Assessment of wood load condition by Near Infrared (NIR) spectroscopy

The assessment of the mechanical properties of wood using non-destructive evaluation (NDE) tools has been widely developed and refined. These NDE tools mainly rely on vibrational, ultrasonic or stress-wave approaches. Vibrational techniques generally show higher correlations between the estimated modulus of elasticity (MOE), or modulus of rupture (MOR), and the measured MOE, or MOR, than stress-wave techniques. They are, however, relatively difficult to apply in the field due to boundary conditions common in many timber structures. Thus, improved tools for assessing timber structures are still needed. Recently, near infrared (NIR) spectroscopy (500–2400 nm) has shown promise for predicting the MOE and MOR of wood. This work focuses on extending the use of NIR for measuring the load applied to small wood beams. The reflectance NIR spectrum was measured as the applied load was increased. Good correlations (r > 0.96) between the measured load and the predicted load were obtained using spectra taken from both the tension and compression surfaces of the small wood beams.     

In situ assessment of structural timber using semi-destructive techniques

This paper summarizes the test recommendations for selected semi-destructive testing techniques as developed by members of the RILEM Technical Committee AST 215 “In-situ assessment of structural timber”. The recommendations cover resistance drilling, core drilling, glue line test, tension micro-specimens, screw withdrawal, and several hardness tests. The paper includes a matrix of common non-destructive testing to assess structural timber. The discussion of each technique is intended to provide users with sufficient information to understand the theoretical basis, typical equipment set up, and basic capabilities and limitations.     

The influence of wind-induced compression failures on the mechanical properties of spruce structural timber

Compression failures (CF) are defects in the wood structure in the form of buckled fibres. They are a well-known 'natural' phenomenon in softwood trees exposed to frequent or strong winds, but their influence on the utilisation of timber is still debated. While a reduction of the mechanical properties in bending and tension at the fibre level and in small clear specimens is generally acknowledged, the effect is less obvious with structural timber in the presence of other defects such as knots or grain deviations. In the presented case study a statistically significant reduction of the moduli of rupture and elasticity in bending is observed in a sample of 563 squared timber beams, but the characteristic values of the mechanical properties still exceed the limits for the strength classes of visually graded structural timber (according to the Swiss standard SIA 265). Nevertheless, because of the potential safety risk by the more brittle fracture behaviour, it is recommended to exclude timber with detected CF from use in load bearing structures stressed in tension or bending.     

Cross‐laminated Timber Plates Subjected to Concentrated Loading

Since wood products for structural elements, especially cross‐laminated timber (CLT), have gained importance in the building sector, the need for appropriate and reliable design codes for such wood products has become essential. For the improvement and development of design concepts, a profound knowledge about the mechanical behaviour of these products is necessary. For this reason, this work focuses on global failure mechanisms and the corresponding evolution of different crack modes in CLT plates, depending on geometric and/or material related properties. Therefore, plate‐bending experiments on three‐ and five‐layered CLT plates were carried out. In addition to standard evaluation methods, each specimen was cut into small cubes to identify the failure modes inside the plates. Regions with dominant shear failure, tensile failure, delamination and mixed failure modes could be captured and connected to geometry and loading situation. Based on this evaluation, well‐known but not yet in detail described effects, such as the ductile structural behaviour of CLT plates, can be explained. Moreover, the evolution of rolling shear failure modes as well as from which point the progressive failure highly affects the plate stiffness is investigated and analysed in detail.     

大尺寸安全销实物单面剪切试验装置及方法

介绍了轧机等大型关键设备所配备的大尺寸安全销实物单面剪切试验装置与试验方法。实物单面剪切试验与小试样性能试验两种方法的结果对比表明，大尺寸安全销实物单面剪切试验是无法用小试样性能试验替代的。     

Determination of dynamic elastic moduli and shear moduli of aged wood by means of ultrasonic devices

Due to ecological and environmental factors, re-using aged wood is becoming more and more important, also in applications where mechanical strength plays a central role. The aim of this study was to examine specific mechanical parameters of naturally aged and dried wood and to better understand the influence of aging on the elastic behaviour of wood. To this aim, measurements on boards and on small, clear wood specimens were carried out. Ultrasound velocities of longitudinal and, in some cases, of transversal waves were measured to determine dynamic elastic moduli and shear moduli. The measurements were performed on structural timber of aged Norway spruce (aged wood) and compared with specimens of recently cut and kiln dried timber of the same species (recent wood) as a reference with comparable density properties and average annual ring width. The measurements revealed higher values of dynamic elastic modulus for aged wood in the longitudinal and radial directions, but no significant difference was found in the tangential direction or in the shear moduli. It is supposed that the difference is more likely a consequence of variability in densities and the structure parameters (annual ring structure, microfibril angle, growth conditions) rather than a consequence of the wood age. The relation between the dynamic elastic modulus in the longitudinal direction and wood density was nearly the same for aged and recent wood specimens, so with increased prudence, grading methods developed for recent wood can also be applied for aged wood.     

Fracture characterisation of green-glued polyurethane adhesive bonds in Mode I

Unseasoned (green) spruce timber side boards of size 25 × 120 × 600 mm were flatwise-glued with a one-component PUR adhesive. Glued pairs of boards were then kiln-dried to 12 % moisture content. A special small-scale specimen for testing the fracture properties of the adhesive bond in Mode I was developed in order to evaluate the adhesive bond properties. The complete force versus deformation curve, including both the ascending and the descending parts, could be obtained with these small-scale specimens, enabling the strength and fracture energy of the bond line to be calculated. In addition, the fractured specimens were examined by scanning electron microscope. Results show that both the tensile strength and the fracture energy of the green glued PUR adhesive bonds were equal to those of the dry glued bonds. The methodology developed and used in the present study gives new possibilities for analysis of the mechanical behaviour of wood adhesive bonds, and particularly of their brittleness and its correlation with the type of fracture path. This is in sharp contrast to the use of standardised test methods (e.g. EN 302, ASTM D905) with specimens having relatively large glued areas. Using such types of specimens, it is not possible to obtain the complete force versus deformation response of the bond. In addition, when using such test methods, failure takes place in the wood or in the fibres near the bond, thus making it impossible to obtain detailed information about the bond line characteristics.     

Determination of Moisture Content in Ceramic Brick Walls Using Ground Penetration Radar

The moisture content in ceramic masonry plays an important role in their performance and durability, also serviceability is severely affected by moisture. The presence of moisture represents one of the major issues associated with structures fabricated with ceramic masonry. The intrinsic characteristic of the material along with the age of the structure plays an important role in the amount of moisture absorbed by the walls. Traditional methods for determining the moisture content of in-situ masonry walls are many times destructive or semi-destructive and provide limited information. In this study, ground penetrating radar is used as a non-destructive technique for measuring the moisture content in ceramic masonry walls. Control samples were used as calibration in the laboratory and field measurements were performed to determine the moisture content of the ceramic walls. The developed methodology was applied to real size ceramic walls, were in-situ measurements showed areas with different moisture levels. The methodology developed showed to be fast and easy to use in the field for measuring the moisture content of ceramic brick walls.     

On the role of pin geometry in microstructure and mechanical properties of AA7075/SiC nano-composite fabricated by friction stir welding technique

The main purpose of this research is to investigate the influence of pin geometry on the macrostructure, microstructure and mechanical properties of the friction stir welds, reinforced with SiC nano-particles. Toward this end, friction stir welding (FSW) conducted using five geometrically different pin tools. Other welding parameters were remained unchanged. Microstructural evaluation using optical microscopy (OM) and scanning electron microscopy (SEM) revealed a banded structure consisting of particle-rich and particle-free regions in stir zone (SZ). The most uniform particle distribution was found in the case of using threaded tapered pin tool. On the other hand, it was discovered that the reinforcements had severely accumulated in the SZ of specimen friction stir welded (FSWed) with four-flute cylindrical pin tool. Moreover, threaded tapered and four-flute cylindrical specimens showed the highest and the lowest microhardness, respectively. In spite of four-flute cylindrical specimen, other specimens failed in base metal during tensile testing. Besides, the highest ultimate tensile strength (UTS) was recorded for the specimen FSWed with triangular pin tool. Fracture surface of tensile specimens were also studied employing SEM technique.     

Basalt FRP rods for reinforcement and repair of timber

Limited research has been undertaken into the use of basalt fibre reinforced polymer (FRP) materials for the strengthening and repair of structural timber elements. This paper describes an experimental test programme in which the flexural performance of low-grade glued laminated timber was reinforced using bonded-in basalt FRP rods. Tension test results show that basalt FRP rods compare extremely well to the mechanical characteristics of glass FRP rods. Strengthened and repaired beams exhibited considerable ductility in contrast to brittle tension behaviour of the unreinforced sections. With the use of a modest reinforcement percentage of 1.4% strategically located in circular routed out grooves at the soffit of the beam, mean stiffness enhancements of 8.4% and 10.3% for the global and local measurements were achieved respectively and a mean improvement in the ultimate moment capacity of 23% was achieved in comparison to the unreinforced glulam beams. The distance of the reinforcement to the neutral axis was shown to be highly influential on the mechanical enhancements. The use of basalt FRP rods is seen to be highly effective as a repair technology for damaged timber elements. Strain profile readings from the beams which included the reinforcement demonstrated improved utilisation of the compression characteristics of the timber. In all testing, a good quality bond is reported between the basalt FRP and wood. There exists significant potential for the development of environmentally friendly engineered structural elements by combining timber based products with other natural materials such as basalt fibre reinforced polymers.     

Cellulose microfibril angles in a spruce branch and mechanical implications

The nanostructure of the wood cell wall and, in particular the tilt angle of the cellulose fibrils versus the longitudinal cell axis (microfibril angle, MFA), are known to play a key role in determining the mechanical properties of wood. A variation of microfibril angles during growth may therefore be regarded as a means to adapt to different loading situations. In the present study, a branch of Norway spruce (Picea abies) was used as a model system. The change of microfibril angles with increasing age and size of the branch and therefore increasing gravitational load was systematically investigated. Small angle X-ray scattering (SAXS) was applied to obtain a map of MFA all over the branch as a function of the distance from the trunk within each annual ring. It was found that in compression wood the MFA decreased continuously from the trunk towards the tip in all annual rings. In opposite wood, however, the course of microfibril angles was found to change considerably with the age of the branch: in the outer annual rings, very small microfibril angles occurred in the middle part of the branch. The results are discussed in view of the mechanical implications of different microfibril angles.     

Optimization of processing variables and mechanical properties in rubber-wood particles reinforced cement based composites manufacturing technology

The manufacturing processes of rubber crumb–wood particle reinforced cement based composites (RWCC) which shortened the pressing cycle and enhanced the properties were completely investigated. Meanwhile, optimal processing conditions as well as properties of strength were tested to validate the predicted variables and properties. Mechanism effects of interacting process variables [density, duration of pressure time (namely, pressure time) and pressure] on properties of RWCC were opened out. The RWCC performance was evaluated by measuring its flexural strength (FS), modulus of elasticity (MOE) and internal bond (IB) strength. The experimental results were statistically analyzed by using Response Surface Method (RSM) software to identify the significant manufacturing process variables for RWCC. In the process variables, density and duration of pressing time had more significant influences on mechanical properties (FS and MOE and IB), but had little effect on strength performance caused by pressure. In addition, the microstructure of RWCC was clearly examined by using a scanning electron microscope (SEM). The mechanism effects were also revealed through analyzing the microstructure of the interface of rubber crumb/wood particle/cement. The results of a comprehensive evaluation for properties of RWCC with the highly active polymeric methylene diphenyl isocyanate adhesive (PMDI) as binder system not only present excellent mechanical properties, but also possess some of functional properties such as optimized acoustic properties and energy conservation. Finally, the optimal manufacturing process parameters were obtained by means of the maximizing mechanical properties.     

Ultrasonic guided waves in bone

Recent progress in quantitative ultrasound (QUS) has shown increasing interest toward measuring long bones by ultrasonic guided waves. This technology is widely used in the field of nondestructive testing and evaluation of different waveguide structures. Cortical bone provides such an elastic waveguide and its ability to sustain loading and resist fractures is known to be related to its mechanical properties at different length scales. Because guided waves could yield diverse characterizations of the bone's mechanical properties at the macroscopic level, the method of guided waves has a strong potential over the standardized bone densitometry as a tool for bone assessment. Despite this, development of guided wave methods is challenging, e.g., due to interferences and rnultiparametric inversion problems. This paper discusses the promises and challenges related to bone characterization by ultrasonic guided waves.     

Biaxial testing of high strength carbon fiber composite cylinders for pulsed magnet reinforcement

A methodology is introduced to test carbon-fiber-reinforced, hoop-wound composite cylinders for their biaxial mechanical properties under axial compression and hoop tension. The understanding of the behavior of these composites under biaxial loads is extremely important in the design of pulsed magnets. These composites are used as reinforcements for both the inner conducting layers and as an overall exterior reinforcement. Testing of actual pulsed magnets to ascertain design change effects of composite reinforcement schemes on the maximum attainable field can be expensive; hence, a standard biaxial testing method is desirable which is relevant to the design of pulsed magnets. In this investigation, an attempt was made to produce a standard testing procedure aimed at measuring the biaxial mechanical properties (elastic, plastic, and failure envelope) of composite materials. This methodology was applied to two different carbon/epoxy based composites. The results of these tests (elastic properties and failure points) are compared with theoretical predictions, specifically those due to Tsai-Wu.     

Prüftechnik für Hochtemperatur–Konstruktionswerkstoffe

Test Techniques for High Temperature Structural Materials The progress in the development of high-temperature resistant materials for practical application at temperatures between 1000°C and 2000°C requires also further development of high-temperature testing and measuring techniques for the evaluation of the necessary material data. Selected characteristic testing problems in the evaluation of mechanical properties at temperatures above 1000°C are dealt with, and technical solutions and exemplary results are presented.     

An optional connection material in timber structures: densified poplar

The development of eco-friendly connection material instead of steel is a challenging problem in timber structures. Following densification, the mechanical properties of low-density species can be significantly improved. Densified wood may be a potential connection material in timber structures. This paper reviewed the different processing for densified wood, and obtained favorable mechanical properties and dimensional stability based on small specimen sizes, which are much less than the applicable sizes in practice. A densification processing with alkali pretreatment was adopted for poplar widely cultivated in the world to produce the densified poplar, which has been rarely reported as connection material. Various specimens of densified poplar were tested to obtain their main mechanical properties such as strength and deformability. The set recovery of densified poplar was also measured to observe their dimensional stability. In addition, the hygroscopic swelling strains for the diameter of densified poplar dowel were measured to present their moisture-dependent behavior. The improved mechanical properties and dimensional stability confirmed the fact that densified poplar with alkali pretreatment can be an optimal connection material.

Graphical abstract

     

Effect of plasticity on the residual stress measurement using the groove method

The mechanical methods based on milling rectilinear or annular grooves on a component's surface and measurement of relaxed strains are some of the most used semi-destructive methods for the determination of residual stresses. These are evaluated from the relaxed strains by means of equations based upon the linear elastic theory. In this paper the errors due to yielding localised at the bottom of the groove have been investigated. The analyses were carried out by means of the finite element technique varying the most important parameters involved. The experimental results show a good agreement with the numerical ones.     

Wood—a 45th anniversary review of JMS papers. Part 1: The wood cell wall and mechanical properties

The first part of a comprehensive review of the literature on wood published in the Journal of Materials Science since its inception in 1966 is presented. Papers are reviewed by subject ranging from the determination of the microfibril angle in the wood cell wall through to the evaluation of fatigue life. The role of moisture content in determining mechanical properties of wood is explored and mechanical properties are reported including creep, fatigue and fracture. It is concluded that JMS has played a key role in disseminating state of the art literature on new developments in the understanding of the structure-related properties of wood.     

Testing and modeling of a traditional timber mortise and tenon joint

The structural safety and behaviour of traditional timber structures depends significantly on the performance of their connections. The behaviour of a traditional mortise and tenon timber joint is addressed using physical testing of full-scale specimens. New chestnut wood and old chestnut wood obtained from structural elements belonging to ancient buildings is used. In addition, the performance of different semi and non-destructive techniques for assessing global strength is also evaluated. For this purpose, ultrasonic testing, micro-drilling and surface penetration are considered, and the possibility of their application is discussed based on the application of simple linear regression models. Finally, nonlinear finite element analysis is used to better understand the behaviour observed in the full-scale experiments, in terms of failure mode and ultimate load. The results show that the ultrasonic pulse velocity through the joint provides a reasonable estimate for the effectiveness of the assembly between the rafter and brace and novel linear regressions are proposed. The failure mechanism and load–displacement diagrams observed in the experiments are well captured by the proposed non-linear finite element analysis, and the parameters that affect mostly the ultimate load of the timber joint are the compressive strength of wood perpendicular to the grain and the normal stiffness of the interface elements representing the contact between rafter and brace.     

Resistographic inspection of ancient timber structures for the evaluation of mechanical characteristics

Among non-destructive testing methods for the analysis of ancient timber structures, the Resistographic method is gaining great diffusion. This method is based on the registration of density variations through timber sections by means of an electronic drill and nowadays it is generally employed only to detect the presence of defects and damage. The present study is about the use of Resistographic method to evaluate mechanical characteristics of timber and, with this aim, an experimental program on wood-worm damaged timber has been carried on. Results show the usefulness of this non-destructive testing method as it allows the investigation through the interior part of timber and thus can give a quite reliable idea of local strength of wood in the examined section. That gives an useful parameter for the detection of recoverable elements in standing timber structures to be restored. In the present research, beech compressive strength has been evaluated from small clear wood specimens, while the evaluation of strength of dimensional lumber is outside the scope of the work.

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Résumé La méthode d'essai non-destructif dite Résistographique, est désormais la plus répandue pour l'analyse d'anciennes constructions en bois. Son application, qui se base sur l'enregistrement des variations de densité à l'intérieur d'une section en bois, à l'aide d'une perceuse électronique, n'est à présent utilisée que pour repérer d'éventuels défauts et endommagements. Cette étude concerne l'utilisation de la méthode Résistographique permettant d'évaluer les caractéristiques mécaniques du bois de construction. Dans ce but, on a expérimenté cette méthode sur des éléments de construction en bois de hêtre, en partie vermoulus. Les résultats démontrent l'utilité de cette méthode d'essai non-destructif qui permet d'examiner l'intérieur des bois, et d'en vérifier avec efficacité la résistance. Tout ceci nous permet d'individualiser tous les éléments récupérables dans les constructions en bois devant être restaurées. Dans cette recherche, on a derterminé la résistance du hêtre grace à des échantillons prismatiques, mais la résistance du bois des poutres en dimension d'usage réel, étant en dehors de l'objet de cette étude, n'a pas été mesurée.

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Editorial Note The University of Naples is a RILEM Titular Member.