Ultrasonic indirect method for evaluating clear wood strength ...NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 Ultrasonic indirect

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

Ultrasonic indirect method for evaluating clear wood strength and stiffness

José MACHADO1, Pedro PALMA1, Sofia SIMÕES1

1 Laboratório Nacional de Engenharia Civil, Lisboa, PORTUGAL, e-mail [email protected]

Abstract Strength and stiffness assessment of timber members in-service is a crucial task in the

rehabilitation of a timber structure. Unsatisfactory results given by traditional procedures applied for appraisal of in-service structural timber members are a strong motivation for research on non-destructive techniques (NDT). For assessing strength and stiffness of clear wood zones of structural members an indirect ultrasonic method is proposed, given the advantage of requiring the access to only one surface of the element.

This paper presents results of a study comprising three different wood species (oak, maritime pine and european spruce) with two different cross-sections and 150kHz transducers. Different arrangements of the transducers on the same surface were tested and the results compared with the ones obtained with a direct method (transducers placed at opposite ends of the specimens). The results obtained are discussed having in mind the ones already obtained in almost clear structural timber elements as well as in small size clear wood specimens.

Résumé L'évaluation de la résistance et de la rigidité des éléments en bois en service est une tâche

cruciale dans le processus de réhabilitation d’une structure en bois. Les résultats insuffisants donnés par des procédures traditionnelles pour l'évaluation des membres en bois en service sont une motivation forte pour la recherche sur les techniques non destructives (TND). Pour évaluer la résistance et la rigidité des zones en bois sans défaut des éléments structurels, une méthode ultrasonore indirecte est proposée ; elle présente l'avantage d'exiger l'accès à seulement une surface de l'élément.

Ce document présente des résultats d'une étude comportant trois espèces en bois différentes (chêne, pin maritime et sapin européen) avec deux sections transversales différentes et utilisant des sondes de 150kHz. Différentes implantations des sondes sur la même surface ont été examinées et les résultats ont été comparés à ceux obtenus avec une méthode directe (sondes placées aux extrémités des corps d’épreuve). Les résultats obtenus sont discutés en ayant en tête les résultats déjà obtenus en éléments de bois de construction presque sans défaut aussi bien que dans les petits échantillons en bois sans défaut.

Keywords non-destructive testing; wave propagation; pine ; spruce ; oak

1 Introduction Timber inspection is conducted with the aim of providing valuable information for safety

analysis of timber structures. Reliable tools for assessing mechanical behaviour of timber members are specially needed in the case of old timber structures where no information is available on wood species and characteristic strength and stiffness values. Long-established procedure for assignment of strength and stiffness values to structural timber members is


based on application of visual strength grading standards. This approach underestimates the actual strength capacity of timber members and gives unsatisfactory answers.

Assessment of strength and stiffness of timber members is therefore a challenge. On this topic a national research project began in 2008, having in mind the development of a probabilistic safety analysis model for old timber structures. The project is composed of several tasks, being one related with the development of tools for predicting the strength and stiffness capacity of timber elements, as a joint distribution of weak and clear wood zones.

Evaluation of strength and stiffness of clear wood zones will be studied by using non-destructive (velocity of ultrasonic wave propagation), and semi-destructive testing (extraction of wood material to perform tension testing, identification of wood species and determination of density). Ultrasounds application to in-service timber members should have in mind that in most situations only one face of the element will be available and therefore standard direct method (longitudinal wave propagation between ends of the element) used for sawn timber grading is not applicable and an alternative method should be envisaged, Fig. 1.

Figure 1. Indirect (left) and direct (right) methods applied to timber elements

Indirect methods are often used with concrete. PUNDIT manual mentions that for concrete the pulse velocity determined by the indirect method of testing will be lower than that using the direct method, being possible however to point out a conversion factor of 1.05 (Vdirect ∼ 1.05 Vindirect).

Indirect method was already used in some studies carried out by the research team. Application of ultrasounds on the assessment of clear wood zones combined with information from knots characteristics show to improve the capacity of predicting strength and stiffness of structural dimension pine elements [1]. Ultrasonic testing of small clear pine specimens also show a good capability of predicting the compression strength parallel to the grain [2] and an acceptable capacity for predicting stiffness in tension and compression parallel to grain of clear wood chestnut specimens [3].

Although the positive results obtained so far with the application of indirect method for assessing the stiffness of clear wood zones, questions as the effect of the distance between transducers, effect of specimen volume and dimensions and wave propagation mode, among others, have to be studied in respect with the equipment available (PUNDITplus) and test-setup proposed.

The objective of this paper is to analyze the characteristics of the signal (velocity and frequency) that is captured by the receiver transducer, evaluate the effect of distance between transducers (in order to maintain the actual procedure or to modified it) and to evaluate the influenced that wood surface and deeper layers have in respect to propagation mode.


2 Experimental work The test pieces, Table 1, were conditioned in a 20ºC±2ºC temperature and 65%±5%

relative humidity environment until constant mass was achieved (less than 1% variation in 2 hours), prior to began ultrasonic testing.

At one edge of each test piece eight marks were drawn (four marks placed symmetrically to the other four in relation to the middle of the test piece, Fig. 2), corresponding to a distance between transducers of 10cm, 20cm, 30cm and 40cm.

Table 1. Characteristics and number of test pieces used Number of test pieces (N)

dimensions (mm) Wood species 50 x 150 x 500 25 x 150 x 500

Maritime pine (Pinus pinaster Ait.) N = 3 N = 3

European spruce (Picea Abies L.) N = 3 N = 3

White oak (Quercus alba L.) N = 3 N = 3

Figure 2. Setup used for indirect testing. Detail of compression spring placed on top of the

transducer

Ultrasound waves were generated using a PUNDITplus equipment, on one-shot basis as pulse mode, with two flat contact 150kHz transducers (transmitter and receiver) under a constant pressure of 60N assured by a controlled deformation of a compression spring placed on top of the each transducer, Fig. 2. The equipment was combined with a digital oscilloscope and a laptop allowing the signal acquisition at the receiver transducer. Time-of-flight was taken directly from the display of PUNDITplus equipment. Ultrasound velocity (V) was determined according with Eq. 1, taking the time-of-flight (t) and distance (d) between transducers. In each of the four positions five readings were determined.

tdV = (1)

After performing this first stage, in each test piece at the middle of its length a 10mm

depth cut was performed, Fig 3, and all measurements were repeated.


Figure 3. 10mm cut made at the middle-length of the edge of each test piece

3 Results and discussion Fig 4 shows typical frequency spectrums obtained for a distance between transducers of

10cm and 40cm. In the first case it is recognized from the figure a strong peak around 25kHz and small peak around 130kHz whereas for a distance of 40cm only a peak at 25kHz is visible.

0 0.5 1 1.5 2 2.5x 105

0

0.01

0.02

0.03

0.04

0.05

Frequency (Hz)

FFT Amplitude

Teste piece S1d = 10cm

0 0.5 1 1.5 2 2.5x 105

0

0.02

0.04

0.06

Frequency (Hz)

FFT Amplitude

Test piece S1d = 40cm

Figure 4. Characteristic frequency spectrums obtained for spruce test piece S1

These results are inline with previous results [4] using a standard PUNDIT equipment and 37kHz, 54kHz and 150kHz transducers that have shown that actual frequency of transducers could be quite different from those expected in advance (from information on resonant frequency of piezoelectric crystals), Fig. 5. The significant loss of high frequency content even for a distance between transducers of only 10cm is related with the dispersive nature of wood material and the fact that most of the signal energy is transmitted to the back-wall (inferior edge) of the test piece being only a small part of the energy of the signal transmitted along the edge surface of the test piece. For a distance of 40cm all the high frequency of the signal is lost (for all the wood species tested).

0.5 1 1.5 2 2.5x 10

5

0

2

4

x 1012

Frequency (Hz)

FFT Amplitude

Figure 5. Frequency spectrum of a 150kHz transducer(right) as result of a direct contact

(gel couplant) between transmitter-receiver transducers [5]

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 Fig. 6 shows the relative differences obtained between the velocity using indirect method

(for all the distances between transducers) and longitudinal velocity, given by the direct method.

Test pieces 50mmx150mmx500mm

δ (P1,50) δ (P2,50) δ (P3,50) δ (S1,50) δ (S2,50) δ (S3,50) δ (O1,50) δ (O2,50) δ (O3,50)

10 20 30 40

Distance betweentransducers (cm)

-12

-8

-4

0

4

8

12

Rel

ativ

e di

ffere

nce

(%)

Test pieces 25mmx150mmx500mm


10 20 30 40


-8

-6

-4

-2

0

2

4

6

8

Rel

ativ

e di

ffere

nce

(%)

Test pieces 50x150x500mm


10 20 30 40


-35

-30

-25

-20

-15

-10

-5

0

5

10

Rel

ativ

e di

ffere

nce

(%)

Test piece 25mmx150mmx500mm


10 20 30 40


-35

-30

-25

-20

-15

-10

-5

0

5

Rel

ativ

e di

ffere

nce

(%)

Figure 6. Relative differences (mean and 95% confidence limits) observed between velocity

values obtained by the indirect and the direct method (above - before making the cut; below – after making the cut; P – pine; S – spruce; O - oak)

For the first part of the study (test pieces without cut) the relative difference between velocities (indirect versus direct method) can be considered for all test pieces and distances between transducers as constant in a range of ± 10%. The results obtained after making the 10mm cut on the edge (eliminating surface transmission) show that as the distance between transducers increases the effect of the cut decreases and for a distance of 40cm the results with or without cut could be considered the same. This results seems to indicate that as the distance becomes larger the influence of deeper wood layers in velocity of wave propagation velocity increases. These findings support the use of indirect method for predicting the longitudinal velocity moreover the use of 40cm distance between transducers, distance already used in a previous study [1], as it provides a good correlation, Fig. 7, between the indirect and direct velocity readings.


VL = 1351.5 + 0.75 Vind,40

3000 4000 5000 6000

Indirect velocity d=40cm - Vind,40 - (m/s)

3000

4000

5000

6000

Long

itudi

nal v

eloc

ity -

VL -

(m/s

)

r2 = 0.90

Figure 7. Correlation between indirect velocity and longitudinal velocity adopting a distance between transducers of 40cm (pieces without cut) (dash lines – 95% confidence

interval limits)

4 Conclusions The results obtained show the feasibility of using a indirect transmission method with

transmitter and receiver transducers 40cm apart for predicting the longitudinal velocity in cases where the ends of the timber elements are not available. The study also shows that indirect testing modifies significantly the characteristics of the frequency spectrum of the wave received. Although more studies are needed for a full comprehension of the propagation mode in the indirect method, the results now obtained give a strong support to the use of the method within the project for assessment of strength and stiffness of clear wood zones of structural dimension timber elements.

Acknowledgements The authors gratefully acknowledged the support given by Fundação para a Ciência e a

Tecnologia (FCT) to the project PTDC/ECM/66527/2006 entitled “Safety evaluation of timber structures through non-destructive methods and stochastic analysis”.

References 1. Machado, J. S. (2007) “Strength appraisal of wooden members in service by combining

new and old technology”, Proc. of Structural Studies, repairs and maintenance of heritage architecture X, Ed by C. A. Brebbia, 4-6 July 2007, Prague, Czech Republic, pp. 279-287.

2. Machado, J. S., Costa, D., Cruz, H. “Evaluation of pine timber strength by drilling and ultrasonic testing”, Proc. of the International Symposium Non-destructive Testing in Civil Engineering, Ed by German Society for Non-Destructive Testing and Federal Institute for Materials Research and Testing, 16-19 September 2003, Berlin, Germany, CD-ROM.

3. Artur, F., Lourenço, P., Machado, J. S. (2007) “Non-destructive evaluation of the mechanical beavior of chestnut wood in tension and compression parallel to grain”, International Journal of Architectural Heritage, Vol. 1, Issue 3, pp. 272-292.

4. Machado, J. S. (2001) “ Avaliação da variação das propriedades mecânicas de pinho bravo (Pinus pinaster Ait.) por meio de ultra-sons” (in Portuguese), PhD Thesis, Universidade Técnica de Lisboa, Lisboa, Portugal, 265p.

IntroductionIndirect (left) and direct (right) methods applied to timber

Experimental workCharacteristics and number of test pieces usedSetup used for indirect testing. Detail of compression sprin10mm cut made at the middle-length of the edge of each test

Results and discussionCharacteristic frequency spectrums obtained for spruce test Frequency spectrum of a 150kHz transducer(right) as result oRelative differences (mean and 95% confidence limits) observCorrelation between indirect velocity and longitudinal veloc

Conclusions

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Ultrasonic indirect method for evaluating clear wood strength ...NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 Ultrasonic indirect

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