Engineering methods for assessing fatigue crack initiation and crack growth
When faced with a structural system that does not meet service life goals because of fatigue cracking, Hill Engineering recommends taking a stepwise approach to improve performance. While fatigue analysis — using methods like stress-life, strain-life, or damage tolerance — are part of our work, starting fatigue engineering at a higher level often benefits the customer. The steps we recommend are: discovery, manufacturing assessment, stress analysis, spectrum development, fatigue analysis, and fatigue life extension.
As a first step in working with your team, we develop an understanding of the design and the problem. Our process for discovery consists of:
- Interview and interactive discussion – We work with your team to understand the breadth of the problem and the known facts. Our experience allows us to ask the right questions to efficiently define the problem.
- Failure analysis – Failed systems or components are often critical in determining the cause of structural failure, and often point toward a solution. Our staff can perform a comprehensive failure analysis, or make use of failure analysis reports already available. For a new design, this step may be used to assess prior models or prototype parts or systems.
- Work plan development – Given the nature of the problem and its cause, we work with your team to define additional work required to develop a solution.
The outcome from the discovery step is a work plan aimed to address your needs. Typically, the work plan makes use of resources from your team and ours.
Fatigue performance can be highly dependent on the characteristics established by manufacturing. Raw material form, material orientation, machining process, surface treatments, and final coatings each have a role in fatigue (and corrosion) performance. Our experience in aerospace, nuclear, petrochemical, and other industries gives us a broad background in assessing how manufacturing details may be affecting fatigue performance.
An accurate stress analysis is an integral part of mechanical design. Fatigue is very sensitive to stress, and ensuring the accuracy of load and stress estimates is an important step in fatigue engineering. Hill Engineering staff perform stress analysis on a routine basis, often refining design analysis developed by our customer. The magnitude and distribution of stresses caused by applied loads provide input to fatigue analysis and also reveal opportunities for design improvements. State of the art stress analysis uses finite element methods, and Hill Engineering relies on accepted commercial software and experienced staff to deliver high quality analysis results. We can include nonlinearity from plasticity, load transfer, or changing contact conditions. The level of complexity required is often dictated by the characteristics of the design and loading. Often, we can use a customer’s existing analysis as a starting point, to more rapidly meet project needs. An accurate stress analysis determines the locations where stresses are highest, and where subsequent fatigue analysis will be focused. Because fatigue is notorious at revealing structural weakness, it is important to recognize that solving a fatigue problem in one area may move the problem to a different area. The stress analysis must consider an area large enough that the overall program objectives can be met, in all areas.
Time-varying load fluctuations are a distinguishing feature of fatigue failure. A typical sequence of loads versus time, called a spectrum, is a key input to fatigue analysis. For simple machines, a spectrum can be determined from the principles of operation, but more complex systems require monitoring of instrumented components during operation. Whether we proceed from simple assumptions or work to derive a spectrum using data from instrumented components, our staff have the expertise to define a spectrum suitable for fatigue analysis.
Service life estimates are developed from stress analysis, spectrum, and fatigue data using a fatigue analysis performed to a specified basis. We work an analysis that is appropriate to your project, depending on applicable code, industry guidance, materials, consequences associated with failure, and other factors.
An analysis that estimates the time required to form a crack in a component is referred to as a durability analysis, and is performed using either a stress-life or strain-life formulation. Stress-life analysis assumes elastic material behavior and is applied when stress levels are low and fatigue life is long. The more advanced strain-life analysis accounts for elastic-plastic material behavior, which has advantages when stresses are moderate or high, and when it is necessary to account for localized yielding that occurs at a notch or other stress concentration. For irregular histories of stress versus time, cycle counting and damage accumulation are used to estimate durability fatigue life from fatigue data developed under constant amplitude loading.
A crack growth analysis is used to estimate the time required for a crack to extend from an initial size, typically on the order of 0.05 inch (1.2 mm), to a size large enough to cause failure. Crack growth analysis provides the amount of time a structural member exhibits slow, stable crack growth, which quantifies its damage tolerance. The forecast of crack size versus time is used in planning structural inspections that ensure safe operation of aircraft, petrochemical, and other systems. Crack growth analysis is also used in design of pressure vessels, to ensure that cracks will grow through wall and leak, to relieve pressure, rather than fracture.
Hill Engineering has experience in all aspects of fatigue engineering and analysis. Past projects range from relatively simple to highly complex. A simple project used stress-life analysis and predicted the initiation and growth of a single, one-dimensional crack using paper and pencil calculations as specified by a design standard. A complicated project used a spectrum derived from strain gage data, assessed crack initiation using strain-life analysis, rainflow cycle counting and damage accumulation, and then applied finite and boundary element software to assess three-dimensional crack growth through a complex structure. Regardless of the degree of complication required for your project, we can likely support it.
Fatigue Life Extension
Having applied fatigue engineering principles to assess a particular objective, customers may need to enhance fatigue performance through a program of fatigue life extension.