**4. Applications**

In the last several years, numerous studies were carried out on the application of UT NDT for defect detection in low-velocity impacted composite material laminates.

In 1998, the estimation of impact induced damage under low-velocity impact (impact energy: from 3 to 30 J) in carbon fibre reinforced polymer (CFRP) laminates was investigated in [41] through UT C-scans using the pulse-echo immersion method. Delamination areas were accurately quantified by processing the UT image data and the correlation between impact energy and delamination extension was established.

In [42], an UT NDT system for delamination evaluation in CFRP, glass fibre reinforced plastic (GFRP) and aramid fibre reinforced plastic (AFRP) laminates subjected to low-velocity impact tests (impact energy: 2, 3, 5 J) is described. The UT NDT analysis was performed using two different probes (5 and 15 MHz) to evaluate the influence of frequency on the reliable evaluation of delamination in these composites. The results confirmed the NDT system capabilities in terms of damage detection, location and evaluation.

In [40], the authors demonstrated that a combination of normal and oblique incidence pulseecho UT techniques provide highly detailed volumetric images of the damage (matrix cracks and delaminations) induced in composite laminates by low-velocity and low-energy impacts. The tested specimens (quasi-isotropic carbon/polyetheretherketone (PEEK) laminates) were immersed in water and scanned at normal (to detect delaminations) and oblique (to identify matrix cracks) incidence using a focussed broadband transducer (3.2 mm diameter, 18 mm focal length) with a centre frequency of 22 MHz.

A comparative analysis of two different NDT techniques, UT air-coupled C-scan and X-ray radiography, applied to thin carbon/epoxy composite laminates, utilised in naval structures, for the detection of low-energy impact damage was carried out by [43]. The damage area was identified by the two NDT techniques but the UT inspection provided for an easier, faster and more accurate damage characterisation.

In [44], the response of CFRP laminates with different stacking sequences (unidirectional, crossply, quasi-isotropic and woven laminates) at low impact velocity and under low-temperature conditions was examined. Low-velocity impact tests at different temperatures were carried out using an impact energy range from 1 to 13 J. After the impact tests, the damage extension was measured by UT C-scan inspection and the damage mechanisms were studied by optical and scanning electron microscopy. The results showed the influence of temperature, ply reinforcement architecture and stacking sequence on the mechanical behaviour of the CFRP laminates subjected to low-velocity impulsive loads.

The pulse-echo technique allows to characterise the matrix material proprieties (volume fraction, moisture content and porosity) of a composite by evaluating the UT velocity and/or attenuation. Knowing the composite thickness, the attenuation coefficient can be evaluated by measuring the amplitude reduction of the multiple back echoes, and the UT velocity by

A limitation of UT inspection consists of the difficulty to identify defects located very close to the front surface of the part (known as "dead zone") where the pulse length is approximately equal to the time period. This problem can be limited by using shorter pulses or immersion testing procedures. The anisotropic and inhomogeneous properties of composite laminates cause high attenuation of the UT waves, internal UT reflections and UT velocity variations due to the presence of different materials (fibres and matrix) and interfaces (fibre-matrix and inter-ply interfaces).

In the last several years, numerous studies were carried out on the application of UT NDT for

In 1998, the estimation of impact induced damage under low-velocity impact (impact energy: from 3 to 30 J) in carbon fibre reinforced polymer (CFRP) laminates was investigated in [41] through UT C-scans using the pulse-echo immersion method. Delamination areas were accurately quantified by processing the UT image data and the correlation between impact energy

In [42], an UT NDT system for delamination evaluation in CFRP, glass fibre reinforced plastic (GFRP) and aramid fibre reinforced plastic (AFRP) laminates subjected to low-velocity impact tests (impact energy: 2, 3, 5 J) is described. The UT NDT analysis was performed using two different probes (5 and 15 MHz) to evaluate the influence of frequency on the reliable evaluation of delamination in these composites. The results confirmed the NDT system capabilities

In [40], the authors demonstrated that a combination of normal and oblique incidence pulseecho UT techniques provide highly detailed volumetric images of the damage (matrix cracks and delaminations) induced in composite laminates by low-velocity and low-energy impacts. The tested specimens (quasi-isotropic carbon/polyetheretherketone (PEEK) laminates) were immersed in water and scanned at normal (to detect delaminations) and oblique (to identify matrix cracks) incidence using a focussed broadband transducer (3.2 mm diameter, 18 mm

A comparative analysis of two different NDT techniques, UT air-coupled C-scan and X-ray radiography, applied to thin carbon/epoxy composite laminates, utilised in naval structures, for the detection of low-energy impact damage was carried out by [43]. The damage area was identified by the two NDT techniques but the UT inspection provided for an easier, faster and

In [44], the response of CFRP laminates with different stacking sequences (unidirectional, crossply, quasi-isotropic and woven laminates) at low impact velocity and under low-temperature

defect detection in low-velocity impacted composite material laminates.

determining the time spacing between them.

58 Characterizations of Some Composite Materials

and delamination extension was established.

in terms of damage detection, location and evaluation.

focal length) with a centre frequency of 22 MHz.

more accurate damage characterisation.

**4. Applications**

A multi-functional non-linear UT testing approach was presented in [45] for in-situ and exsitu detection of diverse defects (micro-cracking, delamination and disbonding) generated by different damage inducing loads (stress, impact and heat) in CFRP materials and structures for aeronautical applications. The impact tests were conducted using several impact loadings ranging from 4 to 69 J impact energy. The applied UT methodology proved to be a useful tool for the identification of damage for impact energy below 30 J where the visual evidence of damage is lacking.

The effect of temperature on low-velocity impact resistance properties and post-impact flexural performance of CFRP laminates was studied in [46] using UT C-scan and micro-focus X-ray computed tomography. A correlation between the impact temperature and the damage area was validated by the results obtained with the two NDT techniques.

A sparse digital signal model was presented in [47] as an efficient model for the estimation of UT measurements obtained from multi-layered composites. A CFRP laminate with stacking sequence [0/90]4S was impacted in a drop weight tower with 3.8 J impact energy. The laminate was excited by a low-frequency UT sine-burst with central frequency 5 MHz. The UT response signals were utilised for the validation of the developed digital signal model in order to obtain the damage identification. In [48], a multi-level Bayesian method was utilised to identify the through-the-thickness position and the effective mechanical properties of the damaged layers in the same composite laminates using through-transmission UT measurements.

In [49], the authors experimentally tested three composite structures with barely visible impact (BVI) damage and delaminations, using different NDT techniques including UT scanning, piezoelectric sensing, thermography and vibration-based inspection in order to analyse their applicability in the environmental conditions of aircraft elements inspection. The applied UT technique provided a detailed damage evaluation in terms of damage depth, size and location.

Infrared thermography and phased array UT techniques were employed in [50] to detect the impact damage in CFRP composites. Three values of impact energy (18, 29 and 39 J) were chosen for the tests. Both NDT methods presented advantages and limitations. Thermography is fast in detecting the impact damage over large panels, but it is affected by loss of contrast in case of deep defects. The UT technique is more effective in the estimation of thickness and in the inspection of thick parts, but it can be applied only over smooth surfaces and requires a coupling medium.

A laser-ultrasound (LU) scanner was used in [51] to obtain high-quality images of damage in CFRP composites subjected to low-velocity impact with energies 25 and 50 J. X-ray tomograms were also carried out for comparison with the results of the LU study. The high-speed and high-resolution LU scanning method proved to be efficient for in-situ non-contact imaging of the internal materials structure with resolution higher than 1 ply.

In [52], the response to repeated low-velocity impacts was studied for two types of hybrid laminates made of metal and composite layers specifically designed for aircraft structural applications. The damage was evaluated using visual inspection and UT C-scan procedures. Three categories of impact damage were observed: visible deformation without internal or external damage, visible internal damage (C-scan) without external damage and visible internal and external damages.

An UT technique was used in [53] to investigate the delamination caused by low-velocity impact tests on poly(lactic acid)/jute woven fabric composite laminates obtained by conventional film stacking and compression moulding techniques. Square specimens, 100 × 100 mm, were impacted in a falling dart test machine using 5 impact energy values: 2, 5, 10, 12 and 15 J. Delamination damage was evaluated through an UT technique employing a linear phased array probe. The delaminated area was correlated with both the impact energy and the measured indentation depth. The results allowed to identify a threshold energy value beyond which internal damage was detected. Moreover, a linear relationship between delaminated area, energy and indentation depth was found.

A delamination prediction method for composite laminates, utilised for application in unmanned aerial vehicles, subjected to low-energy impact was presented in [54]. UT C-scan tests were carried out with UT beam propagation direction from the bottom laminate surface to the top laminate surface that received the impact. Numerical models were built to simulate the delamination behaviour of the composite laminates, showing a good correlation with the experimental UT results. Delamination prediction can contribute to the evaluation of composite residual strength and the optimization of aircraft structures.

In [55], an UT NDT system was utilised to carry out the metrological characterisation of quadriaxial non-crimp fabric (NCF) CFRP composite laminates subjected to low-velocity impact. The scopes of the UT inspection were thickness estimation, stacking sequence and fibre orientation verification, and composite quality assessment in terms of impact damage development within the whole material volume. The same UT NDT system was considered in [33, 55, 56] for diverse UT testing procedures. **Figure 9** illustrates the specially designed hardware and custom-made software of the UT system operating as follows: the UT oscillator/ detector excites the piezoelectric immersion UT probe which is displaced by a 6-axis robotic arm. After interacting with the tested material, the reflected UT pulses return to the oscillator/ detector which forwards them to a digital oscilloscope for visualisation and digitisation of the UT waveforms. The digitised UT waveforms are then transferred to a PC where a custommade software code provides for UT waveform signal storage and analysis.

Low-velocity impact tests were performed on rectangular composite specimens under a falling weight machine using a cylindrical indenter with hemispherical nose at different impact energies: 9, 12, 16, 20, 25, 30 and 40 J.

drop weight low-velocity impact tests with energy 9, 20 and 40 J, respectively. Each of the four images represents the internal structure of 1/4 (i.e. 1 mm) of the NCF laminate thickness starting from the upper surface (first image on the left) down to the opposite lower surface (last image on the right). In particular, in every figure, image (a) represents the surface damage, images (b) and (c) the internal damage and image (d) the in-plane projection of the total

**Figure 10.** Four UT images for low-energy (9 J) impacted NCF laminate. Each image reports the internal structure of

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**Figure 9.** Specially designed UT NDT system.

1 mm thickness from upper (a) to lower laminate surface (d).

After impact testing, pulse-echo immersion FV-UT scanning was carried out on the impacted specimens with a focused high-frequency transducer (15 MHz) over a 110 × 155 mm area with scan step 1 mm. The delaminated area was measured through UT image processing. In **Figures 10**–**12**, four UT images of the impacted quadriaxial laminates are reported for Non-Destructive Testing of Low-Velocity Impacted Composite Material Laminates… http://dx.doi.org/10.5772/intechopen.80573 61

**Figure 9.** Specially designed UT NDT system.

In [52], the response to repeated low-velocity impacts was studied for two types of hybrid laminates made of metal and composite layers specifically designed for aircraft structural applications. The damage was evaluated using visual inspection and UT C-scan procedures. Three categories of impact damage were observed: visible deformation without internal or external damage, visible internal damage (C-scan) without external damage and visible inter-

An UT technique was used in [53] to investigate the delamination caused by low-velocity impact tests on poly(lactic acid)/jute woven fabric composite laminates obtained by conventional film stacking and compression moulding techniques. Square specimens, 100 × 100 mm, were impacted in a falling dart test machine using 5 impact energy values: 2, 5, 10, 12 and 15 J. Delamination damage was evaluated through an UT technique employing a linear phased array probe. The delaminated area was correlated with both the impact energy and the measured indentation depth. The results allowed to identify a threshold energy value beyond which internal damage was detected. Moreover, a linear relationship between delaminated

A delamination prediction method for composite laminates, utilised for application in unmanned aerial vehicles, subjected to low-energy impact was presented in [54]. UT C-scan tests were carried out with UT beam propagation direction from the bottom laminate surface to the top laminate surface that received the impact. Numerical models were built to simulate the delamination behaviour of the composite laminates, showing a good correlation with the experimental UT results. Delamination prediction can contribute to the evaluation of compos-

In [55], an UT NDT system was utilised to carry out the metrological characterisation of quadriaxial non-crimp fabric (NCF) CFRP composite laminates subjected to low-velocity impact. The scopes of the UT inspection were thickness estimation, stacking sequence and fibre orientation verification, and composite quality assessment in terms of impact damage development within the whole material volume. The same UT NDT system was considered in [33, 55, 56] for diverse UT testing procedures. **Figure 9** illustrates the specially designed hardware and custom-made software of the UT system operating as follows: the UT oscillator/ detector excites the piezoelectric immersion UT probe which is displaced by a 6-axis robotic arm. After interacting with the tested material, the reflected UT pulses return to the oscillator/ detector which forwards them to a digital oscilloscope for visualisation and digitisation of the UT waveforms. The digitised UT waveforms are then transferred to a PC where a custom-

Low-velocity impact tests were performed on rectangular composite specimens under a falling weight machine using a cylindrical indenter with hemispherical nose at different impact

After impact testing, pulse-echo immersion FV-UT scanning was carried out on the impacted specimens with a focused high-frequency transducer (15 MHz) over a 110 × 155 mm area with scan step 1 mm. The delaminated area was measured through UT image processing. In **Figures 10**–**12**, four UT images of the impacted quadriaxial laminates are reported for

nal and external damages.

60 Characterizations of Some Composite Materials

area, energy and indentation depth was found.

energies: 9, 12, 16, 20, 25, 30 and 40 J.

ite residual strength and the optimization of aircraft structures.

made software code provides for UT waveform signal storage and analysis.

**Figure 10.** Four UT images for low-energy (9 J) impacted NCF laminate. Each image reports the internal structure of 1 mm thickness from upper (a) to lower laminate surface (d).

drop weight low-velocity impact tests with energy 9, 20 and 40 J, respectively. Each of the four images represents the internal structure of 1/4 (i.e. 1 mm) of the NCF laminate thickness starting from the upper surface (first image on the left) down to the opposite lower surface (last image on the right). In particular, in every figure, image (a) represents the surface damage, images (b) and (c) the internal damage and image (d) the in-plane projection of the total

materials were illustrated and compared in terms of accuracy, resolution and performance. Applications were presented and discussed for industrial areas where composite materials

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1 Fraunhofer Joint Laboratory of Excellence on Advanced Production Technology

2 Department of Industrial Engineering, University of Naples Federico II, Naples, Italy

3 Department of Chemical, Materials and Industrial Production Engineering, University of

[1] Hong S, Liu D. On the relationship between impact energy and delamination area.

[2] Cairns DS, Minuet PJ, Abdallah MG. Theoretical and experimental response of composite laminates with delaminations loaded in compression. Composite Structures.

[3] Choi HY, Chang FK. A model for predicting damage in graphite/epoxy laminated composites resulting from low-velocity point impact. Journal of Composite Materials.

[4] Liu S, Kutlu Z, Chang FK. Matrix cracking and delamination propagation in laminated composites subjected to transversely concentrated loading. Journal of Composite

[5] Stout MG, Koss DA, Liu C, Idasetima J. Damage development in carbon/epoxy laminates under quasi-static and dynamic loading. Composites Science and Technology.

[6] Liu S. Quasi impact damage initiation and growth of thick-section and toughened composite materials. International Journal of Solids and Structures. 1994;**31**(22):3079-3098 [7] Castellano A, Fraddosio A, Piccioni MD. Ultrasonic goniometric immersion tests for the characterization of fatigue post-LVI damage induced anisotropy superimposed to the constitutive anisotropy of polymer composites. Composites Part B: Engineering.

usage is highly relevant.

Tiziana Segreto1,2\*, Roberto Teti1,3 and Valentina Lopresto3

\*Address all correspondence to: tsegreto@unina.it

Experimental Mechanics. 1989:115-120

(Fh-J\_LEAPT UniNaples), Naples, Italy

Naples Federico II, Naples, Italy

1993;**25**:113-120

1992;**26**(14):2134

1999;**59**:2339-2350

2017;**116**:122-136

Materials. 1993;**27**(5):436

**Author details**

**References**

**Figure 11.** Four UT images for medium energy (20 J) impacted NCF laminate. Each image reports the internal structure of 1 mm thickness from upper (a) to lower laminate surface (d).

**Figure 12.** Four UT images for high energy (40 J) impacted NCF laminate. Each image reports the internal structure of 1 mm thickness from upper (a) to lower laminate surface (d).

internal damage. The analysis of the UT images shows that: (i) the impact damage develops differently at interfaces between layers characterised by diverse fibre orientations; (ii) the delamination area increases with rising distance (depth) from the impact surface as well as with growing impact energy and (iii) the delamination outline exhibits the well-known hatshaped configuration [20]. The UT analysis also reveals the absence of delamination in a small zone directly below the impact surface contact point.
