Strain Measurements within Fiber Boards. Part I: Inhomogeneous Strain Distribution within Medium Density Fiberboards (MDF) Loaded Perpendicularly to the Plane of the Board

Abstract

Internal bond strength testing is a widely used approach for testing quality traits of wood based panels. Generally, failure of internal bond specimens is due to adhesion and/or wood failure in the specimen. It has been reported that a composite product with a large variation in the vertical density profile fails in the center part of the board which is either the middle of the core layer or the transition zone between core layer and face layer. The density in the failure zone is typically 50% lower than the maximum density in the face layers. The aim of this study was to analyze the strain distribution in a specimen under tension perpendicular to the panel plane. The results showed that a high variety of strain magnitude occurred in the specimen. The strain is either aligned with the tension direction or a tension zone is built in one of the edge zones leading to failure. Vector graphics of the specimen show the problematic test setup of internal bond strength measurement. Strain spots in the edges lead to the assumption of an uneven stress distribution due to the momentum which results from non-perfect alignment or irregularities in the test setup.

Adhesives from Waste Protein Biomass for Oriented Strand Board Composites: Development and Performance

“…It is plausible that the relatively weaker adhesion strength observed by these two formulations played a role for the significantly low core density of the OSB and in essence for the density variation. According to Rathke et al, large variations between zones across the thickness could lead to premature failure under mechanical stress in the center part of the panel that includes the core layer as well as the transition zone between the core and the face layer. With regard to this variation, the difference observed for the control, 40, 50, and 60% formulations was about 17% and it was not statistically significant.…”

Strain Measurements within Fibre Boards. Part II: Strain Concentrations at the Crack Tip of MDF Specimens Tested by the Wedge Splitting Method

“…Three different methods were applied: First, we used the average values of the load and the initial slope as input data for the FE simulation to determine an average isotropic modulus of elasticity ( E = 298 MPa). This approximated modulus of elasticity was used in combination with the internal bond strength σ ib = 0.51 ± 0.19 MPa [ 31 ] to calculate the critical strain at failure. The calculation of corresponding strain yielded …”

“…This conclusion is supported by the well-known correlation between the modulus of elasticity and the density. The density profile shown in part one of this series [ 31 ] is approx. constant within ±10 mm from center; therefore the isotropic FE-simulation might describe the material and setup correctly within the measurement plane and region.…”

“…The analysis of the total sample deformation was performed as described in Part 1 of the trilogy [ 31 ].…”

Strain Measurements within Fibreboard. Part III: Analyzing the Process Zone at the Crack Tip of Medium Density Fiberboards (MDF) Double Cantilever I-Beam Specimens

“…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.…”