|NDT.net Apr 2006 Vol. 11 No.4|
Defect localisation in wood with low frequency Ultrasonic Echo TechniqueA. Hasenstab (firstname.lastname@example.org), M. Krause
Federal Institute for Materials Research and Testing (BAM), Berlin, (Germany)
Technical University of Berlin (Germany)
14th International Symposium on Nondestructive Testing of Wood
AbstractThe use of low-frequency ultrasonic echo technology with longitudinal and transverse waves for the investigation of wooden construction components is described. Further more the different influences of the wood fibres on the sound velocity of the longitudinal and transversal waves is pointed out on the basis results of measurements. The efficiency of the ultrasonic echo technique is shown on wooden specimens. Especially the use of the transversal transducers which need no coupling agent, will be shown on a historic monastery. There it is possible to detect the damage from the undamaged side of the specimen by ultrasound echo. As reference procedures the drilling resistance method is used.
With these methods very exact information about the damage at the location of the measurement can be obtained. The disadvantage is that an already damaged construction will be further damaged by these measurements. A large construction also requires numerous measurements that take time. Sometimes it is also difficult to assess the damage to the entire construction on the basis of data obtained from a few drilled holes with a diameter of 3 mm.
Non-destructive techniques to detect hidden cavities in wood are high-level radiation, usually X-rays, and ultrasound transmission technique are used , , , . Being non-destructive, these methods permit unlimited numbers of tests e.g. to check the integrity at different times. Most of the nondestructive methods are based on the principle of transmission, requiring that both sides of the specimen are accessible. In contrast to this, ultrasound echo technique only needs access to one side.
For a long time now, low frequency ultrasonic echo technique (80 to 200 kHz) are already been used for non-destructive testing on concrete elements, for example for thickness measurement or for the localisation of compaction faults, reinforcement rebars and tendon ducts , , , .
Fundamental experiments with ultrasonic echo technique applied to wooden test specimen have been done in cooperation between the department of construction materials at the Technical University of Berlin and the Federal Institute for Materials Research and Testing (BAM).
Experimental ProcedureThe ultrasonic echo technique is based on the reflection of the acoustic waves on material inhomogenities like back wall of the specimen or at any other interfaces. With this technique it is possible to get indirect information about the condition of the construction unit or internal damage from the received signals. The relative results of measurements can be calibrated by well-known dimensions of the construction or by drilling resistance. The small density of wood (?pine << ?concrete << ?steel) is caused by a high cavity density, which makes probes with high performance and low-frequency (50-200 kHz) necessary. A low frequency result in a small absorption of the signal, has however a large wavelength as a consequence, which goes with a larger minimum size of the still detectable error.
The experiments were carried out with longitudinal waves and transverse waves. The longitudinal wave transducers were coupled by Vaseline, ultrasonic gel and glycerine. Here the centre frequency was optimised 100 kHz. The transversal wave transducers need no coupling agent, since they are coupling by point contact and the centre frequency was optimised at 55 kHz. Because of this practical use here most shown results were made with transverse waves without coupling media. The results with longitudinal waves can be seen in .
The experimental set-up Figure 2 consists of a function generator to generate the signal, a preamplifier and a amplifier, a transducer and a PC for the data equalisation. For the measurements longitudinal and transverse waves were used. The probes were excited by a programmed RC2-impuls with different centre frequency.
The longitudinal wave probes from Krautkraemer (Figure 3) are activated with the centre frequency 100kHz and coupled by Vaseline or ultrasonic gel. In the case of a measurement perpendicular to the fibre with v = 1.8km/s results with l=c/f a wavelength of 1,8cm. Thus damage smaller than 1cm diameter usually cannot be detected directly. The transverse waves are produced by spot contact probes as single probes (Figure 4) or as an array consisting of 24 spot contact probes (Figure 5). The transmitting receiving unit consists of 12 spot contact probes as transmitter and 12 probes as a receiver. This kind of probe has the advantage that no coupling agent is necessary and thus a faster measurement without pollution of the measuring surface like with coupling agents is possible. The probes are activated with the centre frequency 55kHz, which results in a wavelength of 2,5cm in the case of a measuring direction perpendicularly to the fibre with v = 1.4 km/s and l=c/f.
Tabelle 1: forms of different waves in matter 
Anisotropy of woodIt is well known, that wood is a anisotropic material. One can distinguish between three different directions in fibre direction (axial), perpendicular to fibre (radial) and tangential to fibre. In addition to the dependence of the acoustic velocity to the wood direction, attenuation is very high due to the high content of air in wood, which means powerful low frequency transmitters are necessary.
Basic experiments for verifying the anisotropy of velocity of longitudinal waves from ,  show that the used probes create the same results. They show, that the velocity increases from tangential to radial up to axial wood direction  (Figure 6). This well-known result doesnt confirm at all with results of tests with transversal waves (Figure 7). In this case no significant variations of the velocities were found corresponding to the different wood directions. One reason could be that wood is a axial isotropic medium and the waves oscillate in direction of fibre and then there is no influence of the annual rings. The probes for longitudinal waves are much larger so just view measurements are possible.
Ultrasound Echo techniqueThe transmission technique evaluates the transit time of the transmitted pulse in the range of 100kHz through the specimen. It is however difficult to locate the position of damage within the specimen, as the transit time resulting from damage close to the surface can be identical to the transit time resulting from deeper damage . In wood it also can be difficult to distinguish between one large knot or a cluster of small ones.
The echo technique allows the direct localization of a reflector, like a back wall or an inhomogeneity. After many tests made at BAM it can be assumed that a clear echo from the back wall shows that the specimen is free of defects.
An echo with a short transit time among a series of echo with long transit time can be caused by a defect. If the velocity of the material and dimensions of the specimen are known, the defect can be located. To obtain a good echo, broad beam transducers with high attenuation in the transducer and short signals are necessary. Also the fact, that the back wall echo is shadowed can indicate inhomogenities or defects.
Ultrasonic echo results are often represented as A-scans or B-scans. An A-scan (Figure 8 right) shows the transit time and the intensity of the pulse. A B-scan (Figure 8 left) is a composite of a number of A-scans. It is a two-dimensional cross section through the specimen and shows the transit time along the line of measurement. If the measured transit time is multiplied with the known velocity of the sound wave in a certain material, the result is the way of flight of the sound wave. So a B-scan can show directly the position of in homogeneities.
After the successful measurements on an undamaged specimen, the measurements on a specimen from a construction which had a visually undamaged surface and a back wall which was damaged in some areas were presented.
Figure 9 shows a measurement along the undamaged surface with a crack. The crack perpendicular to the direction of the measurement is visible in the B-scan as there are no surface waves. The absent echo of the back wall can be a hint for a damage in the specimen. In reality the specimen was damaged by rot.
During the history of the building, humidity was transported into the upper floors of the building by the wheels of horse carts and the outermost beam, resting against the wall (Figure 10), was damaged by rot. The transmission measurements with ultrasonic could obtain no sufficient results, measurements were done by ultrasonic echo.
The results of the measurement along the wooden beam at position 1 (Figure 10) is shown in Figure 11 as a Bscan. Along the whole measurement clear back wall echos were received. The two short ranges without high intensity echos of the back wall are caused by branches on the surface.
Alongside close to the external wall the measurements for the B-scan (Figure 12) at position 2 were created. Here at the beginning the intensity of the echo was high, even a multiple echo was received. Later the intensity of the echo went down and no echo was received.
The other measurements were done at x=160cm (Figure 14) where line 1 had a clear echo, line 2 had none. As expected left had a clear echo and near to the wall, there was no echo. The results were confirmed with drilling -resistance.
ConclusionIt is possible to use low frequency ultrasonic echo techniques successfully with longitudinal (100kHz) and with transverse (55kHz) waves on wood. So it is possible to detect the back walls of the specimens as well as the inserted cavities inside the structures .
The relative results can be calibrated by measuring the dimension of the specimen on the area were it is possible to use drill resistance. So it is possible to check the thickness of a beam without cracks even though the back wall is not accessible. It is therefore possible to determine the dimensions and inner structure of large constructions. Detailed tests are only necessary in areas with no or an unexpectedly early echo from the back wall, because this can be a sign of damage.
With a combination of the described procedures, damage can be examined rationally: The ultrasonic echo procedure can be used for the investigation of large areas. Thus areas with irregularities can be determined under certain conditions, such as accessibility, few cracks, and undamaged surfaces. The expansion of these areas can be examined exactly by X-ray  and the position in depth and the kind of defects using drilling resistance. The present investigation shows that the ultrasonic echo technique has the capability to be utilized for the preservation of large wooden structures, e.g. monuments and historic buildings (frameworks, churches, ). The early detection of defects and their extend make it possible to precisely locate hidden damage that would normally only be discovered during restoration. The possibilities of discovering hidden damage before commencement of work permits a more precise estimation of costs, and therefore a more economic allocation of funds.
AcknowledgementsSpecial thanks go to Mr. Saar who made the measurements possible at the State Office of preservation of monuments and historic buildings of Bavaria in Thierhaupten and offered the possibility to check the ultrasonic echo measurements by drilling resistance. Thanks also go to Mr. Lutz who made the measurements in Thierhaupten possible with the ultrasonic equipment of the University of Darmstadt. Mr. Smith assisted with the translation.