Acoustic technique, application for lumber and …...Course in Non Destructive Testing of Wood 03...

Course in Non Destructive Testing of Wood 03 Acoustic téchnique – Pág. 1ETSI Montes, ETS Arquitectura – Universidad Politécnica de Madrid Madrid, Junio 2005

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Acoustic technique,

application for lumber and structural member evaluation

Coupling

• Little air gap between sample and sensor reduce the signal amplitude

• Goal: no air gap between the sensor and specimen

• Materials: any elastic material, low attenuation: water, grease, fat, soft wax,glue, etc.

• Simple solution: bee wax and paraffin oil


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Coupling by compression

• Spike

• Pressure

0

20

40

60

80

100

120

0 50 100 150 200 250

Force (N)

Siga

nal a

mpl

itude

(mV)

The transducers of FAKOPP 2D microsecond timer


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Pizofilm vibration sensor

www.piezofilm.com

Piezo sensor, accelerometer

• Components:- piezo ceramics (BaTi)- mass- spring- house- connector


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90 kHz ultrasonic signal

Time determination


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40 kHz ultrasonic signal

perpendicular parallel to the grain

Transducers distance is 10 cm

Signal amplitude

Amplitude2 is proportional to the signal energy


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Artificial saw cut

For the energy measurement uniform start signal is necessary

Pendulum for uniform stress wave signal


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VISUALIZATION OF THE STRESS WAVE PROPAGATION IN WOOD

1.) Grid points are marked on the sample surface.

2.) P-wave transit time measurement between the starter (position not changed) and receiver sensors.

3.) Step-by-step the receiver placed at all the grid points, and transit time was recorded in a time matrix.

4.) The matrix is visualized by Excel surface plot. A simple pendulum provided the uniform impact.

The time matrix


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P-wave propagation in intact and hollow oak disk. Grid size is 2x2 cm, time resolution: 20 microseconds,

means that the time width of a strip is 20 ms.

P-wave propagation in longitudinal direction in lumber, around a knot. Material is spruce, dimension 10x41cm, grid size 1x1cm, time

resolution is 3 microseconds.


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Sound propagation in log

Longitudinal wave propagation in LR plane

33 cm


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Result: elliptical wave surface

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Longitudinal wave propagation, 3D model


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Stress wave velocity in 1D and 3D solids

1D: rod, long beam

3D: mature tree (?)

Poisson ratio, dry soft wood, νLR=0,39 νLT=0,46first index is the direction of the force,

second index represents the direction of the deformation

ρMOEV D =1

)21)(1()1(

3 υυυ

ρ −+−

=MOEV D


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+++ - - -+++ - - -

+ : compression

- : tension

Free dilatation and

contraction is possible:

Stress wave propagation in 1D

ρMOEV D =1

MOE : Modulus of elasticity

ρ : density

)21)(1()1(

3 υυυ

ρ −+−

=EV D

Stress wave propagation in 3D

source

Ball wave+ compression

- tension

Free dilatation and contraction is NOT possible:

ν : Poisson ratio


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Stress wave propagation in mature tree close to the bark

mature tree

+ + +

- - -

1D or 3D ?

Simple approach: 3D, but Poisson ratio (ν ) is reduced to the half because dilatation and compression is partly possible.

Poisson ratio, dry soft wood, νLR=0,39 νLT=0,46first index is the direction of the force,

second index represents the direction of the deformation

Effective poisson ratio: ν=0,195

11,05

1,11,15

1,21,25

1,31,35

1,41,45

1,5

0 0,1 0,2 0,3 0,4 0,5

Poisson ratio

V 3D /

V 1D

1D and 3D velocity ratio as a function of Poisson ratio(Time of Flight (TOF) and resonance velocity ratio)


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Figure from M.K. Andrews paper, 13th International Wood NDT

Symp.

Industrial Research Ltd., New Zealand

Experimental determination of V3D/V1D

Stress wave velocity V3D determined on log by TOF slope technique, then a 4 by 4 cm test bar was cut and V1D measured by the same technique

Result: V3D/V1D = 1,02


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Stress wave velocity measured by TOF and resonance technique (longitudinal vibration)

Lumber description VTOF/Vresonance

spruce (full of knots) 1,088

spruce (clear) 1,025

beech (clear) 1,041

robinia (clear) 0,986

spruce (clear) 1,010

Reason? Dispersion?

velocity depends on frequency

Dynamic MOE determination by longitudinal vibration

2, VMOE longdyn ρ=

LfV 2=ρ: density

V: longitudinal velocity

L length of the specimen

f: frequency of the longitudinal vibration

Frequency measurement by FFT analyzer


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Longitudinal vibration and the spectra of a defect free spruce lumber.

Length (L): 3 m.

Resonance velocity determination:

V=2Lf1=2Lfn/n

Calculated velocity depends on mode number.

Which frequency?

f1

f2

fn

PC based FFT analyzer


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Applications

Log grading based on acoustic speed, resonance technique, sensor is accelerometer

Director 2000 tool by Fiber-gen, NZ

Measures longitudinal vibration frequency


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Timber grading by acoustic technique

DynaGrage tool by Dynalyse AB, Sweden

Longitudinal vibration, Lumber grader

Microtec, ViSCAN


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Longitudinal vibration, Portable Lumber Grader (PLG)

Sylvatest, lumber grader


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Sylvatest Duo, ultrasonic timer

Ultrasonic timer for testing sidlings


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Demonstration

• MOE determination• MOE=ρV2

• Velocity determination by:• - Sylvatest• - Fakopp Microsecond timer• - Longitudinal vibration• - Static bending

Acoustic technique, application for lumber and …...Course in Non Destructive Testing of Wood 03...

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Transcript of Acoustic technique, application for lumber and …...Course in Non Destructive Testing of Wood 03...