**1. Introduction**

Low-velocity impact is one of the most subtle threats to composite materials integrity. Due to the weak bonds between the plies, even small energies imparted by out-of-plane loads can result in hardly detectable damages, such as matrix cracks, delamination and fibre breakage, causing considerable stiffness and strength losses in tension and, especially, in compression and severely reducing the material structural integrity. Generally, the main observable

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

damage affecting a laminate subjected to low-velocity impact is delamination, mainly responsible for compression strength decay. For this reason, diverse research works have been devoted to the mechanisms of delamination initiation and growth [1–6]. During impact, more than one delamination in the thickness direction generally develops in a composite laminate, depending on the impact energy and the laminate stacking sequence. Hence, it is crucial to understand the mechanisms of impact damage onset and growth in composite laminates.

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

Non-Destructive Testing of Low-Velocity Impacted Composite Material Laminates…

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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

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

A thorough study of the behaviour of composite laminates subjected to dynamic loads, was carried out by [1–6, 12–14], with the aim to understand the complex mechanisms of damage initiation and propagation under low-velocity impact loading. Many parameters are involved in an impact event and the diverse induced damages, together with their interaction, are very complex to investigate. Moreover, there are instances where impact damage, though seriously present inside the material, is barely visible or not at all visible from the outside.

An extensive experimental testing campaign was carried out on different composite material systems by increasing the initial kinetic energy up to the complete material penetration [16]. This allowed the study of the initiation and the propagation of the complex failure modes related to impact damage. The starting point was the study of the load-deflection curves recorded during impact testing for all the different test conditions. From the curves, the relevant impact parameters were obtained: first failure load and energy, maximum load and energy, absorbed and penetration energy. The influence on the impact parameters, exercised by the composite system, the material constituents, the thickness and the laminate stacking sequence as well as the constraint conditions and the tup diameter were evaluated. Destructive and non-destructive testing were applied to investigate the failure modes, and the observed damage was correlated to the relative energies and the other relevant

Indentation was found to be a function of the impact energy on the basis of the perforation energy. The latter represents the minimum kinetic energy necessary to completely penetrate

by the interlaminar transverse shear stress (σ23) in the layer above the interface.

and residual material properties [2, 9, 12, 16–18].

**2.1. Experimental characterisation of impact damage**

structures.

parameters.

To date, non-destructive testing (NDT) techniques play a fundamental role in diverse industrial areas (such as aerospace, automotive, naval and sporting goods, etc.) for the detection of defects in composite material components in order to ensure their integrity during both the manufacturing phase and the service life [7]. Many types of NDT methods are used for flaw analysis, including ultrasonic inspection, X-ray, acoustography, shearography, acoustic emission, etc. [8].

Ultrasonic testing is the most widely utilised NDT procedure for the detection of flaws in composite materials, allowing the identification and characterisation of internal and external damages without cutting apart or otherwise altering the composite material. The main advantages of UT NDT include [9]: high penetration capacity, which allows to inspect parts of large size; high sensitivity, permitting to detect extremely small defects; only one surface of the part needs to be accessible for UT testing and no hazards exist for the operator or the test materials. The disadvantages of UT NDT comprise: need for expert operators; difficulty in inspecting rough surfaces with irregular or too small shapes; need for a coupling medium between the UT probe and the test part and reference standards are required for both instrument calibration and defect characterisation.In this chapter, the non-destructive characterisation and assessment of lowvelocity impact damage in composite material laminates is investigated through UT inspection. A description of low-velocity impact damage generation and development in composite materials is presented in Section 2. Section 3 gives an overview of the UT testing methods, describing the basic principles, the UT inspection systems, the defect identification capabilities and the UT data representation; moreover, the UT NDT techniques applied to composite materials are illustrated. In the last section, the research studies of the last several years on the detection of defects generated in low-velocity impacted composite materials are presented and discussed.
