Papers

Innovation in heavy equipment and engine technology

“With its ability to accurately predict fatigue life from measured strain data, safe4fatigue™ has proven to be a powerful ally to fe-safe™ and our overall fatigue process.”  Bill Moser (Sr. Specialist) VPD Technology, Applied Research of Caterpillar Product Development Centre Of Excellence.  

Innovation, integration and fatigue assessment

Caterpillar uses a wide range of techniques to develop powerful, fuel-efficient and reliable engines. Real-world engineering is rarely as simple as: design, analyze, test, manufacture. A real opportunity for innovation lies in integrating and understanding how simulation and test correlate, and determining how to best predict real-world behaviour.  

The genesis of the new-design process at Caterpillar is heavily computer-aided, with an integrated CAD and FEA process since the early 1990s. In 2002, Caterpillar selected Sheffield based Safe Technology’s fe-safe™ software for fatigue life prediction and it is now an integral part of their overall design process.  

In addition, Caterpillar employs a number of other methods to improve and support existing designs in the field. An example of this involved a piston experiencing a few field failures somewhat earlier than anticipated. The original fatigue prediction for this component had only involved hand calculations. This was a good opportunity to use a test-based process and Caterpillar engineers used the advanced fatigue analysis and signal processing module in fe-safe™, safe4fatigue™, measuring strains to predict fatigue life.  

Lab engineers arranged instruments around the piston skirt with rosette gauges at three locations. Signals were measured corresponding to the three components of strain.  

Complex loads, signal processing and measured strains  

The test-loading of a piston is complex, due to the various stages of the internal combustion process. A load cycle consists of two complete revolutions of the crankshaft. As the piston is loaded and unloaded, each strain gauge produces three signals. The signals are then processed by safe4fatigue™ to create valid strain histories. The measured strain approach has the valuable advantage that strains directly reflect the physical hardware and actual loading; whereas the accuracy of the finite element approach entails careful application of loads and consideration of the mesh density near the areas of high strain. 

Material Properties and fatigue prediction  

Material properties add complexity to accurate fatigue assessment and prediction. The piston skirt is made of proprietary cast aluminum, which exhibits behavior “somewhere inbetween” brittle and ductile behavior. In order to consider situations for both material types, Caterpillar assessed the fatigue life in safe4fatigue™ using both maximum principal stress algorithm (based on NASA MSFC-388 S-N curve data) and Brown-Miller method (based on material data from an ASM book).  

The former algorithm is appropriate for brittle material while the latter one (Brown-Miller) is more appropriate for ductile material. The effect of mean stress correction was alsoinvestigated using three different methods—Goodman correction, the traditional hand calculation method and a mean stress correction curve for cast AISI material. In addition, the effect of surface finish was also considered in the safe4fatigue™ calculations. 

Results and insights

The minimum life was predicted by safe4fatigue™ at the notch area of the piston skirt. This correlated with field observations. Using the maximum principal stress method, life was predicted at ~1.3 million cycles; the Brown-Miller method predicted ~1.6 million cycles until failure. In areas of high compressive stress—which is the case at the notch—the effect of mean stress was considerable. The hand calculation, which used a default mean stress correction curve, had predicted a fatigue life at the notch of more than 2.5 times the life predicted by safe4fatigue™, which used a more accurate mean stress correction.

Considering the effect of surface finish also proved vital. Fatigue life decreased about 60% between “polished” (Ra<0.25m) and “as-cast” (Ra~30m) surface finishes (the polished surface has the higher fatigue life).  

Overall, Caterpillar found that safe4fatigue™ accurately predicted fatigue life from strain gauge data, and proved to be a useful complement to fe-safe™ and its other predictive and test methods for product design.

About Safe4fatigue™:safe4fatigue™ is a powerful signal processing tool providing advanced fatigue and durability analysis from measured or simulated signals. It is available as a stand-alone product and is also included in Safe Technology’s durability analysis software suite, fe-safe™, at no additional cost.

About Safe Technology Ltd: Safe Technology Limited is a technical leader in the design and development of durability software and is dedicated to meeting its customers'' most demanding applications. As a private company, it is able to take a long-term view of software development and the research and industry collaboration needed to solve real world, industrial applications. In depth knowledge on fatigue combined with expertise in software development allows Safe Technology to provide outstanding service - with basic and advanced training, software support, and consulting services provided by fatigue experts. Safe Technology develops, markets and supports its software products directly from offices in the UK and USA, by a network of independent distributors worldwide and via the worldwide Simulia network.

www.safetechnology.com contact: info@safetechnology.com

About Caterpillar Inc: With sales and revenues in 2008 of $51.3 billion, Caterpillar Inc. leads the world in construction and mining equipment, industrial gas turbines, and diesel and natural gas engines. Caterpillar is the foremost manufacturer of medium-speed engines and a leader for high-speed diesel engines, ranging from 50 to 20,000 horsepower. Besides Cat® construction and mining equipment, other OEMs find uses in trucks, boats, ships, and notably, electrical systems. Caterpillar helps power off-shore oil rigs, desert mining operations and even entire remote communities. To support innovation in machinery and engine technology, Caterpillar continues to advance its leadership in computer-aided engineering whilst leveraging physical testing to improve its overall design process.