What does the stress vs strain curve for rubber look like?
Stress-Strain Curve for RubberWhen you stretch a rubber cord to a few times its natural length, it returns to its original length after removal of the forces. That is, the elastic region is large and there is no well defined plastic flow region. Substances having large strain are called elastomers. This property arises from their molecular arrangements. The stress-strain curve for rubber is distinctly different from that of a metallic wire.Firstly, there is no region of proportionality. Secondly, when the deforming force is gradually reduced, the original curve is not retraced, although the sample finally acquires its natural length. The work done by the material in returning to its original shape is less than the work done by the deforming force. This difference of energy is absorbed by the material and appears as heat. This phenomenon is called elastic hysteresis.Source : Google
What are ductile and brittle materials? Explain with stress-strain curve.
Ductile materials are those which can be drawn into wires when tension is applied ,And can be drawn into sheet when compressive load is appliedFor the above mentioned properties the material should have a tendency to elongatei.e for lower value of stress ,the strain produce is much higherSo from point B to D the region is called elastic region .As material have to be elastic to possess the property of ductilityAnd so called ductile materialsBrittle materials are those which can't elongateas much the ductile materials because they don't have such a long region b/w point B and D and some material even don't have point D they just have fracture point after point BOrthey have higher value of stress for a little value of strain ,Here point 4 is Point D and 3 is Point BSo the distance between them is smaller then 1st fig therefore they are brittle material as they can't elongate as much
What is the difference between yield point and elastic limit?
Elastic Limit and Yield Point may convey the same state of material that is but the latter is used for an engineering convenience. Elastic limit is a value of stress upto which material can be deformed elastically under load, after unloading it will return to it's original dimension. Beyond elastic limit, material will start deform plastically which is characterized with permanent deformation. It is marked by the breakage of bonds. Technically, it is a stress at which first of such bond breaks. Limitations of measuring such delicate phenomena has made engineers to define another point at which permanent deformation can be measured which is known as Yield Point (Offset).The yield strength is defined as the stress which willproduce a small amount of permanent deformation, generally equal to a strain of 0.002 or 0.2%.This is an Engineering stress-strain curve for a ductile material.Source : Mechanical metallurgy by G.E. Dieter.So, for your question, in the region between the elastic limit and the yield point, material just deforms plastically, although on an extremely small scale.P.S. Source that I have mentioned also contains explanation about stress-strain curve from the perspective of molecular level. It is quite an interesting read.
How do I determine the elastic and plastic strain values?
Elastic deformation: This type of deformation is reversible. Once the forces are no longer applied, the object returns to its original shape. Elastomers and shape memory metals such as Nitinol exhibit large elastic deformation ranges. Soft thermoplastics and conventional metals have moderate elastic deformation ranges, while ceramics, crystals, and hard thermosetting plastics undergo almost no elastic deformation. Linear elastic deformation is governed by Hooke's law, which states: σ = E x ε , Where σ is the applied stress, E is a material constant called Young's modulus, and ε is the resulting strain. This relationship only applies in the elastic range and indicates that the slope of the stress vs. strain curve can be used to find Young's modulus. Engineers often use this calculation in tensile tests. The elastic range ends when the material reaches its yield strength. At this point plastic deformation begins.
Is it possible for a polymer to have the tensile strength and break point at the same position in a stress-strain test?
Yes its the case with elastomers/rubbers.The stress at which the elastomers/rubber dumb bell specimen breaks (Elongation at break) is generally measured as Tensile strength.Stress at break - Tensile strength.Strain at break - Elongation at break.These are the two of the important properties obtained from stress-strain graph for elastomers.