Fracture in polymers
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.
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 abrupt change.
[2] In most cases DMTA (Dynamic mechanical thermal analysis) can be used to determine the viscoelastic behavior of samples as a function of time.
Fractures can be characterized by a series of concentric crack growth bands that grow from the surface initiation site.
[3] Polymers are viscoelastic by nature, and exhibit mechanical hysteresis even at moderate strains due to continuous elongation and contraction.
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.
As industries make the shift to implementing polymeric materials, a greater understanding of failure mechanisms for these polymers is needed .
Microstructurally, metals contain grain boundaries, crystallographic planes and dislocations while polymers are made up of long molecular chains.
These secondary bonds (van der Waals) play an important role in the fracture deformation at crack tip.
Many materials, such as metals, use linear elastic fracture mechanics to predict behavior at the crack tip.
Elastic-plastic fracture mechanics relates to materials that show a time independent and nonlinear behavior or in other words plastically deform.
The initiation site for fracture in these materials can often occur at inorganic dust particles where the stress exceeds critical value.
Under standard linear elastic fracture mechanics, Griffiths law can be used to predict the amount of energy needed to create a new surface by balancing the amount of work needed to create new surfaces with the sample's stored elastic energy.
Yielding through crazing is found in glassy polymers where a tensile load is applied to a highly localized region.
High concentration of stress will lead to the formation of fibrils in which molecular chains form aligned sections.
[11][12] This is due to the fact that polymers have viscoelastic behavior and poor conductivity of heat, and they are more sensitive to their cyclic loading conditions than metal.
[13] However, mode III geometry has also been applied to test on twisted rubber disks for further understanding of its fracture behaviors.
In this mechanism, the crack-tip yielding is limited by the brittle material properties and each loading cycle breaks a specific amount of bonds allowing the crack front to advance.
In this mechanism, during loading and unloading, the polymer the stress-strain curve will act as a hysteresis loop as shown in Figure 2, creating energy on the material as discussed before.
The magnitude at which the crack front will advance is largely dependent on the amount/magnitude of cycles, glass transition temperature of the material and the thermal conductivity of the polymer.
A S-N curve represents the amount of cycles being applied along with the stress amplitude and can derived from the Goodman relationship.
In cases where polymers such as PVC follow the rules of linear elastic fracture mechanics, Paris law may be used to relate the fatigue crack propagation rate to the magnitude of stress intensity being applied.
The stable crack growth regime represents the linear region of the red curve which is described using the Power Law model where ‘A’ is a pre-exponential factor.
(Power Law Regime Equation) This process can be caused as a consequence of extensive movement of chain segments like in case or work hardening of materials.
When a Nylon component is subjected to conditions of tensile fatigue, failure occurs when a minimum strain is reached.
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.
However, the mechanical properties of blends, especially the modulus, follow the ‘rule of mixture’ Voigt model and the morphologies show coarsed dispersion.
