Introduction
The sandwich structures
The materials
Units | Low density <60> | Medium density <80> | High density <100> | |
---|---|---|---|---|
Density | (kg/m3) | 60 | 80 | 100 |
Compressive modulus | (MPa) | 69 | 104 | 130 |
Compressive strength | (MPa) | 0.90 | 1.45 | 2.0 |
Compressive strain at yield | 0.013 | 0.014 | 0.015 | |
Shear modulus | (MPa) | 22 | 30 | 40 |
Shear strength | (MPa) | 0.85 | 1.2 | 1.7 |
Shear strain at yield | 0.039 | 0.040 | 0.043 |
Manufacturer | Reference | Style | Nominal (g/m2) | Primary fibre type |
---|---|---|---|---|
Gurit, UK | ‘XE603’ | + 45/− 45 biaxial fabric | 601 | E-glass |
The configurations used for the core of the sandwich structures
Experimental
Results
Flexural test results
The three-point bend flexural tests
The various sandwich structures
The sandwich structure with a <80:80:80> layer foam core
The sandwich structure with a <60:100:60> layer foam core
The sandwich structure with a <100:60:100> layer foam core
The sandwich structure with a <60:80:100> layer foam core
The sandwich structure with a <100:80:60> layer foam core
Comparisons of the various configurations of sandwich structures under three-point bending
Configuration | Initial elastic stiffness (± 30) (N/mm) | First peak force (± 50) (N) | First peak displacement (± 0.5) (mm) | First force drop displacement (± 0.5) (mm) | Energy absorption up to first force drop (± 0.5) (J) | Type of failure |
---|---|---|---|---|---|---|
<80:80:80> | 530 | 2890 | 7.9 | 16.2 | 38.3 | Core crushing, moderate skin indentation, skin fracture |
<60:100:60> | 470 | 1850 | 6.6 | 38.7 | 61.3 | Core crushing, extensive skin indentation, skin fracture |
<100:60:100> | 520 | 2300 | 6.4 | 18.2 | 34.9 | Core crushing, extensive skin indentation, skin fracture, shear cracks, skin/core debonding |
<60:80:100> | 460 | 1850 | 8.8 | 12.6 | 15.7 | Core crushing, a few small cracks in the top layer and skin indentation, shear cracks, skin/core debonding |
<100:80:60> | 460 | 2100 | 5.4 | 6.3 | 6.7 | Many small shear cracks in bottom layer, shear crack, skin/core debonding |
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The initial elastic stiffness is very similar in value for all core configurations, since the average core densities of all the five configurations are very similar. The GFRP skin material, which is same for all the different core configurations, plays a significant role in the elastic part of the test.
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The <80:80:80> sandwich panel exhibits the highest first peak force.
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The <60:100:60> sandwich panel exhibits the highest energy absorption (i.e. 61.3 J) up to the first load drop. This is because the two low-density outer layers offer protection by spreading the indentation load and suppressing the onset of shear cracking and skin/core debonding. However, although greater energy absorption is observed, there is a reduction in the maximum load that this configuration of sandwich structure, with the two low-density outer layers, can withstand in three-point loading.
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The displacements corresponding to the first force peak for all the configurations are very similar in value. The reason for that is that the yield strains of the different density foams are very similar in value.
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The values of the energy absorption up to the first force drop are higher for the symmetric core configurations (i.e. the <80:80:80>, <60:100:60> and <100:60:100> core configurations) compared with the non-symmetric configurations (i.e. the <60:80:100> and <100:80:60> core configurations). This is because the failure mechanisms of shear cracking, core crushing and skin/core debonding tend to occur more readily in the low-density foam layer when a non-symmetric configuration is employed. Further, these mechanisms are particularly likely to occur more readily in such configurations if the low-density layer is the bottom layer of the three layers that form the core. Since the low-density layer then provides a relatively easy fracture path for a shear crack to grow parallel to the rear GFRP skin and then easily cause debonding of this core layer from the skin.
The four-point bend flexural tests
The various sandwich structures
The sandwich structure with a <80:80:80> layer foam core
The sandwich structures with <60:100:60> and <100:60:100> layer foam cores
The sandwich structures with <100:80:60> and <60:80:100> layer foam cores
Comparisons of the various configurations of sandwich structures under four-point bending
Configuration | Initial elastic stiffness ± 40 (N/mm) | First peak force ± 30 (N) | First peak displacement ± 0.5 (mm) | First force drop displacement ± 0.5 (mm) | Energy absorption up to first force drop ± 0.5 (J) | Type of failure |
---|---|---|---|---|---|---|
<80:80:80> | 970 | 3790 | 12.8 | 20.9 | 74.8 | Core crushing, extensive skin indentation, skin fracture |
<60:100:60> | 720 | 2990 | 11.0 | –a | –a | Core crushing, moderate skin indentation, shear cracks in the outer layers, skin/core debonding |
<100:60:100> | 980 | 3220 | 12.1 | –a | –a | Core crushing, extensive skin indentation, shear cracks in the middle layer, skin/core debonding |
<60:80:100> | 810 | 2210 | 4.0 | 4.4 | 6.6 | Core crushing, extensive skin indentation, shear cracks in the upper layer, skin fracture, skin/core debonding |
<100:80:60> | 830 | 2340 | 4.2 | 4.7 | 6.8 | Shear cracks in the bottom layer, skin/core debonding |
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The values of the elastic stiffness of the sandwich structures using the various core configurations are very similar. This again reflects the fact that the average core densities of all the five configurations are very similar.
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The highest peak force and highest energy absorption, up to the first force drop, is recorded for the <80:80:80> core configuration.
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The <60:100:60> configuration of sandwich structure also performed very well with good energy absorption but, for this case of four-point flexural bend loading, there was no load drop. Therefore, it was not possible to give a definitive value of energy absorption up to the first load drop. However, from the results obtained, the indications are that it was improved in much the same way as for the case of three-point flexural bend loading.
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Sandwich structures with a symmetric core (i.e. <80:80:80>, <60:100:60> and <100:60:100>) have a higher measured value for the first peak displacement than sandwich structures with a non-symmetric core configuration (i.e. the <60:80:100> and the <100:80:60> core configurations.)
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Sandwich structures with the <100:80:60> and <60:80:100> core configurations have the lowest energy absorption up to the point corresponding to the first force drop, compared to the other core configurations.
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Compared to three-point flexural bend loading, then generally four-point flexural loading removes the likelihood of indentation related failures. Also, as observed for the three-point bend loading tests, energy absorption was better for the symmetric core configurations than for the non-symmetric core configurations.
High-velocity impact results
Core configuration | Velocity/energy of projectile | General observations | Perforation of the rear GFRP skin |
---|---|---|---|
<80:80:80> | 178 m s−1/288 J | Slight core crushing, slight skin indentation | No |
<80:80:80> | 204 m s−1/378 J | Core crushing, skin indentation | No |
<80:80:80> | 215 m s−1/420 J | Shear cracks, fibre breakage on both skins | Yes |
<80:80:80> | 230 m s−1/481 J | Shear cracks, fibre breakage on both skins | Yes |
<60:100:60> | 178 m s−1/288 J | Core crushing, skin indentation | No |
<60:100:60> | 204 m s−1/378 J | Moderate core crushing, skin indentation | No |
<60:100:60> | 215 m s−1/420 J | Extensive core crushing, fibre breakage on impacted (i.e. front) skin face | No |
<60:100:60> | 230 m s−1/481 J | Core crushing, shear cracks, fibre breakage on both skins | Yes |
<100:60:100> | 178 m s−1/288 J | Slight core crushing, slight skin indentation | No |
<100:60:100> | 204 m s−1/378 J | Moderate core crushing, shear cracks, fibre breakage on impacted (i.e. front) skin face | No |
<100:60:100> | 215 m s−1/420 J | Moderate core crushing, shear cracks, fibre breakage on impacted (i.e. front) skin face | No |
<100:60:100> | 230 m s−1/481 J | Core crushing, shear cracks, fibre breakage on both skins | Yes |
<60:80:100> | 178 m s−1/288 J | Slight core crushing, slight skin indentation | No |
<60:80:100> | 204 m s−1/378 J | Extensive core crushing, shear cracks, fibre breakage on impacted (i.e. front) skin face | No |
<60:80:100> | 215 m s−1/420 J | Extensive core crushing, shear cracks, fibre breakage on impacted (i.e. front) skin face | No |
<60:80:100> | 230 m s−1/481 J | Extensive core crushing, shear cracks, some very fibre breakage on impacted (i.e. front) skin face | No |
<100:80:60> | 178 m s−1/288 J | Slight core crushing, slight skin indentation | No |
<100:80:60> | 204 m s−1/378 J | Extensive core crushing, shear cracks, fibre breakage on impacted (i.e. front) skin face | No |
<100:80:60> | 215 m s−1/420 J | Core crushing, shear cracks, fibre breakage on both skins | Yes |
<100:80:60> | 230 m s−1/481 J | Core crushing, shear cracks, fibre breakage on both skins | Yes |
Conclusions
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The Digital Image Correlation (DIC) technique is very effective at delivering quantitative values of the deformation, strain and onset of damage for the quasi-static three-point bend and four-point bend flexural tests, and the high-velocity impact experiments.
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Under three-point and four-point bend loading, the use of the low-density core in the <60:100:60> layered configuration reduces the likelihood of failure of the sandwich structure by a sudden force drop when compared with the uniform (i.e. homogeneous) <80:80:80> layered core configuration.
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In the four-point flexural bending experiments, the sandwich structures exhibited a failure mechanism less dominated by an indentation failure of the GFRP skin but, instead, with shear cracking of the foam core of more importance, compared to the three-point flexural bending tests.
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The graded-density core sandwich structures, e.g. the <60:100:60> layered core configuration, fail under the quasi-static flexural bending tests through a relatively stable failure mechanism. Hence, they can typically withstand larger deflections before final failure than the sandwich structure with a uniform-density core, although at the cost of a decrease in flexural strength.
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When subjected to a high-velocity impact, the sandwich structure using the <60:80:100> layered core configuration, with the low-density layer on the impacted side, results in the best impact performance, compared to all the other core configurations that were examined. This arises from a failure mechanism which involves compression occurring in the low-density core layer at an early stage which enables the sandwich structure to resist penetration by the impacting projectile.
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Thus, the overall conclusion from this research is that a symmetric graded-density configuration (e.g. the <60:100:60> layered configuration) or a uniform core configuration (e.g. the <80:80:80> layered configuration) gives the best performance for the sandwich structures when subjected to quasi-static flexural loading.
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However, when subjected to an impact by a compliant projectile travelling at a relatively high-velocity, there is an appreciable benefit in employing a graded-density <60:80:100> core configuration for the sandwich structure, with the low-density foam layer of the graded core on the impacted side. Shukla et al. [29, 30] and Rolfe et al. [31] have also shown that a similar configuration of a graded core, with the lowest density on the front face, is beneficial for blast mitigation of composite sandwich structures. They found that in a blast situation the low-density core next to the impacted face-layer provides a cushion absorbing the incident blast wave.
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From testing the various core configurations that have been employed for the sandwich structures, there appears to be no correlation between the performance from the quasi-static three-point and four-point flexural bending tests of the various sandwich structures and the results from the impact tests. Thus, the high-velocity impact behaviour of such sandwich structures cannot be predicted from quasi-static flexural bending tests.
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These findings are very relevant for the testing and design of such sandwich structures which may experience high-velocity impact threats when employed, for example, as forward-facing components on aircraft, marine structures exposed to wave-slam loading and the leading edges of wind-turbine blades.