# Nondestructive Evaluation of Aluminium Foam Panels Subjected to Impact Loading
## 1 Introduction  
Sandwich structures are widely used in the transportation industry (automotive, aerospace, shipbuilding industries) for their lightweight and crashworthiness ability, so it is necessary to assess their energy absorption capabilities [1, 2]. The impact resistance of aluminium honeycomb [3, 4, 5] and foam [6, 7, 8, 9] sandwich panels were investigated by means of low-velocity impact (LVI) tests. The comparison of the responses of different types of sandwich panels is reported in [10]. Composite foam sandwich panels were investigated for railway applications, for rail vehicle car bodies [11] and protective devices subjected to low-velocity impact of ballast projectiles [12], showing that such materials have the potential to enhance the impact strength of components in this field. [PERSON] et al. [13] proposed an innovative Metallic Foam Shell (MFS) protective device against flying ballast impact damage in railway axles. MFS protective device can absorb up to 90% of the initial impact energy with total protection of the axle. Closed-cell aluminium foams (AFs) were chosen by [PERSON] et al. [13] for MFS because they have interesting combinations of properties, such as being lightweight and having high energy absorption capacity. The next step of this research activity was to study the application of the proposed MFS protective device in the offshore structures. [PERSON] et al. [14] compared the anticollision performance of four types of fenders (aluminium, rubber, AF-rubber and rubber-AF) with an offshore wind turbine (OWT) tripod impacted by a ship. They found that AF was the best choice for the fender. The authors observed that both the correct positioning of the AF inside the fender and the optimal thickness of the AF had beneficial effects compared to other materials, i.e., a positive influence on the anticollision protocol, making the tripod less exposed to damage and avoiding the progressive collapse of the ship during secondary disasters. The AF applications in marine structures were described in detail in [15] and [16]. The main applications identified by the authors were helicopter-landing pads, floating navigation signs, components of fast patrol vessels or rescue boats, acoustic/thermal shielding of exhaust and transport containers for hazardous materials. In addition to the mentioned applications, a new foam containing fiberglass was proposed as a potential insulating material for onboard fire doors in shipbuilding [17]. Using the newly developed fiberglass-containing foam, the authors achieved a significant reduction of the thermal bridges for a thinner (37%) and lighter (61%) fire door than a traditional one (with rock-wool). In addition, they noticed that acoustic and mechanical properties increased with the use of this cellular material. As reported in [9; 14], ship and offshore structures may frequently suffer from repeated impact loadings during their service life, which are due to different reasons, such as helicopter wheel landings, ice floe impact, slamming on the bow, green water on the deck, sloshing and jet flow in liquefied natural gas (LNG) tank, collisions between supply vessels and offshore platforms. Consequently, the structural safety of marine structures under repeated impact loads attracted considerable attention [18; 19]. Current studies of AF panels are mainly focused on the dynamic responses under single impact, whereas the dynamic response under repeated impacts was studied only in a few papers. [PERSON] et al. [20] and [PERSON] et al. [21] conducted single and repeated impacts on aluminium foam sandwich (AFS) plates at room (20 \({}^{\circ}\)C) and low (\(-\)60 \({}^{\circ}\)C) temperatures. The authors observed that the single impact deflections of AFS plates at \(-\)60 \({}^{\circ}\)C were smaller compared to those obtained at 20 \({}^{\circ}\)C. They associated this decrease with the increase in AF yield stress at low temperatures.