Creep & fatigue

A testing method of plastic product.
creep & fatigue properties.
long term properties.

1. Creep & Fatigue

2. Introduction Creep & Fatigue are long term mechanical properties. Long-term mechanical properties are used to determine the performance characteristics of a material subjected to mechanical force & deformation under different modes for longer time periods or duration. The deterioration of mechanical properties is measured under stress or strain with respect to time.

3. Creep When a plastic material is subjected to a constant load, it deforms quickly to strain, roughly predicted by its stress-strain modulus and then continues to deform slowly with time indefinitely or until rupture or yielding causes failure . This phenomenon of deformation under load with time is called creep.

4. Significance Plastics are being increasingly used in many high performance applications under constant loading for long duration. Therefore the information and data for long term loading and varying temperature, such as creep, is very important for design of various components likely to be under constant load for long duration. Creep will influence the dimensional stability of components and the data generated by the test can be useful for material characterization, their selection, designing and ascertaining the failure condition.

5. Generalized creep curve Total creep curve is divided in four continuous stages First stage (OP): It represents instantaneous elastic deformation, immediately after load is applied. This strain is the sum of elastic and plastic strain. Second stage (PQ): In this stage strain occurs rapidly but at a decreasing rate. At this stage creep rate decreases with time. It is referred to as primary creep . Third stage (QR): The straight portion of curve QR is characterized by a constant rate creep. This process is called as cold flow. Fourth stage (RS): In this stage creep rate increases until fracture occurs.

6. Test method &Test Specimen and conditioning ASTM D 2990: Standard Test Methods for Tensile, Compressive, Flexural Creep of Plastics. Molded or machined specimens, as used in tensile test (ASTM D 638) or flexural (ASTM D 790), are taken depending upon the type of creep test selected. Specimens are conditioned at a standard atmosphere of 23 ± 2 C and 50 ± 5% RH for specified duration prior to testing.

7. Procedure Tensile creep Specimen is fixed in tensile creep test assembly and strain gauge (or dial gauge) is mounted to measure the extension with respect to time. Test temperature of chamber is maintained and required test load is applied on to the specimen. Increase in length between two gauge marks is monitored and recovered at specified time interval. The percentage creep strain is obtained by the formula % creep strain = Extension(mm) x100 Gauge length(mm)

8. Tensile creep testing

9. Flexural creep Flexural creep measurement is done by applying a constant load to a standard flexural test specimen and measuring it’s deflection as a function of time. Test specimen is placed on three point flexural mode as shown in Fig. Test load to be applied is calculated by the formula, at given stress, P= 2Sbd 2 3L Where, P = Load S = Test stress at which test to be carried out b = width of specimen d = thickness of specimen L= span length

10. Flexural Creep Testing

11. Fatigue Resistance The failure or decay of mechanical properties after repeated application of stress or strain is known as fatigue. In actual application, a number of components come across repeated cyclic load, for example, flexing of shoe sole, gear teeth, inflating and deflating of balloon. Fatigue life is defined as the number of cycles of deformation required to bring about failure to the test specimen under given set of oscillation condition.

12. Significance Fatigue test data is applied in designing of specific parts. Plastic materials are largely being used for designing and manufacturing of plastic components in various applications where cyclic loading conditions are involved such as plastic gear, shoe sole, pulleys etc.

13. Equipment Flexural fatigue : (ASTM D 671): The feature of fatigue machine is an unbalanced, variable centric mounted on a shaft rotating at constant speed by motor. Machine is capable of producing constant amplitude of force on test specimen. It is fitted with RPM counter and it also consist of cantilever such that specimen can be fixed in appropriate position.

14. Tensile fatigue : Different types of machines can be used for determining tensile fatigue. Test specimen is held in a cantilever in the vice at one end so that on rotating of equipment, shaft specimen is bend a concentrated load through Yoke fastened at the opposite end. The number of test cycles required to produce failure is determined. Test is repeated by varying the stress level. A curve of stress versus number of cycle (S-N curve) is plotted from the test result. A typical S.N. Curve is shown in Fig

15. Factors Affecting Fatigue test results depend upon the test specimen preparation method as well as specimen thickness. Specimen thickness greatly influences the fatigue strength. Fatigue test is affected by change in test temperature, test frequency and rate of heat transfer. The fatigue life of polymer is generally reduced by increase of the temperature . Amount of stress and number of test cycle greatly affect the test result. The higher the amount of stress, the lower will be the number of cycles to failure