Notch (engineering)

A notch in mechanical and materials engineering refers to a deliberately introduced v-shaped, u-shaped or circular defect in a planar material whereby stress is concentrated. Notch geometries play an important role in fracture mechanics and materials characterisation. Notches are most often found in impact testing of materials where the presence of a crack origin of controlled morphology is necessary in order to achieve standardised characterisation of fracture toughness. This is most commonly realized using the Charpy impact test , in which the impact of a pendulum hammer (striker) with a horizontal notched specimen and the height of its subsequent swing-through is used to infer the energy absorbed in fracture of the specimen. Impact testing of notched bars is also applied in the Izod impact strength test, typically applied to a cantilever configuration with a vertically positioned specimen. Charpy testing is conducting with U- or V-notches whereby the striker contacts the specimen directly behind the notch, whereas the now largely obsolete Izod method involves a circular notch facing the striker. Notched specimens find use in other characterization protocols, including tensile testing and fatigue.

Types of notches

The type of notch introduced to a specimen depends on the material and characterisation employed. For standardized testing of fracture toughness by the Charpy impact method, specimen and notch dimensions are most often taken from American standard ASTM E23, or British standard BS EN ISO 148-1:2009. For all notch types, a key parameter in governing stress concentration and failure in notched materials is the notch tip curvature or radius.[1]

Sharp tipped V-shaped notches are often used in standard fracture toughness testing for ductile materials, polymers and for the characterisation of weld strength.The application of such notches for hard-steels is problematic due to sensitivity to grain alignment, which is why torsional testing may be applied for such materials instead.

A U notch is an elongated notch having a round notch-tip, being deeper than it is wide. This notch is also often referred to as C-notch, and is the most widely form of introduced notch, due to the repeatability of results obtained from notch specimens. Correlating U-Notch performance to V-Notch equivalent is challenging and is carried out on a case by case basis, there is no standardized correlation between performance values obtained with the two notch types.[2]

A keyhole notch is typically considered as a slit ending in a hole of a given radius. This type of notch is most often considered in numerical models.[3] Fracture toughness results obtained from Keyhole notch testing are often higher than those obtained from V-notched or pre-cracked specimens.

Modelling

The geometry of notches and the associated concentration of stress at notch tips represent an active field of research in the domain of mechanical engineering. By developing new numerical techniques and finite element analysis, such research aims generally to establish relationships between defect morphologies and mechanical performance of materials. Stress concentration at a notch front differs fundamentally from that at crack tips. For notches, a region of stress concentration replaces the stress singularity of a crack tip. When the magnitude of peak tensile stress, which depends on the notch radius, at this localized area exceeds the yield strength of the material, brittle failure occurs.[1] The magnitude of this stress concentration is dependent on notch geometry and modelling stress concentration at such notch tips is therefore valuable towards the meaningful prediction of fatigue life in diverse components.

Crack repair

Drilling a circular hole at the tip of a propagating crack can be used as a method for reducing crack tip stress concentration and preventing further propagation.[1] This is particularly relevant for components subjected to cyclic loading conditions.[4] By drilling a hole, the crack effectively becomes a round tip notch [1] with fatigue life thus considerably improved .

References

  1. 1 2 3 4 Liu, M.; et al. (2015). "An improved semi-analytical solution for stress at round-tip notches" (PDF). Engineering Fracture Mechanics. 149: 134–143.
  2. ↑ "Charpy U-notch impact testing". Retrieved 15 September 2017.
  3. ↑ "Study on stress fields near V-notch tip with end-hole under in-plane and out-of-plane conditions".
  4. ↑ "Fatigue Life Extension by Crack Repair Using Stop-hole Technique under Pure Mode-I and Pure mode-II Loading Conditions".
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