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

Abstract

This is the second part of an article series where the mechanical and fracture mechanical properties of medium density fiberboard (MDF) were studied. While the first part of the series focused on internal bond strength and density profiles, this article discusses the fracture mechanical properties of the core layer. Fracture properties were studied with a wedge splitting setup. The critical stress intensity factors as well as the specific fracture energies were determined. Critical stress intensity factors were calculated from maximum splitting force and two-dimensional isotropic finite elements simulations of the specimen geometry. Size and shape of micro crack zone were measured with electronic laser speckle interferometry. The process zone length was approx. 5 mm. The specific fracture energy was determined to be 45.2 ± 14.4 J/m2 and the critical stress intensity factor was 0.11 ± 0.02 MPa.

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

“…The coefficient of variation is significantly lower than that of the internal bond strength determined in part 1 of the trilogy [ 4 ], where the coefficient of variation was 37.2%. 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/m 2 ) of the numbers which were generated with the DCIB experiment.…”

“…The resulting wedge splitting specimens are similar stiff as the DCIB specimens for similar crack lengths. The ligament length was set to 90 mm as with the test set-up in Sinn et al [ 24 ] and a total of six specimens were tested. The results revealed specific fracture energy numbers of 114 ± 9 J/m 2 which is much closer to the results gained by means of DCIB testing than the results from Sinn et al [ 24 ] measured with the wedge splitting set-up simply reinforced by steel plates G f = 45.2 J/m 2 ± 14.4 J/m 2 .…”

“…Despite the fact that wood [ 5 , 8 ] and wood based panels [ 13 ] show non linear characteristics, which refers to the formation of a large process zone and fiber bridging behind the crack tip [ 25 ], most publications use the concept of linear elastic fracture mechanics [ 6 , 7 , 12 , 13 , 24 ]. In most cases the determined stress intensity factors K IC are calculated from single edge notched bending specimen.…”

“…As described in Rathke et al [ 4 ] and Sinn et al [ 24 ], the deformation of the sample surface causes a change in the phase difference and therefore a new speckle pattern. To calculate an image with the typical fringe pattern, each image pattern is subtracted from the previous one [ 28 ].…”

“…The aim of this study is (1) to compare the well-known wedge splitting set-up [ 5 ] with the DCIB set-up [ 21 , 22 , 23 ] based on the example of MDF data from part 1 [ 4 ] and 2 [ 24 ] of the trilogy. Stress intensity factor and specific fracture energy will be evaluated.…”