Table of Contents ICAC'98
On the efficiency of current NDT methods for impact damage detection and quantification in thermoplastic toughened CFRP materialsX.E. Gros, K. Takahashi
Research Institute for Applied Mechanics
Kyushu University, 6-1 Kasuga-koen, Kasuga-shi 816-8580, Japan
M.-A. De Smet
GIE NDT Expert, France
Corresponding Author Contact:
Email: email@example.com, URL: http://www.riam.kyushu-u.ac.jp/fracture/xav01.htm
|TABLE OF CONTENTS|
The resistance to impact damage of a thermoplastic toughened CFRP material currently used in the manufacture of the Boeing 777 was evaluated non-destructively. Several [0/45/-45/90] 8-plies panels of thermoplastic toughened CFRP material were impacted with energies ranging from 2 to 14 Joules, and non-destructive characterisation of these samples was carried out using some of the most currently used NDT methods by industry. These include visual examination, ultrasonic C-scan, infrared thermography, radiography, shearogaphy and eddy currents. These NDT methods were used to quantitatively evaluate the resistance to impact damage of this new thermoplastic toughened CFRP material. In addition, a comparison of the efficiency of these methods to accurately detect and quantify impact damage in thermoplastic toughened CFRP material was made and the needs of industry for composites evaluation discussed.
Impacts were generated using a computer controlled drop tower mechanism. Impact energy (J) was recorded as a function of time (ms) and load (N) as a function of time (ms) and defelection (m); such plots are shown on figure 1.
Recorded plots of impact energy versus time, load versus time and deflection for a [0/45/-45/90] reinforced composite panel submitted to an impact of 6 Joules.
|Fig 2. Plots of the detected impact damage length against impact energy as detected by visual examination on the front and back surfaces of the specimens|
|Fig 3. Oscilloscope display of eddy current signals from the inspection of a 10 Joules impacted specimen|
|Fig 4. Ultrasonic C-scans of a 10 (top) and 14 J (bottom) impact damage on CFRP panels|
|Fig 5. Infrared thermograph of a toughened CFRP panel damaged by an impact of 14 J|
|Fig 6. Real-time X-ray radiograph of a toughened CFRP panel damaged by a 14 J impact|
|Fig 7. Typical shearographic image of an impacted composite material|
The naked eye visual examination offers a low cost rapid survey but is limited to the detection of impact damage of energy greater than 4 Joules. Quantification is difficult and varies greatly depending whether inspection is carried out on the front or back surface of the material. Indeed, for an equal impact energy of 14 Joules, the detected damage length was 76% greater on the back surface than on the front surface. Also, because impact damage on this material generates only limited visual damage, such examination can be very limited.
Eddy current testing of CFRP materials is a low cost sensitive technique capable of providing both qualitative and quantitative impact damage information. However, this technique was difficult to apply using commercial equipment on this newly reinforced composite. Signal interpretation can be very straightforward by analysing an analogue signal display on the cathode ray tube of an oscilloscope, or in the form of a colour coded mapping of the surface tested. Only qualitative information was gathered, and for impact of low energies, it was difficult to estimate the degree of delamination. If carefully developed, it is thought that electromagnetic techniques present a low cost non-contact alternative to other more costly NDT methods.
Ultrasonic C-scan examination is regularly used for the testing of composites and provides remarkable results for both on-site and during manufacture testing. However, it was noticed that ultrasonic waves were greatly attenuated by this new material. This may be due to the high toughness resin used in the manufacture of this material. Therefore, existing ultrasonic equipment would have to be adapted for an improved examination. Indeed, this wave attenuation may result in a partial quantification of the delaminated area.
Infrared thermography was fast and non-contact and provided results in real-time in a colour coded format. However, damage caused by impacts of less than 8 Joules were not detectable. A limiting factor to be kept in mind for a technique which is considered as a rapid and efficient NDT method by industry. Additional research work is required, especially on the image processing side, to be able to detect low energy impact damage. The main advantage of this technique remains in the fact that direct colour images of the material inspected are generated. Although, such images may seem easy to interpret, in certain cases, image processing operations are necessary for proper thermograph interpretation.
Without the use of dye penetrant, conventional real-time X-ray radiography is inadequate for the detection of impact damage in CFRPs. Nevertheless, the special technique from the JFCC allowed the detection of damage resulting from a 8 Joules impact. This is remarkable considering the low attenuating factor of this new material. Unfortunately, this laboratory equipment could be difficult to use for on-site testing and still present some radiation hazards. With the apparatus used, radiographs were displayed on a TV monitor, thus removing the photo processing phase often encountered with conventional film radiography. However, interpretation of the radiograph can sometimes be subjective and is limited by the resolution of the TV monitor used.
Despite encountering minor difficulties (mainly due to the first time testing of this new material), the shearographic examination gave promising results. Among all the techniques used, it offers (together with naked eye visual examination) one of the best impact damage detection (see Table 1) and also gives a direct rough estimation of the damaged area. Here again, signal output is in a visual format (not analogue) and can be very straightforward for an experienced operator.
Table 1: Minimum impact damage energy detected on thermoplastic toughened thermoset resin polymeric matrix panels for different NDT techniques
||NDT technique||Minimum impact energy detected (Joules)
||Visual (naked eye)||4
As advanced composite materials are being developed, existing NDT techniques may face new challenges to efficiently detect and quantify impact damage. Judging by the rapid pace to which new advanced materials are being developed, it will become more and more necessary to adapt and modify existing NDT techniques to meet new requirements as well as to be able to efficiently detect and characterise damage in these new materials such as thermoplastic toughened thermoset resin polymeric matrix composites.
The authors would like to thank M. Matsushima from the National Aerospace Laboratory for his help in ultrasonic testing. Toray Industries Inc. is also acknowledged for kindly supplying CFRP specimens. The advice of T. Sakagami from Osaka University for the infrared thermographic testing, and the help of Y. Ikeda from JFCC for radiographic testing are also acknowledged.