1. COMBINING RESULTS FROM VISUAL INSPECTION, NON-DESTRUCTIVE TESTING & SEMI-DESTRUCTIVE TESTING TO PREDICT THE MECHANICAL PROPERTIES OF WESTERN HEMLOCK
Can predictions of mechanical properties be improved by combining results?
Which results can be most usefully be combined?
Dan Ridley-Ellis
David Gil-Moreno
Mike Bather
The aim of this early years PhD research is to combine non-destructive testing (NDT), visual inspection and the testing of small and very small (micro) samples to make a good predictive model for static modulus of elasticity (MOE), bending strength (modulus of rupture – MOR) and density.
VISUAL INSPECTIONS
The visual inspections focus on just three characteristics of timber: knots, rate of growth (ring width) and slope of grain. Four ratios relating to knots were obtained, and the two most useful of these were found to be tKAR (total knot area ratio of a knot group) and mKAR (margin knot area ratio of a knot group, mKAR). MiCROTEC software was used to calculate values based on an assumption that knots up to 150mm apart are considered as a knot group. Rate of growth, general and local slope of grain were found to have particularly weak predictive powers.
150 STRUCTURAL SIZED JOISTS
50 x 100 x 3100mm
135 MICRO CLEAR SPECIMENS
6.5mm dia x 91mm
COMBINED
(adjusted R2)
R2=0.94
R2=0.93
Joist dynamic MOE + tKAR
0.51
R2=0.61
Joist dynamic MOE + mKAR
Ring width
Density of full cross section
Mean MOE of micro clears A+B
0.32
R2=0.90
Mean density of micro clears A+B
6.5mm dia. cut outs in supports to create curved bearing surfaces
Joist dynamic MOE
MICRO CLEAR SPECIMENS
mKAR
R2=0.72
MODULUS OF ELASTICITY OF JOISTS TESTED TO DESTRUCTION
Ring width + tKAR
Pairs of cores taken from blocks to create standard sized specimens
R2=0.66
Mean micro MOE + tKAR
R2=0.27
General slope of grain
R2=0.25
R2=0.67
Ring width + mKAR
tKAR
R2=0.00
Local slope of grain
0.57
Mean micro MOE + mKAR
R2=0.29
R2=0.02
Mean MOE of micro clears A+B
Ring width
R2=0.69
Coring reduces dimensional differences between specimens
Micro clear specimen just after failure in three point bending
Density of full cross section
Joist dynamic MOE + tKAR
R2=0.68
Joist dynamic MOE + mKAR
R2=0.21
R2=0.15
Joist dynamic MOE
Scatter plot relating static modulus of elasticity (MOE) of small clear and micro clear specimens and dynamic MOE of structural sized joists with static MOE of structural sized joists
R2=0.31
mKAR
0.50
Mean MOR of micro clears A+B
0.56
0.33
tKAR + ring width
BENDING STRENGTH OF JOISTS TESTED TO DESTRUCTION
General slope of grain
0.53
tKAR + mean micro MOE
tKAR + mean micro MOR
R2=0.27
R2=0.01
0.56
tKAR
Local slope of grain
R2=0.29
R2=0.07
TO DO LIST (conclusions)
Find which test results combine to give the best predictions of mechanical properties
 Devise a method of measuring knots focusing on the surface location and appearance of knots in place of knot area ratios
 Try combining knot measurements with dynamic stiffness measurements using insitu time of flight measurements
0.53
Density of micro clear B
Ring width
R2=0.74
Scatter plot relating bending strength (modulus of rupture, MOR) of small clear and micro clear specimens with bending strength (MOR) of structural sized joists
Mass of complete joist
R2=0.70
Mean density of micro clears A+B
R2=0.22
DENSITY OF BLOCKS CUT FROM JOISTS (EN408)
DOWNLOAD
THE PAPER
HERE!
REFERENCES
MiCROTEC. Web Knot Calculator v2.2.
Piazza, M. and Riggio, M.: Visual strength-grading and NDT of timber in traditional structures. Journal of Building Appraisal, 3:267–296, 2008
White, R. H. and Ross R. J.: Wood and timber condition assessment manual, 2nd Ed. General Technical Report FPL-GTR-234. Forest Products Laboratory. Wisconsin, U.S., 2014
EN 384, EN 408, EN 1310, EN 13556, EN , BS 373, BS 4978, CP 112-2
R2=0.68
Density of micro clear A
RESULTS
The diagram represents the coefficients of determination, R2 as circles:
BEST CORRELATION
GOOD CORRELATION