Lauri I. Salminen

Quantifying fatigue generated in high strain rate cyclic loading of Norway spruce

Papermaking, especially mechanical pulping, consumes much energy. To reduce this energy consumption one has to understand and exploit the phenomena present during the pulping. An important phenomenon to understand is wood fatigue. We quantitatively measure the fatigue generated during high strain rate cyclic loading of spruce wood performed under conditions resembling those present during mechanical pulping. We impacted the samples with 5% strain pulses at 500 Hz. The radial direction stiffness drop in the samples was quantified by 500 kHz ultrasonic through-transmission postimpacting. The depth profile of the generated fatigue was also determined. A dependency of the amount of fatigue generated during cyclic straining on the moisture content was detected. A hypothesis about the temporal and spatial evolution of the fatigue during the process is presented. The results, supporting the hypothesis, provide insight into wood behavior under mechanical pulping conditions.

Significance of fatigue for mechanical defibration

The fatigue induced by high-frequency cyclic loading on the compressibility and tensile properties of wood and wood cell walls was quantified. The non-elastic behavior of fatigued and reference samples was similar, whereas their elastic behavior differed, as expected. Next, the effects of the dynamic fatigue on the mechanical pulping process were quantified by grinding fatigued and untreated samples and by comparing the paper strength produced by the two pulps against the consumed pulping energy. Pre-introducing fatigue increased the energy efficiency of grinding and may allow designing a more energy efficient mechanical pulping process.

Effect of fatigue and annual rings’ orientation on mechanical properties of wood under cross-grain uniaxial compression

The mechanics of fresh wood with and without a fatigue pre-treatment that mimics a mechanical pulping process was experimentally studied. The mechanical properties of Norway spruce samples under compression are considered with the macroscopic stress–strain data and from local strain properties via digital image correlation technique. The results highlight the effects of the orientation of the wood annual rings compared to the loading direction and of the pre-fatigue. The wood presents a low yield point when the annual rings are tilted compared to the load axis, but the Young’s modulus and yield stress are higher when the annual rings are either parallel or perpendicular to the load direction. In the last case, buckling of softest layers occurs. The fatigue treatment makes the wood less stiff as deduced from the decreases of Young’s modulus and yield stress, whatever the orientation of annual rings. Secondly, it creates a thin and localized softened layer.

Repetitive impact loading causes local plastic deformation in wood

The relationship between the impactor velocity and the amount of strain localization in a single impact compression of cellular solids is known. However, few studies report on the effects of repeated high frequency compression. We therefore studied the mechanical behavior of Norway spruce, a cellular viscoelastic material, before, during, and after cyclic high frequency, high strain rate, compression. A custom made device applied 5000-20 000 unipolar (constrained compression and free relaxation) fatigue cycles with a 0.75 mm peak-to-peak amplitude at 500 Hz frequency. The consequences of this treatment were quantified by pitch-catch ultrasonic measurements and by dynamic material testing using an encapsulated Split-Hopkinson device that incorporated a high-speed camera. The ultrasonic measurements quantified a stiffness modulus drop and revealed the presence of a fatigued low modulus layer near the impacting surface. Such a localized plastic deformation is not predicted by classical mechanics. We introduc...

A 3D micromechanical study of deformation curves and cell wall stresses in wood under transverse loading

The deformation of wood is analyzed using the finite element method to quantify the phenomena in wood cells and cell walls. The deformation curves of computed microstructures are compared to experimental observations in two different loading cases: compression and combination of shear and compression. Simulated and experimental shapes of deformation curves match qualitatively and the deformation shapes exhibit a similar response to change in the loading mode. We quantify the intra-cell-wall stresses to understand the effects of the different layers during the deformation. The results benefit the development of energy efficient mechanical and chemo-mechanical pulping processes for pulp, board, and composite manufacture. In addition, the aspects of cell deformation can be exploited to dismantle the wood to accelerate chemical reactions in biorefinery.

Thermal conductivity of wood: effect of fatigue treatment

The effect of fatigue treatment on the thermal conductivity of wood was studied. Fresh Norway spruce samples both with and without fatigue were analyzed in a temperature rise experiment by means of an infrared camera. The experimental temperature profiles were compared to finite-element simulations of heat conduction. The temporal features of temperature profiles indicate an increase in the conductivity of fatigued wood, which points to changes in the cellular structure of wood. The importance of fatigue for thermal conductivity and consequently for mechanical pulp-making is discussed.

Influence of strain rate, temperature and fatigue on the radial compression behaviour of Norway spruce

Abstract A dynamic elastoplastic compression model of Norway spruce for virtual computer optimization of mechanical pulping processes was developed. The empirical wood behaviour was fitted to a Voigt-Kelvin material model, which is based on quasi static compression and high strain rate compression tests (QSCT and HSRT, respectively) of wood at room temperature and at high temperature (80–100°C). The effect of wood fatigue was also included in the model. Wood compression stress-strain curves have an initial linear elastic region, a plateau region and a densification region. The latter was not reached in the HSRT. Earlywood (EW) and latewood (LW) contributions were considered separately. In the radial direction, the wood structure is layered and can well be modelled by serially loaded layers. The EW model was a two part linear model and the LW was modelled by a linear model, both with a strain rate dependent term. The model corresponds well to the measured values and this is the first compression model for EW and LW that is based on experiments under conditions close to those used in mechanical pulping.

Cyclic impulsive compression loading along the radial and tangential wood directions causes localized fatigue

We report for the first time on the existence of a localized reduction in elasticity caused by repeated compression impaction applied along the tangential wood direction. Previous research indicates that localized strain profiles are generated by such cyclic impacting on wood along its radial direction. This finding is significant for the paper/board-making industry where wood is exposed to cyclic unipolar compression during grinding. However, the effect of the impacting direction, with respect to the orientation of the annual rings, on the localization phenomenon is unknown. In addition, the shape of the developing fatigue layer is unclear. We revisit the localization phenomenon with a focus on tangential impacting. We employed ultrasonics and x-ray tomography to quantify the induced fatigue. An interlacing technique increased the precision of the ultrasonic stiffness depth profiling technique. We studied both radial and tangential wood annual ring geometries. We used ultrasound to quantify the drop in s...

Cyclic loading and fatigue of wood

INTRODUCTION The creep rate of some hygroscopic materials has a strong dependence on fluctuations in the ambient relative humidity (RH). Wood [1], paper [2] and individual wood fibers [3] are known examples. This phenomenon, known as mechanosorptive creep, threatens the integrity of any hygroscopic material structure under constant load, and particularly shortens the storage-life of corrugated boxes [4]. Previously, many models for describing the generic mechanisms of mechanosorptive creep, as well as mechanisms particular to paper, have been proposed. Mechanosorptive creep in cellulosebased materials has been attributed to physical ageing of glassy materials [5], macroscopic moisture gradients and associated enhanced stresses [6], various fiber-level processes introducing stress concentrations [7,8,9] and more [6,10]. This work is focused on testing the predictions of the previously proposed models, and particularly identifying the length-scale at which the dominant mechanosorptive creep mechanism is found; sample size-level, fibril-level, or subfibrillevel. To avoid the complexity of hierarchical microstructures typical to wood or paper, we use nanofibrillated cellulose (NFC) films and aerogels [11] as model systems.