Nondestructive Evaluation of BSCC Artificial Heart Valves

. The subject of investigation is a set of four artificial heart valves made of electrically conductive material, namely the alloy Ti-6Al-4V. Notches of defined geometry with gradually increasing depth were formed on these BSCC (Bjork-Shiley Convexo-Concave) valves. These defects are created on the so-called output strut, which serves as a support mechanism for the controlled movement of the so-called occluder disc and they have been investigated. The method which was used is ultrasonic infrared thermography, where the object was excited by the UTVIS EDEVIS ultrasonic system and the response was detected by the FLIR SC 7200 infrared camera. The postulate for the use of this method is that the excitation signal may be periodically pulsate or in any other way modulated by certain amplitude frequencies, called "lock-in frequency". Lock-in thermography is a type of active lock-in method. The results of damage detection on these relatively complicated objects are compared and discussed.


Introduction
Non-destructive testing of materials (NDT) plays an important role in the quality management system of the production process. It enables the early detection of internal defects in the product or semi-finished product, which could prevent its effective use or cause a structural accident after a certain period of operation. We use a number of methods using different physical principles to detect and quantify the detected errors in the field of non-destructive testing. The advantage of non-destructive methods over destructive methods is that the product remains unchanged after further testing for further use. NDT test methods for individual types of semi-finished products and products are prescribed in the relevant standards and regulations. From this point of view, the decisive factor is usually the customer's request, which determines according to which standard or regulation the product is to be assessed for NDT.

LOCK-IN thermography
The essential for the utilize of this technics is that the excite signal could be periodically throbbed or in some other method modulated by particular amplitude frequencies, called "lock-in frequency" f lock−in . Lock-in thermography is a kind of active lock-in technique. This means that to excite specimenby heat, this heat must be generated periodically and correlation usage the lock-in method is applied to every heat subscription of each individual pixel on the view of the registered device. Digital lock-in form correlation comprise of averaging the consequence of the recorded values F k and method of weighting factors K k to the overall amount of recorded values M [1]: There for is the output signal. When the excitation signal is harmonic then the most convenient correlation function is also harmonic (sine, cosine) function. This type of lock-in correlation is named sin/cos or narrowband correlation. It could be obtained either by narrowing the bandwidth of the recorded signal or utilized the amount of harmonic functions for K k in the formula (1) [1]. The principal benefit of the sin/cos correlation is that it makes possible to user to take into account the phase of the signal after the evaluation (off-line) when it is used two-channel correlation. Two-channel correlation intends that there were applied two kinds of weighting factors, one approximates a sin-function and the second approximates cos-function. Correlation is transferred twice in parallel with both kinds of weighting factors [1][2][3][4][5]: Next the first channel measures the component in-phase with the sin-function, and the other channel measures the component in-phase with the cos-function, which is π/2 phase-shifted to the sin-function [13]. If formulas (2,3) are inputted into formula (1) than the outcome of the two correlations over a complete number periods is [5][6][7][8][9][10]: Where S 0 is named the in-phase signal and S π 2 is usually named the amplitude signal. Both signals could be either positive or negative [14][15][16][17][18].

UTVIS EDEVIS
Ultrasound lock-in thermography is a strong measure method for detect indication of cracks, disbonds, or delaminations. UTVIS is based on digital high-power ultrasound generators and converters as excitation sources and high-sensitive infrared camera. Ultrasound thermography (or Vibrothermography) uses the interaction of mechanical and thermal waves to detect material defects. If the defect in the component absorbs the injected high-energy ultrasound waves, it is heated locally (selective defective dark field method). The resulting temperature gradient on the sample surface is measured with an infrared camera, which visualizes the scattered energy. Depending on the application, there are two derivatives of this method: the very fast analysis by the decay phase and the sensitive (1) (2) (3) Lockin method. In both cases, the evaluation calculates the time delay between the injected energy and the thermal response, leading to a robust and reliable technique that is constant against surface properties or ultrasonic distribution [19].

Measurement
As the samples werechosen4 artificial heart valves made of electrically conductive material, namely the alloy Ti-6Al-4V. Three of samples were damaged by cutting they were created with gradually increasing depth. The sample was placed under ultrasound exciter and is subsoiled by a holder suitable shape (Fig. 2). Than the sample was excited by choice modulated frequency while the defect was displayed. The entire operationof excitation wasrecorded by infrared camera (Fig. 3). Setup of infrared camera was the same for every single excitation of frequency. The 50 Hz of frame rate was selected on infrared camera. The emissivity value was chosen1 in the software environment because this quantity does not affectof the measurement result. The program, that was used to evaluate every single image, is called the DisplayImg and it was supplied with the excitation equipment. This program uses the theoretical foundations of the lock-in technique, as it is explained in the first section of this article. Since a first form of excitation was chosen sweep excitation. This is type of excitation which get along through whole area of modulated frequencies. It is supposed to notice the whole process of excitation and then select the most suitable modulated frequency at which are detected damages on the measured subject. After detect the suitable modulated excitation frequency, resolution of radiation energy was succeeded drawn in the program like in phase and amplitude display [10][11][12][13][14].Furthermore, only the displays in phase were evaluated. In the first sample can be damage observed by the light area, with excitation frequency 0.03 Hz (Fig. 4). On the second sample it is possible to see damage when excitation frequency is 0.006 Hz (Fig. 5). The third sample was excited by frequency 0.065 Hz when damage was displayed (Fig. 6).

Conclusion
Non-destructive testing of material was carried out for the purpose of this measurement. As the samples were selected four artificial heart valves made of electrically conductive material, namely the alloy Ti-6Al-4V. Damages of defined geometry were created on valves. These damages have increasing depth. Device with an ultrasonic excitation UTv is which work by the lock-in method for NDT was used as excitation device that test the samples. At first it was executed ultrasonic excitation through the total area of modulated frequency, by this excitation was selected modulated frequency that was able find defects on the samples. For important facts can be regarded the knowledge that, as of oscillation (if modification of the excitation area frequency, then when the exciting frequency correspond, with its own than is a displayed resonances) and in this sort of excitation (ultrasound) is a reaction in the IR field dependents on instant variables of the excitation signal. Defect was created by the light fields by variation the radiation structure on interpreted pictures. The method has proper to be anappropriate to solution for NDT testing to find defectson heart valves.