Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
1998-07-16
2001-05-01
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S084000, C703S007000
Reexamination Certificate
active
06226597
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to maintenance of components that are subject to fatigue failure.
BACKGROUND OF THE INVENTION
It is well known that component fatigue life is a major factor in the design of many mechanical, fluid, and electrical devices and systems. In systems that require high reliability, component fatigue life becomes even more critical. Aircraft gas turbine engines are an example of systems that require high reliability and for which component fatigue life is a critical factor. Component fatigue life is especially critical for the high energy rotor components of gas turbine engines because they pose a significant threat to aircraft safety should an uncontained failure occur. Given the potentially disastrous consequences of hardware failures in any aircraft gas turbine engine, the aircraft engine industry has developed very sophisticated design methodologies which attempt to insure that all critical engine components can reliably meet service life expectations for a given set of operating conditions. Thus, fatigue failure due to repeated duty cycles is a failure mode of great interest to the engine designers because it directly influences the reliability and life cycle cost of the end product. Accordingly, there is heavy emphasis placed upon designing components which safely maximize their fatigue life.
The primary approach to designing fatigue susceptible hardware which has evolved over many years within the aircraft engine community is commonly referred to as the “safe life” method. The safe life method is based on the principle that the minimum number of load cycles that can be sustained before the generation of a fatigue crack or other fatigue indication may be deterministically calculated for any given design. This minimum number of load cycles must take into account variations in hardware dimensions, material properties and operating environments (ambient conditions). In the safe life method, once this minimum number of operating cycles is determined, a retirement limit or life for the hardware is established. Retirement limits are typically set less than the minimum number of load cycles to provide a margin of safety. At least one engine manufacturer has typically used a safety factor of three to determine its retirement limits. It would be understood by those skilled in the art that the safe life method may be applied to any mechanical, electrical or fluid component.
One typical procedure for determining the safe life of a new component for a gas turbine engine involves the following:
Determine the expected duty cycle.
Establish minimum engine and deteriorated engine thermodynamic conditions.
Perform a transient heat transfer analysis using the thermodynamic conditions.
Perform a transient finite element stress analysis using the heat transfer results.
Establish the maximum operational strain ranges in the component for several locations, accounting for a “mission mix” of ambient conditions and engine deterioration.
Determine the minimum fatigue life based on the strain ranges and existing specimen fatigue data.
Apply a safety factor to the minimum life based on service experience, test experience, etc. to determine the retirement limit of the component.
While theoretically ensuring a high degree of reliability, there are some disadvantages to the safe life method. For example, since the method is deterministic in nature and assumes minimum values throughout, the vast majority of components are forced to retire long before they have developed cracks or other failure indications. For the aircraft engine industry, this is not cost effective in that many engines are forced off wing and torn down to have hardware with remaining useful life removed and discarded.
In addition, experience has shown that despite the application of the safe life approach, fatigue failures in service can occur. For aircraft engines, this discrepancy is usually the result of one of several factors, including:
Inaccurate heat transfer or stress analysis.
Improper duty cycle definition or operators who employ duty cycles other than that assumed.
Hardware failures in the control system which allow engine operation at other than the assumed thermodynamic conditions.
Control logic “bugs” which allow engine operation at other than the assumed thermodynamic conditions.
Also, since the declared safe life for a given component is calculated assuming no need for inspection, that component is generally not inspected prior to reaching its retirement limit. This is often the case even if the hardware is available for some other maintenance reason. If the original analysis on which the safe life limit is based should turn out to be non-conservative, valuable opportunities to detect negative fatigue trends in a fleet of engines are lost and oftentimes the first indication of a problem is an actual failure. Alternatively, if the analysis upon which the limit is defined proves to be overly conservative, a complicated and time consuming program of forced removals and inspections is required in order to gather data to support incremental life extensions.
SUMMARY OF THE INVENTION
It is the principal object of the invention to provide a new and improved method of maintaining components subject to fatigue failure during use in the field. More specifically, it is an object to provide a method of maintaining fatigue critical components in a system that maintains or increases the level of reliability or safety of the system while reducing the operating cost of the system for the system users.
Another object is to provide an enhanced approach to establishing life limits of fatigue critical components. It is another object of the invention, to provide a method of maintaining fatigue critical components that allows for the detection of unexpected or non-nominal failure modes. It is a further object to provide a method that allows for easy life extensions of fatigue critical components for a real time health monitoring capability. Yet another object is to provide a tool for numerically evaluating the effect of various maintenance scenarios during the life cycle of a component or product, such as a turbine engine. A further object of the invention is to reduce the number of forced component removals.
One or more of the above objects are achieved in a method that uses statistical distributions in a simulation of in service use of a fleet of components to predict the failure rate of the fleet over a fixed time increment for an assumed inspection program. These predictions are used to determine an operating plan, including an inspection program, that will maintain an acceptable failure rate by detecting components with fatigue indications and removing the components prior to failure. The inspection program provides actual fatigue data resulting from in service use. As the inspection data base grows, the simulation is revised to incorporate the data. The predicted failure rates and the operating plan are then updated based on the revised simulation.
More specifically, one or more of the above objects are achieved in a method for maintaining components subject to fatigue failure during in service use. The method includes the steps of:
a) determining a probabilistic distribution of a fatigue indication occurrence and a probabilistic distribution of a fatigue failure life for a given component that is subject to fatigue failure from actual in service use;
b) setting an acceptable in service failure rate for the given component;
c) forecasting each given component that will be in service during a first time increment;
d) simulating in service use and inspection of the given components over the first time increment to determine an acceptable operating plan for the given components based on
1) the probabilistic distributions of the fatigue indication occurrence and fatigue failure life,
2) a probability of detecting a detectible fatigue indication during inspection of any one of the given components,
3) the forecasted components that will be in service during the first time increment, and
4) the accept
Eastman Donald G.
Elgin Richard L.
Hao Beilene
Assouad Patrick
Hamilton Sundstrand Corporation
Wood Phillips VanSanten Clark & Mortimer
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