Objective: To study Stress relaxation behavior of elastomer.Apparatus: (a) Universal testing machine(b)Polymer sample(c) Vernier CalliperTheory:Loaded materials that are subjected to constant strain sometimes realize a decrease in stress as a function of time, this is referred to as Stress Relaxation. The cause of Stress Relaxation is that viscous flow in the polymeric materials internal structure occurs by the polymer chains slowly sliding by each other, by the breaking and reforming of secondary bonds between the chains, and by mechanical untangling and recoiling of the chains. The amount of Stress Relaxation that occurs within a material is highly dependent on Temperature. = 0exp (-t/)where = stress after time t (near study state), o = peak stress and = relaxation time constant. The rate at which stress relaxation occurs depends on the Relaxation Time Constant () which is a material property and is defined as the time needed for the instantaneous stress () to decrease to 0.37 (1/e) of the initial stress . Stress relaxation describes how polymers relieve stress under constant strain. Because they are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion. This nonlinearity is described by both stress relaxation and a phenomenon known as creep, which describes how polymers strain under constant stress. Fig.1Polymer sample Fig.2 UTMProcedure: (1)Measure the dimensions of the specimen provided (Thickness, Length and Width) (2)Set the elastomer sample in the UTM (3)Set the appropriate loading conditions (4)Run the test. (5)Plot the curves stress vs. time and strain vs time

Observations:

Objective: To study Creep properties of elastomers.Apparatus: (a) Universal testing machine(b)Polymer sample(c) Vernier CalliperTheory:The rate of deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode.It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and generally increases as they near their melting point.

DimensionsStress relaxationCreep

Length(mm)4040

Width(mm)6.146.01

Thickness(mm)3.103.28

Procedure:(1)Measure the dimensions of the specimen provided (Thickness, Length and Width) (2)Set the elastomer sample in the UTM (3)Set the appropriate loading conditions (4)Run the test. Observations:

Objective: To study mechanical hysteresis of elastomers.Procedure:(1)Measure the dimensions of the specimen provided (Thickness, Length and Width) (2)Set the elastomer sample in the UTM (3)Set the appropriate loading conditions (4)Run the test.

In stress-strain coordinates, the geometrical locus of operational point becomes a closed loop, known as hysteresis loop. Since materials absorb elastic energy under cyclic stress, the unloading curve is always situated underneath the loading curve, the hysteresis loop configuration depending on the inelasticity type. The area of the surface enclosed within the hysteresis loop always equals the amount of energy dissipated in the material upon one loading-unloading cycle. This loop evolves with the number of stress cycles and may offer useful information upon the material state. Theoretical and experimental investigation of this dependence is the main task of this paper. Hysteresis variation is clearly highlighted upon modification of test parameters for various load cycles. This shows the fact that time variation of material load causes increase in fatigue, decrease in mechanical strength and, implicitly, modification of hysteresis loop. Perfect elastic materials possess an ideal linear stress-strain characteristic. A cyclic stress generates a strain in these materials which is cyclically variable and in phase with the stress. Inelasticity is always present although very fine measurements are required to detect it. As a result of inelasticity, a phase shift occurs between strain and stress. In stress-strain coordinates, the geometrical locus of operational point becomes a closed loop, known as hysteresis loop. Since materials absorb elastic energy under cyclic stress, the unloading curve is always situated underneath the loading curve, the hysteresis loop configuration depending on the inelasticity type.

Number of cycleHysteresis loss

10.370

20.270

30.263

40.260

50.258

Conclusions: 1. The value of stress is found to be decreasing with time at constant strain value. 2. The value of strain is found to be increasing with time in creep test ( constant stress) 3. The consecutive mechanical loading cycles are found to have lesser area under curve than the previous cycles, which implies that the energy absorbed is decreasing per cycle.