Short Answer:

Stress concentration is caused by sudden changes in the shape or geometry of a material that disturb the uniform flow of stress. When a part has holes, notches, sharp corners, keyways, or cracks, the load is not distributed evenly, and certain points experience much higher stress.

These geometric irregularities make the stress lines crowd together, increasing stress at those locations. This condition can lead to failure or cracks even when the average applied stress is within the material’s safe limit. Therefore, stress concentration occurs mainly due to structural discontinuities or irregularities in a component.

Detailed Explanation :

Causes of Stress Concentration

Stress concentration occurs when there is a local increase in stress around discontinuities or irregularities in a material or structure. These irregularities disturb the normal flow of stress through the component and create points where stress becomes much higher than the average stress. This localized high stress can cause cracks, deformation, or even failure of the component. The major causes of stress concentration are related to geometry, material properties, and type of loading.

Below are the main causes explained in detail:

1. Holes and Cutouts

Holes are one of the most common causes of stress concentration. When a plate or structure with a hole is subjected to a tensile or compressive load, the stress lines that would normally flow smoothly are interrupted by the hole. This interruption causes the stress to crowd near the edges of the hole, increasing the local stress value.
For example, in a flat plate with a circular hole under tension, the stress at the edge of the hole can be up to three times higher than the nominal stress in the rest of the plate.

2. Sharp Corners and Notches

Sharp corners or notches act as stress raisers. When there is a sudden change in geometry such as at a notch, groove, or corner, the load-carrying area changes abruptly. This causes stress lines to become dense at the notch tip, increasing the local stress.
In mechanical components like keys, shafts, or couplings, sharp corners are often replaced by fillets or rounded edges to reduce the effect of stress concentration.

3. Keyways and Grooves in Shafts

In rotating shafts, keyways and grooves are used for power transmission. However, these cutouts create discontinuities in the shaft, disturbing the stress distribution. The edges of these grooves experience concentrated stresses which can lead to crack formation or fatigue failure under repeated loading.

4. Cracks and Surface Defects

Cracks or small surface defects are very serious causes of stress concentration. Even a very tiny crack in a component can lead to an enormous increase in local stress at the crack tip. Under cyclic or fluctuating loads, these cracks may propagate rapidly and cause fatigue failure. Hence, cracks must be detected early and repaired to prevent breakdowns.

5. Sudden Change in Cross-Section

When a component changes from a thick section to a thin one abruptly, the stress flow is disturbed. The stress lines become crowded near the transition region, creating localized high stresses. For example, in stepped shafts or beams with sudden thickness variations, stress concentration occurs at the junctions of different sections. Smooth transitions or tapered connections are used to avoid this issue.

6. Keyholes, Slots, and Threaded Portions

Slots, keyholes, and threaded areas are common in mechanical parts. These features reduce the effective cross-sectional area and change the stress path. Thread roots in bolts or screws experience higher stresses because of their sharp notches. That is why rounded threads or special fillet designs are used in high-stress applications.

7. Material Discontinuities

If the material is not uniform, such as when there are inclusions, voids, or impurities inside, stress concentration may occur even without visible geometric irregularities. Different materials joined together can also create discontinuities because of different elastic properties. This mismatch in stiffness causes localized stresses at the joint.

8. Improper Manufacturing and Surface Finish

Manufacturing defects such as machining marks, scratches, weld undercuts, or rough surfaces act as tiny notches. These imperfections disturb the stress flow and lead to high localized stress. Proper finishing and surface treatments like polishing, shot peening, or heat treatment are used to minimize stress concentration caused by manufacturing.

9. Loading Conditions

Sometimes, the nature of applied load also causes stress concentration. For example, a point load or impact load creates a highly localized area of high stress. Uneven or eccentric loading also increases stress at certain areas of the component, especially when the load is not applied through the centroidal axis.

10. Thermal and Residual Stresses

Temperature differences within a component can cause expansion or contraction in certain areas, leading to thermal stress concentration. Similarly, residual stresses left from welding or heat treatment can add to mechanical stresses, creating concentrated stress zones even before the component is loaded.

Prevention of Stress Concentration

While it cannot always be completely avoided, stress concentration can be minimized by:

• Using fillets instead of sharp corners
• Designing gradual changes in cross-section
• Applying relief holes at the ends of slots or cutouts
• Using better surface finishing to remove machining marks
• Avoiding sudden geometry changes in the design

These design improvements allow the stress to flow more uniformly through the component, reducing chances of failure.

Conclusion

Stress concentration is mainly caused by geometric discontinuities such as holes, notches, grooves, cracks, or sudden section changes. These irregularities disturb the normal stress flow and cause localized high stresses which can lead to failure or fatigue cracks. Factors like material defects, poor surface finish, and uneven loading can also increase stress concentration. Proper design techniques like smooth transitions, fillets, and improved surface finishing help reduce these effects and make the component more reliable and durable.