Fatigue Testing and Analysis
Overview
The type of loading that a fatigue critical structure is subjected to depend largely on what the function of the structure is and what controls the loading applied. In some cases, for instance rotating machinery for power generation, the loading can be adequately simplified and represented by a constant load amplitude cycle. In this case, loading is dictated by function on an angular rotation-by-rotation basis. This is contrasted to the case of a fighter airplane where the loading is dictated by external aerodynamic loads combined with highly variable pilot inputs. In many cases, the use and function of a structure can significantly impact the loading. An example of this is the case of a passenger aircraft where taxis, takeoff, cruise at altitude and landing dictate a primary loading cycle. Given that most of the service time is spent at cruise, the magnitude of the repeated load for the fuselage is largely driven by the cabin pressurization.
Whatever the source and magnitude of the cyclic loading, the challenge for the structural engineer is to determine the amplitude of the variable amplitude spectrum loading and sim- plify it in a manner that can then be combined with analytical fatigue design approaches to adequately size the structure. Design tools and approaches have been available for many years to assist in accomplishing this objective. The term 'fatigue' was originally coined by Wohler in the 1840's when he examined railroad car axle failures. Sixty years later Goodman and Basquin examined the mean stress effect and developed the stress-life approach to de- sign. The second World War spurred a significant amount of activity including development of the concept of linear damage as postulated by Palmgren and Miner. Methods to design and maintain aircraft when cracks are growing in a structure were developed after a series of high profile failures in the 1970's by the USAF. All of these developments have culminated in the design tools now available to treat fatigue crack initiation and propagation under variable amplitude loading.
Applying these fatigue design strategies often requires a significant mechanical testing effort to tune the analytical models to predict actual laboratory observation. Hampering this process is the absence of any standardized test method to perform fatigue testing under spectrum loading conditions. While standards have been developed to characterize material responses to fatigue loading, no methods yet exist for the more complicated spectrum loading test and laboratories have consequently developed their own custom approaches. Although little standardization also exists for fatigue analyses, there are some accepted methods and techniques available for treating variable amplitude loading. Nevertheless each organization that performs this type of work tends to have customized the methods to suit their needs and specific approaches. One of the primary goals of this symposium was to provide a forum to communicate amongst technical professionals involved in this type of work. Applications reported on include those focused purely on testing, fatigue design techniques/approaches as well as a combination of both.
The technical papers in this book represent peer reviewed and approved papers of those presented at an international symposium focused on fatigue testing and analysis under vari- able amplitude spectrum loading conditions. To aid in assimilating this information, the papers are categorized into six sections: Fatigue Testing, Aerospace Applications, Design Approach and Modelling, Other Applications, Load Interaction, and Probabilistic and Mul- tiaxial Approaches.
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Overview
The type of loading that a fatigue critical structure is subjected to depend largely on what the function of the structure is and what controls the loading applied. In some cases, for instance rotating machinery for power generation, the loading can be adequately simplified and represented by a constant load amplitude cycle. In this case, loading is dictated by function on an angular rotation-by-rotation basis. This is contrasted to the case of a fighter airplane where the loading is dictated by external aerodynamic loads combined with highly variable pilot inputs. In many cases, the use and function of a structure can significantly impact the loading. An example of this is the case of a passenger aircraft where taxis, takeoff, cruise at altitude and landing dictate a primary loading cycle. Given that most of the service time is spent at cruise, the magnitude of the repeated load for the fuselage is largely driven by the cabin pressurization.
Whatever the source and magnitude of the cyclic loading, the challenge for the structural engineer is to determine the amplitude of the variable amplitude spectrum loading and sim- plify it in a manner that can then be combined with analytical fatigue design approaches to adequately size the structure. Design tools and approaches have been available for many years to assist in accomplishing this objective. The term 'fatigue' was originally coined by Wohler in the 1840's when he examined railroad car axle failures. Sixty years later Goodman and Basquin examined the mean stress effect and developed the stress-life approach to de- sign. The second World War spurred a significant amount of activity including development of the concept of linear damage as postulated by Palmgren and Miner. Methods to design and maintain aircraft when cracks are growing in a structure were developed after a series of high profile failures in the 1970's by the USAF. All of these developments have culminated in the design tools now available to treat fatigue crack initiation and propagation under variable amplitude loading.
Applying these fatigue design strategies often requires a significant mechanical testing effort to tune the analytical models to predict actual laboratory observation. Hampering this process is the absence of any standardized test method to perform fatigue testing under spectrum loading conditions. While standards have been developed to characterize material responses to fatigue loading, no methods yet exist for the more complicated spectrum loading test and laboratories have consequently developed their own custom approaches. Although little standardization also exists for fatigue analyses, there are some accepted methods and techniques available for treating variable amplitude loading. Nevertheless each organization that performs this type of work tends to have customized the methods to suit their needs and specific approaches. One of the primary goals of this symposium was to provide a forum to communicate amongst technical professionals involved in this type of work. Applications reported on include those focused purely on testing, fatigue design techniques/approaches as well as a combination of both.
The technical papers in this book represent peer reviewed and approved papers of those presented at an international symposium focused on fatigue testing and analysis under vari- able amplitude spectrum loading conditions. To aid in assimilating this information, the papers are categorized into six sections: Fatigue Testing, Aerospace Applications, Design Approach and Modelling, Other Applications, Load Interaction, and Probabilistic and Mul- tiaxial Approaches.
Download
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