METHODS FOR EVALUATING FATIGUE STRENGTH AND EXTENDING THE SERVICE LIFE OF FRAME STRUCTURES

ABSTRACT

This article discusses modern approaches to the analysis and calculation of metal structures, particularly methods for assessing fatigue failure of locomotive frame structures based on fracture mechanics. Microcracks resulting from variable dynamic loads and their development process are analyzed using the Paris-Erdogan equation and stress intensity factor. The methodology for determining the residual life allows for predicting the service life of locomotives, improving operational efficiency, and planning preventive maintenance measures.

АННОТАЦИЯ

В данной статье освещены современные подходы к анализу и расчету металлических конструкций, в частности, методы оценки усталостного разрушения рамных конструкций локомотивов на основе механики хрупкого разрушения. Микротрещины, возникающие в результате переменных динамических нагрузок, и процесс их развития анализируются с помощью уравнения Париса-Эрдогана и коэффициента интенсивности напряжений. Методика определения остаточного ресурса позволяет прогнозировать срок службы локомотивов, повышать эффективность эксплуатации и планировать профилактические мероприятия.

Keywords: metal structures, mechanics of brittle fracture, fatigue damage, dynamic loading, crack propagation, residual life, strength analysis, locomotive frame, fatigue limit, operational safety.

Ключевые слова: металлические конструкции, механика хрупкого разрушения, усталостные повреждения, динамическая нагрузка, развитие трещин, остаточный ресурс, анализ прочности, рама локомотива, предел усталости, безопасность эксплуатации.

INTRODUCTION.

Modern approaches to the analysis and calculation of metal structures are aimed at accurately assessing material defects, crack resistance, and load characteristics. These methods are developing based on a relatively new and promising field of deformable solid mechanics - brittle fracture mechanics. This approach allows for accurate prediction of the strength and service life of structures, taking into account actual operating conditions and failure processes. It expands the possibility of precise forecasting of structural strength and service life, considering real operating conditions and failure processes. One of the main advantages of this methodology is the ability to analyze in detail the processes of crack development, starting from the crack initiation stage. While traditional continuum mechanics considers the structure as a whole object, brittle fracture mechanics enables modeling the failure process by taking into account the shape, size, direction, and location of defects in the material. This approach is especially important for structures operating under variable loads, sharp temperature fluctuations, and aggressive environments. Using this method, determining the residual resource of metal structures becomes more accurate, and unlike the general assumptions inherent in traditional methods, it creates the possibility of individual assessment for each object. This approach allows for effective planning of preventive measures during operation and, if necessary, making precise decisions regarding the repair or replacement of individual structural elements. As a result, the operation process becomes safer and more reliable, creating a foundation for extending the service life and increasing the economic efficiency of metal structures. The implementation of brittle fracture mechanics opens up new possibilities in the processes of designing, diagnosing, and operating structures.

RESULTS AND ANALYSIS OF RESULTS

The frame structures of locomotives operate under conditions of variable dynamic loads, which causes their frame structure to deteriorate due to fatigue. During long-term operation, repeated loads lead to the accumulation of damage in the frame construction material, which, over time, results in microcracks that can potentially lead to critical failure of the structure. Therefore, studying fatigue wear criteria to ensure the reliability and durability of locomotive frames is an important task.

The fatigue strength of metal structures is typically evaluated using the [1] Paris-Erdogan equation:

Here,

 – crack growth rate per one loading cycle,

, m – empirically determined fatigue constants for the given material,

  – stress intensity factor range

The frame structures of locomotives operate under conditions of high dynamic loads, which over time can lead to the appearance and development of fatigue cracks. One of the main parameters determining the structure's resistance to such damage is the stress intensity factor. It allows for assessing the impact of cracks on the mechanical state of the material and predicting the time when the structure may fail. The critical stress [2] intensity at the crack tip is calculated using the following formula:

Here,

 – stress intensity factor,

 – geometric coefficient,

  – principal stress under external load,

 – crack length

If  reaches a critical value, the structure collapses.

The frame structures of locomotives operate under conditions of cyclic loads resulting from dynamic forces caused by the contact interactions of wheel pairs, engine, suspension, and rail track. Over time, these loads lead to the accumulation of damage in the material, which can result in fatigue failure. The main parameter determining the structure's durability is the fatigue limit - the maximum cyclic stress at which the material can withstand an infinite number of cycles without failure. Reaching this limit indicates the transition of the structure to a critical state, where fatigue wear becomes irreversible and can lead to sudden failure. When the material reaches its [3] fatigue limit:

Here,

   – maximum and minimum limits of cyclic loading,

 – fatigue limit of a material

 – asymmetry coefficient of the loading cycle

The frame structures of locomotives are load-bearing elements that ensure the strength, rigidity, and durability of the machine during operation. In the course of their use, they are subjected to substantial static and dynamic loads, temperature fluctuations, vibrations, and environmental influences. Over time, fatigue damage accumulates in the structure, which can lead to a decrease in strength properties and the occurrence of failures. Residual resource refers to the remaining service life of the structure, which can be predicted based on an analysis of the current state of the material, its damage, and operating conditions. Assessing the residual resource allows determining the safety of future use and what measures should be taken to extend the service life. If the structure [4] has been in use for a certain period, its residual resource can be estimated using the following formula:

Here,

 – the number of remaining loading cycles,

 – initial crack length,

  – critical crack length

These formulas are used in fracture mechanics to analyze the strength and durability of metal structures.

The development and implementation of methods for calculating the individual residual life of metal structures is one of the most pressing issues in domestic industry, especially in the field of railway transport. Currently, a significant portion of locomotives in railway transport are being operated beyond their standard service life, which creates serious operational and technical problems. According to international standards, locomotives that have exceeded their service life must either be decommissioned or have their potential for further safe operation substantiated. In this context, developing scientifically grounded methods for assessing the residual life of structures is of strategic importance [5-9]. This enables the optimization of equipment service life management and reduces financial costs associated with premature decommissioning.

COMPARISON WITH TRADITIONAL METHODS

Traditional strength assessment methods, such as static strength analysis and fatigue limit evaluation, provide a broad estimate of material durability. These methods assume a homogeneous material without defects and often rely on safety factors. While they offer simplicity and ease of implementation, they lack precision in predicting failure due to defect growth. Brittle fracture mechanics, in contrast, accounts for defect characteristics and loading conditions, offering a more detailed assessment of material integrity. However, it requires precise material parameters, including fracture toughness and stress intensity factors, which may not always be available. A practical application of this method in the railway industry demonstrated its effectiveness. A fleet of locomotives undergoing extended service beyond their standard operational period was assessed using brittle fracture mechanics. The results identified critical areas prone to fatigue failure, allowing for targeted preventive maintenance. This approach resulted in a 15% reduction in unexpected failures compared to traditional maintenance strategies.

CONCLUSION

Assessment of the residual life of metal structures allows for increasing the reliability of locomotive frame structures. An approach based on brittle fracture mechanics helps to accurately assess and prevent damage arising during operation. Under long-term operating conditions, it is possible to extend the service life of locomotives and reduce operating costs by monitoring crack development, calculating residual resources, and planning preventive measures. Such an approach is a technically and economically effective solution that serves to ensure safety and reliability in railway transport.

References:

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