Reliability Of Life Prediction Research Articles

Modeling fatigue small-crack growth with confidence – A multistage approach

Measurement of fatigue small-crack growth on the surface of smooth or notch specimens has evolved into a mature experimental technique. However, post data processing and model correlation for fatigue small-crack growth has not progressed in a parallel manner to the measurement technique. A new physics-based, mathematically precise fatigue small-crack growth modeling method is proposed for the specifics of surface small-crack growth in heterogeneous metals and rigorous statistical analysis. A multistage fatigue modeling philosophy was implemented and the fatigue damage incubation, small-crack growth, and long-crack growth regimes were identified. The uncertainty in measurement errors and the effects of random variation of microstructural features on the small-crack growth were explored using Monte Carlo (MC) simulations. The reliability of life prediction is directly correlated to the uncertainty-sources of measurements and the random nature of material microstructures through uncertainty propagation in the model correlation process. Fatigue damage incubation life is proven to be an essential quantity, both mathematically and physically, and it is introduced into the modeling process when correlating multiple experiments under the same loading condition. The proposed model is demonstrated using fatigue surface crack growth measurements of a dual-phase Ti-alloy.

Spectrum load fatigue—underlying mechanisms and their significance in testing and analysis

The constant amplitude fatigue process is driven by cyclic slip and surface chemistry. These in turn are affected by crack geometry, crack closure and crack-tip stress state, including residual stress. The sensitivity of these factors to spectrum loading determines durability and residual life under service load conditions. Cyclic slip forms the basis for the Rainflow cycle-counting technique and is therefore an integral part of any spectrum load fatigue analysis or test. Assessment of the relative significance of operative mechanisms impacts quality and cost effectiveness of spectrum load tests and also, the reliability of life predictions.

Quality Of Life:Activities At Volvo Aero Corporation To AssessAnd Improve The Accuracy Of Assumptions AndMethods Employed In Calculating The ServiceLife Of Aero Engine Components

We will discuss the quality system for service life prediction of modern aerospace engines introduced at Volvo Aero. The requirements on a quality system for such complex analyses must satisfy high demands on flexibility and extendibility, while not compromising reliability, clarity and ease of use. The main focus of this paper is on how software, methods instructions and a quality ranking system are used for assessing and improving the accuracy and reliability of life predictions. While cost and bottom line profit are driving factors, this quality system for life predictions benefits the engineer in his daily work. He saves time by using a structured system with access to a company-wide methods library. The risk of operator errors is much lower, because of the structured system, and the quality ranking gives consistent information about the assessment level of each analysis. Continuous improvement is facilitated by identifying weak links in the analysis chain, and because improved or new methods are easily communicated to all life prediction engineers. The dataflow and workflow of the life prediction analysis are described as a process chart. Ongoing work includes ranking and quantitative assessment of common assumptions and models, in collaboration with national and international research associates. Damage & Fracture Mechanics VI, C.A. Brebbia, A.P.S. Selvadurai, (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-812-0