Creep description

Creep description - Long term performance of polymers 1.0...

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1 Long term performance of polymers 1.0 Introduction Polymer materials exhibit time dependent behavior. The stress and strain induced when a load is applied are a function of time. In the most general form can be thought of as a 3 dimensional surface. The stress-strain-time relationship, or constitutive law, can be determined by loading a polymer specimen with constant stress (creep) or constant strain (stress relaxation or isometric). We can also construct isochronous curve by cross section of the surface with constant time levels as shown in Figure 1. Figure 1. Constant stress-strain-time coordinates [1] 1.1 Creep, constant load When a plastic material is subjected to a constant load, it deforms continuously (Figure 2). The initial strain is roughly predicted by its stress-strain modulus. The material will continue to deform slowly with time indefinitely or until rupture or yielding causes failure. The primary region is the early stage of loading when the creep rate decreases rapidly with time. Then it reaches a steady state which is called the secondary creep stage followed by a rapid increase (tertiary stage) and fracture. This phenomenon of deformation under load with time is called creep. Of course, this is an idealized curve. Some materials do not have secondary stage, while tertiary creep only occurs at high stresses and for ductile materials. [1] All plastics creep to a certain extent. The degree of creep depends on several factors, such as type of plastic, magnitude of load, temperature and time. The standard test method for creep characterization is ASTM D2990. In this Log Time Stress Strain constant strain section isometric section constant time section isochronous section constant stress section creep curve
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2 test procedure, the dimensional changes that occur during time under a constant static load are measured. Figure 2. Creep curve for plastics, a constant load is applied [1] If the applied load is released before the creep rupture occurs, an immediate elastic recovery equal to the elastic deformation, followed by a period of slow recovery is observed (Figure 3). The material in most cases does not recover to the original shape and a permanent deformation remains. The magnitude of the permanent deformation depends on length of time, amount of stress applied, and temperature. Figure 3. Creep curve with recovery. A constant load is applied at t 0 and removed at t 1 The creep rupture is basically similar to a creep test with the exception that it is continued until the material fails. Since higher loads are used, creep rates are higher and the material fails in a shorter time (usually terminated in 1000h [1]). This test is useful in establishing a safe envelope inside which a creep test can be conducted. The basic information obtained from the stress rupture test is the time to failure at a given stress. Based on this data, a safe stress can be determined below which it is safe to operate,
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Creep description - Long term performance of polymers 1.0...

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