• CIRCULAR VERSUS SQUARE SPECIMENS
Figure 6 shows histories of absorbed energy, central displacement, contact force, contact stiffness for the circular GLARE 5 (3/2) specimens with the cross-ply [0°/90°]s, unidirectional [O4], angle-ply [+45°/-45°]s and quasi-isotropic[0°/+45°/ -45°/90°] layup sequences. Figures 7 and 8 depict corresponding C-scan results, back-side (non-impacted) views, as well as cross-sectional views of specimens of those configurations. Tables 3 summarizes experimental results of the impact-induced crack lengths measured on the impacted side (the top aluminum sheet) and the non-impacted side (the bottom aluminum sheet), respectively, along with the post-impact permanent central deflection in GLARE 5 (3/2) composite materials with different stacking sequences and geometries subjected to 40J impact energy. Comparing these results to those of the square specimens, the following remarks can be found.
The energy history curves were in a very close pattern to those of the square specimens. The central deflection curves for the circular unidirectional and quasi-isotropic specimens increased noticeably in comparison with the corresponding panels of square specimens. This enhancement was very negligible for the cross-ply and angle-ply panels. The peak force values for the circular specimens were lower than those of the corresponding square specimens. The contact time was also raised by changing the geometry from square to circular. This difference was quite noticeable for the unidirectional and quasi-isotropic panels but it was relatively negligible for the cross-ply and angle-ply panels. The contact stiffness decreased by changing the geometry from square to circular. The effect of the above mentioned differences could be revealed by comparing the cross sectional, C-scan and back side views of the specimens with the ones for the square specimens. Comparing the cross-sectional views of the two different geometries for the cross-ply and angle-ply specimens, the overall damage patterns were similar except in circular geometry there was no debonding between the non-impacted aluminum and the adjacent prepreg layer. For the circular unidirectional specimen the induced damage pattern was different from the corresponding square specimen. Prepreg damages among the aluminum layers were noticeably increased. Also unlike the unidirectional square specimens, in which the prepreg close to the non-impacted side were broken into several pieces by through-the-thickness cracks, it did not happen for the circular specimen. The major differences in damage pattern were obvious for the circular quasi-isotropic specimen. By evaluating the cross-sectional views for the quasi-isotropic square and circular specimens, a dramatic change could be seen in the induced damages. Unlike the square specimen, which only the bottom aluminum damaged, all the aluminum layers failed for the quasi-isotropic circular specimen. It is worth noting that the impacted-side aluminum failed in two places. Another important difference was that the delamination was negligible for the circular specimens whereas it was relatively notable for the square specimens.
Fig.6. Impact responses of the cross-ply, unidirectional, angle-ply and quasi- isotropic GLARE 5 (3/2) circular specimens under 40J impact energy.
Fig.7. Back side (non-impacted) view and corresponding C-scan view of the cross-ply, unidirectional, angle-ply and quasi-isotropic GLARE 5 (3/2) circular specimens under 40J impact energy.
The C-scan results revealed that the damage contour decreased for the cross-ply and angle-ply circular specimens. For the unidirectional panels of the two geometries, the C-scan shapes were almost similar to each other with the exception that for circular geometry, the length of the damage along the 0° fiber direction was longer (Table 3, Figs. 4 and 7). In fact, the induced crack in the unidirectional circular specimens passed the grip area of the specimen. For the quasi-isotropic specimens, the crack at the non-impacted side of the circular specimen was noticeably longer than the square specimen. By transition from square to circular geometry, the damage contour changed from almost a circular shape to an elliptical shape with major axis along the 0° fiber direction. Due to change of the geometry, the central deflections were higher for the circular specimens compared to those of square specimens.
Fig.8. Cross-sectional view of the cross-ply, unidirectional, angle-ply and quasi-isotropic GLARE 5 (3/2) circular specimens under 40J impact energy. The symbol ® means that the fiber direction is perpendicular to the cross sectional view picture.
Table 3. Crack lengths and permanent deflections of the cross-ply, unidirectional, angle-ply and quasi-isotropic GLARE 5 (3/2) under 40J impact energy.
|
Stacking Sequence & Geometry |
Impact Energy (Joule) |
Crack length |
in outer layer (mm) |
Permanent Deflection (mm) |
|
Impacted side |
None impact side |
|||
 |
40 |
9 |
18 |
6.75 |
 |
40 |
69 |
75 |
7.65 |
 |
40 |
7 |
16 |
6.65 |
 |
40 |
0 |
12 |
5.30 |
 |
40 |
8.5 |
16 |
6.8 |
 |
40 |
83 |
83 |
7.7 |
 |
40 |
8.5 |
16 |
6.8 |
 |
40 |
7.5 |
20 |
5.80 |
CONCLUSIONS
This study presents an experimental investigation on the impact response of GLARE 5 (3/2) composite materials considering the effects of stacking sequence and geometry through using drop weight impact tester. The following remarks can be concluded from this study.
• GLARE 5 made of unidirectional fibers had the worst impact resistance; followed by cross-ply and angle-ply configurations, while the quasi-isotropic lay-up showed the best resistance to impact.
• By introducing circular geometry, damage patterns and impact behaviors were changed. This was very obvious for the panels with quasi-isotropic layup configuration, i.e. [0°/+45°/-45°/90°].
• Only the profile of damage zone could be detected through the ultrasonic C-scan. The mechanical- sectioning technique must be adopted in order to get the details of damage inside the fiber-metal laminates. The drop-weight induced damage included indentation around impact center, delamination between aluminum and glass-epoxy composite, cracks in aluminum layers, and damage in composite layers. More severe damage occurred on the non-impacted side of fiber-metal laminates.
