How do we find ultimate properties?

Transcription

1 Introduction Why ultimate properties? For successful product design a knowledge of the behavior of the polymer is important Variation in properties over the entire range of operating conditions should be known

2 How do we find ultimate properties? Failure tests are the answer Various tests are carried out until the material fails Failure tests carried out with some degree of simulation between test and end use

3 Properties Rigidity Ultimate strength Toughness Creep (long term deformation resistance) Resistance to thermal degradation

4 Ultimate Strength This is the stress at failure

5 Ultimate Strength It is never the limiting factor for any application polymer never subjected to a single steady deformation in absence of aggressive environment failure generally takes place due to repeated stresses, impact, penetration by sharp objects or propagation of tear

6 Toughness It is the energy absorbed before failure It is the area under the stress strain curve Energy may be stored elastically or maybe dissipated as heat (as in permanent deformation of a crystalline material) Units J/m 3. Thus it is the energy required to break unit volume of material Tough materials absorb a lot of energy when fractured and brittle materials absorb very little energy

7 Breaking Energy total energy required to cause rupture. Is a measure of the toughness Impact tester used to measure the breaking energy a hammer used to apply load to a sample until the sample breaks. The energy used in breaking the sample is proportional to the difference in height before and after the break

8

9 Izod Test similar to impact tester except that it uses a cantilever beam Impact energy = energy absorbed = mass of pendulum * g * (h( 1 h 0 ) where g = accelaration due to gravity

10 Charpy Test simple beam is used

11 Charpy Machine

12 Charpy Chart

13 Brittleness Temperature Test Pendulum Arm used to break rubber & soft plastics Temperature progressively lowered until the sample fractures

14 Comparison of Toughness

15 Dumbbell Tests for strength

16

17 Advantage Failure will take place at the centre and will not be affected by the stress concentration at the jaws Disadvantage Accurate measurement of strain difficult

18 Ring Specimen Used for rubbery samples Rate of strain is uniform thus stress time record is also stress strain record Disadvantage stress concentration at holder is difficult to avoid even when holder is lubricated or rotated Stress needed to deform the ring into an oval masks the stress due to deformation initial portion of the stress strain curve maybe distorted

19

20 Trouser Tear test

21 Trouser Tear (ASTM D 1938) The Trouser Tear test measures primarily crack propagation. Force is applied to the specimen in the same direction as the separating jaws of the test equipment. The Trouser Tear test gives a better measure of tear strength because the sample legs are much wider helping to minimize sample elongation during testing. If the sample breaks across one of the legs of the specimen, rather than between the legs, the test is no longer measuring the resistance to tear propagation.

22 Advantage Stress to propagate tear stays at constant value. This makes it easy to estimate the tear propagated than some others in which the tear propagates so rapidly that only a peak stress can be read.

23 Creep Failure Creep is development of additional strains in a material over time Creep is most prevalent under high stresses and temperatures, and is not necessarily a failure mode Creep test gives information on long term dimensional stability of a load bearing element When combined with temperature the test measures deflection temperature

24 Test: Material is subjected to prolonged constant tension or compression loading at constant elevated temperature. Deformation is recorded at specified time intervals and a creep vs. time diagram is plotted. Slope of curve at any point is creep rate.. If failure occurs, it terminates the test and the time for rupture is recorded. If specimen does not fracture within the test period, creep recovery may be measured.

25 Creep Curves

26 Fatigue Fatigue is a process by which a material is weakened by cyclic loading Fatigue testing gives much better data to predict the service life of materials Fatigue testing can be thought of as simply applying cyclic loading to the test specimen to understand how it will perform under similar conditions in actual use. The load application can either be a repeated application of a fixed load or simulation of the service loads. The load application may be repeated millions of times and up to several hundred times per second

27

28 Fatigue Strength

29 WLF Equation It gives the effect of temperature on viscosity log ηt 10 ηt g = 17.44( T ( T If a change in the material property with temperature arises with change in the viscosity then it is possible to apply the WLF equation e.g. Stress Relaxation The force required to maintain a fixed strain at a constant temperature will decay with time owing to decrease in viscosity of molecules. A measure of the stress relaxation is the relaxation time i.e. time taken by the material to relax to 1/e of its stress on application of strain T T g g ) )

30 WLF Equation (contd.) log θt 10 θt g = 17.44( T ( T T T g g ) ) WLF = Williams, Landel and Ferry

31 Fracture of Glassy Polymers Two mechanisms are involved Shear Bands caused due to shear deformation. These bands have very little void volume Crazes these are fine cracks at right angles to the applied stress. They are narrow zones of highly deformed polymer. A craze can contain 20-90% voids the rest being fibrils which are threadlike elements that make up the structure of fibers Unlike actual cracks, crazes and shear bands are capable of supporting stresses because of the oriented polymer involved

32 Critical Stress σ = π. c K IC = Plane strain fracture toughness Y = constant to accommodate the geometry c = crack length if σ f <σ c then fracture will not occur c Y K IC

33 Fracture toughness can be increased by various inclusions e.g. rigid fibers or particles These can spread the applied force over a larger zone Heterophase systems (partly crystalline and partly amorphous) are used when toughness is a major criterion

34 Fibers They exhibit viscoelastic behavior like amorphous polymers Viscoelasticity: A combination of viscous and elastic properties in a material with the relative contribution of each being dependent on time, temperature, stress and strain rate

35 Crystalline fibers have high melting temperature due to a polar structure e.g. rayon, nylon, polyesters, acrylics, cotton, wool and silk contain ester, amide or hydroxyl groups that can form hydrogen bonds Moisture and heat will have a large effect on the physical properties of fibers

36

37 Rubber Two types of rubber natural and styrene butadiene rubber Natural rubber crystallizes on stretching even though it is above its melting point and the crystallites melt on release of stress. It is self reinforcing i.e. it is stronger at highly stressed point Addition of fillers (e.g. carbon black) increases the tensile strength only slightly for self reinforcing polymers. But tear strength and abrasion resistance are very much improved

38 Rubber (contd.) For styrene butadiene rubber addition of fillers like carbon black increases the tensile strength more than for natural rubber

39 Composites Composite materials are combination of materials They are made by combining two or more materials in such a way that the resulting material has certain desired properties e.g. glass fiber reinforced plastics (GRP) Polymers when combined with glass fibers result in Polymer Matrix Composites (PMC)

40 Composites (contd.)

41 Composites (contd.)

42 Composites (contd.)