In ANSYS calculations, stress concentration and stress singularity are two different concepts, but both are related to abnormal stress increase in local regions.
1. Stress Concentration
Definition
Stress concentration refers to the phenomenon where local stress is much greater than nominal stress at locations where the cross-sectional geometry of a part changes suddenly. It is an important factor causing structural failure and the main failure location of structures.
Causes and Effects
Usually caused by sudden changes in geometric shape (such as openings, slots, steps, etc.), material defects (inclusions, pores, cracks, etc.), and residual stress.
Under static load, plastic materials may not need to consider the effect of stress concentration, while brittle materials need to consider it. Under loads that change periodically with time or impact loads, the effect of stress concentration must be considered for both plastic and brittle materials.
Treatment Methods
(1) Optimize geometric shape: such as adding fillets, avoiding sharp corners, etc. Increase the radius of transition arcs to reduce stress concentration. Add appropriate radius fillets at shaft shoulders, hole edges, and other positions of mechanical parts to reduce stress concentration factors. At connections between different cross-sectional sizes, design reasonable transition structures, such as gradual conical or curved surface transitions. Change right-angle connections between plates and beams to arc transitions to reduce stress concentration caused by sudden cross-section changes.
(2) Use high-order elements: can better capture stress changes. For stress concentration problems, choosing appropriate element types is also important. Generally, high-order elements (second-order elements) compared to low-order elements (first-order elements) can better simulate complex stress changes. Using high-order elements in stress concentration regions can improve calculation accuracy.
(3) Fatigue analysis: For structures subjected to alternating loads with stress concentration, perform fatigue analysis to evaluate their fatigue life. Consider the effect of stress concentration on fatigue life, and then take corresponding measures (such as increasing structural dimensions, optimizing geometric shape, changing materials, etc.) to improve the fatigue performance of the structure.
2. Stress Singularity
Definition
Stress singularity refers to the phenomenon where stress values in local regions continuously increase and do not converge as the mesh is refined, eventually tending toward infinity. It is a pathological phenomenon in finite element calculations that cannot exist in actual structures.
Causes and Effects
The force application area tends to 0, such as concentrated forces acting on points or lines.
Geometric shape discontinuity, with features such as right angles and sharp contact points.
At connections of different materials, such as bonded connection positions of different material components in assemblies.
Stress singularity will cause very slow convergence speed of finite element solutions and make calculation results lose physical meaning.
Treatment Methods
(1) Rationality of constraints: Ensure that applied constraint conditions conform to actual physical situations, avoiding stress singularity caused by over-constraints or unreasonable constraints. For some complex structures, more reasonable constraint methods may be needed, such as elastic supports, flexible connections, etc., to simulate actual boundary conditions.
(2) Adjust loads: If loads act directly near stress singularity points, it may cause stress singularity to intensify. Consider appropriately dispersing loads to larger areas, or using more reasonable load distribution methods, such as converting concentrated forces to distributed forces.
(3) Refine the model: For regions near stress singularity points, such as sharp corners, small holes, or junctions of different geometric shapes, appropriately refine the geometric shape of the model to make it more consistent with actual structures, reducing stress concentration caused by geometric mutations.
(4) Fillet transition: Add appropriate fillets to the geometric model to avoid sharp edges and corners. Fillets can disperse stress, make stress distribution more uniform, and reduce the degree of stress singularity.
(5) Model simplification: such as using beam elements, shell elements.
(6) Ignore stress singularity: If stress singularity points do not affect the overall performance of the structure, the stress value at that point can be ignored, focusing on other key regions.
(7) Adjust structure: If stress singularity occurs at dangerous parts of the structure and cannot be accurately calculated by numerical methods, consider optimizing and improving the structure to reduce stress concentration.
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