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necking phenomena or internal cracks, cavities or voids decrease the effective
area of the specimen. These all lead to a decrease in the effective cross-
sectional area and an increase in strain rate. Strength is quickly lost in this stage
while the material’s shape is permanently changed. The acceleration of creep
deformation in the tertiary stage eventually leads to failure, which is frequently
termed rupture.

Figure 2.3: Stages in creep failure

d. Impact strength
The impact strength of a material is defined as the ability to resist a sudden applied
load or force. It is normally state as the amount of mechanical energy absorbed in the
process of deformation under the applied impact loading and is known as energy lost
per unit of thickness. Impact strength is also used to determine whether a material
will act in a brittle or ductile manner when subjected to sudden applied loads.

A material's impact strength or toughness may be measured through any of the
following tests:

Charpy impact test
The Charpy impact test is a high strain-rate test that involves striking a standard
notched specimen placed in horizontal position with a controlled weight pendulum
swung from a set height. The impact test helps measure the amount of energy
absorbed by the specimen during fracture.

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