HOW TO USE A NONDESTRUCTIVE
EVALUATION of timber structures
Petr
Horáček
Abstract
A
variety of NDE techniques can be employed by an inspector in order to determine
the condition of an timber structure. Advances are
needed to improve the effectiveness of predicting timber properties and overall
structural capacity from various NDE methods. The goal of this paper is to
describe a combination of techniques that will provide a more effective
prediction of timber structure condition and capacity.
Key words: Nondestructive evaluation, wood properties, timber
structures
1 INTRODUCTION
Nondestructive
evaluation techniques (NDE) utilizing different methods has successfully been
used to improve the
assessment of the integrity of wood structures [1]. NDE
techniques for wood differ greatly from those for homogeneous, isotropic
materials such as metals, plastics, and ceramics. In such materials, whose
mechanical properties are known and tightly controlled by manufacturing processes, NDE techniques are used only to detect the
presence of discontinuities, voids or inclusions. However, in wood, these
irregularities occur naturally and are further induced by degradative agents in
the environment. Therefore, NDE techniques for wood are used to measure how
environmentally induced irregularities interact in a wood member to determine
its mechanical properties. The purpose of this document is to provide
guidelines on the application and use of the NDE method in locating and
defining areas of decay in timber structure members [2].
Timber
bridges are often exposed to harsh environmental conditions. Over time, this
exposure can lead to deterioration resulting from decay, insect attack,
weathering, and mechanical damage [3]. Early decay is difficult to recognize
because it is visually subtle and often occurs on the interior of a timber or
below ground. Non visual methods of identifying early decay include culturing
the fungus from the infected wood and various physical tests, such as those
that indicate a change in strength properties, acoustic changes, and electrical
changes.
2 NDE OF timber members
Inspection
methods are currently a combination of tests for physical detection of decay
(for example, sounding, boring), mechanical detection (compression test,
splinter test), electrical detection (moisture meter, x-ray), or acoustic
detection (acoustic emissions, stress wave time). Laboratory detection methods,
such as culturing, microscopy, and serological tests, though definitive for
early stages of decay and valuable to the inspection process, have historically
not lent themselves to field use [3].
The fundamental hypothesis governing NDE of wood materials was first put forth by [4]. He proposed that the energy storage and dissipation properties of wood materials, which can be measured nondestructively by using any of several vibration-or acoustic-type techniques, are controlled by the same mechanisms that determine the static behavior of the materials. A variety of inspection techniques can be employed to locate damage and decay in timber members in order to maintain structural performance [3]: visual inspection, stress wave / ultrasonic, drill resistance, radiography, microwave / ground penetrating radar, vibration.
2.1 Visual
inspection
Visual inspection is the simplest NDE technique, and should be the first step in assessing a timber bridge. Using visual inspection, technical personnel can quickly develop a qualitative assessment of the relative structural integrity of individual members. Obvious deficiencies can be easily identified, Including external damage, decay, crushed fibers in bearing, creep, or presence of severe checks and splits. Results of visual inspection can be employed to guide further NDE.
Visual inspection requires strong light and is
suitable for detecting intermediate or advanced surface decay, water damage,
mechanical damage, or failed members. Visual inspection cannot detect early
stage decay, when remedial treatment is most effective [5].
2.2 Stress
wave / ultrasonic technique
Stress wave propagation in wood is a dynamic
process that is directly related to the physical and mechanical properties of
wood. In general, stress waves travel faster in sound and high quality wood
than in deteriorated and low quality wood. By measuring wave transmission time
through a timber in the given direction, the internal condition of the tree can
be fairly accurately evaluated. If decay is present in the member, the
attenuation and propagation time of the stress wave passing through the member
is increased. The increase in propagation time, in extensively decayed wood,
may be as great as 10 times the propagation time for solid wood [1].
Table 1
summarizes recent research on stress wave transmission times for various
species of wood. In general, the reference information can be summarized into
two groups: softwoods and hardwoods. Generally, sound travels faster in
hardwood species than in softwood species. As a rule in living trees, the
baseline transmission time is 1,000 µs/m for softwoods
and 670 µs/m for hardwoods [Table 1] [9]. Measured
transmission time (per length basis) less than this would indicate a sound
wood. A study conducted by [11] demonstrated that a 30% increase in
stress wave transmission time implies a 50% loss in strength. A 50% increase
indicates severely decayed wood [9]. For timber structures such threshold
criteria is still missing.
Table 1: Reference stress wave
velocities and transmission times for various species at moisture content
6-12%: sound wood [10],
decayed wood calculated according to [11].
|
Wood species |
Stress wave transmission time (ms.m-1) |
||
|
Sound wood [10] |
Moderate decay [11] |
Severe decay [11] |
|
|
Spruce |
700-930 |
910-1209 |
1000-1375 |
|
Fir |
625-1070 |
940-1390 |
1085-1605 (1574 10) |
|
Pine |
1075 |
1390 |
1610 (1640 10) |
|
Oak |
570-645 |
740-835 |
855-970 |
|
Beech |
600 |
780 |
900 |
|
Maple |
670 |
871 |
1005 |
Because
wood is an organic substance, the speed of wave propagation varies with grain
direction. The speed of sound across the grain is about one-fifth to one-third
of the longitudinal value. [9]. Remember that longest transverse-to-grain
transmission time is found at a 45° orientation to the annual rings. The
shortest is in the radial direction; stress wave speed is about 15-30% faster
than that in the tangential directions. [9] Stress wave transmission time (TT, ms.m-1) is given by:
TT = 106 c-1 = 106 (E/r)-1/2 (1)
where c – stress wave velocity (m.s-1), E – modulus of elasticity (N.m-2), r – wood density (kg.m-3).
2.3 Drilling
resistance
Drill resistance is a quasi-nondestructive test that has been used to determine density end detect decay in trees and timber. It is classified as quasi-nondestructive because a small diameter (1.5mm -3mm) hole remains in the specimen after testing. However, this hole is small enough to have only negligible structural effects on the remaining cross-section and may be sealed to prevent access for agents of decay. Drill resistance (RD, Nm.s.rad-1) devices operate under the premise that resistance to penetration is correlated with material density. Drill resistance is determined by measuring the power required to cut through the material:
RD = T / w (2)
where T – drilling torque (Nm), w – angular speed (rad/s).
Plotting drill resistance versus drill tip depth results in a drill-resistance profile that can be used to evaluate the internal condition of a tree or timber member and identify locations of various stages of decay. The resistance profile can also be used to estimate member density and compares favorably with radiographic.
Due to the invasive nature of the drill resistance technique, and the fact that it provides a vary localized measure of density, this technique maybe best employed if used in conjunction with NDE methods that provide qualitative condition assessment (e.g., visual inspection) or regional condition assessment (e.g., stress wave or ultrasonic inspection). In such a scenario, visual or stress wave inspection could be used to locate expected regions of decay. Drill resistance measurements could then be taken at a limited number of key locations to determine the through-thickness condition of the wood. These measurements could be combined to predict MOE and possibly member strength.
An overall analysis of data from different
timber specimens led to the development indices of decay for the test specimens. It seems to be difficult to
distinguish between different stages of decay within narrow range of 0-25%
[Table 2].
Table 2: Deterioration assessment based on drilling resistance
(indices represents nearly 85% of the specimens tested) [8].
|
Drilling resistance (Nm.s.rad-1) |
Deterioration index |
|
25-100% |
Sound wood |
|
10-25% |
Moderate deterioration |
|
0-10% |
Severe deterioration |
3 Quantification
of biodegradation
Another attempt there is to develop an NDE methodology whereby the presence and extent of biodegradation could be quantified [6]. It is easy to add an additional constraint to the objectives since we wanted to develop the technique in such a way that it could quantify biodegradation at inaccessible locations.
Since in exterior use we cannot control the environmental variables, such as temperature and moisture content, for in-situ measurements, we needed to develop an NDE which can quantify the biodegradation while at the same time is insensitive to temperature and moisture variations.
In a general form, e.g. the vibration signal output (V), is assumed to be a function (f) of a number of major material and environmental variables. In mathematical form [7]:
V = f (D, B, M, T, G, C, ....) (3)
where D – density, B – biodegradation, M - moisture content, T – temperature, G – geometry, C - boundary condition.
In the final form the biodegradation is quantified by a decay ratio (DR). The decay ratio is defined as the fraction of decay occupying the cross section. Based on the above outlined criteria the final form of the mathematical prediction model chosen is [7]:
DR = a+b (P/S) + c(G) (5)
where P - NDE parameters, S - species parameter, G - geometry parameter, a, b, c - numerical constants.
Line A
Line B
Line C
Figure 1: Assessment of wood deterioration (Douglas-fir) by several stress-wave tools [12].
The above decay ratio is computed at several orientations at a given cross section. This way the decay location can be mapped to scale. Note that the process can be repeated more times rotating the measurements at some degrees each time. The decay ratio approach gives excellent results between actual and NDE-predicted decay ratios.
One example of application of above mentioned
approach there is comparison of stress wave transmission plots and interior
photographs of wood specimen revealing areas of different grade of
deterioration (Figure 1). Figure 1 illustrates results obtained from a specimen
that showed mostly sound wood with a pocket of moderate–severe deterioration at
approximately 240 mm from the specimen end after being cut open. Nearly all
measured stress-wave transmission times were within the sound to slightly
moderate decay zones. However, the stress-wave transmission time measured was
in the severe to slightly moderate decay zone for one device, whereas it was in
the severe decay zone for another devices [12].
Anyway, a good approximation of threshold criteria distinguishing a sound and
deteriorated wood there is the stress-wave transmission time around 1000 ms.m-1
(softwoods) and 700 ms.m-1
(hardwoods). For more details refer to [10].
Results on comparative performance characteristics such as accuracy, reliability, and ease of use in detecting internal decay in the timber structures provide good potential for interpretation of the testing results. Regardless of the unit used, the user must be careful to differentiate the presence of decay from internal splits, cracks, or ring shake in the timbers. It is recommended that an increment corer or resistance drill be used to confirm the exact levels and locations of decay [12].
4 conclusion
Professionals may be satisfied to produce a report on the conducted NDE research. However, eventually a question is raised whether the results could be used to produce a tool that can be put to practical application. While the professional may not be the one who produces the NDE tool, he or she should be familiar with the questions raised by the one who will consider the feasibility of such an undertaking. Some of the more important elements of the feasibility evaluation are discussed in the paper:
• Stress-wave timing technologies can be used successfully to detect the presence and level of internal decay for timber bridge components.
• Stress-wave timing measurements perpendicular to the grain provide an excellent tool to assess the extent of internal decay in timber bridge components.
• There were, however, differences in the level of variability and the decay thresholds for different equipment.
• Any nondestructive testing tool or device must be used as part of a comprehensive condition assessment that incorporates an in-depth visual inspection, knowledge of prior use of the structure, and a working knowledge of fundamental engineering properties of structural wood products.
• When used with visual and probing techniques, NDE technique provides a very accurate description of the internal condition of timber structures.
acknowledgment
This paper was elaborated with the financial support of Ministry of Education, Youth and Sport of Czech Republic, No. MSM 6215648902.
LITERATURE
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L., Ross, R. J.: NONDESTRUCTIVE
EVALUATION TECHNIQUES FOR TIMBER BRIDGES.
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Montreux
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[7] Bodig, J.: The Process of NDE Research for Wood and Wood Composites. In: Proceedings of 12th International Symposium on Nondestructive Testing of Wood, 2000, NDT.net – March 2001, Vol. 6 No. 03
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Brashaw, B. K., Ross, R. J., Pellerin, R. F.:
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Petr Horáček, Doc. Dr. Ing., horacek@mendelu.cz, +420-545134049, Faculty of Forestry and Wood Technology, Mendel University of Agriculture and Forestry Brno, Zemědělská 3, CZ-61300 Brno, Czech Republic.