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Non-Destructive Testing of Load-Carrying Metalworks of Objects which have Completed their Terms V.A. Kulish, E.S. Krylov State scientific research and design institute of coal industry "UkrNIIproekt", Ukraine, Kiev 03142, acad. Palladin avenue, 46/2, tel. 444-21-84, telefax (044) 451-00-23 Contact

Problems connected with safe operation of equipment and constructions with completed terms of service are especially pressing in respect to mining objects, such as excavators, spreaders, aerial tramways supports, transport trestles, overhead and gantry cranes.
Load-carrying steelworks are the most important component of the foregoing objects and account for 50-70 % of the machines total weight. In addition structural failure is much more dangerous as compared with mechanisms; therefore, the technical state of structures is the major factor that limits the machines serviceability [1, 2].
As practice shows the main factor reducing the rated service life of load-carrying metalworks is physical wear and tear and mainly fatigue and corrosion failure.
Scientifically grounded prolongation of the equipment service life is based on the conception of "non-dangerous structure damage" that has been predominating lately. According to this conception it is not dangerous to permit the operation of damaged structures if defects that have appeared and developed in the process of work do not lead to failure.
With regard for this conception the work of the materials can be permitted under conditions of specified failure, that, for cracks, corresponds to area BC (Fig. 1); the cracks opening width in this area normally exceeds 10^-3 mm [3].


Fig 1: Change of retained strength of structure (1) and crack size (2) in process of cyclical loading Areas: OD - rated life of structure - 100 %; OA - growth of crack core - 10 % of rated life; AB - development of sub-microcrack into microcrack - 30 % of rated life; BC - growth of fatique crack - up to 50 % of rated life (sensitivity of ultra-sonic testing 10 -3 mm); CD - critical size of fatique crack - up to 10 % of rated life (is detected visually)

At present in accordance with requirements of standards, tolerances relative to welding defects that are detected by traditional methods of non-destructive testing (NDT) (ultra-sonic testing, radiographic method of testing) depend on weld joints category (each weld joint being classified by the type of loading). In most cases, due to the complex scheme of structure loading, it is not possible to refer a weld joints to either category and, therefore, to evaluate the results of NDT.
Acoustic emissive (AE) testing differs advantageously from the traditional methods, because defects detected by this method (cracks, maximal cross-sectional thinning etc.) are progressive and, therefore, especially dangerous. The AE method makes it possible to evaluate how dangerous are the defects.
As the experience of UkrNIIproekt shows, the combined application of AE method and traditional methods of NDT permits to increase trustworthiness and reduce working hours in testing of load-carrying metalworks, owing to the fact, that only those metalworks are subjected to traditional testing, which have progressing defects detected by acoustic emission testing. Inspection of metalworks by traditional methods is conducted in correspondence with norms and instructions developed by UkrNIIproekt.
These norms permit to solve following tasks:

• identify the load-carrying metalworks for top priority testing;
• choose the most effective testing method and monitoring equipment depending on the type of defects and structures tested;
• set up the sequence and procedure of testing;
• draw up paperwork on the results of the testing;
• evaluate the technical state of metalworks;
• organize testing service in the enterprise.
To implement AE method, a testing device UAEKM-m has been designed and is being used. The device includes acoustic post that switches on piesotransducer matched with preamplifier (AE detector), simulator of AE signals, processing unit and PC.
The UAEKM-m device is able to carry out following functions:

• checking the serviceability and tuning the instrumentation by means of simulator;
• gaining and processing information received during testing of load-carrying metalworks;
• detection of defects, progressing under load in steelwork elements;
• displaying the testing results.
The AE simulator provides the simultaneous remote control of serviceability in acoustic and electronic sections.
The design of UAEKM-m detector is covered by UA patent 7250 A [4].
The UAEKM-m device has been successfully tested on the bucket-wheel and walking excavators, as well as on overhead and gantry cranes.


Fig 2: Arrangement of monitored unit of bucket-wheel excavator ERP - 2500 Example of parts designation: 12L - Leftpart;1LR- Left and right parts relative to vertical longitudinal plane of excavator (sight at bucket-wheel)

Fig. 2 illustrates the arrangement of monitored parts of bucket-wheel excavator ERP-2500. Loading modes charts of load-carrying metalworks when tested by UAEKM-m device are given in the table.


Unit of excavator	Unit of steelwork	Arrangement of AE and strain-gauge transducers on monitored units of steelwork	Modes of loading in different positions of boom structures and at various modes of operation		Additional stresses in steelwork elements at various modes of loading, MPa		Instrumentation indication during measurements
			Prelimininary	While testing	Prelimininary (s1)	while testing (s2)
ERP-2500. Counter-weight boom	16L, 16R		Hoisting of bucket-wheel from low to horizontal position	Straight-line motion forwards or backwards within 60 seconds	4,0	12,0-20,0	Angle of boom inclination -0°
ERP-2500. Big balance beam	17,18				6,0	15,0- 25,0	Angle of rotation of superstructure -15° Angle of swing of discharge boom -90°
			Setting of bucket-wheel boom at an angle of 15° and discharge boom at an angle of 90° to longitudinal axis	Straight-line motion forwards or backwards within 60 seconds
Table : Loading modes charts of load-carrying steelworks of bucket-wheel excavator ERP-2500 when tested by means of UAEKM-m device

Conditions and modes of loading in process of testing are set proceeding from the possibility to create maximal additional stresses that are required for excitation of AE signals.
Technical state of load-carrying steelworks is evaluated on results of their combined inspection with application of traditional and AE methods and checking calculation. On this basis trouble-shooting recommendations are worked out and conclusion is made about possibilities of further operation.
Table. Loading modes charts of load-carrying steelworks of bucket-wheel excavator ERP-2500 when tested by means of UAEKM-m device

Conclusions

1. Combined application of traditional and AE methods of NDT during inspection of load-carrying steelworks of objects which have completed their terms, as well as conducting of checking strength analysis permit to increase trustworthiness of diagnostics of steelworks and, consequently, to prolong safe operation of these objects.
2. To determine technical state of load-carrying steelworks of objects wich have completed their terms, guidelines and norms which apply to traditional and AE methods of testing, as well as the UAEKM-m device for acoustic emissive testing of load-carrying steelworks have been worked out by the Institute "UkrNIIproekt".

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