Department of Materials Science and Engineering
Creep of Polymers
The aim of this laboratory is to demonstrate the phenomenon of creep in materials and its effect on the long term mechanical performance of materials.
YOU MUST READ ASTM STANDARD D2990-01 BEFORE THE LAB
Most of the materials experience time dependent permanent deformation as a function of time at a constant stress and this phenomenon is called creep. For example, metals and ceramics exhibit creep at high temperatures (>0.5 Tm, where Tm is melting temperature), whereas polymers exhibit creep even at room temperature. Some soft and ductile metals such as lead undergo creep even at room temperature. As a result of creep, the mechanical properties of most of the materials deteriorate over a period of time. Understanding and quantifying creep is important from the point of designing materials for long term use. For example, a steam turbine or a gas turbine blade has to be characterized before it is put into service as most of this kind of components are subjected to high stresses and temperatures on the order of thousands of hours (~10,000 or 100, 000 hours).
In this laboratory, we are going to study the creep of a high density polyethylene using a simple creep apparatus (see fig.1). The sample is loaded into the right most portion of the lever (for details see Fig.1) and below the dial indicator. Note down the inner indicator (small needle) reading and adjust the outer indicator in such a way that it reads zero at the beginning. Measure the length of the loading points on the lever.
Dog bone shaped tensile samples will be used for testing. Four samples will be provided per group as each one will be tested at different load.
Measure the gage length and thickness and width of the gage section.
Insert the specimen in to the apparatus. Apply the load in less than 5 seconds and note down...
References: 1. George E. Dieter, Mechanical metallurgy, McGraw-Hill, 2nd edition, 1976.
2. Lawrence E. Nielsen and Robert F. Landel, Mechanical properties of polymers and composites, 2nd edition, Marcel Dekker, Inc., 1994.
3. ASTM standard D2990-01, Standard test method for Tensile, Compressive, and Flexural creep and creep-rupture of Plastics.
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