Mode-II progressive fracture of sawn timber and timber-adhesive bond: experimental evaluation of end notched flexure (ENF) testing using DIC

Abstract

The end-notched flexure (ENF) testing technique has been used in this paper to investigate the Mode II fracture behaviour of sawn timber and timber-adhesive bonds. Mode-II strain energy release rates were obtained by using classical data reduction schemes based on actual crack lengths captured using digital image correlation (DIC) technique. The compliance-based beam method (CBBM), which does not require the actual crack length measurements, was also evaluated for its suitability for Mode-II fracture of timber. Utilizing forward shear slip measurement from DIC at every millimetre of crack length, a simple yet effective and innovative technique to identify crack tip opening and to monitor crack length has been proposed herein. Three different initial notch widths were used in the current study to examine their suitability in producing reliable crack initiation and consistent bond delamination in solid timber and glued timber. The strain energy release rates calculated from the actual crack and equivalent crack length values showed similar trends in fracture resistance curves, which are commonly known as R curves, but the initial strain energy release rate (GII0) and the critical strain energy rate (GIIC) obtained from the two methods showed some discrepancies. Equivalent crack method CBBM was found to be inadequate in determining a consistent initial strain energy release rate despite producing a stable and steady fracture energy after crack initiation. Experimental results showed that Mode-II timber-adhesive bond line fracture is relatively more brittle than timber fracture, and the average critical strain energy release rate of timber-adhesive bond (GIIc) was 14% greater than that of sawn timber. Based on experimental observations, specific test criteria for ENF testing of timber structures are proposed in this paper.