Fracture Toughness

Nigel,

Fracture toughness (or K1C as it is often referred to) is a parameter based on Linear Elastic Fracture Mechanics (LEFM), Which means that your sample must have a linear stress/strain curve up to the point of fracture. This being the case it means that only brittle polymers such as PS or PMMA in plain strain or those well below their Tg can be used. There cannot be any plastic deformation during failure. The concept and its measurement are exactly the same as for metals and ceramics.

A good text, although very mathematical is Chapter 7 of "Stress Analysis of Polymers" by J G Williams (1980)Published by Ellis Horwood in the U.K. and distributed by John Wiley & Sons in the USA and Canada.
ISBN 0-85312-169-9

Regards,

Peter P

Nigel,

Sorry that should have been plane strain.

See
Regards,

Peter P

Hi PrentP,

Thanks for the reply.

Nigel

Your question is quite vauge, but here goes

Fracture toughness will depend on quite a few variables, including how you induce the fracture.

Most plastics data shows energy at break by a pendulum swinging against a sample held in a vice. Most real world fractures occur by a very different mode, which is more realistically represented by dropped dart tests or elongation at break figures.

For the same plastic including the same grade, and real world failures, the biggest accross the board variable is notches in the design. Compared to metals, plastics are very notch sensitive. Various plastics have varying degree of notch sensitivity.

There are several other important variables that only apply to some types of plastic. These are:-
Degree of crystalinity.
Type of crystals.
Size of crystals.
Environmental stress cracking.
Fatigue.
Exposure to UV light.
Long term effects of exposure to heat and air.
Moisture conditioning.
Skin layer effects.
Moulded in stress.
Anealing.
Changes in section thickness.
molecular orientation vs load (dependant on flow direction while moulding)
Reinforcing fibre orientation (if present).

Factors dependant on grade of polymer used are:-
Molecular weight.
Plasticisers.
Impact modifiers.
Fillers and reinforcements.
Other additives such as flame retardants.
Colours.
Alloys.
Neucleating agents.
Lubricants.
Chemical type of polymer.
Degree and type of cross linke if any.
Molecular weight distribution, including pressence of very low molecular weight components

Factors depending on test conditions:-
Temperature at which tests done.
Thickness of sample.
Cut or moulded notch.
High speed low weight or low speed high weight impact.

That's all I can do now of the top of the head.

If you have more questions can you please be much more specific




Regards
pat

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Hi Patprimmer,

The question is specifically PVC pipe and the C-Ring test.

Have you had any experience or do you know of this test/


Thanks for your help

Nigel

Sorry Nigel

My experience is mostly confined to engineering plastics and injection moulding.

It has mainly evolved by finding remedies for in field failurs.

I don't know the C-ring test.

Mechanical properties of PVC pipe grades as I understand them are dependant a lot on the "K" value (dependant on molecular weight) and additive package. Extrusion grades used rfor the pipe generally have better properties than injection moulding grades used for fittings, but this is not my specific field.

Regards
pat

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nigel228,

Are you referring to BS 3505/3506? Pat gave you a good list of variables that would affect the test. If you want an estimate for the fracture toughness of rigid PVC, use a value around 5-6 MPa m[sup]0.5[/sup].

Regards,

Cory

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