Fatigue Analysis Software11 September 2009
Fatigue analysis software can breathe new life into road assets and save time and money during the design process. But how reliable are the readings, particularly for testing loads on the nation’s roads? Paul French investigates.
Since being created to help car manufacturers in the 1950s, fatigue management software has developed from an in-house pet project to a viable commercial product. Today, many large companies in transport, aerospace and mining use the technology to reduce costs and man-hours when designing new plants and machinery.
"Fatigue used to be the art that produced financial black holes," says Professor John Draper, chairman of analysis software company Safe Technology and author of Modern Metal Fatigue Analysis.
"Potential errors were hidden behind the euphemism of 'safety factors' and manufacturers paid the price for overweight components that cracked prematurely, a seemingly endless series of prototype developments, unpredictable warranty claims and loss of customer confidence.
"Traditionally, fatigue failures have been fixed by over-design, but increasingly, engineers are under pressure to 'design down' to save weight and material costs. Over-design is no longer a viable option and the need for sophisticated fatigue analysis tools has become increasingly apparent."
Fatigue analysis software can help reduce development times by making sure designs are right before you actually cut a component, according to Professor Phil Irving, head of the damage tolerance group at the UK’s Cranfield University. Irving, with other professionals, has been working to find ways of making fatigue analysis solutions.
"It can make the difference between doing a dozen fatigue tests and one or two, which saves a lot of time and money during development," he says. "In the aerospace industry, for example, certain regulations require testing and analysis of fatigue liability in sections of the aircraft. Fatigue software allows you to demonstrate analysis in that instance and it also helps to calculate life extensions for big structures such as oilrigs. If you have good information you can re-calculate fatigue life and extend the life of your assets. We’re seeing a lot of that in the North Sea at the moment."
How does it work?
There are three major inputs to fatigue analysis software – component load measurements, stress analysis of the structure and material fatigue properties.
As Draper explains: "The software interfaces directly to FEA [finite element analysis] software and is supplied complete with a database of material fatigue properties to which users can add their own data," says Professor Draper. "It calculates where and when fatigue cracks will occur – the fatigue hot-spots – plus the factors of safety on working stresses for rapid optimisation and the probability of survival at different service lives, otherwise known as the 'warranty claim' curve. The results are presented as contour plots of fatigue lives, stress safety factors and probabilities of failure and plotted using standard FEA viewers and graphics software."
The road load data process can be complex and expensive but represents an important part of the product development cycle. Software companies such as nCode collaborate with major original equipment manufacturers (OEM) for years in order to provide systems for test design, data acquisition, test analysis, data management, fatigue analysis and reporting.
When a major elastomer components manufacturer was needed to validate its products against extensive road load data supplied by OEMs, the company turned to nCode’s ICE-flow GlyphWorks. The software is used to process the vast amount of raw data coming from several OEMs.
Using the fatigue management technology – and particularly its ability to define specific material damage curves and mean stress corrections needed for these multi-material components – test signals were validated and then compared in terms of damage severity.
Using the software’s fatigue editing tool, they were able to reduce these signals allowing sensible gains in rig testing. The analysis procedures were saved and the flows then sent to other international users.
By rapidly processing the real road load data, the manufacturer is able to quickly validate anti-vibration parts, using the flexibility of the GlyphWorks material property functionality. According to nCode the whole procedure is now automated, standardised and the resulting analysis flows are commonly used by several international sites.
New advancements and innovations
For years fatigue development software has been unable to cope with one of engineering’s most difficult areas – multiaxial states of stress. However, developers like nCode and Safe Technology have made huge advancements in this area since 2006.
"Fatigue analysis software research is in an exciting phase," says Professor Draper. "The last two to three years have seen a lot of the gaps in our knowledge filled. We are now able to operate without making too many approximations and have software capable of taking into account things like multiaxial states of stress, so the results are much truer."
Acquiring the information to feed into fatigue analysis software packages can be a costly business. Measuring load components and conducting stress analysis involves employing teams of people, keeping the subject operational and installing systems to conduct trials, which can run into thousands of pounds. Because of this, the data is not always as good as it should be, which can affect the accuracy of the fatigue life predictions.
"Fatigue analysis software is not always wonderfully accurate," says Professor Irving. "People get around this by inputting data for different materials or reducing the level of stress to get the optimum calculated fatigue life. This life can then be calibrated against an experimentally measured life. The effects of changes in material or stress can be accurately assessed; but absolute fatigue life can be difficult to calculate accurately.
"We did an experiment once where we asked 12 organisations to use their fatigue analysis software to calculate the fatigue life of a sample component on a helicopter. We then did an experimental measurement to find the correct answer, which was 400 flight hours. Whilst some of the companies were within a few hours, some suggested the fatigue life should be 250 hours and one said it was 12,000!"
However, Professor Draper believes that the advancements in research over the last couple of years mean that the accuracy of fatigue analysis software is on a par with anything else in engineering. "We can estimate what an allowable stress is to an accuracy of within 5% now," he says. "That puts us on a par with anything else in the field."
Leading IT researcher Gartner expects software spending to increase 1.5% in 2010. However, the picture for the niche fatigue analysis software market looks far rosier, according to Draper.
"It’s very difficult to put a figure on it but I’d say this market is growing by about 30% each year," he says. "I think this kind of growth will continue into the future as more companies and more industries are being forced into reducing costs. The high cost of raw materials means you can’t just throw metal at problems like fatigue anymore."
Although acknowledging that cost of procuring information to feed into the software may prevent some companies from using it, Irving also predicts a bright future for the market. "Fatigue analysis software will become more widely adopted as it becomes further integrated with CAD packages," he says. "I think there needs to be more transparency and a set of standards introduced, so there is more uniformity on fatigue life predictions. But ultimately, as a service experience, I expect the market will grow and the software will become more accurate as time progresses."