Finite element analysis (FEA) is an engineering tool used in performing analyses such as calculating stresses or deformations in complex structures under load. FEA utilizes a computer solid model of a design or component, which is stressed in a way that could be expected in reality, resulting in a stress scheme of the material which is a result of the input state. FEA uses what is known as finite element modeling (FEM). FEM is a numerical technique using many simple element equations over small domains, named finite elements, to approximate the solution of a complex equation over a larger domain. The finite element method is a numerical technique for finding approximate solutions to boundary value problems. The boundary conditions as well as the geometry of the specimen being analyzed are the critical inputs for a FEA problem. Computer calculations are performed over many small subdomains to determine the overall reaction to the given boundary conditions. This analysis is useful for new product design, as well as when improving or determining the expected behavior of current items.
Finite element analysis uses a complex system of points called nodes, which make a grid called a mesh across the surface of the computer model of the specimen being analyzed. In other words, FEA, as applied in engineering, uses mesh generation techniques for physical systems, dividing a complex structure into smaller elements. This mesh is programmed to contain the material and structural properties, which determine how the material will react to any given loading conditions. The accuracy of the analysis depends on the density of the nodes throughout the region under study. Often times, regions which will be receiving large amounts of stress have a higher node density than the areas which will experience little stress. Also, points of interest such as fillets, corners, areas of high detail, or areas known to fail, often have a high node density. The higher the node density, the more accurate the results of the analysis, but a balance must be found so that a solution can be obtaining using the computing power which is available. FEA provides detailed visualization of the structure showing the meshes, as well as the stress and deformation distributions throughout the domain.
Finite element analysis has become a great alternative to actually building prototypes and testing them in the laboratory. It allows equipment design to be fully developed and optimized prior to the manufacturing stage, thereby saving significant production costs. Potential failure due to stresses as well as problem areas within a specimen can be predicted with a great deal of accuracy, allowing designers to see the flaws or concerns within their designs. The finite element method of product design and testing is far superior to the manufacturing costs which would accrue if each sample was individually built and tested. Aero Nav Laboratories has finite element analysis capabilities that could potentially lower the cost of your design process drastically.
Equipment reliability is defined as the ability of a component or system to perform its intended function, under a given set of conditions, for a specified period of time. A reliability analysis will predict the probability of failure or the frequency of failure. Reliability often defines the maintainability of a system, and plays a large part in the cost-effectiveness of the system. Due to the large ranges of uncertainty involved in reliability analysis, (especially when multiple components, each with their own reliability, make up a single system) quantitative methods for reliability prediction and measurement are often avoided.
Reliability engineering uses many techniques such as hazard analysis, failure mode and effects analysis (FMEA), stress, fatigue, corrosion, and wear analyses, etc.
FMEA is of particular interest. It consists of identifying, for each component, the individual failure modes and the resulting effects on the overall system. The analysis provides early identification of critical subsystem or system failure modes. This allows their elimination or minimization during the development effort.
The more complex a system, the more elaborate the approach for reliability analysis of the system must be. The approach may (or may not) consider the following: failure analyses, requirement specifications, mechanical design, manufacturing methods, adequate testing, proper maintenance of the system, careful transport and storage, correct usage practice of the mechanism, etc. Since mathematical and statistical approaches are often avoided for reliability analyses of complex systems, it is only through engineering experience, skills, and knowledge, that an effective reliability analysis could be conducted.
A reliability program plan is used to define the reliability goals and to set up a program to achieve these goals. The plan enables the detailed identification of program goals and to specify the required tasks, as well as budgets, schedules, manpower requirements, etc. It is normally approved by management and becomes a guiding document throughout all stages of the program.
Computational fluid dynamics (CFD) is a branch of the field of fluid mechanics. It addresses the flow of liquid and gases under various conditions. Included among these conditions are the following:
- Flow with and without heat transfer
- Laminar, transitional, and turbulent flow regimes
- Subsonic, transonic, and supersonic regimes
- Gas/liquid mixtures
- Incompressible and compressible flow
- Non-Newtonian fluids
CFD uses numerical methods and algorithms to solve fluid flow problems.
Problem solving is performed in accordance with the following steps:
- Define the physical geometry of the problem
- Establish a mesh which fills the volume occupied by the fluid
- Define the specific fluid and heat transfer (if any) conditions relative to the process for which a solution is desired.
- Define the boundary conditions by specifying the fluid properties at the boundary
- Perform the computer simulation for the analysis and flow visualization of the solution.
The interaction between different areas of fluid flow, or the interaction between a fluid and a surface can be predicted using the Navier-Stokes equations. CFD software uses boundary conditions entered by the user to calculate solutions to this set of equations over a series of pre-defined elements into which the model being analyzed has been subdivided. Similarly to the finite element method for stress analysis, computational fluid dynamics results in a solution only as accurate as the number of elements over which calculations have been performed. Hence, the more elements, the more reliable the results are likely to be.
Complex simulations can easily be computed with high-speed computers, resulting in accurate representations of fluid flow in two or three dimensions. Furthermore, certain simplifications can be made to the Navier-Stokes equations in order to facilitate execution of the simulation. Computational fluid dynamics techniques are quite useful in the product design process, since fluid interactions could be predicted with a high degree of accuracy, without going through the task of prototype manufacturing and wind tunnel testing.
Modal analysis is the field of studying the dynamic properties of mechanical structures when subjected to vibrational excitation. It consists of measuring and analyzing the dynamic response of structures when excited by an input. Vibration modal analysis uses the mass and stiffness distributions to determine natural frequencies, damping, and mode shapes at the various natural frequencies. The excitation input signals can be impulse, broadband, sine, swept sine, etc., using an impact hammer or a vibration shaker.
- Impulse - narrow, spike waveform
- Broadband - wide waveform
- Sine - sine waveform
- Swept sine - sine waveform sweeping over a range of frequencies.
Testing and data acquisition can be done using the following methods:
- Single excitation point and multiple output points (SIMO)
- Multiple input points and a single output point (MISO)
- Multiple input points and multiple output points (MIMO)
The SIMO technique was the classical approach to obtaining vibration data. Subsequently MISO became popular. MISO and SIMO are identical due to the theory of reciprocity. MIMO has currently become popular where partial coherence analyses can identify which part of the response came from which excitation source.
Data is acquired by the use of accelerometers, load cells, or non-contact laser vibrometers.
Fourier analysis is used to analyze the output vibration signals to yield information regarding resonant frequencies, damping, and mode shapes. The resonant frequencies represent the discrete frequencies at which the structure will vibrate as exhibited by high displacements. Damping is a measure of how readily the structure dissipates the vibrational energy and subsequently returns to rest upon removal of the excitation. The mode shapes represent the actual shapes taken by the structure at each of the natural frequencies.
The results of vibration modal analyses can be correlated with the results of finite element stress analyses to provide a complete description of the behavior of a structure under static and dynamic loading conditions.
AERO NAV LABORATORIES has prepared a number of interesting blogs for our reader’s attention concerning topics of importance. Watch for them!
Vibration modal analysis
Computational fluid dynamics
Finite element analysis
Vibration fixtures 1
Vibration fixtures 2
To view our previously posted blogs go to www.aeronavlabs.blog.com
If you have any questions regarding the above topics please contact:
- The laboratory contains many different types of equipment capable of performing the wide range of testing specified by customers.
- To ensure that the proper test equipment is used for a particular test, it is important that the customer provide the following information:
- Item weight
- Item configuration
- Detailed test specifications and parameters
- Operating conditions
- Fluid media for testing pneumatic or hydraulic components
- Electrical and fluid power requirements
- Weight and description of any auxiliary components such as dummy weights, etc.
- Ensure that all information reflects the current equipment status
- Test procedures are prepared by the laboratory’s engineering staff to provide instruction to the laboratory’s mechanics for performing the required tests. The procedures are based on information provided by the customer prior to preparation of the procedures.
- The test procedure is formatted to provide information such as descriptions of the unit under test, the laboratory equipment which will be used, detailed test procedures, special fixtures, instrumentation, etc.
- A copy of the test procedure is forwarded to the customer for their review and approval prior to the beginning of testing.
- The review and approval phase allows the customer to ensure that their requirements will be met. It also provides an opportunity for possibly reducing costs during testing by eliminating unnecessary steps or procedures, or by making clarifications to the procedures.
No. 6 in a Series:
Test letters or reports are provided at the conclusion of a test program. The following issues should be considered by customers when addressing the requirements for a test letter or report:
- A test letter can be provided for most programs at no cost. It provides only compliance statements with no test data.
- A test report provides test methods, description of the test setup, photographs, typed data sheets (handwritten data sheets are not provided), a test equipment list, and test results.
- The format of the Aero Nav test report meets or exceeds the requirements of U.S. government specifications for reporting.
- The laboratory serves as an independent third party, and makes no conclusions or recommendations based on the results of the test program.
No. 5 in a Series:
Sending of customer representatives to witness and operate units under test can be a costly experience. The following issues should be considered:
- Where the equipment to be operated is relatively straightforward, consideration should be given to having the laboratory’s personnel operate the equipment. Instructions shall be provided in writing to the laboratory.
- Where the operation is complex, requiring special training and/or instrumentation, customer personnel should perform the required steps.
- Where customer personnel will be utilized it is wise not to overstaff the witness team. The laboratory facilities are not structured to accommodate relatively large numbers of people.
No. 4 in a Series:
Test scheduling is an important issue in a testing program. The customer should be aware of the following:
- A program is placed on the laboratory test schedule upon receipt of a purchase order, approval of procedure, and either receipt of a firm date for delivery of the equipment or actual equipment delivery.
- Scheduling is done on a “first-come first-served” basis, upon availability of laboratory test equipment.
- In the event of a failure during testing, the test will be stopped. The unit under test will remain on the test stand pending instructions from the customer. However, charges may be incurred awaiting disposition from the customer.
- Upon disposition, the test may be resumed or stopped and the equipment removed for correction of the failure mode. Upon receipt of the revised test item, the test may be resumed or restarted if the laboratory test equipment is available. If not the test will be placed on the laboratory schedule for future testing.
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