Fatigue Analysis Techniques
Fatigue Life Prediction Methods and Techniques
Designing your products to withstand the maximum load they encounter in service might not be sufficient, even with a substantial safety margin. If the loads vary during use, your product could experience fatigue damage over time, leading to unexpected failure. Metallic components, when subjected to loads, undergo a small amount of microscopic plastic deformation. This can happen due to imperfections in the base material, welds, or discontinuities from machining or manufacturing processes.
Fatigue failure can be predicted through analytical modeling, which falls into two main categories: stress-based and strain-based fatigue analysis. In stress-based fatigue, a Stress-Life (SN) curve for the material maps the damage caused by individual load cycles. Using Rainflow Cycle Counting, a load sequence can be decomposed into stress cycles, each contributing to the overall damage. By accumulating this damage, it's possible to estimate the approximate lifespan of the component. Stress-based fatigue analysis is fundamentally a linear technique and yields accurate results primarily for components with a long life, known as High Cycle Fatigue. Most analysis software claiming to offer Fatigue Analysis typically provides only Stress-Based predictions, which have limited applicability.
Strain-based fatigue analysis is a non-linear approach that separately accounts for the elastic and plastic strain components. These strains are calculated at critical locations where failure is likely to occur, considering the material properties in various conditions such as as-machined or as-forged. Damage is assessed based on the hysteresis loops on a stress-strain curve, allowing for an accurate prediction of the component's lifespan. This method can also predict the life of non-metallic components, such as rubbers and composites.
Fatigue Design
Effective fatigue design must be integrated early in the product development process and cannot be easily or cheaply added later. It should be part of the initial concept design, considering various factors that impact fatigue performance.
We support your product design process in several ways:
- Conducting independent assessments of fatigue-critical features to ensure that design modifications do not increase fatigue failure risk or reduce fatigue strength as the design matures.
- Performing sizing and layout studies that focus on fatigue inputs such as inertial loads, sources of dynamic load, load paths, static loads, loads from shipping, and installation to provide a holistic assessment of the product's fatigue life.
- Optimizing joint design for best load path performance, durability, and cost efficiency, including manufacturing and raw material costs.
- Engineering the manufacturing process to minimize its impact on fatigue and reduce residual stress.
- Selecting materials that are optimized for manufacturing, cost, durability, and appearance.
Accelerated Fatigue Testing
Fatigue testing can be time-consuming, and if products fail just before the required sign-off period, the testing time nearly doubles. Additionally, products entering new markets may require further testing to ensure performance and uphold their reputation, necessitating careful test planning. Fatigue-Expert assists your team by designing optimal fatigue tests that maximize information while minimizing testing time. Accelerating fatigue tests is complex because increasing the loading frequency can alter the failure model and test life, rendering results inapplicable to product design. We collaborate closely with your engineers to understand the product's operational performance, design, and materials, performing a baseline analysis to model fatigue performance. Based on these results, test optimization is conducted and applied to subsequent test series.
In-Depth Stress Analysis
Before conducting a detailed fatigue study, it's crucial to fully understand the stresses in your product. This can be achieved through prototype measurements, which avoid some of the assumptions in modeling. However, early in product development, prototypes may not be available, and preparing one for testing might take too long to be useful.
An FE Model is not a cure-all for stress-related issues. The stresses must be suitable for fatigue analysis, as fatigue is often driven by local, frequently plastic stresses. Determining what to model and what to exclude requires advanced structural analysis skills and deep knowledge of fatigue methods.
Our capabilities include linear and non-linear stress analysis, hand calculations, fatigue modeling, optimization tools, and a variety of presentation and reporting options.
For fatigue-focused stress analysis tailored to your product development needs, contact us for a confidential and free discussion.
Expertise and Historical Context in Fatigue Consulting
FEDYNE offers CAE methods to replicate damaging loads for correlation with Lab results, supporting clients in fatigue durability evaluation and design optimization
Harsha Kolar, owner of FEDYNE, brings decades of experience in FE Based and fatigue analysis dating back to 2004, including collaboration with pioneers of the Frequency domain based fatigue with D.Neil Bishop. FEDYNE has contributed to the development of many commercial fatigue algorithms and has been providing consulting services since 2015