| Balsam fir (Abies balsamea (L.) Mill.) is deemed as an underutilized softwood species due toits low density. The thermo-hydro-mechanical (THM) densification technology has been usedto improve most mechanical properties of wood. THM technology can be described ascompressing wood with the aid of temperature and moisture until the thickness decreases to agiven value. To decrease the manufacturing cost of the laminated wood products made ofdensified wood, a desirable product design is to produce a two-layer laminated wood productwith the densified wood as surface and undensified wood as substrate. However, such anasymmetrical laminated wood product could cause dimensional instability when it is subjectedto moisture change. This could be reflected in the thickness recovery of the densified woodlayer caused by residual stresses and the cupping of the laminated wood product due to thedifference in material properties and geometrical configuration.The overall goal of this dissertation was to develop a deep understanding of the mechanismdictating the dimensional stability of laminated products made by layers of densified andundensified wood. To achieve this overall goal five specific objectives were achieved in thisdissertation:1) to investigate the residual stresses existing in the mechanically densifiedbalsam fir;2) to optimize the densification parameters [i.e., temperature, compression ratio(CR), and pressing time] used in the THM densification process;3) to measure the elasticparameters of the densified balsam fir wood in the radial and tangential (RT) plane [e.g., moduliof elasticity (ERand ET), shear modulus (GRT), and Poisson’s ratios (μRTand μTR)] by means of adigital image correlation (DIC) method;4) to develop an approach to calculate the cuppingvalues of two types of specimens containing a densified wood layer of different thickness via aMATLAB-based image batch processing algorithm; and5) to study the interfacial normal stress(INS) and interfacial shear stress (ISS) along the bond line and cupping using a finite element(FE) model.The major findings were:1) a CR-based mathematical model developed could well simulate therelease of mechanically induced residual stress with increasing time, which indicates that theresidual stress increases with the increase of CR;2) optimal densification was achieved with aCR of60%, and a pressing temperature of230oC and duration of20minutes;3) the measuredER, ET, GRT, μRTand μTRof densified balsam fir were284MPa,2551MPa,21MPa,0.10, and0.33;4) the specimen with the thickness ratio of0.18had better dimensional stability than theone of0.58; and5) the FE model results showed that INS and ISS increased with increasingthickness ratio and large INS and ISS concentrated at the two ends of the bond line. The major contributions made in this dissertation were1) differentiation between the physicallyinduced and mechanically induced residual stresses in densified softwood and constitution oftheir relationship;2) direct measurement of the elastic properties of small densified woodspecimens by using a compression-DIC combined testing method; and3) development of acomputer-based image batch processing algorithm for automatically and accurately measuringthe cupping in a series of images taken by a computed tomography (CT) scanner. |
