If the products have shock absorbers, then when choosing the duration of the impact acceleration, the lower resonant frequencies of the products themselves, and not the protection elements, are taken into account.

The parameters to be checked are chosen, by changing which one can judge the shock resistance of the electronic equipment as a whole (distortion of the output signal, stability of the functioning characteristics, etc.).

When developing a test program, the directions of impacts are set depending on the specific properties of the tested REA. If the properties of the REA are unknown, then the test should be carried out in three mutually perpendicular directions. In this case, it is recommended to choose (from the range specified in the TS) the duration of the shocks that cause resonant excitation of the tested REE.

Impact strength is evaluated by structural integrity (eg, no cracks, contact). Products are considered to have passed the impact test if, after testing, they meet the requirements of standards and PI for this type of test.

The impact test is recommended to be carried out after the impact test. Often they are combined. In contrast to the impact strength test, the impact resistance test is carried out under an electrical load, the nature and parameters of which are established in TU and PI. At the same time, the control of the REA parameters is carried out during the impact to check the performance of products and identify false positives. Products are considered to have passed the test if during and after it they meet the requirements established in the standards and PI for this type of test.

2.3. Task three.

To study devices for testing electronic equipment for impact /1. pp.263-268. 2. pp. 171-178. 3. p.138-143/

Devices for testing. Impact stands are classified according to the following criteria:

By the nature of the reproducible blows - stands of single and multiple blows;

According to the method of obtaining shock overloads - stands of free fall and forced acceleration of the platform with the tested product;

According to the design of brake devices - with a rigid anvil, with a springy anvil, with shock-absorbing rubber and felt pads, with collapsible deformable brake devices, with hydraulic brake devices, etc.

Depending on the design of the shock stand and in particular on the brake device used in it, shock pulses of half-sinusoidal, triangular and trapezoidal shape are obtained.

To test REA for single impacts, impact test benches are used, and for multiple impacts, cam-type test benches that reproduce impacts of a half-sinusoidal shape are used. These stands use the principle of free fall of the platform with the product under test on shock-absorbing pads.

The main elements of the impact stand of the pile type (Fig. 2.3.1.) are: table 3; base 7, which serves to dampen the speed of the table at the moment of impact; guide 4, which ensures the horizontal position of the table at the moment of impact; gaskets 5, forming a shock impulse.

The energy required to create an impact is accumulated as a result of lifting the table with the tested product fixed on it to a predetermined height. For lifting the table and its subsequent dropping, the stand is equipped with a drive and a reset mechanism. Kinetic energy acquired by the body in the process

Sound insulation that reduces the level of sound pressure to established standards;

Ground loop, resistance not 40m;

Concrete foundation.

4. During operation, the shock stand must be

installed on the foundation.

5. Power supply of the unit from the AC mains

voltage 220± V, frequency 50 Hz.

6. Electrical power consumption (maximum) is not

more than 1kW.

7. The installation provides combinations of accelerations and

Impact mechanism. In the mechanics of an absolutely rigid body, impact is considered as a jump-like process, the duration of which is infinitely small. During the impact, at the point of contact of the colliding bodies, large, but instantly acting forces arise, leading to a finite change in the momentum. In real systems, finite forces always act during a finite time interval, and the collision of two moving bodies is associated with their deformation near the point of contact and the propagation of a compression wave inside these bodies. The duration of the impact depends on many physical factors: the elastic characteristics of the materials of the colliding bodies, their shape and size, the relative speed of approach, etc.

The change in acceleration with time is commonly called a shock acceleration impulse or a shock impulse, and the law of change in acceleration with time is called the form of a shock impulse. The main parameters of the shock pulse include peak shock acceleration (overload), the duration of the shock acceleration and the shape of the pulse.

There are three main types of product response to shock loads:

* ballistic (quasi-damping) mode of excitation (the period of EI natural oscillations is greater than the duration of the excitation pulse);

* quasi-resonant mode of excitation (the period of EI natural oscillations is approximately equal to the duration of the excitation pulse);

* static mode of excitation (the period of EI natural oscillations is less than the duration of the excitation pulse).

In the ballistic mode, the maximum value of the EM acceleration is always less than the peak acceleration of the impact pulse. Quasi-resonant The quasi-resonant excitation mode is the most rigid in terms of the magnitude of the excited accelerations (m is more than 1). In the static mode of excitation, the response of the ED completely repeats the acting pulse (m=1), the test results do not depend on the shape and duration of the pulse. Tests in the static region are equivalent to tests for the effects of linear acceleration, since it can be seen as a stroke of infinite duration.

Drop tests are carried out in a quasi-resonant mode of excitation. Impact strength is evaluated by the integrity of the design of the power plant (no cracks, chips).

Impact tests are carried out after impact tests under electrical load to verify the ability of the ED to perform its functions under mechanical shock conditions.

In addition to mechanical shock stands, electrodynamic and pneumatic shock stands are used. In electrodynamic stands, a current pulse is passed through the excitation coil of the moving system, the amplitude and duration of which are determined by the parameters of the shock pulse. On pneumatic stands, impact acceleration is obtained when the table collides with a projectile fired from an air gun.

The characteristics of shock stands vary widely: load capacity, load capacity - from 1 to 500 kg, number of beats per minute (adjustable) - from 5 to 120, maximum acceleration - from 200 to 6000 g, duration of blows - from 0.4 to 40 ms.

Estimate the time of elastic impact of solid bodies, considering the collision of a rod that hits an immovable non-deformable wall (Fig.).

Most often in problems it is assumed that the elastic impact of solids occurs instantly, but it is quite obvious that this assumption is an idealization.
The collision of real bodies always takes a finite amount of time τ . In fact, if the change in the momentum of the body during the collision occurred instantly,
F = mΔv/t →0 → ∞
then the force of interaction of bodies upon impact would be infinitely large, which, of course, does not happen.
What can determine the duration of the collision? Let us assume that we consider the reflection of an elastic body from a non-deformable wall. During the collision, the kinetic energy of the body during the first half of the collision is converted into the potential energy of the elastic deformation of the body. During the second half, the deformation energy is converted back into the kinetic energy of the bouncing body.

This idea was embodied in the testing problem 2005. Solve this problem to understand this moment.
 A task. Two perfectly elastic washers with masses m 1 \u003d m 2 \u003d 240 g each slide translationally on a smooth horizontal surface towards each other with speeds, the modules of which v 1 \u003d 21 m / s and v 2 \u003d 9.0 m / s. Maximum value of potential energy E elastic deformation of the washers during their central collision is equal to ... J.

Therefore, it is obvious that the elastic properties of the body play a certain role in a collision. So, we can expect that the impact duration depends on the Young's modulus of the material of the body E, its density ρ and its geometric dimensions. It is possible that the duration of the blow τ also depends on the speed v with which the body hits the obstacle.
It is easy to see that it is not possible to estimate the collision time using dimensional considerations alone. Indeed, even if we take a ball as an incident body, the dimensions of which are characterized by only one parameter - the radius R, then from the quantities E, ρ , R and v it is possible to compose an innumerable set of expressions having the dimension of time:
τ = √(ρ/E) × f(ρv 2 /E), (1)
where f− arbitrary function of dimensionless quantity ρv 2 /E. Therefore, to find τ dynamic consideration is needed.
It is easiest to carry out such a consideration for a body that has the shape of a long rod.
Let a rod moving with speed v, butt-ends on a fixed wall. When the end section of the rod comes into contact with the wall, the velocities of the particles of the rod lying in this section instantly vanish. At the next moment of time, the particles located in the neighboring section stop, and so on. The section of the rod, the particles of which have already stopped by this moment, is in a deformed state. In other words, at this moment of time, that part of the rod is deformed, to which the wave of elastic deformation has reached, propagating along the rod from the point of contact with the barrier. This deformation wave propagates along the rod at the speed of sound u. If we assume that the rod came into contact with the wall at the time t = 0, then at the time t the length of the compressed part of the rod is ut. This part of the rod in Fig. a shaded.

In the unshaded part of the rod, the velocities of all its particles are still equal v, and in the compressed (shaded) part of the rod, all particles are at rest.
The first stage of the process of collision of the rod with the wall will end at the moment when the entire rod turns out to be deformed, and the velocities of all its particles become zero (Fig. b).

At this moment, the kinetic energy of the projectile rod is completely converted into the potential energy of elastic deformation. Immediately after this, the second stage of the collision begins, in which the rod returns to the undeformed state. This process begins at the free end of the rod and, propagating along the rod at the speed of sound, gradually approaches the barrier. On fig. in

the rod is shown at the moment when the unshaded part is no longer deformed and all its particles have a speed v pointing to the left. The shaded area is still deformed, and the velocities of all its particles are equal to zero.
The end of the second stage of the collision will come at the moment when the entire rod turns out to be undeformed, and all the particles of the rod acquire speed v, directed opposite to the speed of the rod before impact. At this moment, the right end of the rod separates from the barrier: the undeformed rod bounces off the wall and moves in the opposite direction with the same modulo speed (Fig. G).

In this case, the elastic deformation energy of the rod is completely converted back into kinetic energy.
It is clear from the foregoing that the duration of the collision τ is equal to the time of passage of the elastic deformation wave front along the rod back and forth:
τ = 2l/u, (2)
where l is the length of the rod.
The speed of sound in the rod u can be determined as follows. Consider the rod at time t(rice. a) when the deformation wave propagates to the left. The length of the deformed part of the rod at this moment is equal to ut. With respect to the undeformed state, this part is shortened by the value vt, equal to the distance traveled by this moment by the still undeformed part of the rod. Therefore, the relative deformation of this part of the rod is equal to v/u. Based on Hooke's Law
v/u = (1/E) × F/S, (3)
where S− cross-sectional area of ​​the rod, F is the force acting on the rod from the side of the wall, E− Young's modulus.
Since the relative deformation v/u is the same at all times while the rod is in contact with the barrier, then, as can be seen from formula (3), the force F constant. To find this force, we apply the law of conservation of momentum to the stopped part of the rod. Before contact with the barrier, the considered part of the rod had momentum ρSut.v, and at the moment of time t its momentum is zero.
That's why
ρSut.v = Ft. (4)
Substituting force from here F into formula (3), we obtain
u = √(E/ρ). (5)
Now the expression for time τ . The collision deformation of the rod with the wall (2) takes the form
τ = 2l√(ρ/E). (6)
Collision time τ can be found in another way, using the law of conservation of energy for this. Before the collision, the rod is undeformed and all its energy is the kinetic energy of translational motion mv 2 /2. After some time τ/2 from the beginning of the collision, the velocities of all its particles, as we have seen, vanish, and the entire rod appears to be deformed (Fig. b). The length of the rod has decreased by the amount Δl compared to its undeformed state (Fig. d).

At this moment, the entire energy of the rod is the energy of its elastic deformation. This energy can be written as
W = k(Δl) 2 /2,
where k− coefficient of proportionality between force and deformation:
F = kΔl.
This coefficient, using Hooke's law, is expressed in terms of Young's modulus E and rod dimensions:
σ = F/S = (∆l/l)E,
F = SEΔl/l and F = kΔl,
from here
k = ES/l. (7)
Maximum deformation Δl is equal to the distance over which the particles of the left end of the rod move during the time τ/2(rice. d). Since these particles are moving at a speed v, then
Δl = vτ/2. (8)
We equate the kinetic energy of the rod before the impact and the potential energy of deformation. Considering that the mass of the rod
m = ρSl,
and using relations (7) and (8), we obtain
ρSlv 2 /2 = ES/(2l) × (vτ/2) 2,
where for τ again we obtain formula (6).
This collision time is usually very short. For example, for a steel rod ( E \u003d 2 × 10 11 Pa, ρ \u003d 7.8 × 10 3 kg / m 3) length 28 cm calculation by formula (6) gives τ = 10 −4 s.
Strength F, acting on the wall during impact, can be found by substituting the speed of sound in the rod (5) into formula (4):
F = Sv√(ρE). (9)
It can be seen that the force acting on the wall is proportional to the speed of the rod before impact. But for the applicability of the above solution, it is necessary that the mechanical stress of the rod F/S did not exceed the elastic limit of the material from which the rod is made. For example, for steel, the elastic limit
(F/S) max = 4 × 10 8 Pa.
Therefore, the maximum speed v steel rod, at which its impact with the barrier can still be considered elastic, turns out to be, according to formula (9), equal to 10 m/s. This corresponds to the free fall speed of a body from a height of only 5 m.
Let us indicate for comparison that the speed of sound in steel u = 5000 m/s, i.e. v<< u .
The time of collision of the rod with a fixed barrier (in contrast to the force) turned out to be independent of the speed of the rod. This result, however, is not universal, but is related to the specific shape of the body in question. For example, for an elastic ball, the time of collision with the wall depends on its speed. The dynamic consideration of this case turns out to be more complicated. This is due to the fact that both the contact area of ​​the deformed ball with the wall and the force acting on the ball during the collision do not remain constant.

Punch Power - Momentum, Speed, Technique and Explosive Strength Drills for Fighters

Punch Power - Momentum, Speed, Technique and Explosive Strength Drills for Fighters

The issue was filmed in the Leader-Sport fitness club

Pavel Badyrov, the organizer of the punching power tournament, master of sports in powerlifting, multiple champion and record holder of St. Petersburg in bench press, continues to talk about punching power, punching speed, and also shows exercises for explosive strength for fighters.

Hit

Impact is a short-term interaction of bodies, during which the kinetic energy is redistributed. It often has a destructive character for interacting bodies. In physics, impact is understood as such a type of interaction between moving bodies, in which the interaction time can be neglected.

Physical abstraction

Upon impact, the law of conservation of momentum and the law of conservation of angular momentum are satisfied, but usually the law of conservation of mechanical energy is not fulfilled. It is assumed that during the impact the action of external forces can be neglected, then the total momentum of the bodies during the impact is preserved, otherwise the impulse of external forces must be taken into account. Part of the energy is usually spent on heating bodies and sound.

The result of a collision of two bodies can be fully calculated if their motion before the impact and the mechanical energy after the impact are known. Usually, either an absolutely elastic impact is considered, or the energy conservation coefficient k is introduced, as the ratio of the kinetic energy after the impact to the kinetic energy before the impact when one body hits a fixed wall made of the material of another body. Thus, k is a characteristic of the material from which the bodies are made, and (presumably) does not depend on the other parameters of the bodies (shape, speed, etc.).

How to understand the impact force in kilograms

Momentum of a moving body p=mV.

When braking against an obstacle, this impulse is “quenched” by the impulse of the resistance force p=Ft (the force is not constant at all, but some average value can be taken).

We get that F = mV / t is the force with which the obstacle slows down the moving body, and (according to Newton's third law) the moving body acts on the obstacle, i.e. the impact force:
F = mV / t, where t is the impact time.

Kilogram-force is just an old unit of measurement - 1 kgf (or kg) \u003d 9.8 N, that is, this is the weight of a body weighing 1 kg.
To recalculate, it is enough to divide the force in newtons by the acceleration of free fall.

ONCE AGAIN ABOUT THE POWER OF IMPACT

The vast majority of people, even with a higher technical education, have a vague idea of ​​what impact force is and what it can depend on. Someone believes that the impact force is determined by momentum or energy, and someone - by pressure. Some confuse strong blows with blows that cause injury, while others believe that the force of the blow should be measured in units of pressure. Let's try to clarify this topic.

Impact force, like any other force, is measured in Newtons (N) and kilogram-forces (kgf). One Newton is the force due to which a body of mass 1 kg receives an acceleration of 1 m/s2. One kgf is a force that imparts an acceleration of 1 g = 9.81 m/s2 to a body weighing 1 kg (g is the free fall acceleration). Therefore, 1 kgf \u003d 9.81 N. The weight of a body with mass m is determined by the force of attraction P, with which it presses on the support: P \u003d mg. If your body weight is 80 kg, then your weight, determined by gravity or attraction, P = 80 kgf. But in common parlance they say “my weight is 80 kg”, and everything is clear to everyone. Therefore, often they also say about the impact force that it is some kg, but kgf is meant.

The force of impact, unlike the force of gravity, is rather short-term in time. The shape of the shock pulse (during simple collisions) is bell-shaped and symmetrical. In the case of a person hitting a target, the shape of the pulse is not symmetrical - it increases sharply and falls relatively slowly and in waves. The total duration of the impulse is determined by the mass invested in the blow, and the rise time of the impulse is determined by the mass of the percussion limb. When we talk about impact force, we always mean not the average, but its maximum value in the process of impact.

Let's throw a glass not very hard at the wall so that it breaks. If it hits the carpet, it might not break. In order for it to break for sure, it is necessary to increase the force of the throw in order to increase the speed of the glass. In the case of the wall, the blow turned out to be stronger, since the wall is harder, and therefore the glass broke. As we can see, the force acting on the glass turned out to depend not only on the strength of your throw, but also on the rigidity of the place where the glass hit.

So is a man's blow. We only throw our hand and the part of the body involved in the strike at the target. As studies have shown (see "Physical and Mathematical Model of Impact"), the part of the body involved in the impact has little effect on the force of the impact, since its speed is very low, although this mass is significant (reaches half the body mass). But the impact force was proportional to this mass. The conclusion is simple: the impact force depends on the mass involved in the impact, only indirectly, since it is with the help of just this mass that our impact limb (arm or leg) is accelerated to maximum speeds. Also, do not forget that the momentum and energy imparted to the target upon impact is mainly (by 50–70%) determined by just this mass.

Let's get back to punching power. The impact force (F) ultimately depends on the mass (m), dimensions (S) and speed (v) of the striking limb, as well as on the mass (M) and stiffness (K) of the target. The basic formula for the impact force on an elastic target is:

It can be seen from the formula that the lighter the target (bag), the lower the impact force. For a 20 kg bag, compared to a 100 kg bag, the impact force is reduced by only 10%. But for bags of 6–8 kg, the impact force already drops by 25–30%. It is clear that by hitting the balloon, we will not get any significant value at all.

You will have to basically take the following information on faith.

1. A straight punch is not the strongest of punches, although it requires good technique and especially a sense of distance. Although there are athletes who do not know how to hit the side, but, as a rule, their direct hit is very strong.

2. The force of a side impact due to the speed of the striking limb is always higher than that of a direct one. Moreover, with a delivered blow, this difference reaches 30-50%. Therefore, side punches are usually the most knockout.

3. A backhand blow (like a backfist with a turn) is the easiest in execution technique and does not require good physical preparation, practically the strongest among hand strikes, especially if the striker is in good physical shape. You just need to understand that its strength is determined by a large contact surface, which is easily achievable on a soft bag, and in real combat, for the same reason, when hitting a hard complex surface, the contact area is greatly reduced, the impact force drops sharply, and it turns out to be ineffective. Therefore, in combat, it still requires high accuracy, which is not at all easy to implement.

Once again, we emphasize that the blows are considered from a position of strength, moreover, on a soft and large bag, and not on the amount of damage inflicted.

Projectile Gloves reduce hits by 3-7%.

Gloves used for competition attenuate impacts by 15-25%.

For reference, the results of measurements of the strength of delivered strikes should be as follows:

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#boxing_lessons

Impact force - momentum, speed, technique and explosive strength exercises for fighters from Pavel Badyrov updated: January 6, 2018 by: Boxingguru

In mechanics, impact is the mechanical action of material bodies, leading to a finite change in the velocities 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 or greater than the interaction time.

During impact interaction at the points under consideration, impact accelerations, speed or displacement are determined. Together, such impacts 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 impact loads act on an object in two or three mutually perpendicular planes simultaneously. Impact loads on an aircraft can be both non-periodic and periodic. The occurrence of shock loads is associated with a sharp change in the acceleration, speed or direction of movement of the aircraft. Most often in real conditions there is a complex single shock process, which is a combination of a simple shock pulse with superimposed oscillations.

The main characteristics of the shock process:

• laws of change in time of impact acceleration a(t), velocity V(t) and displacement X(t) peak shock acceleration;
• duration of shock acceleration front Tf - time interval from the moment of occurrence of shock acceleration to the moment corresponding to its peak value;
• coefficient of superimposed fluctuations of shock acceleration - the ratio of the total sum of absolute values ​​of increments between adjacent and extreme values ​​of shock acceleration to its doubled peak value;
• impact acceleration impulse - the integral of impact acceleration over a time equal to the duration of its action.

According to the shape of the curve of the functional dependence of motion parameters, 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 approximating the dependence of acceleration on time (half-sinusoidal, cosanusoidal, rectangular, triangular, sawtooth, trapezoidal, etc.).

A 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 damped natural oscillations in it. The value of overload upon impact, the nature and rate of stress distribution over the structure of the aircraft are determined by the force and duration of the impact, and the nature of the change in acceleration. 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 the aircraft structural elements is determined. A simple impact can cause destruction due to the occurrence of strong, albeit short-term overstresses in the material. A complex impact can lead to the accumulation of fatigue microdeformations. Since the aircraft design has resonant properties, even a simple impact can cause an oscillatory reaction in its elements, also accompanied by fatigue phenomena.

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

The fight against the harmful effects of mechanical overloads is carried out in various ways: 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. Measures to protect 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 influences.

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

The general task of testing an aircraft for impact loads is to check the ability of an aircraft and all its elements to perform their functions during and after impact, i.e. maintain their technical parameters during impact and after it within the limits specified in the regulatory 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 natural operating conditions and the reproducibility of the impact.

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

Test rigs that simulate individual impacts followed by vibrations constitute a special class of equipment for mechanical testing. Impact 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 - according to the type of reproducible shock loading;

IV - according to the principle of action;

V - according to the energy source.

In general, the scheme of the shock stand consists of the following elements (Fig. 2.52): the test object, mounted 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 of shock overload; primary converters; auxiliary devices for adjusting the modes of operation of the tested object; power supplies necessary for the operation of the tested object and recording equipment.

The simplest stand for impact testing in laboratory conditions is a stand that operates on the principle of dropping a test object fixed on a carriage from a certain height, i.e. using the earth's gravity to disperse. In this case, the shape of the shock pulse is determined by the material and shape of the colliding surfaces. On such stands it is possible to provide acceleration up to 80000 m/s2. On fig. 2.53, a and b shows the fundamentally possible schemes of such stands.

In the first version (Fig. 2.53, a) a special cam 3 with a ratchet tooth is driven 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 give it a blow. Impact overload depends on the height of the fall H, the stiffness of the braking elements h, the total mass of the table and the test object M and is determined by the following relationship:

By varying this value, you can get different overloads. In the second variant (Fig. 2.53, b), the stand works according to the drop method.

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

The principle of operation of the stand with an air gun (Fig. 2.54, a) is as follows. Compressed gas is supplied to the working chamber /. When the predetermined pressure is reached, which is controlled by the manometer, the automat 2 releases the container 3, where the test object is placed. When exiting the barrel 4 of the air gun, the container comes into contact with the device 5, which allows you to measure the speed of the container. The air gun is attached to the support posts through shock absorbers b. The given braking law on the shock absorber 7 is implemented by changing the hydraulic resistance of the flowing fluid 9 in the gap between the specially profiled needle 8 and the hole in the shock absorber 7.

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

As an accelerating device, rubber shock absorbers, springs, and, in some cases, linear asynchronous motors can be used.

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

1. The impact of a test object with a rigid plate is characterized by deceleration due to the occurrence 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 growth (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 are used in the form of a plate or gasket lying on a rigid base. The materials of these gaskets can be steel, brass, copper, lead, rubber, etc. (Fig. 2.55, b).

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

4. To reproduce an impact with a relatively large deceleration path, a braking device is used, consisting of a lead, plastically deformable plate located on the rigid base of the stand, and a hard tip of the corresponding profile that is embedded in 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, e) installed on the movable part of the shock stand can be used as a braking device. This type of braking provides relatively small half-sine overloads with a duration measured in milliseconds.

6. A punched 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 small 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 large deceleration paths for an object (up to 200 - 300 mm) with small deformations of the washer.

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

10. In the practice of shock testing, a braking device in the form of a hydraulic shock absorber has become widely used (see Fig. 2.54, a). When the test object hits the shock absorber, its rod is immersed in the liquid. The liquid is pushed out through the stem point according to the law determined by the profile of the regulating needle. By changing the profile of the needle, it is possible to realize different types of the braking law. 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 sealing devices, etc. Therefore, the calculated profile must be experimentally corrected. Thus, the computational-experimental method can be used to obtain the profile necessary for the implementation of any braking law.

Impact testing in laboratory conditions puts forward a number of special requirements for the installation of the object. So, for example, the maximum allowable movement in the transverse direction should not exceed 30% of the nominal value; both in impact resistance tests and in impact strength tests, the product must be able to be installed in three mutually perpendicular positions with the reproduction of the required number of shock impulses. The one-time characteristics of the measuring and recording equipment must be identical over a wide frequency range, which guarantees the correct registration of the ratios of the various 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 a shock pulse of different shapes. This means that there is no one-to-one correspondence between some acceleration time function and the shock spectrum. Therefore, from a technical point of view, it is more correct to specify specifications for shock tests that contain requirements for the shock spectrum, and not for the time characteristic of acceleration. First of all, this refers to the mechanism of fatigue failure of materials due to the accumulation of loading cycles, which may be different from test to test, although the peak values ​​of acceleration and stress will remain constant.

When modeling impact processes, it is expedient to compose a system of determining parameters according to the identified factors necessary for a fairly complete determination of the desired value, which can sometimes be found only experimentally.

Considering the impact of a massive, freely moving rigid body on a deformable element of a relatively small size (for example, on a brake device of a bench) fixed on a rigid base, it is required to determine the parameters of the impact 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 give the law of conservation of momentum, two - the laws of conservation of mass and energy, 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 assuming the state of the deformable volume to be isentropic, one can exclude entropy from the number of determining parameters. Since only the motion of the center of mass of the body is considered, it is possible not to include the velocity components Vx, Vy among the determining parameters; Vz and coordinates of points L", Y, Z inside the deformable object. The state of the deformable volume will be characterized by the following defining parameters:

• material density 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 stress 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 TS-theorem, eight parameters, three of which have independent dimensions, can be used to compose five independent dimensionless complexes:

Dimensionless complexes composed of the determined parameters of the impact process will be some functions of the independent 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 relations:

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

If, upon impact, no residual deformations appear in the sections of the body outside the contact zone, then the deformation will have a local character, and, consequently, the complex R5 = pY^/E can be excluded.

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

The force coefficient of resistance to plastic deformation Cp is directly related to the force characteristic index N (the coefficient of compliance of the material, 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 = m/(pa?) is the reduced relative mass of the colliding bodies, which characterizes the ratio of their reduced mass M to the reduced mass of the deformable volume in the contact zone; xV is a dimensionless parameter characterizing the relative work of deformation.

The function Cp - /z (R1 (Rr, R3, R4) can be used to determine overloads:

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

will be criteria for the similarity of these processes.

When these conditions are met, the numerical values ​​of the functions /b/g./z» L» me- will also be the same at similar moments of time -V CtZoimax-const; ^r= const; Cp = const, which makes it possible to determine the parameters of one impact process by simply recalculating the parameters of another process. Necessary and sufficient requirements for physical modeling of impact processes can be formulated as follows:

1. The working parts of the model and the natural object must be geometrically similar.
2. Dimensionless complexes, composed of defining para meters, must satisfy condition (2.68). Introducing scaling factors.

It must be borne in mind that when modeling only the parameters of the impact process, the stress states of bodies (natural and model) will necessarily be different.