Data processing: structural design – modeling – simulation – and em – Simulating nonelectrical device or system – Mechanical
Reexamination Certificate
1998-09-17
2001-04-03
Teska, Kevin J. (Department: 2763)
Data processing: structural design, modeling, simulation, and em
Simulating nonelectrical device or system
Mechanical
C703S001000, C703S008000, C703S002000
Reexamination Certificate
active
06212486
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method of dynamic durability analysis and fatigue area identification and, more specifically, to a method of dynamic durability analysis and fatigue area identification using modal techniques for a structure.
2. Description of the Related Art
The trend in the vehicle industry, and in particular the motor vehicle industry, is to reduce new product development cycle time, from conception to sale. From an engineering perspective, increasingly sophisticated analysis techniques are being utilized in simulating a vehicle structure to predict vibrational characteristics of a motor vehicle. For example, in the field of motor vehicle dynamics, a widely known method of finite element analysis is utilized to identify natural modes and frequencies and predict stress responses and fatigue life of the vehicle structure. The finite element model commonly utilizes simple loads, for static as well as quasi-static (inertia relief) methods, to determine stresses and fatigue life in the vehicle structure. However, these methods may not identify all durability-related areas of the motor vehicle that are dynamic in nature and respond with elastic vibrations. The finite element model may also use modal analysis, including transient modal analysis, to identify natural modes, frequencies and corresponding stress responses. However, this technique is limited to only a few seconds of dynamic durability road load data or for a few known elements of the structure.
These techniques are not feasible for predicting potential high stress responses within a large structure, such as a complete motor vehicle, due to the prohibitive amount of computer time and memory necessary to analyze the data for such a vehicle structure. Fatigue life prediction needs a complete structural stress time response history, therefore, the computational time is significant. For example, a finite element model of a motor vehicle may include approximately 200,000 elements and a vehicle durability test route may last 30 to 60 minutes. Determining the dynamic stress time history of such a model using conventional methods would require over 200 days of computer processing time. The computational magnitude of determining the stress response for each element within the vehicle structure renders current methods impractical. The problem is described in a Society of Automotive Engineers paper entitled, “Dynamic Durability Analysis of Automotive Structures” by L. Huang, H. Agrawal and P. Kurudiyara, SAE No. 980695, presented Feb. 23-26, 1998, which is hereby incorporated by reference. Thus, there is a need in the art for an efficient method of calculating a dynamic stress time history for a structure to predict fatigue life within the structure.
SUMMARY OF THE INVENTION
Accordingly, the present invention is a method of dynamic durability analysis and fatigue area identification for a structure. The method includes the steps of simulating a dynamic finite element model of the structure to determine modal stresses and modal displacements for an element of the structure and performing a modal transient analysis using the modal displacements. The method also includes the steps of determining a stress bound for the element from the modal stresses and modal transient analysis, determining if a stress bound for the element is greater than a predetermined value and identifying the element as a critical element if the stress bound for the element is greater than the predetermined value. The method further includes the steps of determining a stress time history for the critical element and using the stress time history to perform a fatigue analysis to identify an area of fatigue within the structure.
One advantage of the present invention is that the method is applicable to a large structure, such as a motor vehicle structure, subject to complex dynamic loads due to the diverse road conditions experienced over the life of the motor vehicle. Another advantage of the present invention is that it uses well known finite element analysis techniques to perform the dynamic stress calculations required to obtain the stress time history of the structure. Yet another advantage of the present invention is that the method is computationally efficient by determining a stress bound to select a critical stress element, and by calculating the stress time history of only the critical elements.
Other features and advantages of the present invention will be readily appreciated as the same becomes better understood after reading the subsequent description when considered in connection with the accompanying drawings.
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Agrawal Hari
Huang Liping
Ford Global Technologies Inc.
Kelley David B.
Teska Kevin J.
Thomson William
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