combined non-destructive and destructive tests for the mechanical

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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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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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


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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)


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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]


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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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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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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)


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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


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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


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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


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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


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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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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)


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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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) [%]


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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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)


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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


elastic cycles

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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


failure cycles

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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


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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


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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


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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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


failure cycles

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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


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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


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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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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)


coefficients of variations

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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


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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


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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


30



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


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conclusions


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

combined non-destructive and destructive tests for the mechanical

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