This section introduces some related ultrasound equations used to verify the experimental test. First, the ultrasound longitudinal velocity was obtained by calculation using A36 carbon steel Young’s modulus, and Poisson’s ratio, in another hand, the same ultrasound longitudinal velocity was obtained from the actual experiment. Second, the velocity can further calculate by temperature coefficients added every 10°C from 30°C 250°C, a similarly actual experiment was also conducted at every 10°C step up to 250°C as well as for ultrasonic attenuation. This section describes the methodology and setup used to determine ultrasound velocity and attenuation caused by temperature rise on carbon steel by experiment.
2.1. Ultrasonic attributes
The typical sound waves for ultrasonic testing are longitudinal, share, and lamb waves. Longitudinal wave, also known as straight beam, was used to detect material loss, internal lamination, or corrosion. Ultrasonic thickness gauging, pulse-echo A-scan ultrasonic testing, and phased array ultrasonic testing utilise the longitudinal wave.
Longitudinal waves are compressional waves that propagate in a medium by making the particles parallel to the wave’s propagation direction. In other words, the particles oscillate back and forth along the wave’s axis. This sort of wave motion produces alternating zones of compression and rarefaction, resulting in high-and-low pressure regions.
Because of their capacity to penetrate deep into materials and provide useful information about their internal condition, longitudinal waves are often used in ultrasonic testing. One of the most common applications of longitudinal waves is ultrasonic thickness gauging, which involves sending a wave through a material and measuring the time it takes for the wave to travel through the material and reflect back. This approach is frequently used in a variety of sectors to evaluate the structural integrity of pipelines, tanks, and other structural components.
Longitudinal waves are essential in ultrasonic testing because they provide a reliable and effective method of detecting material loss, lamination, and corrosion. Longitudinal waves are excellent for a wide range of applications due to their ability to penetrate deep into materials and provide vital information about their interior state. Understanding the features and applications of longitudinal waves allows ultrasonic testing specialists to optimise their inspection processes and provide reliable structural integrity assessments in a variety of sectors.
The first equation represents the basic link between frequency (f), wavelength (), and phase velocity (v). It describes the interaction of these three characteristics for an ultrasonic wave travelling through a material.
The phase velocity of an ultrasonic wave is directly proportional to its frequency and wavelength, according to this equation. The phase velocity is the rate at which the ultrasonic wavefronts propagate across the medium.
This equation can be used to derive a variety of helpful relationships. If the frequency and wavelength are known, for example, the phase velocity can be determined. If the phase velocity and frequency are known, the wavelength can be calculated. This equation is crucial in understanding the behaviour of ultrasonic waves and their interactions with various materials. It is widely utilised in ultrasonic testing and other applications where the propagation characteristics of ultrasound waves must be analysed and comprehended.
where:
V = velocity;
= frequency;
= wavelength
Equation (2) provides an alternative method for estimating ultrasonic velocity in addition to the previously discussed equation. This equation connects the velocity of an ultrasonic wave to the time it takes the wave to traverse a certain distance through a material. The distance travelled, indicated as (x), reflects the round-trip distance of the signal via the sample and back.
To compute the velocity using Equation (2), first, determine how long it takes the ultrasound signal to travel the round-trip distance. This is accomplished by calculating the time delay between successive echoes received from the substance. The interval between echoes is equal to twice the sample thickness.
The velocity of the ultrasonic wave can be properly estimated by applying Equation (2) and measuring the essential parameters, such as round-trip distance and duration of travel. This velocity information is critical in ultrasonic testing because it allows for the determination of other key properties such as the material’s attenuation and the ultrasound wave’s reflection and transmission coefficients.
Equation (2) establishes a key relationship that enables quantitative measurement of ultrasonic wave propagation within materials. Using this equation and exact measurements, practitioners can extract significant information about the material’s properties, allowing for the optimisation of ultrasonic testing techniques as well as the enhancement of detectability and sensitivity in a variety of applications.
Ultrasonic velocity measurements can provide useful information on a variety of fundamental material properties. Ultrasonic velocity measurements can be used to derive parameters such as Young’s modulus, Poisson’s ratio, acoustic impedance, and other factors when the density of a solid material is known.
The modulus of elasticity, Poisson’s ratio, and material density all play important roles in determining how fast bulk waves propagate in a material. Equations (3) and (4), as referred to in [
27], give mathematical formulations for computing longitudinal and shear wave velocities, respectively. As input parameters, these equations use the modulus of elasticity, Poisson’s ratio, and density values.
It is now possible to estimate and analyse crucial material properties by properly monitoring ultrasonic velocities and using the necessary equations. This data can be extremely useful in a variety of domains, including materials science, engineering, and non-destructive testing, allowing for a better knowledge of material behaviour and helping informed decision-making in practical applications [
28].
where:
σ = Poisson’s ratio, ρ = material density, G = Shear modulus, E = Young’s modulus Vl = longitudinal velocity, Vs = transverse (shear) velocity
Poisson’s ratio is known as transverse contraction per unit width divided by longitudinal extension per unit length under simple stress. The Poisson’s ratio can also be calculated when the longitudinal and share velocities are known, as described in Equation (5) [
29]. In Equation (6), determining Young’s modulus requires material density and sound velocity [
30].
where: σ = Poisson’s ratio, ρ = material density, G = Shear modulus, E = Young’s modulus of elasticity Vl = longitudinal velocity, Vs = transverse (shear) velocity.
The phenomenon of ultrasonic attenuation happens during the propagation of sound waves and is impacted by factors such as absorption and diffraction. As a result of these circumstances, energy is lost when the sound wave travels and reflects back. Analysing the attenuation record, which calculates the attenuation in decibels (dB) based on the first two echoes of the data signal, yields the overall attenuation of the sound wave [
31]. The energy loss per unit length, reported in decibels per metre, is frequently used to quantify attenuation. This parameter provides useful information regarding the rate at which the energy of a sound wave decreases as it travels through a medium.
Understanding and characterising ultrasonic wave attenuation is critical in a variety of applications, including non-destructive testing and material evaluation. Researchers and practitioners can acquire insights into the qualities of the tested materials and make educated decisions about the detectability and sensitivity of ultrasonic testing techniques by analysing attenuation behaviour.
The attenuation factor
can be calculated by Equation (7), where V1 is the amplitude without loss, and V2 is the amplitude with loss. The attenuation dB can also be obtained by increasing or reducing it until it reaches the reference level, such as 80% FSH by particle [
32].
In addition to the above-listed attributes, ultrasonic velocity measurements can provide information about the following material characteristics:
Grain Size: The grain size of polycrystalline materials can be estimated via ultrasonic velocity measurements. The interaction between ultrasonic waves and grain boundaries influences the velocity of the waves, allowing the average grain size of the material to be calculated [
33].
Phase Transformations: Under certain temperature or pressure circumstances, some materials undergo phase transformations, such as solid-solid or solid-liquid transitions. Ultrasonic velocity measurements can detect these phase transitions by detecting variations in velocity as the material transforms, making phase diagrams and material behaviour easier to investigate [
34].
Analysis of Stress and Strain: Ultrasonic velocity data can be used to evaluate the effects of stress and strain on the material. Variations in velocity can reflect changes in the mechanical characteristics of a material, providing insight into the presence of residual stresses or deformation mechanisms [
35].
Elastic Anisotropy: Elastic anisotropy refers to the directional dependency of certain materials’ elastic characteristics. Ultrasonic velocity measurements can help characterise and quantify this anisotropy, providing vital information on the mechanical behaviour of the material in different directions [
36].
This paper aims to determine the theoretical calculation of velocity change and attenuation when the temperature rises to 250°C. Hence, the following formulae are essential to calculate ultrasound attributes with temperature coefficients added [
37].
Equation (8) gives a broad calculation not limited to sound velocity but other ultrasound attributes [
38]. Where M is one of the sample’s mechanical properties, such as Young’s modulus, Shear modulus, Poisson’s ratio, and bulk modulus;
is the ultrasound attribute in solids at a given temperature,
is the reference temperatures,
= solids’ temperature change, and
is the temperature dependence coefficient. The
reported by many researchers for Young’s modulus is -0.0803 GPa/°C, and for Poisson’s ratio is 2.4 x10
-5/°C [
39]. The theoretical calculation in this paper was first to obtain Young’s modulus and Poisson’s ratio in different temperatures using Equation (8), and after that, use Equation (3) to calculate longitudinal velocity in different temperatures.
2.2. Experiment test
The experiment was conducted on the phased array ultrasonic testing equipment with a 5LA12-0-L 5 MHz probe and a high-temperature wedge. The selected test specimen was a 200mm x 200mm x 25mm thick carbon steel plate. Portable spectroscopy was carried out to verify the carbon steel material grade and obtain the material’s chemical properties, as shown in
Table 2. The chemical properties matched the carbon steel listed in ASME II—Table TM-1 and Table PRD.
The ASME II—Table TM-1 given the carbon steels with carbon content <0.30% material grade Young’s modulus is 201.73GPa in ambient temperature (30°C) with interpolation calculation, ASME II—Table PRD shows the material density for carbon steels is 7750Kg/m3, and Poisson’s ratio is 0.3. Therefore, the theoretical longitudinal velocity and shear velocity can obtain by calculating according to Equation (3) and Equation (4), respectively.
The test specimen was first tested at ambient temperature; it used the principle of Equation (2) to measure two back wall echoes (round trip) via pulse-echo A-Scan with known thickness. It shows that the longitudinal velocity by experiment is 5915.8 m/s, the calculated longitudinal velocity is 5919.5 m/s at 30°C, and the difference is 0.06%.
Additional parameters such as density values, Young’s modulus, and Poisson’s ratio were calculated for the temperature range of 30°C to 250°C to validate the accuracy of the theoretical calculations. To determine these characteristics, the thermal coefficient was added to Equation (8), which took into consideration the material’s thermal expansion.
In addition, Equation (3) was utilised to compute the theoretical longitudinal velocity at various temperatures. Based on the material parameters and temperature, this Equation may estimate the longitudinal wave velocity.
The test specimen was put on a heating element in the experimental setup, as shown in
Figure 1. Three thermocouples were strategically placed on the top, centre, and bottom of the material to measure the temperature increase. At each phase, the temperature was allowed to stabilise for at least 25 minutes, ensuring an accuracy of 0.5°C.
The precise back wall approach was used for the experimental measurements, which involve measuring the time it takes for the ultrasonic wave to travel through the specimen and reflect back from the opposite surface. The temperature was gradually raised from 30°C to 250°C in 10°C increments.
The author hoped to test the correctness and dependability of the theoretical models and expand their understanding of ultrasonic wave behaviour in changing temperatures by undertaking these experiments and comparing the measured results with the theoretical calculations.
The experiment was performed after the temperature verification by thermocouples and a hand-held thermos meter in each step to record the ultrasound longitudinal velocity at the temperature change.
The amplitude of the sound echo might decrease as the temperature rises; this process is known as ultrasonic attenuation. To measure and record the attenuation, a reference point is established at ambient temperature by establishing the echo amplitude as the baseline or reference level at 80% of the full-screen height (FSH) [
40].
The echo amplitude tends to decrease as temperature rises due to a variety of variables such as increased energy absorption and scattering within the material. To effectively evaluate ultrasonic attenuation, the echo amplitude is increased to maintain it at 80% FSH during each succeeding test at higher temperatures. The difference in dB measurements from the reference level offers useful information regarding the attenuation of the ultrasonic signal as it changes with temperature.
Researchers and engineers can determine the level of attenuation induced by temperature differences by monitoring and analysing dB changes. This data is critical for understanding the behaviour of materials at various temperatures and can be used to optimise ultrasonic testing methodologies and equipment settings for accurate fault discovery and evaluation.