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How can you use numerical analysis to predict mechanical component fatigue?

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Numerical analysis is a powerful tool for mechanical engineers who want to predict and prevent component fatigue, which is the progressive damage caused by repeated loading and unloading cycles. Fatigue can lead to cracks, fractures, and failures that compromise the safety and performance of mechanical systems. In this article, you will learn how to use numerical analysis to model the stress and strain behavior of materials, apply fatigue criteria and methods, and estimate the fatigue life of components.

本文章的要点总结

Critical plane assessment:

To predict fatigue, use the critical plane approach which identifies where maximum damage occurs under multiaxial stress. This insight directs maintenance and strengthens component design.
Damage ratio calculation:

Employ a damage ratio method for high cycle fatigue by using finite element analysis to calculate stress and compare it with permissible cycles from S-N curves. It's a quantitative way to ensure component resilience.

本摘要由 AI 和以下专家提供支持

Shahab Zangeneh, PhD

Fitness for Service (FFS) assessment
Milad Hajimohammad

Senior Piping, Pipe Stress & FE…

1 Numerical analysis basics

Numerical analysis is the branch of mathematics that deals with finding approximate solutions to complex problems using algorithms, computer programs, and numerical methods. Numerical analysis can handle problems that are too difficult or impractical to solve analytically, such as nonlinear equations, differential equations, integrals, and optimization. Numerical analysis can also provide more accurate and efficient results than experimental or empirical methods, especially for large-scale or complex systems.

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Milad Hajimohammad

Senior Piping, Pipe Stress & FE Analysis Engineer, expert in using CAESARII, PRG FEPipe and NozzlePro, AVEVA PDMS, AVEVA E3D
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Using FEA with a code or a standard, can lead to very accurate results for fatigue failure mode evaluations. ASME SCE8-Div 2-part 5 covers both high cycle fatigue and low cycle fatigue (Ratcheting) failures with both linear and non-linear assessments. In linear assessment for high cycle fatigue, first the total equivalent stress amplitude defined as one-half of the total stress range of (Pl+Pb+Q+F) is calculated for each cycle using FEA. Then the permissible number of cycles for the calculated stress amplitude is obtained using appropriate S-N curve. The damage ratio is calculated then for each cycle using Df=(actual number of cycles/ permissible number of cycles). Finally, sum of all damage ratios of all cycles must be lower than 1.0

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Daniel Morais

Coordenador de Manuten??o | Engenheiro de Confiabilidade | Especialista de Confiabilidade | Analista de PCM |
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A análise numérica, um ramo matemático, concentra-se em encontrar solu??es aproximadas para problemas complexos por meio de algoritmos, programas de computador e métodos numéricos. Ela é particularmente útil para abordar desafios que s?o difíceis ou impraticáveis de resolver analiticamente, como equa??es n?o lineares, equa??es diferenciais, integrais e otimiza??o. Além disso, a análise numérica pode oferecer resultados mais precisos e eficientes do que métodos experimentais ou empíricos, destacando sua aplicabilidade em sistemas complexos ou de grande escala.

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2 Stress and strain modeling

One of the main applications of numerical analysis in mechanical engineering is to model the stress and strain behavior of materials under different loading conditions. Stress is the internal force per unit area that a material experiences when subjected to an external load. Strain is the deformation or change in shape or size of a material due to stress. Numerical analysis can help you calculate the stress and strain distributions in a component using methods such as finite element analysis (FEA), which divides the component into small elements and solves the equilibrium equations for each element. FEA can also account for different material properties, geometries, boundary conditions, and load types.

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Daniel Morais

Coordenador de Manuten??o | Engenheiro de Confiabilidade | Especialista de Confiabilidade | Analista de PCM |
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A análise numérica desempenha um papel crucial em engenharia mecanica, especialmente na modelagem do comportamento de tens?es e deforma??es de materiais sob diversas condi??es de carregamento. Tens?o, representando a for?a interna por unidade de área, e deforma??o, indicando a altera??o na forma ou tamanho do material devido a tens?es, s?o fundamentais nesse contexto. A análise numérica, notavelmente através da análise de elementos finitos (FEA), permite calcular distribui??es de tens?o e deforma??o em componentes, dividindo-os em pequenos elementos e resolvendo equa??es de equilíbrio.

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3 Fatigue criteria and methods

To predict the fatigue behavior of a component, you need to apply a fatigue criterion or method that relates the stress and strain history to the fatigue damage. Depending on the type and level of loading, the material characteristics, and the failure mode, there are different fatigue criteria and methods that can be used. The stress-life method is suitable for high-cycle fatigue, where the stress is below the yield strength of the material. It uses the S-N curve or the W?hler curve to relate the stress amplitude and the number of cycles to failure. The strain-life method is suitable for low-cycle fatigue, where the stress is above the yield strength of the material and plastic deformation occurs. It uses the ε-N curve or the Manson-Coffin curve to relate the strain amplitude and the number of cycles to failure. Finally, in cases of crack initiation and propagation, where stress is concentrated at a flaw or defect in the material, you can use a fracture mechanics method which uses the stress intensity factor and crack growth rate to relate stress range and crack size to failure.

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Shahab Zangeneh, PhD

Fitness for Service (FFS) assessment
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In API 579, the critical plane approach is employed to evaluate the fatigue life of a component subjected to multiaxial loading. This method entails the identification of the critical plane, where the maximum damage or stress occurs within the material. The critical plane may not necessarily align with the principal stress or strain axes and can vary based on the loading conditions. The API 579 standard offers guidance on conducting the critical plane approach, encompassing techniques for identifying the critical plane and assessing fatigue damage at that location. It considers the multiaxial stress state and factors in the impact of mean stress, stress ratio, and other influences fatigue life.

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Daniel Morais

Coordenador de Manuten??o | Engenheiro de Confiabilidade | Especialista de Confiabilidade | Analista de PCM |
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Para antecipar o comportamento de fadiga de um componente, é essencial aplicar um critério ou método de fadiga que vincule o histórico de estresse e tens?o ao dano de fadiga. Diversos critérios e métodos de fadiga est?o disponíveis, dependendo do tipo e nível de carregamento, características do material e modo de falha. O método stress-life, apropriado para fadiga de ciclo alto, utiliza a curva S-N ou a curva de W?hler para correlacionar a amplitude de tens?o e o número de ciclos até a falha. O método strain-life, adequado para fadiga de ciclo baixo com deforma??o plástica, emprega a curva ε-N ou a curva de Manson-Coffin.

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4 Fatigue life estimation

Once you have chosen a fatigue criterion or method, you can use numerical analysis to estimate the fatigue life of a component. This is the number of cycles or the time that a component can withstand before failure. To do this, you need to define the loading history or spectrum, which is the sequence of loading cycles or events that a component experiences during its service. You can use methods such as rainflow counting or load-time history to simplify and classify the loading history into stress or strain ranges and cycles. Additionally, you must calculate the damage or crack growth for each loading cycle or event using the chosen fatigue criterion or method such as the Palmgren-Miner rule or the Paris law. Then, compare this damage or crack growth with the failure criterion, which is the threshold or limit that defines when a component fails. Methods such as the Goodman diagram or the fracture toughness can be used to determine this failure criterion for a given material and geometry. Lastly, calculate the fatigue life as the number of cycles or the time that corresponds to the failure criterion; interpolation or extrapolation can be used to find this value from an S-N curve, an ε-N curve, or a crack growth curve.

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Daniel Morais

Coordenador de Manuten??o | Engenheiro de Confiabilidade | Especialista de Confiabilidade | Analista de PCM |
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Após a escolha de um critério ou método de fadiga, a análise numérica é empregada para estimar a vida de fadiga de um componente, representando o número de ciclos ou tempo que o componente pode suportar antes da falha. Para isso, é crucial definir o histórico ou espectro de carregamento, que é a sequência de ciclos de carregamento ou eventos experimentados pelo componente durante seu servi?o. Métodos como contagem de fluxo de chuva ou histórico de tempo de carregamento s?o utilizados para simplificar e classificar o histórico de carregamento em faixas e ciclos de tens?o ou deforma??o.

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5 Numerical analysis benefits and challenges

Numerical analysis can provide several advantages for predicting and preventing mechanical component fatigue, such as reducing the need for physical testing, increasing the accuracy of fatigue predictions, enhancing design and optimization, improving maintenance and inspection strategies, and extending the service life of components and systems. However, numerical analysis also has its challenges and limitations, such as requiring a high level of mathematical and computational skills, depending on the quality and availability of input data and parameters, involving uncertainties due to approximations and assumptions, and needing validation with experimental or empirical data.

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Daniel Morais

Coordenador de Manuten??o | Engenheiro de Confiabilidade | Especialista de Confiabilidade | Analista de PCM |
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A análise numérica oferece várias vantagens cruciais para prever e prevenir a fadiga de componentes mecanicos. Estas incluem a redu??o da dependência de testes físicos, aumento da precis?o nas previs?es de fadiga, melhoria no projeto e otimiza??o, aprimoramento das estratégias de manuten??o e inspe??o, além de estender a vida útil de componentes e sistemas. No entanto, é importante reconhecer os desafios e limita??es associados à análise numérica, como a necessidade de habilidades matemáticas e computacionais avan?adas, a dependência da qualidade e disponibilidade dos dados e parametros de entrada, a presen?a de incertezas devido a aproxima??es e pressupostos, e a exigência de valida??o por meio de dados experimentais ou empíricos.

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Matheus Lima

Automotive and Industrial Maintenance reliability/Mechanical Engineering/Continuous improvement
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Um desafio real para a análise numérica é adaptar a realidade dos ambiente no qual o equipamento trabalha. Muitas variáveis naturais influenciam o comportamento do material, mas também há mudan?as operacionais, que podem ser mais difíceis de considerar dentro das análises, pois há muita influência da cultura organizacionais onde o equipamento opera.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How can you use numerical analysis to predict mechanical component fatigue?

1

2

3

4

5

6

1 Numerical analysis basics

2 Stress and strain modeling

3 Fatigue criteria and methods

4 Fatigue life estimation

5 Numerical analysis benefits and challenges

6 Here’s what else to consider

Mechanical Engineering

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How can you use numerical analysis to predict mechanical component fatigue?

1

2

3

4

5

6

1 Numerical analysis basics

2 Stress and strain modeling

3 Fatigue criteria and methods

4 Fatigue life estimation

5 Numerical analysis benefits and challenges

6 Here’s what else to consider

Mechanical Engineering

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