The strength, stiffness and density of wood vary to a great extent. For Norway spruce the variation in bending strength can be between 10 and 90 MPa. To be able to use this
material in load-bearing structures it is necessary to have a better knowledge of the properties. The wood material is produced by nature and it is therefore not possible to control the variation in properties by changing the manufacturing process. For wood it is instead necessary to get an estimate of the properties and grade the material into different strength classes.
The characteristic strength value for all materials is normally defined as the 5 %-fractile in the distribution of strength, see Figure 2.25.
By grading the material into different strength classes it is possible to:
• improve the control of timber characteristics such as strength and stiffness • have a common classification within a market
• optimise the yield from the raw material • optimise the use (good enough quality).
According to most standards the material is graded based on its bending strength. The mean modulus of elasticity in bending and density also have to be checked so that it is within the limits of the grade. All other parameters are estimated based on these values. The relation- ships between the characteristic bending strength fm,k and other strength and stiffness
properties used in the European standards for softwoods are as shown in Table 2.3.
Figure 2.25: Principle of the variation in strength for timber and the material split into three strength classes. Fr equenc y a b c fx,k,a fx,k,b fx,k,c ungraded material
Table 2.3 Calculation of characteristic strength according to SS-EN 338 for softwoods
Tensile strength parallel to grain Compression strength parallel to grain Compression strength perpendicular to grain Modulus of elasticity parallel to grain
Mean modulus of elasticity perpendicular to grain Mean shear modulus
Mean density
There are currently two types of grading; visual strength grading and machine strength grading.
Visual strength grading
The visual strength grading technique relies on the assumed correlation between visually detected defects and strength. In principle there are grading rules establishing which type and size of defects can be allowed in each strength class. The first visual strength grading rules were established in the US in the 1920’s and visual grading rules were also established in several European countries during the 1930’s. The grading rules are of course different for different wood species, number of grades and so on. Traditionally visual strength grading has been conducted by human inspectors looking at each piece of timber as it passes them on a conveyor. This of course gives problem in accuracy since the grader only has a couple of seconds to assign the board to a certain grade. It also has the natural drawback of only seeing the visual defects, defects within the material is not possible to detect. The method only allows for the use of simple combinations of defects. Today, there are scanning techniques possible to use together with the visual strength grading rules that improve the technique.
Machine strength grading
The machine strength grading technique relies on running the pieces of timber through a machine that measures one or several parameters non-destructively. These parameters are then used to predict the strength and possibly stiffness. The idea to use non-destructively measurements as a basis for prediction of strength and stiffness was presented in the 1950’s in both USA and Australia. The reason was a wish to improve the accuracy of the grading. The most commonly used parameters to measure non-destructively are modulus of elasticity and density.
The best predictor for strength is the stiffness or modulus of elasticity. The coefficient of determination varies between 0,5 and 0,7 between different studies; the difference is mainly dueto different measurement methods for the MOE. A typical result from a test of bending strength and stiffness can be seen i Figure 2.26.
2.5.1 Relationship between strength, stiffness and other parameters
The strength grading principles are built on measuring one or more parameters non-
destructively and use this measure to predict the strength (and possibly stiffness and density). This means that to use the grading principles it is necessary to have a good knowledge about the relationship between these parameters and strength and the influence of different kind of natural characteristics on these relationships. For clear wood the following relationships have been found between strength, density and modulus of elasticity (MOE):
• Bending strength – modulus if elasticity R2 = 0,70 – 0,75
• Bending strength – density R2 = 0,60 – 0,65
• Modulus of elasticity – density R2 = 0,60 – 0,65
The properties in timber, however, vary to a large degree between different trees, logs and even within a tree or log. The natural characteristics of wood also influence the parameters and their relationships, and it is not only the size of the defects but also their location that has an influence. In visual grading the occurrence of knots is the single most important parameter for assigning boards into different strength classes. Knots have also shown to be the most important factor in true strength tests. In strength tests of Norway spruce it has been shown that approximately 90 percent of the failures start at the position of a knot. Spiral grain angle is the cause of failure in some cases. Compression wood has a pronounced effect on the relationships between these parameters, it has high density and low MOE but the strength is only marginally decreased.
There are a number of different non-destructively measurable parameters that can be used in strength grading. Numerous tests have been done to establish the relationship between different parameters and the bending or tension strength of sawn timber. Studying a few of these test series gives the following coefficient of determination between bending strength and other parameters, see Table 2.4.
Table 2.4 Coefficient of determination between bending strength and a non-destructively measured parameter based on a number of studies on sawn timber (Johansson 2003)
Measured parameter R2
Knots 0,16 – 0,27
Annual ring width 0,20 – 0,44
Density 0,16 – 0,40
MOE, bending or tension 0,53 – 0,72
Knots and density 0,38
80 60 40 20 0 0 5 10 15 20 R2 = 0.52 Bending s tr egnth (MP a)
Modulus of elasticity (edgewise) (GPa)
Figure 2.26: The relationship between the modulus of elasticity (edgewise) and bending strength for 380 studs of the dimension 45 x 70 mm.
The difference can also be attributed to different knot sizes in the material in different studies. The modulus of elasticity includes information about knots (lower MOE at the position of the knot), spiral grain angle (lower MOE) and compression wood (lower MOE) which is why it works better than the other parameters. It is of course also possible to combine different parameters when grading timber. Adding information about knots together with information about the average density strengthens the prediction of strength. Adding more information to the MOE does not strengthen the prediction to any higher degree, probably because the MOE already includes this information.
2.5.2 Machine strength grading principles
There are many techniques used in strength grading machines; static flat-wise bending, MOE determined from vibration or measurement of density by radiation are the most commonly used techniques.
2.5.2.1 Static flat-wise bending
Flat-wise bending of the timber is the principle that has been used the longest and was the dominating technique among the grading machines up until the early 21st century. The
technique is based on measuring the flat-wise MOE in three-point bending. The boards are continuously feed flat-wise through a machine with three rollers and the MOE is evaluated based on either 1) measurement of deformation for a constant load or 2) measurement of the load necessary to create a pre-set deformation, see Figure 2.27. This gives a measure of the variation of the MOE along the board. With a known MOE it is possible to estimate the strength of the board and assign it into a specific strength classes.
Timber
Reference roller Reference
roller Loadcell Computer
Load roller Air Press roller Displacement Air
Figure 2.27: Principle of a static bending machine.
2.5.2.2 MOE determined from vibration
The dynamic modulus of elasticity of a board can be determined by introducing vibrations into it and evaluate its resonance frequency. The principle is built on theory for vibration of prismatic beams and the equation for determining the MOE from the first resonance frequency for free-free support conditions in the axial direction can be expressed as:
where EA1 is the modulus of elasticity, ρ is the density at the measurement time, fA1 is the first resonance frequency in the axial direction and l is the beam length. For other support
conditions than the free-free and for higher order frequencies the equation has to be modified. It is also possible to use transverse vibrations. For clear wood specimens the dynamic MOE correlates well with the true MOE, the same has been seen in several studies also for timber. The method, however, has the drawback of measuring an “average” MOE of the board while the static bending machines have, at least in theory, the capacity to localise the position with the minimum MOE along the board. Several studies have shown that for material with limited knot size the techniques works as well as or better than the technique using static bending.
2.5.2.3 Other measurement techniques
With x-ray, γ-rays or microwaves it is possible to measure the density variation within a piece of timber. Several of the natural characteristics of wood imply a density that is different from normal wood. Knots and compression wood show for example a higher density and juvenile wood has a lower density. Using enough accuracy in the measurement procedure it is possible to characterise, for example the size, position and shape of the knots within a piece of timber. With known knot structure and threshold values for knot areas it is possible to create a machine that can grade timber into different strength classes.
There are also other types of techniques that can be used. One is based on the wave velocity; this technique is based on measuring the time it takes for a pulse, generated by a hammer, or
measuring MOE with a radiation technique to find the size and location of knots. There are a number of grading machines on the market; most of them are built on vibration techniques but almost as many are built on flat-wise bending and x-ray. Some of these machines also have the capability to use two of the techniques in combination.