Fatigue Design and Evaluation Committee 

Structural Analysis Division - Mary Wickham

Mary announced the new Structural Analysis Division vice-chairman of is Jin Qian. Greg Glinka presented "Stress Concentration and Stress Distribution In Weldments," based on discussions with John Deere and Ralph Stephens. The approach to structural analysis of complex cross sections to find local point stresses at weld toe and apply standards with nominal stress is ok for test specimens, but difficult to apply to real components. Therefore, he uses hot spot stresses or average stresses. In offshore applications for tubular structures definition of nominal stress is not unique. Finding nominal stresses is not easy; and some other method is needed. He can use detailed numerical analysis and get nominal stress from sections of interest equal to hotspot stresses. He uses a method proposed by a Japanese analyst, where mean stresses are calculated away from the weld toe, with two stress concentration factors and linear extrapolation to the section of interest all along the surface. He could also linearize the stress distribution through the thickness. Greg's method uses membrane stress and bending stress obtained from a shell element analysis of the structure, as a direct output of hot spot stress. He can then find stress concentration factors from membrane and bending stresses. For example, fatigue problems in a crane arm were caused by local stresses not by a nominal bending stresses. A shell finite element model of the entire box gave stresses in each cross section. He calculated peaks stresses for stress concentration factors from pure bending and stress concentration factors for tension. Peak stresses with were obtained with Neuber analysis, and then applied to damage calculations. Another example was a T-joint loaded in bending and tension. The stress concentration factor for fillet depends on weld height and toe radius. Greg applied his method to a T-joint modeled by Jin Qian with finite element shell elements and found results compared closely to FEA results. The key point is to split the stress into membrane and bending stresses from a shell finite element analysis and apply to crack growth. Finite element stresses are for uncracked sections. Greg also integrated weight functions and compared to results from the Paris equations. He assumed 0.3 to 0.5mm internal cracks in the weight functions.

Jim McConville, from Mechanical Dynamics Inc., presented " A Survey of FEA Based Stress Recovery Methods in ADAMS." The primary purpose of engineering analysis is to "prevent nasty surprises." Structural models are only as good as the loading. A "good analysis "rule is that it

needs all three of these qualities "good, fast, and cheap." But you

usually only get two at a time. As an example Jim described a flexible airframe landing simulation that was modeled with a Nastran finite element model. The model was a condensed structure with 60 retained modes, out of 2400 degrees of freedom. 2.1 Hz through 1572 Hz. It used Greg Brampton's hard points, static superelement condensation, eigenvalue extraction with a Lagrangian approach to show the effect of landing stresses. Jim showed a comparison of landing gear loads for the rigid model and flexible aircraft models for a force-based model and a displacement based model. Force based linear elastic structural analysis methods depend on loads the interface points. The accuracy of force based linear elastic structural analysis methods depend on loads at the interface points and are incomplete because it cannot account for accelerations. The FEA solution is valid if reactions at the supports are equal to zero or small or compared to the applied loads. The displacement based model method is linear elastic. Flexible coupling uses a reduced degree of freedom set. A MSC Nastran example problem showed supports are arbitrary if support reactions are small. Automotive customers typically inertia relief methods. Jim concluded that flexibility is important. The free-free modal behavior of a condensed model agreed closely with the behavior of a full model. Retained modes of interest work the notches. Highly condensed models can yield very accurate results. One simulation predicted fatigue life at 7.1 hours compared to test life of 12 hours. These analysis results were good for low frequency damped structural simulations. Jim described an "Autoflex" process that automates the modeling process that makes early predictions of fatigue life feasible and practical.

Planning Session Results. The activity plan, generated during the meeting, is shown below:

SAE FD&E  -- Structural Analysis Division

Planning Session -- Fall 2001

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