Plastics History and Applications

8/14/2019 Plastics History and Applications

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Strain

The measurement of how much the part bends or changes size under load compared to theoriginal dimension or shape is called strain. Strain applies to small changes in size; VSR hasthe ability to redistribute surface strains by up to 100% within a few minutes of

treatment.

Strain = (Final Length Original Length) / Original Length= Change in length or Deformation / Original length

If the change in size is in millimeters and the original dimension was in millimeters, then theunits for strain are mm per mm.Stress, Strain and Modulus are related to each other by the following equation. The modulus or stiffness of a material can be determined when the material is loaded in different ways, such astension, compression, shear, flexural (bending) or torsion (twisting). They will be called TensileModulus, also known as plain Modulus, Flexural Modulus, Torsional Modulus, etc.

Modulus = Stress / StrainOr

Modulus = Load / change in shape when loaded. (Stiffness).

The stress / strain equation is the equation used by designers to predict how a part will distort or change shape when loaded. Predicting the stress and strain within an actual part can becomevery complex . Fortunately, reducing the stress and redistributing the strains can be veryeasy with the correct application of VSR.

Some additional terms used to describe material behavior.

Yield Point

The yield point is that point when a material subjected to a load, tensile, compressive, etc. gives(yields) and will no longer return to its original length or shape when the load is removed.Some materials fail before reaching yield for example certain cast irons and die castaluminums.

Tensile Strength

The maximum strength of a material without breaking when the load is trying to pull it apart.This is the system often used by material suppliers to list tensile properties in their salesliterature. Unlike thermal stress relieving VSR will not reduce the tensile strength of certain materials

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Elongation is always associated with tensile strength because it is the increase in the originallength at fracture and is expressed as a percentage. For an example the HDPE pipe as tested inthe previous tests has an elongation factor of >600% whilst at yield it is only 16%. It willtherefore undergo considerable deformation before fracture takes place.

Compressive Strength

The maximum strength of a material without breaking when the material is loaded. This term becomes less meaningful with some of the softer materials. Teflon, for example, does notfracture.Compressive strength would be the maximum force required to deform a material prior toreaching the yield point.

Flexural Strength

The strength of a material when a beam of the material is subjected to bending. The material inthe top of the beam is in compression, while the bottom of the beam is in tension. Somewherein between the stretching and squeezing there is a place with no stress and it is called theneutral plane. A simple beam supported at each end and loaded in the middle is used todetermine the flexural modulus given in properties tables . Stress relieving normally increasesthe flexural strength of materials

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Conclusions

This paper has been a brief attempt at explaining the history and many applications of

plastics in todays environment. Similar problems as are encountered with the design andmanufacture of steel components are often encountered using plastics, the most common of which appears to be dimensional stability and creep rupture. Both are commonly associatedwith high stress concentrations within the material.

As the creep ratios are affected by time and temperature with plastics this is an important point to bear in mind as with pipelines containing either water or gas as they are under aconstant stress (internal pressure). It therefore stands to reason that careful considerationmust be given to material selection and the possible stress relieving of pipes.

As described within the paper many of these problems can be overcome by using careful

material selection, good design and good engineering practice during manufacture and. Itmust be born in mind that no form of stress relieving is a replacement for poor design and

bad engineering practice during manufacture.

As the tests carried out have clearly demonstrated stress levels in plastics can be reduced bythe application of VSR, and material stability is restored.

An interesting aspect of the tests, and it is proven by the attached stress / strain / creepformulas is that plastics appear to suffer from the affect of material ageing.

It would however be recommended to carry out more detailed tests in a more controlledenvironment before recommending VSR as a method of stress relieving.

Although VSR has a sixty-year proven track record on conventional materials, itsapplications on plastics and similar materials are yet to be exploited. Owing to the hugesavings in component downtime, VSR offers considerable advantages if proven successfulin this unique field.

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Plastics History and Applications