Abstract
This paper is the third part of a study dealing with the mechanical and fracture mechanical characterization of Medium Density Fiberboards (MDF). In the first part, an analysis of internal bond strength testing was performed and in the second part MDF was analyzed by means of the wedge splitting experiment; this part deals with the double cantilever I beam test, which is designed for measuring the fracture energy as well as stress intensity factor in Mode I. For a comparison of isotropic and orthotropic material behavior, finite element modeling was performed. In addition to the calculation of fracture energy the stress intensity factor was analyzed by means of finite elements simulation and calculation. In order to analyze strain deformations and the process zone, electronic speckle pattern interferometry measurements were performed. The results revealed an elongated process zone and lower results for KIC if compared to the wedge splitting experiment. The Gf numbers are higher compared to the wedge splitting results and can be explained by the thicker process zone formed during the crack propagation. The process zone width on its part is influenced by the stiff reinforcements and yields a similar crack surface as with the internal bond test.
Highlights
The mechanical characterization of wood based panels includes, according to European standards [1,2,3]tensile, bending, and internal bond strength tests
In total six specimens were analyzed for specific fracture energy and stress intensity factor calculation and three specimens were analyzed for electronic laser speckle interferometry (ESPI) measurements
The data for specific fracture energy gained by means of the wedge splitting experiment in part 2 of the trilogy [24] reveals only approx. 31.8% (45.2 ± 14.4 J/m2) of the numbers which were generated with the Double Cantilever I-Beam (DCIB) experiment
Summary
The mechanical characterization of wood based panels includes, according to European standards [1,2,3]tensile, bending, and internal bond strength tests. Internal bond strength testing (IB) is a simple test where specimens are adhesively bonded to braces and tested in tension perpendicular to the panel plane This testing procedure includes a high number of effects which can bias the IB values to a large degree, such as for instance overlap of the adhesive at the edges, incomplete bonding of the specimen to the braces, varying bond line thickness and inhomogeneous transfer of stresses from the braces to the specimen [4]. The latter induces stress concentrations within the specimen, which in turn influences the IB value measured. No further information, such as for instance Young’s modulus or the fracture energy, can be derived from this procedure
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