Page 357 - Introduction to Marine Engineering

Engineering materials 329

Figure 16.S Impact test

bar with a vee-notch cut in the centre of one face. The specimen is
mounted horizontally with the notch axis vertical (Figure 16,3). The test
involves the specimen being struck opposite the notch and fractured. A
striker or hammer on the end of a swinging pendulum provides the blow
which breaks the specimen. The energy absorbed by the material in
fracturing is measured by the machine.

The hardness test measures a material's resistance to indentation. A
hardened steel ball or a diamond point is pressed onto the material
surface for a given time with a given load. The hardness number is a
function of the load and the area of the indentation. The value may be
given as a Brinell number or Vickers Pyramid number, depending upon
the machine used.

Creep test

Creep is the slow plastic deformation of a material under a constant
stress. The test uses a specimen similar to that for a tensile test. A
constant load is applied and the temperature is maintained constant.
Accurate measurements of the increase in length are taken often over
very long periods. The test is repeated for various loads and the material
tested at what will be its temperature in service. Creep rate and limiting
stress values can thus be found.

Fatigue test

Fatigue failure results from a repeatedly applied fluctuating stress which
may be a lower value than the tensile strength of the material. A specially
shaped specimen is gripped at one end and rotated by a fast revolving
electric motor. The free end has a load suspended from it and a ball race
is fitted to prevent the load from turning. The specimen, as it turns, is
therefore subjected to an alternating tensile and compressive stress. The