Introduction
Materials
Foam Material Properties
Material | Density | QS/HR elastic | QS/HR tensile | QS/HR tensile |
---|---|---|---|---|
(kg/m3) | modulus | breaking | breaking | |
(MPa) | stress (MPa) | strain (%) | ||
M100 SAN | 108 a
| 107/143 | 2.69/5.67 | 7.82/5.66 |
M130 SAN | 140 a
| 161/245 | 3.70/7.81 | 7.26/3.67 |
M200 SAN | 200 a
| 258/438 | 5.63/8.44 | 5.57/2.06 |
C70.90 PVC | 100 b
| 80.0/177 | 2.73/7.84 | 9.38/4.43 |
110SL PMI | 110 c
| 156/462 | 3.89/8.71 | 5.59/2.06 |
Face-Sheet Properties
Material | Density (kg/m3) | Elastic modulus | Tensile breaking | Inter-laminar shear | Compressive |
---|---|---|---|---|---|
(GPa) | stress (MPa) | strength (MPa) | strength (MPa) | ||
Glass fibres | 2550 a
| 80 a
| 2000 a
| – | 1450 b
|
Amreg22 resin | 1140 c
| 3.74 c
| 72.2 c
| 53.3 c
| 462 c
|
QE1200 quadriaxial GFRP Ply | 1803 | 16 | 391 | 53.3 | 385 |
Innegra IS-940 PP fibres | – | 14.8 d
| 667 d
| – | – |
Woven Innegra PP ply | – | 6.02 | 234 | 53.3 | 462 |
Interfaces
Experimental
Name | Sandwich panel | Areal density | Frame rate |
---|---|---|---|
core thickness | (kg/m2) | of the camera | |
(mm) | (s −1) | ||
M100 SAN Core | 40 | 8.8 | 5400 |
C70.90 PVC Core | 40 | 8.8 | 5400 |
110SL PMI Core | 40 | 8.8 | 5400 |
Graded SAN Core | 30 | 9.1 | 7000 |
PP Interlayer | 40 | 9.7 | 7000 |
Varying Polymer Type Study
Graded Density Core Study
Polypropelene Interlayer
Test Layout
Name | Stand-off distance to cause rear face-sheet tensile failure (m) | Stand-off distance to cause core cracking (m) | Stand-off distance to cause front face-sheet compressive failure (m) |
---|---|---|---|
M100 SAN Core | 17 | 28 | 19 |
C70.90 PVC Core | 17 | 26 | 20 |
110SL PMI Core | 17 | 20 | 15 |
M130 SAN Core | 19 | 27 | 18 |
Instrumentation
Post-Blast Damage Assessment
Results
Blast Loading Response
Varying polymer types
Graded density core
Polypropelene interlayer
Post-Blast Damage Assessment
Varying polymer types
Graded density
Polypropelene interlayer
Discussion
Name | Reflected blast impulse (kPa.ms) | Maximum deflection (mm) | Maximum pull-out (mm) | Positive displacement time (ms) |
---|---|---|---|---|
SAN Core | 962 | 89 | 45 | 12.8 |
PVC Core | 817 | 103 | 83 | 12.4 |
PMI Core | 732 | 101 | 92 | 14.1 |
Graded Core | 817 | 103 | 79 | 11.2 |
Polypropelene interlayer | 732 | 92 | 65 | 10.7 |
Name | Number of front face-sheet cracks | Number of through thickness core cracks | Total amount of debonding (%) |
---|---|---|---|
SAN Core | 1 | 34 | 19 |
PVC Core | 5 | 82 | 41 |
PMI Core | 8 | >200 | >75 |
Graded Core | 0 | 30 | 18 |
Polypropelene Interlayer | 0 | 110 | 29 |
Conclusion
-
By using high speed cameras with frame rates greater than 2000 s −1 the maximum out of plane displacement and pull-out can be accurately measured, and the deceleration of the sandwich panel rebound caused by core cracking is visible.
-
The use of speckles on a rigid section of the test cubicle allows rigid body motions of the test structure during the test to be removed in the DIC processing, so that the actual deflection of the sandwich panel can be determined. There is still twisting present in the cubicle front, which can be seen in the edges of some of the contour plots taken from DIC, and these could be reduced by speckling all of the inside sections of the cubicle front, rather than just the centre section.
-
In a comparison of SAN, PVC and PMI foam cores of equal density and thickness, it was found that all cores offered good resistance to blast at this charge size and stand-off distance, but that SAN offers the best blast resistance with minimal out of plane displacement and pull-out, and suffers the least damage from blast testing. The PMI core offers the best stiffness and strength, so if used within the elastic limit would offer the most suitable properties. The overriding factor for the foam in blast response is the strain to failure, as the stiffness of the foam is very low in comparison to the face-sheets.
-
The use of a stepwise graded density foam core reduces the amount of core damage due to interfaces inhibiting crack propagation through the core, thus reducing the amount of out of plane displacement and pull-out of the sandwich panel. Furthermore, the majority of core cracking takes place in the lower density foam layers, allowing for a smoother rear face-sheet deformation.
-
By placing compliant PP plies between the GFRP plies in the front face-sheet, cracking of the front face-sheet is prevented. This happens despite the same amount of core damage present in the compliant face-sheet panel as in the single density PVC core sandwich panel to which the comparison was made. This conclusion is important for marine applications due to the prevention of front face-sheet cracking retaining the structural integrity of the sandwich panel.