Shock damping means. Cushioning the boat bottom to absorb shock loads. Protecting structures with shock absorbers and dampers

The invention relates to the field of shock testing of shock absorbers and can be used in the design of shock protection devices made of composite materials. The aim of the invention is to obtain characteristics of shock absorbers, showing the efficiency of their operation under shock impacts (coefficients of the efficiency of shock damping of shock absorbers associated with structural damping, damping in materials, as well as due to different acoustic stiffness various elements shock absorber, etc.) Tests are carried out on an installation, the quality factor of which is not less than an order of magnitude higher than the quality factor of the shock absorber. The required coefficient is equal to the product of coefficients associated with various physical properties of the shock absorber. At the same time, replacing the damping liners with liners made of various materials with previously known damping properties makes it possible to determine each of the coefficients as a result of the analysis of impact spectra obtained during impact tests. The technical effect is to improve the quality of the study of the process of shock absorbers during shock impacts. 6 ill.

The proposed technical solution relates to the field of testing shock absorbers made of composite materials to determine their damping properties under impact. Recently, the use of new materials (metal-rubber, carbon-fiber reinforced plastics, etc.) in systems of protection against vibro-shock loads on ships, airplanes, and spacecraft requires a sufficiently accurate determination of the effectiveness of each of the shock absorber elements. Currently known different ways determining the damping properties of shock absorbers. For example, in the study of shock absorbers operating under rather slowly changing external influences, the method of estimating the absorption coefficient by analyzing the hysteresis loop is used (IM Babakov "Theory of oscillations", pp. 153-154, Moscow: Nauka, 1968). However, these tests consider the energy dissipation over a full cycle of oscillations. To protect equipment from shock effects (often of an explosive nature), shock absorbers are used, which should primarily reduce the amplitude of the leading front of the shock wave of deformations. Reducing secondary vibration is usually not a big problem. The most suitable in this case is the analysis of the amplitude-frequency characteristics or the total values ​​of the impact before and after the shock absorber. For example (A. Nashif et al. Damping of vibrations, p. 190, M .: Mir, 1988, prototype), the method for constructing the amplitude-frequency characteristic consists in exciting vibrations in the test sample, measuring the exciting force applied at a given point, determining the dynamic response using accelerometers and strain sensors, and then comparing the amplitude-frequency response before and after the shock absorber. The use of a harmonic Fourier analyzer, as well as similar computational techniques, as a rule, is valid only for the case of "aftereffect" (when the impact has already ended and the secondary vibration is being investigated). In addition, the use of installations with a sufficiently low quality factor for testing (for example, vibration stands) leads to an overestimation of the damping properties of shock absorbers. The method described above also does not allow separating the scattering of external influences due to various physical properties of the shock absorbers (structural damping, reflection from boundaries, etc.). The purpose of this technical solution is to partially eliminate the above disadvantages, which will allow for a better study of the process of shock absorbers operation under shock effects. The proposed technical solution differs in that the shock absorber is loaded on an installation, the quality factor of which is not less than an order of magnitude higher than the quality factor of the shock absorber, and the tests are carried out sequentially, first obtaining the relationship between the forces and deformations in the shock absorber under shock impact, then determining the acoustic stiffness of the shock absorber at different levels loading, after which the tests are carried out with liners of the same design made of different materials with predetermined damping properties, and the assessment of the impact damping efficiency is made by comparing the shock acceleration spectra at the control points, while the impact damping efficiency coefficient is presented as a product of the coefficients , each of which is determined by analyzing the shock spectra of test accelerations of the previously mentioned liners. The essence of the proposed technical solution is illustrated by drawings, where Fig. 1 shows a shock absorber made of metal rubber 7VSh60 / 15, Fig. 2 shows the relationship between forces and deformations p- (hysteresis loop), Young's modulus (as the tangent of an angle) and the speed of sound in the material; FIG. 3 shows a diagram of the experimental setup; FIG. Figures 4-6 show the cumulative coefficient of the impact damping efficiency, the coefficient obtained due to structural damping, and the coefficient obtained due to dissipation in metal rubber. Consider, as an example, a shock absorber made of metal rubber (Fig. 1) and try to estimate the damping properties of the shock absorber using the proposed algorithm. When the deformation wave approaches the shock absorber, both its reflection due to various shock stiffnesses and scattering in the material (metal rubber of the shock absorber) and due to the structural damping of the shock absorber itself (tightening degree, clearances, etc.) occur. Let be the total coefficient of the impact damping efficiency. i = 1i 2i 3i,

Where 1i is a coefficient associated with structural damping;

2i - coefficient associated with the values ​​of acoustic stiffness;

3i is a coefficient related to material scattering. Obviously, for the materials used, 3i = 1 (except for metal-rubber, since the dimensions of the inserts are small, and scattering in the material begins to affect only at L> 1 m, and even then making up 1-2% per 1 m. O.D. Alimov and Other Impact, Propagation of Deformation Waves in Impact Systems (Moscow: Nauka, 1982). The damping efficiency coefficient itself according to the shock spectrum is understood as the amplitude-frequency characteristic of the ratio of the shock spectra of accelerations of the VIP before and after the shock absorber:

1 = A B1i / A B2i. Coefficient

Shows the effectiveness of various liners, since 1i = const (the same shock absorber), and for all liners, except metal rubber, 3i = 1, then

Ij = (1i 2i 3i) / (1j 2j 3j) = 2i 3i / 2j. Consider a material whose acoustic stiffness is equal to the acoustic stiffness of metal rubber, then

That is, we get the coefficient of damping of the shock wave, which characterizes the properties of metal rubber. As you know (LG Shaimordanov. Statistical mechanics of deformable fibrous nonwoven porous bodies. Krasnoyarsk, 1989), metal rubber is a material with pronounced nonlinear characteristics. In addition, the damping properties of a material can be influenced by speed (shock and explosive) and the type of loading. At the same time, the hysteresis loop (its limiting right branch) for a metal-rubber shock absorber in the region of limiting deformations does not depend on the loading rate. Thus, knowing the dependence of P- (hysteresis loop) and the magnitude of the impact (in the form of a force impulse), it is possible to obtain for any moment of time Young's modulus and, consequently, the speed of sound (Fig. 2). By selecting different values ​​of impacts and values ​​of acoustic stiffness, it is possible to obtain the coefficients of the damping of the impact impact depending on the strength of the external impact. Obviously, in such tests, the scattering of external influences should be minimal. There is a known formula connecting the quality factor Q and the logarithmic decrement of oscillations: Q = 3.141 ... /, a = lnA1 / A2, where A1 and A2 are the amplitudes of two adjacent oscillations. Whence it can be seen that even with an increase in the figure of merit by an order of magnitude (80-100, for conventional structures about 8-10), the energy dissipation in the experimental setup can be neglected. The use of the concept of a shock spectrum of accelerations to assess the efficiency of shock absorbers under shock influences makes it possible to correctly analyze the operation of shock absorbers both at the time of application of the load and after the end of its action (OP Doyar "Algorithm for calculating the shock spectrum" in the collection Dynamics of systems. Numerical Methods of Investigation of Dynamical Systems, Nistru: Kishenev, 1982, pp. 124-128). An example of the practical implementation of the proposed method. According to the proposed method, the damping coefficients were determined for the 7VSh60 / 15 shock absorber used in the vibration protection belt of one of the spacecraft developed by NPO PM (Fig. 1). The diagram of the test setup is shown in Fig. 3, where 1 - waveguides, 2 - shock absorber 3 - ABC-052 accelerometers. Fifteen bolt blasting operations were carried out. The force momentum for the bolt was obtained earlier. Dynamic deformations of the shock absorber were recorded using the high-speed photo-registration method. The dependence of the density of the material (metal rubber) on the effort was taken according to the passport data of the shock absorber. Liners made of steel, bronze, aluminum, textolite, fluoroplastic were used for replacement. A bursting bolt 8x54 was used as a source of impact. When replacing a metal-rubber liner with a steel liner (body material and fasteners), you can immediately obtain a coefficient associated with structural damping, because other scattering effects are excluded. FIG. 4, 5 show graphs of the total impact damping factor and the damping factor associated with structural damping, and FIG. 6 shows the coefficient obtained due to the dispersion of the impact in the metal rubber. The impact level was 6 kN. Measurement range in amplitude up to 6000g, and in frequency up to 10,000 Hz. The total measurement and processing error did not exceed 9-11%.

CLAIM

Strong hydrodynamic overloads, in simpler terms - the impact of waves in the bottom, have become one of the main problems of modern boat building, which impede the growth of travel speeds. The creators of high-speed planing boats fought against excessive overloads mainly in two directions: they were looking for such hull contours that would soften the force of impacts by reducing the area of ​​the bottom that touched the water and giving it a wedge-shaped cross-section, or they sought to raise the hull above the ridges waves, tear off the bottom from the surface of the water. As a result of the development of the first direction, "deep V" type contours, catamarans, Fox sleighs, "Sea Knife", etc. appeared. In the second direction, small hydrofoils and hovercraft, ekranoplanes developed.

But both of these directions in the design of planing vessels are associated with tangible energy costs. To achieve high speed, both a deep V boat and a hydrofoil or hovercraft require additional engine power compared to traditional low-dead-lift boat types.

Meanwhile, there is still a way to reduce the force of hydrodynamic shocks in the bottom, which does not require an increase in engine power or reinforcement of the body structure. Its essence lies in the use of shock absorption, damping of shock loads using elastic structural elements introduced into the body. With damping, the impact force is reduced due to the increase in the duration of the increased hydrodynamic pressure on the bottom. The magnitude of the overload, measured by the number g - the acceleration of the body's free fall - is almost directly proportional to the time the pressure acts on the boat. So: the elastic elements of the structure make it possible to reduce the overload on the hull of the planing boat when sailing on waves by almost 2 times compared to the hull having a traditional "rigid" design.

The authors have carried out a number of design studies of damping elements, which can be successfully applied to the hulls of recreational and tourist and sports boats. In some cases, they make it possible to make the case lighter and cheaper, which will require less material and labor intensity for its manufacture than serial designs.

One of possible options body of "elastic" design, proposed by the authors, is shown in Fig. 1 (see Inventor's Certificate No. 1070048, published in the "Bulletin of Inventions" No. 4 1984). Damping occurs by installing hollow cube-shaped elements in the sponsons between two layers of elastic bands. Thanks to the elastic structure, the bottom of the sponsons follows the profile of the wave, which reduces splashing, and the pitch becomes smoother.

The bow end of the vessel is a narrow central hull 1, turning into a monoski 2 and having side sponsons 3, smoothly turning into a sharp-chinned hull in the stern. In the middle part, the sponsons are filled with cubic waterproof elements 5, which are connected in the upper and lower parts with elastic strips 6 (it is possible to use rubber bands, reinforcement with steel cord). Cubic elements can move in the side guides of 7 sponsons in the vertical direction. Above, the cubic elements are spring-loaded with shock absorbers 8. The ends of the lower flexible strips 6 are rigidly fixed on the sponson line, in the upper ones they remain free.

With low excitement, the blows will be small; the waves, acting on the elastic strip 6, will transfer impact energy through the elements 5 to the spring shock absorbers 8.

In case of significant excitement, simultaneously with the elastic sponsons, the central building 1 will also enter the work, which has bottom contours in the nose with increased deadlift. Elastic sponsons dampen the impact energy at the initial moment and do not allow the central hull to significantly submerge in the wave, reducing the overall resistance of the vessel. Elastic bands follow the wave profile, while spring dampers absorb the vibrational energy of the elements. This, in combination with a narrow central hull, turning into a monoski, will allow the vessel to operate in high seas at high speed. By reducing shock loads, the strength of the body braces can be reduced. If this does not lead to weight savings, then it compensates for the mass of flexible structures.

This technical solution is especially useful for planing trimaran and catamarans. True, a known disadvantage is the difficulty of using the volumes of hollow damping elements, which occupy a part of the total useful volume of the body.

In another version, the elastic element is made in the form of longitudinal corrugations in the side metal lining (article number 1088982, published in the "Bulletin" No. 16 1984). The corrugated insert extends along the entire length of the bead, starting from the nasal quarter; the corrugations are filled with elastic material (Fig. 2).

The bottom sheathing is reinforced with longitudinal stiffening ribs, which are supported by floras 3. They are fixed to the bottom panel of the side sheathing 4 below the corrugated insert 5. Above the insert, the side sheathing is reinforced with stringer 7 and pusher pads 8.

Hydrodynamic shocks perceived by the bottom panels are transmitted to the flora and, accordingly, to the side skin. Most of the impact energy is absorbed during the deformation of the side inserts 5 and elastic filler 6. Due to the "pliability" of the bottom plating, the loads perceived by it are less than with a rigid structure, and the boat can develop more high speed on waves without the risk of damaging the hull.

This option is most promising for small planing motorboats and boats. Its implementation is not hindered by any technical difficulties - it is enough to stamp longitudinal corrugations with a certain rigidity in the side sheathing. The described invention was used, for example, in the development of a modernized version of the motorboat "Neman-sport" (), preliminary tests prototype which showed a noticeable improvement in operational characteristics (first of all - comfort when sailing in waves) compared to the base model.

For motorboats and boats, it is also possible to recommend the installation of flexible longitudinal stiffening ribs (article number 1100000, Bulletin No. 19.) % compared to the traditional longitudinal set design. This allows you to reduce the size of the strong connections of the bottom floor and, in fairness, - by 30% the thickness of the outer skin.

Compliant longitudinal ribs are made in the form of stampings from a thin aluminum sheet C-shaped profiles connected to each other through shock-absorbing elements (Fig. 3, a). The development of such a design is the use of shock-absorbing C-shaped elements in combination with corrugated bottom skin (in. P. No. 1106724, "Bulletin" No. 29, 1984). Here, the hydrodynamic loads, which are perceived by the corrugated bottom sheathing, transfer it to the C-shaped shock absorbers, which are supports for the corrugations on the transverse floras 6 (Fig. 3, b). Floras, in turn, are supported on stringers 6 and keel 7.

Due to the elasticity of the C-shaped plates 4 and the elastic spacers 5 installed between them, at the time of the hydrodynamic impact on the wave, elastic deformation of the bottom skin occurs. The spacers 4 can be made of synthetic rubber and reinforced with steel cord. Due to the elastic deformation of the bottom skin, the magnitude of the stresses acting in the skin and the set of stresses is halved.

Above, only general technical solutions to the problem of increasing the reliability and reducing the mass of the hulls of planing motorboats and boats were presented. There is still a painstaking experimental work, the results of which will make it possible to create a reliable method for choosing the dimensions of the body connections, taking into account the flexibility of elastic elements.

The invention can be used in the field of mechanical engineering to absorb and reduce shock loads. The damper contains a rod 2 with a cutting device fixed on it, consisting of a support sleeve 5, a knife head 7 and a sleeve 10 of a plastic material installed between them. Wedge-shaped teeth 9 are made on the end face 8 of the knife head 7 in contact with the sleeve 10, and the sleeve 10 is equipped with an annular shoulder 11. When the damper is operating, the teeth 9 of the knife head 7 cut off the shoulder 11 of the sleeve 10, reducing the impact loads acting on the damped object. The technical result consists in increasing the energy consumption of the damper, eliminating its jamming when the damped object is subjected to loads directed at an angle, maintaining the damping ability of the device under the action of repeated shock loads. 2 c.p. f-ly, 3 dwg

The invention relates to the field of mechanical engineering and can be used in the design of devices for absorbing and reducing shock loads. Known is a damper containing a cylindrical body and a rod with friction pads placed in it, which are connected to the rod and interact with the inner surface of the body (see and.with . No. 297518, class F 16 F 11/00, 1969). The disadvantage of this device is the instability of the damping characteristics due to large fluctuations in the coefficient of friction depending on the state of the rubbing surfaces (ambient temperature, the presence of dirt on surfaces, coatings, As a result of the analysis of scientific, technical and patent literature, as a prototype of the claimed device, a well-known device for absorbing the energy of an impact of a car was adopted, containing a cylindrical body and a rod placed therein and a cutting device consisting of a knife head, fixed on the stock, and a set of cutting elements, interacting them with the inner surface of the case (see. French patent No. 2137258, cl. F 16 F 7/00, 1972 - prototype). The disadvantages of this device are also the instability of the damping properties, possible jamming of the cutting elements in the body of the cylindrical body due to the unevenness and uncertainty of the depth of penetration of the cutting elements into the side surface of the body, especially under shock loads acting at an angle on the shock-absorbing structure, since the cutting head of the cutting device is fixed immovably on the rod. Jamming can lead to a loss of damping properties of the device and even to breakage of cutting elements when they penetrate into the body. This damper has a relatively low energy consumption due to the limited stroke of the cutting elements along the axis of the body and the significant resistance of the body metal (albeit plastic) to the penetration of cutting elements into it. In addition, the known damper reduces loads only with a single shock impact and cannot reduce repeated loads vibrational damping character, which usually occur after the first impact, maximum in its amplitude value. The purpose of the proposed device is to obtain more stable damping properties compared to the prototype, to increase the energy intensity of the damper and expand its scope (the ability to reduce vibrational loads and loads acting under angle to the damper axis) .To achieve this goal in the proposed device, the process of introducing (cutting) cutting elements into the body material is replaced by a cut of a thin-walled collar of a sleeve made of a plastic material, for example, from aluminum mini alloy type AMts or AD. For this, a cutting device is installed on the rod, fixed on the casing of the damped structure, consisting of a cutter head, a support sleeve and a sleeve made of plastic material installed between them. Wedge-shaped teeth are made on the end face of the cutter head in contact with the sleeve made of plastic material, and on the sleeve made of plastic material there is an annular band or bead. Moreover, the knife head is mounted on the rod coaxially with the sleeve made of plastic material, it covers it due to the larger diameter, i.e. centered on its outer diameter, and, in addition, has the ability to move relative to it in the axial direction. In the initial position, the wedge-shaped teeth of the knife head with their tops rest (contact) on the annular collar of the sleeve and during the operation of the damper, i.e. under the action of shock loads, they interact with it, namely, they cut the grooves in the collar of the sleeve and cut it off with their side surfaces. more stable and definite damping properties of the device. In the proposed device there is no possibility of jamming, because even under the action of loads directed at an angle to the damper axis, the cylindrical body of the cutter head will move along the lateral surface of the sleeve under the action of the axial component of the load. The choice of the bushing material with certain mechanical (plastic) properties and the thickness of its flange (and, therefore, the flange cut area) make it possible to unambiguously determine the impact force leading to a full or partial shear of the annular flange, and by varying the height and angle at the apex of the wedge-shaped teeth cutting the flange, it is possible to provide the necessary stroke of the damper to absorb the impact energy, thereby ensuring its required energy consumption. Making grooves in the collar of the sleeve and pre-installing the tops of wedge-shaped teeth in these grooves improves the characteristics of the damper, because in this case, the tops of the teeth do not cut through the initial grooves (in this case, undesirable bending and creasing of the bead may occur), but immediately begin to cut the bead of the sleeve with their lateral surfaces (a “clean” cut occurs). with a damped structure and a washer of the rod fastening nut, ensures the installation (return) of the rod with the support to its original position after the first impact on the support. This allows to reduce not only single shock loads, but also possible repeated loads. Figure 1 shows a general view of the damper in the initial state. A variant of the device with pre-made grooves in the collar of the sleeve and with the tops of the teeth of the knife head installed in them. Figure 2 shows a general view of the damper after operation with a partial cut of the collar of the sleeve (such a cut of the collar is possible after the first blow). Figure 3 shows a general view. The damper is installed on the casing 1 of the shock-absorbing structure and fixed on it through the rod 2 by nut 3 and washer 4. One end of the rod 2 is fixed to the body 1, at the other end of the rod there is a support 6, which receives shock loads acting on the structure. The cutting device of the damper consists of a support sleeve 5, a knife head 7, at the end 8 of which wedge-shaped teeth 9 are made, and a sleeve 10 of plastic material, equipped with an annular shoulder 11. Support sleeve 5, a knife head 7 and the bushing 10 are mounted on the rod 2, and the bushing 10 is located between the cutter head 7 and support sleeve 5. In this case, the inner diameter of the cutter head 7 is made larger than the outer diameter of the sleeve 10, the body of the cutter head 7 covers the body of the sleeve 10, thereby centering on the outer diameter of the sleeve 10 to ensure a uniform cut of the collar 11 and to ensure free movement cutter head 7 relative to (along) the sleeve 10 when the damper is triggered. The contact of the cutter head 7 and the sleeve 10 is carried out in such a way that the wedge-shaped teeth 9, made on the end face 8 of the cutter head 7, are mounted with their tops 12 on the collar 11 and are in contact with it. The support sleeve 5 serves as a support for the sleeve 10, the diameter of the sleeve 5 must be no larger than the diameter of the sleeve 10 to ensure that its collar 11 is cut off by the teeth 9 of the knife head 7 and the teeth 9 of the knife head 7 can move freely along the sleeve 10 when the damper is triggered. the collar 11 of the sleeve 10 is pre-made grooves 13 in which the tops 12 of the teeth 9 of the knife head 7 are installed. The number of teeth on the end face 8 of the knife head 7 is equal to the number of slots 13 of the collar 11 of the sleeve 10. In this case, when the damper is triggered, the cut of the collar 11 of the sleeve 10 occurs directly by the lateral surfaces of 14 teeth 9. The compression spring 15, covering the support sleeve 5, the knife head 7 and the sleeve 10 made of plastic material (cutting device) and installed on the rod 2 between the body 1 of the shock-absorbing structure and the washer 4 of the nut 5, provides the installation of the rod 2 , washers 4, nuts 3 and support 6 to their original position after the initial impact for the next d damping of possible repeated shocks. The damper works as follows. When the support 6 hits an obstacle, shock loads on the body 1 of the shock-absorbing structure are transmitted through the damper, namely through the support 6, nut 3, washer 4, rod 2. Under the action of the axial component of the shock load, the knife head 7 with the rod 2 moves along the sleeve 10. At the same time, its teeth 9 with their tops 12 cut grooves in the collar 11 of the sleeve 10 and with their lateral surfaces 14 during the subsequent movement along the sleeve 10 they cut off its collar 11 (see. Figures 2 and 3) due to its wedge-shaped shape (the width of the teeth increases with a change in the height of the teeth from their top to the base). The cut of the flange sections between the teeth can be partial or complete, depending on the impact force and the geometric parameters of the flange 11 and the mechanical properties of the material of the sleeve 10. In the case of preliminary execution of the grooves 13 in the shoulder 11 of the sleeve 10 and the installation of the tops 12 of the teeth 9 of the cutter head 7 ( see figure 1), when the damper is triggered, the flange 11 will be cut directly by the side surfaces of the 14 teeth 9. rod 2, washers 4, nuts 3 and support 6 to their original position by spring 15, which is compressed under the action of shock loads (movement of the cutter head 7 relative to the sleeve 10), after the end of the action of the shock loads, the spring 15 is expanded. In this case, the knife head 7 partially cuts off the collar 11 of the sleeve 10 after the first impact (see figure 2) and with subsequent impacts continues to further cut the bead (see figure 3). Thus, the shock load acting on the body 1 of the structure is reduced due to forces of plastic shear of the flange sections of the sleeve by the teeth of the knife head. The claimed device, in comparison with the technical solution adopted as a prototype, makes it possible to effectively reduce both axial loads and loads directed at an angle to the damper axis, as well as shock loads of a repeated nature, eliminating the possibility jamming of cutting elements (there is no penetration of teeth into the material of the sleeve body, there is only a cut of its shoulder). At the same time, the energy intensity of the damper increases and the stability of its damping properties improves. Calculations carried out by the authors, as well as field tests of the device as part of standard products and bench tests as part of working products, have shown significant efficiency of the proposed technical solution for damping shock loads.

Claim

1. A damper containing a housing, a rod and a cutting device placed on it, interacting with the inner surface of the housing, characterized in that the cutting device is made in the form of a knife head with wedge-shaped teeth, a support sleeve and a sleeve of plastic material installed between them, provided with an annular shoulder , moreover, the cutting head is centered on the outer diameter of the sleeve with a collar with the ability to move relative to it, and the wedge-shaped teeth of the knife head interact with the collar of the sleeve with their tops. The damper according to claim 1, characterized in that grooves are made in the annular collar of the sleeve, into which the tops of the wedge-shaped teeth of the knife head are installed, while the teeth interact with the collar of the sleeve with their lateral surfaces. A damper according to claims 1 and 2, characterized in that a spring is installed on the rod, which surrounds the cutting device.

In mechanics, a shock is the mechanical effect of material bodies, which leads to a finite change in the speeds of their points in an infinitely small period of time. Impact motion is a motion that occurs as a result of a single interaction of a body (medium) with the system under consideration, provided that the smallest period of natural oscillations of the system or its time constant are commensurate with or greater than the interaction time.

In the case of shock interaction, shock accelerations, velocity, or displacement are determined at the points under consideration. Collectively, such influences and reactions are called shock processes. Mechanical shocks can be single, multiple and complex. Single and multiple impact processes can affect the apparatus in the longitudinal, transverse and any intermediate directions. Complex shock loads affect the object in two or three mutually perpendicular planes at the same time. Aircraft impact loads can be both non-periodic and periodic. The occurrence of shock loads is associated with a sharp change in acceleration, speed or direction of the aircraft movement. Most often, in real conditions, a complex single shock process occurs, which is a combination of a simple shock pulse with superimposed oscillations.

The main characteristics of the impact process:

• the laws of time variation of shock acceleration a (t), velocity V (t) and displacement X (t) \ duration of impact acceleration t is the time interval from the moment of appearance to the moment of disappearance of shock acceleration, satisfying the condition, a> an, where an is peak shock acceleration;
• the duration of the front of the shock acceleration Tf is the time interval from the moment of the appearance of the shock acceleration to the moment corresponding to its peak value;
• the coefficient of superimposed shock acceleration oscillations is the ratio of the total sum of the absolute values ​​of the increments between adjacent and extreme values ​​of the shock acceleration to its doubled peak value;
• shock acceleration impulse - integral of shock acceleration for a time equal to the duration of its action.

According to the shape of the curve of the functional dependence of the parameters of movement, shock processes are divided into simple and complex. Simple processes do not contain high-frequency components, and their characteristics are approximated by simple analytical functions. The name of the function is determined by the shape of the curve that approximates the dependence of acceleration on time (half-sinusoidal, cosanusoidal, rectangular, triangular, sawtooth, trapezoidal, etc.).

Mechanical shock is characterized by a rapid release of energy, resulting in local elastic or plastic deformations, excitation of stress waves and other effects, sometimes leading to malfunction and destruction of the aircraft structure. The shock load applied to the aircraft excites rapidly decaying natural oscillations in it. The value of the impact overload, the nature and rate of stress distribution over the aircraft structure are determined by the force and duration of the impact, and the nature of the acceleration change. Impact, acting on the aircraft, can cause its mechanical destruction. Depending on the duration, complexity of the impact process and its maximum acceleration during testing, the degree of rigidity of aircraft structural elements is determined. A simple blow can cause destruction due to the occurrence of strong, albeit short-term, overvoltages in the material. A complex impact can lead to accumulation of fatigue microstrains. Since the aircraft design has resonant properties, even a simple impact can cause an oscillatory response in its elements, which is also accompanied by fatigue.

Mechanical overloads cause deformation and breakage of parts, loosening of joints (welded, threaded and riveted), loosening of screws and nuts, movement of mechanisms and controls, as a result of which the adjustment and setting of devices changes and other malfunctions appear.

The fight against the harmful effects of mechanical overloads is carried out in various ways: by increasing the strength of the structure, using parts and elements with increased mechanical strength, using shock absorbers and special packaging, and rational placement of devices. Protection measures against the harmful effects of mechanical overloads are divided into two groups:

1. measures aimed at ensuring the required mechanical strength and rigidity of the structure;
2. measures aimed at isolating structural elements from mechanical stress.

In the latter case, various shock-absorbing means, insulating gaskets, compensators and dampers are used.

The general task of testing an aircraft for the impact of impact loads is to check the ability of the aircraft and all its elements to perform their functions during and after impact, i.e. maintain their technical parameters during and after shock impact within the limits specified in the normative and technical documents.

The main requirements for impact tests in laboratory conditions are the maximum approximation of the result of a test impact on an object to the effect of a real impact in full-scale operating conditions and reproducibility of impact impact.

When reproducing shock loading modes in laboratory conditions, restrictions are imposed on the shape of the instantaneous acceleration impulse as a function of time (Fig. 2.50), as well as on the permissible limits of the impulse shape deviations. Almost every shock impulse on a laboratory bench is accompanied by a pulsation, which is a consequence of resonance phenomena in shock installations and auxiliary equipment. Since the spectrum of the shock pulse is mainly a characteristic of the destructive effect of the shock, even a small pulsation superimposed can make the measurement results unreliable.

Test rigs that simulate single shocks followed by vibrations constitute a special class of mechanical testing equipment. Shock stands can be classified according to various criteria (Fig. 2.5!):

I - according to the principle of shock impulse formation;

II - by the nature of the tests;

III - by the type of reproducible shock loading;

IV - according to the principle of action;

V - by energy source.

In general, the shock stand diagram consists of the following elements (Fig. 2.52): a test object fixed on a platform or container together with a shock overload sensor; acceleration means for communicating the required speed to the object; braking device; control systems; recording equipment for recording the investigated parameters of the object and the law of change in shock overload; primary converters; auxiliary devices for adjusting the operating modes of the test object; power supplies necessary for the operation of the tested object and recording equipment.

The simplest stand for shock tests in laboratory conditions is a stand that works on the principle of dropping a test object fixed on the carriage from a certain height, i.e. using gravity to accelerate. In this case, the shape of the shock pulse is determined by the material and shape of the colliding surfaces. Such stands can provide acceleration up to 80,000 m / s2. In fig. Fig. 2.53, a and b show possible schematic diagrams of such stands.

In the first version (Fig. 2.53, a), a special cam 3 with a ratchet tooth is rotated by a motor. When the cam reaches the maximum height H, the table 1 with the test object 2 falls on the braking devices 4, which impart a shock to it. The shock overload depends on the height of fall H, the stiffness of the braking elements k, the total mass of the table and the test object M and is determined by the following relationship:

By varying this value, various overloads can be obtained. In the second version (Fig. 2.53, b) the stand works according to the dropping method.

Test benches using a hydraulic or pneumatic drive to accelerate the carriage are practically independent of the action of gravity. In fig. 2.54 shows two options for pneumatic shock stands.

The principle of operation of the stand with a pneumatic gun (Fig. 2.54, a) is as follows. Compressed gas is supplied to the working chamber /. When the set pressure is reached, which is controlled by a manometer, the automatic 2 release of the container 3, where the test object is located, is triggered. When exiting the barrel 4 of the air gun, the container contacts the device 5, which makes it possible to measure the speed of the container movement. The air gun is attached to the support legs through shock absorbers b. The predetermined law of braking on the shock absorber 7 is implemented by changing the hydraulic resistance of the overflowing liquid 9 in the gap between the specially profiled needle 8 and the hole in the shock absorber 7.

The structural diagram of another pneumatic shock stand, (Fig. 2.54, b) consists of a test object 1, a carriage 2 on which the test object is installed, a gasket 3 and a brake device 4, valves 5, which allow creating specified gas pressure differences on the piston b, and gas supply systems 7. The braking device is activated immediately after the collision of the carriage and the gasket to prevent the carriage from retracing and distorting the shock waveforms. Management of such stands can be automated. They can reproduce a wide range of shock loads.

As an accelerating device, rubber shock absorbers, springs can be used, as well as, in individual cases, linear induction motors.

The capabilities of almost all shock stands are determined by the design of the braking devices:

1. The impact of the test object with a rigid plate is characterized by deceleration due to the appearance of elastic forces in the contact zone. This method of braking the test object makes it possible to obtain large values ​​of overloads with a small front of their rise (Fig. 2.55, a).

2. To obtain overloads in a wide range, from tens to tens of thousands of units, with their rise time from tens of microseconds to several milliseconds, deformable elements in the form of a plate or gasket are used, lying on a rigid base. The materials for these gaskets can be steel, brass, copper, lead, rubber, etc. (Fig. 2.55, b).

3. To ensure any specific (given) law of variation of n and t in a small range, deformable elements in the form of a tip (crusher) are used, which is installed between the slab of the impact stand and the test object (Fig. 2.55, c).

4. To reproduce an impact with a relatively long braking path, a braking device is used, consisting of a lead, plastically deformable plate located on a rigid base of the stand, and a rigid tip of the corresponding profile that penetrates into it (Fig. 2.55, d), fixed on the object or platform of the stand ... Such braking devices make it possible to obtain overloads in a wide range of n (t) with a short rise time, up to tens of milliseconds.

5. An elastic element in the form of a spring (Fig. 2.55, d) installed on the moving part of the shock stand can be used as a braking device. This type of braking provides relatively small overloads of a half-sinusoidal form with a duration measured in milliseconds.

6. A punctured metal plate, fixed along the contour at the base of the installation, in combination with a rigid tip of the platform or container, provides relatively low overloads (Fig. 2.55, e).

7. Deformable elements installed on the movable platform of the stand (Fig. 2.55, g), in combination with a rigid conical catcher, provide long-term overloads with a rise time of up to tens of milliseconds.

8. A braking device with a deformable washer (Fig. 2.55, h) makes it possible to obtain long braking distances of an object (up to 200 - 300 mm) with small deformations of the washer.

9. The creation in laboratory conditions of intense shock impulses with large fronts is possible when using a pneumatic braking device (Fig. 2.55, s). The advantages of a pneumatic damper include its reusable action, as well as the ability to reproduce shock impulses of various shapes, including those with a significant predetermined front.

10. In the practice of carrying out shock tests, a braking device in the form of a hydraulic shock absorber is widely used (see Fig. 2.54, a). When the test object strikes the shock absorber, its rod is immersed in the liquid. The liquid is pushed out through the stem point according to a law determined by the profile of the regulating needle. By changing the needle profile, it is possible to realize different kind the law of inhibition. The profile of the needle can be obtained by calculation, but it is too difficult to take into account, for example, the presence of air in the piston cavity, friction forces in the sealing devices, etc. Therefore, the calculated profile must be experimentally corrected. Thus, by the computational and experimental method, it is possible to obtain the profile necessary for the implementation of any law of inhibition.

Conducting shock tests in laboratory conditions also puts forward a number of special requirements for the installation of an object. For example, the maximum permissible transverse movement should not exceed 30% of the nominal value; both during tests for impact resistance and during tests for impact strength, the product should be able to be installed in three mutually perpendicular positions with the reproduction of the required number of shock impulses. One-time characteristics of the measuring and recording equipment must be identical over a wide frequency range, which guarantees correct registration of the ratios of different frequency components of the measured pulse.

Due to the variety of transfer functions of different mechanical systems, the same shock spectrum can be caused by shock impulses of different shapes. This means that there is no one-to-one correspondence between some temporal function of acceleration and the shock spectrum. Therefore, from a technical point of view, it is more correct to set the technical conditions for impact tests, containing requirements for the impact spectrum, and not for the time characteristic of acceleration. This primarily relates to the mechanism of fatigue failure of materials due to the accumulation of loading cycles, which can be different from test to test, although the peak values ​​of acceleration and stress will remain constant.

When modeling shock processes, it is advisable to compose the systems of determining parameters according to the factors identified, which are necessary for a sufficiently complete determination of the desired value, which sometimes can only be found experimentally.

Considering the impact of a massive, freely moving rigid body on a deformable element of a relatively small size (for example, a stand brake device), fixed on a rigid base, it is required to determine the parameters of the shock process and establish the conditions under which such processes will be similar to each other. In the general case of the spatial motion of a body, six equations can be compiled, three of which are given by the law of conservation of momentum, two are the laws of conservation of mass and energy, and the sixth is the equation of state. These equations include the following quantities: three velocity components Vx Vy \ Vz> density p, pressure p and entropy. Neglecting dissipative forces and considering the state of the deformed volume to be isentropic, it is possible to exclude entropy from the defining parameters. Since only the motion of the center of mass of the body is considered, it is possible not to include the components of the velocities Vx, Vy among the defining parameters; Vz and coordinates of points Л ", Y, Z inside the deformable object. The state of the deformable volume will be characterized by the following defining parameters:

• the density of the material p;
• pressure p, which is more expedient to take into account through the value of the maximum local deformation and Otmax, considering it as a generalized parameter of the force characteristic in the contact zone;
• the initial impact velocity V0, which is directed along the normal to the surface on which the deformable element is installed;
• current time t;
• body weight t;
• free fall acceleration g;
• the modulus of elasticity of materials E, since the stressed state of the body upon impact (with the exception of the contact zone) is considered elastic;
• characteristic geometric parameter of the body (or deformable element) D.

In accordance with the mc-theorem, out of eight parameters, among which three have independent dimensions, one can make up five independent dimensionless complexes:

Dimensionless complexes composed of the determined parameters of the shock process will be independent by some functions] dimensionless complexes P1 - P5.

The parameters to be determined include:

• current local deformation a;
• body speed V;
• contact force P;
• tension within the body a.

Therefore, we can write functional relationships:

The type of functions / 1, / 2, / e, / 4 can be established experimentally, taking into account a large number of defining parameters.

If, upon impact, residual deformations do not appear in the sections of the body outside the contact zone, then the deformation will have a local character, and, therefore, the complex R5 = pY ^ / E can be excluded.

The complex Jl2 = Pttjjjax) ~ Cm is called the relative body mass coefficient.

The coefficient of the force of resistance to plastic deformation Cp is directly related to the indicator of the force characteristic N (the coefficient of material compliance, depending on the shape of the colliding bodies) by the following relationship:

where p is the reduced density of materials in the contact zone; Cm = t / (pa?) Is the reduced relative mass of colliding bodies, which characterizes the ratio of their reduced mass M to the reduced mass of the deformed volume in the contact zone; xV is a dimensionless parameter characterizing the relative work of deformation.

The function Cp - / s (R1 (R, R3, R4) can be used to determine the overloads:

If we ensure the equality of the numerical values ​​of the dimensionless complexes IJlt R2, R3, R4 for two shock processes, then these conditions, i.e.

will represent the similarity criteria for these processes.

If the indicated conditions are fulfilled, the numerical values ​​of the functions /b / r./z »A» te- at similar moments of time -V CtZoimax- const will also be the same; ^ r = const; Cp = const, which makes it possible to determine the parameters of one shock process by simply recalculating the parameters of another process. The necessary and sufficient requirements for physical modeling of shock processes can be formulated as follows:

1. The working parts of the model and the full-scale object should be geometrically similar.
2. Dimensionless complexes composed of determining pairs, meters, must satisfy condition (2.68). Introducing scale factors.

It should be borne in mind that when modeling only the parameters of the shock process, the stressed states of bodies (nature and model) will necessarily be different.