RU2168722C2 - Method of nondestructive test of multilayer articles and gear for its realization - Google Patents

Abstract

FIELD: nondestructive test of multilayer articles made of metals, plastics and their combinations. SUBSTANCE: method consists in pick-off of useful information on elastic properties of article in zone of impact from point of surface of article on which blow was delivered and exactly at moment of delivery of blow. Striker proper executing free oscillatory motion ( free fall-blow- recoil ) with strictly dosed pulse replenishment of energy at moment of recoil is carrier of this information. Free falling striker made of ferromagnetic material is employed to deliver mechanical blows. Gear for realization of method has striker, electromagnet coil, microphone, former of current pulse of electromagnet, amplifier- former of synchropulses and meter measuring period of strike pulses. Striker is positioned vertically in space of electromagnet coil. EFFECT: enhanced sensitivity, locality and authenticity of test. 3 dwg

Description

 The invention relates to the field of non-destructive testing of materials and products and can be used for non-destructive testing of multilayer plastic metal products and their combinations obtained by diffusion welding, soldering, gluing, etc. It is intended to detect local defects in the connection of individual layers and uses shock -acoustic control method, which consists in striking a controlled product and analyzing the reaction of the product to a shock pulse.

 Known commercially available flaw detectors that implement the shock-acoustic method: AD - 20UP, AD - 50U and others [1-3]. They use a drummer, driven by an electromagnet and delivering blows at a certain frequency on the surface of the controlled product, and a sensing element that perceives elastic waves propagating in the product after impacts. As such a sensitive element, a metal probe with a piezoelectric element is used, which is in continuous contact with the product surface near the point of impact and perceives surface vibrations excited by the impact. Further, the signal is analyzed by a single-channel (AD-20UP) or multi-channel (AD-50U) filters and is indicated using a pointer (AD-20UP) or a set of gas-discharge linear indicators, which allow displaying the spectral composition of the signal.

 The main disadvantages of these flaw detectors are insufficient sensitivity, reproducibility and locality of control. The reasons for these shortcomings are: the presence of a significant distance (about 10 mm) between the point of impact and the pick-up point of the excited free vibrations of the product surface (defined by the "base" of the primary transducer, that is, the distance between the striker and the probe of the sensor), the occurrence of spurious vibrations of the sensor when the probe slides along the controlled surface due to its roughness, as well as an additional spurious signal caused by vibrations of structural elements of the primary educator, excited by the movements of the striker (including reactive forces, which it is impossible to get rid of) and transmitted to the sensing element. In addition, due to the significant mass of the impactor, these flaw detectors create a high level of noise during operation and can damage the surface of the product (especially if the top layer is a thin plastic plate or film). For the same reason, the frequency of impacts is limited, and therefore the control performance when scanning a controlled surface.

 Partially, they tried to overcome these shortcomings in the AD-60C flaw detector [4] by eliminating a mechanical probe with a sensitive element in contact with a controlled surface. Instead, a microphone was used, located near the drummer, which perceived the acoustic signal arising from an impact through the air, without requiring mechanical contact with the controlled surface.

 It is this flaw detector that was chosen as the prototype, since it is the closest in technical essence and the achieved results to the proposed invention. The signal from the microphone, after preliminary amplification, branches out into 12 narrow-band filters tuned to different frequency bands and together covering almost the entire sound range. Each channel contains a narrow-band filter, a peak detector, and a direct current amplifier controlling a linear gas discharge indicator, the height of the luminous column of which is proportional to the level of the amplifier output signal. All 12 indicators are combined into a light panel that displays the instant spectrum of the acoustic signal. In addition, the outputs of the most informative channels (at the operator’s choice) can be connected to the inputs of an analog adder controlling the defect detector. The sensitivity of the adder for each input can be adjusted.

 However, experimental studies conducted by the authors and other researchers [5] showed that, even with a more advanced method of spectral analysis of the signal and its further processing by computer numerical methods, the sensitivity and locality of control remain not high enough.

 This can be explained by the fact that the acoustic signal perceived by the microphone is excited not only by that point on the surface of the product that is hit, but practically by the entire surface of the controlled product, along which elastic waves propagate at a much greater speed than through air, and have a very small the damping, and therefore the spectrum of these oscillations, depends on the properties of the product (the presence of a joint defect between the layers) not only directly below the point of impact, but also in a very wide area around it. (To confirm this, a very common way of detecting cracks in glass, porcelain and earthenware "by sound" when tapping it at any point, and not necessarily in the area where this crack is located, can be given. In addition, the sound is made by the drummer himself and the structural elements of the entire primary transducer vibrating during impacts, creating additional interference that worsens sensitivity. It should be expected that under workshop conditions, when automating defect recognition, sensitivity will be further reduced due to the presence of external acoustic noise, which in the workshop can reach intensities comparable to or even exceeding a useful acoustic signal.

 The technical problems to which the invention is directed are to increase the sensitivity, locality and reliability of control.

 The solution to these problems is achieved by the fact that a new method of removing useful information when using the acoustic shock method and the device implementing it is proposed.

 The proposed method consists in the fact that useful information about the elastic properties of the product in the impact zone is removed from the point on the surface of the product on which the blow is applied, and exactly at the moment when it is applied, and the carrier of this information is the drummer himself, making free oscillatory motion (free fall - shock - rebound) with a strictly dosed pulse energy recharge at the moment of rebound.

 The invention is illustrated by drawings, where Fig. 1 is a structural diagram of a device, Fig. 2 is a combined time diagram of the speed of a hammer and the forces acting on it, and Fig. 3 is a schematic drawing of a primary transducer.

 The device (Fig. 1) that implements this method consists of a primary transducer 6, which includes a steel drum 1, an electromagnet coil 2 and a microphone 3, a sync pulse amplifier-shaper 7 corresponding to the leading edge of the shock pulse, an electromagnet current pulse shaper 8 and a meter shock pulse period 9.

 The primary transducer (Fig. 3) consists of a cylindrical ferromagnetic impactor 1 that can freely move along the vertical axis in the channel of the coil 2 and, in the absence of current in the coil, is supported by the lower end rounded to provide a central point impact on the product 5 so that the center of gravity of the striker A is located below the center point of the coil B. In addition to the coil in the housing 4 is a microphone 3.

The device operates as follows. In the initial position, the lower end of the striker 1 rests on the surface of the controlled product and its center of gravity is below the center of the coil of the electromagnet (so that the striker does not fall out of the cavity of the coil in the absence of the product, its upper part has a larger diameter, and the lower part of the cavity of the electromagnet has a diameter smaller than the diameter of the upper part of the hammer, and in the presence of a controlled product, the upper part of the hammer does not reach the lower part of the channel of the coil, which has a smaller diameter). When the initial triggering voltage pulse is applied to the electromagnet, the striker will be pulled into the coil, and then falling freely, it will strike on the surface of the product. The shock pulse F _y τ _y will lead to a rebound of the striker. The sound from the shock is picked up by the microphone 3, which converts it into an electrical impulse. An amplifier-driver 7 of a clock pulse amplifies this pulse and generates a very short synchronizing pulse along its leading edge, which arrives at the inputs of an electric current pulse generator 8 and a shock pulse period meter 9. An electric current generator 8 generates an electromagnet current pulse at a given input amplitude and adjustable duration, which is supplied to the coil of electromagnet 2. The electromagnet creates a momentum of force F _em • τ _em acting on the impact to at the time of his bounce from the controlled surface and directed upwards. This impulse gives the drummer additional acceleration at the very beginning of its upward movement. From the moment the electromagnetic pulse ends, the projectile moves up inertia due to the kinetic energy stored in it, overcoming the force of gravity. When the kinetic energy is completely converted into potential, the free fall of the striker will begin downward. In the future, the process will be repeated, becoming self-oscillating. The amplitude of the vibrations of the striker will increase until the additional kinetic energy imparted to the striker by an electromagnetic pulse is equal to the energy loss during the shock spent on creating elastic shock waves in the controlled product, the striker and the surrounding space, plastic deformation of the surface layers of the colliding bodies and. etc. With the steady-state mode of vibration of the projectile, their period will cease to increase and when hit on the same point of the product and the electromagnetic pulse is constant, certain amplitude and frequency of vibrational movements of the projectile will be established, which will further depend only on the elastic properties of the surface of the controlled product at the point of impact . By changes in the period of oscillations of the impactor during scanning of the surface of the product, one can judge the change in the elastic properties caused by the presence of internal defects in the impact zone (voids, delaminations, poor-quality adhesive, welded or soldered joints between layers, etc.).

где m - масса ударника;
V₀ - скорость ударника в момент нанесения удара.At the time of striking, the kinetic energy of the striker is:

where m is the mass of the striker;
V ₀ - the speed of the striker at the time of striking.

где v_0↑ - - скорость ударника в момент отскока.At the moment of rebound, the kinetic energy of the projectile is equal to:

where v _{0 ↑} - is the velocity of the striker at the time of the rebound.

W _{0 ↑} <W _{0 ↓} , because part of the energy of the projectile was spent on inducing vibrations in the colliding bodies, which decay due to the forces of internal friction.

где g - ускорение силы тяжести,
H - высота, на которую поднимается ударник в режиме установившихся колебаний, когда удары наносятся по одной и той же точке изделия.The velocity v _{0 ↓} can be determined from the equality of the potential and kinetic energies of the projectile:

where g is the acceleration of gravity,
H - the height to which the striker rises in the mode of steady-state oscillations, when strikes are made at the same point of the product.

Без электромагнитного импульса сердечник отскочил бы от изделия, поднявшись на высоту h < H, которая так же может быть найдена из равенства кинетической и потенциальной энергий.It follows that:

Without an electromagnetic pulse, the core would bounce off the product, rising to a height h <H, which can also be found from the equality of kinetic and potential energies.

откуда

Величина ударного импульса, появляющегося при соударении двух тел, зависит не только от их масс и скоростей до удара, но и от упругих свойств соударяющихся тел; эти свойства характеризуются величиной, называемой коэффициентом восстановления [6], который определяется как:

По теореме об изменении количества движения для ударника для отрезка времени, равного τ_y, справедливо следующее уравнение:
m•[v_0↑-(-v_0↓)] = F_y•τ_y;
m•(v_0↑+v_0↓) = F_y•τ_y;
учитывая (4), получаем

или

откуда

На отрезке времени τ_y на ударник действует и сила тяжести, однако ее действием за время удара можно пренебречь, в силу того, что она много меньше ударной силы [6].

where from

The magnitude of the shock impulse that appears during the collision of two bodies depends not only on their masses and velocities before the impact, but also on the elastic properties of the colliding bodies; these properties are characterized by a value called the recovery coefficient [6], which is defined as:

By the theorem on the change in the momentum for the striker for the time interval equal to τ _y , the following equation is true:
m • [v _{0 ↑} - (- v _{0 ↓} )] = F _y • τ _y ;
m • (v _{0 ↑} + v _{0 ↓} ) = F _y • τ _y ;
taking into account (4), we obtain

or

where from

In the time interval τ _y , gravity also acts on the projectile, however, its action during the impact can be neglected, due to the fact that it is much less than the impact force [6].

Applying the theorem on the change in the momentum of a projectile for a time interval τ _em , we obtain the following equation:
m • (v _{↑ τem} -v _{0 ↑} ) = (F _em -F _T ) • τ _em . (6)
where v _{↑ τem} is the speed of the striker at the end of the electromagnetic pulse,
F _T - gravity.

откуда

Очевидно, что кривая изменения скорости ударника (см. фиг.2) за пределами действия ударного и электромагнитного импульсов будет симметрична относительно момента T₀, когда ударник находится в крайнем верхнем положении, различаясь лишь знаком вектора скорости, поскольку в это время ударник движется лишь под действием инерции и силы тяжести. Это означает, что в любой точке C траектории движения ударника, расположенной ниже крайнего верхнего положения (рассматривается положение центра тяжести ударника) и выше той точки, где заканчивается действие электромагнитного импульса при движении ударника вверх, скорости ударника. v_c↑ и v_c↓ будут равны по модулю.Substituting in (6) the value v _{0 ↑} from (5), we obtain:

where from

It is obvious that the curve of the change in the velocity of the projectile (see FIG. 2) outside the action of the shock and electromagnetic pulses will be symmetrical with respect to the moment T ₀ when the projectile is in its highest position, differing only in the sign of the velocity vector, since at this time the projectile moves only under by inertia and gravity. This means that at any point C of the trajectory of the striker, located below the extreme upper position (the position of the center of gravity of the striker is considered) and above the point where the action of the electromagnetic pulse ends when the striker moves upward, the velocity of the striker. v _{c ↑} and v _{c ↓} will be equal modulo.

Сдвигая точку C вниз до той точки, в которой при движении ударника вверх заканчивается действие электромагнитного импульса, получим из условия равенства скоростей вверх и вниз:

откуда

Начиная с точки C, ударник движется вверх и вниз только под действием инерции и силы тяжести. Время движения тела, подброшенного вертикально вверх от точки C с начальной скоростью v_c↑ = v_↓τэм, до момента возвращения его в точку С равно

Это время будет составлять большую часть периода установившихся свободных автоколебаний ударника. Действительно временем самого удара можно пренебречь, поскольку оно очень мало по сравнению с полным временем движения ударника вверх и вниз, длительность действия электромагнитного действия принудительно устанавливается достаточно малой (во всяком случае такой, чтобы до его окончания центр тяжести ударника не успел бы достичь центра катушки, т. к. после прохождения центра катушки электромагнитная сила будет уже не ускорять, а тормозить движение ударника). В этом случае полное время движения ударника вверх будет равно:

Движение ударника вниз ниже точки C уже не будет полностью симметрично движению вверх на этом участке пути, т.к будет продолжаться, как и до точки C, только под действием силы тяжести. Поэтому и отрезок времени τ_c↓, необходимый для преодоления расстояния от точки C до соприкосновения с поверхностью изделия, не будет равен τ_эм. Однако ввиду малости τ_эм по сравнению с

разницей между τ_эм и τ_c↓ можно пренебречь, ввиду чего период установившихся колебаний будет равен

При этом время свободного падения ударника с высоты H будет определяться выражением [7]:

Как это было показано выше, оно (при малых τ_эм) приблизительно равно 0,5•T, откуда:

В этих выражениях величину

можно рассматривать как обобщенный параметр, определяющий упругие свойства контролируемого изделия в точке удара. Именно он и несет полезную информацию. Все остальные величины, входящие в выражения (11) и (12), являются постоянными, причем F_эм и τ_эм - легко регулируемыми.Denoting the difference between H and point C by h _s , we obtain

Shifting point C down to the point at which the action of the electromagnetic pulse ends when the striker moves up, we obtain from the condition that the velocities are equal up and down:

where from

Starting from point C, the hammer moves up and down only under the action of inertia and gravity. The time of motion of a body thrown vertically upward from point C with an initial velocity v _{c ↑} = v _{↓ τem} until it returns to point C is

This time will make up most of the period of steady free oscillations of the drummer. Indeed, the time of the impact itself can be neglected, since it is very small in comparison with the total time of movement of the striker up and down, the duration of the electromagnetic action is forcibly set sufficiently short (in any case, such that before its end the center of gravity of the striker would not have time to reach the center of the coil, since, after passing through the center of the coil, the electromagnetic force will no longer accelerate, but inhibit the movement of the striker). In this case, the total upward movement time of the striker will be:

The movement of the striker downward below point C will no longer be completely symmetrical to the upward movement in this section of the path, because it will continue, as to point C, only under the influence of gravity. Therefore, the time interval τ _{c ↓} necessary to overcome the distance from point C to the contact with the surface of the product will not be equal to τ _em . However, due to the smallness of τ _em compared with

the difference between τ _em and τ _{c ↓} can be neglected, because of which the period of steady-state oscillations will be equal to

In this case, the free fall time of the striker from a height H will be determined by the expression [7]:

As shown above, it (at small τ _em ) is approximately 0.5 • T, whence:

In these expressions, the value

can be considered as a generalized parameter that determines the elastic properties of the controlled product at the point of impact. It is he who carries useful information. All other quantities in expressions (11) and (12) are constant, and F _em and τ _em are easily adjustable.

зависящую от упругих свойств изделия в точке удара и несущую информацию о наличии и характеристиках подповерхностного дефекта.Thus, by measuring the period T or the amplitude H of the steady-state oscillations of the projectile, one can determine

depending on the elastic properties of the product at the point of impact and carrying information about the presence and characteristics of the subsurface defect.

 More convenient for measurement is the period T of self-oscillations of the projectile, for which it is sufficient to measure the period between current pulses supplied to the electromagnet coil.

If F _em • τ _{em is} constant, each value of P _y will have its own values of T and H. By adjusting the magnitude of the electromagnetic pulse F _em • τ _em , you can increase or decrease T and H. In principle, this allows you to measure P _y from the magnitude of the electromagnetic pulse required to achieve the set values of T or H. However, this method of measurement will be less productive, since it will require for each point on the surface to regulate F _em • τ _em (amplitude and duration of the current pulse supplied to the electromagnet coil) until One of the self-oscillations of the striker does not equal the set value. But by adjusting the electromagnetic pulse it is very convenient to adjust the impact force and the range of variations of the period T when controlling various objects with significantly different elastic properties.

 Significant distinguishing features of the proposed method are the implementation of the striker in the form of a freely falling body of ferromagnetic material and its use as a carrier of information about the elastic properties of the controlled product at the point of impact by measuring the period or amplitude of its steady-state oscillations during pulse replenishment of its energy at the instant of rebound with strictly dosed electromagnetic pulses.

 The high sensitivity of this method and the device that implements it is achieved, firstly, due to the almost complete absence of interfering or destabilizing factors, and secondly, due to the fact that all useful information is concentrated in one measured parameter - the period of steady-state vibrations of the hammer, and is not washed out throughout the spectrum of the excited acoustic signal and, thirdly, due to the possibility of measuring the oscillation period with very high accuracy (using the digital measurement method).

и параметров электромагнитного импульса F_эм и τ_эм. Последние можно задавать с весьма высокой точностью. Из неучтенных при выводе данного выражения факторов можно отметить лишь трение ударника о стенки канала катушки электромагнита. Однако выбором оптимального зазора и подбором материала каркаса катушки электромагнита, обеспечивающим малый коэффициент трения со сталью (например, фторопласт), а также благодаря центрирующему действию электромагнитного импульса на ударник (вследствие наведения в нем вихревых токов возникают силы, стремящиеся расположить его строго по оси катушки) влиянием этого фактора можно пренебречь. Единственным существенным ограничением является необходимость строго вертикального расположения электромагнита. Сканирование желательно осуществлять путем перемещений изделия при неподвижном первичном преобразователе, хотя при обеспечении равномерности поступательного движения электромагнита это не имеет значения. Высокая локальность контроля обеспечивается тем, что вся полезная информация об изделии снимается именно с той точки его поверхности, по которой наносится удар, и именно в момент удара, путем измерения той части энергии удара, которая поглощается изделием, поскольку именно эта величина трансформируется в непосредственно измеряемый параметр - период установившихся колебаний ударника.The first of these factors merits a more detailed consideration. From the expression (10) it can be seen that the period of steady-state oscillations of the striker depends only on a useful informative parameter

and parameters of the electromagnetic pulse F _em and τ _em. The latter can be set with very high accuracy. Of the factors unaccounted for in the derivation of this expression, we can note only the friction of the striker against the channel walls of the electromagnet coil. However, the choice of the optimal gap and the selection of the material of the frame of the coil of the electromagnet, providing a low coefficient of friction with steel (for example, fluoroplastic), and also due to the centering action of the electromagnetic pulse on the impactor (due to the induction of eddy currents in it, forces arise that tend to position it strictly along the axis of the coil) the influence of this factor can be neglected. The only significant limitation is the need for a strictly vertical arrangement of the electromagnet. Scanning is preferably carried out by moving the product with the primary transducer stationary, although it does not matter if the electromagnet is moving uniformly. The high locality of control is ensured by the fact that all useful information about the product is removed from the exact point on its surface that is hit, and precisely at the moment of impact, by measuring the part of the impact energy that is absorbed by the product, since it is this quantity that is transformed into directly measured parameter - period of steady oscillations of the striker.

 The authors conducted experimental studies of a prototype device that implements the proposed method, confirmed the high sensitivity and locality of control. The studies were conducted on samples of two types with artificial defects. The first sample is a honeycomb panel made of titanium alloy with a cell diameter of 10 mm, a thickness of the upper skin of 0.5; 0.75; 1.0 mm, the thickness of the lower skin 2 mm and a total thickness of 17.5 mm, made by diffusion welding. Artificial defects were obtained in the manufacture of the panel by laying between the honeycomb filler and the upper skin of a thin film in the form of a circle with a diameter of 35 mm. The film, made by special technology, prevented diffusion during welding, as a result, the filler and skin, being in mechanical contact, did not have a welded joint.

 The samples of the second type were two- and three-layer panels of sheet duralumin. The connection was carried out by means of an adhesive film VK-25. The total thickness of the panels is 1.2; 1.9 and 2.6 mm. Artificial defects were obtained in two ways: by cutting out strips of the adhesive film with a width of 15 mm and "zhirivanie" the same strips before gluing.

 In the first case, voids formed under the sheathing layer, and in the second case there were no voids, but the bond strength between the layers in the places of “greasing” sharply decreased.

 On the honeycomb panel, all defects were reliably detected at all thicknesses of the upper skin layer. Moreover, at the defect-free places, the joints of the upper skin with the walls of the honeycomb filler and the places corresponding to the centers of the honeycomb cells were easily distinguished, and defective places were detected even from the back of the panel. The boundaries of defects were localized with an accuracy of 1-2 mm (depending on the thickness of the skin). On samples of the second type, for all thicknesses of the upper layer, cut-type defects were detected with 100% reliability. Defects of this type led to a change in the oscillation period by 80–90%, the boundary of the defects being localized with an error of up to 0.5 mm, and defects of the “greasing” type led to changes in the period of steady-state oscillations by 12–15% (depending on the thickness of the upper layer plating).

 Comparative tests with the AD-60C serial flaw detector showed that only defects with a diameter of 35 mm were detected reliably in the honeycomb panel, and even then with a skin thickness of 0.5 and 0.75 mm. The honeycomb structure of the filler did not differ at all. No defects were detected at all on the samples of the second type. These results are in good agreement with the results of studies of a similar flaw detector with a more advanced method of spectral decomposition and signal analysis using a computer [5].

 Thus, experimental studies have confirmed that the assigned technical problems are solved.

Literature
1. Lange Yu.V. Acoustic spectral method of non-destructive testing. Defectoscopy, 1978, N 3, pp. 7-14.

 2. Lange Yu. V., Ustinov E.G. Auth. testimonial. N 557318. Bull. fig. 1977, N 17.

 3. Lange Yu. V., Ustinov EG Acoustic spectral flaw detector. - Flaw detection, 1978, N 4, p. 27-33.

 4. Lange Yu. V., Ustinov E.G. Low-frequency acoustic flaw detector AD-60S. - Flaw detection, 1982, N 1, p. 12-15.

 5. Lange Yu. V., Voropaev S.I., Muzhitsky V.F. and others. The use of spectral analysis in low-frequency acoustic flaw detectors. - Flaw detection, 1995, N 10, p. 74-83.

 6. Targ S.M. Short course theoretical mechanics. - M .: Nauka, 1968, 415 p.

 7. Alexandrov EV, Sokolinsky VB Applied theory and calculations of shock systems. - M .: Nauka, 1969, 71 p.

Claims (2)

 1. A method for non-destructive testing of multilayer products, which consists in inflicting mechanical impacts on the surface of the controlled product and analyzing the reaction of the product to these impacts, characterized in that the impacts are applied by a freely falling impactor made of ferromagnetic material, and at the moment of rebound they are affected by a strictly dosed electromagnetic pulse, which gives the striker additional upward acceleration and compensates for the energy loss upon impact, due to the energy spent on creating the product and the environment space of elastic acoustic waves (internal friction) and plastic deformation of the surface layers of colliding bodies, the magnitude of these losses being determined by measuring the period of steady-state vibrations of the impactor or the amplitude of these vibrations.  2. A device for non-destructive testing of multilayer products, containing a steel drum, an electromagnet coil, a microphone and an electromagnet current pulse generating device, characterized in that a sync pulse amplifier-generator and a shock pulse period meter are additionally introduced, the input of the sync pulse generator-amplifier being connected to a microphone, and its output is with the inputs of the electromagnet current pulse shaper and the shock pulse period meter, and the drummer itself is located vertically in strips These are electromagnet coils with a gap providing free movement of the striker along the axis of the electromagnet coil.

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