Process Zone Size and Crack Growth Measurement in Fiber Cements


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Title: Process Zone Size and Crack Growth Measurement in Fiber Cements

Author(s): Roger M. L. Foote, Yiu-Wing Mai, and Brian Cotterell

Publication: Special Publication

Volume: 105


Appears on pages(s): 55-70

Keywords: cements; composite materials; cracking (fracturing); fibers; crack propagation; measurement; microcracking; wood; Structural Research

Date: 12/1/1987

In analytical modeling of crack growth resistance (KR) curves for fiber cements, it is important to determine the size of the matrix fracture process zone (FPZ), in addition to the characteristics of the fiber-bridging zone. New experimental techniques are given for identifying and measuring crack growth and FPZ in a low-modulus wood-fiber cement. A computerized data acquisition system has been developed to investigate the nature of crack growth with a grid of closely spaced conductive bars screen-printed onto the specimen surface using colloidal graphite. As the crack path progresses through the grid, the position of the crack tip is automatically recorded and the discrete cracking behavior of crack growth is shown. Crack lengths measured in this way are in good agreement with results obtained using optics. The extent of the FPZ can be determined by cutting thin strips of the specimen normal to the crack path in the vicinity of the crack tip and measuring the bending stiffness of each strip as a function of distance away from the tip. The presence of microcracking is easily detected by this technique and the size of the FPZ can be determined. Experimental results show that the process zone is approximately 30 to 40 mm in a compact tension geometry.