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epoxy composites containing interlaminar paper inclusions
Acoustic emission from graphite/epoxy composites containing interlaminar paper inclusions J.H. Williams, Jr and S.S. Lee The results of a preliminary study of the acoustic emission m o n i t o r i n g of graphite fibre-reinforced laminates, w i t h and w i t h o u t backing-paper inclusions, are presented. A m p l i t u d e d i s t r i b u t i o n analysis provides some striking distinctions between the paper flawed and the unflawed laminates.
Graphite fibre/epoxy composites are frequently produced by laminating a number of prepreg laminae. The prepreg is generally purchased on rolls where the rolled prepreg is separated by a carrier backing (release) paper. If the backing paper is not removed prior to the laminating process, it becomes laminated into the structure, thus producing an interlaminar flaw. Whereas such a flaw may only marginally affect the in-plane tensile strength, it may have substantial effects on the flexural, fatigue, and dynamic behaviour of the composite. This paper presents a summary of the results of a preliminary study to assess the potential of acoustic emission monitoring in the non-destructive detection of interlaminar flaws such as backing paper inclusions.
Equipment, specimens, and procedure The experimental system consisted of an Instron Universal Testing Machine (89 000 N rating), acoustic emission detection and processing equipment (Acoustic Emission Technology, Inc), and other recording and display apparatus. The emissions were detected by a PZT piezo-electric sensor (AC 175) having a resonant peak at 175 kHz at a sensitivity of - 70 dB (re 1 V//zBar). The transducers were held on the specimens with rubber bands at a constant force of about 15 N, and coupled with AET-5C6 viscous resin. The AE signal was bandpass-filtered (125-250 kHz). The total system amplification was maintained at 80 dB and a constant threshold voltage of 0.6 V (after amplification) was used. The Source Locator (AET Model 3000) was used for AE linear location and the Amplitude Distribution Analyzer (AET Model 203) was used for amplitude distribution analysis. The test section of the specimens was 229 mm along the tensile axis, 76 mm wide, and 0.76 nm~ thick. The laminates Professor Williams and Dr Lee are in the Composite Materials and Nondestructive Evaluation Laboratory in the Department of Mechanical Engineering at the Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
NDT INTERNATIONAL. FEBRUARY 1979
consisted of six laminae having the following stacking sequence: [+ 45 °, - 45 °, + 45 °] sInterlaminar flaws in the form of backing paper having dimensions 76 mm by 38 mm were introduced into some of the specimens. Each flawed specimen contained two pieces of completely encased backing paper which were placed between the first and second laminae, and the fourth and fifth laminae, respectively. The backing paper was symmetrically disposed with respect to the tensile and width directions of the specimen. Aluminium tabs were epoxied to the specimens outside of the test section. The specimens were tensile loaded at a crosshead rate of 8.5 x lO-Tm/s. In all tests the load was monotonically increased until either the specimen ruptured or it was judged that sufficient ndt data had been obtained.
Results and discussion Spectral analysis Spectral analysis was performed on a few of the AE signals from both the paper-flawed* and the unflawed* specimens. The equipment and the analysis techniques which were employed were the same as those used by Williams and Egan. 1 The centre frequency of the average of three AE spectra from a paper flawed specimen was lower than the centre frequency of the average of three AE spectra from an unflawed specimen. However, because the statistical analysis of large numbers of signals which are generally required for conclusive AE spectral analysis was not conducted, these results must be considered tentative and therefore will not be presented in detail.
AE ringdown counts It is important to be able to detect flaws at load levels which are much less than the rupture load. During loading in the *The terms unflawed and paper flawed are usedfor convenienceas both types of specimensobviouslyhavea number of different types of material and fabrication fl~ws
0308-9126/79/010005-03 $02.00 © 1979 IPC Business Press
5
might appear that the unflawed specimen generates a higher count than the paper-flawed specimen. This is, in fact, the case for the event count. However, the large number of higher amplitude events in the paper flawed specimen results in a higher ringdown count (as mentioned above). Thus, although the paper flawed specimen may generate a lower event count (which is probably due to higher attenuation for those signals passing through the paper region en route to the sensor), the fact that the paper-flawed specimen also generates more energetic events will probably produce a higher ringdown count. These observations suggest a number of interesting and potentially fruitful studies.
Range of load during which AE was recorded 4 4 5 0 - 5340 N
• ....
bJ < ,
.J
J~ Bottom of specimen
,._,l.t,.
•
_,,t
.
.
.
.
.I. ,I I.. l., I!1..., I
l
"I
Top of specimen
Extent of paper inclusion Fig. 1 Location data for interlaminar paper flawed specimen for the indicated load ranges
The most striking distinction between the AE data from the paper flawed and the unflawed specimens resulted from the amplitude distribution slope analysis. This is shown schemati-
range 0-2225 N (all specimens had a rupture load greater than 5340 N), the AE ringdown count for a paper flawed specimen was 715, whereas the value for the unflawed specimen was 435. Although the data are limited, they do suggest that some distinction can be made between the unflawed and paper flawed specimens at load levels which are less than half the rupture strength. It is interesting to note that in general, a larger than typical AE ringdown count for a given load range is indicative of flaws. 2 Another pair of paper flawed and unflawed specimens was tested and produced AE ringdown counts of 685 and 450, respectively. These counts were obtained over the load range 0-2225 N, as before, and are consistent with the counts from the respective types of specimens.
Linear location Based on the difference in the arrival times of the AE signxl at two aifferent transducers, the AE source in a tensile specimen can be located. AE events in the paper-flawed specimens tended to occur more frequently in the vicinity of the upper and lower edges of the paper. For the unflawed specimens, events tended to occur randomly throughout the specimen length except at the dominant inherent flaw(s) which was often the site of specimen rupture. These results, together with those shown in Figs 1 and 2, are consistent with what would be expected from a detailed stress analysis.
Range of load during which AE was recorded 5340-5915 N
.M J~ i
0-4450 N
I1,1. Fig. 2 ranges
6
J ......ILL, J.., JJ. T Location of specimen rupture
Top of specimen
Location data for unflawed specimen for the indicated load
Paper flawed specimen JI 1 iII
1 I1_
Paper flawed specimen
I
Range of load during which AE was recorded 4 4 5 0 - 5540 N 0-4450 N
,_1
LU
5540-5915 N
~J
Fig. 3 illustrates an interesting and important consequence of the choice of AE data presentation. First of all, it should be observed th,t only the 4450-5340 N load range is common for the two specimens. Even for this common load range it *The total energy in the AE signal depends upon the amplitude and the c~uration; assuming equal ringdown times for each analysed signal from the same sensor, the amplitude distribution analysis is equivalent to an energy distribution 'ranking'
.
Bottom of specimen
Amplitude distribution analysis AE events can be ranked according to their amplitude using amplitude distribution analysis.* The results for paper flawed and unflawed specimens are shown in Fig. 3; for the paper flawed specimen, the AE amplitude distribution was double peaked, and single peaked for the unflawed specimen. The results indicate a more frequent occurrence of a higher percentage of the more energetic AE signals from the paper flawed specimen.
[ i
Unfiawed
Least energetic
Increasing amplitude i.-
4450-5540N
Most energetic
Fig. 3 Amplitude distribution data for the paper flawed and unflawed specimens for the indicated load ranges
NDT I N T E R N A T I O N A L . FEBRUARY 1979
Table 1. Slope of log AE total event count as a function of amplitude for various load ranges L o a d range (N)
Paper f l a w e d s p e c i m e n
Unflawed specimen
0-2225
- 0.30
-
2225-4450
- 0.50
- 0.78
4450-fracture
-
- 0.77
0.56
1.00
J
Amplitude Fig. 4
Sketch showing amplitude distribution slope analysis
cally in Fig. 4 where the slope of the log AE total event count versus amplitude is indicated. A smaller magnitude* of the slope represents a higher percentage of high amplitude AE. The results of the amplitude distribution slope analysis are summarized in Table 1. Note that the two ranges of slope do not overlap. Furthermore, the magnitudes of the slopes versus load have opposite trends; the paper-flawed specimen shows an increasing trend and the unflawed specimen shows a decreasing trend. Thus, it appears from Table 1 that the use of amplitude distribution analysis is promising for the non-destructive detection of inteflaminar paper inclusions.
analysis provides some striking distinguishing AE characteristics between paper flawed and unflawed laminates. Other AE techniques such as linear location and cumulative AE within a load range give some encouragement also. Although further investigation is required in order to be able to locate paper inclusions with confidence, it appears that the choice of AE techniques and system parameters can be optimized for the detection and analysis of a variety of composite flaws, including interlaminar paper inclusions.
Acknowledgements
The test specimens were provided by the David W. Taylor Naval Ship Research and Development Center. The authors' research in non-destructive evaluation is generally supported by the Materials and Structures Division of the NASA Lewis Research Center.
Conclusions
A preliminary investigation of the AE monitoring of graphite fibre/epoxy laminates, with and without intedaminar backingpaper inclusions, has been conducted. Amplitude distribution
References
1 2
*This discussion is based on the assumption that the slope is negative, as is generally the case
NDT INTERNATIONAL.
FEBRUARY
1979
J.H. Williams,Jr and D.M. Egan 'Acoustic emission analysis of fiber composite failure mechanisms'Mater Evaluation (in press) J.H. Williams,Jr and S.S. Lee 'Acoustic emission monitoring of fiber composite materials and structures' J Composite Mater 12 (October 1978) p 348
7
Graphite fibre/epoxy composites are frequently produced by laminating a number of prepreg laminae. The prepreg is generally purchased on rolls where the rolled prepreg is separated by a carrier backing (release) paper. If the backing paper is not removed prior to the laminating process, it becomes laminated into the structure, thus producing an interlaminar flaw. Whereas such a flaw may only marginally affect the in-plane tensile strength, it may have substantial effects on the flexural, fatigue, and dynamic behaviour of the composite. This paper presents a summary of the results of a preliminary study to assess the potential of acoustic emission monitoring in the non-destructive detection of interlaminar flaws such as backing paper inclusions.
Equipment, specimens, and procedure The experimental system consisted of an Instron Universal Testing Machine (89 000 N rating), acoustic emission detection and processing equipment (Acoustic Emission Technology, Inc), and other recording and display apparatus. The emissions were detected by a PZT piezo-electric sensor (AC 175) having a resonant peak at 175 kHz at a sensitivity of - 70 dB (re 1 V//zBar). The transducers were held on the specimens with rubber bands at a constant force of about 15 N, and coupled with AET-5C6 viscous resin. The AE signal was bandpass-filtered (125-250 kHz). The total system amplification was maintained at 80 dB and a constant threshold voltage of 0.6 V (after amplification) was used. The Source Locator (AET Model 3000) was used for AE linear location and the Amplitude Distribution Analyzer (AET Model 203) was used for amplitude distribution analysis. The test section of the specimens was 229 mm along the tensile axis, 76 mm wide, and 0.76 nm~ thick. The laminates Professor Williams and Dr Lee are in the Composite Materials and Nondestructive Evaluation Laboratory in the Department of Mechanical Engineering at the Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
NDT INTERNATIONAL. FEBRUARY 1979
consisted of six laminae having the following stacking sequence: [+ 45 °, - 45 °, + 45 °] sInterlaminar flaws in the form of backing paper having dimensions 76 mm by 38 mm were introduced into some of the specimens. Each flawed specimen contained two pieces of completely encased backing paper which were placed between the first and second laminae, and the fourth and fifth laminae, respectively. The backing paper was symmetrically disposed with respect to the tensile and width directions of the specimen. Aluminium tabs were epoxied to the specimens outside of the test section. The specimens were tensile loaded at a crosshead rate of 8.5 x lO-Tm/s. In all tests the load was monotonically increased until either the specimen ruptured or it was judged that sufficient ndt data had been obtained.
Results and discussion Spectral analysis Spectral analysis was performed on a few of the AE signals from both the paper-flawed* and the unflawed* specimens. The equipment and the analysis techniques which were employed were the same as those used by Williams and Egan. 1 The centre frequency of the average of three AE spectra from a paper flawed specimen was lower than the centre frequency of the average of three AE spectra from an unflawed specimen. However, because the statistical analysis of large numbers of signals which are generally required for conclusive AE spectral analysis was not conducted, these results must be considered tentative and therefore will not be presented in detail.
AE ringdown counts It is important to be able to detect flaws at load levels which are much less than the rupture load. During loading in the *The terms unflawed and paper flawed are usedfor convenienceas both types of specimensobviouslyhavea number of different types of material and fabrication fl~ws
0308-9126/79/010005-03 $02.00 © 1979 IPC Business Press
5
might appear that the unflawed specimen generates a higher count than the paper-flawed specimen. This is, in fact, the case for the event count. However, the large number of higher amplitude events in the paper flawed specimen results in a higher ringdown count (as mentioned above). Thus, although the paper flawed specimen may generate a lower event count (which is probably due to higher attenuation for those signals passing through the paper region en route to the sensor), the fact that the paper-flawed specimen also generates more energetic events will probably produce a higher ringdown count. These observations suggest a number of interesting and potentially fruitful studies.
Range of load during which AE was recorded 4 4 5 0 - 5340 N
• ....
bJ < ,
.J
J~ Bottom of specimen
,._,l.t,.
•
_,,t
.
.
.
.
.I. ,I I.. l., I!1..., I
l
"I
Top of specimen
Extent of paper inclusion Fig. 1 Location data for interlaminar paper flawed specimen for the indicated load ranges
The most striking distinction between the AE data from the paper flawed and the unflawed specimens resulted from the amplitude distribution slope analysis. This is shown schemati-
range 0-2225 N (all specimens had a rupture load greater than 5340 N), the AE ringdown count for a paper flawed specimen was 715, whereas the value for the unflawed specimen was 435. Although the data are limited, they do suggest that some distinction can be made between the unflawed and paper flawed specimens at load levels which are less than half the rupture strength. It is interesting to note that in general, a larger than typical AE ringdown count for a given load range is indicative of flaws. 2 Another pair of paper flawed and unflawed specimens was tested and produced AE ringdown counts of 685 and 450, respectively. These counts were obtained over the load range 0-2225 N, as before, and are consistent with the counts from the respective types of specimens.
Linear location Based on the difference in the arrival times of the AE signxl at two aifferent transducers, the AE source in a tensile specimen can be located. AE events in the paper-flawed specimens tended to occur more frequently in the vicinity of the upper and lower edges of the paper. For the unflawed specimens, events tended to occur randomly throughout the specimen length except at the dominant inherent flaw(s) which was often the site of specimen rupture. These results, together with those shown in Figs 1 and 2, are consistent with what would be expected from a detailed stress analysis.
Range of load during which AE was recorded 5340-5915 N
.M J~ i
0-4450 N
I1,1. Fig. 2 ranges
6
J ......ILL, J.., JJ. T Location of specimen rupture
Top of specimen
Location data for unflawed specimen for the indicated load
Paper flawed specimen JI 1 iII
1 I1_
Paper flawed specimen
I
Range of load during which AE was recorded 4 4 5 0 - 5540 N 0-4450 N
,_1
LU
5540-5915 N
~J
Fig. 3 illustrates an interesting and important consequence of the choice of AE data presentation. First of all, it should be observed th,t only the 4450-5340 N load range is common for the two specimens. Even for this common load range it *The total energy in the AE signal depends upon the amplitude and the c~uration; assuming equal ringdown times for each analysed signal from the same sensor, the amplitude distribution analysis is equivalent to an energy distribution 'ranking'
.
Bottom of specimen
Amplitude distribution analysis AE events can be ranked according to their amplitude using amplitude distribution analysis.* The results for paper flawed and unflawed specimens are shown in Fig. 3; for the paper flawed specimen, the AE amplitude distribution was double peaked, and single peaked for the unflawed specimen. The results indicate a more frequent occurrence of a higher percentage of the more energetic AE signals from the paper flawed specimen.
[ i
Unfiawed
Least energetic
Increasing amplitude i.-
4450-5540N
Most energetic
Fig. 3 Amplitude distribution data for the paper flawed and unflawed specimens for the indicated load ranges
NDT I N T E R N A T I O N A L . FEBRUARY 1979
Table 1. Slope of log AE total event count as a function of amplitude for various load ranges L o a d range (N)
Paper f l a w e d s p e c i m e n
Unflawed specimen
0-2225
- 0.30
-
2225-4450
- 0.50
- 0.78
4450-fracture
-
- 0.77
0.56
1.00
J
Amplitude Fig. 4
Sketch showing amplitude distribution slope analysis
cally in Fig. 4 where the slope of the log AE total event count versus amplitude is indicated. A smaller magnitude* of the slope represents a higher percentage of high amplitude AE. The results of the amplitude distribution slope analysis are summarized in Table 1. Note that the two ranges of slope do not overlap. Furthermore, the magnitudes of the slopes versus load have opposite trends; the paper-flawed specimen shows an increasing trend and the unflawed specimen shows a decreasing trend. Thus, it appears from Table 1 that the use of amplitude distribution analysis is promising for the non-destructive detection of inteflaminar paper inclusions.
analysis provides some striking distinguishing AE characteristics between paper flawed and unflawed laminates. Other AE techniques such as linear location and cumulative AE within a load range give some encouragement also. Although further investigation is required in order to be able to locate paper inclusions with confidence, it appears that the choice of AE techniques and system parameters can be optimized for the detection and analysis of a variety of composite flaws, including interlaminar paper inclusions.
Acknowledgements
The test specimens were provided by the David W. Taylor Naval Ship Research and Development Center. The authors' research in non-destructive evaluation is generally supported by the Materials and Structures Division of the NASA Lewis Research Center.
Conclusions
A preliminary investigation of the AE monitoring of graphite fibre/epoxy laminates, with and without intedaminar backingpaper inclusions, has been conducted. Amplitude distribution
References
1 2
*This discussion is based on the assumption that the slope is negative, as is generally the case
NDT INTERNATIONAL.
FEBRUARY
1979
J.H. Williams,Jr and D.M. Egan 'Acoustic emission analysis of fiber composite failure mechanisms'Mater Evaluation (in press) J.H. Williams,Jr and S.S. Lee 'Acoustic emission monitoring of fiber composite materials and structures' J Composite Mater 12 (October 1978) p 348
7