ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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Fig. 4
Calculated ECP signals for scans perpendicular to a 0.46 mm (0.018 in.) long by 0.10 mm (0.004 in.)
deep slot
Fig. 5 Calculated ECP signals for scans parallel to a 0.46 mm (0.018 in.) long by 0.10 mm (0.004 in.) deep slot
Example 1: Surface Flaws in a Flat Plate.
The sensitivity of the ECP method to very small surface flaws in a low-conductivity material was illustrated with flaws in
a flat-bar Ti-6Al-4V specimen (Ref 3). The ECP signals obtained by scanning a probe linearly past three electrical
discharge machined (EDM) notches are shown in Fig. 6. The 0.47 mm (0.0184 in.) long by 0.17 mm (0.0066 in.) deep
notch in this specimen gave a very prominent ECP signal. More important, recognizable signals were also obtained from
the two small notches--one 0.20 mm (0.0079 in.) long by 0.06 mm (0.0022 in.) deep and the other 0.22 mm (0.0085 in.)
long by 0.05 mm (0.0020 in.) deep. The background signal in the region where no flaws were present was caused by the
inhomogeneity of the specimen material. From these data, the inherent sensitivity of the ECP method for detection of very
small surface flaws was established.
Fig. 6 ECP signals from EDM notches in a Ti-6Al-4V flat-bar specimen. Flaw dimensions are length by depth.
Example 2: Flaw Detection in Fan-Disk Blade Slots.
Evaluations of the ECP method were performed on blade slots in the first-stage fan disk of the F100 gas-turbine engine
(Ref 3). Figure 7 shows a photograph of this disk, which was fabricated from Ti-6Al-2Sn-4Zr-6Mo material. The blade-
slot configuration is illustrated in Fig. 8. An 8.89 mm (0.350 in.) radius on each side of the blade slot blended into a flat
area on the bottom of the blade slot, and the primary area of interest for surface-flaw detection was at the tangency point
of this radius. Notches were electrical discharge machined in two blade slots at the tangency point for use as target flaws.
The ECP probe scan path was approximately in line with each EDM notch (Fig. 8). Figure 9 shows the ECP signals from
two blade slots with surface EDM notches measuring 0.46 mm (0.0182 in.) long by 0.27 mm (0.0105 in.) deep and 0.27
mm (0.0105 in.) long by 0.15 mm (0.0058 in.) deep. Prominent signatures were obtained from both the larger EDM notch
(Fig. 9a) and the smaller EDM notch (Fig. 9b); this demonstrated the sensitivity for detecting small flaws in this engine
component.
Fig. 7 F100 first-stage fan disk
Fig. 8 F100 first-stage fan-disk blade-slot configuration showing a flaw location and ECP probe scan track
Fig. 9 ECP signals from EDM notches in F100 first-stage fan-disk
blade slots. (a) Signal from the 0.46 mm
(0.0182 in.) long by 0.27 mm (0.0105 in.) deep notch. (b) Signal from the 0.27 mm (0.0105 in.) long by 0.15
mm (0.0058 in.) deep notch
Example 3: Surface Flaws in Thread Roots.
The ECP method was evaluated for the detection of flaws in the thread roots of a titanium helicopter rotary wing-head
spindle (Ref 4). The threaded end of the spindle is illustrated in Fig. 10. Notches were electrical discharge machined into
the thread roots, as shown in Fig. 10. The ECP probe was positioned on the crest of the threads, resulting in a liftoff of
1.22 mm (0.048 in.) from the root. For scanning, the probe was held stationary while the spindle was simultaneously
rotated and translated axially so that the probe followed the threads.
Fig. 10 Threaded end of a helicopter spindle showing a flaw location
The ECP data were obtained on a spindle containing three rectangular notches designated A, B, and C (Fig. 11). These
notches were spaced 120° apart around the circumference in the root of one thread. As shown in Fig. 11, prominent ECP
signals were obtained from all three defects, including the smallest one, which measured 0.64 mm (0.025 in.) long by 0.43
mm (0.017 in.) deep. These data exhibit several important characteristics. First, the signal background was far above
electronic noise and was highly reproducible for repeat scans. The ECP sensitivity was limited only by the signal
background obtained from the spindle itself, not by electronic noise. Second, signals were obtained when the probe
passed directly over the defects in a thread and when the probe was located at adjacent threads, as indicated by the
satellite signals designated A', B', and C' in Fig. 11. Signal polarity reversals were obtained when the probe was
positioned away from the flaws in the adjacent threads; the satellite signals can be distinguished from the primary flaw
signals by this polarity reversal. These characteristics are typical of the ECP method and demonstrate that it is not
adversely affected by the complex specimen geometry imposed by the spindle threads.
Fig. 11
ECP signals from flaws A, B, and C in a spindle thread root with a probe on the crest of the threads.
Signals obtained when the probe was located over the adjacent thread are designated A', B', and C'.
Flaw
dimensions are length by depth.
Example 4: Flaw Detection through Spindle Wall Thickness.
The helicopter rotary wing-head spindle described in Example 3 was also inspected with an ECP probe located in the
spindle bore. In this evaluation, flaws in the thread roots were detected through the 9.32 mm (0.367 in.) spindle wall
thickness.
Data obtained from a spindle that contained a flaw 7.75 mm (0.305 in.) long by 2.21 mm (0.087 in.) deep in the root of
the first thread are shown in Fig. 12; the flaw is identified as I. A flaw signal was obtained when the probe passed directly
over the flaw, and satellite signals (I') were obtained when the probe was located at the adjacent threads. The flaw-to-
sensor distance was large because the inspection was performed through the wall thickness. This resulted in the satellite
signal polarity reversal (evident in Example 3) occurring outside the region shown in the plot.
Fig. 12
ECP signals from defect I (7.75 mm, or 0.305 in., long by 2.21 mm, or 0.087 in., deep) in the spindle
thread root with a probe located in the spindle bore. The signals obtained when the probe was located at
adjacent threads are designated I'.
Example 5: Second-Layer Fastener-Hole Cracks.
The ECP method was evaluated on a C-5A aircraft spanwise splice-joint specimen to demonstrate the detection of
fastener-hole defects in the second layer with the probe positioned on the outer surface of the first layer (Ref 5). A cross
section of the two-layer splice joint is illustrated in Fig. 13. The aluminum specimen had top and bottom layer thicknesses
of 3.8 mm (0. 15 in.) (in the joint region) and a single row of 4.77 mm (0.188 in.) diam tapered titanium fasteners with
countersunk heads. The specimen contained 2.5 mm (0.10 in.) deep and 5.1 mm (0.20 in.) long through-thickness radial
notches in the second layer.
Fig. 13 Schematic of a two-layer structural-fastener configuration
The ECP probe was positioned on the top layer of the specimen and was scanned linearly along the row of fasteners (Fig.
14), with the sensor scan track 13.5 mm (0.53 in.) from the edge of the fastener holes. Strong signatures were obtained
from hole No. 11, with a 5.1 mm (0.20 in.) notch oriented toward the scan track side of the fastener hole, and from hole
No. 8, with a notch of the same size oriented toward the opposite side of the fastener hole from the scan track. A
prominent signal was also obtained from the 2.5 mm (0.10 in.) notch in hole No. 4. Signals from unflawed holes are very
small and are not evident at the sensitivity used to obtain the data in Fig. 14. These results show the potential for a rapid,
linear scan inspection of fastener holes.
Fig. 14 ECP signals from second-layer flaws in a C-5A wing-
splice specimen with the probe on the top layer.
Flaw dimensions are the radial length of EDM notches.
References cited in this section
3.
R.E. Beissner, G.L. Burkhardt, and F.N. Kusenberg
er, "Exploratory Development of Advanced Surface Flaw
Detection Methods," Final Report, Contract No. F33615-82-6-5020, Report No. AFWAL-TR-84-
4121,
Wright-Patterson Air Force Base, Sept 1984
4.
C.M. Teller and G.L. Burkhardt, Application of the Electric C
urrent Perturbation Method to the Detection of
Fatigue Cracks in a Complex Geometry Titanium Part, Rev. Prog. Quant. NDE, Vol 2B, 1983, p 1203-1217
5.
C.M. Teller and G.L. Burkhardt, NDE of Fastener Hole Cracks by the Electric Current Perturbation Method,
Rev. Prog. Quant. NDE, Vol 1, 1982, p 399-403
Electric Current Perturbation NDE
Gary L. Burkhardt and R.E. Beissner, Southwest Research Institute
Flaw Characterization
Both experimental data and theoretical modeling results suggest the potential capability of the ECP method for flaw
characterization (Ref 6). Signal features related to crack characterization are most apparent when the scan direction is
parallel with the crack length (perpendicular to j
0
) and directly over it, although these features can be extracted from other
scan orientations. When a defect is scanned perpendicular to j
0
, a bipolar signal is obtained, as in the scan shown in Fig.
11. The peak-to-peak signal amplitude and the peak-to-peak spacing can be related to the flaw interfacial area and the
flaw length, respectively. These relationships will be discussed in this section. The details of theoretical modeling are
discussed in Ref 1.
Experimental results were obtained from a single, half-penny-shaped fatigue crack grown with a laboratory fatigue
machine in a Ti-6Al-4V rod-type tensile specimen. Fracture of identical specimens containing cracks grown under similar
conditions showed that true half-penny-shaped fatigue cracks (2:1 aspect ratio) were obtained.
Theory and experiment were compared for the peak-to-peak signal amplitude as a function of the interfacial area of the
crack (the planar area exposed if the specimen were fractured) and for the peak-to-peak separation as a function of crack
surface length. These data are shown in Fig. 15. In Fig. 15(a), it can be seen that the theory predicts an approximately
linear relationship between signal amplitude and the interfacial area of the crack. Experimental results show very close
agreement with theory, indicating that the crack interfacial area for cracks of this type can be estimated by measurement
of the ECP signal amplitude.
Fig. 15 Flaw characterization of half-
penny cracks with the ECP method. (a) Comparisons between the ECP
signal amplitude and the crack interfacial area. (b) Comparisons between the ECP signal peak-to-
peak
separation and the crack length
A plot of the ECP peak-to-peak separation as a function of crack surface length is shown in Fig. 15(b) for both theory and
experiment. The theoretical model again predicts an approximately linear relationship between the peak-to-peak
separation and the crack surface length over the range of lengths shown in the plot. Experimental data show a similar
relationship, although some departure from linearity is observed. In both cases, a direct relationship is apparent between
the peak-to-peak separation and the crack surface length. This shows that the surface length of the defect can be estimated
by measuring the peak-to-peak separation of the ECP signal.