Multi-scale characterization of chestnut timber elements
3.2 Phase 1 results
(
2 1)
1 2 1
X,0 A w w
F F E l
−
⋅
−
= ⋅ (3.5)
where (F2 − F1) is an increment of load (N) on the straight line portion of the load deformation curve, (w2 − w1) is the increment of deformation (mm) corresponding to (F2 − F1), A is the cross sectional area of the central cross section (mm2) and l1 is the gauge length (mm) for the determination of EX,0 (either compression Ec,0, or tension Et,0) (N/mm2).
In compression, the deformation was measured over a central gauge length of four times the smallest cross section dimension of the sample, using one pair of LVDT's placed on opposite faces to eliminate the effect of possible distortion. In tension specimens, deformation was measured over a central gauge length of ten times the smallest cross section dimension, using a clip gauge extensometer. Compression and tension parallel to the grain strength, respectively fc,0 and ft,0, were calculated as:
A
fX,0 = Fmax (3.6)
where Fmax (N) is the maximum applied load until failure and the A (mm2) is the cross sectional area near the section in failure.
Before being tested, all samples were conditioned in a climatic chamber capable of keeping a temperature of 20 ± 2ºC and a humidity of 65 ± 5%, until constant mass was obtained. Constant mass is considered to be attained when the results of two successive weighting, carried out at an interval of six hours, do not differ by more than 0.1% of the mass of the test piece. Weightings were made to ten specimens chosen randomly, using a weighting scale with a precision of 0.01 g in intervals of six hours until the conditioned state was achieved.
3.2 Phase 1 results
This experimental phase allows, by use of NDT and SDT, the definition of the mechanical properties found for structural size elements accounting for a common state of conservation in existing old timber structures.
3.2.1 Non and semi-destructive testing Visual inspection
The first step of the visual inspection comprised the geometrical characterization of each element. For that purpose, the beginning and end of each segment was measured using a three side ruler (precision of mm). By using this tool, it was possible to obtain a reasonable
definition of the geometry of each cross section including wane with only two measurement positions (Figure 3.10).
The length of the elements varied between 4 m and 6 m with a mean value of 5.32 m (COV = 11.8%). The average values for the nominal cross section dimensions were 18.0 cm (COV = 3.1%) for height and 13.0 cm (COV = 6.0%) for width. Even if the variation in the nominal cross section dimensions within each element was low, significant wane was found. This wane was mainly consequence of the initial sawing process rather than from deterioration (elements still presented sharp edges) and did not pose problems to the existing connections to other structural elements.
Figure 3.10: Measurement of the cross section dimensions: example of two consecutive sections.
In each 40 cm segment, the significant parameters for visual strength grading were reported. Along the length of a timber element, it is well noticeable the variation in both quantity and size of defects, anomalies and decay (Figure 3.11) and therefore even if a section presents damage that may limit its structural behaviour, others may still present a satisfactory condition.
a) b) c) d)
Figure 3.11: Examples of defects and anomalies found in the wood elements:
a) deterioration of internal fibers by xylophagous; b) decay by fungi;
c) superficial attack of xylophagous; d) knots and fiber misalignment.
Visual grading was considered for the residual cross section, thus not considering the decayed external layers and also assuming a rectangular section without wane. By this process it is intended to obtain the mechanical properties related to the material itself and its defects rather than to the state of conservation. Another purpose of this classification
15,4
13,6 11
18,8
12,4 7
1,7
6,35
4,4 13,3
3,8
2,1
[cm]
without the parameters of external damage is to obtain reference values for comparison with the small samples that will be taken in the following steps of the experimental campaign. In order to have a qualitative comparison along the timber beams length but also between different beams, the percentage of segments that is included in a given visual class is accounted for each beam (see Figure 3.12).
A B C D E F G H I J K L M N O P Q R S T
0 20 40 60 80 100
% of segments
old beam element
I II III NC
Figure 3.12: Percentage distribution of segments of old beams included in each visual grading class regarding UNI 11119 (UNI, 2004).
In this case, only beams C, G and L have segments with grade NC, resulting from the presence of significant knots. With exception of beam G, a minimum percentage of 70%
for the sum of classes I and II is found for each beam.
Penetration impact tests
The results of the impact tests evidenced a mean value of 11.3 mm penetration depth, moreover presenting that measurements made within the same segment had lower variation compared to the measurements made within member and between members. In almost all beams the coefficient of variation is higher when analysing the values along the total beam (mean COV = 17.6%) than when analysing the measurements within a segment (mean COV = 13.1%). This is due to the local nature of the test and its dependency to the decay level. Since decay is not evenly spread through the length of the beam the variation increases when assuming the global values rather than the local measurements.
By complementing a traditional visual inspection with NDT results permitted, in this case, to verify that the decay found in some parts of the timber beams was essentially superficial, since the difference between visible decayed and non-decayed segments had an average increase of approximately 1.1 mm in the penetration depth.
Drilling resistance tests
The drilling resistance tests were performed in the ending parts of the beams (segments that presented higher extent of visible decay) in a section that still presented a defined cross section, obtaining values of resistance measure, RMdecay. A mean value of 245 bit
with COV of 22.6% was found, while neither significant voids nor lower resistant sections in the inner regions of the cross section were found in the drilling resistance profiles.
Drilling resistance tests were useful to estimate the depth of the decayed layer, even if it should be noted that the measurements are made regarding only the drilling path of the device needle, and therefore only evaluate the material locally.