Notch effect

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The notch effect is a physical phenomenon in materials that leads to destruction through fracture or crack and starts at a notch .

The susceptibility to notch effects depends on the material and z. B. very large in aluminum and its alloys .

Notches in a workpiece can be due to the design, through machining processes and / or arise unintentionally.

Types and occurrences of notches (examples)

  • A constructional notch consists e.g. B. on a screw in the area of ​​the transition from the shaft to the head. Note: A screw that is tightened too tightly always breaks off at the head or in the cut thread.
  • A tear in a workpiece caused by external circumstances represents a notch at its end that can trigger the notch effect and lead to the tear “continuing”.
  • Stress cracks can also trigger notch effects.
  • A notch created by machining is z. B. the trace ("crack") of the scriber on a workpiece.

(The use of scribers is regularly prohibited in aircraft construction; only special “lead” or colored pens that do not scratch the surface of the material may be used to mark lines).

  • Notches through tool marks (turning tools, milling cutters, engraving tools, thread cutting tools etc. - all cutting tools)
  • With angular openings in a component, there is a risk of a notch effect in every corner.

Origin and consequences of the notch effect

At the deepest point of the notch, a crack (structural separation) occurs on the atomic / crystal level, which propagates through the material and leads to a weakening of the material at this point and to long-term destruction. Notch effects are often the trigger for a fatigue fracture at mechanically stressed areas of a component.

Prevention of the notch effect

  • "Rounding" of notches through suitable measures. This is preferred for screws in the transition area between shaft and head. Although this often results in a reduction in cross-section, the screw is more resilient than without this measure.
  • Rolling or pressing threads instead of cutting (rounding the edges)
  • Prohibition of the use of scriber (see above) as well as avoidance of any surface damage to the workpiece (e.g. grooves during machining!).
  • Existing cracks in a workpiece (if the crack is not yet a problem) are "drilled out" at the end, ie a hole with a certain diameter is drilled in order to prevent the crack from "continuing".
  • Rounding (e.g. by drilling) the corners of angular openings in workpieces.
  • Use of materials that are largely insensitive to the notch effect.

Closing remarks

Although the notch effect is generally undesirable, it offers the possibility of deliberately and easily dividing materials: After notching the surface, it can be deliberately separated by breaking (e.g. stone processing).