Combined non-destructive
and destructive tests
for the mechanical characterization of
old structural timber elements
Beatrice Faggiano
Maria Rosaria Grippa
Anna Marzo
Federico M. Mazzolani
Department of Structural Engineering (DIST)
University of Naples “Federico II”, Italy
speaker Dr. Paolo Negro
Joint Research Centre, Ispra, Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
contents
  introduction
  material and methods
  non-destructive tests (NDT)
Aim of the study….
identification of a methodological process for
in situ mechanical characterization of ancient
timber members by means of combined nondestructive techniques.
  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.
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
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
  Global Test Methods (GTM)
- ultrasonic and vibration methods
GTM
LTM
  Local Test Methods (LTM)
- electronic penetrometers
- superficial hardness systems
experimental activity (DIST Laboratory, University “Federico II”, Naples, Italy)
evaluation of the mechanical properties of old structural timber by means of NDT compared techniques
  Non-destructive tests (NDT)
- hygrometric
- ultrasonic
- sclerometric
- resistographic
NDT
DT
  Destructive tests (DT)
- compression parallel to the grain and bending
  Correlations between NDT and DT parameters
Faggiano et al.
Department of Structural Engineering (DIST)
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University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
chestnut structural elements
material
ancient timber roofing trusses
inclined strut
king post
standard dimensions (UNI EN 408)
14 specimens type C for compression tests
6D
D
L/D≅6
10 specimens type B for bending tests
king post
inclined strut
L/D≥19
D
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
geometric survey and visual inspection
material
Visual inspection
qualitative assessment of the conservation state
Defects and alterations
  knots and fibre deviation
  longitudinal splitting
  ring shakes
  biological damage and 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]
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
longitudinal fissures
insect
attacks
knot
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
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
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
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.
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
hygrometric tests
non-destructive tests (NDT)
testing equipment
Tramex Professional device
  digital pin type resistance meter
testing set-up
  resistor pins aligned along the
direction parallel to the grain
  10 readings per specimen
  moisture content readings:
7% - 40%; 0.1% increments
  the professional is callibrated on
wood at an ambient temperature
of 20 °C
temperature Adjustment chart
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
ultrasonic investigations
non-destructive tests (NDT)
testing equipment
Ultrasonic System CMS
testing set-up
direct method
transducers
TSG-20
parallel to the grain
  5 readings per specimen
in longitudinal direction
RSG-55
perpendicular to the grain
  11 tested sections
  4 readings per section
  44 readings per specimen
in transversal direction
signal acquisition software (Pocket Sonic)
1
2
3
4
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
ultrasonic investigations
non-destructive tests (NDT)
data processing
Data Sonic processing software → signals in ASCII format → diagrams in scale time [usec] – Amplitude [mV]
typical stress waveform
Transversal signals
discovery of weak
and critical zones
TF
cross-section
fissures
T3
T2
T1
Dynamic parameters
•  TF: “Time of Flight” (transmission time)
•  SWS: Stress Wave Speed = L/TF
•  Edyn: dynamic elasticity modulus =
SWS2 ρ
T4
(*) L: distance between the transducers
ρ: wood density
Faggiano et al.
Department of Structural Engineering (DIST)
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University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
sclerometric tests
non-destructive tests (NDT)
testing equipment
testing set-up
grid area
Wood Pecker mechanical hammer
longitudinal tests
  2 tested end-sections
  9 shots per area
  18 shots per specimen
  blunt metallic needle:
25 mm diameter; 50 mm length
  shot by means of 5 spring blows
  equipped by a comparator system
pin penetration depth (PD)
  PD = Ln – (Lc+Dc) [mm]
transversal tests
  2 orthogonal directions
  5 tested areas
  9 shots per area
  90 shots per specimen
(*) Ln: pin length
Lc: comparator reading
Dc: depth of protective cap
Faggiano et al.
Department of Structural Engineering (DIST)
11
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
resistographic measurements
non-destructive tests (NDT)
testing equipment
IMLRESIF400 System
testing set-up
wax paper strip
electronic unit
longitudinal measurements
  2 tested end-sections
  5 measures
per section
  10 measures
per specimen
transversal measurements
  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
  11 tested sections
  2 measures
per section
  22 measures
per specimen
  data recorder on a wax paper strip at a scale of 1:1
and stored on a electronic unit as graphic profiles
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
resistographic measurements
non-destructive tests (NDT)
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
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
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
longitudinal profile
similar drilling resistance on homogeneous parts
damaged zone
transversal profile
strong variation of drilling resistance due
the crossing of the growth rings
hollow area
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)
14
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
experimental results
non-destructive tests (NDT)
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) [%]
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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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
compression tests parallel to the grain
destructive tests (DT)
testing equipment
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)
load cell
top fixed
head
test methods
The compression tests were performed in two step:
1.  Elastic cycles, for the determination of the elasticity
specimen
base plate
LVDTs
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
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
destructive tests (DT)
compression tests parallel to the grain
elastic cycles
testing set-up
test procedure: 3 load ranges
elasticity modulus
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
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)
Faggiano et al.
Department of Structural Engineering (DIST)
17
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
destructive tests (DT)
compression tests parallel to the grain
failure cycles
testing set-up
typical F-w curves
compression
strength
2 LVDTs (5, 6) are placed on loading-base
plate of the testing machine, measuring the
base displacements (w) of the samples
envelope curves and mean curve
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)
Faggiano et al.
Department of Structural Engineering (DIST)
18
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
destructive tests (DT)
compression tests parallel to the grain
collapse mechanisms
longitudinal splitting
fractures in radial direction
cleavage along the annual rings
multidirectional rupture
The propagation of the failure patterns
was developed for the presence of
defects, such as longitudinal cracks,
ring shakes, knots and slope of grain
Faggiano et al.
Department of Structural Engineering (DIST)
19
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
bending tests
destructive tests (DT)
testing equipment
Mohr & Federhaff machine
actuator
steel profile
support
specimen
test arrangement (UNI EN 408)
test methods
  four-points static scheme
The bending tests were performed in two step:
  simply supported beam by means of two
semi-cylindrical hinges
1.  Elastic cycles, for the determination of the
  loaded beam with two concentrated forces
local (Em,l) and global (Em,g) elasticity modulus
2.  Failure cycles, for the evaluation of the bending
by interposition of a steel profile between
strength (fc,0), post-elastic behaviour and collapse
the actuator and the specimen
modes of the tested specimens
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
bending tests
destructive tests (DT)
elastic cycles
testing set-up
test procedure: 3 load ranges
local elasticity modulus
global elasticity modulus
typical F-w curves
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
Faggiano et al.
Em,l (I) = 12391 N/mm2
Em,l (II) = 12375 N/mm2
Em,l (III) = 12068 N/mm2
Em,l (mean) = 12278 N/mm2
Department of Structural Engineering (DIST)
Em,g (I) = 10307 N/mm2
Em,g (II) = 11036 N/mm2
Em,g (III) = 11011 N/mm2
Em,g (mean) = 11023 N/mm2
University of Naples “Federico II”
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Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
bending tests
destructive tests (DT)
failure cycles
testing set-up
typical F-w curves
bending
strength
LVDT5
LVDT4
LVDT (4) is placed at the mid-span of the
beam; LVDT (5) measures the loadingactuator displacements.
envelope curves
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)
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
bending tests
destructive tests (DT)
collapse mechanisms
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
Specimen B1
Specimen B4
Specimen B6
Specimen B8
Specimen B5
Specimen B9
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
bending tests
destructive tests (DT)
collapse mechanisms
specimen B3
rupture at the beam tension side: tear of the more stressed fibres
II cycle
V cycle
VII cycle
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IX cycle
Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
bending tests
destructive tests (DT)
collapse mechanisms
specimen B2
rupture at the beam compression side: buckling mode and slipping phenomena
I cycle
Faggiano et al.
III cycle
Department of Structural Engineering (DIST)
25
IV cycle
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
experimental results
destructive tests (DT)
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)
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
coefficients of variations
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
applied methodology
mechanical identification
by NDT- DT parameters
NDT parameters (X)
DT parameters (Y)
longitudinal and transversal average values:
  SWS: ultrasonic stress wave speed [m/sec]
  PD: sclerometric penetration depth [mm]
  Am: resistographic drilling-resistance [%]
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
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
estimation of wood density
mechanical identification
by NDT- DT parameters
regression-coefficients (R2) between density and NDT parameters
sclerometric and, especially, resistographic techniques may be taken in practical applications
for a quick and quantitative estimation of wood density
resistographic tests
density
density
sclerometric tests
penetration depth
Faggiano et al.
Department of Structural Engineering (DIST)
drilling resistance
University of Naples “Federico II”
28
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
prediction of elasticity modulus
mechanical identification
by NDT- DT parameters
regression-coefficients (R2) between elasticity modulus and NDT parameters
the prediction of the stiffness properties could be allowed by means of ultrasonic investigations
static elasticity modulus
static elasticity modulus
ultrasonic tests
stress wave speed
Faggiano et al.
Department of Structural Engineering (DIST)
dynamic elasticity modulus
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
prediction of strength
mechanical identification
by NDT- DT parameters
regression-coefficients (R2) between strength and NDT parameters
the drill-resistance method appears mostly suitable for a non-destructive estimation of strength
bending strength
compression strength
resistographic tests
mean amplitude
Faggiano et al.
Department of Structural Engineering (DIST)
drilling resistance
University of Naples “Federico II”
30
Italy
Combined non-destructive and destructive tests
for the mechanical characterization of
old structural timber elements
conclusions
on the basis of experimental results…
sclerometric
definition of a methodology for in situ mechanical characterization of old timber members based on NDT techniques
strength (f)
resistographic
×
ultrasonic
density (ρ)
SWS2
=
dynamic elasticity
modulus (Edyn)
static elasticity
modulus (E)
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Faggiano et al.
Department of Structural Engineering (DIST)
University of Naples “Federico II”
Italy
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