Icke-förstörande tester av gran infekterad av rotröta med ultraljud och akustiska metoder

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Icke-förstörande tester av gran infekterad av

rotröta med ultraljud och akustiska metoder

Nondestructive testing (NDT) of Norway spruce with respect to infection by

root and butt rot using ultrasound and acoustic methods.

Växjö 12/10- 2011 Claes Sturesson Avdelningen för Skog och Trä

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Organisation/ Organization Författare/Author(s)

LINNÉUNIVERSITETET Claes Sturesson

Institutionen för teknik

Linnaeus University

School of Engineering

Dokumenttyp/Type of document Handledare/tutor Examinator/examiner

Examensarbete/ Diploma work Harald Säll Göran Petersson

Titel och undertitel/Title and subtitle

Icke-förstörande tester av gran infekterad av rotröta med ultraljud och akustiska metoder/

Nondestructive testing (NDT) of Norway spruce with respect to infection by root and butt rot using ultrasound and acoustic methods.

Sammanfattning (på svenska)

Ultraljud och akustisk impedans användes för att empiriskt undersöka rötad frisk trä. Trädslaget som användes var gran (Picea abies (L.) Karst.) och de rötade granproverna var kraftigt infekterade med rotröta

(Heterobasidion spp. (Fr.) Bref.). Mätningarna i denna studie visade att ultraljudshastigheten minskade drastiskt i rötat trä jämfört med friskt trä. På motsvarande sätt ökade tiden för signalen att färdas genom trädet i rötat trä jämfört med friskt trä. Resultaten från spektrumet av den akustiska impedansen visade på skillnader som kan hänföras till rötan i trädet. Det krävs emellertid fler mätningar för att säkerställa skillnaderna mellan friskt och rötat material med metoden akustisk impedans.

Nyckelord Icke-förstörande test, gran, rotröta, ultraljud

Abstract (in English)

Ultrasound and acoustic impedance were used to empirically investigate decayed and sound wood. The tree species used was Norway spruce (Picea abies (L.) Karst.) and the decay constituted of root and butt rot (Heterobasidion spp (Fr.) Bref.). It was shown that the ultrasound velocity decreased drastically in decayed wood as compared to sound wood. Correspondingly, the time-of-flight (TOF) of the signal was increased in decayed wood as compared to sound wood. The resulting spectra of the acoustic impedance measurements showed some differences between sound and decayed wood that can be attributed to the decay in the wood.

However, further studies are needed to support the resulting differences using acoustic impedance.

Key Words Nondestructive testing, Norway spruce, root and butt rot, ultrasound

Utgivningsår/Year of issue Språk/Language Antal sidor/Number of pages

2011 English 25

Internet/WWW http://www.lnu.se

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Icke-förstörande tester av gran infekterad

av rotröta med ultraljud och akustiska

metoder

Nondestructive testing (NDT) of Norway spruce with respect to

infection by root and butt rot using ultrasound and acoustic

methods.

Växjö, 12/10-2011 15 hp Skogs- och träteknik Handledare: Harald Säll, Linnéuniversitetet, Institutionen för teknik Examinator: Göran Petersson, Linnéuniversitetet, Institutionen för teknik

Claes Sturesson

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Organisation/ Organization Författare/Author(s) Linnéuniversitetet Claes Sturesson Institutionen för teknik

Linnaeus University School of Engineering

Dokumenttyp/Type of Document Handledare/tutor Examinator/examiner Examensarbete/Diploma Work Harald Säll Göran Petersson

Titel och undertitel/Title and subtitle

Icke-förstörande tester av gran infekterad av rotröta med ultraljud och akustiska metoder/

Nondestructive testing (NDT) of Norway spruce with respect to infection by root and butt rot using ultrasound and acoustic methods.

Sammanfattning (på svenska)

Ultraljud och akustisk impedans användes för att empiriskt undersöka rötad frisk trä. Trädslaget som användes var gran (Picea abies (L.) Karst.) och de rötade granproverna var kraftigt infekterade med rotröta (Heterobasidion spp. (Fr.) Bref.). Mätningarna i denna studie visade att ultraljudshastigheten minskade drastiskt i rötat trä jämfört med friskt trä. På motsvarande sätt ökade tiden för signalen att färdas genom trädet i rötat trä jämfört med friskt trä. Resultaten från spektrumet av den akustiska impedansen visade på skillnader som kan hänföras till rötan i trädet. Det krävs emellertid fler mätningar för att säkerställa skillnaderna mellan friskt och rötat material med metoden akustisk impedans.

Nyckelord Icke-förstörande test, gran, rotröta, ultraljud

Abstract (in English)

Ultrasound and acoustic impedance were used to empirically investigate decayed and sound wood. The tree species used was Norway spruce (Picea abies (L.) Karst.) and the decay constituted of root and butt rot (Heterobasidion spp (Fr.) Bref.). It was shown that the ultrasound velocity decreased drastically in decayed wood as compared to sound wood. Correspondingly, the time-of-flight (TOF) of the signal was increased in decayed wood as compared to sound wood. The resulting spectra of the acoustic impedance measurements showed some differences between sound and decayed wood that can be attributed to the decay in the wood. However, further studies are needed to support the resulting differences using acoustic impedance.

Key Words Nondestructive Norway spruce root and butt rot ultrasound

Utgivningsår/Year of issue Språk/Language Antal sidor/Number of pages 2011 English 25

Internet/WWW http://www.lnu.se

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Summary

Forestry plays a major part in the economy of Sweden. The most economically important tree species in Sweden is Norway spruce (Picea abies (L.) Karst.). The quality of the wood product is essential for its applicability to a specific purpose and it is therefore of interest to detect imperfections. Root and butt rot (Heterobasidion spp. (Fr.) Bref.) causes severe damages to Swedish forests.

Ultrasound and acoustic impedance were used to empirically investigate decayed and sound wood. The tree species used was Norway spruce (Picea abies (L.) Karst.) and the decay

constituted of root and butt rot (Heterobasidion spp (Fr.) Bref.). It was shown that the ultrasound velocity decreased drastically in decayed wood as compared to sound wood. Correspondingly, the time-of-flight (TOF) of the signal was increased in decayed wood as compared to sound wood. The resulting spectra of the acoustic impedance measurements showed some differences between sound and decayed wood that can be attributed to the decay in the wood. However, further studies are needed to support the resulting differences using acoustic impedance.

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Sammanfattning

Skogsnäringen har en stor betydelse för Sveriges ekonomi. Det mest ekonomsikt betydelsefulla trädslaget i Sverige är gran (Picea abies (L.) Karst.). Kvaliteten av träprodukten är väsentlig för dess speciella ändamål och det är därför av intresse att kunna detektera defekter. Rotröta (Heterobasidion spp. (Fr.) Bref.) orsakar stora skador på den svenska skogen.

Ultraljud och akustisk impedans användes för att empiriskt undersöka rötad frisk trä. Trädslaget som användes var gran (Picea abies (L.) Karst.) och de rötade granproverna var kraftigt

infekterade med rotröta (Heterobasidion spp. (Fr.) Bref.). Mätningarna i denna studie visade att ultraljudshastigheten minskade drastiskt i rötat trä jämfört med friskt trä. På motsvarande sätt ökade tiden för signalen att färdas genom trädet i rötat trä jämfört med friskt trä. Resultaten från spektrumet av den akustiska impedansen visade på skillnader som kan hänföras till rötan i trädet.

Det krävs emellertid fler mätningar för att säkerställa skillnaderna mellan friskt och rötat material med metoden akustisk impedans.

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Abstract

Ultrasound and acoustic impedance were used to empirically investigate decayed and sound wood. The tree species used was Norway spruce (Picea abies (L.) Karst.) and the decay

constituted of root and butt rot (Heterobasidion spp (Fr.) Bref.). It was shown that the ultrasound velocity decreased drastically in decayed wood as compared to sound wood. Correspondingly, the time-of-flight (TOF) of the signal was increased in decayed wood as compared to sound wood. The resulting spectra of the acoustic impedance measurements showed some differences between sound and decayed wood that can be attributed to the decay in the wood. However, further studies are needed to support the resulting differences using acoustic impedance.

Keywords: Nondestructive Norway spruce root and butt rot ultrasound

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Preface

The grant from ”the Foundation of Anna and Nils Håkansson” for studying acoustics applied to root and but rot in trees is greatly acknowledged. The skilful measurement competence of Docent Peter Ulriksen is highly appreciated.

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1. Introduction

Forestry plays a major part in the economy of Sweden. The most economically important tree species in Sweden is Norway spruce (Picea abies (L.) Karst.). The quality of the wood product is essential for its applicability to a specific purpose and it is therefore of interest to detect imperfections. Root and butt rot (Heterobasidion spp. (Fr.) Bref.) causes severe damages to Swedish forests. It has been estimated that the loss for Swedish forest owners is of the magnitude of 500 - 1000 MSEK annually (Thor et al., 2004). However, with knowledge of the frequency of root and butt rot in the forest, the effect can be minimized by planning of operations such as commercial thinning (Piri and Korhonen, 2008).

There are both destructive and nondestructive techniques available when identifying root and butt rot in a stand. Destructive methods have obvious drawbacks and it is advantageous to perform a nondestructive evaluation of the trees.

1.1 Purpose of this study

The purpose of this study was to evaluate the feasibility of using ultrasonic and acoustic tools in sound and decayed wood. In particular, it was of interest to assess infection of root rot (Heterobasidion annosum (Fr.) Bref.) in Norway spruce (Picea abies (L.) Karst.) by measuring signal parameters. In this study, no attempt has been made to relate the observed signal with the degree of the decay.

1.2 Techniques used for nondestructive testing (NDT)

of wood

Several techniques have been tested for nondestructive investigations of wood (e.g.

Bucur, 2003). The methods include for example:

• Visual

• Manual sounding

• Mechanical (e.g. microdrill resistance, pin penetration)

• Electrical

• Nuclear (X-ray, Gamma-ray, magnetic resonance)

• Microwaves

• Acoustic tomography

• Ultrasound

The technique chosen can depend on cost, practicality, information needed, accuracy and speed of analysis.

Visual and manual sounding methods are dependent on the experience of the tester and are to a large extent subjective. Mechanical drilling is quasi-invasive in the sense that you have to drill several holes to obtain sufficient and reliable

information. Electrical tools (e.g. Rotfinder ® and Shigometer®) are slow and not

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so practical in the forest and also subject to some uncertainty concerning the results. Nuclear- and microwave techniques are costly and impractical in the forest.

In addition, these techniques have inherent high energy levels and are therefore not so safe. Acoustic tomography (e.g. Picus ® and Arbotom ®) generate

comprehensive information in two dimensions but are costly and too slow for field work.

Ultrasound refers to a frequency above around 20 kHz (e.g. Cracknell, 1980), i.e. a sound with a frequency greater than detected by human hearing. Fast and reliable information, and at a low cost, can be obtained by measuring e.g. the time elapsed in transmission or reflection of ultrasound through a trunk of a tree.

1.3 Literature survey on ultrasound for detection of

decay in wood

Early research using ultrasound for defect detection in wood date back some 30 or 40 years ago. The pioneering efforts were made at the USDA Forest Product Laboratory.

Around this time an International Symposium on Nondestructive Testing (NDT) of Wood was instigated and the Proceedings of these reoccurring Conferences are referenced to as literature.

Examples of pertinent studies include e.g.: McCracken (1985) showed that the transit time for ultrasonic pulses in ash increased for a decayed stem compared to a healthy stem at frequencies of 54 and 150 kHz. Ultrasound velocity measurements were used Kazemi-Najafi et al. (2009) to study internal decay in beech. Advanced decay was simulated by manually creating holes in the discs studied. The frequency used was 16 kHz pulsed longitudinal waves with transmission time measurements. The results showed that the velocity decreased by increasing the hole diameters.

Several previous studies have investigated wood decay using ultrasound. However, to the knowledge of the author, no such work has been done on Norway spruce (Picea abies (L.) Karst.) infected by root rot (Heterobasidion spp (Fr.) Bref.).

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2. Background

2.1 Acoustics/ultrasound in wood

The propagation of sound waves is an elastic phenomenon determined by the mechanical properties in the solid wood specimen. It can therefore be utilized for information about imperfections, such as decay, in wood.

Wood is an inhomogeneous and anisotropic medium and because of symmetry denoted orthotropic. The theory of waves in isotropic and anisotropic solids can be found in the literature (e.g. Rose, 1999).

There are two types of elastic waves (See Figures 1a and b).

Figure 1a. Longitudinal or Compressional waves, Primary (P) – waves.

Figure 1b. Transversal or Shear waves, Secondary (S) –waves.

Because of the anisotrophy of wood the ultrasound velocity travels faster in the fiber, or grain, direction compared to the radial or tangential direction. Thus, for solid wood the ultrasound velocity is dependent on the fiber orientation. The ultrasound velocity is around 1000-2000 m/s across the grain and 5000 – 6000 m/s along the grain (Beall, 2002). The wave velocity can be approximated as proportional to the elastic modulus (e.g. Koponen et al., 2005) according to:

Y = ρ vl2 (1)

where Y is the Young´s modulus, ρ is the average sample density and vl is the longitudinal wave sound propagation velocity. Correspondingly, a shear wave sound propagation velocity (vs) can be approximated as:

G = ρ vs2 (2)

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where G is the shear modulus.

Sound waves propagate faster in solid wood than in decayed wood (e.g. Pellerin et al, 1985). The time-of-flight (TOF) can therefore be used to determine imperfections present in the material. In addition to TOF measurements, the attenuation of the signal can be measured and used to detect decay in wood since the presence of decay increases the damping properties of the system (Bucur, 2006). The attenuation behaves differently in different directions in the wood. The attenuation is higher for signals travelling across the fiber compared to along the fiber direction (Schafer, 2000). The upper usable frequency level in wood is 100 – 200 kHz because of attenuation (Beall, 2002).

The ultrasound output signal depends on the: type of fungus; tree type; age of the tree, moisture content; density; fiber orientation and geometry (e.g. Beall, 2002). It is of interest in this study to investigate the influence of decay only and it is therefore important to keep all other parameters as constant as possible.

Issues:

• Difficult to determine initial stages of decay (See Section 2.2).

• Incorrect placement of sensors, especially when using one receiver, can lead to incorrect results.

• Signal can dissipate (depending on frequency and sample diameter and condition).

• Keeping all other parameters affecting the signal constant (apart from decay).

• Reproducibility.

2.2 Classification of decayed wood

The decay in wood can be classified into different stages. The traditional classification includes the following categories (e.g. Shain, 1971): incipient (aniline wood),

intermediate and advanced decay. From a mechanical point of view, however, four stages have been defined as pointed out by Axmon (2004). See Table 1.

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Stage Characteristics

Color zone Wood shows slightly violet discoloration

(aniline wood, incipient decay (Shain, 1971); scarcely any change in mechanical properties or structure.

Light hard rot Wood shows light-brownish

discoloration; its mechanical properties are affected.

Dark hard rot Wood shows brownish-red

discoloration; the structure is preserved, but the mechanical properties are severely impaired.

Soft rot Wood is brown, soft, and spongy.

Table 1. Stages of decay in wood from Axmon (2004).

The loss of mechanical properties depends on the severity of the decay (Pratt, 1979).

Other classifications include (e.g. Bucur, 2006): initial, early, medium and advanced decay.

The degrading of wood can be detected in early stages using ultrasonic velocities (Bucur, 2006, pp 259) for a certain type of decay. In a study by Emerson et al. (2002) it was found that ultrasonic wave attenuation can be used to identify incipient through advanced decay. Furthermore, it was concluded that the wave velocity can identify the presence of moderate to advanced decay but cannot be used for incipient decay.

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3. Materials and methods

3.1 Sample preparation

Norway spruce (Picea abies (L.) Karst.) samples were collected at the Trolleholm estate in Scania, Sweden. See detailed map in Appendix 1. A sound piece of wood and a decayed piece of wood were sampled and tested. The pieces of wood were sealed in double plastic bags in the field and kept in a cooling room until tested.

The wood specimen had a diameter of approximately 25 - 30 cm and a length of approximately 50 - 75 cm. The length was chosen to avoid effects from the edges when measuring.

The decay was determined by photography as a percentage of discolored wood of the total area at a cross section and classified according to Table 1. See Figures 2 and 3.

The decayed portion of the cross section of the upper and bottom side of the wood had a diameter of 198 and 206 mm, in the direction of measurement. The wood diameter of the sound wood was 239 mm at the measuring height.

(The decay can also be determined by calculating the area of colonies of root tick. This is done using a microscope after the disc had been incubated for a week.)

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Figure 2 a and b. Cross-sections of sound wood sample.

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Figure 3a and b. Cross-sections of decayed wood sample.

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3.2 Methods

Ultrasound and acoustic impedance were used to study the differences in signal output between sound and decayed wood.

The distance between the tip of the acoustic horns was assessed by measuring the distance between the backside of the beam probes and by measuring the length of the horns. This was done using a caliper. The horns were manually attached to the wood bark and held during measurements. A mechanical experimental device is under construction for better control of the position of the acoustic horns. However, this rig has not been used in this study.

Factors affecting TOF include bonding between specimen-transducers; length of sample; surface condition of sample; temperature and humidity; frequency.

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4. Experimental

4.1 Ultrasound

4.1.1 The experimental equipment and setup

The electrical signal is transformed to an ultrasonic pulse in the piezoelectric transducer. This pulse travels through the tested piece of wood and is then again converted into an electrical signal which can be monitored in an oscilloscope. Pulses of compressional or longitudinal waves were used in the experiments. Even though the used exponential horns are designed for compressional waves it can be expected that parts of the signal is converted into shear waves when the pulse propagates into the wood specimen.

The equipment used consists of the following (See Figures 4 - 6):

• Low frequency straight beam probe with exponential horn from General Electric (GE). The length was measured to 96,9 mm, including the exponential horn.

Exponential (acoustic) horns were used for better contact with the wood surface. See Figure 4.

• Transmitter: HP pulse generator (HP 33120A), RITEC amplifier

• Receiver: oscilloscope (HP 54600A), preamplifier (Bruel & Kjaer 2635), bandpassfilter (Krohn-Mite 3905B Multich), laptop.

Figure 4. Acoustic exponential horns used for ultrasound measurements.

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Pulse generator HP 33120A

Amplifier Ritec

Wood

Ultrasonic Preamplifier B&K 2635 Bandpassfilter Krohn-Mite 3905B Multich

Digital Oscilloscope HP 54600 A Laptop Computer

Transmitting Transducer

Receiving Transducer

Figure 5. Schematic depiction of the experimental ultrasound setup.

The bandpassfilter was set to 80 – 110 kHz in the experiments using the frequency of 92 kHz and to 50 – 110 kHz when using the frequency of 60 kHz. The preamplifier was set to 1000 mV/unit out when using 92 kHz and 100 mV/unit out when using 60 kHz. The Ritec amplification was set to 6.0 in all cases.

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Figure 6. Ultrasonic measurement system used to measure ultrasound transmission in spruce wood.

4.1.2 Signal processing

Several parameters of the ultrasonic pulse can be analyzed at the receiving end of the signal. These include:

• Time of flight (TOF) of velocity measurements.

• Amplitude (attenuation)

• Waveform pattern

• Frequency (possible frequency dependence of mechanical properties)

• Peak voltage

• Average signal level and root mean square voltage (energy)

Proper signal processing is essential for the results. Time-of-flight (TOF)

measurements were conducted in this study. Spectrum analysis can be used for more advanced measurements such as attenuation and phase.

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4.1.3 Experimental validation

The measurement method repeatability was validated by measuring the same sample several times and observing the difference in the resulting of TOF.

By taking samples of several healthy trees taken from the same stand information about the spreading of measurement can be obtained. This was not performed in this study.

4.2 Acoustic impedance

Acoustic impedance measures the “acoustic resistance” in the material and is the force divided by the velocity. It is measured using a dynamic signal analyzer (DSA) which controls a vibrator and records the signal from an impedance head installed between the vibrator and the tree. The impedance head measures simultaneously the force and the acceleration. The acceleration is integrated to velocity in the DSA. The unit for acoustic impedance is Ns/m².

The acoustic impedance equipment (DSA, vibrator and impedance head) were from Bruel & Kjaer and can be operated from 1 to 18.000 Hz. The impedance head and vibrator were attached to a screw driven into the wood. See Figure 7.

Figure 7. Setup of acoustic impedance measurements.

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5. Results and discussion

5.1 Characteristics of exponential horns

The inherent characteristics (modes) of the exponential horns were evaluated from the spectrum received when exciting the transmitting transducer. A peak of strong

magnitude was found at approximately 92 kHz and a weaker at approximately 62 kHz.

See Figure in Appendix 2. By using the higher frequency, more energy is conveyed into the system and a better resolution can be expected. At the same time, however, the signal attenuation increases. Because of this situation, a good compromise between frequency and resolution is required (Bucur, 2005).

5.2 Ultrasound

5.2.1 Sound wood

In order to determine the repeatability of the measurements, five measurements were conducted at a mid position of the sound Norway spruce piece. The horns were deattached between each measurement. The distance between the backside of the horns were measured using a caliper and for all five measurements the distance was 424 mm. Thus, the distance travelled by the ultrasound signal was 230,2 mm since the length of the horns were 96,9 mm each. The time-of-flight was measured from the first arrival time of the transmitted and received signal. See below example in Figure 8.

Measurements were conducted at a frequency of 92 kHz.

Figure 8. Example of transmitted and received signals for sound wood.

The measured TOF is reported below in Table 2.

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Parameter/Measurement 1 2 3 4 5

Distance (mm) 230,2 230,2 230,2 230,2 230,2

TOF (µs) 228 223 227 227 226

Velocity (m/s) 1010 1032 1014 1014 1018

Table 2. Results from sound piece of wood at the same measuring point.

The obtained radial velocity correlates well with what is expected across the grain.

According to Mattheck and Bethge (1993), the radial velocity for spruce is 931 – 1085 m/s. Also a good repeatability is shown.

5.2.2 Decayed wood

The decayed wood measurements are reported for four different heights or levels in the piece of wood. The used frequency was 60 kHz. This frequency was used due to the high attenuation of the signal when using 92 kHz because of the high degree of decay in the wood. It can be of interest to investigate the possible frequency

dependence on the TOF. This could be done by using the lower frequency of 60 kHz in the sound wood and comparing the results of TOF with the results using 92 kHz.

Hovewer, this experiment was not performed in this study.

Figure 9. Example of transmitted and received signals for decayed wood.

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Parameter/Measurement 1(7) 2(8) 3(9) 4(10)

Distance (mm) 270,2 265,2 264,2 260,2

TOF (µs) 1469 1341 1341 1491

Velocity (m/s) 184 198 197 174,5

Table 3. Results from decayed piece of wood at different heights.

As can be seen from Table 3, the velocity decreases significantly in the case of the decayed wood as compared to the sound wood specimen, as presented in Table 2. It can be expected that the signal travels along the circumference of the wood and thus to a minor degree through the decayed volume. This has been suggested previously in the literature (e.g. Kazemi-Najafi et al., 2009).

5.3 Acoustic impedance

The resulting spectra of the measurements of the acoustic impedance are shown in Figure 10. The reported mobility is the inverse of the impedance.

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Figure 10. Impedance and mobility of sound and decayed wood. (Blue = sound, Red

= decayed)

The “jig-saw” pattern of the spectrum at frequencies between around 300 – 4000 Hz, shown in Figure 10, can be attributed to the decay in the wood. The peak patterns at higher frequencies are attributed to the inherent spectrum of the screw system. See Figure 11.

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Figure 11. Impedance and mobility with only the screw mounted.

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6. Conclusions

Ultrasound measurements of the radial time-of-flight (TOF) revealed significant differences between sound and decayed Norway spruce (Picea abies (L.) Karst.). It was found that the TOF of the ultrasound signal increases drastically for decayed wood as compared to the sound wood specimen. The root and butt rot (Heterobasidion spp (Fr.) Bref.) in the decayed sample was advanced. In order to detect incipient decay, more advanced signal processing can be employed, such as analysis of attenuation and amplitude.

The resulting spectra from the acoustic impedance measurements show some features which can be attributed to the decay in the wood. Hovewer, more data samples are required to draw conclusions regarding the differences of the impedance spectra of sound and decayed wood.

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7. Nomenclature

Symbols

G – Shear modulus (N/m²)

vl - longitudinal wave sound propagation velocity (m/s) vs - shear wave sound propagation velocity (m/s) Y – Young´s modulus (N/m²)

Greek symbols ρ - density (kg/m³)

Abbreviations

DSA – Dynamic signal analyzer NDT – Nondestructive testing TOF – Time-of flight (s)

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8. Referenser

Axmon, J., Two-Dimensional Signal Root Estimation in Blind Multichannel Impact Response Analysis, PhD thesis, Departement of Electroscience, Lund University, (2004).

Beall, F.C., Overview of the use of ultrasonic technologies in research on wood properties. Wood Science an Technology, 36, 197-212 (2002).

Bucur, V., Nondestructive Characterization and Imaging of Wood, ISBN 3-540- 43840-8 Springer- Verlag Berlin Heidelberg New York (2003).

Bucur, V., Ultrasonic techniques for nondestructive testing of standing trees, Ultrasonics, 43, 237-239,(2005).

Bucur, V., Acoustics of Wood, ISBN-10 3-540-26123-0 Springer- Verlag Berlin Heidelberg New York (2006).

Cracknell, A. P., Ultrasonics, Wykeham Publ., London, (1980).

Emerson, R.; Pollock, D.; McLean, D.; Fridley, K.; Pellerin, R.; Ross, R., Ultrasonic of inspection of large bridge timbers. For. Prod. J. 52 (9): 88-95, (2002).

Kazemi-Najafi, S.; Shalbafan, A.; Ebrahimi, G., Internal decay assessment in standing beech trees using ultrasonic velocity measurements. Eur. J. Forest Res. 128:345-350 (2009).

Koponen, T.; Haeggström, E.; Karppinen, T.; Saranpää, P.; Serimaa, R., Ultrasonic study on modulus of elasticity and nonlinearity parameter (B/A) in Norway spruce as a function of year ring, Review of Quantitative Nondestructive Evaluation, Vol. 24, p.

1493, (2005).

Mattheck, C.G., Bethge, K.A., Detection of decay in trees with the Metriguard Stress Wave Timer, Journal of Abroriculture, 19, 6, 374-378, (1993).

McCracken, F., Using sound to detect decay in standing hardwood trees. Proc. 5^th Symp on Nondestructive Testing of Wood, Washington State University, Pullman, pp.

477 – 489, (1985).

Pellerin R.F.; DeGroot R.C.; Esenther, G.R, Nondestructive stress wave

measurements of decay and termite attack in experimental wood units. Proceedings, Fifth NDT of Wood Symp., Pullman WA, pp. 319-352 (1985).

Piri, T.; Korhonen, K., The effect of winter thinning on the spread of Heterobasidion parviporum in Norway spruce stands, Can. J. For. Res., 38, (10), 2589-2595, (2008).

Pratt, J.E., II. Loss of strength of wood in various categories of rot, Forestry, vol. 52, 1, 31-45, (1979).

Rose, J.L., Ultrasonic waves in solid media. Cambridge University Press , Cambridge, (1999).

Schafer, M. E., Ultrasound for detection and grading in wood and lumber, 2000 IEEE Ultrasonics Symposium, 771, (2000).

Shain, L., The response of sapwood of Norway spruce to infection by Fomes annosum, Phytopathology, vol.61, 301-307, (1971).

Thor, M.; Ståhl, G.; Stenlid, J., Räkna med rotröta – nytt hjälpmedel för skoglig

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planering. Resultat från Skogforsk nr. 13. (2004)

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10. Appendices

Appendix 1: Map of the Trolleholm estate where the tree samples were collected.

Appendix 2: Characteristics of the exponential horns.

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Appendix 1 (number of pages: 1)

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Appendix 2 (Number of pages: 1)

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Institutionen för teknik 351 95 Växjö

tel 0772-28 80 00, fax 0470-76 85 40

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