The influence of material’s surface modification on the structure’s dynamics-initial test results

Laser Doppler Vibrometry (LDV) is a still inexhaustible source of research knowledge. Continuous development of the hardware and software makes it possible to employ this technique to new applications. Among plenty of LDV research ideas, the analysis of the influence of material’s surface modification on the structure’s dynamics is not widely investigated. The aim of the paper is to verify a research hypothesis, which assumes that by applying a thin layer of material (coating) to an existing structure or by physical modification of structure’s surface, it is possible to measurably change the dynamics features of the analysed structure in terms of values of natural frequencies and oscillations amplitudes. To perform the assumed research and measurement of vibration a doppler laser vibrometer Polytec PSV-500 was used. Measurement outcomes for different titanium alloy plates surface modifications are depicted. In article’s summary research-based conclusions are formulated.


Introduction
Laser Doppler Vibrometry (LDV) has its origins in fluid velocity measurements reported by Yeh and Cummins at Columbia University in 1964 [1].Doppler frequency shift that occurs when light is scattered by a moving surface is the basis of LDV.This frequency shift is directly proportional to the surface velocity and so its detection enables convenient and non-contact measurement of vibration velocity [2].Since the development of the LDV took place, the tremendous progress in this measurement technology can be noticed, mainly by the development of computer and automatic science.Rotor vibrations and bearings [3] are the most classic application but nowadays, LDV are widely employed in nearly every area of science.The advantages of LDV over traditional contact sensors are particularly visible for measurement of rotating or hot elements where attaching traditional sensors could be very difficult or impossible.Thin and soft structures could be added to the list, but this would still neglect the special benefits now routinely exploited where high frequency operation, high spatial resolution or remote transducer operation is required.
Beneath a short review of the examples of the LDV'S current utilizations will be presented to depict the versality and importance of this measurement system: -health diagnostic: LDV has proved to be an effective diagnostic tool in damage detection in body because of its ability to make non-contact measurement over a dense grid of points [1].More and more popular are examples where LDV was used in medicine.In [4] LDV was exploited to diagnose aortic stiffness, whereas in [5] the teeth mobility has been measured.
-structural defect: LDV is widely used for non-contact detecting of defects or cracks.This feature is very interesting for aircrafts inspection.
-micro-electro mechanical systems (MEMS) [1]: In dynamic MEMS systems, a part of the system is driven to vibrate, often at resonance, with applications including surface acoustic devices, micromirror arrays and sensors.Here, LDV's distinct advantages over microscopy-based techniques include three-dimensional measurements [9,10] of picometre displacement, over a wide area [11] and at GHz frequencies.
-rotating parts: all measurements of rotating elements are usually problematic.One of the most known advantage of LDV is possibility of non-contact vibration measurement of rotating elements even with high rotational speed.Widely investigated are turbine blades, machines rotors or particularly in the past, hard discs [12,13].
-acoustics: Acoustics is strictly connected with vibrations, which LDV can precisely measure.From beginning of LDV's technology loudspeakers were commonly analyzed [3].Nowadays 3D technics make it possible to analyze more sophisticated objects like whole instruments or noise made by objects [14].
-other applications: presented grounds of LDV possibilities make it clear that there are nearly no limitations for use of LDV.Nowadays they are becoming more accessible for more researchers and companies.Exemplary applications of LDV could be military, for detecting landmines [15], valve kinematics in a high-performance internal combustion engine [16] or verification of fruits quality [17].
As was presented above, LDV has tremendous research capability which is commonly being discovered and boosting by constant development of the accessible hardware.
The idea of the paper is to use LDV to analyze the influence of material's surface modification on the structure's dynamics.By a huge development in coatings technology which has been done in recent decades, this type of surface modification has become popular.It is widely use to [18]: -change the friction coefficient (usually diminish it, to extent the durability of the parts, lower value of the forces) -boost wear, oxidation, corrosion, hardness properties -change bio or chemical properties -prevent from heat [19] -modify stresses Great achievements in machines parts wear preventing (usually because of hardness and low friction coefficient) led to the situation when many parts in nowadays cars or aircrafts have modified surface by coatings.
The accessible bibliography doesn't indicate that the proposed idea to analyze the dependence of coatings on the dynamics behavior of the structure has been studied.Some papers investigated only the influence of the material layer on accuracy of the LDV [20].
The perspective of potential possibility of changing/adjusting dynamic nature of the present structures/objects without noticeable change of the object's mass and dimensions is very interesting.It could make it possible to change the natural frequencies of the objects to avoid resonance.

Experimental setup
To perform research and measurement of vibration a high tech doppler laser Vibrometer Polytec PSV-500 [21] was used -figure 1.The modified substrates were tested in terms of the dynamic characteristics.Polytec PSV-500 makes it possible to measure vibration in an ultra-wide frequency range (up to 25 MHz), capturing vibrational velocities up to 30 m/s.Research of the structures was performed in a frequency domain, using plate-attached inducer (figure 1a).
An initial assessment of the measurement results was provided by frequency-selective 3D animations of the deflection shapes in false colors or with a photo-realistic object texture.Software initial post-processing functions enabled the evaluation of transfer-functions and damping values.
Crucial outcomes were the operating deflection shapes (amplitudes) and identified resonant frequencies of the structures.

Structures description
For the tests a Ti6Al4V titanium plates are used, widely employed in aviation and automotive industry.The material was characterized in terms of mechanical properties and phase composition using the nanoindentation and X-ray diffraction, respectively (results not presented in this paper).
The surface modification of Ti6Al4V alloy plates included: -shot peening.The method is similar mechanically to sandblasting, though its purpose is not to remove material, but rather it employs the mechanism of plastic hardening to achieve its goal, with each particle functioning as a ball-peen hammer.The shot peening process can noticeably increase the fatigue life of the element.
-deposition of hard coating with the use of PVD (Physical Vapor Deposition) method.TiW coating with a thickness of 300 nm was deposited on the surface of Ti6Al4V plate.

Methodology verification
For the tests 150x150 mm Ti6Al4V alloy plates with a thickness of 0.5 mm and 1 mm of thickness were prepared (see figure 2b).However, because of significant cost of titanium alloy plate, initial tests were performed with steel plate of 1 mm thickness (and the same size of 150x150 mm).As figure 2a depicts, piezooelectronic transducer was attached.Then, using Polytec PSV 500 scanning vibrometer system, bult-in generator and attached piezooelectronic transducer, the surface vibration was measured.
The outcome of the measurement is presented in the figure 3. To verify the results and to avoid gross errors a contact verification has been executed.The structure (with attached piezooelectronic transducer) was excited by hammer.During this test the transducer was connected not to vibrometer but digital oscilloscope.Tension generated by the transducer and registered by the oscilloscope is presented in figure 3b in frequency domain.The highest peak at 180 Hz indicates the 1 st mode shape.Comparing both, the vibrometer outcome (178.1 Hz) and the oscilloscope nearly full correspondence of both methods can be stated.Based on this conclusion further vibrometer investigation with titanium alloy plates were conducted.Main natural frequency shape of steel plate is presented in the figure 4.

Test results
The first investigated sample was Ti6Al4V alloy plate of 0.5 mm thickness and without any surface modification.Deflections magnitude in frequency domain are presented in figure 5a.The 1 st mode shape is visible at 110.9 Hz.Corresponding structure mode shape is depicted in figure 5b.Second analysed sample was 1 mm thick Ti6Al4V.FFT spectrum (figure 6a) shows two significant excitations.Taking into account that the second value is exactly at piezooelectronic transducer main frequency, this one can be neglected.The 1 st mode shape (264.1 Hz) is presented in the figure 6b.The first executed surface modification (for 1 mm thick titanium alloy plate) was shot peening.The characteristic surface appearance can be seen in figure 7. The shot granulation was 0.4 ÷ 0.8 mm.The process of shot peening was performed in 3 series of 12 minutes.In each series, the plate was rotated 4 times.The last analyzed sample was the one with TiW coating (300 nm thick) deposited on 0.5 mm thick titanium alloy plate.The deposition was made to both sides of plate.The 1 st mode shape occures at 96.875 Hz (see figure 9) which is less comparing the plate without any surface modification.The second presented surface modification, (deposition of 300 nm thick TiW coating), led to drop of the main natural frequency as well -from 110.9 Hz to 96.875 Hz which is 12.65% of shift.
In both cases the surface modifications made the structure more stiff what prompted to drop of the main natural frequency.What has not change is the shape of structures' vibrations -equally after shot peening and TiW coating deposition, as well as before those procedures, the way how the plates oscillates at main natural frequency, were the same.
The presented research outcomes allowed to conclude that the surface modification can significantly modify the dynamics features of the analyzed structures.During design process of thin metal structures, the influence of the surface modification has to be taken into account as the factor affecting their dynamic properties.

Figure 1 .
Figure 1.General view of the test stand.

Figure 3 .
Figure 3. a) Preliminary vibrometer test results with steel sheet plate-FFT spectrum (dimensions in [nm] in frequency domain [Hz]); b) Oscilloscope verification of natural frequency.

Figure 4 .
Figure 4. Preliminary vibrometer test results with steel plate-main natural frequency shape: 178 Hz.

Figure 8
Figure 8 presents the corresponding test results.The 1 st mode shape now appears at 239.1 Hz which is 25 Hz shift in comparison to the same plate without any surface modification.It is worth emphasising that the shot peening did not influence the vibration shape of the plate.