**2.1. Experimental characterisation of impact damage**

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.

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.

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

Due to their brittleness and anisotropy, composite laminates are particularly sensitive to low-velocity impact damage caused by accidental loadings imparted during fabrication or

**2. Impact damage in composite materials**

46 Characterizations of Some Composite Materials

impact energy and to limit the decelerations on the human body.

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 parameters.

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 the laminate and is evaluated as the area under the complete load-displacement curve at penetration [22]. This is a fundamental parameter to be known in order to gather information about the impact energy that causes the loss of material mechanical properties [16, 23].

#### **2.2. Load-displacement curve analysis**

The load-displacement curve recorded during experimental impact tests is a fundamental tool to obtain information about the impact response and behaviour of composite material samples or structures under service conditions. Some characteristic points on the recorded curve are correlated with the evolution of the impact damage inside the material. In correspondence of these points, the first failure load and energy, the maximum load and energy, the absorbed and the penetration energy, were calculated. The influence of the thickness, the laminate stacking sequence, the matrix type and content, the fibre type and orientations and the impact conditions (impactor tup, diameter of sample support and load speed) was clearly evidenced by comparing the load-displacement curves obtained under the different test conditions. The examination of the load-displacement curves evidence that, notwithstanding the differences in thickness, material composition and reinforcement architecture, there are typical features common to all composite laminates subjected to impact testing [24]. **Figure 1** shows a schematic view of a typical load-displacement curve with the characteristic points identified by arrows and letters ("a", "b", "c", "d", "e").

In **Figure 2**, four curves from low-velocity impact tests on carbon fibre reinforced polymer (CFRP) laminates with different thicknesses are overlapped: despite the thickness difference, common features can be clearly noted. Up to point "a", the curve shows no evidence of damage developing inside the material. A different behaviour between thin and thick laminates can be observed due to the increase of the initial laminate rigidity with increasing thickness (**Figure 2**). The thinner laminates display a clear non-linear response for very low displacement

values, due to the larger amount of displacement at low impact force in comparison with the thicker laminates [25]. At the end of the elastic phase, a load drop occurs, the more clearly when the material thickness is sufficiently high (point "a" in **Figure 1**). This behaviour is difficult to appreciate for the lowest thickness where the load remains substantially constant with increasing displacement or a different slope is evidenced. However, in both cases, a local

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The successive load drop is an indication of fibre breakage and/or damage propagation in the form of matrix cracking, delamination, fibre breakage, fibre/matrix debonding and fibre pull out (point "b" on the curve). Matrix cracking in the resin pockets are the first type of damage developed during an impact [25] and the presence of matrix cracks does not affect the overall laminate stiffness [26]. However, matrix cracks represent the initiation point for delamination [4, 21] and fibre breakage which dramatically change the stiffness of the composite laminate [27]. All the energy exceeding the one necessary for these damage initiation phenomena is employed for damage propagation. After the first failure, the load increases again, although the laminate rigidity is reduced. Then, a series of load drops are noted, resulting in oscillations in the force-displacement curve, which correspond to extensive propagation of failures of fibres and in the resin through-the-thickness. In the range from points "b" to "d" (**Figure 1**), the different damages propagate through all the layers, until the complete perforation is achieved (point "d"). The slope of the load-displacement curve begins to rapidly decrease when composite material perforation occurs. The maximum force (point "c") is generally achieved between points "b" and "d", even for the thicker laminates (12 layers or more); point "b" is often found coincident with point "d", which means that the first significant fibre failure frequently occurs

rigidity variation happens, denoting damage in the laminate.

**Figure 2.** Load, F, versus displacement, d, curves for different CFRP laminate thickness, t.

at maximum force [24].

**Figure 1.** Schematic view of the impact load-displacement curve at penetration.

**Figure 2.** Load, F, versus displacement, d, curves for different CFRP laminate thickness, t.

the laminate and is evaluated as the area under the complete load-displacement curve at penetration [22]. This is a fundamental parameter to be known in order to gather information about the impact energy that causes the loss of material mechanical properties [16, 23].

The load-displacement curve recorded during experimental impact tests is a fundamental tool to obtain information about the impact response and behaviour of composite material samples or structures under service conditions. Some characteristic points on the recorded curve are correlated with the evolution of the impact damage inside the material. In correspondence of these points, the first failure load and energy, the maximum load and energy, the absorbed and the penetration energy, were calculated. The influence of the thickness, the laminate stacking sequence, the matrix type and content, the fibre type and orientations and the impact conditions (impactor tup, diameter of sample support and load speed) was clearly evidenced by comparing the load-displacement curves obtained under the different test conditions. The examination of the load-displacement curves evidence that, notwithstanding the differences in thickness, material composition and reinforcement architecture, there are typical features common to all composite laminates subjected to impact testing [24]. **Figure 1** shows a schematic view of a typical load-displacement curve with the characteristic points

In **Figure 2**, four curves from low-velocity impact tests on carbon fibre reinforced polymer (CFRP) laminates with different thicknesses are overlapped: despite the thickness difference, common features can be clearly noted. Up to point "a", the curve shows no evidence of damage developing inside the material. A different behaviour between thin and thick laminates can be observed due to the increase of the initial laminate rigidity with increasing thickness (**Figure 2**). The thinner laminates display a clear non-linear response for very low displacement

**2.2. Load-displacement curve analysis**

48 Characterizations of Some Composite Materials

identified by arrows and letters ("a", "b", "c", "d", "e").

**Figure 1.** Schematic view of the impact load-displacement curve at penetration.

values, due to the larger amount of displacement at low impact force in comparison with the thicker laminates [25]. At the end of the elastic phase, a load drop occurs, the more clearly when the material thickness is sufficiently high (point "a" in **Figure 1**). This behaviour is difficult to appreciate for the lowest thickness where the load remains substantially constant with increasing displacement or a different slope is evidenced. However, in both cases, a local rigidity variation happens, denoting damage in the laminate.

The successive load drop is an indication of fibre breakage and/or damage propagation in the form of matrix cracking, delamination, fibre breakage, fibre/matrix debonding and fibre pull out (point "b" on the curve). Matrix cracking in the resin pockets are the first type of damage developed during an impact [25] and the presence of matrix cracks does not affect the overall laminate stiffness [26]. However, matrix cracks represent the initiation point for delamination [4, 21] and fibre breakage which dramatically change the stiffness of the composite laminate [27]. All the energy exceeding the one necessary for these damage initiation phenomena is employed for damage propagation. After the first failure, the load increases again, although the laminate rigidity is reduced. Then, a series of load drops are noted, resulting in oscillations in the force-displacement curve, which correspond to extensive propagation of failures of fibres and in the resin through-the-thickness. In the range from points "b" to "d" (**Figure 1**), the different damages propagate through all the layers, until the complete perforation is achieved (point "d"). The slope of the load-displacement curve begins to rapidly decrease when composite material perforation occurs. The maximum force (point "c") is generally achieved between points "b" and "d", even for the thicker laminates (12 layers or more); point "b" is often found coincident with point "d", which means that the first significant fibre failure frequently occurs at maximum force [24].

In [5], it was demonstrated that an interaction between matrix cracking and delamination initiation exists. Delamination propagation starting from intralaminar cracks was found mainly in thin laminates [5, 28] where the membrane contribution is important. In **Figure 5**, low (a) and high (b) magnification micrographs of dynamically loaded CFRP samples are reported showing matrix cracks and delamination starting from the cracks in the resin pocket and con-

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As found in several research works by different authors [6, 21], the evolution of damage in a composite laminate subjected to a concentrated dynamic load is driven by intralaminar tensile and shear cracks occurring in the layers farther from and nearer to the contact point. From these cracks, delaminations were found to be generated at interfaces between differently oriented plies, mainly propagating in the direction of the fibres in the lower ply and

**Figure 5.** Low (a) and high (b) magnification micrographs of dynamically loaded CFRP laminate with thickness t = 2 mm.

**Figure 6.** Typical damage zone after impact (back laminate surface). Laminate thickness t = 1.92 mm. Impact energy

extending the more sideways with respect to the contact point.

nected by intralaminar cracks [5].

U = 15.8 J.

**Figure 3.** Fibre failures indicated by the black arrows.

In **Figure 3**, examples of fibre failures are shown. The decrease in contact load between points "d" and "e" corresponds to the penetration process. Finally, beyond point "e", the contact load decreases slowly: the cylindrical body of the impactor slides through the penetrated sample. The penetration energy necessary to completely penetrate the laminate, given by the area under the load-displacement curve at penetration, is conventionally calculated at point "e". Both **Figures 1** and **2** refer to impact test cases where complete perforation occurred. In case of non-perforating impacts, during the loading phase the maximum displacement is reached and then the displacement decreases during unloading (**Figure 4**). After the first load drop (arrows in **Figure 4**), the unloading part is different from the loading one since a fraction of the energy is stored in the material for damage formation.

**Figure 4.** Load-displacement curves for a not penetrated CFRP laminate (t = 3 mm): (a) impact energy level U = 5 J; (b) impact energy level U = 15 J.

In [5], it was demonstrated that an interaction between matrix cracking and delamination initiation exists. Delamination propagation starting from intralaminar cracks was found mainly in thin laminates [5, 28] where the membrane contribution is important. In **Figure 5**, low (a) and high (b) magnification micrographs of dynamically loaded CFRP samples are reported showing matrix cracks and delamination starting from the cracks in the resin pocket and connected by intralaminar cracks [5].

As found in several research works by different authors [6, 21], the evolution of damage in a composite laminate subjected to a concentrated dynamic load is driven by intralaminar tensile and shear cracks occurring in the layers farther from and nearer to the contact point. From these cracks, delaminations were found to be generated at interfaces between differently oriented plies, mainly propagating in the direction of the fibres in the lower ply and extending the more sideways with respect to the contact point.

In **Figure 3**, examples of fibre failures are shown. The decrease in contact load between points "d" and "e" corresponds to the penetration process. Finally, beyond point "e", the contact load decreases slowly: the cylindrical body of the impactor slides through the penetrated sample. The penetration energy necessary to completely penetrate the laminate, given by the area under the load-displacement curve at penetration, is conventionally calculated at point "e". Both **Figures 1** and **2** refer to impact test cases where complete perforation occurred. In case of non-perforating impacts, during the loading phase the maximum displacement is reached and then the displacement decreases during unloading (**Figure 4**). After the first load drop (arrows in **Figure 4**), the unloading part is different from the loading one since a fraction

**Figure 4.** Load-displacement curves for a not penetrated CFRP laminate (t = 3 mm): (a) impact energy level U = 5 J;

of the energy is stored in the material for damage formation.

**Figure 3.** Fibre failures indicated by the black arrows.

50 Characterizations of Some Composite Materials

(b) impact energy level U = 15 J.

**Figure 5.** Low (a) and high (b) magnification micrographs of dynamically loaded CFRP laminate with thickness t = 2 mm.

**Figure 6.** Typical damage zone after impact (back laminate surface). Laminate thickness t = 1.92 mm. Impact energy U = 15.8 J.

A different behaviour is noted for thin and for thick laminates. In thin laminates, bending stresses are more important whereas shear stresses predominate in thick laminates and delaminations without evidence of intralaminar cracks were found at mid-thickness.

*f* = \_\_1

*λ* = \_\_*<sup>v</sup>*

wavelength of the UT waves introduced into it.

*VL* <sup>=</sup> <sup>√</sup>

**Figure 7.** Basic parameters of an UT wave.

The relationship between *v*, *f* and *λ* is given by Eqs. (2) and (3).

UT velocity, *v*, in a perfectly elastic material at a given temperature and pressure is constant.

*λ* = *vT* (3)

In UT NDT, the shorter wavelength resulting from an increase in frequency will usually provide for the capability to detect smaller discontinuities. As a general rule, a discontinuity must be larger than one-half the wavelength in order to be detected.Based on the particle displacement mode, UT waves are classified as longitudinal, shear, surface, and Lamb waves. Longitudinal waves are compressional waves where the particle motion is parallel to the propagation direction of the wave. Shear waves are present when the oscillation direction is perpendicular to the propagation direction. Surface (Rayleigh) waves have an elliptical particle motion and travel across the surface following the profile of the material. Plate (Lamb) waves have a complex vibration occurring in materials where the thickness is less than the

UT propagation velocity in a medium and UT wave attenuation (loss of amplitude and energy)

\_\_\_\_\_\_\_\_\_\_ \_\_\_\_\_\_\_\_\_\_ *E*(1 − *υ*)

depend on the medium itself. In solids, the velocity of longitudinal waves, *VL*

where *E* = Young's modulus; ν = Poisson's ratio; ρ = density of the material.

*<sup>T</sup>* (1)

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*<sup>f</sup>* (2)

*<sup>ρ</sup>*(<sup>1</sup> <sup>+</sup> *<sup>υ</sup>*)(<sup>1</sup> <sup>−</sup> <sup>2</sup>*υ*) (4)

, is given by:

In **Figure 6**, a typical impact damage, visually observed on to the back surface of the impacted laminate, is reported, where the classical visible diamond-shaped delaminated area has attained its maximum size. The delamination axes coincide with the warp-weft fibre directions of the surface fabric layer.
