Numerical Analysis Of Damage In FRP/Metal Hybrid Laminates Under Low Velocity Impact

Fiber Reinforced Metal Laminates (FRMLs) are new type structural materials that were firstly created in 1980's in Holland. They were intended to partly replace aluminium alloys in airplane for their excellent anti-fatigue properties. FRMLs consist of aluminium alloy sheets and fiber-reinforce plastics (FRP) layers. These two kinds of constituents are bonded together to form the hybrid laminates. FRMLs possess reduced density and better fatigue resistance as compared with metal materials. On the other hand, FRMLs are superior to non-metal composites in machinability. Although the general mechanical properties of FRMLs have been studied and realized widely, their strength behaviors are far from clear, such as the impact property, fatigue mechanisms, and notched strength features. Owing to the increasingly extensive application of FRMLs in the airplane structure, the study of these fields is an important and urgent subject. FRMLs can be damaged when they suffer the surprised impact (many instances are low velocity impact) during the usage and maintenance process. The low velocity impact damage is usually difficult to discover, so they are latently dangerous. Laminates under low velocity impact (LVI) is one of the important topics of the concern, but the achievement for FRMLs is very limited. The damage process in a composite under low velocity impact is very complicated. Firstly, it includes the problem of impact-contact. Secondly, we should consider several types of damages in the laminate, including matrix crack and delamination. Thirdly, to construct an appropriate model and develop corresponding analytical methods is necessary. On the basis of three-dimension dynamic finite element method, the relation of impact-contact is established in this paper by meshing the impactor and laminates. To solve the dynamical equation, the explicit center-time difference method is used. The mechanism of matrix crack and delamination is analyzed in this paper. The modified matrix crack criterion and the delamination criterion proposed by Hashin and Choi-Chang are utilized in the analysis, and the two damage forms(matrix crack and delamination)can be distinguished in the process of impact. The influence of damage is captured by decreasing the laminate stiffness and releasing freedom degree between coupling nodes. Several numerical examples are worked out, and we can draw the following primary conclusions: 1) The damage model for LVI established in this paper can not only effectively simulate the dynamical response of laminates under low velocity impact, but also deal with the damage (matrix crack and delamination) growth, by decreasing the laminate stiffness and releasing freedom degree between coupling nodes. The analysis results are in accordance with the experimental observations in the literature. 2) There exists an energy threshold of impact for a laminate. The laminate do not suffer any damage unless the impact energy is larger than the energy threshold. The energy threshold of impact and the area of delamination are two indicators representing the anti-impact properties of laminates. FEM Results show that the anti-impact property of FRMLs is superior to FRP. 3) Owing to the fact that aluminium alloy sheets in the FRMLs can absorb majority energy of impact by plastic deformation, the anti-impact property of FRMLs is better than FRP. Comparing with the FRP, the FRMLs with equivalent volume (replace the surface layers of FRP by Al sheets with the same thickness) or with equivalent mass (replace the surface layers of FRP by Al sheets with the same mass) are both superior to FRP in the aspect of anti-impact, and the anti-impact property is improved with the increasing thickness of Al sheets in the FRMLs.