What does an Aerospace Stress Engineer Do in Aircraft Industry?

No one airplane can be finally designed and no one part or assembly drawn and turned to manufacturing process without obtaining of the reliable aircraft stress analysis results. In more simple words, - it is obvious that before producing something we have to ensure it will withstand the loads which will most likely occur during its life cycle.

The modern commercial or military airliner, or a space vehicle is a highly scientific machine and the combined knowledge and experience of hundreds of engineers and scientists working in close cooperation is necessary to insure a successful product. Typically engineering department of any big Aircraft company consists of many groups of specialists whose education and background covers all fields of engineering education such as Physics, Chemistry and Metallurgy, Mechanical, Electrical and, of course, Aeronautical Engineering.

In general, the engineering department of an aerospace company can be broken down into six large rather distinct sections, which in turn are further divided into specialized groups, which in turn are further divided into smaller working groups of engineers:

(1) Aerodynamics Group

(2) Structures Group

(3) Weight and Balance Control Group

(4) Power Plant Analysis Group

(5) Materials and Processes Group

(6) Controls Analysis Group

Group of engineers performing stress analysis of aircraft components is a part of a Structures Group, - number (2) in the list above. Essentially the primary job of the group of stress engineers is to help specify or determine the kind of material to use and the thickness, size and cross-sectional shape of every structural member or unit on the airplane or missile, and also to assist in the design of all joints and connections for such members.

What Does a Stress Engineer Do?

An aerospace stress engineer performs static, dynamic and fatigue calculations, is responsible for checking various types of airplane parts and assemblies: Wing and Empennage skins, ribs, spars, stringers, various clips, fasteners, joints and fittings. Same for the fuselage – skins, frames, frame clips, joints, stringers and attachment brackets. Also Engine attachments, landing gear assemblies and parts, even cabin internal equipment. All the analysis is performed in accordance with the correspondent articles of FAR / CS Airworthiness requirements, i.e., FAR/CS 23 – small airplanes, FAR/CS 25 – large airplanes, FAR/CS 27 – helicopters / rotorcraft.

Stress engineer is also responsible for the structural strength test performance and correlation of its results with “theoretical” analysis.

What are structural analysis skills?

Stress engineer must have excellent knowledge of mechanics, mathematics, properties of materials and even basics of Chemistry and Metallurgy. Must be able to perform both analytical “by-hand” calculations and to professionally use the Computer-aided engineering (CAE) tools , understand how is the analyzed part or assembly working, which loads it perceives, must have good communication skills for being in contact with manufacturing and design engineers , read and understand engineering drawings, manufacturing, certification and different normative documentation and standards.

What is stress in engineering?

Many “common” words was said, - so let us consider what is stress in engineering and briefly explain the very basic principles of Strength of Materials.

So, let us consider simple round bar with cross-section area of A, under an action of two equivalent forces P:

In any arbitrary section (x-x) in this case, the reaction R is equal to the applied force P.

Normal Stress (typically marked by the Greek symbol Sigma) is the relation of the reaction force R to the cross-section area A:

Any material has its own stresses, upon reaching the values of which its destruction begins. Destruction of material is a broad concept. Depending on the purpose and working conditions, not only the rupture of the part, but also its plastic (residual) deformations may be unacceptable. In particular, according to the Airworthiness requirements , aircraft parts under operational loads must work only in the elastic zone under the effect of the most important relation in stress engineering – Hooke's law.

This law states that within the limit of elasticity, the stress induced (σ) in the solid due to some external force is always in proportion with the strain (ε). In other words, the force causing stress in a solid body is directly proportional to the solid's deformation.

If to plot the graph for the stress (σ) versus strain (ε) for the round bar considered above:

As seen in the graph above, the stress curve versus strain curve is linear within the material limit of elasticity. It means that for the force P below this limit, the stress in the bar is in proportion with the strain.

Acting and Allowable stresses are differentiated during stress calculations. Acting stress is the stress under the value of Acting force: σ = P/A. Allowable stress is the Max stress the part can withstand, typically marked by [σ].

Margin of Safety (MS) is the relation between the Allowable and Acting stress:

MS = [σ] / σ – 1

It is obvious that MS should be always positive. Negative MS value means that the part cannot withstand the acting loads – this situation requires making the immediate decisions in design (geometry changing) or selection of stronger material.

In addition to analytical “by-hand” calculations, stress analysts often use Computer-aided Engineering tools, especially Finite-element modelling (FEM) tools. The essence of the finite element method is as follows: The loaded object is represented as a composition of elements with finite dimensions. The number of these elements is also finite. The coordinates of its nodes specify each element, called the “Finite Element” (FE). In other words, a solid body is geometrically represented in space by the coordinates of some of its material points, which are the vertices (nodes) of finite elements:

Each finite element in the model has properties that are as close as possible to the properties of the material of the investigated part. Element Stiffness Matrix represents its properties:

Main equation of Finite-element method is the following:

{ Δ } – Matrix of Element’s Nodal Displacements,

[K] -1 – Inverted Stiffness Matrix of the element,

{F} – Matrix of loads applied to the element

Global Stiffness matrix of all Elements is being assembled from Stiffness matrices of all elements in the model. System of thousands of equation is being solved to obtain the Nodal displacements, which, in turn, can easily be converted into Stresses or Forces required for stress evaluation. Obviously, this system of equations is solved by numeric methods such as Gaussian Elimination or Cholesky Decomposition with use of computer.

Airplane Jet Engine Finite-Element Analysis results (displacements) in one of FE-Tools is shown in figure below:

The approximate (and not complete) list of the most popular CAE (FEM) software used in engineering is the following:

ABAQUS . This software is one of the leading in the large industry. Offers detailed documentation and a wide range of solution options. Allows the user (engineer) to create subroutines (extensions) "for personal use" for almost all basic functions. The interface looks somewhat "old-school". For most tasks when working with ABAQUS, the FORTRAN language is used.

ANSYS . One of the oldest and most widely used software in stress engineering. Works well with almost all kinds of simulations found in the industry. The documentation covers up to 90% of possible tasks. ANSYS interface is more “user-friendly” than ABAQUS interface.

MSC NASTRAN . NASA STRucture ANalysis. Initially developed by NASA and widely used in the aircraft and space industries. Nastran is open source, so the solver is used in many other CAE packages. Primarily focused on solving aerospace thin-walled structures analysis problems.

SolidWorks Simulation . Despite the fact that SolidWorks is a Computer-aided Design (CAD) system, its CAE Simulation module has become popular due to its ease of use (as a solver for relatively simple, preliminary calculations and checks).

LS Dyna . One of the first CAE systems, in which the developers offered a full dynamic analysis and an option for contact pairs automatic search.

Typical Aircraft Stress Analysis procedure

Let’s consider the aircraft assembly stress analysis procedure on an example of the Static part of aircraft wing stress analysis.

Assume all the loads on the Wing for all required load cases to satisfy the Airworthiness requirements have already been obtained from the Aerodynamics analysis , - typical tasks of the Stress Group are the following:

- Create and analyze the FE-Model of the Wing to obtain the Loads on all Wing parts

- Analyze all parts: Skins, Ribs, Stringers, Stringer Clips, Wing-to-Fuselage Fittings, - - - - Fasteners, Access Covers, Baffle plates etc. using “classical” analytical methods with creation of the analysis templates for typical/repeating parts typically in Excel , MathCAD or MATLAB .

- Perform the definition and clarification of parts final dimensions, thicknesses and materials (so-called “sizing” procedure), communicate with the Design Group.

- Check and sign all the Design Drawings for all analyzed parts and assemblies from Stress positions (make sure everything is “covered” by the analysis).

- Perform the Certification reports for final parts configuration for FAA , EASA or other national Airworthiness Regulator .

An example of commercial airplane Wing Global FEM is shown below:

An example of “by-hand” analysis template:

It should also be noted that after obtaining the reliable results of Static Stress analysis it is required to analyze the Aircraft structure for Fatigue and Damage Tolerance. Briefly, the most important things to know in Fatigue analysis are (a) Stress Concentrations and (b) Fatigue Life (S-N Curve).

Stress concentration is the phenomenon of the occurrence of increased local stresses in areas of abrupt changes in the shape of an elastic body, as well as in contact areas. The area in which these high stresses occur is called a “stress concentrator”:

For example, the Hole is a stress concentrator for the regular plate:

Ϭ min here is the “regular” stress in the plate, P/A.

Ϭ max – highest stress which occurs at concentrator (hole area).

The relation between Ϭ max and Ϭ min is called the Effective Stress Concentration Factor:

K = Ϭ max / Ϭ min

Another basic thing to know is SN-Curve (sometimes written S-N Curve). This is a plot of the magnitude of an alternating stress versus the number of cycles to failure for a tested specimen of given material:

Fatigue Life (number of cycles to failure) of the particular specimen is calculated using the S-N Curves, K-factor, different empirical formulas of damage summation (see Palmgreen-Miner’s Rule as an example).

Obviously, it will take millions of words to explain all the aspects of Strength of Materials, Engineering Mechanics, Theory of Plates and Shells, Elasticity and Plasticity,  Mechanics of Multi-Layer Composite materials, - relatively new separate and very important part, - so, summarizing all of the above, it can be said that the Aerospace stress engineer is a real vocation: this engineer combines the “computer-working” at the office with visiting the objects, production workshops, communicates with design engineers and many other colleagues. Companies that are looking for aerospace stress specialists have serious requirements for their qualification.