**2. Impact damage in composite materials**

By considering that for many composite materials applications, such as body panels of cars, trucks, rail vehicles and aircraft fuselage, the designer of the composite structure must ensure the prevention of penetration by foreign objects of known mass and velocity. Accordingly, the knowledge of penetration energy becomes a critical issue. Moreover, the absorbed energy is a fundamental parameter in impact situations where it is necessary that the mechanical shock is not transferred to the human body, such as in motorcycle helmets and race car frames, with the aim to ensure the driver's safety in case of high-speed crashes. Accordingly, for these applications, laminated composites must be designed to absorb as much as possible the impact energy and to limit the decelerations on the human body.

Due to their brittleness and anisotropy, composite laminates are particularly sensitive to low-velocity impact damage caused by accidental loadings imparted during fabrication or service. This has led to numerous studies concerning impact dynamics [10–12], mechanisms of failure initiation and propagation [12–15] and correlation between impact energy, damage and residual material properties [2, 9, 12, 16–18].

Delamination is the most important and crucial damage caused by dynamic loading conditions. Matrix cracking consists in cracks that develop in the resin rich areas between two adjacent composite layers. It has been observed that delamination occurs when a threshold energy is reached in presence of matrix cracking [19]. Even if there is a common agreement on the mechanisms of initiation and growth of this failure mode during an impact event, and several research studies are devoted to this topic [15, 20], a general approach to predict the damage mechanisms and interaction in order to prevent catastrophic failures, is absent. The complexity of the stresses in the vicinity of the point of impact complicates the analysis. In [21], it was shown that delamination growth is governed by interlaminar longitudinal shear stress (σ13) and transverse in-plane stress (σ22) in the layer below the delaminated interface and by the interlaminar transverse shear stress (σ23) in the layer above the interface.

A critical aspect of impact damage is the fact that it is difficult to detect by visual inspection: a composite structure can be severely damaged without any external sign. The only external indication of an impact is indentation, that is, the plastic deformation of the laminate surface due to the contact, left by the impactor during the loading phase. This has led to the concept of "barely visible impact damage", usually adopted in the design of aeronautical structures.
