RO121752B1 - Thermomechanical fatigue test installation - Google Patents

Abstract

The invention relates to a thermomechanical fatigue test installation for a refractory heat-resistant low-alloy steel with cycles of mechanical and thermal fatigue independently applied to a test specimen, in the temperature range of 20...1000 A C. According to the invention, the installation comprises some fixed and mobile beams (1, 2 and 19), respectively, forming a rigid frame, two columns (3 and 4), a guiding column and a displacement column, a fixed beam (2) being in contact with a power box (18) continued with an upper gripping end (7) where a test specimen (5) is fixed, the diametral deformation thereof being measured by means of a diametral deformation transducer (6), the test speciment (5) being also fixed in a lower gripping unit (7) axially acuated by means of a ball screw (8) and a worm reducer (9), by a stepper motor (11), the displacement of the lower unit (7) being measured by a lower induction system (12), an air-cooled halogen lamp (23) being mounted in the upper gripping end (7) in front of the test specimen (5).

Description

The invention relates to a test facility for thermomechanical fatigue of poorly alloyed, heat-resistant and refractory steels with mechanical and thermal fatigue cycles applied independently to a test in the temperature range + 20 ° C ... + 1000 ° C, in order to determine the curve of non-isothermal oligocyclic fatigue, which is a material characteristic, used in the design codes of the thermomechanically required equipment. The achievements of similar equipment of MTS, Amsler-Wolpert, Instron, etc. These test equipment have several disadvantages, namely:

- either they have heating systems with electric oven with heat radiation to the test tube with isothermal regime (without temperature variation);

- either work in a non-isothermal regime, use the cylindrical tube with full section and the heating is achieved by passing the current through the tube, disturbing the fatigue degradation;

- either use a heater with CIF (high frequency currents) installations which are very expensive, consume a lot of electricity (have the power of about 30 Watt), with a very difficult heating of the test on a calibrated test length (with 80 -90 welded thermocouples on its surface).

For the part of fatigue pulse, which performs the cycles of mechanical fatigue, the electro-hydraulic valves with valves made by most of the mentioned companies are known. This system is very good for fatigue at ambient temperature in the polycyclic domain, where cycles with high demand frequency (1 ... 50 Hz) are required, but for the oligocyclic domain, where the request frequency is low (below 0.1 Hz) it has important disadvantages:

- requires high energy consumption (approx. 30 KW);

- high cost, large gauge, noise and major vibrations, etc.

The solution of the heating furnaces outside the test tube does not provide high heating speed and does not have reliability in the succession of the thermal cycles.

There is also known an installation protected by the patent RO 119798, in which the system for performing cycles of elasto-plastic mechanical deformation is due to the partial blocking of the test heads by an imposed stiffness spring, which allows to determine the resistance to thermal fatigue. . The heating of the test is done by inserting a halogen lamp inside the test. The retrieval of the force and diameter deformation information is carried out each with a translator.

The technical problem that the invention solves is to achieve the improvement of the system for obtaining mechanical fatigue cycles.

The test installation, according to the invention introduces to the installations presented above the following new features:

- the mechanical fatigue cycle charging system is based on an electromechanical system comprising a DC motor drive, from a drive, belt drive, screw reducer and a ball screw drive. Thus, the test specimen with axial loads in the oligocyclic domain (with frequencies below 0.1 Hz) is ensured. The ball screw system avoids the possibility of axial play on both directions of tension (traction-compression);

- the control of the thermal cycles (heating-maintenance-cooling) and the taking of the force signals, the diametrical deformation and the temperature of the test is done by means of a personal computer with interface (separate channels analogical digital converter and analogical digital converter);

The installation is presented using two figures:

FIG. 1, the constructive scheme of the test group,

FIG. 2, block diagram of command, acquisition of signals and tracing of the hysteresis loop.

RO 121752 Β1

The construction scheme of the installation (fig. 1) consists of a rigid frame with three sleepers 1 (one movable-19 and two fixed-1 and 2), two guide columns and two displacement columns (pos. 3 and 4). fitted with the upper clamping end 7 of the test specimen 5 is 3 interposed the force dose 18. The diameter deformation is measured using the diameter deformation transducer 6. 5

The system for performing mechanical fatigue cycles consists of a step-by-step electric motor 11, with reversible rotation direction, belt transmission 10, screw gear reducer 9 and 7 ball screw transmission 8, which act axially (upwards, respectively in bottom) lower clamping tray 7. The installation also includes an inductive system 12, for measuring the displacement of the lower clamping tray 9.

The test system with axial fatigue cycles (traction-purchase-11) can provide control in three distinct ways: with force control (from force dose 18), with deformation control (from diameter 6 deformation transducer) and with displacement control 13 (from inductive displacement transducer 12). On the calibrated portion with the minimum outer diameter of the specimen 5, a thermocouple (chromel-alumel) is welded in points to take the temperature signal.

The control of the cycles of mechanical and thermal fatigue, as well as the retrieval of the signals 17 of temperature, force, diameter deformation and displacement is realized by software from a personal computer 15.16 through an interface. The amplification of the force signals, 19 diameter deformation and displacement is performed with the help of an amplifier bridge 14, and the recording of the voltage-deformation hysteresis loop is made on the computer monitor 21 personally.

The thermal cycles applied to the test can be: 23

- in isothermal regime (T = const);

- in saw teeth: heating from minimum temperature T _min to maximum temperature T _max , 25 followed by cooling to T _min , etc.;

- semi-trapezoid: additional with maintenance at the maximum temperature T _max with the time of 27 maintenance t _men (during the maintenance period at T _max there is a relaxation of the tensile stresses);

- trapezoid: additional with maintenance at maximum temperature T _max a maintenance time 29 t _men1 and with maintenance at minimum temperature T _min a maintenance time t _men2 .

The principle of operation of the installation is that of the request of the cylindrical test tube 31 with thermal and mechanical fatigue cycles independent of each other. the thermomechanical fatigue test may be with hard request (with mechanical deformation control) 33 or with soft request (with force or tension control). Combinations of independent thermal and mechanical cycles of request for oligocyclic fatigue under 35 isizerm respectively:

- isothermal oligocyclic fatigue; 37

- non-isothermal oligocyclic fatigue (thermomechanical fatigue):

- in phase; 39

- in opposition to the phase,

The test group receives the command for the software heating semicycle from the personal computer via a CNA (numerical-analog converter) channel of the interface to the thyristor power control system of the test element heating element 43. The heating is carried out by radiation from a halogen lamp with a power of 1kW.

The test group receives the order for the software cooling semicycle from the computer 45 via other CNA channels of the interface to the coil control system of a pneumatic distributor that allows access to air (or inert gas) on the calibrated portion 47 of the test specimen. . The control of maintaining at temperatures T _max (respectively T _min ) is given by the software with the appropriate adjustment of the heating element. 49

RO 121752 Β1

The test reaction (feadback) is performed by controlling the temperature with the help of a chrome-aluminum thermocouple welded by points on the calibrated portion of the test specimen, the signal of which is taken over by a thermocouple adapter, which in turn releases a signal. electrical in the range of 2-10 V usable by a channel of the analog numerical converter of the interface by the computer. The system allows heating semi-cycles up to 1000 ° C, with a fast reaction and with a maximum measurement error of ± 1%. The control of the mechanical cycles of the test is done from the computer through the interface to the electrical system that operates the electric motor of direct current in both directions of rotation, allowing its request in both traction and compression. The command and control of the request is made either at hard request (control of the deformation, by taking the signals of diameter deformation and transforming them into axial deformation), or at soft request (control of the force or tension at the force dose).

The force and diameter deformation signals are taken over by a channel of the amplification bridge in the +10 V. field. The calibration diagrams of the two measurement translators are entered in the computer memory, the intermediate values obtained by linear interpolation. On the monitor 14 of the computer you can see the variation of the cycles of temperature, force (voltage) and axial deformation (total or plastic). The hysteresis loop 17 of elasto-plastic deformation oe _x for the prescribed cycles can be recorded on the computer monitor.

The test can be stopped automatically when the value of the voltage amplitude in the current request cycle decreases to 50% of the one in the first request cycle, or by the command from the console when macrofissures with a minimum length appear on the calibrated portion of the specimen. of approx. 10% of the diameter of the calibrated portion of the test specimen. The solution chosen for the heating cycle with a halogen lamp allows very high speeds to be achieved. 100 ° C / s and has the following advantages:

- power supply directly from the network;

- sufficient power for the 1 KW lamp for heating up to 1OOO'C and thus reduced electricity consumption;

- high reliability and relatively low cost;

- quick and easy replacement in case of damage;

- standardized execution and availability.

The software system that runs the entire process of conducting the tests ensures that the following functions are fulfilled:

- acquisition of temperature, diameter deformation and mechanical tension values;

- real-time temperature adjustment according to the cyclic profile imposed by the operator;

- dialogue with the operator for starting and stopping the test block, setting the test parameters and other commands;

- display during the test of measured and calculated data in numerical and graphical form (temperature variation diagram, hysteresis loop, etc.);

- archiving on disk the main data obtained during the test, for further processing;

- statistical calculations of the experimental results;

- drawing the hysteresis loop of elasto-plastic deformation;

- editing the test report.

The test facility presented is used to perform the tests of thermomechanical fatigue in order to raise the oligocyclic fatigue curve, which is a characteristic material determinant that the designer of installations that works under the conditions of the thermomechanical demands uses for the calculation of the respective lifetime.

Claims (1)

Claim 1 Test facility for thermomechanical fatigue, controlled by a personal computer (15), consisting of a test group, consisting of a load frame with two displacement columns (4), two guide columns (3) , two fixed sleepers (1, 2) and one movable (19), two fastening heads (7) one lower and one upper, between which a test specimen (5), a force transducer (18) and a fastener are fixed; a diameter 7 deformation transducer (6), a halogen lamp (23) placed inside the specimen, characterized by the fact that it also has an inductive transducer system (12) for measuring the displacement of the bottom feed tray (7) and a system for performing mechanical fatigue cycles consisting of a step-by-step electric motor (11), with reversible rotation direction, a belt drive (10), an 11 screw reducer (9) and a screw transmission with balls (8), which axially actuate the clamping rod (7) in cold, and on the specimen (5), on the calibrated portion with the minimum outside diameter 13, a chromel-aluminum thermocouple is welded in points for taking the temperature signal, so that the request through independent mechanical and thermal cycles allows both control. tension σ, as well as that of the axial deformation e _x , allowing the determination of the resistance to non-isothermal oligocyclic fatigue, which is a material characteristic for the design of the equipment.