Software that Fights Fatigue

Fatigue analysis software can breathe new life into assets and save time and money during the design process. But how reliable are the readings, particularly for the military sector?

Since being created to help car manufacturers in the 1950s, fatigue management software has developed from being an in-house pet project to a viable commercial product. These days, many large companies in defence, aerospace and mining use the technology to reduce costs and man-hours when designing new equipment and machinery.

"Fatigue used to be the black art that produced financial black holes," says Professor John Draper, chairman of 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."

Development benefits

Fatigue analysis software can help reduce development times by making designs correct before a component is actually cut, according to Professor Phil Irving, head of the damage tolerance group at Cranfield University, UK.

"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, 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 like 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."

"Fatigue analysis software can help reduce development times by making designs correct before a component is actually cut."

How does it work?

There are three major inputs on fatigue analysis software: component load measurements, stress analysis of the structure and material fatigue properties.

"The software interfaces directly to finite element analysis software and is supplied complete with a database of material fatigue properties to which users can add their own data," says Draper. "It calculates where and when fatigue cracks will occur – the fatigue hotspots – 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."

Analysing MRAV design

When Alvis Vickers Limited required an analysis of its latest multirole armoured vehicle (MRAV), it employed Integrated Design & Analysis Consultants and Krauss Maffei Wegmann to help create a 3D model of the vehicle. The object of the analysis was to determine the suitability of the vehicle structure for about 75 load cases as well as simulating the vehicle's operational loads. The process resulted in several modifications being made to the design of the MRAV, saving time and money.

The analysis was done using the 3D design programme Pro/ENGINEER, which created the mass, beam, shell and solid elements of the MRAV. Shell elements, for example, were used to model the main hull structure, while solid elements were used for model castings and fabrications. For areas where castings and fabrications were attached to the hull platework, the attachment was made using shell elements that were used to simulate the weld connectivity.

The results of the analysis focused mainly on the welded joints within the vehicles. Any potentially problematic joints were scrutinised further using the third-party application FEWeld, which takes the ANSYS results and evaluates the weld stress in further detail. From this, a stress contour was created that clearly highlighted possible areas of weakness.

The analysis was overall said to have provided confidence in the structural design of the MRAV and proved so successful the technique was later used to investigate the cost and weight-saving measures of the design.

Fatigue challenges

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 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."

Irving and his team conducted an experiment once where 12 organisations were asked 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," Irving adds. "While 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."

Industry recognition

However, Draper believes that the advancements in research over the last couple of years mean the accuracy of fatigue analysis software is equal to anything else in engineering, once the data entry is right.

"The accuracy of fatigue analysis software is equal to anything else in engineering, once the data entry is right."

"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."

The industry is also becoming aware of this. 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. "This kind of growth will continue 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."