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Diagnostics of steel structures with the dynamic non-destructive method

The method of the assessment of steel structures mechanical properties is considered with the nondestructive test by conical indentation. The nondestructive evaluating of the steel mechanical properties in real structures is widely spread in many applied problems. Dynamic indentation method is one of the most effective because of compatibility and accuracy. For this purpose, the static and dynamic problem of axisymmetric elastic-plastic truncated cone indentation is solved, and the results are compared with finite element analysis and experimental data. The method of nondestructive evaluating of mechanical characteristics is suggested and devise of the realization of the method is tested at real structures. The method is tested on steel railway bridges that are being operated for more than 60 years. As a result, zones with lower strength values were identified, and recommendations for strengthening the design were given.


Introduction
One of the main goals in civil engineering is to improve the quality of buildings and structures, and to ensure their reliability. There are many accidents of oil and gas pipelines have become more frequent because of the strength failure of the material. Evaluation of mechanical properties of metal structures for various purposes is one of the most important instruments at any stage of their design and operation. The most important mechanical properties are yield strength, ultimate strength, elongation, Charpy impact strength and hardness. It needs to know these characteristics at all stages of the life cycle structure [1,2] and at any point. Non-destructive testing methods are prioritized especially for existing structures [3]. Recent researches show the possibility of assessment of the parameters of creep of materials for subsequent calculations [4].
The quality management of structures is an effective way of ensuring the reliability of buildings and construction objects that specifies the strict compliance of the characteristics of materials with the requirements of design project at each stage of the technological process. In the process of operation, the structures are exposed to a number of influences: power, shock, heat and others [5]. The properties of materials change under the circumstances, and the further changing must be thoroughly monitored directly on the structure at all stages of the product life cycle: design, manufacture and operation.
The procedure for mechanical properties determining in structural steel using indentation test with spherical indenter was considered in [6,7]. For investigating mechanical properties in steel weld zone, the indentation approach was applied in [8,9]. Instrumented indentation tests were performed across the weld zone and the mechanical properties (E, H,  y , and n) were then determined from their load-depth data. It should be noted that the spherical indenter is very sensitive to the degree of surface roughness and gives a large error on rough asperity.
However, existing methods for assessing the mechanical state of materials do not match modern requirements for express testing on real structures. The of accuracy and the range of the applicability do not allow the use of express methods outside laboratories. Determination of the yield strength, tensile strength and elongation require equipment for uniaxial tensile testing, determination of C V Charpy V-notched absorbed energy and fracture toughness or crack resistance -complex laboratory equipment and qualified personnel. Thus, a new method has to be developed for express estimation of the steel structure mechanical state at any point.

Materials, methods and device for mechanical properties assessment
Theoretical and experimental data obtained in [10] allow us to consider the problem of control of mechanical properties (MP) of materials in a wider aspect. The organization of any kind of design diagnostics is associated with the measurement of MP structural elements. In this case, these properties are determined by the manufacture and testing of special samples. Since such samples cannot be cut from an already manufactured structure, their properties cannot be measured, and the condition of the structure is uncertain.
The most important structural elements are undergone by complex processing (thermal, mechanical, chemical), after which the properties of the material vary greatly, and their values need to be known at the design, manufacturing and operation stages.
In [10] we introduced a device for MP assessment by impact conical indentation. The device (Figure 1) consists of handle 1, spring 2, striker 3, body 4, indenter holder 5, sensor 6 and 9, body part 7, damper 8. Moving the handle 1 upward starts the mechanism. At the same time, the rod of the striker rises until the pawl snaps into place, which is pressed against the spring. Further, when the handle 1 is lowered down, the pawl is withdrawn to the side and releases the striker, which under the action of the spring 2 hits the holder of the indenter 5. The movement of the holder of the indenter 5 is registered by a sensor 6 whose signal is processed by an electronic unit.
Long-term operation of metal structures requires monitoring and periodic diagnosis of their technical condition. In most cases, it is not allowed to make test specimens from existing steel structure. Under the influence of external loads, temperatures, corrosion, the metal changes its properties that have to be determined at any point of the structure.
Let us consider several real constructions where the method of determining the complex of mechanical characteristics by the nondestructive shock imprint method was applied. Recently, we conducted a testing of metal structures of the football stadium overpass to the World Cup 2018 (Figure 2). Next example is survey of 2 bridges across the Don River in the area of Novocherkassk and Belaya Kalitva, Russia (Fig. 3). Both bridges were built in 1948 and during the period of operation to the present they experienced cyclic loading from the passing railway trains. The bridges are made of bridge steel, which for 70 years of operation was corroded and could change its properties due to fatigue damage.

Results and discussion
Analysis of the results of the tests of the steel structure of highway overpass (Figure 2) presented in Table 1 allows us to conclude the following. Minimum values of mechanical characteristics were observed at the metal at point 1 with σ yield = 363 MPa, σ u = 496 MPa, δ 5 = 17%.
Mechanical characteristics correspond to those stated in the project documentation.  Given that bridges were operated without stopping traffic, the only acceptable method is non-destructive testing by portable devices. The measurements were carried out in the zones of the lower belt of trusses, the main longitudinal and transverse beams.
The determination of the mechanical characteristics in each mentioned element of the span structure was carried out in all elements entering the element under consideration in two sections: at the end of the element (non-stressed zone) and in the most stressed section along the first row of rivets.
The tests were carried out in the zones between the rivets of the extreme transverse row, where it was possible, and in the zone above this row. In the cross sections of the elements where the tests were carried out, the numbers of the rolling elements and the measurement points in the stressed and non-stressed zones were taken into account.
In longitudinal and lateral beams, the mechanical characteristics of the metal of rolling elements were determined in the middle sections in accordance with the scheme.

Conclusions
The method of non-destructive testing based on the impact indentation of a conical indenter is considered on the example of steel structures of the road overpass of the Rostov-Arena stadium and steel structures of bridges across the Don river. An important feature of the considered structures is that the road overpass has just been built, and the bridges across the don river were built 70 years ago and operated all this time without stopping.
Results of impact indentation of a conical indenter for several kinds of steels allowed to determine the main mechanical characteristics at various points of structural elements. The elements with the lowest strength characteristics are determined. This approach allows regular studies of mechanical characteristics on real structures [11,12,13] and draw conclusions about their work capacity.
The work was financially supported by Russian Foundation for Base Research (project 18-01-00715-а).