The focus of this study is investigation of the probability of inclusion detection. [1, 2]. Because it would be applied in a production environment, a practical design of the test procedure was necessary.
The Eddy Current method needs a small sensor for the detection and determination of the exact position of the defect. With the use of automated C-scan and imaging techniques, it is possible to improve test speed and accuracy, and reduce the subjective aspect of validation.
Another focus is Ultrasonic Testing with the investigation of repeatability, consisting mainly of probe frequency variation and test speed. A technique for testing of industrial parts has been developed which should be applicable to the testing of critical sections in one step, e.g., scanning with multiple probes.
Depending on the manufacturing process and structure of fibre reinforced
light metals various kind of defects can occur:
Under certain conditions such defects can cause a fracture of the components.
Abb 2. Shot
Abb 3. Pore
Abb 4. Delamination
Abb 5. fibre-free zone
Abb 6. Fibre concentration
Abb 7. In-depth deviation of fibres
By using focused probes, and a high dynamic range, it is possible to achieve a precise lateral resolution, necessary for testing fibre reinforced materials . The system provides a high pulser voltage together with a short rise time. The system uses a focused 25 MHz PVDF immersion probe and with the broadband features of this set up, the capability for finding small defects in high absorbing materials is assured .
A C-scan image presents the results of an echo amplitude (A) over the planar coordinates x and y, formula C = f (x, y, A). This allows good interpretation of defect sizes. Instead of echo amplitude (A) a so-called D-scan image presents the results of echo transit time (tz), formula D = f (x, y, tz) . Interpretation of defect depth can be determined more easily from a colour image.
3.1.1 Detection of defectsDetermination of position resolution capability by using a reference body
A specimen containing 12% Saffil is used for this determination. Artificial flat bottom holes (FBH) between 50 µm to 400 µm are drilled into the specimen. The smallest detectable drill is 100µm. Using a 25 MHz probe, we can calculate a factor of d/wavelength = 0,3, which is a normalized figure. That formula facilitates comparison between reflection (good detection) and scattering (bad or no detection).
The detection of Shots was tested on material reinforced with fiberfrax. We see that with increasing depth the sensitivity of detection increases. Also the factor d/wavelength decreases, improving sensitivity. The reason is that the focused probe gains a better resolution in the area of its focus point. An equivalent circle diameter was defined to get a better determination of the size. That means the real defect plane was used to calculate its equivalent circle diameter. The smallest Shot of 121 µm equivalent circle diameter was detectable (fig. 8), as long as the defect is located in the focus of the probe.
Fig 8. C-image with Shots
An artificial foreign body of iron was added to the specimen. Iron wires with a length of 10 mm up to 16 mm and diameters of 100 µm, 200 µm, 300 µm and 500 µm were used. Image 9 shows a result of a 200 µm diameter.
Fig. 9. C-scan image with inclusion of iron.
The detection of fibre concentration is very difficult. For its detection, size is not the only factor; density can influence results even more. Based on the metallographic method it was determined that detection was possible only if the fibre concentration deviation is 10% of the average. The smallest detectable fibre concentration was 340 µm (fig. 10).
Fig. 10. C-scan image with fibre concentration
This investigation shows that delaminations are possible to detect for a minimum of 35% fibre density (fig. 11).
|Fig. 11. C- scan image with delamination|
The results show that it is difficult to detect fibre-free areas due to the
low reflection coefficient
at the defect boundary (water to fibre-free area). The interface echo was
evaluated on a part which showed a visible fibre-free area at its surface
In-depth deviation of fibres
The detection of in-depth deviation of fibres was possible using D-imaging techniques. That measures the time of flight up to the fibre reflection. That could be proven on a sample which showed a higher fibre density in a deeper area of the part.
3.1.2 Influence of the part's roughness on the detection of Shots
The geometry and roughness of the specimen's surface has an essential influence on the result of the ultrasonic testing method. That can limit the ability to detect small defects, necessitating a surface treatment. The ultrasonic method needs a constant echo amplitude (constant coupling energy) free from influence of the surface condition.
Especially when a small wave length is used, the surface effect is strong. Higher scattering occurs on higher surface roughness - that results in a loss of energy. The capability for detection of small defects decreases. This is intensified when high frequency focused probes are used.
The investigation shows that with a roughness of Rm= 18 µm the detection sensitivity decreases significantly. (Test frequency 25 MHz, Filter frequency 25 MHz).
3.1 Statistical investigation for the detection of shots with Ultrasonics
| The investigation of a specimen containing 20% Fibrefrax fibres used a
focused probe IAP-F 25.3.1 with a nominal frequency of 21 MHz and an
filter of 25 MHz.
Figure 1 compares the results of detected shots of 8 groups of different provided sizes, determined with metallography. The size tolerance of each group was 30 µm [1, 2]. Since the smallest detectable defect was approx. 120 µm, the statistical investigation used shots bigger than 90 µm.
Virtual improvement of detection can be seen in the 121 µm and 150 µm range. Most of these defects were located at a depth of 2 mm. Because the probe focus also lies in this area, the detection of those defect sizes was easier than in other areas. A statistically determined safe detection of 200 µm sizes can be realised.
Fig 1: Comparison of metallography detected defects with the Ultrasonic results .
For this unusual method for defect sizing was used - the DGS method with its diagrams. The principle is not explained here, since there is a great deal of in-depth literature on the subject. [6, 7]. The investigation continued with the precondition that the DGS method can be applied by considering the conditions of focused probes. The investigation used a focused probe with a 25 MHz filter.
The specimen is fibrefrax reinforced aluminum with a fibre density of 20%. The sample contained some defects. To obtain good repeatability it is necessary to adjust the probe on a reference body. The results show a correlation between echo amplitude and defect size. The result is applicable to defects located at the focus point or not more than 0.6 mm from it. (behind the focus point in the direction of propagation). On defects farther away, that area will appear as interference, and thus influence the results. An exact determination of the defect size will not be possible.
The centre of the equipment is the microprocessor controlled eddy current unit "eddyMax" manufactured by TMT. Using new software it was possible to control the eddy current board and the manipulator. The generated data set can be calculated for a C-scan image and plotted, saved and post processed.
Use of the Scan equipment makes essential improvement possible due to its capability for investigation of sensors, repeatability and probability of detection.
The sensitivity was clearly increased by reduction of the effective sensor width. For the effective area of the sensor field - besides the test frequency, the conductivity and the magnetic properties of the material (ferromagnetic or paramagnetic) - sensor construction is especially important. A small defect can easily influence the field of a small sensor. An eddy current probe type AN 05-01 with an 0,5 mm core size, well suited to the high lateral resolution needed for C-scan imaging was used.
|Test frequency:||200 kHz|
|Test speed:||40 mm/s|
It was possible to detect all of the 6 drills. The drills were placed in
the area between non-reinforced and reinforced material. The C-scan image
could show clearly with its grey/colour scale the position of drills in the
non-reinforced and reinforced areas (fig. 3).
A conclusion of this result is that natural porosities of approx. 50 - 100 µm (simulated by the drills) can be detected as long as they are located on the surface.
Fig 5: Drill (dia 200 µm) in the area between reinforced and non-reinforced material
|Probe :||AN 05-01|
|Test speed :||50 mm/s|
|The detection of a specific shot was possible with all frequencies. The best result was achieved by use of 100 - 200 kHz (Fig. 4). A detected shot measured 400 µm which was determined by metallography.|
Fig. 18: Shot on the surface (Test frequency 200 kHz)
3.2.4 Detection of areas free of fibres with Eddy Current TestingSpecimens with artificially provided fibre free areas were used. Detection of the fibre free areas was possible up to the minimum of 100 µm in one direction (Fig 19).
fig 5: Fibre-free area on the surface (width approx. 100µm)
|The investigation was carried out on a specimen reinforced with 20% Fibrefrax with fibre free areas on the surface. Due to the different capabilities of each method it is possible to detect the fibre free areas on the surface with Eddy Current and the shots inside the material with Ultrasonics. (Fig. 20).|
Fig. 20 : Comparison of C-scan images of a specimen a). with Ultrasonics b). with Eddy Current
By using a hardware divider for the trigger pulses it was possible to increase the test speed from 40 mm/s up to 125 mm/s. To investigate the influence of this speed a specimen with approx. 270 µm fibre-free area was scanned; other parameters were not changed. That was carried out with 10 different speeds in the 25 mm/s up to 125 mm/s range .
The investigation used the following parameters for all tests :
|Test frequency||200 kHz|
|The comparison between the C-scan images (fig. 21 and fig. 22), showed that with speed increased three times the fibre-free area was clearly detected. However, related to the scan process (spiraling scanning of the specimen), a directional relation to the direction of the scan axis occurs. This can influence the presentation of the fibre-free area of the C-scan image. The detection of the fibre-free areas as material inhomogeneity is still possible with this test speed.|
Fig. 21 u. 22: C-scan image of a fibre-free area
test speed 37,5 mm/s (left) test speed 125 mm/s (right)
Since increasing the speed was not possible with the existing
another solution was sought to achieve the needed speed of 1 m/s.
A turntable was modified for realising the speed up to 1m/s. The
Eddy Current unit Defectoscope AF with an AN 05-01 probe was used.
An engine piston was used as a specimen , provided with a symmetrical
reinforced ring which made a simple adjustment of the system possible. This
piston contained two fibre-free areas on the surface.
The eddy current signals were transmitted from the Defectoscope AF and an A/D converter (16 bit resolution) with a sampling rate of 50 kHz to the PC port. Post processing generated the presented images.
Because the investigation chose the x-axis as the influence of the take-off
parameter, only the y-part of the Eddy Current signal was used for the
Two fibre-free areas (A, B) clearly produce higher voltage, resulting from higher conductivity in the fibre-free area. Also an area of lower conductivity indicative of fibre concentration could be detected.
To provide a clear comparison of different test speeds, the investigated area was reduced to one circumference.
The test results show that with increasing speed detection is more difficult and depends on defect size. Large fibre-free areas ( approx. 750 µm), scanned with a speed of 1 m/s, still generated a clear signal/noise ratio, while an area of 380 µm was already difficult to detect with a speed of 0,5 m/s. Two diagrams (fig. 23) show this result for a speed of 0,1 m/s and 1m/s (maximum).
|Prof. Dr.-lng. habil. Horst - Dieter Tietz|
Dr. Horst-Dieter Tietz studied material science and material testing in Magdeburg, where he received his doctorate in 1965. He continued there and was tenured in 1970. He has also served as manager of an industrial material institute in the heavy-steel market in Magdeburg. From 1989-1992 he was director of the Institute for Materials and Quality control, Director of the TH Zwickau in 1991-1992, before founding the University for Technique and Economics Zwickau (FH) and serving as its Director until 1996. He is presently the Director of the Westsächsischen University Zwickau (FH), the Technical Ceramic Application Center. Dr. Tietz continues to teach at several Universities. He is the author of many professional books in the fields of materials and nondestructive testing.
FH Zwickau Homepage http://www.fh-zwickau.de (German)
Special expertise in "Prüfung von Kompaktwerkstoffen, Schichtverbunden und Grünlingen".
|| UTonline | |Top ||