaxial tension and compression. The results of this investigation will lead to a better understanding of the physical process involved in the fracture of concrete and will make it possible to optimize performance of concrete by proper selection of an appropriate microstructure. It is also intended to use these results to form a constitutive model for concrete.
Mat. Res. Soc. Symp. Proc. Vol. 114. '1988 Materials Research Society
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UNIAXIAL COMPRESSION Holographic Interfero metry The hologram is made with the simplified se
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