| Since the birth in twentieth century, because of its high strength-weight ratio and modulus-weight ratio, good fatigue resistance, anti-vibration performance and effective ability of absorbing impact energy, as well as the meeting requirements under the various conditions of use, the sandwich structure has been the engineering structure "material" in aviation and aerospace. Nowadays, the application of sandwich structures has been throughout the aerospace, shipbuilding, transportation, construction, electronics, sports and entertainment.Sandwich structure is characterized in mechanics as the main bearing skins, while the role of core material is to be subjected to shear force, with firmly bonded together with the skins. One of the key technologies is the effective combination of skins and the core material. Only the effective combination is achieved, can we obtain the excellent performance of sandwich structural materials.It is well known that the traditional laminated plates have the problem of delamination and are easy to defect. On the other hand, it is difficult to gain the effective combination between the skins and the core material by conventional adhesion, which leads to structural weakness. The invention of three-dimensional orthogonal panel makes the composites to be free of delamination failure and offers a better integrity of the sandwich structure because of the existence of the Z-yarn in weaving.The skins of the sandwich are made of Kevlar fibers and the core material is the phenolic foam. The matrix is the phenolic resin. The preform of sandwich composite is fabricated in the three-dimensional orthogonal loom. The foam sandwich composites are obtained by two methods: hand lay-up and vacuum assisted resin transfer molding (VARTM) and then mechanic and flame tests are done.In the low-speed impact test, there are two levels of impact energy are used: 12 J and 19 J. The impact damage process and form are analysed through the load-time curve and energy-time curve. We can know that there are three stages in the impact damage process:crushing or damage of resin and upper skin; damage of foam core; damage of the lower skin. When the impact energy is 12 J, there are damage of upper skin and part damage of core material while in the impact energy of 19 J, the lower skin begin to be damaged in the process. From the analysis, it can be known that the impact property of sandwich composites made by VARTM is better than the sandwich composites made by hand lay-up method, which may attributes to the better infiltration of resin in the VARTM progress.In the three-point beading test, there are several failure modes can be seen from analysing the load-displacement curve:compression or tensile failure of skins; shear failure of core; compression or tensile failure of core. However, the maximum bending load is small, which maybe resulted from several factors, such as some imperfection in the skins, the weak combination between the skins and core and the more possible of shear failure than bending failure in the test.There are two test methods are used to indicate the flame retardant of sandwich composites: limiting oxygen index (LOI) test and level of combustion method. The LOI of the sandwich composites made by the two curing methods both can reach 49 and the burning level of the two are HB, both of which suggest that the sandwich composites have the excellent flame retardant performance. |
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