**3.2. UT inspection system**

The basic equipment of an UT inspection system consists of diverse functional units: pulser/ receiver, transducer and display devices. A pulser/receiver is an electronic device generating short, high amplitude electric pulses which are converted by the transducer into highfrequency UT energy. The sound energy is introduced into the test material and propagates through the material in the form of UT waves. If there is a discontinuity (e.g. a crack) in the UT wave path, part of the energy is reflected back from the flaw surface. The reflected UT wave signal reaches the transducer which transforms it into an electrical signal that can be recorded and/or displayed on a screen [36].

The control functions associated with the pulser circuit include the pulse length or damping and the pulse energy, whereas the control functions in the receiver phase are related to the refinement, filtering and amplification of the return signals.

Selection of the appropriate UT transducer is the first significant step to be considered for UT inspection of a part. Two main categories of transducer are available: contact and immersion transducers. The first category refers to transducers utilised for direct contact inspections which are generally hand manipulated by a skilled operator. Diverse contact transducers are commercially available and their selection depends on the characteristics of the contact surface and the thickness of the part as well as on the aims of the UT inspection. The most common contact transducers are: flat contact, dual element and angle-beam transducers. Immersion transducers are designed to operate in a liquid environment and consequently are typically utilised inside a water tank or as part of a squirter system for UT NDT scanning applications. These transducers can be equipped with cylindrically or spherically focused lens. A focused transducer has the property to concentrate the sound energy onto a small area in order to improve sensitivity and axial resolution.

Two basic quantities are measured in UT testing: the time-of-flight (TOF) corresponding to the amount of time for the sound to travel through the sample, and the amplitude of the received signal. Based on velocity and round trip time-of-flight through the material, the material thickness, *S*, can be calculated as follows:

$$\mathbf{S} = \frac{v \, t\_s}{2} \tag{6}$$

where *v* = material sound velocity; *t <sup>s</sup>* = time-of-flight.

The speed of transverse (or shear) waves, *VT*, depends on the shear deformation under shear stress (shear modulus) and the density of the medium, defined by the following formula:

In isotropic materials, the elastic constants are the same for all directions within the material. However, most materials are anisotropic and the elastic constants differ with each direction. ASTM E494 - 15: "Standard Practice for Measuring Ultrasonic Velocity in Materials" covers a test procedure for measuring UT velocity in materials with conventional UT pulse-echo flaw detection equipment. In this practice, tables with longitudinal and shear velocities are

UT attenuation is the decay rate of the UT wave as it propagates through a material. It is mainly due to absorption (conversion of sound energy into other forms of energy) and scattering (reflection of sound in directions other than the original propagation direction) phenomena. The amount of attenuation through a material is a critical parameter for the selection of

The basic equipment of an UT inspection system consists of diverse functional units: pulser/ receiver, transducer and display devices. A pulser/receiver is an electronic device generating short, high amplitude electric pulses which are converted by the transducer into highfrequency UT energy. The sound energy is introduced into the test material and propagates through the material in the form of UT waves. If there is a discontinuity (e.g. a crack) in the UT wave path, part of the energy is reflected back from the flaw surface. The reflected UT wave signal reaches the transducer which transforms it into an electrical signal that can be recorded

The control functions associated with the pulser circuit include the pulse length or damping and the pulse energy, whereas the control functions in the receiver phase are related to the

Selection of the appropriate UT transducer is the first significant step to be considered for UT inspection of a part. Two main categories of transducer are available: contact and immersion transducers. The first category refers to transducers utilised for direct contact inspections which are generally hand manipulated by a skilled operator. Diverse contact transducers are commercially available and their selection depends on the characteristics of the contact surface and the thickness of the part as well as on the aims of the UT inspection. The most common contact transducers are: flat contact, dual element and angle-beam transducers. Immersion transducers are designed to operate in a liquid environment and consequently are typically utilised inside a water tank or as part of a squirter system for UT NDT scanning applications. These transducers can be equipped with cylindrically or spherically focused lens. A focused transducer has the property to concentrate the sound energy onto a small area in order to improve sensitivity and axial resolution.

\_\_ \_\_ *G*

*<sup>ρ</sup>* (5)

*VT* <sup>=</sup> <sup>√</sup>

reported for metal and ceramic materials [35].

the appropriate UT transducer for an application.

refinement, filtering and amplification of the return signals.

**3.2. UT inspection system**

and/or displayed on a screen [36].

where *G* = shear modulus of elasticity.

54 Characterizations of Some Composite Materials

Measurements of the relative change in UT signal amplitude can be used for sizing flaws or measuring the material attenuation properties.

### **3.3. Variables in UT inspection for defect detection**

The major variables to be considered in UT NDT include the characteristics of the utilised UT waves and the proprieties of the part being inspected. UT equipment type and capability interact with these variables; often, different types of equipment need be selected to accomplish different inspection objectives. Generally, a compromise must be made between favourable and adverse effects to achieve an optimum balance and to overcome the limitations imposed by equipment and test material [37].

The frequency of the utilised UT waves affects the inspection capability in several ways:


Sensitivity, resolution, penetration and beam spread are largely determined by the selection of the transducer and are only slightly modified by changes in other test variables.

#### **3.4. UT inspection methods and data representation**

A first difference between UT inspection techniques can be made with reference to the transducer or probe position [34, 36, 37]:

• Contact technique, where the probe is placed directly on the surface of the part to be examined.

• A-scan. It provides a quantitative display of UT signal amplitudes (y axis) and time-offlight information (x axis) obtained by UT material interrogation at a single point on the part surface. The A-scan can be used to analyse the type, size and location (chiefly depth) of flaws. A discontinuity in the material is indicated by a peak (echo) the distance of which from the zero of the time axis is proportional to the path that the UT beam performs before encountering the discontinuity itself. The amplitude of this defect peak is proportional to

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• B-scan. This format provides a quantitative display of time-of-flight data reported along the y axis obtained during a linear scan (x axis) of the part. A B-scan provides information about the part thickness and the depth of a defect for a single plane that normally intersects

• C-scan. A semi-quantitative or quantitative display of UT signal amplitudes obtained over an area of the part surface is represented using a C-scan. The information can be used to map out the position of flaws in an UT image representing the plan view of the part. A C-scan format also records time-of-flight data, which can be converted and displayed by

• D-scan. It is similar to a C-scan, but in this case the time-of-flight data obtained over an area of the part is utilised for UT image generation instead of the signal amplitude data.

• FV-scan. Full volume scan (FV-scan), or volumetric scan, is based on the detection and storage of the entire UT waveform in the propagation direction (z-direction) during x-y scanning of the part surface. FV-scan provides for the 2½ D representation of the material internal structure, based on the generation of C-scans at any depth along the z-axis for any

Due to the non-homogeneous and anisotropic nature of composites materials, the frequency range utilised in UT NDT of composites is markedly reduced due to the high damping and attenuation of the high-frequency signals. Usually, the employed frequency in industrial applications is 5 MHz or less, limiting the possibility to detect small flaws. The typical defects present in composite materials are: delamination, cracks, fibre-matrix debonding and fibres fractures [6, 12–15]. Delamination is probably the most investigated failure mode in composite material laminates [1, 4, 5, 16]. During UT NDT of a composite part, the presence of an extended delamination corresponds to a UT waveform with a reduction of the back echo amplitude together with the appearance of a defect echo located at the delamination depth. Other smaller defects such as voids and inclusions cause a loss of the UT back echo amplitude and/or can be weakly reflected [38, 39]. Flaws (e.g. delamination) lying parallel to the surface of the part subjected to UT inspection can be easily detected utilising normal incidence probes, whereas defects (e.g. cracks and fibre fractures) lying perpendicular to the surface are difficult to detect due to their small reflecting surface (this problem can be solved using angle-beam transducers) [40].

By employing UT through-transmission or pulse-echo techniques, it is possible to locate and size the defects based on the measurements of UT signal amplitude and/or time-of-flight.

image processing techniques to provide information on flaw depth.

the acoustic energy reflected by the discontinuity.

the part arranged along the scan direction.

portion of the material thickness.

**3.5. UT inspection of composite materials**

• Immersion technique, where the probe is immersed in a liquid substance that separates it from the part surface.

The main operating techniques of UT NDT are the through-transmission method and the pulse-echo (or reflection) method.

In the through-transmission technique, two probes, positioned at opposite sides with respect to the part, are used: one probe transmits the UT beam into the part and the other probe receives it. A defect, reflecting a part of the incident beam, causes a decrease in the UT energy detected by the receiving probe. The presence of the defect is highlighted by comparing the received signal with a reference signal obtained from a standard, flaw-less sample. In this technique, two opposite surfaces of the part under examination must be accessible to the transducers.

The pulse-echo technique is based on the property of the UT beam to be reflected whenever it encounters a discontinuity or a defect in its path. The amount of reflected energy highly depends on the reflecting surface size, that is, on the dimensions of the encountered discontinuity perpendicularly to the UT beam propagation direction. To perform the test, it is sufficient that only one surface of the part is accessible, since a single probe is used to send the incident UT beam and, at the same time, receive the reflected UT signal. In **Figure 8**, the typical UT waveform generated during UT pulse-echo inspection of a defective part is shown. The UT waveform enters the material and a first echo, called interface or front echo, is visualised. The back echo corresponds to the final (last) surface of the part under examination. If a discontinuity is encountered inside the material, a defect echo is visualised between the front and the back echoes.

Pulse-echo UT inspection can be accomplished with longitudinal, shear, surface or Lamb waves. Straight-beam or angle-beam techniques can be used, depending on the part shape and the inspection objectives. The detected UT data can be analysed to obtain the required information on defect characteristics, such as type, size, location and orientation.

Diverse representations of UT data are available. The most common formats utilised are: A-scan, B-scan, C-scan, D-scan and FV-scan [8, 9, 34, 36, 37].

**Figure 8.** UT waveform generated during UT pulse-echo inspection of a defective part.


#### **3.5. UT inspection of composite materials**

**Figure 8.** UT waveform generated during UT pulse-echo inspection of a defective part.

A-scan, B-scan, C-scan, D-scan and FV-scan [8, 9, 34, 36, 37].

• Contact technique, where the probe is placed directly on the surface of the part to be

• Immersion technique, where the probe is immersed in a liquid substance that separates it

The main operating techniques of UT NDT are the through-transmission method and the

In the through-transmission technique, two probes, positioned at opposite sides with respect to the part, are used: one probe transmits the UT beam into the part and the other probe receives it. A defect, reflecting a part of the incident beam, causes a decrease in the UT energy detected by the receiving probe. The presence of the defect is highlighted by comparing the received signal with a reference signal obtained from a standard, flaw-less sample. In this technique, two opposite surfaces of the part under examination must be accessible to the

The pulse-echo technique is based on the property of the UT beam to be reflected whenever it encounters a discontinuity or a defect in its path. The amount of reflected energy highly depends on the reflecting surface size, that is, on the dimensions of the encountered discontinuity perpendicularly to the UT beam propagation direction. To perform the test, it is sufficient that only one surface of the part is accessible, since a single probe is used to send the incident UT beam and, at the same time, receive the reflected UT signal. In **Figure 8**, the typical UT waveform generated during UT pulse-echo inspection of a defective part is shown. The UT waveform enters the material and a first echo, called interface or front echo, is visualised. The back echo corresponds to the final (last) surface of the part under examination. If a discontinuity is encountered inside the material, a defect echo is visualised between the front

Pulse-echo UT inspection can be accomplished with longitudinal, shear, surface or Lamb waves. Straight-beam or angle-beam techniques can be used, depending on the part shape and the inspection objectives. The detected UT data can be analysed to obtain the required

Diverse representations of UT data are available. The most common formats utilised are:

information on defect characteristics, such as type, size, location and orientation.

examined.

transducers.

and the back echoes.

from the part surface.

pulse-echo (or reflection) method.

56 Characterizations of Some Composite Materials

Due to the non-homogeneous and anisotropic nature of composites materials, the frequency range utilised in UT NDT of composites is markedly reduced due to the high damping and attenuation of the high-frequency signals. Usually, the employed frequency in industrial applications is 5 MHz or less, limiting the possibility to detect small flaws. The typical defects present in composite materials are: delamination, cracks, fibre-matrix debonding and fibres fractures [6, 12–15]. Delamination is probably the most investigated failure mode in composite material laminates [1, 4, 5, 16]. During UT NDT of a composite part, the presence of an extended delamination corresponds to a UT waveform with a reduction of the back echo amplitude together with the appearance of a defect echo located at the delamination depth. Other smaller defects such as voids and inclusions cause a loss of the UT back echo amplitude and/or can be weakly reflected [38, 39]. Flaws (e.g. delamination) lying parallel to the surface of the part subjected to UT inspection can be easily detected utilising normal incidence probes, whereas defects (e.g. cracks and fibre fractures) lying perpendicular to the surface are difficult to detect due to their small reflecting surface (this problem can be solved using angle-beam transducers) [40].

By employing UT through-transmission or pulse-echo techniques, it is possible to locate and size the defects based on the measurements of UT signal amplitude and/or time-of-flight. 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 determining the time spacing between them.

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

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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

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

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 [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

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 imag-

ing of the internal materials structure with resolution higher than 1 ply.

area was validated by the results obtained with the two NDT techniques.

in the same composite laminates using through-transmission UT measurements.

laminates subjected to low-velocity impulsive loads.

damage is lacking.

coupling medium.

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).
