# Delamination Assessment in Composite Laminates through Local Impulse Excitation Technique (IET)
## 1 Introduction  
In recent decades, there has been an increasing demand for composite materials in the industrial landscape. Composite materials satisfy the need for lightweight yet highly performing materials with fiber-reinforced materials, which have demonstrated abilities to have a flexible design and be used in almost every type of industrial application. Also, composites are characterized by enhanced damage tolerance with respect to standard metal alloys. However, the wide application of composite materials still has limitations due to the lack of proper strategies for damage quantification. Due to their mixed compositions and the large difference between the fiber and matrix properties, composite materials present several damage mechanisms, each affecting a specific scale, such as fiber breakage and matrix cracking at the micro-scale or delamination at the laminate scale, which demand accurate and local investigations. As the damage determines a degradation in macro-scale material properties, several nondestructive techniques (NDTs) have focused on the assessment of the damage and its severity by investigating the material's macro-scale response. Among these, we can mention ultrasonics [1, 2], acousto-ultrasonics [3, 4], acousto-ultrasonics in combination with neural network algorithms [5, 6], the Detecting Damage Index [7], thermography [8, 9, 10], resistivity-based measurements [11, 12, 13, 14] and optical methods, such as Shearography and Digital Image Correlation [15, 16]. The reader can refer to [17] for details on the advantages and limitations of each NDT. Although theyprovide quantitative information on the damage severity as a degradation of the elastic, thermal, or electrical response of the material, a main drawback of these methodologies is the cost related to the complex equipment needed for each technique. Furthermore, it is usually necessary to transport the component to a specific location for nondestructive tests, which further increases the costs. The authors have recently proposed an NDT based on the Impulse Excitation Technique, which aims to determine the local variation in the elastic properties of the damaged region of a composite component (local-IET) [18; 19; 20]. By reversibly clamping its extremities through a vacuum, the vibrational response of the inspected region is isolated, with the measured resonant frequencies only being a function of the elastic characteristics of the investigated area. This allows for the sensitivity of the vibrational test to be enhanced in the presence of local damage [20]. Furthermore, from the local vibrational response, the local variation in the elastic properties can be assessed, and the damage severity is quantified in terms of the degradation of the elastic properties. Since it is a vibrational test, the equipment of the local-IET has a relatively low cost, and the methodology allows for on-site analyses of composite components with a significant cost reduction [17; 18; 20]. This paper aims to investigate the sensitivity of a local-IET in the presence of delamination. Delamination, i.e., the debonding between adjacent laminae, is among the most critical damage mechanisms of a composite as it prevents the transfer of forces and moments between plies, thus affecting the strength of the laminate. For example, under compressive loads, buckling instability is likely to occur if the delaminated ply has a free surface on one side [21]. Delamination can occur both during the manufacturing process, as contaminations due to dirt cause poor bonding between laminae [22; 23], and during the in-service life of the component. Under fatigue loads, delamination occurs as matrix cracks lengthen with the cycles and interact with each other. Under lateral loading, as in the case of impact loads, delamination is caused either by interlaminar stresses due to bending or by matrix cracks in proximity of plies interfaces [24]. As such, the proper identification of the presence of delamination within a composite component is crucial for a health state assessment.