**3.1. Basic principles**

UT NDT is based on the measurement of the energy variations associated with mechanical waves, with frequencies ranging between 50 kHz and 25 MHz, generated by a piezoelectric transducer. The UT beams are introduced into the material by a coupling medium (oil, grease and water) and the variations of the reflected and/or transmitted UT energy are used to identify defects within the material which represent discontinuities in the UT path. When an atomic or molecular particle is displaced from its equilibrium position due to UT waves propagation in the material, the internal (interatomic or intermolecular) forces tend to bring it back to its original position. The displacement of a particle causes the dislocation of those placed in the neighbourhood, and thus the propagation of the UT waves in all the material is determined [8, 34].

In **Figure 7**, the basic parameters of a continuous UT wave are shown. The distance between two consecutive peaks of an UT wave is the wavelength, *λ*, while the number of UT oscillations per unit time is the frequency, *f*. The time required to complete a full cycle is the period *T*. The relation between frequency and period in a continuous wave is given by:

$$f = \frac{1}{T} \tag{1}$$

UT velocity, *v*, in a perfectly elastic material at a given temperature and pressure is constant. The relationship between *v*, *f* and *λ* is given by Eqs. (2) and (3).

$$
\lambda = \frac{v}{f} \tag{2}
$$

$$
\lambda = vT \tag{3}
$$

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 wavelength of the UT waves introduced into it.

UT propagation velocity in a medium and UT wave attenuation (loss of amplitude and energy) depend on the medium itself. In solids, the velocity of longitudinal waves, *VL* , is given by:

$$V\_{\perp} = \sqrt{\frac{E(1-\nu)}{\rho(1+\nu)(1-2\nu)}}\tag{4}$$

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

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

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

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 direc-

During World War I, underwater detection systems using high-frequency acoustic waves and quartz resonators for submarine detection were developed by Langevin [29] as a consequence to the tragic sinking of the Titanic in 1912. In 1928, Sergei Y. Sokolov proposed the use of a through-transmission UT technique for flaw detection in metals [30]. Mulhauser firstly patented an UT device employing separate transmitter and receiver transducers to detect flaws in solids [29]. In 1940, Firestone was the first to realise the UT reflection or pulse-echo technique [31]. In 1948, extensive study of UT medical imaging started in the United States and Japan. One of the first UT testing apparatuses using piezoelectric crystal transducers for the detection of defects was patented by McNulty in 1962. This apparatus was capable of isolating defect signals from high level noise signals and providing an alarm upon occurrence of a defect signal [32]. Since those times, technology improvements led to remarkably enhanced UT non-destructive testing (NDT) allowing to detect surface, subsurface and internal flaws (cracks, delaminations, cavities, pores, inclusions and fractures) in diverse types of materials (metals, composite materials and plastics) [33]. In the manufacturing industries, UT NDT techniques are widely applied for the quality control of components and structures as well as

UT NDT is based on the measurement of the energy variations associated with mechanical waves, with frequencies ranging between 50 kHz and 25 MHz, generated by a piezoelectric transducer. The UT beams are introduced into the material by a coupling medium (oil, grease and water) and the variations of the reflected and/or transmitted UT energy are used to identify defects within the material which represent discontinuities in the UT path. When an atomic or molecular particle is displaced from its equilibrium position due to UT waves propagation in the material, the internal (interatomic or intermolecular) forces tend to bring it back to its original position. The displacement of a particle causes the dislocation of those placed in the neighbourhood, and thus the propagation of the UT waves in all the material is determined [8, 34].

In **Figure 7**, the basic parameters of a continuous UT wave are shown. The distance between two consecutive peaks of an UT wave is the wavelength, *λ*, while the number of UT oscillations per unit time is the frequency, *f*. The time required to complete a full cycle is the period

*T*. The relation between frequency and period in a continuous wave is given by:

delaminations without evidence of intralaminar cracks were found at mid-thickness.

tions of the surface fabric layer.

52 Characterizations of Some Composite Materials

for the characterisation of materials.

**3.1. Basic principles**

**3. Ultrasonic non-destructive testing**

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:

$$V\_r = \sqrt{\frac{G}{\rho}}\tag{5}$$

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

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

*<sup>s</sup>* = time-of-flight.

Measurements of the relative change in UT signal amplitude can be used for sizing flaws or

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

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

related to the pulse length; resolution usually improves with increasing frequency.

ity, is generally increased by using high frequencies (short wavelengths).

posites, due to the resultant scattering of the UT waves.

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

ducer or probe position [34, 36, 37]:

• Sensitivity, or the capability of an UT inspection system to detect a very small discontinu-

• Resolution, or the ability of the UT system to generate simultaneous and distinct indications from discontinuities located close to each other within the material or located close to the front surface of the part, is directly proportional to the frequency bandwidth and inversely

• Penetration, or the maximum depth in a material from which useful indications can be detected, is reduced by the use of high frequencies; this effect is most pronounced in the inspection of metals with coarse grain structure or inhomogeneous materials, such as com-

• Beam spread, or the divergence of an UT beam from its central axis, is also affected by frequency: as frequency decreases, the shape of an UT beam increasingly departs from the ideal of zero beam spread. This characteristic is observed at almost all frequencies used in UT inspection. Other factors, such as transducer diameter and the use of focusing lens, also affect beam spread.

Sensitivity, resolution, penetration and beam spread are largely determined by the selection

A first difference between UT inspection techniques can be made with reference to the trans-

of the transducer and are only slightly modified by changes in other test variables.

<sup>2</sup> (6)

http://dx.doi.org/10.5772/intechopen.80573

55

material thickness, *S*, can be calculated as follows:

*<sup>S</sup>* <sup>=</sup> *<sup>v</sup> <sup>t</sup>* \_\_\_*<sup>s</sup>*

measuring the material attenuation properties.

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

where *v* = material sound velocity; *t*

by equipment and test material [37].

where *G* = shear modulus of elasticity.

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 reported for metal and ceramic materials [35].

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 appropriate UT transducer for an application.
