The analytical use of stress waves for the detection of decay in

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The analytical use of stress waves
for the detection of decay in
standing trees
G. LAWDAY1 AND P.A. HODGES2
1 Computing
Department, Buckinghamshire Chilterns University College, Queen Alexandra Road, High
Wycombe, Bucks HP11 2JZ, England
2 Forest Products Research Centre, Buckinghamshire Chilterns University College, Queen Alexandra Road, High
Wycombe, Bucks HP11 2JZ, England
Summary
This paper describes the use of an innovative method to detect decay in trees. The criteria for the
project required the decay detecting process to be non-invasive and to provide an estimation of the
position and extent of decay. A measured impulse from a hammer blow was used to produce stress
waves within the stem of a tree. The stress waves were detected by an accelerometer and the
resulting amplitude-time signals were recorded on a portable oscilloscope. The time of flight and
attenuation of the stress wave were determined and the signal was transformed on a computer
hosting a commercial software package. Six measurements were taken between different coordinates positioned around the circumference of a tree stem that was suspected of containing decay.
Analysis of the amplitude spectrum of the stress waves provided an indication of the presence of
sound or decayed wood between each set of co-ordinates. Estimation of the position of decay in the
stem was achieved by calculating a short-time Fourier transform on each temporal signal and
mapping the path of the stress waves through sound wood. The information produced by the
analysis of stress waves was confirmed by felling the tree.
Introduction
The hazard posed by a tree suffering from decay
depends on the ratio of the infected to healthy
tissue within the stem. Guidance notes (Minnesota Department of Natural Resources and the
USDA Forest Service, 1996) suggest that a 25 mm
ring of sound wood is required for every 150 mm
of stem diameter at any point on the stem. If the
proportion of decayed wood to sound wood
exceeds this level then action may need to be
taken to minimize the hazard posed by the tree.
Visual appraisal and rudimentary assessment
© Institute of Chartered Foresters, 2000
instruments have traditionally been used to detect
decay in trees. Some systems are invasive, such as
the use of an increment borer that provides a core
sample for examination (Mattheck and Breloer,
1997). The deep holes produced by increment
borers and drills can enable decay fungi to
become established and help spread any compartmentalized infection to sound wood (Toole
and Grammage, 1959).
Aural assessment of decay in trees is considered
to be non-invasive. This method relies on the
tester’s ability to interpret the acoustic emission
from a hammer blow and make a deduction
Forestry, Vol. 73, No. 5, 2000
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F O R E S T RY
based on the sound produced. This process can be
considered as time-frequency signal analysis
where the cochlea and auditory cortex of the
inner ear sense fluctuations in air pressure.
Several advanced acoustic techniques have
been developed for the purpose of decay detection
in trees. This equipment typically measures the
time-of-flight of a pulse of ultrasound or a stress
wave across a tree stem. Any deviation from the
expected transit time is indicative of a peculiarity
or degradation of a wave path. However, until
recently, the interpretation of the data produced
by these techniques has not been able to provide
accurate information about the extent of decay
within a stem.
Waid and Woodman (1957) showed how the
measurement of ultrasonic wave attenuation was
a potential technique for the location and identification of incipient heart and butt rot in standing
trees. The measurement method was based on
observations of significant loss of transmission
energy in diseased wood. Current ultrasonic tree
testing tools exploit this phenomenon by measuring the increased transit time when ultrasonic
waves circumvent decay within a tree trunk
(Sandoz and Lorin, 1996). However, the system
is limited to a time-of-flight value that provides
insufficient detail to position or identify the type
of decay.
The work of Pellerin and Bozhang (1996)
included the Fourier analysis of similar resonant
stress wave signals and the mechanical properties
of sound and decayed timber. Stress waves are
generated in a material when it is disturbed with
a force component parallel to the direction of
wave propagation. The natural frequencies of
longitudinal vibrations are independent of the
area and cross section of the test piece, making
the test very convenient for irregularly shaped
objects. The authors associated the spectral components in stress wave signals with the mechanical properties of sound and decayed wood. Their
approach to positioning the decay in trees was to
perform multiple measurements on two axes of
the timber and plot the results on a three-dimensional map of decay position and amplitude.
The current work detailed in this paper is based
on the signal analysis of stress waves set up in the
stem of a standing tree. The evaluation method
requires the velocity of stress waves in wood to
be fairly constant. This was found to be the case
by Hearmon (1948) who predicted that the velocity of plane waves in timber was in the order of
1300–1500 ms–1. It has been recognised that variations can occur between and within tree species
with regard to this property (Bucur, 1995).
However, for the following experiment it was
assumed that a stress wave velocity of 1300 ms–1
provided sufficient accuracy. The adopted
approach uses the Fourier transform, where a
signal (expressed as a function of time) is transformed by spectral analysis. This technique was
used to discover the spectral composition of the
stress waves. Short-time Fourier transform analysis enables a signal to be divided into time slots
and analysed. The spectral analysis of each time
slot produces a dual density distribution of frequency and time, which can be shown graphically
as a spectrogram. Interpretation of the spectrogram allows the extent and position of decay
within the stem to be estimated. Further information on this method of time frequency analysis can be found in Cohen (1995).
Method
Tree measurement
The work detailed in this paper relates to the
examination of a mature ash tree (Fraxinus excelsior). Stress wave data were obtained from the
standing tree using the equipment prior to the tree
being assessed by the examination of bored
samples. The tree was felled and the freshly
exposed cross section enabled the accurate
determination of the decay pattern and extent of
infection. Measurements from the standing tree
and subsequent cross-sections were taken
approximately 1 m above ground level.
Equipment
A calibrated impulse hammer with a head mass
of 1 kg was used to excite the wood and produce
the stress waves. Detection of the resultant stress
waves was undertaken with a piezoelectric
accelerometer. Two short, large diameter metal
screws were screwed into the tree to allow
measurements to be taken between the reference
co-ordinates. An accelerometer was attached to
one, and the other was struck with the hammer.
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The amplitude time signals were recorded on a
portable storage oscilloscope so that both timeof-flight and wave attenuation could be determined. Subsequent transformation of the dataset
was carried out on a personal computer using a
commercial software package.
Assessment procedure
Four measurement positions around the circumference of the tree were selected and allocated a
reference co-ordinate (A, B, C or D). The location
of each co-ordinate was noted and the circumference of the tree was determined. A quantifiable
impulse was applied at each of the co-ordinates
and the resultant stress wave was recorded. Two
methods were used to monitor the quality of the
impulse. First, the response was measured in real
time and, second, the recording oscilloscope
could, if required, automatically average multiple
measurements, thus minimizing the variability of
the applied impulses. The equipment gave reproducible oscilloscope measurements of 20 mV per
4 N of applied force to provide the necessary
degree of experimental replication. The oscilloscope sampling rate was set at 50 000 samples per
second (Ss–1) and the average of 32 measurements
was recorded between each pair of positions.
A dataset of six readings was obtained by
applying measured impulses between the screws
at the different positions around the stem. The
location of the co-ordinates, shown in Figure 1,
enabled data to be collected along four chords
and across the diameter of the stem. An increment
borer was used to take a core boring of the wood
in the radial direction from each co-ordinate position. The core borings were examined to determine the presence of any decay within the stem.
Finally, the tree was felled to determine the exact
extent of decay within the stem.
Results and discussion
Decay pattern within the tree
The results of the boring trial shown in Table 1
indicated a central zone of decay surrounded by
a ring of sound wood. However, upon visual
assessment of the standing tree it was suspected
that the biodeteriogen had entered the trunk via
Figure 1. Transverse section of the tree under
investigation with an area of decay shown crosshatched (not to scale). Dimensions in cm.
Table 1: Depth of sound wood (cm) in the radial
direction determined by boring from each
co-ordinate position
Co-ordinate
A
B
C
D
Depth of sound wood (cm)
18
7
21
24
a side wound and that the extent of decay was
greater than that deduced from the boring
samples. The true pattern of decay, established
upon felling the tree, is indicated by the crosshatched section shown in Figure 1. Approximately 38 per cent of the cross section of the tree
was affected by decay at the assessment position.
This extended from a canker at the surface,
between co-ordinates B and D, into the heartwood. The decay zone did not extend to the
chords set by co-ordinates B to C, A to C and A
to D.
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Variations in stress wave velocity
The velocity of each impulse between the co-ordinates is shown in Table 2. It should be noted that
the distances given in this table were determined
with reference to the positions of the metal interface screws. The distances allow for the reduced
stress wave propagation path produced by the
depth of penetration of these screws. The velocities were calculated by measuring the distance
between each co-ordinate and recording the time
of flight of the particular stress wave. Velocities in
the region of 1300 ms–1 were determined between
co-ordinates B to C, A to C and A to D. This
finding was within the expected velocity range for
sound wood as previously detailed by Hearmon
(1948) and Bucur (1995). The velocities of the
stress waves between positions B to D, C to D and
A to B were considerably lower than 1300 ms–1.
The lower stress wave velocities indicated the
presence of decayed wood between these positions. This view was substantiated upon felling
the tree when it was found that the chords
between co-ordinates B to D, C to D and A to B
passed through the decayed section of the stem.
The concept that reduced stress wave velocity
and increased time-of-flight can indicate the presence of decay has been used in the development
of a number of decay-detecting instruments such
as stress wave timers. However, such equipment
cannot be used to determine the extent and position of the decay.
Interpretation of the temporal signals and signal
spectra
The temporal waveforms shown in Figure 2 indicate the presence of sound and decayed wood
within the stem of the tree. The reciprocal of the
time of flight was used to give the resonant wave
frequency.
The waveforms measured for an impulse travelling between positions B to C, A to C and A to
D are shown in Figures 2a, b and c. The resonant
wave frequency of the stress wave between these
co-ordinates was consistently centred at 2 kHz on
the three waveforms. These results were typical of
the response to sound wood when subjected to an
impulse, as the chords between these co-ordinates
were not located in the decayed section of the
stem.
The waveforms obtained for stress waves travelling along chords between positions B to D, C
to D and A to B were different from the waveforms established for stress waves travelling
through sound wood. The intricate waveform
shown in Figure 2d relates to the measurements
made between co-ordinates B to D on the circumference of the stem. The resonant wave frequency peaked at 2 kHz and indicates the
presence of sound wood along this chord.
However, unlike the readings determined for
sound wood, resonant wave frequencies lower
than 2 kHz were also present in this waveform.
These were notably centred at approximately
1 kHz on the horizontal axis and this portion of
the waveform was characteristic of the decayed
wood that was found between the two positions.
This waveform indicated that the stress wave was
travelling through the decayed portion of the
stem and also circumventing the region of decay.
Another intricate waveform (shown in Figure
2e) was recorded for the impulse between position C and position D. The predominant resonant wave frequency was centred around 1 kHz
on this waveform, which is lower than that
expected for sound wood. Similarly, the waveform in Figure 2f shows that the spectrum had
Table 2: Basic measurements made on the sample tree to calculate the velocity of stress
waves between different positions
Chord
Distance (cm)
Time-of-flight (s)
A to D
A to C
B to C
A to B
B to D
D to C
58
52
51
80
61
81
440
400
385
730
865
720
Calculated velocity (ms–1)
1318
1300
1325
1096
705
1125
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Figure 2. Waveforms derived for the stress wave set up between (a) positions A and D, (b) positions A and
C, (c) positions B and C, (d) positions B and D, (e) positions D and C and (f) positions A and B.
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Figure 2. Continued.
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Figure 2. Continued.
453
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shifted towards the lower frequencies (less than
2 kHz) in comparison with the waveforms determined for sound wood. However, taken in isolation, the waveform was not wholly indicative of
the decayed portion between co-ordinates A and
B. This result emphasises the need to compare the
response with other measurements in order to
make a decision regarding the presence of decay.
A good correlation was made between the
waveforms derived for stress waves between
various positions on the circumference of the tree
and the presence of decay between these
positions. The waveform for sound wood was
typically uncomplicated and exhibited a peak resonant wave frequency at 2 kHz. The waveform
obtained for stress waves travelling through
decayed wood tended to be more intricate and
had resonant wave frequencies below 2 kHz.
The amplitude spectrum was used to identify
decay within a tree and the co-ordinate method
described in this paper allowed the position of the
decayed section to be approximately predicted.
However, this method of signal analysis could not
be used to position the decayed area accurately or
be used to evaluate the extent of decay within a
tree trunk. In order to be an effective diagnostic
tool the analysis of the stress waves was developed further.
Short-time Fourier transform analysis
The spectrogram in Figure 3 shows a short-time
Fourier transform (STFT) of the signal obtained
between co-ordinates B and D. The chord
between these positions was in the zone of
decayed wood within the stem. This particular
transform method enabled the resonant wave frequencies to be plotted against the time taken to
receive the signal. The spectrogram shows spectral densities and times of flight for the stress
wave travelling between these co-ordinates. The
region at the higher frequencies on the spectrogram relates to the stress wave that circumvented
the decayed portion of the stem. This was found
to have a resonant wave frequency (shown on the
vertical axis) centred at approximately 2 kHz.
The region at the lower frequencies relates to the
stress wave that travelled through the decayed
part of the stem. This had a lower resonant wave
frequency that was centred at approximately
1 kHz.
The time taken to receive the signals is shown
on the horizontal axis of Figure 3. Accurate
determination of the time difference between the
two spectral regions was determined on the computer using orthogonal cursors. This technique
enabled interference terms to be ignored and
detected each spectral density peak. A time displacement of 1.25 ms was found to exist between
the spectral regions determined for sound wood
(frequency 2 kHz) and for decayed wood (frequency 1 kHz). Since the stress wave velocity
for sound wood has been found to be constant at
1300 ms–1, the path length of the stress wave that
circumvented the decayed region could be calculated. The calculated path length for the stress
wave to travel around the decayed portion was
found to be 1.6 m. The circumference of the
section of decayed wood on the cross section was
measured after the tree was felled and found to
be 1.56 m. Taking into account the co-ordinate
positions in relation to the area of decay it is
unlikely that the calculated path length of the
stress wave would have exactly equalled the circumference of the decayed area. However, this
information allowed the mapping of the stress
wave path that goes around the decayed area. An
assessment was then made regarding the extent
and position of decay within the stem.
The current mapping of decay relies on the
intuitive use of the combined information derived
from basic and advanced stress wave analysis.
This is only satisfactory as the system is still at the
development stage. Continuing research in the
Computing Department at Buckinghamshire
Chilterns University College will enable a
mapping system to be developed that provides the
operator with a visual picture of the predicted
decay pattern that is overlaid on the cross-section
of the stem.
Conclusions
Basic stress wave analysis was used to determine
the presence of decay in trees. This process
allowed the non-destructive evaluation of trees
and could be used to indicate whether an invasive
assessment technique is necessary. However, this
technique only provided limited information
regarding the position and extent of decay within
the stem. Advanced stress wave analysis using
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Figure 3. Spectrogram of the short-time Fourier transform for the stress wave set up between positions B
and D.
short-time Fourier transforms enabled the path
length of stress waves in sound wood to be accurately predicted. Further interpretation of the
data using short time Fourier transforms provides
a good indication of the extent of decay in tree
trunks and the possibility of accurately mapping
the decayed regions.
The technique exploited the attributes of living
tree wood and the fundamental properties of
elastic wave propagation. In particular, it was discovered that trees gave the same identifiable spectral component for sound wood in the radial and
tangential direction. This spectral component
became the discriminant for sound and decayed
wood.
The results showed that the selected method of
undertaking stress wave measurement of a tree
trunk gave sufficient information for the
detection of a central region of decay. Six
measurements allowed an estimation of the position and extent of the decayed portion of the
stem. The accuracy of this novel decay detection
process was confirmed by felling the tree and
comparing the suspected decay pattern with the
actual decay pattern.
References
Bucur, V. 1995 Acoustics of Wood. CRC Press, New
York, 121 pp.
Cohen, L. 1995 Time-Frequency Analysis. Prentice
Hall, New Jersey.
Hearmon, R. 1948 The elasticity of wood and plywood.
Forest Products Research. Special Report No 7.
HMSO, London.
Mattheck, C. and Breloer, H. 1997 The Body Language
of Trees. HMSO, London.
Minnesota Department of Natural Resources and
USDA Forest Service 1996 How to Recognize
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456
F O R E S T RY
Hazardous Defects in Trees. USDA Forest Service
NA-FR-01–96, Washington, DC, 20 pp.
Pellerin, R. and Bozhang, S. 1996 Non-destructive
evaluation of the degree of deterioration in wood:
stress wave frequency spectrum analysis. In Proceedings of the 10th International Symposium on Nondestructive Testing of Wood, Lausanne, Switzerland,
pp. 99–115.
Sandoz, J. and Lorin, P. 1996 Tares internes de bois sur
pied: detection par ultrasonics. Tech. For. 3,
231–240.
Toole, E.R. and Grammage, J.L. 1959 Damage from
increment borings in Bottomland hardwoods. J. For.
57, 909–911.
Waid, J. and Woodman, M. 1957 A non-destructive
method for detecting diseases in wood. Nature 80,
45–75.
Received 30 June 1999