NESC tech bulletin

National Aeronautics and Space Administration

www.nasa.gov

Applicability

Structural analysis Agency-wide.

Background

Advances in modeling and simulation, new finite element 

software packages, modern computing platforms, computing 

engines, and powerful computers are providing opportunities 

to interrogate complex designs in a very different manner and 

in a more detailed approach than ever before. The current 

trend in the structural design process is increasing reliance 

on modeling and simulation to assess local stress states and 

evaluate margins of safety. In addition, there is also a tendency 

to perform three-dimensional (3D) analyses under the 

assumption that detailed 3D models inherently provide higher 

fidelity and more accuracy than two-dimensional (2D) and 

shell models. Furthermore, aerospace structural components 

are inherently complex; typically local stress concentrations, 

free edges, skin-stiffeners, varying thickness shells, etc. are 

par for the course. Global- or system-level structural models 

of these components often include connections between and 

among finite elements of different dimensionality (e.g., beam 

element connected to a plate/shell/solid element). Quite 

often negative stress margins are calculated and reported 

from these analyses. The reported negative margins raise 

questions about the adequacy of the structural design and 

may, in fact, initiate separate independent assessments of the 

design, a redesign of the component(s), or both. Alternatively, 

in many instances these stress values may be prescribed as 

input to a life-prediction analysis and tools, and the predicted 

outcome may be an inadequate design life, driven in part by 

these artificially high local stress values. As a consequence, 

schedule delays may result and costs may increase due to 

perceived necessity to redesign. 

Findings and Conclusions:

Recent studies show that in some, but not all cases, these 

negative stress margins computed using local stresses are 

inaccurate and are artifacts of modeling and analysis. The 

areas where negative margins are frequently encountered 

are often near stress concentrations; point loads and load 

discontinuities; near locations of stress singularities; in areas 

having large gradients but with insufficient mesh density; in 

areas with modeling issues and modeling errors; in areas with 

connections and interfaces; in areas of 2D-3D transitions; 

near bolts, due to details of bolt modeling; and near areas of 

complex boundary conditions. Now, more than ever, structural 

analysts need to examine, interrogate, and interpret their 

analysis results and perform basic “sanity checks” to determine 

if these negative margins are, in fact, real or they are just 

artifacts of modeling and analyses. Knowledge of the behavior 

of structures and the theory of elasticity, the ability to formulate 

an estimate of expected results before they are obtained, the 

awareness of consequences of modeling assumptions, etc.

are essential to interpret the numerical results.

Another disturbing aspect noted in the recent past is the

inability to prescribe appropriate boundary conditions by 

widely available desktop software packages. The reported 

positive margins by these software packages may, in fact, be 

false positive. These packages are inexpensive and may not 

have all the analysis options and capabilities that the widely 

used general-purpose software packages (such as NASTRAN,

ANSYS, ABAQUS, etc.) offer. The margins evaluated with these 

desktop packages need to be confirmed by performing a

reanalysis with the widely used packages and also ensuring 

that proper boundary conditions are prescribed.

References

Raju, I. S.; Lee, D. S.; and Mohaghegh, M.: “Negative Stress 

Margins – Are They Real?”, AIAA-2011-1808-588, Paper 

Presented at the 52nd AIAA SDM conference, Denver, Colorado, 

April 4-7, 2011.

For more information, contact the NESC at www.nesc.nasa.gov

Aircraft empennage

Rocket structures Composite Crew Module

External tank stringer

V4i

Structural Analyses and Margins of Safety 

Examples of complex structural analysis models

There is an increasing reliance on modeling and simulation to verify, quantify, and certify designs of complex 

structures. The availability of a range of commercial modeling and simulations tools and packages with a variety 

of capabilities, in conjunction with increased computational resources, is allowing analysts to rapidly perform 

detailed analyses. However, care should be taken to understand specific tool limitations, assumptions, and 

boundary conditions as erroneous results can be generated without being recognized by the analysts. In addition, 

the reported margins of safety should be carefully interrogated to identify any false positive or negative margins 

and highlight any areas for structural concern.

NASA Engineering and Safety Center Technical Bulletin No. 12-02