Strain distribution in wood during chipping

To be able to understand more in detail what actually happens during chipping, the strain field in a chip during chipping was studied by means of the Digital Speckle Photography (DSP) technique. In addition to recording the strain field, the load on and displacement of the chipping tool was also recorded. The equipment used in this study was a DSP system, an MTS servo hydraulic testing machine and a specially developed chipping device. Displacement controlled testing was performed with a crosshead speed of 1.0 mm/s. The results are promising but needs some improvement regarding resolution of the digital images in a vicinity of the knife-edge. The cutting speed in this investigation is low in comparison to normal industrial cutting speeds and since wood in general exhibits a viscoelastic material behaviour this might give a slightly different strain field as compared to an industrial chipping process. However, it is believed that using DSP as a tool for studying the deformations during chipping, even under quite restricted conditions, will increase the understanding of the chipping process. The present study is a part of a larger project aimed at a better understanding of the chip formation and wear mechanisms of wood chipping knifes. INTRODUCTION One requirement from the pulp and paper industries is that the chip size (thickness) distribution should be as narrow as possible because it affects for example the packing degree in the compression screws used to transport the chips. It is generally accepted that the chip quality affects the whole pulp production and thereby the properties of the pulp. The ideal chips have a high bulk density and a narrow size distribution. To improve chip quality with regard to chip dimensions and fibre damage, the mechanics of chip formation must be better understood. The purpose of this paper is to present a method, based on digital speckle photography (DSP), for investigating the deformation fields in wood during chipping. There is experimental evidence indicating that when varying the length of the wood chips in the chipping process (keeping all other parameter constant), the ratio of length to thickness for the chips will be constant in some average sense [1,2]. Depending on what type of chipper that is used and the wood quality, one will have a range of chip lengths and thicknesses, which may be more or less narrow. Variation in the properties of the wood gives a natural variability of 20% [3-5]. When the chipper knife starts to penetrate the log, compression stresses develops parallel to the wood fibres. This compression stress, and the wood’s resistance to splitting into individual chips, determines the chip thickness to chip length ratio produced, as well as pin chips and fines generation [6]. In the present study, the strain fields of crack propagation were measured in order to improve the understanding of crack growth in wood and to see if digital speckle photography (DSP) is suitable for this investigation. The system used for the deformation analysis is based on image analysis of the undeformed and deformed images of the wood specimen from which chips are cut. By numerical differentiation of the displacement field, the strain field can be determined. DSP has found great applicability in a number of interesting applications. Thuvander et al. [7] used it to study crack tip strain field in wood at the scale of annual growth rings, Jernkvist and Thuvander [8] studied stiffness variation across annual growth rings in Picea abies, Ljungdahl et al. [9] studied transverse anisotropy of compressive failure in european oak and Dumail et al. [10] analysed rolling shear of spruce wood. By using DSP it is possible to evaluate quantitatively the in-plane displacement and strain fields. By eliminating the rigid body movement of the specimen, the local deformations on the specimen surface can be determined. Of interest is the local deformation (strain), up to the instant when a chip is formed. Depending on the size of the selected test area, smaller cracks will be captured and visualized as local strain concentrations during the evaluation as well. The cutting speed in this investigation is low in comparison to normal industrial cutting. The cutting speed has a significant effect on pin chip production as it has on the production of over thick chips [4,11]. Low chipping speed almost certainly results in a high sliding frictional component of the work of chipping [12]. There are other parameters which affects the material behaviour such as temperature, moisture content etc., which have not been considered in this preliminary investigation. In this first study, some preliminary test values are presented.