Semi impregnated micro-sandwich structures* A novel composite configuration for low-cost panels with improved toughness

Transcription

1 Semi impregnated micro-sandwich structures* A novel composite configuration for low-cost panels with improved toughness P. Nieri 1, I. Montanari 2, A. Terenzi, L. Torre, J. M. Kenny 3 1 Delta-Tech srl loc. Rifoglieto, 60/A Altopascio, Italy 2 Strada Nazionale, Collecchio (PR), Italy 3 University Of Perugia UdR INSTM Loc. Pentima Bassa, Terni, Italy jkenny@unipg.it DRY FLEECE IMPREGNATED FLEECE PATENT PENDING *Presented at the SAMPE Europe 29 th International Conference and Forum - SEICO 08 (Hotel MERCURE Paris Porte de Versailles - March 31 st - April 02 nd )

2 Summary A number of different micro-sandwich panels were fabricated, using glass fabric prepreg and nonwoven needle-punched PET fleece, either pre-impregnated and dry, within an investigation program addressed to the development of low-cost laminates for automotive applications. Among all different panel configurations tested, a number of panels, containing different levels of dry fibre within the non-woven sandwich core thickness, showed very interesting results. In particular, these panels showed outstanding impact strength combined with low maximum force, which makes them good candidates for the fabrication of automotive external panels, or composite laminates for personal protection. 1. Introduction Thin sandwich structures, containing long fibre composite skins and low-cost fleece core have been widely used in the past, both in automotive and sporting good applications including helmet and footwear [1], although very little literature exists on this subject. In comparison to traditional sandwich structures [2-5], these kind of panels are normally thinner and can be easier to manufacture. In previously reported examples, the non-woven fleece was meant to be completely wetted and bonded by the matrix. Although substantial porosity often remained within the fleece, in all cases fibres were all wetted and bonded by the matrix, in order to obtain the desired stiffness. In the course of this investigation, thick plies of dry non-woven fleece were used, in combination with thin woven prepreg plies, in order to maximize thickness and reduce both weight and cost. This led to relatively thick structures with different extent of dry fibre content within the core thickness. When fleece impregnation level was too low and not enough stiff bridging could form between the two composite skins, the resulting panel was soft and characterized by very poor dimensional stability. On the other side, once sufficient matrix bridging could form between the skins, panels showed good stiffness and dimensional stability which, in case of small deformations, seemed to be independent on the level of core impregnation. Further investigation on the mechanical properties of these panels showed that the presence of dry regions within the fleece can dramatically affect failure mechanism and impact behaviour. 2. Experimental 2.1 Materials used A glass fabric prepreg was used with the following properties: Fibre: Glass roving 300 tex Fibre areal weight: 380 g/m 2 Weaving style: Twill 2x2 Matrix: Delta-Preg DT806R Resin content: 36,4% ; 40,9% ; 49,4% respectively The fleece was 400 gsm needle-punched, based on PET staple fibre and exempt from binder or sizing. It was used either dry or pre-impregnated with 60% by weight of the same matrix used on the glass fabric. 1

3 2.2 Panel fabrication All panels were made using the same layup and cure procedure. One glass prepreg was laid first over a flat metal surface, treated with release agent, followed by one ply of nonwoven and another ply of glass prepreg. A metal caul plate, treated with release agent, was then placed on top of the laminate. The whole assembly was placed into a vacuum bag, having care to allow the fleece in the laminate to communicate with the vacuum system. The panels were autoclave cured at 100 C for 90 min, with heating ramp 2 3 C/min and 4 bar pressure. Once autoclave pressure was reached, the vacuum bag was vented to atmospheric pressure. Four panel sets were fabricated: Glass fabric resin content (%) 36,4 Panel set A Non-woven PET resin content (%) 0 Glass fabric resin content (%) 36,4 Glass fabric resin content (%) 40,9 Panel set B Non-woven PET resin content (%) 0 Glass fabric resin content (%) 40,9 Glass fabric resin content (%) 49,4 Panel set C Non-woven PET resin content (%) 0 Glass fabric resin content (%) 49,4 Glass fabric resin content (%) 36,4 Panel set D Non-woven PET resin content (%) 60,0 Glass fabric resin content (%) 36,4 Table 1: Materials produced and composition 2.3 Testing A first morphological analysis was performed by optical microscopy on the cross section of each sample, using a Leica MZ 12,5 stereo-microscope, fitted with DFC 320 R2 digital camera. The presence of dry regions within the core ply was visually documented and evaluated in size. A complete mechanical characterization was performed in order to analyse the mechanical behaviour of the different materials and to correlate this behaviour to the structure of the microsandwich. More in detail, tensile tests were performed in samples with the external skin at 45 angle respect to fabric warp following the ASTM D3039 reference [6]. Flexural tests were performed in samples with the external skin at 90 angle respect to warp according to the reference ASTM D 790 [7]. The previous tests were performed in an electronic dynamometer LLOYD model LR30K, for tensile tests a load cell of 30kN was selected and the crosshead speed was fixed at 5 mm/min., whereas for flexural test a load cell of 500N was used and the crosshead speed was 1,7 mm/min. Impact test was performed by a ball drop testing machine, on 20 cm x 40 cm size panels; the impact was produced by a 15,5 kg impact head dropping from up to 8 m height. Impact energy and impact force profile were recorded versus time. Finally, penetration tests were carried according to UNI-EN [8]; the test was performed using the same testing machine used for tensile and flexural analysis, and the testing fixture represented in figure 5. All tests were performed at room temperature conditions and at least five samples were tested for each measurement, in order to have a correct statistical evaluation of material properties. 2

4 3. Test results 3.1 Optical microscopy The results of the optical microscopic analysis, reported in figure 1, clearly show the different degree of fleece impregnation in the different samples, as well as the different presence stiff matrix- impregnated bridges, connecting the external skins. Figure 1: Optical microscope images of the cross section for: (a) Set Panel A (b) Set Panel B (c) Set Panel C (d) Set Panel D (a) (b) (c) (d) The dry zone percentage in the core resulted directly related to the level of the impregnation of the glass fabric used for the skins. Panel D, instead, shows no presence of dry regions. 3.2 Tensile tests The results of tensile tests are reported in figure 2 and the relative data are summarized in table 2. By observing the figure 2 it is possible to notice that toughness is directly related to the morphology of the material. In fact, the higher the dry region content, the higher is failure strain; whereas tensile strength shows opposite behaviour; it increases with the decrease of the dry region content. By comparing the different stress-strain curves, it is quite evident that the presence of different levels of dry regions within the core affects the deformation and failure mechanism in tensile conditions. In fact, while the initial slope, corresponding to the linear elastic region, is the same for all samples, the difference of yeld strain and subsequent curve slope are directly correlated with the different presence of dry regions. The nonwoven material is able to give very high deformation without failure, thanks to a sliding mechanism of dry fibres in the fleece. When the latter is completely impregnated, sliding is not allowed as fleece fiber is totally bonded by the matrix and the deformation mechanism is similar to traditional composites. It is evident that the presence of the dry zones in the microsandwich causes a hybrid deformation behaviour where the dry zones act as residual bonding points after the stiff bridges between panel skins have failed. The macroscopic consequence is a high toughness and a very progressive failure mechanism. 3

5 Figure 2: Stress-Strain curves obtained in the tensile tests for all material tested Stress [MPa] Material Young Modulus [MPa] Tensile Strength [MPa] Failure Strain [%] A 7207 ± ,9 ± 2,3 17,85 ± 0,46 B 7560 ± ,3 ± 3,2 15,06 ± 1,06 C 8292 ± ,5 ± 3,2 12,63 ± 0,53 D 9163 ± ,3 ± 5,9 7,72 ± 0,33 Table 2: Material properties achieved by the tensile test It is clear that the higher is the impregnation level, the lower is the chance for the fiber to slide and the lower is the failure strain; therefore the failure mechanism becomes more and more similar to that of typical composites. 3.3 Flexural tests The results of flexural tests are reported in figure 3 and the relative parameters are reported in table 3. Also in this loading conditions, it is possible to observe that the failure mechanism is quite progressive. In correspondence with the failure of the external skin an important decrease of the load curve is achieved, but the progressive failure and the sliding mechanism of the nonwoven fiber causes a progressive fracture of the laminate. Again the braking behaviour is directly related to the microstructure and the dry region content. Figure 3: Curve achieved with the flexural tests Load [N] Material Flexural Young Modulus [MPa] Flexural Strength [MPa] A ± ,3 ± 16,5 B ± ,7 ± 18,6 C ± ,9 ± 9,7 Table 3: Material properties in flexural conditions 4

6 3.4 Impact tests In this kind of impact tests it is possible to measure the impact energy and to record the force profile versus time. This enables evaluating also maximum force reached during the tests. The results of the tests are summarized in table 4; it is important to point out that the results for material D should be regarded as an upper limit, in fact it was not possible to determine the minimum failure impact energy, since this was lower than the minimum measureable by our instrumentation. By observing the data reported in table 4, it is possible to notice the influence of the dry zones on impact energy. In material A the fracture is quite progressive, thanks to the presence of the dry zones in the nonwoven; material D instead shows opposite behaviour with a sudden fracture in correspondence of the impact. Such different behaviour clearly involves completely different mechanism. In the first case the adsorbed energy is the highest, whereas the maximum force reached during the impact is the lowest; vice versa, in the second case the adsorbed energy is the lowest while maximum force is the highest. An important conclusion of this test is that the presence of dry regions confers a high potential to adsorb energy during the impact. This characteristic is very important for applications in the automotive industry, in particular concerning those related to safety. Moreover it is important to notice that the impact response of the material can be easily adjusted by tuning the extent of core impregnation level. In fact the materials B and C show intermediate behaviour respect to materials A and D. This adds to the flexibility of the application of these semi-impregnated micro-sandwich structures in different industrial applications. Figure 4: Typical Load time curve obtained during the impact test Table 4: Results of the impact test with a ball drop instruments Material Impact energy [J] Maximum Force [N] A ± 142 B ± 150 C ± 179 D < ± 352 Stress [MPa] Time [s] Penetration tests In figure 5 a schematic representation is reported of the tool used to perform penetration tests. Test results are reported in figure 6 and table 5; penetration strength is the maximum force before the first failure of the material occurs, and the failure displacement is the corresponding displacement. Also in this kind of tests the presence of the dry zones causes significant difference in material behaviour.

7 Figure 5: Schematic representation of the penetration tools used in penetration test Figure 6: Typical Load time curve obtained during the impact test Load [N] Material Penetration Strength [N] Failure Displacement [mm] A 325 ± 20 1,75 ± 0,16 B 340 ± ± 0,18 C 494 ± 28 1,51 ± 0,08 D 477 ± 111 2,30 ± 0,44 Table 5: Penetration tests results Looking at the results, it is possible to notice that both penetration strength (and relative penetration displacement) and penetration mechanism after the first perforation are closely related to the impregnation level of the nonwoven core. In particular, the higher the impregnation level, the higher is the penetration strength and the lower is penetration displacement. Besides, at higher impregnation level the resistance to further penetration decreases quickly after the first failure. Instead, the presence of some dry zones within the core fleece causes higher resistance to further penetration; this is evidenced by the presence of a force plateau after initial failure. 6

8 4. Conclusion The properties of the materials produced and tested in this investigation are strongly influenced by the microstructure achieved during the manufacturing process. These materials can exhibit very high toughness due to the deformation mechanism in the dry regions of the nonwoven. In particular the fibre in the dry regions can slide respect to each other without showing a neat fracture; this mechanism causes high level of failure strain of the material and high level of energy adsorbed during the deformation. The possibility to adjust the sandwich microstructure, by changing the impregnation level of the non-woven core, offers a flexible tool for tuning material properties during the manufacturing process. The favourable combination of good mechanical performance and low cost makes this type of thin sandwich panels a very attractive option for industrial applications, especially those where low specific weight, low cost and high toughness are requested, like in transportation and personal protection. References [1] Patent US2002/ A1 Thin composite laminate and use thereof in making sports articles, especially boots [2] Extending the concept of sandwich structures, Reinforced Plastics February 1997 [3] C. S. Lee, D. G. Lee, Manufacturing of composite sandwich robot structures using the co-cure bonding method, Composite Structures 65 (2004) [4] K. F. Karlsson, B. T. Astrom, Manufacturing and applications of structural sandwich components, Composites Part A 28A (1997), [5] O. Ronzant, P. E. Bourban, j. A. E. Manson, Manufacturing of three dimensional sandwich parts by direct thermoforming, Composites Part A 32 (2001) [6] ASTM D 3039 M [7] ASTM D 790 M [8] UNI-EN DeltaTech S.p.A. Località Rifoglieto 60/A int. 1 I Altopascio (LU) - Italia Tel Fax info@delta-tech.it