·Table of Contents
·Materials Characterization and testing
Effects of porosity on the Mechanical Strength and Ultrasonic Attenuation of CF-Peek fibre Placed CompositesA. Demma,
Department of Mechanical Engineering, Imperial College, Exhibition Road, London, SW7 2BX, England
Center for Nondestructive Evaluation, The Johns Hopkins University, Baltimore, MD, 21218, USA
The aim of this study is to contribute to improvements in the ultrasonic assessment of porosity. The purpose of this paper is to determine whether porosity effects on the mechanical strength can be correlated with the porosity induced ultrasonic attenuation of unidirectional CF-peek composites obtained from tow placement of prepregs.
The samples used in this investigation were 12 specimens cut from a CF-Peek composite ring containing a wide range of void content. The specimens were cut from locations where the ultrasonic attenuation was fairly uniform; the locations were chosen with the aim of covering the broadest porosity range. The composite ring was obtained from tow placement of prepregs using hot air. The composite was unidirectional and the fibers were placed along the circumference of the ring. The dimensions of the ring were:
f=615 mm, t=5,7mm, L=86mm
where f is the inner diameter, t is the thickness and L is the length
|Fig 1: Ring dimensions|
The coupons dimensions were:
1"x 0.25"x 0.25"
where 1" is the length along the fibers direction.
The composite ring was tested along a length of L1 = 38mm (fig.1) over the entire circumference. An ultrasonic bridge system in water jet configuration using the through transmission method was used to scan the cylinder. The ring was clamped to a turntable so that the transducers were motionless during the scans. A 1/4" nozzle orifice was used. The pixel dimensions on the C-scan final image were 0.38 mm x 0.38 mm. The scans were carried out using narrow band generation at three different frequencies (1 MHz, 2.25 MHz, and 5 MHz). No signal average was performed.
The C-scan image relative to a single frequency was divided into four sectors with 90º coverage. Fig. 2 shows the sector 0º-270º at 2.25 MHz. The images were subsequently processed in order to obtain five grey level maps (0-24, 25-49, 50-74, 75-99, 100-127) from each image. The 12 specimen locations were chosen using a visual analysis of the discretized images. Two criteria were used to choose a suitable area: the first was the need to cover the broadest grey levels range possible and the second was to take areas showing uniform ultrasonic attenuation.
|Fig 2: C-scan image at 2.25 MHz from 0º to 270º|
The most common method to obtain morphological data of the pores is microscopy.
Before carrying out the microscopy experiments all the specimens were polished finely along the fiber direction on one face. A morphological analysis was performed on 144 microscopy images (12 for each sample) with a dimension of 1.7 mm x 1.3 mm.
|Fig 3: Shear load diagram|
The presence of pores affects the shear strength more than the axial strength. Consequently a short beam shear test (SBS) [ASTM-D 2344, 1997] was chosen to excite shear stress on the composite coupons. The shear test specimen is center-loaded as shown in Fig.3. In Fig. 3 and Table 1, l is the length of the sample, a is the span, b is the width, h is the thickness, c is the loading nose width and r is the relative radius.
|Table 1: Setup and specimen dimensions|
The h and l dimensions of the specimen were carefully chosen to obtain the required failure mode (interlaminar shear crack) [Demma, 1999]. An extra sample was tested to verify the geometrical parameters chosen. Some of the experiments (specimens 4-5-6-8-10-11) were run putting rubber under the loading nose to reduce the severity of the stress concentration (Cui et al., 1994).
|Table 2: Average grey level|
Table 3 summarizes the statistical analysis results obtained from the blob analysis. lMstat is the value of the minor axis and it corresponds to the value of a generic cord of circumference.
The radius value (a) is obtained considering (1):
|Table 3: Dimension main axis and radius|
Fig. 4a shows the concentration distribution of the pores. The distribution shows a strong tendency for the voids to align parallel to the plies. Fig 4b shows the aspect ratio distribution of the pores (ratio between length and diameter). The average value of the aspect ratio is 11. Therefore, the pores can be modelled as infinite elliptical cylinders whose axis is aligned with those of the fibres. No clear correlation between microscopy and ultrasonic attenuation was found, probably because little information was acquired about an individual specimen during microscopy.
|Fig 4a: Histogram of pore orientation||Fig 4b: Aspect ratio distribution|
The reduced scattering cross section G for our case was obtained based on the theory on diffraction by cylindrical obstacles (Tan, 1976) and its application in porosity estimation (Nair et al.1989). The hypotheses usually used to simplify the two-dimensional elastodynamic diffraction problem in the case of a cylindrical obstacle inside an infinite elastic transversely isotropic medium were thereby verified (Nair et al., 1989).
The reduced scattering cross section G in the simple case of a circular cylindrical cavity is shown in Fig. 5 (k is the wave number and a is the pore radius).
|Fig 5: Reduced scattering cross section G for circular cavity|
Fig. 5 is plotted for a value G equal to 0.533 where h is the ratio between shear velocity and longitudinal velocity in the plane perpendicular to the fiber direction (Nayfeh, 1995).
Fig. 5 shows how ultrasonic attenuation increases with frequency and demonstrates that the biggest slope corresponds with the 2.25 MHz frequency.
The presence of interlaminar cracks was noticed in all the specimens after the mechanical tests. Five out of six of the specimens tested using rubber under the loading nose broke during the test. In the composite coupons, load increased slowly when the load was about 1500 N. At this critical value cracks and delaminations were propagating, which was audible during the tests.
From Fig. 6 a, b, and c it can be seen that the frequency used during the ultrasonic tests affected the correlation between ultrasonic and mechanical tests. Ultrasonic attenuation and therefore porosity is higher when the C-scan image becomes darker. We did not compare the correlations at different frequencies. In order to avoid the complications involved in estimating frequency corrections we considered the narrow band generation as though it was a single frequency generation. Fig. 6a (correlation at 1 MHz) presents a data concentration due to the low energy scattered. No apparent correlation was encountered in this case. At 2.25 MHz a broading of the grey levels range can be seen. That is because at this frequency the scattered energy is higher than in the previous case. Fig. 6.b shows how well ultrasonic and mechanical tests correlate at this frequency. At 5 MHz we observed a further broadening of the grey levels range although at this frequency we lost the correlation found at 2.25 MHz. This can be explained by examining the reduced scattering cross section (Fig. 5). At 5 Mhz the scattered energy is almost constant with the porosity dimension whereas at 2.25 MHz the slope of the scattered energy as a function of ka becomes steeper. It appears that porosity detection sensitivity changes with the signal frequency used in the test and in our case it was optimized at 2.25 MHz.
| Fig 6c:
||Fig 6: Correlation between ultimate load and grey levels
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