| 1. |
- Bengtsson, Rhodel, et al.
(författare)
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Comparison of measured creep in a wooden beam with finite element predictions based on orthotropic viscoelastic material model
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Creep is of concern for long-term deformations of wooden structures. Since wood is anisotropic and creeps in several material directions, it may not be sufficient to include only axial creep along the grain even for deformations in beam-like components. A bottle-neck is that creep characterisation in all material directions is both costly and complicated. Multiscale modelling from cell-wall creep including the main contributing features (density, ray content, microfibrillar angle) can contribute to fill to complete material models for wood creep. In the present study, we have chosen a four-point bending test of a Norway spruce beam to represent a loaded wooden component in a structure. Digital image correlation was used to gather data on strain and displacement fields during the creep test. The experimental results were compared with finite element predictions based on a 3D orthotropic viscoelastic model obtained by multiscale homogenisation. There was generally good agreement in the strain fields between the finite element simulations and experimental observations. However, the numerical predictions exhibits slightly greater stiffness in terms of displacement, suggesting the need for further refinement of the multiscale model or a combination of materials creep charactrisation and multiscale modelling.
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| 2. |
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| 3. |
- van Blokland, Joran, et al.
(författare)
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Bending properties and strain fields around knots in thermally modified timber
- 2018
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Konferensbidrag (refereegranskat)abstract
- Thirty-two (32) boards of Norway spruce with cross-sectional dimensions of 145×45 mm2 were first tested non-destructively in a four-point static bending test, were then thermally modified according to the ThermoWood® process, and were finally tested destructively in the mentioned test set up. For one of these boards, the 2D strain fields occurring due to pure bending were recorded, both before and after thermal modification, over the surface of a knotty part of the board using a non-contact optical deformation measurement system. The objectives were to get more insight into the static bending behaviour of thermally modified timber (TMT), specifically with regard to the local and global modulus of elasticity (MOE) and their respective relationship to bending strength, and the strain development around a cluster of knots. The bending strength was significantly reduced by thermal treatment, whereas the effect on the MOEs was limited. Linear regression analyses demonstrated that bending strength of TMT can be predicted by employing stiffness as indicating property. Strain field measurements showed that at the examined levels of loading the quantity and distribution of strains in a knotty area were not influenced by thermal modification. It was therefore suggested that the influence of thermal modification on global stiffness, as well as on local stiffness around knots, is limited.
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| 4. |
- van Blokland, Joran, 1989-, et al.
(författare)
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Crack formation, strain distribution and fracture surfaces around knots in thermally modified timber loaded in static bending
- 2020
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Ingår i: Wood Science and Technology. - : Springer. - 0043-7719 .- 1432-5225. ; 54, s. 1001-1028
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Tidskriftsartikel (refereegranskat)abstract
- The effect of thermal modification (TM) on the chemistry, anatomy and mechanical properties of wood is often investigated using small clear samples. Little is known on the effect of growth-related and processing defects, such as knots and checks, on the bending strength and stiffness of thermally modified timber (TMT). Nine boards of Norway spruce with different combinations of knot types were used to study the combined effects of checks and knots on the bending behaviour of TMT. Digital image correlation (DIC) measurements on board surfaces at sites of knots subjected to bending allowed to study strain distribution and localise cracks prior to and after TM, and to monitor development of fracture (around knots) in TMT to failure. DIC confirmed that checking in knots was increased after TM compared to kiln-dried timber, specifically for intergrown knots and intergrown parts of encased knots. Effects appear local and do not affect board bending stiffness at these sites. Bending failure in TMT initiated mainly at knot interfaces or besides knots and fractures often propagated from checks. Scanning electron microscopy analyses of fracture surfaces confirmed this, and fractures were typically initiated around knots and at knot interfaces due to crack propagation along the grain in the longitudinal–radial plane (TL fracture) under mixed mode I and II loading, such that boards failed in simple tension like unmodified timber. Images of fracture surfaces at the ultrastructural level revealed details of the brittle behaviour of TM wood. This was especially apparent from the smooth appearance of transwall failure under mode I loading across the grain.
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| 5. |
- Van Blokland, Joran, et al.
(författare)
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Electrical resistance characteristics of thermally modified wood
- 2022
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Ingår i: European Journal of Wood and Wood Products. - : Springer Science and Business Media LLC. - 0018-3768 .- 1436-736X. ; 80, s. 749-752
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents electrical resistance characteristics of ThermoWood (R) Thermo-D Norway spruce for wood moisture content measurements below fibre saturation point. Electrical resistance along the grain of small clear wood specimens was measured at various moisture content levels of similar to 4-15% for thermally modified and similar to 5-25% for unmodified spruce. The results show that-similar to unmodified wood-the relationship between wood moisture content and electrical resistance can be well described by a first order polynomial function obtained from simple linear regression. This provides accurate resistance-based predictions of thermally modified wood moisture content. In addition, established temperature corrections derived previously from unmodified wood seem applicable to thermally modified wood.
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| 6. |
- van Blokland, Joran, et al.
(författare)
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Embedment properties of thermally modified spruce timber with dowel-type fasteners
- 2021
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Ingår i: Construction and Building Materials. - : Elsevier. - 0950-0618 .- 1879-0526. ; 313
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates the effect of thermal modification, ThermoWood Thermo-D treatment versus no treatment, on the embedment properties of Norway spruce timber (Picea abies [L.] Karst.) with dowel-type fasteners. The test specimens were reinforced to prevent splitting of the wood. The influence of density, load direction, test specimen configuration (full hole versus half hole), moisture content, gauge points and calculation method were also evaluated. Thermal modification primarily affected the embedment strength parallel to the grain, which was ∼ 25% higher after thermal modification, mainly due to the change in physical properties because of the treatment, i.e. the lower equilibrium moisture content. The influence of the investigated parameters on embedment properties of thermally modified spruce followed similar trends as for unmodified spruce. It was for example seen that the density–embedment strength relationships still hold after the treatment despite the decrease in density and the increase in embedment strength parallel to the grain. However, after thermal modification, the influence of load direction on embedment strength was ∼30% larger and the influence of calculation method (yield versus ultimate strength) on embedment strength perpendicular to the grain was ∼10% smaller.
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| 7. |
- van Blokland, Joran, et al.
(författare)
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Evaluation of non-destructive test methods to predict bending properties of thermally modified timber
- 2018
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Ingår i: 2018 World Conference on Timber Engineering (WCTE), August 20-23, 2018, Seoul, Republic of Korea. - : World Conference on Timber Engineering (WCTE). ; , s. 8-
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Konferensbidrag (refereegranskat)abstract
- Thermally modified wood is available through a number of manufacturers in Europe on today’s market for interior and exterior building products. Thermal modification of wood allows for improvement of dimensional stability and durability, but a considerable decrease in strength properties occurs. Despite this loss in strength, thermally modified wood shows potential to be further exploited in structures exposed to loading. For such applications, accurate prediction of its static bending behaviour is essential. This paper studies the applicability of two different non-destructive test (NDT) techniques in estimating the bending properties of thermally modified timber (TMT). The study was done on 100 Norway spruce logs. One hundred (100) boards (i.e. one from each log) were thermally modified and the mirrored 100 boards were used as controls. After modification, resonance-based and time-of-flight measurements of axial wave velocity were carried out. Subsequently, all 200 boards were bent to failure following European standard EN408. This study shows that although TMT has a lower bending strength than unmodified timber, predictions of bending strength and stiffness using the NDT techniques are possible and with sufficient accuracy. The resonance-based method gave better predictions of the bending properties of TMT in respect to time-of-flight method.
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| 8. |
- van Blokland, Joran, et al.
(författare)
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Machine learning-based prediction of internal checks in weathered thermally modified timber
- 2021
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Ingår i: Construction and Building Materials. - : Elsevier. - 0950-0618 .- 1879-0526. ; 281
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Tidskriftsartikel (refereegranskat)abstract
- This study investigated possibilities to predict the presence of internal checks in thermally modified Norway spruce timber after 2.5 years of weathering based on the initial properties of the boards. Machine-learning classification enabled sorting the input parameters based on their relative importance for accurate predictions. The parameters of thermally modified timber with the highest relative importance were annual ring width followed by initial moisture content, density and dynamic stiffness. Whereas after kiln drying these were, density, annual ring width, initial moisture content and acoustic velocity. The results showed that predictions are possible, and an accuracy of 67% was achieved by using annual ring width combined with density and initial moisture content, or acoustic velocity that can be determined after either kiln drying or thermal treatment. (C) 2020 Published by Elsevier Ltd.
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| 9. |
- Van Blokland, Joran, et al.
(författare)
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Machine learning-based prediction of surface checks and bending properties in weathered thermally modified timber
- 2021
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Ingår i: Construction and Building Materials. - : Elsevier BV. - 0950-0618. ; 307
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Tidskriftsartikel (refereegranskat)abstract
- Machine learning (ML)-based models, decision tree and ANFIS, were used to predict the degree of surface checking and bending properties of 30-month weathered thermally modified timber. The results showed that the investigated initial board properties did not allow accurate predictions of surface checks. ML regression and clustering analysis confirmed important variables for accurate predictions of bending properties were dynamic stiffness, acoustic velocity, density and lowest local bending modulus. ML models performed better than conventional regression models used for timber grading, and a prediction accuracy of 80–90% for bending stiffness and 50–70% for bending strength could be achieved.
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| 10. |
- van Blokland, Joran, et al.
(författare)
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Performance of thermally modified spruce timber in outdoor above-ground conditions : Checking, dynamic stiffness and static bending properties
- 2020
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Ingår i: Applied Sciences. - Basel, Switzerland : MDPI. - 2076-3417. ; 10:11, s. 1-25
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies have shown that thermally modified wood (TMW) performs well in outdoor, above‐ground conditions in terms of resistance to wood‐decaying fungi. Yet, little is known about the development of defects such as checks and the corresponding mechanical properties of TMW in this condition. This experiment focused on the effect of 30 months outdoor above‐ground exposure (weathering) on the degree of checking, dynamic stiffness and static bending properties of thermally modified timber (TMT) of Norway spruce. Two board pairs per log were cut from 190 logs; one board of each pair was thermally modified and the other used as control. Then, 90 board pairs were exposed to the weather in south Sweden. Surface checking and axial stiffness were monitored at six‐month intervals by using digital photography and non‐destructive tests (time‐of‐flight and resonance method) to monitor changes in the material upon weathering. Finally, all boards were tested destructively in a 4‐point static bending test following EN 408 standard. Results showed that weathering had no significance influence on static bending properties of TMT even though the degree of checking was considerably higher in TMT than unmodified timber after weathering. In particular, checks along growth rings were deeper, longer and more common in TMT after weathering, especially on the pith side of boards. The maximum depth of these checks did not depend on board orientation (i.e., which side was exposed) and exceeded limits given in strength grading standards for 7% of the modified boards included. Axial dynamic stiffness determined at 6‐month intervals was less influenced by fluctuations in moisture content for TMT compared to unmodified timber, but did not confirm the increase in the degree of checking of TMT. The presence of checks from weathering did influence failure modes in TMT; horizontal shear failure became more frequent and some boards failed in compression.
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