Non-destructive test apparatus with eddy current transducer rotary head and field homogenizing conductive ring for scanning metal test materials

A rotary head for scanning the surface of elongated test materials (62) by eddy current transducers (60) is additionally provided with means for magnetization of the test material (62) over an intended test range making it now possible to achieve sensitive testing of non-bare ferromagnetic bars and wires, testing for "smeared cracks" where surface has closed above the material separation, and testing of welded austenitic pipes. For the magnetization, parts of the housing (10) as well as a rotating hollow shaft (40) are employed in the conduction of magnetic flux, and special measures are taken for the homogenization of the magnetic field in the test range.

BACKGROUND OF INVENTION 
1. Field of the Invention 
The invention relates to a rotary head for scanning the surface of 
elongated metal test materials and, more particularly to such a rotary 
head in a non-destructive eddy current defect test apparatus. 
2. Description of Related Art 
For many years, rotary heads have been known for use in non-destructive 
testing of materials, and are widely used. They serve there for testing, 
in particular, of bare curved surface materials of steel and non-ferrous 
substances to detect defects located at any point below the surface and 
extending up to the surface. During long periods of application, such 
rotary heads have gained a high technological performance providing 
excellent results. On bare surfaces, e.g., cracks having a depth of only 
30 micrometers can be reliably detected, and this accomplished at rather 
high running speeds. 
On the other hand, there are present limits, which would be desirable to 
overcome, since this would lead to further possible applications. For 
instance, sensitive testing of "non-bare" ferromagnetic steel is not 
possible with known eddy current rotary heads because of the high 
interference level encountered. Another example relates to cracks in 
ferromagnetic materials, wherein further processing, as rolling, drawing 
or peeling, will result in a surface being closed again. In this case, the 
low penetration depth of eddy currents for the high frequencies required 
to achieve desired sensitivity accounts for the fact that such "smeared" 
cracks are impossible to detect in most cases. Difficulties have also been 
encountered in the testing of welded austenitic pipes, wherein ferrous 
impurities are found to generate a high interference level in the area of 
the weld seam, and, therefore, prevent an efficient testing of such pipes. 
SUMMARY OF THE PRESENT INVENTION 
According to the invention, the test material is magnetized resulting in a 
considerable reduction of the magnetic permeability of the test material, 
and, if possible, magnetized to reach the saturation range. There is a 
risk, however, in doing this that the ferrite cores of eddy current 
transducers will also become saturated, and as a result that the 
sensitivity of the latter will be undesirably affected. This is 
compensated for in the invention by locating a ring of magnetically 
conductive material between the magnetization coil and the eddy current 
transducer. This ring homogenizes the magnetic field in the test range, 
and, therefore, the radial component of the magnetic field above the test 
material surface, will stay small. 
By reducing the magnetic permeability of the test material, a comprehensive 
suppression of all interference sources caused by magnetic inhomogeneities 
of the material surface, such as scale, heat zones, weld seams or cold 
consolidation, for example, is achieved. It is now possible, for the first 
time, to also test "non-bare" ferromagnetic material, using rotating eddy 
current probes. For "smeared cracks" (i.e. with closed surfaces) the 
strong reduction of the magnetic permeability, which may be 50 times or 
more, causes a corresponding increase in the penetration depth of the eddy 
currents, making defects detectable, which would have normally remained 
hidden. Also, now for the first time welded austenitic pipes are now fully 
accessible to testing with eddy current rotary heads, since by the 
reduction of the magnetic permeability, the influence of ferrous 
impurities can be made ineffective. 
A first embodiment of the invention provides interchangeable protective 
sleeves supported by the housing and having a borehole adapted to the 
diameter of the test material, which sleeves serve as conductors of the 
magnetic flux. According to another embodiment, a protective sleeve 
extends into the borehole of a hollow shaft allowing the magnetic flux to 
be conducted in an optimum manner via the rotating hollow shaft into the 
test material. A still further embodiment requires that between the 
protective sleeve and the hollow shaft, there is provided a narrow air gap 
at least at one position, such that a low-loss transfer of the magnetic 
flux is possible. 
According to a particularly advantageous and further embodiment of the 
invention, the homogenization ring covers the side of the magnetization 
coil directed toward the eddy current transducers up to a predetermined 
distance from the axis of rotation, which corresponds approximately to a 
maximum of double the radial distance of the eddy current transducers from 
the axis of rotation. An important aspect of this embodiment is that the 
homogenization ring is in magnetically conductive connection with those 
parts of the housing conducting the magnetic flux. 
According to yet another aspect of the invention, the protective sleeves 
are totally or partially coated with a thin layer of magnetically 
non-conductive material thereby "sticking" of the magnetized ferromagnetic 
test material to the protective sleeves is prevented, and a smooth passage 
of the latter is achieved. A still further aspect of the invention comes 
about from the rotary section comprising a rotary disk rigidly connected 
to the hollow shaft, the rotary disk eddy current transducers being 
suspended, and the rotary disk being made of magnetically non-conductive 
material. Therefore, magnetic poles are prevented from being formed at the 
disk, and no eddy currents are produced in the adjacent housing sections, 
which would necessarily lead to heating-up and braking effects. 
An essential aspect of the invention is that, in the test range covered by 
the apparatus magnetic field, there is also provided a stationary eddy 
current transducer assembly which may be integrally mounted onto the 
interior of a protective sleeve. In this way the space and cost-economical 
combination of stationary and rotating eddy current transducers is made 
possible in a simple manner. Therein, it is particularly advantageous that 
both can use the same d.c. pre-magnetization to lower the interference 
level. 
A stationary eddy current transducer assembly can be adapted as either 
absolute-type as well as difference-type continuous coils. On the other 
hand, segment coils distributed over the transducer periphery can also be 
provided. Absolute-type coils can, in addition to the various defect test 
possibilities, now also detect material mistakes, e.g., thiciness. 
Difference-type continuous coils are particularly suitable for detecting 
external defects, holes in pipe walls and inclusions of foreign matter. 
It is advantageous that the various systems can complement each other. 
Furthermore, for ferromagnetic material, stray flux coils can be used 
instead of the eddy current continuous coils. In the latter case, coils 
built up in a similar way as with the eddy current continuous coils, in 
that the same coils may also be used for simultaneous reception of eddy 
current and stray flux signals. 
The d.c. pre-magnetization has, in addition to the objects already 
described, the additional object of producing a magnetic stray flux at the 
defect positions, which stray flux is detected by the coils mentioned 
above. Thus, the detection level for lateral cracks is further reduced. 
Beside external cracks, internal cracks are now also detectable and local 
wall thickness reductions can be detected as well.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 2, there is shown a rotary head in 
cross-section and in a front elevational view, respectively. A housing 10 
including a base plate 12, walls 14, a housing plate 16, a cover plate 18, 
spars 20, a receiving body 22, and a bearing body 24 forms the bearing 
component of the rotary head. The mentioned parts of the housing 10 are 
made of ferromagnetic steel, and are connected to each other by welding or 
other suitable means. The cover plate carries a drive motor 26 secured to 
a flange plate 28. The rotational movement of the motor 26 is transferred 
from a drive shaft 30 to a drive wheel 32 which is protected against 
access by a cover 34. The drive belt 36 transfers the drive force to a 
driven wheel 38 unitarily connected to a hollow shaft 40. The hollow shaft 
40 is supported in the bearing body 24 by two suitable roller bearings 42 
which maintain each other over a ferromagnetic support bushing 44 and 
which are fixed to the hollow shaft 40 and to the bearing body 24 by rings 
46 and 48, respectively. The air gap between the bearing body 24 and the 
support bushing 44 is small, in order that the magnetic flux will be 
subject to low impedance when passing from one to the other. 
A rotary disk 50 of light metal is rigidly fixed to the hollow shaft 40. It 
forms, together with the hollow shaft 40 and the driven wheel 38, a rotary 
section 51. Two probe levers 52 are mounted in bearing blocks 56 to rotate 
about two pins 54, which bearing blocks are fixed to the rotary disk 50. 
For the sake of clarity, only one of the two probe levers 52 is shown in 
FIG. 1. 
At the front end of the probe levers 52, a probe beam 58 is mounted 
carrying five eddy current transducers 60 which are located, during 
operation, immediately adjacent to the surface of the test material for 
scanning it in spiral tracks, when along with the rotating rotary section 
51, the rotary head is passed by the test material. As an example of a 
test material, a generally cylindrical bar 62 leaving the rotary head in 
direction of arrow 61. 
The signals of the rotating eddy current transducers 60 are conducted over 
a rotary transmitter assembly 64 to the outside, where they are connected 
by a cable to an electronic evaluator unit (both not shown). The rotary 
transmitter assembly may be of the type described in detail in a parallel 
patent application DE 36 32 395 comprising a rotor disk 66 and a stator 
disk 68, the former being rigidly connected to the hollow shaft 40 and 
secured by a bushing 70, and the latter being fixed by a carrying plate 72 
to the receiving body and thereby located closely adjacent the rotary disk 
66. The outputs of the eddy current transducers 60 are connected with 
primary windings 74 included in the rotor disk 66, whereas secondary 
windings 76 included in the stator disk 68 are conducted over a plug (not 
shown) connection to the cable mentioned above. 
The housing plate 16 (omitted in FIG. 2 in order to show the probe levers 
52) has to fulfill a double function. On one hand, it closes the opening 
in the front wall 14 except for a passage 78 through which the test 
material 62 passes. On the other hand, the plate space for receiving a 
magnetization coil 80 may be produced as a free-supporting winding 
introduced into the space of the housing plate 16 intended therefor, and 
held by a homogenization ring 82. 
An additional feature of the homogenization ring 82 is explained as 
follows. Into the passage 78 of the housing plate 16 there is inserted a 
front protective sleeve 84. A rear protective sleeve 86 is fixed to the 
rear wall 14, and extends into the borehole of the hollow shaft 40. It is 
a primary object of both protective sleeves 84, 85 to guide the test 
material 62 safely. Furthermore, the protective sleeves 84, 86 are made 
from a magnetically conductive material to offer an important contribution 
for the conduction of the magnetic flux. The rear protective sleeve 86 
comprises, for this latter purpose, at its front end a limited length 
portion having a slightly smaller diameter than that of the borehole of 
the hollow shaft 40, so that a narrow air gap 88 of low magnetic impedance 
is formed. 
The protective sleeves 84, 86 are interchangeable, and provided with a 
precisely dimensioned selection of boreholes. Accordingly, on one hand, an 
accurate guiding of the test material 62 is achieved, and, on the other 
hand, an effective introduction of the magnetic flux into the test 
material is guaranteed. The magnetic flux generated in the magnetization 
coil 80 passes over the housing plate 16, front wall 14, base plate 12 and 
cover plate 18, respectively, or over the side walls, over the spars 20, 
receiving body 22, bearing body 24, support bushing 44 and hollow shaft 
40. The immediate introduction of the magnetic flux into the test material 
62 is performed, as mentioned above, via the protective sleeves 84, 86. 
The boreholes of the protective sleeves 84, 86 are each coated with a thin 
layer of magnetically non-conductive material or, optionally as in the 
present example, abrasion-resistant sleeves 90 and 92, are provided. In 
this manner, it is achieved that the magnetized ferromagnetic test 
material will not "stick" in the interior of the protective sleeves 84, 
86, and can be pulled through the rotary head without exhibiting a large 
resistance. 
The test range, over which the test material 62 is fully magnetized, is 
limited, substantially, by the ends of the two protective sleeves 84, 86. 
The magnetic flux in the test range should be as homogeneous as possible, 
and, in particular, in the space above the test material 62 where the eddy 
current transducers 60 are located, the radial component of the magnetic 
field should be held within narrow limits. This is achieved by the 
introduction of the homogenization ring 82 absorbing a large fraction of 
the lines of force, which would otherwise, starting from the edge of the 
magnetization coil 80, be shunted immediately to the test material 62 or 
to the rear protective sleeve 86. In order that this measure will be 
effective even for the most unfavorable application, i.e. for the largest 
diameter of the test material, it is recommended to cover the 
magnetization coil 80 up to a distance from the axis of rotation by the 
homogenization ring, corresponding approximately to twice the maximum 
radial distance of the eddy current transducers 60 from the axis of 
rotation. 
FIG. 3 refers to the case where, in addition to the rotating eddy current 
transducers 60, there are employed stationary eddy current transducers 
within sensing range of the object being tested. For this purpose, the 
front protective sleeve is of a different design. Since all other parts of 
the rotary head remain unchanged, only a modified protective sleeve 100 is 
shown mounted into the housing plate 16. The latter may, as was described 
above, be used in a rotary head according to FIGS. 1 and 2. The protective 
sleeve 100 is deeper on the right-hand side. The central free space 
accommodates the eddy current transducer assembly 102. The latter 
comprises a coil body 104 of a suitable insulating material. In the bottom 
of the coil body 104, there are provided three grooves, into each of which 
are inserted eddy current receiver coils 106, 108. One of these coils may 
be an absolute-type coil 106 whereas the two outer coils (108) may be 
differentially connected. An eddy current excitation coil 112 is wound on 
the receiver coils 106, 10B and separated therefrom by an insulating layer 
110. Wires 114 connect the receiver coils 106, 108 and the excitation coil 
112 with a junction box 116 allowing for the connection of the stationary 
eddy current transducer assembly 102 to an electronic unit. As described 
for the protective sleeves 84, 86, the protective sleeve 100, too, is 
protected by a thin sleeve 118 against magnetic "sticking". 
Instead of the described eddy current transducer assembly 102 including the 
test material 62, there can also be mounted in the space provided within 
the protective sleeve 100, a set of eddy current segment coils distributed 
over the periphery. It is also possible to employ the described assembly 
of transducers 102 as stray flux transducers where magnetic flux generated 
in the test material 62 by the magnetization coil 80 produces magnetic 
stray fluxes at the defect positions. The stray fields can be detected, 
e.g., by the coil 106, and can be processed in known manner.