Stress Analysis and Fracture Mechanics

DEI is an industry leader in the stress analysis of components, structures, and systems. Several staff members have extensive experience in applying classical and numerical stress analysis techniques to practical engineering problems. Our strong backgrounds in applied mechanics, theory of elasticity, and mechanical properties of materials allow us to knowledgeably apply the finite element method, a powerful numerical analysis technique, for a variety of situations. We routinely tackle difficult problems that require linear and nonlinear elastic stress analysis, fracture mechanics, steady-state and transient thermal analysis, and modal analysis.

Examples of some of our work in this area include:

Optimization of pressure vessel stud tensioning
Elastic-plastic welding simulation
System dynamic analysis
Fracture mechanics

Optimization of Pressure Vessel Tensioning
Many large pressure-vessel enclosures (e.g., nuclear reactor vessel heads, nuclear containment closures, and heat exchanger heads) are fastened with flange-stud connections that require each stud to be tightened ("tensioned") to ensure proper closure. This is often completed using multiple tensioning "passes," an approach that requires significant time and effort. At nuclear plants, this extra time leads to significant radiation exposure for workers and sometimes to costly outage extensions.

DEI developed an optimized stud-tensioning procedure for pressurized water reactor and boiling water reactor vessel heads using a nonlinear finite element analysis model. For each reactor head design, we create a computer model of the reactor head to determine the system response to the current tensioning procedure—and then develop an improved tensioning process that reduces the number of passes while still properly tightening all studs. DEI has developed optimized tensioning procedures for more than 40 sites.

Elastic-Plastic Welding Simulation
Residual stresses are often higher than the stresses imposed by operating conditions such as pressure and temperature. DEI is an industry leader in determining the residual stress fields due to welding using finite element models. Because residual stresses are often higher than the stresses experienced during normal operation, they are a significant factor when considering stress-corrosion cracking, fatigue, and crack growth.

Our welding simulations include thermal analyses to determine the temperature distribution in the weld and adjacent base metal as each weld pass is applied. We incorporate these temperatures as inputs to the stress analyses, which in turn apply elastic-plastic material properties to account for the high strains that are developed during weld cooling.

DEI has analyzed an array of different weld geometries, including nozzle J-groove welds, axial and circumferential welds in shrouds (cylindrical plates), and pipe butt welds among others. The analytical models are typically developed in parametric form, which allows large numbers of similar designs to be evaluated efficiently.

System Dynamic Analysis
DEI performs several types of dynamic analysis, including steady-state, transient, seismic, and modal analyses. These analyses help our clients verify the design of existing equipment in order to select the right replacement components. The results are particularly important because replacement parts often have a different weight or geometry, raising questions about how well they will perform in the system.

Fracture Mechanics
DEI staff have been active in the field of fracture analysis since the development of modern linear elastic fracture mechanics (LEFM) in the early 1960s. Our work has included fracture-related research and the practical application of fracture mechanics to engineering analysis. In addition to LEFM, we have experience in extending fracture mechanics to the analysis of elastic-plastic fracture, creep crack growth, fatigue crack growth and corrosion-assisted cracking.

Recent work has included integration of fracture mechanics modeling techniques into elastic-plastic welding simulation analyses such that the effects of relaxation of welding residual stresses with crack growth can be considered.