Assessing Damage to Carbon Fiber Reinforced Polymers Through Acoustic Micro-Imaging: Even the slightest damage to a carbon fiber reinforced polymer piece can be detected through acoustic micro imaging.
For this article, a carbon fiber composite was impact tested by dropping a weight onto the sample and then imaging it to detect the locations and patterns of internal damage. An acoustic micro-imaging tool uses a transducer that scans the surface of the CFR and pulses high-frequency ultrasound into the sample. It receives the echoes returning from internal material interfaces, including those associated with damage, a few millionths of a second later. The returning echo's amplitude gives information about the two materials at the interface, while the echo's arrival time tells the depth of the interface. Because ultrasound at high frequencies does not travel thorough air, the moving transducer is coupled to the surface by a water jet. A pulse traveling into the CFRP is reflected by any material interface. The only likely interface between two solid materials is the interface between the surrounding polymer and a carbon fiber. The amplitude of an echo reflected from this interface and received by the transducer a few millionths of a second later is determined by the properties of the two materials.
But the diameter of the carbon fibers is so small relative to the wavelength of the ultrasound that the echoes scatter in many directions. The result is that some individual fibers may not be imaged. Instead--if there are no defects--there is a low level of background echoes.
But if there is damage--as in the impact-tested CFRP used here--the damage is likely to take the form of breaks in fibers, the delamination of fibers from the surrounding polymer, or similar anomalies. A pulse of ultrasound striking such an anomaly is encountering the interface between a solid and a gas (air), or even between a solid and a vacuum. In either case, the physical properties of the two materials cause nearly 100 percent of the pulse to be reflected. Delaminations, cracks, and other gap-type anomalies are therefore more strongly imaged than other interfaces. In black and white images, such as those presented here, such anomalies are bright white, indicating the highest amplitude of echo signal.
The sample examined here consisted of carbon fibers in a polymer matrix. The fibers were oriented by depth at 0[degrees], 45[degrees], 90[degrees], and 135[degrees]. The sample was flat and measured 11 cm by 15.5 cm, and was 13 mm thick. To induce internal damage, a spherical steel ball having a diameter of 7.62 cm and a weight of approximately 1.8 kg was dropped onto the supported sample from a height of 3.048 m. When it made contact with the sample, the ball was traveling at a speed of 7.73 m/s.
An optical image of the region of the sample where the steel sphere impacted is shown in Figure 1. The light-colored circle is the impact zone. To the right and top of the part of the sample is visible the plate on which the sample was resting.
To image the sample acoustically, a Sonoscan C-SAM[R] acoustic micro-imaging tool was used. The tool's ultrasonic transducer scanned back and forth a few millimeters above the surface. The scan speed at its peak was above 1 m/s. The transducer was coupled to the sample's surface by a constant column of water, a necessity because ultrasonic waves will not travel through air at these frequencies.
Each second as it moved across the sample the transducer sent several thousand pulses of ultrasound into the sample. Each pulse entered the sample at a specific x-y location and, if it encountered a material interface, sent back an echo that was received by the transducer before the next pulse was launched. Pulses that encountered no interface sent back no echo, and software would assign the color black to those x-y locations in the acoustic image. Pulses that encountered the interface between two solids would send back an echo to which some shade of gray would be assigned, and pulses that encountered the interface between a solid and a gas would be assigned bright white pixels in the acoustic image.
When ultrasound is pulsed into a sample, echoes are returned only from material interfaces. If beneath a particular x-y location there is no material interface, the pulse of ultrasound may continue downward into the sample until it has all been absorbed. There is no echo, and the pixel will be black.
Even so, there may be internal structural features at various depths within the sample. To avoid confusion about the depth of a particular feature, the tool operator will, in most cases, limit the echoes accepted for imaging to those echoes from the depth of interest. In many samples, the depth of interest is relatively small compared to the whole thickness of the sample.
But for this composite sample, the interest was in determining the lateral extent of damage at multiple depths. For this reason, the depth to be investigated was divided into 40 horizontal gates. A gate is a single depth defined by the operator from which echoes are accepted for assembling the acoustic image. A gate is a time function: all of the echoes arriving at the transducer between Time A and Time B go into a specified gate. The time is the travel time of the echo from the interface to the top surface of the sample. In this case, 40 gates of equal vertical extent were set from a point just below the sample's surface to a point 2.556 mm below the surface. Each gate thus measured 64 microns in its vertical extent (technically known as its width). Collectively, the gates encompassed 1 9.66 percent of the sample's thickness.
The acoustic image of Gate 1 is shown in Figure 2. Vertically, this gate extends from just below the surface to a depth 64 microns below. Because each gate is very narrow (vertically), all four fiber orientations are not visible in every gate. In this gate, those oriented at 0' and 90* are visible as white or pale gray lines of various apparent widths. The gate width was chosen arbitrarily and not to match the vertical distribution of the fibers.
There is no evidence of extreme damage here. There is, about a third of the way down from the top and near the center, a collection of horizontal fibers that are unusually bright. But regions of the sample far from the impact site randomly display similarly bright fibers, so the bright fibers seen here are probably not the result of the induced physical damage. Even as deep as Gate 8, there is very little if any evidence of physical damage.
At Gate 10, in Figure 3, the evidence of damage suddenly becomes unmistakable. The bright area indicates true gap-type damage--or rather the top of a gap. Since the viewing sequence is moving downward, this feature is probably the upper portion of a delamination lower in the sample. It intrudes into the bottom of Gate 10, but not very far, because over most of its area it casts shadows of fibers that are in Gate 1 0 but above the gap's depth.
In Gate 11 (Figure 4), fibers have been pushed upward or downward entirely out of the depth of this gate. The delamination persists mostly unchanged through Gates 12, 13, and 14, and finally begins to disappear in Gate 1 5. Its overall vertical extent its thus about 350 microns.
In Gate 15, shown in Figure 5, the "old" delamination lies to the right, while a "new" delamination is forming to its left. Note that both delaminations lie in the bottom portion of this gate, as shown by the shadowing of fibers above most of the area of both delaminations. The new delamination appears to have a somewhat larger area and also seems to be extending toward the edge of the sample at the top of the image.
By Gate 20 seen in Figure 6, the new delamination has expanded massively toward both the top and bottom of the image. The old delamination is faintly visible, but an additional delamination has formed at upper left and reaches the edge of the sample. The more recently formed delaminations appear bright white over much of their respective areas, and extend with variations and extensions several gates below Gate 20. These delaminations lie about 1.4 mm below the surface of the sample.
A surprise comes in Gate 34, seen in Figure 7: the delaminations at left are the descendants of the delamination observed in this region in Gate 20. But to their right is a delamination having almost the shape, size, and location of a delamination that was observed vanishing in Gate 15 (Figure 5).
This delamination also makes it easier to visualize the z dimensions within the sample. Gate 34, like all the other gates, is 64 microns in its vertical extent. The delamination reflects ultrasound, with the result that features below the delamination are not visible. A single fiber can be seen crossing the delamination; the fiber must lie in the few microns between the delamination and the upper limit of the gate.
Then ability to divide a sample nondestructively into gates, and then to obtain an image of each gate, makes acoustic micro imaging a useful tool for visualizing and analyzing internal features. In this investigation 40 gates, all of the same width, were set, but the same sample could have been imaged with 1000 or more gates if needed. Each gate is a time window that may be only a few nanoseconds long. All the echoes collected during that window are used to image, for example, Gate 34. The sample is only scanned once to collect the data for imaging all of the gates.
The step-by-step imaging mode used here creates planar images, but the same tool can create a non-destructive cross-section through a sample such as this. The transducer scans back and forth along a straight line, collecting echoes from successively higher depths. The completed image matches in details and dimensions a physical section along the same line.
By Tom Adams Nordson-Sonoscan, Inc.
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|Title Annotation:||TECHNICAL ARTICLE|
|Comment:||Assessing Damage to Carbon Fiber Reinforced Polymers Through Acoustic Micro-Imaging: Even the slightest damage to a carbon fiber reinforced polymer piece can be detected through acoustic micro imaging.(TECHNICAL ARTICLE)|
|Date:||Oct 1, 2018|
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