The invention relates to a method for ultrasonic testing of elongate hollow profiles, in which a probe provided with piezoelectric transducers is used in a phased array technique.

It is generally known that an ultrasonic probe in a phased array design is provided with a multiplicity of piezoelectric individual transducers, e.g. sixteen or more. The length of the individual transducers, which can be activated to the maximum extent in the probe, including the gaps between two individual transducers is also defined as transducer ruler length. For financial reasons, such probes comprising a multiplicity of transducers are advantageously used in test bodies by reason of their increased testing coverage, said test bodies having to be tested over the entire surface and length.

The material used for the hollow profiles to be tested includes essentially all ultrasound-conducting materials, such as metals and synthetic materials. In particular, hollow profiles consisting of metal, e.g. steel, are considered hereinafter. Hollow profiles consisting of steel are used e.g. as structural pipes in the construction industry or in agricultural machine manufacture.

The hollow profiles produced in a factory can be up to 15 m in length and are cut to length according to customer requirement. Angled hollow profiles in terms of the present invention are characterised by a cross-section which can be triangular, rectangular, square or polygonal, wherein the hollow profiles can be produced e.g. from a welded or hot-rolled seamless round pipe by hot-drawing, cold-drawing or from a metal sheet which is correspondingly bent and welded at the joining edges. In cross- section, these hollow profiles each comprise straight regions in the profile limbs, said regions having a specific edge length, as well as radius regions in the transition of the bent straight profile limbs. The edge radii which are produced e.g. during hot-drawing or bending and which can be more or less large depending upon the production method, wall thickness of the hollow profile and material are typical of such hollow profiles. Typical radii in hot-rolled seamless hollow profiles range e.g. between 1 x wall thickness and 3 x wall thickness, wherein typical wall thicknesses are in the range of 5 to 25 mm. In general, the ultrasonic testing of pipes has been known and established for a long time. Ultrasonic tests are used in order to detect any possible discontinuities in workpieces, such as e.g. laminar imperfections, cracks, notches, scrap marks or pores in the pipe wall or weld seam, as part of the production process. Furthermore, the ultrasonic testing is also used for checking the dimensional stability of pipes, such as e.g. the wall thickness.

In connection with a square billet German patent application DE 33 46 791 A1 describes an ultrasonic testing device in a phased-array technique. A probe of the ultrasonic testing device is arranged in a determined angle to a surface of the square billet. Also, German patent application DE 10 2012 016 607 A1 deals with an ultrasound testing system for defects of square bar steel. Several parallel linear grooves in a surface of the square bar steel are used to conduct a calibration of the system. Further, patent application US 2015/0122029 A1 uses phased-array- technique with piezoelectric elements for ultrasonic testing. The ultrasonic sensor includes a transmission sensor on one side of the solid testing element and a reception sensor on the opposite side of the solid testing element, so that the solid testing element is positioned between those two sensors.

In the case of hollow profiles, it is also important to detect possible discontinuities which can appear in particular as longitudinal defects on the inner and outer surface of the profile limbs and also in the region of the radii. Since the radius regions are subject to greater stresses during the production of the hollow profile, they are more prone to having defects.

During the ultrasonic testing, e.g. according to the pulse-echo method using piezoelectric sound converters, ultrasonic pulses are generated which are introduced into the workpiece via a coupling medium (water or oil). If the ultrasonic pulse generated by an ultrasonic transducer strikes an obstacle, such as e.g. pores or cracks, the original propagation of the ultrasonic signal is influenced at this location by reflection or diffraction. In this manner, the sound beam can propagate in the workpiece such that sound portions return to the intromission location and can then be detected by the same or another ultrasonic converter. In many cases, the ultrasonic converter used for generating the ultrasonic pulse is identical to the ultrasonic converter used for detection purposes.

During the ultrasonic testing, a distinction can be made between direct coupling and indirect coupling. In the case of direct coupling using a probe having a single transducer or a plurality of transducers in a phased-array technique, it is important that the probe lies in a planar manner on the test body surface and that no trapped air bubbles are present in the coupling agent, so as not to cause any measuring errors or measuring inaccuracies. Typically, the coupling agent is applied prior to testing in a thin film onto the surface to be tested and the probe is placed on the test body surface thus wetted. It is also possible to couple the probe, via a solid body as an intermediate piece, to the workpiece provided with coupling agent, in order to permit e.g. angle intromissions. It is also possible to make adjustments to the respective contour of the test body using such an intermediate piece which is also referred to as a lead wedge and can consist e.g. of synthetic material. However, this type of testing is wholly unsuitable for automated testing of the surface of elongate and angled hollow profiles in a synchronised production operation. In addition to the direct coupling of the probe to the test body surface, there is also indirect coupling of the probe via a coupling agent flow length produced within an ultrasonic testing device, i.e. via a liquid column which can be several mm or cm depending upon the testing assignment.

Such an ultrasonic probe is known e.g. from German patent document DE 603 20 365 T2, in which the probe is arranged within a probe cavity, which is filled with a coupling liquid, and having a defined coupling agent flow length. The probe cavity is defined by a tyre-shaped coupling element which consists of soft rubber and is mounted so as to be able to rotate about the probe. As a result, the ultrasonic probe can roll on the workpiece surface via the coupling element.

Furthermore, the so-called immersion technique is known, in which the probe and the surface of the test body to be tested are immersed into a water bath as shown in patent application US 2010/0212430 A1 where a bar material with rectangular cross- section is tested. Immersion technique testing is also used e.g. for automated testing on round pipes comprising rough surfaces which do not permit direct coupling of the probe to the surface of the test body. During the ultrasonic testing, in addition to the straight or planar regions in the profile limbs of the hollow profile, essentially also the inner and outer surfaces of the radius regions of the hollow profile can be tested for longitudinal defects, if a plurality of individual probes is arranged and guided at fixed angles consecutively or

simultaneously corresponding to the hollow profile geometry. For different dimensions of the hollow profiles, the probes would then each have to be rearranged according to the changed geometric conditions. However, this is very complex and thus not cost- effective and not practical for an automated manufacturing sequence.

When testing hollow profiles for longitudinal defects, a different intromission angle must also be adjusted in each case for internal defect and external defect testing in dependence upon the respective curvature in the radius region, in order to achieve optimum detection of the defects and correct testing.

In the case of automated systems with or without a coupling agent flow length, this mechanical tracking of the probes is only possible with considerable outlay, for which reason these methods have hitherto only been used for testing round pipes, e.g. as rotation test rigs.

Therefore, for the purpose of testing elongate, angled hollow profiles there has hitherto not been a method which can be used cost-effectively and by means of which not only the planar regions in the profile limbs but also those of the inner and outer surface of the radius regions of the bent profile limbs can be tested in an automated manner in particular for longitudinal defects. On the whole, the problem with the ultrasonic testing of elongate, angled hollow profiles is that, by using the methods known for round pipes, testing of the entire test body surface of elongate, angled hollow profiles which is cost-effective and at the same time has a greater level of quality assurance and can be automated is not possible or is possible only with very considerable outlay.

Therefore, the object of the invention is to provide a method for the ultrasonic testing of elongate, angled hollow profiles, preferably consisting of metal, in which testing of test body outer and inner surfaces for defects, in particular longitudinal defects, including the radius regions, which has a greater level of quality assurance and can be automated in a cost-effective manner can be achieved by means of probes in a phased-array technique.

This object is achieved by a method comprising the features of claim 1 . Advantageous embodiments of the invention are described in dependent claims 2 to 10.

In accordance with the invention, in the case of a method for the ultrasonic testing of angled hollow profiles consisting of a sound-conducting material, in particular consisting of metal, comprising an ultrasonic testing device, consisting of a probe having transducers arranged on a transducer ruler in a phase-array technique, wherein ultrasonic signals for detecting external and internal defects on the hollow profile are generated by the transducers and are coupled into the surface of the hollow profile, testing which has a greater level of quality assurance and can be automated in a cost-effective manner is achieved by virtue of the fact that for the purpose of detecting external and internal defects in a surface region and in radius regions of the hollow profile, the transducers are controlled in dependence upon their position with respect to the surface region and their position with respect to the radius regions of the hollow profile such that ultrasonic signals are generated having intromission angles adapted to the external and internal defects in the surface region and in the radius regions. Prior to the actual ultrasonic testing of the hollow profile, ultrasonic testing is performed on a reference profile onto which are machined reference defects; the relevant intromission angles of the associated transducers or groups of transducers to be activated are determined by detection of reference defects on the radius region which are arranged externally and internally in the radius region of the reference profile and the determined intromission angles are adopted in dependence upon the respective position of the transducer with respect to the surface region or the radius regions of the reference profile for testing the hollow profile. The determined intromission angles are determined so that echo signal reflected by a reference defect has high amplitude and is clearly distinguished from background noise signal.

In this case, the transducers or groups of transducers are advantageously activated individually in order to adjust the intromission angles. Provision is also

advantageously made that the length of the transducer ruler is selected such that the probe is coupled by means of coupling agent to the entire width of the surface of the hollow profile to be tested comprising the surface region and the two radius regions. Alternatively, the length of the transducer ruler can be smaller than the entire width of the surface of the hollow profile, for example 100 mm when the entire width of the surface of the hollow profile is 400 mm, so that the entire surface of the hollow profile is tested in four sections at least.

Automatic or semi-automatic testing of the surface of the hollow profile both on the surfaces and also in the radii of the corner regions is possible with the invention without any problem. In addition to internal and external defects, in particular longitudinal defects, laminar imperfections in the material, as well as wall thickness measurements, can also be carried out in one operation. Testing for transverse defects and oblique defects is likewise possible using an adapted probe which couples ultrasonic signals in corresponding directions into the surface of the hollow profile.

By means of the invention, a considerable reduction in the amount of work is achieved because many different intromission conditions can be achieved in one testing procedure in dependence upon position and adapted to the respective test object geometry.

Therefore, the core of the invention is that during ultrasonic testing for defects, in particular longitudinal defects, a different intromission angle is adjusted in each case for the internal defect and external defect testing and in particular in dependence upon the respective position of the individual transducer, in relation to the radius region to be tested.

In an advantageous manner, the distance of the respective transducer to the next bent profile edge is selected in this case as a reference plane for establishing the position, wherein a different intromission angle is adjusted in each case for the internal defect and external defect testing, in order to achieve optimum testing in the surface and radius regions.

If the probe array is guided along a reference plane, in particular the profile edge, by means of the device, specific transducers are influenced in dependence upon their position with respect to the profile edge by in each case different delays of the transmission pulse (Delay Laws), in order to permit optimum and correct testing of the profile in the surface regions but also in the radius regions in each case in a different manner for the inner surface and outer surface. In this case, the reference plane extends in parallel with the longitudinal axis of the hollow profile and at a right-angle with respect to the transducer ruler and intersects the beginning of an outer surface of a radius region to be tested.

The ultrasonic testing of, in particular, longitudinally directed internal defects in the radius region of e.g. rectangular hollow profiles is effected by suitably selecting intromission angles and corresponding elements of the probe array.

In this context, the term "suitably" means that the echo signal of the defect has an amplitude which is as high as possible and which is clearly distinguished from the background noise of the echo signal.

The location-dependent intromission angles of the individual transducers or groups of transducers for detecting internal and external defects is established in an

advantageous manner on a previously ultrasonically tested reference profile, in which e.g. a linear array is arranged 90° with respect to the longitudinal axis of the hollow profile and is suitably positioned with the aid of a coupling device. In an advantageous manner, a longitudinal groove, arranged perpendicularly to the inner surface of the reference profile, in the radius region serves as a reference defect or reference reflector for the ultrasonic signals. These longitudinal grooves as reference defects are hard to detect during calibration. At the end this leads to a high measuring accuracy regarding usual defects appearing in hollow profiles which are easier to detect than the elected reference defects.

Instead of longitudinal grooves, which are introduced perpendicularly to the inner surface, as reference reflectors, it is also alternatively possible to arrange e.g.

through-bores, flat base blind bores or grooves which are not introduced

perpendicularly to the surface, should this be advantageous for establishing the location-dependent intromission angles of the individual transducers or groups of transducers. Subsequently, intromission angles, which are suitable for testing for internal defects in the radius region, are selected in dependence upon the position of the individual transducers or the respective excited group of transducers. The selection takes place in such a manner that, taking into consideration the geometry of the sound impact point, intromission angles in the material can be achieved which are at an angle, which is optimum for reflection, for the purpose of detecting internal defects in the radius region. By selecting another intromission angle, corresponding testing of the external defects in the radius region can then likewise be achieved.

Therefore, complete testing of a hollow profile for longitudinally oriented internal defects and external defects in the surface regions and in the radius regions can be achieved over the complete profile cross-section and over the length of the hollow profile. Moreover, in the case of a perpendicular intromission of sound into the surface, wall thickness and laminar imperfection testing is likewise possible using the same array.

In one advantageous embodiment of the invention, the use of a multi-dimensional array, e.g. a 2-dimensional matrix array is provided for implementing oblique defect and transverse defect testing. Holographic excitation of this array can contribute to the further optimization of the testing.

However, on account of physical aspects, only up to half of the ultrasonic signals of the transducers can reach the radius region, as otherwise the signals are no longer reflected by the internal corner of the hollow profile and therefore any possible defect signal from the radius region can no longer be detected.

Therefore, complete ultrasonic testing of the entire surface of the hollow profile and the automation thereof during a single testing procedure can be effected

advantageously using a plurality of ultrasonic testing devices, which are arranged around the test object or around the hollow profile, and a hollow profile which can be moved relative thereto. Therefore, all of the surface and radius regions of the hollow profile are then ultrasonically tested at the same time in a single testing cycle. The coupling agent used for the ultrasonic testing can be e.g. water, oil or emulsions. Specifically, in the case of test bodies consisting of steel, oil can advantageously be used as a coupling agent and at the same time as an anti-corrosion agent.

On the one hand, the lubricating effect of the oil makes it considerably easier to displace the probe on the test body surface and reduce the expenditure of effort required for this purpose and on the other hand, the test body surface is least temporarily protected against corrosion at the same time.

In a further advantageous embodiment of the invention, provision is made that for testing purposes the probe is arranged in a housing which is filled with coupling agent and is moved by means of a lateral guide in the direction of the longitudinal axis of the hollow profile over the surface at a fixed position of the transducers relative to the surface of the hollow profile. In the case of this advantageous type of ultrasonic testing, the ultrasonic testing device consists of a housing which is filled with coupling agent and is mounted so as to be able to rotate about a probe arranged therein. The probe is spaced apart from the inner wall of the housing by a defined coupling agent flow length. The position of the transducers is advantageously established by means of a rotatably mounted guide which is fixedly connected to the housing and rolls on the lateral surface of the hollow profile.

In order to test the hollow profile, the housing is moved in the direction of the longitudinal axis of the hollow profile relative to the surface thereof. On the one hand, the ultrasonic testing device can be moved over the hollow profile and on the other hand, however, the hollow profile can also be moved underneath the ultrasonic testing device. The housing itself is mounted so as to be able to rotate about the probe and therefore the ultrasonic testing device can roll over the housing, which surrounds the liquid chamber, on the workpiece surface. In order to couple the housing to the surface of the test body, said surface is wetted in a known manner with coupling agent, such as e.g. with water. The housing itself can consist e.g. of sound-conducting synthetic material.

In one advantageous development of the invention, provision is made that the housing is formed as a membrane which is permeable for coupling agent and the surface of the hollow profile is wetted during the ultrasonic testing. In this case, the housing must be provided with a connection in order to be able to feed coupling agent continuously to the housing.

Alternatively, the surface of the hollow profile can also be wetted in a separate operation prior to or at the beginning of the ultrasonic testing.

This device comprising an integrated coupling agent flow length has the advantage of a relatively simple and cost-effective structure and of simple handling. Moreover, the device can be used in a simple manner controlled by hand or controlled in a semi- automated or fully automated manner.

Although the device in accordance with the invention is preferably used for ultrasonic testing of metallic hollow profiles, e.g. consisting of steel, it is not limited thereto. It is also feasible to use this device e.g. for testing hollow profiles consisting of synthetic material or other non-metallic but sound-conducting materials.

Further features, advantages and details of the invention are apparent from the description hereinafter of an exemplified embodiment illustrated in the drawing, in which:

Figure 1 shows a schematic cross-sectional view of an ultrasonic testing device in accordance with the invention,

Figure 2 shows a cross-sectional view of an angled reference profile comprising reference grooves,

Figure 3 shows a schematic view of a transducer ruler of the ultrasonic testing device shown in figure 1 with a hollow profile to be tested and

Figure 4 shows an enlarged view of a radius region of the hollow profile in figure 3. Figure 1 shows a schematic cross-sectional view of an ultrasonic testing device 1 1 in accordance with the invention for testing elongate and angled hollow profiles 5 comprising a rectangular cross-section for longitudinal defects. The ultrasonic testing device 1 1 consists of a probe 1 , which is equipped with in this example thirty-two transducers A to G arranged in the manner of a ruler (see figure 3), in a phased-array technique. Typically, the probe 1 has a so-called transducer ruler 4, on which the transducers A to G are arranged. The transducers A to G are also called virtual probes since the transducers A to G each consist of one or more transducer elements. These transducer elements are controlled in order to generate ultrasonic signals 10 in a desired direction. The transducer ruler 4 is arranged in a housing 2 which forms a liquid chamber and is filled with a coupling agent 3. The length of the transducer ruler 4 is dimensioned corresponding to the length or width of a head part 5.1 , a side part 5.2 or 5.4 or a base part 5.3 (see figure 3) of the hollow profile 5 which is to be tested. As usual the head part 5.1 , the side parts 5.2, 5.4 and the base part 5.3 have roughly parallel inner and outer surfaces. The housing 2 of the ultrasonic testing device 1 1 is mounted so as to be able to rotate about the transducer ruler 4 which is mounted in a fixed position in the housing 2, so that the ultrasonic testing device 1 1 or its housing 2 can roll on a surface of the hollow profile 5 and therefore is in contact with the hollow profile 5. In this case, the liquid in the housing 2 and the wall of the housing 2 form a coupling agent flow length between the probe 1 and the hollow profile 5. The housing 2, in particular its wall, consists of sound-conducting synthetic material, in order to be able to couple the ultrasonic signals 10 transmitted by the probe 1 (see figure 3) into the hollow profile 5. In order to couple the housing 2 to the surface of the hollow profile 5, said surface is wetted in a known manner with coupling agent 3, such as e.g. water. The wetting with coupling agent 3 takes place in this case prior to or during the ultrasonic testing by means of a separate operating step in an automated manner or by hand.

The location of the transducers A to G in the ultrasonic testing device 1 1 relative to the surface of the hollow profile 5 to be tested is established by means of a guide roller 6. For this purpose, the guide roller 6 is mounted on the ultrasonic testing device 1 1 so as to be able to rotate about a vertical axis and, during the ultrasonic testing, rolls along the surface of the left-hand side part 5.4 of the hollow profile 5 in the direction of the longitudinal axis L of the hollow profile 5. As a result, the transducers A to G or groups of transducers A to G are oriented in terms of location in relation to the surface of the hollow profile 5 to be tested and can be guided for the ultrasonic testing in the direction of the longitudinal axis L of the hollow profile 5. The head part 5.1 of the hollow profile 5 to be tested and also the side parts 5.2 and 5.4 to be tested and the base part 5.3 in each case at the beginning and end consist of a radius region 5.5 and a surface region 5.6 arranged therebetween (see figure 2). Depending upon whether longitudinal defects are to be detected in the radius region 5.5, in the surface region 5.6 or also in the surface 5.6 but actually in proximity to the radius region 5.5, the ultrasonic signals 10 are to be adapted accordingly in relation to their intromission angles and therefore the respectively locally associated transducers A to G. The adaptation is effected by electronic control of the transducers A to G and with the aid of a reference profile 5' or its dimensions of the radius regions 5.5 and the surface region 5.6 in the direction of the transducer ruler 4. The intromission angle is understood to be the angle which is formed by the direction of the ultrasonic signal 10 proceeding from the surface of the hollow profile 5 to be tested into the material of the hollow profile 5 to be tested and a vertical with respect to the surface of the hollow profile 5 to be tested. The respectively determined intromission angles are then stored for each hollow profile 5 and each side thereof to be tested and if required are used for configuring the intromission angles of the transducer ruler 4. By means of the previously respectively established intromission angles, internal and external defects in the planar surface regions 5.6 and the radius regions 5.5 can be detected in a targeted manner. Typically, the transducer ruler 4 comprises an elongate rectangular shape and is oriented with its longitudinal extension horizontally, in parallel with the surface of the hollow profile 5 to be tested and at a right angle with respect to a longitudinal axis L of the hollow profile 5. During the ultrasonic testing, the transducer ruler 4 is guided along the surface of the hollow profile 5 to be tested, in its longitudinal axis L. In this example, the ultrasonic testing device 1 1 is moved by hand over the hollow profile 5. For this purpose, the ultrasonic testing device 1 1 comprises a handle 7. Figure 2 shows a schematic cross-sectional view of an angled and hollow reference profile 5' comprising reference defects formed as reference grooves. The reference grooves are each arranged in the radius region 5.5 of the hollow profile 5' and are formed as external defects 8, 9 on the inner and outer surface of the radius region 5.5 and in each case perpendicularly to the inner and outer surface of the radius region 5.5. The reference grooves also extend in the direction of the longitudinal axis L of the hollow profile 5'. By means of ultrasonic testing of the reference profile 5' and thus of the reference grooves, the transducers A to G or groups of transducers A to G to be activated in each case and their respective intromission angles are then determined and stored for the dimensions of the reference profile 5', in particular the geometry thereof in the region of the radius region 5.5. The determination takes place in such a manner that, taking into consideration the geometry of the sound impact point, intromission angles in the material of the hollow profile 5' can be achieved which are at an angle, which is optimum for reflection, for the purpose of detecting external defects in the radius region 5.5. In relation to the ultrasonic signals 10, the reference grooves can also be defined as reference reflectors. As previously described, in this case the transducer ruler 4 is arranged with its longitudinal extension 90° to the longitudinal axis L of the hollow profile 5. Subsequently, intromission angles, which are suitable for testing the internal defect 9 in the radius region 5.5, are selected in dependence upon the position of the individual transducers or the respective excited group of transducers.

Figure 3 shows a schematic view of a transducer ruler 4 of the ultrasonic testing device 1 1 shown in figure 1 with a hollow profile 5 to be tested. For the sake of clarity, this view shows only the transducer ruler 4 without the housing 2. It is apparent that a multiplicity of transducers in a phased-array technique, of which some are designated by the letters A to G, are spaced apart along the transducer ruler 4 in its longitudinal direction and uniformly from one another. The transducers generate ultrasonic signals 10 which are directed to the hollow profile 5 to be tested. The hollow profile 5 to be tested is rectangular and comprises two side parts 5.2, 5.4, a head part 5.1 and a base part 5.3. In the present case, the head part 5.1 including half of the radius regions 5.5 adjoining the surface region 5.6 are initially examined for internal and external defects. The length of the transducer ruler 4 of the probe 1 is such that the head part 5.1 of the hollow profile 5 to be tested can be tested as far as to the radius region 5.5. Depending upon the location of the respective transducer, ultrasonic signals 10 comprising different intromission angles are coupled into the surface of the hollow profile 5. However, on account of physical aspects, in each case only up to half of the ultrasonic signals 10 of the transducers A, B, C and D can reach the two radius regions 5.5, as otherwise the ultrasonic signals 10 are no longer reflected by the internal corner of the hollow profile 5 and therefore any possible defect signal from the radius region 5.5 can no longer be detected. The internal and external defects located in the planar surface region 5.6 of the head part 5.1 are detected by means of the ultrasonic signals 10 of the transducers E and F. Of course, in addition to the transducers E and F, still further ones of the transducers are used in the planar surface region 5.6. For reasons of clarity, corresponding ultrasonic signals 10 have not been illustrated. In addition, in accordance with the invention laminar imperfection testing or wall thickness measurements are also conducted using the transducer G.

Figure 4 shows an enlarged view of the radius region 5.5 of the hollow profile 5. For calibration purposes one external defect in the outer radius region 8 and another external defect in the inner radius region 9 are placed in the hollow profile 5. The inventive transducers G, F, D and B are grouped together to so-called virtual probes generate ultrasonic signals 10 with intromission angles adapted to the detect external and internal defects in the first half of the radius region 5.5 being in front of a center line 12 of the radius region 5.5 seen in a direction of detecting. Accordingly, the transducers G, F, D and B can direct their ultrasonic signals 10 starting from the head part 5.1 of the hollow profile 5 into the radius region 5.5 of the hollow profile 5. The center line 12 of the radius region 5.5 defines a line where an ultrasonic signal 10 is just reflected backwards to the transducers A to G and is not diffracted within hollow profile 5. In the embodiment shown in figure 4 transducers G and F produce ultrasonic signals 10 that are reflected from external defects 8, 9 and detected by transducer G or F respectively. However, it is also possible that a neighboring transducer also detects the ultrasonic signals 10 so that transmitter and receiver can but must not be identical.

List of reference signs

1 probe

2 housing

3 coupling agent

4 transducer ruler

5 hollow profile

5.1 head part of the hollow profile 5

5.2 side part of the hollow profile 5

5.3 base part of the hollow profile 5

5.4 side part of the hollow profile 5

5.5 radius region

5.6 surface region

5' reference profile

6 guide roller

7 handle

8 external defect in the outer radius region

9 external defect in the inner radius region

10 ultrasonic signals

1 1 ultrasonic testing device

12 center line of radius region

A to G transducers

L longitudinal axis

R reference plane