Approximation to the relief of tensions by means of vibration (VSR)

The residual tensions of a material or component are the tensions that exist in him when there is not (and sometimes when not even there has been previously) solicitaciones or external loads. The processes of conformed are the most common causes that cause residual tensions. Virtually all the processes of conformed, welding, mechanised, moldeado, thermal sensors treatments, etc. can be entered also later along the life of the structure by procedures of installation or setting, by overload settlement of the floor or subterranean structures, or by dead loads that can arrive to be integral part of the structure.

Can divide in two big groups: macro and microtensiones, being able to find both present at the same time (Figure 1). The macro tensions, that often designate tensions Type I, vary inside the material of the piece along an a lot greater distance that the size of grain. By the contrary, the origin of the microtensiones has to look for in differences in the microstructure of the material. To his time these tensions are used to to be divided in two groups; Type II and Type III. The residual tensions Type II vary in the scale of a grain while the ones of Type III correspond to the tensions arising inside a grain like consequence of the presence of the dislocaciones or other crystalline defects. These micro tensions often have his origin in the presence of different phases or constituents in a material [1].

1) Mechanical: shot peening, hammer peening, needle peening, ultrasonic peening

2) No mechanical: thermal sensors and by vibrations

The relief of tensions by means of thermal sensors treatment realises heating the piece or union soldered to a temperature and to a period of determinate time. It recommends a gradual warming uniform of all the piece, a time of permanence to determinate temperature (that in the case of the steels is around 600 °C during an hour by inch of thickness), followed of an equally gradual cooling and uniform. When the material is subjected to a warming, produces in him a decrease of his elastic limit; like the residual tensions of the cords of welding (especially the longitudinal) can reach values very near to this, the level of the residual tensions will reduce when redistributing to maintain a balance of strengths. Also it is possible that present mechanisms of termofluencia that relax the tensions during the thermal sensors treatment; however, to reach an important effect require prolonged permanences that no always are possible in the practice.

For the relief of tensions by mechanical treatments have the one of preestirado and by means of vibrations.

The possibility to use the vibrations to relieve the tensions discovered around 1943, during the Second World War, when some constructions soldered that they transported from the workshop of welding to the workshop of machines by train and truck presented a lower distortion when they mechanised . To measure that the war advanced, the Germans entered the “transport after the welding” in components of precision of big size. The engineers did not know why occurred this phenomenon, but thought that it was to consequence of the movements in the piece of work and began to experience and develop methods to generate the movements of a more controlled way. The Americans also discovered that through the vibrations induced in the material, the constructions won in precision.

The relief of tensions by vibration is a relatively simple method that induces harmonic vibrations or sub-harmonic with big amplitude in the material. The usual team consists of an electrical source with a unit of control, an electrical engine that turns eccentric ((inductor of the strength) and an accelerometer (transductor) that registers the flow of frequency through the piece of work (Figure 3).

All the metals have a harmonic frequency. The metal will vibrate to this frequency when it exposes to a sudden impact of example to the martillar on a beam. After the hit, the metal vibrates until all the power dissipates through the internal friction. The relief of tensions by vibration affects the similar material to the hit of hammer. The difference finds inside the source of vibration; since it uses an electrical engine that generates a constant supply of vibrations. When a piece of work does vibrate, the heat accumulates and causes an infinitesimal increase of the temperature in the interior of the material, inducing the sufficient power so that the dislocaciones reposicionen and go back to a level of lower tensions. Of this way the residual tensions reduce in the material.

The vibrations induce in the piece of work of two ways. If the piece of work is the sufficiently big, the vibrator can connect directly in her. The piece of work has to be isolated of the floor with pads of rubber to prevent that the vibrations extend to the surroundings. Another form is to join the vibrator to a table where this anchored one or more pieces of work and also will be isolated by pads of rubber, allowing the relief of the tensions of simultaneous way.

The small pieces have to be always anchored to a table or table of work to guarantee that the harmonic frequency was not out of the rank of the vibrator. The frequency of the vibrations depends of the material, size and form of the piece of work. Generally, the frequency of vibration is in the interval of 0-100Hz. The time for the treatment of vibration is often less than 30 minutes (Figure 4). It knows that the application in which they have showed greater effectiveness is in the dimensional control of structures soldered [5].

At present there are two types of available methods of relief of tensions by vibration

During the first method does a vibrated and slow scanning the piece of work from the frequency zero to the maximum frequency, for example 0-100 Hz during 8 minutes. The answer is analysed and his harmonic frequency registered. Generally, they register two or three of these frequencies. The vibrator adjusts and changes the speed to a that it was equal to the first harmonic frequency. The vibration follows a time given, generally 10 minutes. After this the speed of the vibrator increases to a that it is equal to the following harmonic frequency upper and the process repeats .

The another method, called sub-resonant has the same procedure of initiation where register the harmonic frequencies and, afterwards, the frequency of the vibrations keeps slightly underneath of the harmonic, generally underneath of 4 Hz. When it diminishes and it stabilises the first frequency means that it has relaxed the piece of work from the residual tensions. The time so that this occur is of roughly 20-30 minutes, depending of the size of the piece of work (figure 5).

To different of the methods for relief of tensions by thermal sensors treatment, this present method the different advantages eat:

• The team is compact and portable, in this way, can move to any part of the plant of production or even to move it easily to other companies.
• The costs of investment, operation and maintenance are low.
• The hard process between 15 and 30 minutes by location, according to the weight and configuration of the component or structure.
• Is easy to use and does not require the need of extensive instructions for the operario
• can use in a wide range of materials.
• Does not have negative effects in the material.

The table 1 does a comparative of the relief of tensions by vibration with the conventional method.

References

[1] To. M. Irisarri. Effect of the residual tensions on the provision of diverse components. Annals of Mechanics of the Fracture Vol. I, 2006, 245-250.

[2] P. J. Withers and H. K. D. H. Bhadeshia. Residual stress Part 1 – Measurement techniques. Materials Science and Technology, 2001, Vol. 17, 355-365.

[3] Tim Wessel. Stress Relieving Welds. US Army Corps of Engineers.

[4] http://www.gigabiz.co.uk/

[5] F. Casanova, D. Henao, F. Franco. Dimensional control of unions soldered using treatment of relief of tensions by means of vibrations. Magazine Engineering and Investigation, Vol. 28, Number 1, 2008, 22-25.

[6] http://www.metronicnet.com/pdf/presentacion_estabilizador.pdf

[7] Stefan Lindqvist, Jonas Holmgren. Alternative Methods for Heat Stress Relief. Master of Science Programme, Mechanical Engineering Luleå University of Technology. Department of Applied Physics and Mechanical Engineering, 2007, 84 págs.