RU2763858C1 - Method for determining the residual strength of a thin-walled structure - Google Patents

Abstract

FIELD: structural tests.

SUBSTANCE: invention relates to the technique of strength testing of full-scale structures, in particular to a method for a known type of tests for the residual strength of a full-size thin-walled structure. In the process of implementing the proposed method, incisions of reliably detectable sizes are made on the tested structure before it is loaded, in structural elements critical for fatigue conditions. Multifilament crack detectors are mounted on the path of incision development. The structure is loaded stepwise until a signal appears from one of the crack detectors. According to this signal, the structure is automatically unloaded and the position of the vertex of the formed crack is determined by a non-destructive testing method and the crack length is measured. If there is a stable growth of the crack, then the step loading is repeated. Loading is repeated until the crack grows, which, with subsequent loading, can lead to its unstable growth. In this case, the tests are suspended without bringing the structure to its complete destruction. The resulting total size of the incision and crack is taken as a critical size, and the achieved load value is taken as a critical load. Repair of the structure is carried out in the area of the developed crack, and the tests are continued according to the procedure described above until the other incisions grow to critical sizes.

EFFECT: preventing the complete destruction of the structure, as a result of which the costs of performing expensive restorative repairs are reduced, or in case of the impossibility of repair for the manufacture of additional full-size structures, and also leads to a reduction in costs and duration of testing.

1 cl, 4 dwg

Description

The invention relates to the technique of strength testing full-scale thin-walled structures, in particular to a method for determining residual strength by testing. The results of these tests are used in the selection of design, technological and operational solutions in the development of complex structures.

The need to test a damaged structure for residual strength is due to the fact that when calculating the residual strength of complex structures consisting of a large number of different power elements operating under specific conditions, simplifying assumptions inevitably have to be introduced. A number of factors related to the operating conditions of damaged elements in a complex stressed state cannot be taken into account at all. All this is a source of errors in residual strength calculations. This necessitates residual strength tests, which make it possible to determine both the critical size of the damage and the critical value of the load, which lead to the complete destruction of the structure. Due to the risk of complete destruction of the structure during residual strength tests, especially with the possible explosive destruction of hermetic containers when they are pressurized, there is an urgent need to develop a special method that prevents the complete destruction of the structure, which is the object of this invention.

An analogue of the test method is the "Method for studying the crack resistance of materials", copyright certificate SU 1323904 A1, IPC G01N 3/00 (2000-01-01), 07/15/1987. In this method, a sample with a stress concentrator in the form of a notch is loaded until a crack forms, and during loading, the load and displacement of the notch edges are recorded, along which the deformation in the stress concentration zone is determined, and a uniform deformation outside the concentration zone is additionally recorded. A force diagram is built - the difference between local and homogeneous deformations, according to which the moment of crack formation is determined, and the crack resistance of the material is judged by the values of the force and the difference in deformations corresponding to the start of the crack.

The disadvantage of this method, firstly, is that the deformation in the concentration zone is determined indirectly by measuring the displacements of the crack edges. Secondly, crack resistance is judged by the difference in deformations in the concentration zone and outside it at the moment of formation of a crack from a notch. In the case of determining the residual strength of full-scale thin-walled structures damaged by large cracks, this approach leads to large errors in determining the residual strength. This is due to errors in determining the deformations in the concentration zone according to the measurement data of the displacements of the crack faces in the case of its buckling. In addition, the residual strength can only be determined by the moment of crack formation, which does not fully determine the bearing capacity of the damaged structure.

An analogue of the test method is the "Method for determining the residual strength of the structure", copyright certificate SU 1756789 A1, IPC G01M 5/00, G01N 29/04, 08/23/1992, bul. 31. In this method, in order to determine the residual strength of the structure, mainly the wing of the aircraft, resonant vibrations are excited, the vibration frequencies of the tested structure with defects are measured and compared with the vibration frequencies measured on the reference structure. To do this, pre-determine the frequencies of autoresonant vibrations and the breaking loads from bending and torsion, both intact and damaged by various damages of the reference structures. In the obtained range of frequencies and breaking loads, the minimum frequency of bending or torsional vibrations corresponds to the minimum allowable residual strength.

The disadvantage of this method is the need to test one undamaged and several, with various damages, reference structures to determine their frequencies of autoresonant bending and torsional vibrations, as well as breaking loads from bending and torsion. When carrying out such tests, great technical difficulties arise, due to the need to ensure the same distribution of masses, stiffness of the undamaged structure and termination conditions in all tests. In addition, the need to conduct a large number of complex tests of reference structures limits the possibility of applying the method to a wide range of full-size thin-walled structures.

An analogue of the test method is set out in the patent "A method for recognizing critical structural failure sites during static tests and a device for its implementation", patent RU 2 650 749 C2, IPC G01N 3/00 (2006/01), dated 04/17/2018, Bull. No. 11. The method continuously analyzes the process of changing the parameters of the primary sensors from the level of the applied load and when the calculated parameter of any sensor goes beyond the confidence interval, which can occur in the event of plastic deformations, the formation of macrocracks or loss of stability of the structural element, the loading stops and the position of the structure is fixed at the initial stage of destruction while maintaining the integrity of the structure for express analysis or revision.

The disadvantage of this method is that in the case of its application for testing the residual strength of a structure damaged by cracks, it is impossible to preliminarily, by calculation methods with a sufficient degree of accuracy, determine the limiting size of the crack or the limiting load value at which the structure will be completely destroyed. Errors in determining the critical size of a crack or the critical load can lead to the fact that, under loading, the crack will fall into the zone of its unstable development and, as a result, this will lead to the complete destruction of the structure, requiring a very laborious and expensive restoration repair to continue testing.

Closest to the invention in terms of technical essence and the achieved result is the method adopted for the prototype "Method for studying the crack resistance of thin-walled structures", set forth in the author's certificate SU 1104378 A, G01N 3/00, dated 23.07.84, bul. No. 27. This method is essentially a method for determining the residual strength of a thin-walled structure. In this method, a thin-walled structure with a cracking concentrator in the form of a crack of small length (notch) is loaded until the crack begins to grow. During loading, the displacement of the crack faces in the direction perpendicular to the surface of the structure due to buckling is measured. After starting the crack, its length is also measured, and a diagram “displacement of the crack edges-crack length” is plotted. Loading is carried out until the moment of a significant decrease in strength or until destruction. Then, according to the diagram, the load is determined, starting from which, in the specified diagram, the value of the displacement of the crack edges receives a sharp jump, and this load is taken as acceptable for structural studies.

The disadvantage of this method is that in order to determine the residual strength, the loading must be brought to a significant decrease in the bearing capacity or destruction of the structure. In addition, the method has low accuracy due to the fact that it does not take into account the local conditions of fracture at the crack tip. Also, the method does not allow testing a thin-walled structure with a large number of initial cuts due to the risk of complete destruction of the structure during the advanced development of one of them.

The technical result consists in preventing the complete destruction of the structure, resulting in a reduction in the cost of expensive restoration repairs, or, if repair is not possible, in the manufacture of additional full-size structures, and also leads to a reduction in costs and duration of testing.

The technical result is achieved by the fact that in the method for determining the residual strength of a thin-walled structure, including making a notch on the structure, loading the damaged structure, registering the length of the developing crack, the structure is loaded until a crack forms and the load is recorded. Next, step-by-step loading is carried out repeatedly until the crack grows by a given size in the range from 3 to 10 mm and at each step the load is recorded, the structure is unloaded and the length of the crack is recorded, after the crack grows to its precritical state, the tests are stopped, the dependence of the load on the length of the crack is plotted, which determine the critical load and the critical size of the crack, which determine the residual strength of the structure. The size of the crack growth is set in the range from 3 mm to 10 mm.

List of figures:

- in Fig. 1 shows a block diagram of the control of loading and measurements during testing;

- in Fig. 2 shows a diagram of step-by-step loading with registration of crack growth;

- in Fig. 3 shows a) a diagram of notches on the lower surface of the wing; section of the lower spar belt and an incision of 1/3 of the spar wall, b) a section of the butt stringer and an incision of the skin at two interstringer distances, c) a section of the lower spar belt and an incision of 1/3 of the spar wall, as well as an incision of the adjacent skin to the nearest stringer;

- in Fig. 4 shows the cut zone of the lower wing panels with crack and acoustic emission sensors installed.

In FIG. 1 shows: artificial notch 1, multi-filament crack sensor 2, single-filament crack sensor 3, acoustic emission sensor 4, crack 5, crack detector 6, acoustic emission system 7, multi-channel loading system 8.

In FIG. 2 shows: notch size 9, critical crack size plus notch size 10, crack start load 11, incremental loading 12, crack growth curve 13, critical load 14.

In FIG. 4 shows: notch 1, multifilament crack sensor 2, crack 5, acoustic emission sensor 4.

The method is carried out as follows. An incision 1 of size 9 is made on the tested structure. On the predicted path of crack development, multi-filament crack sensors 2 are glued with a pitch between the threads in the range from 3 mm to 10 mm to register the growth of the crack with signaling device 6. The multi-filament crack sensor is glued in such a way that the first thread of the sensor defends about 5 mm from the top of the notch. At a distance of the critical size of the crack growth (approximately 50 mm), a single-filament crack sensor 3 is glued, for emergency unloading of the structure by a multi-channel loading system 8 in the event of a crack growing to a critical size 14. In the vicinity of the crack propagation trajectory, acoustic emission sensors 4 are mounted, the signals of which are recorded by an acoustic system. emission 7 at the time of crack formation. The structure is loaded with multichannel loading system 8 until acoustic emission signals appear from sensors 4. Acoustic emission signals are recorded by acoustic emission system 7 and at the same time multichannel loading system 8 registers the load at which a crack forms.

The loading is continued until the first thread of the multi-thread crack sensor 2 is triggered, caused by the growth of the crack 5, and the load value is recorded. The load is reduced by ≈30% of the achieved value. The magnitude of the load reduction is chosen from the condition of stopping the growth of the crack. Determine the position of the crack tip and measure its length. Next, step-by-step multiple loading 12 of the structure is continued. At each step, the magnitude of the achieved load and the length of the crack are recorded. Due to the risk of complete destruction of the structure, loading is stopped before reaching the critical crack size 10. When the crack approaches the critical size, it is observed that with a small load increment, an increased crack length increment occurs. Increased crack growth is a sign of a pre-critical state of the structure, at which loading is stopped.

The method was tested when testing the full-scale design of the wing of a short-haul aircraft. The tests were carried out in the following sequence. Notches were made on the lower surface of the wing, simulating cracks of reliably detectable sizes 3a, 3b, 3c. Along the edges of the notches, multifilament crack sensors 2 were mounted and acoustic emission sensors 4 were installed.

Upon reaching 40% of the calculated load, the multi-filament crack sensor 2 was triggered. The wing structure was unloaded and the growth of a crack 2 mm long from the cut in the direction of flight was recorded.

Continue loading. Upon reaching 64.4% of the calculated load, the multi-filament crack sensor 2 was triggered. The wing structure was unloaded and the crack grew up to 10 mm from the cut in the direction of flight.

Continue loading. Upon reaching 68.7% of the design load, two multifilament crack sensors were triggered. The wing structure was unloaded and the crack grew up to 28 mm from the cut in the direction of flight and 18 mm against the flight.

Continue loading. Upon reaching 70.9% of the design load, two multifilament crack sensors were triggered. The wing structure was unloaded and the crack grew up to 34 mm from the cut in the direction of flight and 28 mm against the flight.

The tests were stopped due to precritical crack development, which can lead to the complete destruction of the wing.

The tests carried out made it possible to obtain a technical result, which consists in the fact that the residual strength tests determined with sufficient accuracy the critical load and the critical crack size without complete destruction of the wing. Prevention of complete destruction of the wing made it possible to significantly reduce the cost of restoring the crack zone to continue testing in order to determine the critical load and the critical crack size for the remaining damaged zones.

Claims (2)

1. A method for determining the residual strength of a thin-walled structure, including making a notch on the structure, loading the damaged structure, registering the length of the developing crack, characterized in that the structure is loaded until a crack forms and the load is recorded, then stepwise loading is repeatedly performed until the crack grows to a given size and by at each step, the load is recorded, the structure is unloaded and the length of the crack is recorded, after the crack grows to its precritical state, the tests are stopped, the dependence of the load on the length of the crack is plotted, by which the critical load and the critical size of the crack are determined, characterizing the residual strength of the structure. 2. The method according to claim 1, characterized in that the size of the crack growth is set in the range from 3 mm to 10 mm.

Images