1.0 Introduction
Creep is defined as time — dependent deformation of a material under a constant stress or load. At temperature less than 40 percent of the absolute melting point, the extent of creep if negligible, but at temperatures higher than this is commonly thought of as a high temperature test.
The creep material measuring device is a simple unit designed for demonstrating and investigating the creep characteristics of lead polypropylene specimen at room temperature. A temperature module is provided to enable the investigation/effects of the temperature as a creep rate.
A failure may also be due to a phenomenon called creep, which is the plastic flow of a material under load at elevated temperatures in addition, the actual shape of a part may be responsible for failure for example stress concentrations due to sudden changes in contour must be taken into account. This is especially true when dynamic loads with direction reversals exist and the material is not very ductile.
The single specimen creep measuring apparatus/machine is a table top apparatus measuring dimensionally 740 x 470 x 310 mm and approximated weight of 14.5kg. This simple machine uses specimens of lead and various types of plastics which creep significantly at temperature and under loads.
The design is based on a simple lever principle having a mechanical advantage of 8:1 which provides a steady and uniform tensile load. The extension of the specimen under load is measured by a dial guage.
It is made up of vertical column which is the main supporting frame, a lever beam, a stop watch, dial guage, a set of weigh hanger, connecting strip, Aluminum block, Ammeter, circuit breaker, volt meter, temperature control, thermocouple, Heating element and a panel made of mild steel shut to house the above mentioned components. To ensure correct loading of the specimen, the load beam is filted with harden connecting strip which load is applied by means of a set of weight hanger as required. This apparatus is especially designed for teaching.
Generally for satisfactory use of this equipment, a bunch of approximately 1 x 0.6m is recommended.
A creep curve is a plot of the elongation of a tensile specimen versus time, at a given temperature, under a constant load. The four stages of creep curve is
(1) Initial elongation flowing the application at load,
(2) transient or primary creep,
(3) steady state or secondary creep, and
(4) tertiary creep.
Below is the creep curve which shows the four stages.
The Primary Creep
Fig 1.1: This is the stage of the curve over which the creep rate, is de/dt, is continuously decreasing as show in the figure above. This is of interest to a designer since it forms part of the extension reached in a given time and may affect clearance.
The Secondary Creep
Fig 1.2: This section is a period of approximately constant creep rate and is termed secondary creep; the creep rate is at a minimum over this region.
This secondary stage is the region at which creep occurs at more or less “STEADY STATE”. This is the important part of the curve for most applications.
The Tertiary Creep
Fig 1.3: This stage represents a continuously increasing creep rate and terminates in fracture of the specimen as shown in the figure above. This tends to be the final stage of the creep curve when the rate of extension accelerates and finally leads to fracture.
…
