Finite element method simulation

In some situations, for example on samples with complicated structures, ex- perimental results may be insufficient on their own to validate theoretical predictions

IOS detector

Irregularly shaped sample Generation

optics

Three axis prism mounting

(a) Lab setup

Lab jack Irregular sample IOS detector Multi-axis right angled prism Laser source (b) Schematic diagram

Figure 3.6: Experimental setup (a) and schematic diagram (b) for an irregularly shaped test piece showing the IOS detector mounted on a retort stand and the laser source being directed onto the sample via a right-angled prism held in a three axis moveable holder.

about the interactions of ultrasound with defects. In such cases modelling of the physical system is performed, allowing aspects of the simulated system to be con- trolled, such as reducing the amount of backwall reflections in order to aid in the understanding of the interactions observed.

The simulation method used in this thesis is the finite element method, which splits a structure up into discrete elements, each of which represents a small part of the structure and is bordered by other similar elements[242–245]_{, as shown schemat-}

ically in figure 3.7. The method is used to solve a differential wave equation (such as equations 2.23 and 2.24) together with the boundary conditions over an object of complex shape[242]_{. By assuming a variation in the differential equation over}

an individual element, such as that caused by an ultrasonic stress/strain, once the boundary conditions on that element are considered an approximate solution of the wave equations at that element can be obtained[242]_{. If this solution is compatible}

with those for the surrounding nodes then a set of simultaneous equations can be formed and their solution will solve the wave equations at those elements, which can then be used to calculate the wave displacement, particle velocities, associated pressures and many other quantities. The FEM calculated out-of-plane surface dis- placement is the same measured quantity as that which is obtained from experiment but without experimental variations, such as material inhomogeneities, temperature variations, surface damage, etc, that arise in real systems. A detailed mathematical treatment of FEM simulations is found in references[243–245].

FEM models can be produced in 2D or 3D with the wave equations adjusted as appropriate, and are capable of simulating many different forms of ultrasonic

Fixed gen. Det. start Det. end

Absorbing boundary Symmetric boundary Free boundary

Scan direction

Figure 3.7: An example of a FEM simulation mesh for a Rayleigh wave incident on a v-shaped surface-breaking defect, showing the different boundary conditions applied and the scanning process. The number of nodes shown here is not representative of the true number used in a FEM simulation.

interaction depending upon the method used to excite the ultrasound within the simulation and the boundary conditions applied to the model[107]_{. The boundary}

conditions on external surfaces are described as free boundaries when they are al- lowed to interact in a normal fashion with any ultrasonic waves, however, this can produce lots of unwanted scattering events which lead to a complicated ultrasonic signal. To simplify this the boundaries can be made absorbing such that any ultra- sonic wave that interacts with the boundary is absorbed and not propagated back into the model, reducing the complexity of recieved signals; for example in chapter 4 the interaction of a Rayleigh wave with a surface breaking defect is studied in a sample that will produce multiple unwanted sidewall reflections of the Rayleigh wave, and also bulk wave reflections from the backwall - all of which are removed if absorbing boundaries are applied.

In addition, the model can be made artificially larger by making a symmet- rical boundary that mirrors the ultrasonic propagation about the boundary, thereby reducing the computational time required as only half the model volume needs to be solved. In order to avoid numerical dispersion errors, the elements defined in the simulation must be sufficiently smaller than the wavelength of the simulated ultrasound, which is largely dictated by the size of the model used and the availi- able computational resources, and to maximise this the application of symmetric or absorbing boundaries is useful[243].

The implementation of this method by hand would be extremely laborious, involving solving large numbers of simultaneous equations, and therefore this form of simulation is carried out on high powered computers; even then a three dimen-

Absorbing boundary Symmetric boundary Free boundary

Fixed gen. Det. start Scan direction Det. end

Figure 3.8: An example of a FEM simulation mesh for a Lamb wave incident on a surface-breaking defect, showing the different boundary conditions applied. In this case the defect extends only part of the width of the sample, allowing for a diffracted wave to be observed passing around the defect.

sional model may take several hours to complete. A careful balance must be struck between the number of elements used to simulate the system; too few and the model will not converge to the true value, too many and the simulation will take too long to compute.

The FEM simulations presented here were carried out using the commer- cial modelling package PZFlex[246]. The geometry of the FEM simulations were adapted for each experimental setup such that the overall shape was similar to that of the specimen. The FEM models were, however, scaled down in directions that were non-essential to the technique being used through the use of symmetry and by employing absorbing boundary conditions, which were applied so as to reduce the amount of sidewall scattering present. For example, for Rayleigh waves propagating in an aluminium block with a full-face width surface-breaking defect, the opposite side of the block to that with the defect and the two ends of the sample was set to be absorbing, the top surface is set to be free as it is the only surface of interest and symmetry was used about a line perpendicular to the defect, at the mid point of the defect, thus producing a situation that approximates the propagation of Rayleigh waves in a half-space,as shown in figure 3.7.

The simulations were 3D in nature as this better represents the physical systems being modelled in chapters 4, 5 and 6, however, the trade off for this is that the run-time of the simulations is dramatically increased over the 2D alterna- tive. Simulating in three dimensions means that, for defects which do not propagate across the entire width of the sample, the diffraction of ultrasound around the de- fect ends can be studied. This is important for accurately replicating data for full thickness defects when the source and detector are on different sides of the defect.

(a) A-scan

(b) Frequency content

Figure 3.9: A comparison between the wave form structure and subsequent fre- quency content of the experimental data and FEM data produced from a dipole force generation method for a Rayleigh wave in aluminium.

An example of a FEM simulation set-up for a defect that is not the full width of the sample is shown in figure 3.8 for a Lamb wave supporting sample. In this model both the top and bottom faces of the sample have free boundary conditions as the sample is sufficiently thin for the wave to interact with both surfaces.

For each different simulation, only the relevant simulated laser was scanned, with the other being held in a fixed position; for example in the scanning laser de- tection simulations the laser source was kept stationary. This acted to reduce the calculation times as there was no need to recalculate the boundary conditions of generation at each scan position.

Laser generation of ultrasound in section 4.2.1 and chapters 5 and 6 was modelled through the application of a dipole force on a series of nodes, for both Rayleigh and Lamb wave experiments. The dipole force was either applied to a

(a) A-scan

(b) Frequency content

Figure 3.10: A comparison between the wave form structure and subsequent fre- quency content of the experimental data and FEM data produced from a thermal generation method for a Rayleigh wave incident on a 90◦ _defect.

line of nodes or a circular series of nodes depending on the generation mechanism required. This method has been shown to be reliable in replicating the ultrasonic waves observed in experimental approaches[22,95,107,128].

For simulations modelling scanning laser source experiments in section 4.4 the dipole force was replaced with a thermal generation source in order to achieve a closer agreement with the laser generation mechanism, as outlined in section 2.2.4. This generation method applies a thermal gradient, which has a gaussian distribu- tion, to a line of nodes across the width of the line source. The heating duration and penetration depth of the heating was adjusted so that the output ultrasound of the model matched the experimentally observed form[86,95,107].

Before any correlation between the phenomena observed experimentally and those seen in the FEM simulations can be explained, the nature of the ultrasound

Figure 3.11: Experimental and FEM A-scans for Lamb wave propagation in 1.5 mm thick aluminium plates.

produced in the model must be compared to that seen experimentally to ensure that accurate reproduction of the waves has been achieved. This requires varying the gen- eration conditions until the simulated waveform represents the experimental form in both shape and frequency content as closely as possible. In the case of the dipole force this requires varying the rise time and maximum value of the force applied, whereas for the thermal generation mechanism the temperature change, penetration depth and heating duration are varied to produce wave forms that closely match the experimental forms.

The wave structure and frequency content produced by simulation of a dipole force generation mechanism is compared to the equivalent experimental scanning de- tection approach for Rayleigh waves in figure 3.9. The arrival time for the incident Rayleigh wave differs between the experimental and simulation data due to the vary- ing separation used in the simulations. To decrease the calculation time required for the simulations the source to detector distance is smaller than that used in the experiments, giving an earlier arrival time for the Rayleigh wave in figure 3.9.

The general structure of the wave forms between the experimental and sim- ulation A-scans is in good agreement, with the frequency content also showing good correlation. This allows for the direct comparison between the experimental and simulated waveforms when the outcomes of scanning laser detector experiments are considered, as in chapter 4. The number of elements per wavelength (λ= 3.78 mm from peak experimental frequency) used here was 21, which has been shown to produce accurate reconstruction of experimental ultrasound in a sufficiently small calculation time[95,107]_.

(a) Experimental Sonogram

(b) FEM Sonogram

Figure 3.12: Sonograms showing the frequency content of the experimental (a) and FEM (b) data produced from a 3D dipole force generation method for a Lamb wave propagating in a thin sheet.

eration mechanism used for the Rayleigh wave simulations in section 4.4 with the experimental data is shown in figure 3.10. The wave form structure is similar be- tween the experiment and simulation, with the exception of some higher frequency ringing of the thermoelastic model, which occurs just after the Rayleigh wave arrival. Whilst not ideal, the high frequency ringing can be dealt with through appropriate windowing of the incident Rayleigh wave, and the influence it has on the frequency content can be seen to be small, as shown in figure 3.10b.

The ringing features arrive at a constant time after the incident wave and as such will not move into the windowed region, as a reflected or mode converted wave will as the source is moved closer to a defect, as explained in section 4.4. The differ- ence in arrival times between experiment and simulation arises due to the fact that the simulations are carried out with a varying separation between each detection point. This also means that the degree of attenuation experienced between each

consecutive simulation point will vary, and this can account for some differences between the simulated and the experimental data[107]. For the thermal generation method a higher density FEM mesh is required so that the heat flow between ad- jacent nodes is sufficiently accurate, and in this case the number of elements per wavelength was 110.

For the simulation of Lamb waves in thin plates (chapter 5) a dipole force generation mechanism was used, with the resulting comparison between the simu- lation and experiment shown in figure 3.11. A 3D simulation was employed here as the defects were not the entire width of the sample, and diffraction around the tip edges was expected, which requires wave movement in a direction hitherto of only small significance in the Rayleigh wave models. To evaluate the equivalence of these simulations with the experimental data a time frequency representation (a sonogram) was employed, described in section 2.1.5. The sonograms for both ex- perimental and simulated A-scans are shown in figure 3.12 and it can be seen that the frequency content is very similar between the two. Both have frequency content arriving at times that correspond to the arrival times of different Lamb wave modes, as explained in chapter 5. This is shown by the similar shaped frequency features that arrive at the same times relative to one another in both experimental and sim- ulation data. Similarly to the dipole force generation for Rayleigh wave simulations, the number of elements per wavelength used was 21, which has been shown to give an accurate reconstruction[22]_{. The material properties for the standard materials}

used in all simulations were taken from Kaye and Laby[141]. Detection was mod- elled by extracting the out-of-plane displacements from single nodes, giving a higher spatial resolution when compared with the experimental data.

Through a combination of the experimental methods and the finite element method simulations described in this chapter the interactions of various ultrasonic surface waves with different geometry surface-breaking defects were studied, allow- ing the identification of several interesting near-field phenomena and the subsequent characterisation of these defects.