# Finite Element Analysis

Finite element (FE) analysis can be used to calculate internal loads on components of assemblies. It can also be used to calculate the elastic stability of complex structures. Sometimes, when appropriate, detailed FE models can be used to check hand strength analysis or calculated point stresses for performing fatigue or fracture mechanics analysis. If used properly, finite element analysis can be a powerful tool to predict internal loads and component strength or life. If not used properly, finite element analysis will produce garbage masked by fringe plots.

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## Finite Element Loads Models

Finite element loads models are used to predict how an externally applied load is carried through a statically indeterminate structure. Historically, thin-skinned structures were modeled using shear elements and rods. Shear elements were only able to carry shear, while rods were only able to carry axial loads. Rod element section areas contained actually lumped cross-sectional areas of the skin and stringers if they were in tension, and effective areas of the skin and stringers at crippling stresses when in compression. This modeling technique provides clear load distributions for each subassembly, but suffers from requiring separate models for different load conditions.

The current modeling practice of high-performance structures is to use a combination of bars (beams) and plate elements to predict internal load distributions. The internal load of a panel or subassembly can be extracted from the internal load finite element model (IFEM) using a grid point force balance or element loads. Sometimes the IFEM can be modified to contain a more detailed, fine-mesh of the interested part if the strength of the part is highly dependent on its surrounding structure. Normally, the critical internal loads are either applied to a fine-mesh finite element "carveout" model or used they are used in classical "hand" analysis.

## Finite Element Model of Composite Structures

Laminated composite structures are often modeled using layered plate or shell elements. These layered elements take into account coupling between axial, bending and twist from the non-homogenous composite layups. Stiffness matrices are calculated for each element and are represented in the finite element model. For large, complex structures, such as an aircraft, loads can be extracted from the IFEM and applied to a detailed finite element model, or "hand" analysis can be used to calculate strength. Compression strength is usually calculated from a section crippling cutoff and a buckling analysis. Crippling is a semi-empirical solution, while buckling can be calculate using closed-form solutions for orthotropic plates.

## Solid Element Finite Element Model of Life Prediction

For critical metallic structural components that operate in a cyclic environment, predicting durability using fatigue or fracture mechanics is a must. A well-meshed, properly constrained and loaded, finite element (FE) model made of solid elements can predict stress concentrations in complex parts. These models can be used to optimize a component or to document a life prediction.