Fracture in polymers

Polymer fracture is the study of the fracture surface of an already failed material to determine the method of crack formation and extension in polymers both fiber reinforced and otherwise.^[1] Failure in polymer components can occur at relatively low stress levels, far below the tensile strength because of four major reasons: long term stress or creep rupture, cyclic stresses or fatigue, the presence of structural flaws and stress-cracking agents. Formations of submicroscopic cracks in polymers under load have been studied by x ray scattering techniques and the main regularities of crack formation under different loading conditions have been analyzed. The low strength of polymers compared to theoretically predicted values are mainly due to the many microscopic imperfections found in the material. These defects namely dislocations, crystalline boundaries, amorphous interlayers and block structure can all lead to the non-uniform distribution of mechanical stress.

Long term stress or Creep Failure

Taking into account the viscoelastic path at small strain based on thermally activated rate processes. When strain attains higher values, high enough to lead to failure, its slope versus time exhibits an abrubt change. At this specific time the creep function appears a minimum.^[2] In most cases DMTA (Dynamic mechanical thermal analysis) can be used to determine the viscoelastic behavior of samples as a function of time. A classic case is when the rubber hose ruptures due to creep after many years of service. DMTA can be used for o-rings and gaskets to measure the creep rates.

Fatigue Failure

The term fatigue refers to the effect of cyclic or intermittent loads. Cyclic loading due to either oscillating mechanical stress or to alternate heating and cooling, is more detrimental than static loading. Under cyclic load the crack are initialized as localized sites within the part and these extend in size during cycling. Ultimately they expand and join to such an extent that the material can no longer hold and support the stress. Fractures can be characterized by a series of concentric crack growth bands that grow from the surface initiation site. Cyclic loading can bring about failure in polymer due to: chain scission, built up heat due to hysteresis, recrystallization of material and cumulative crack generation.

Chain scission

Chain scission occurs in a polymer as a result of intense localized heat. the chemical bond in a polymer backbone may be broken with the generation of free radicles by heat, ionizing irradiation, mechanical stress and chemical reactions. These scissions multiple in number cause a fracture tip initialization to occur followed by its growth.^[3]

Built-up heat from hysteresis

Polymers are viscoelastic by nature, and exhibit mechanical hysteresis even at moderate strains due to continuous elongation and contraction. Some of this inelastic deformation energy is dissipated as heat within the polymer, and consequently the materials temperature will rise as a function of frequency, testing temperature, the stress cycle and the type of polymer. As the temperature within the polymer rises, the stiffness and yield strength will fall, and thermal failure becomes a possibility as deformation levels become excessive.

Recrystallization

This process can be caused as a consequence of extensive movement of chain segments like in case or work hardening of materials.

Fatigue in Nylon

When nylon component is subjected to conditions of tensile fatigue, failure occurs when a minimum strain is reached. this means that the lifetime of nylon material is dictated by the time under load and not on the number of cycles

Fatigue of Short-Fibre-Reinforced Plastics

Fatigue failure in these reinforced polymers is due to the formation of micro cracks that are easily initiated, and which coalesce into one crack, causing the final failure ^[4]

Impact Fracture

A good polymer is generally defined as one capable of absorbing a large amount of energy before failure. Poly carbonates have one of the highest impact resistance values. However, amorphous polymers exhibit brittle behaviour under impact, especially if the component is notched or is too thick relative to a corner radius. The occurrence of brittle failure can be decreased by: increasing the molecular weight, inclusion of rubber phase, inducing orientation in the polymer and reducing internal defects and contaminants.

Measuring impact strength

Conventional Izod tests are used to measure the energy required to break a notched specimen. however, this is not considered as a satisfactory test. Major limitation being that most polymers as notch sensitive and fail readily under izod test.

Blends

Blended materials can have an increased fracture toughness with balanced stiffness and strength. Usually these are formed from copolymerization or modification with a suitable elastomer. However, the mechanical properties of blends, especially the modulus, follow the ‘rule of mixture’ Voigt model and the morphologies show coarsed dispersion^[5]

References

1. John Scheirs, “john wiley and sons”, 30-oct-2000 “[Compositional and Failure Analysis of Polymers: A Practical Approach]”
2. G. Spathis, E. Kontou, “Creep failure time prediction of polymers and polymer composites”
3. Robert Oboigbaotor Ebewele,"CRC Press,2000" "polymer science and technology"
4. Mandell and Lang
5. Wolfgang Grellmann, Sabine Seidler, “Springer 2001” “Deformation and Fracture Behaviour of Polymers”