combined non-destructive and destructive tests for the mechanical
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Beatrice Faggiano
Maria Rosaria Grippa
Anna Marzo
Federico M. Mazzolani
Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
Department of Structural Engineering (DIST)
University of Naples “Federico II”, Italy
speaker Dr. Paolo Negro Joint Research Centre, Ispra, Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
introduction
material and methods
non-destructive tests (NDT)
destructive tests (DT)
mechanical identification by NDT-DT methods
The research activity has been developed in the framework of the Italian project PRIN 2006 “Diagnosis techniques and totally removable low invasive strengthening methods for the structural rehabilitation and the seismic improvement of historical timber structure”, prof. M. Piazza coordinator.
Research UNIT UNINA “Experimental evaluation of the mechanical properties of wood by means of non-destructive compared techniques for the characterization of existing wooden structures”, research team: prof. B. Faggiano scientific responsible, Ing. M. R. Grippa and Ing. A. Marzo.
contents
Aim of the study…. identification of a methodological process for
in situ mechanical characterization of ancient
timber members by means of combined non-
destructive techniques.
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
introduction
non destructive techniques (NDT) towards diagnosis, structural analysis and consolidation intervention of existing wooden structures
experimental activity (DIST Laboratory, University “Federico II”, Naples, Italy) evaluation of the mechanical properties of old structural timber by means of NDT compared techniques
Global Test Methods (GTM) - ultrasonic and vibration methods
Local Test Methods (LTM) - electronic penetrometers - superficial hardness systems
GTM LTM
Non-destructive tests (NDT) - hygrometric - ultrasonic - sclerometric - resistographic
Destructive tests (DT) - compression parallel to the grain and bending
Correlations between NDT and DT parameters
NDT DT
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
material chestnut structural elements
inclined strut king post
ancient timber roofing trusses
king post
inclined strut
D
L/D≅6
6D
14 specimens type C for compression tests
D
L/D≥19
10 specimens type B for bending tests
standard dimensions (UNI EN 408)
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
material geometric survey and visual inspection
Visual inspection qualitative assessment of the conservation state Defects and alterations knots and fibre deviation longitudinal splitting ring shakes biological damage and insect attacks
longitudinal fissures knot
14 specimens type C for compression tests
square cross-section with large rounded edges Dmean [14.5 cm; 16.0 cm] L (≅ 6D) [87 cm; 96 cm]
knots ring shake
insect attacks
10 specimens type B for bending tests
nearly circular cross-section Dmean [15.0 cm; 16.5 cm]
L (≥ 19D) [300 cm; 340 cm]
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
non-destructive methods (NDT)
hygrometric tests
evaluation of the moisture content of the wood, which severely affects its mechanical properties and its susceptibility to degradation by decay ultrasonic investigations
definition of a direct relationship between the stress wave speed and the elastic properties of the material, based on the theory of acoustic wave propagation sclerometric tests
assessment of the material hardness and superficial consistence by the penetration depth of a blunt pin, fired into the wood resistographic measurements
detection of internal defects and density variations by means of measure of drilling resistance along the path of a small needle inserted into the wood
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive methods (DT)
compression tests parallel to the grain (14 specimens type C)
determination of the modulus of elasticity (Ec,0) and strength (fc,0) in compression bending tests (10 specimens type B)
determination of the local (Em,l) and global (Em,g) modulus of elasticity, together with bending strength (fm)
The full-scale static tests aim at evaluating the structural behaviour of the timber elements in terms of stiffness, load
bearing capacity and collapse mechanism. The tests were performed according to UNI EN 408 standard.
UNI EN 408 (2004) Timber structures – Structural timber and glued laminated timber – Determination of some physical and mechanical properties.
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
hygrometric tests
digital pin type resistance meter
moisture content readings: 7% - 40%; 0.1% increments
the professional is callibrated on wood at an ambient temperature of 20 °C
Tramex Professional device
testing equipment
temperature Adjustment chart
testing set-up
resistor pins aligned along the direction parallel to the grain 10 readings per specimen
non-destructive tests (NDT)
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
non-destructive tests (NDT) ultrasonic investigations
Ultrasonic System CMS transducers
TSG-20
RSG-55
signal acquisition software (Pocket Sonic)
testing equipment testing set-up
direct method
parallel to the grain 5 readings per specimen in longitudinal direction
perpendicular to the grain 11 tested sections 4 readings per section 44 readings per specimen in transversal direction
1 2 3
4
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
non-destructive tests (NDT) ultrasonic investigations
data processing
Data Sonic processing software → signals in ASCII format → diagrams in scale time [usec] – Amplitude [mV]
T3
T4
T1 T2
Transversal signals discovery of weak and critical zones
cross-section fissures
TF
typical stress waveform
Dynamic parameters
• TF: “Time of Flight” (transmission time)
• SWS: Stress Wave Speed = L/TF
• Edyn: dynamic elasticity modulus = SWS2 ρ (*) L: distance between the transducers ρ: wood density
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
non-destructive tests (NDT) sclerometric tests
testing set-up
grid area longitudinal tests 2 tested end-sections 9 shots per area 18 shots per specimen
transversal tests 2 orthogonal directions 5 tested areas 9 shots per area 90 shots per specimen
testing equipment
blunt metallic needle: 25 mm diameter; 50 mm length shot by means of 5 spring blows equipped by a comparator system
Wood Pecker mechanical hammer
pin penetration depth (PD) PD = Ln – (Lc+Dc) [mm] (*) Ln: pin length Lc: comparator reading Dc: depth of protective cap
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
non-destructive tests (NDT) resistographic measurements
testing equipment
wax paper strip
electronic unit
IMLRESIF400 System
drilling resistance measuring system
needle 1.5 to 3.0 mm diameter and 40 mm length
spent energy of drilling device measured as
drilling resistance every 0.1 mm
data recorder on a wax paper strip at a scale of 1:1
and stored on a electronic unit as graphic profiles
longitudinal measurements
testing set-up
2 tested end-sections 5 measures per section 10 measures per specimen
11 tested sections 2 measures per section 22 measures per specimen
transversal measurements
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
non-destructive tests (NDT) resistographic measurements
data processing
F-Tools Pro software → data in ASCII format → graphic profiles in scale Drilling Depth [mm] – Amplitude [%]
wood with high quality presence of internal more resistant part presence of knots on the lateral surface
wood of good quality homogeneous parts
wood of poor quality damaged zone with low amplitude hollow areas with null drilling resistance
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
non-destructive tests (NDT) resistographic measurements
data processing
mean value of amplitude as parameter of wood drilling resistance
Mean amplitude (Am)
ratio between the integral of the diagram area and the depth of the needle path (L)
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
damaged zone
longitudinal profile similar drilling resistance on homogeneous parts
hollow area
transversal profile strong variation of drilling resistance due the crossing of the growth rings
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
non-destructive tests (NDT) experimental results NDT parameters average values for each specimen Hygrometric tests moisture content (MC) [%] Ultrasonic investigations stress wave speed (SWS) [m/sec] Sclerometric tests pin penetration depth (PD) [mm] Resistographic measurements drilling resistance (Am) [%]
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
mean values and variation coefficients
the resistographic parameters are affected by the higher coefficients of variations, due to the presence of defects which inflluence the local measures of the drilling-resistance
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) compression tests parallel to the grain testing equipment
test methods The compression tests were performed in two step: 1. Elastic cycles, for the determination of the elasticity
modulus (Ec,0) 2. Failure cycles, for the evaluation of the
compression strength (fc,0), post-elastic behaviour and collapse modes of the tested specimens
top fixed head
base plate specimen
load cell
LVDTs
Mohr & Federhaff machine
Hydraulic press with capacity of 5000 kN (force controlled tests) Loading cell HBM of 740 kN Displacements transducers (LVDT) with accurancy of 1×10-3 Acquisition system (HBM-Spider 8) and PC for data recording by means of the software Catman (v. 6.2)
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) compression tests parallel to the grain
4 LVDTs (1, 2, 3, 4) are placed on the opposite faces of the specimen to measure the relative displacements (Δw) in the elastic range over the central gauge length (L1)
testing set-up
test procedure: 3 load ranges
typical F-w curves
Ec,0 (I) = 9268 N/mm2
Ec,0 (II) = 9948 N/mm2
Ec,0 (III) = 11016 N/mm2
Ec,0(mean)=10078 N/mm2
elasticity modulus
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
elastic cycles
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) compression tests parallel to the grain
testing set-up
2 LVDTs (5, 6) are placed on loading-base plate of the testing machine, measuring the base displacements (w) of the samples
test procedure several loading-reloading failure cycles in each step, the load is increased up to the failure and applied at a constant base plate speed in such a way that the maximum force is reached within 300±120 sec (UNI EN 408)
typical F-w curves
envelope curves and mean curve
compression strength
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
failure cycles
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) compression tests parallel to the grain
fractures in radial direction
cleavage along the annual rings
multidirectional rupture
longitudinal splitting
The propagation of the failure patterns was developed for the presence of defects, such as longitudinal cracks, ring shakes, knots and slope of grain
collapse mechanisms
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) bending tests testing equipment
test methods The bending tests were performed in two step: 1. Elastic cycles, for the determination of the local (Em,l) and global (Em,g) elasticity modulus 2. Failure cycles, for the evaluation of the bending
strength (fc,0), post-elastic behaviour and collapse modes of the tested specimens
steel profile
actuator
support specimen
test arrangement (UNI EN 408)
four-points static scheme simply supported beam by means of two semi-cylindrical hinges loaded beam with two concentrated forces by interposition of a steel profile between the actuator and the specimen
Mohr & Federhaff machine
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) bending tests
testing set-up
LVDT2 LVDT3
LVDT1
LVDT4
Determination of the local elasticity modulus: 3 LVDTs (1, 2, 3) are placed on the neutral axis to measure the deformations over the central gauge length (w; L1) Determination of the global elasticity modulus: LVDT (4) measures the deflections (w) at the mid-span of the beam
elastic cycles
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
test procedure: 3 load ranges
Em,l (I) = 12391 N/mm2 Em,l (II) = 12375 N/mm2 Em,l (III) = 12068 N/mm2 Em,l (mean) = 12278 N/mm2
typical F-w curves
Em,g (I) = 10307 N/mm2 Em,g (II) = 11036 N/mm2 Em,g (III) = 11011 N/mm2 Em,g (mean) = 11023 N/mm2
local elasticity modulus
global elasticity modulus
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) bending tests
typical F-w curves
envelope curves
bending strength
testing set-up
LVDT (4) is placed at the mid-span of the beam; LVDT (5) measures the loading-actuator displacements.
test procedure several loading-reloading failure cycles in each step, the load is increased up to the failure, reaching the maximum force within 300±120 sec (UNI EN 408)
LVDT4
LVDT5
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
failure cycles
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) bending tests
Specimen B1 Specimen B4 Specimen B5
Specimen B6 Specimen B8 Specimen B9
For almost all the specimens, failure mechanisms triggered around large knots located at the central zone at the tensile edge of the beam cross-section, in some cases anticipated by fibres buckling mechanism at the compression side and slip phenomena
collapse mechanisms
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) bending tests collapse mechanisms
II cycle
V cycle
VII cycle IX cycle
specimen B3
rupture at the beam tension side: tear of the more stressed fibres
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) bending tests collapse mechanisms
I cycle III cycle
IV cycle Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
specimen B2
rupture at the beam compression side: buckling mode and slipping phenomena
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
destructive tests (DT) experimental results
mechanical properties E: elasticity modulus [N/mm2] - Ec,0: in compression - Em,l: local in bending - Em,g: global in bending f: strength [N/mm2] - fc,0: in compression - fm: in bending
compression tests bending tests physical and dynamic properties ρ: wood density [kg/m3] determined on the basis of the measured sizes and weight of the timber specimens Edyn: dynamic elasticity modulus [N/mm2] determined by means of the theoretical relation for homogenous and isotropic elements:
Edyn = SWSL2 ρ
(SWS is the stress wave speed)
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
coefficients of variations
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
mechanical identification by NDT- DT parameters
applied methodology
NDT parameters (X) longitudinal and transversal average values: SWS: ultrasonic stress wave speed [m/sec] PD: sclerometric penetration depth [mm] Am: resistographic drilling-resistance [%]
DT parameters (Y) physical and mechanical properties: ρ: wood density [kg/m3] E: modulus of elasticity [N/mm2] f: strength [N/mm2]
linear regression model estimation of wood density
prediction of elasticity modulus
prediction of strength
regression-coefficient (R2) ranges
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
mechanical identification by NDT- DT parameters
estimation of wood density
sclerometric and, especially, resistographic techniques may be taken in practical applications for a quick and quantitative estimation of wood density
regression-coefficients (R2) between density and NDT parameters
dens
ity
sclerometric tests
penetration depth
dens
ity
resistographic tests
drilling resistance
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
mechanical identification by NDT- DT parameters
prediction of elasticity modulus
regression-coefficients (R2) between elasticity modulus and NDT parameters
the prediction of the stiffness properties could be allowed by means of ultrasonic investigations
stat
ic e
last
icity
mod
ulus
ultrasonic tests
stat
ic e
last
icity
mod
ulus
stress wave speed dynamic elasticity modulus
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
mechanical identification by NDT- DT parameters
prediction of strength
the drill-resistance method appears mostly suitable for a non-destructive estimation of strength
regression-coefficients (R2) between strength and NDT parameters
resistographic tests
com
pres
sion
stre
ngth
mean amplitude
bend
ing
stre
ngth
drilling resistance
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
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Combined non-destructive and destructive tests for the mechanical characterization of old structural timber elements
conclusions
Faggiano et al. Department of Structural Engineering (DIST) University of Naples “Federico II” Italy
on the basis of experimental results… definition of a methodology for in situ mechanical characterization of old timber members based on NDT techniques
density (ρ)
strength (f)
static elasticity modulus (E)
dynamic elasticity modulus (Edyn)
scle
rom
etric
resistographic
× = SWS2
ultra
soni
c
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