Antioxidants stabilize the polymer by terminating the chain reaction due to the absorption of UV light from sunlight. The chain reaction initiated by photo-oxidation leads to cessation of crosslinking of the polymers and degradation the property of polymers. Antioxidants are used for protect from thermal degradation.

UV radiation causes photooxidative degradation which results in breaking of the polymer chains, produces radicals and reduces the molecular weight, causing deterioration of mechanical properties and leading to useless materials, after an unpredictable time.

Oxidative degradation involves the disintegration of macromolecules by the action of oxygen on the substrate (oxidation). Free radicals are formed which react with oxygen-producing oxy- and peroxy-radicals. It takes place as a result of the simultaneous interaction of oxygen with the polymer and high temperature.

What is rubber deterioration? Most elastomers undergo rubber degradation over time and the most common rubber deterioration causes are exposure to light, oxygen (ozone) and heat.

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.

Metals and engineering alloys have high fracture toughness values due to their high resistance to cracks. Engineering polymers are also less tough when it comes to resisting cracking, yet engineering composites of ceramics and polymers show an enhancement in fracture toughness than both components.

Polymers tend to have low elastic moduli. Why do most polymers have intermediate toughness but low fracture toughness? They become more brittle.

Question: Question 9 (5 Points) [Chapter 14-15] What Is Most Likely The Final Thing To Happen As Polymers Fail Under An Applied Stress? Tilting Of The Crystalline Regions.

[…] a brittle material will fail when the maximum tensile stress, σ1, in the material reaches a value that is equal to the ultimate normal stress the material can sustain […] So, under pure shear it fails in tension at a 45° angle.

Aluminum does not have an endurance limit such as ferrous metals. Aluminum will fail if it goes though enough stress cycles. If the stress on the steel is above the endurance limit, the metal will fail and how quickly it fails depends on the magnitude of the stress. I.e. higher the stress the more quickly it fails.

Some metals such as ferrous alloys and titanium alloys have a distinct limit, whereas others such as aluminium and copper, do not and will eventually fail even from small stress amplitudes.

The relation of endurance limit with ultimate tensile strength is an important guide in such design. The endurance limit needs be corrected for a number of factors such as size, load, surface finish, temperature and reliability. The methods for finding these factors have been discussed and demonstrated in an example.

Measurement of fatigue strength The S−N diagram is the most widely used concept when calculating fatigue strength. It is a graph that plots the constant cyclic stress of amplitude S applied to a material specimen against the number of loading cycles N the specimen can withstand before eventually failing.

Fatigue is defined as a process of progressive localized plastic deformation occurring in a material subjected to cyclic stresses and strains at high stress concentration locations that may culminate in cracks or complete fracture after a sufficient number of fluctuations.

Fatigue strength is the ability of a material to resist fatigue failure. ASTM defines it as the limiting value of stress (denoted by SNf) at which failure occurs after Nf number of load cycles. This number of cycles can be from a few cycles up to a large number depending upon the load and material.

Fatigue strength is of importance whenever a high number of deflections occur over the life of the component. Contact springs (or other stressed components) can fail during service when the metal is deflected and released a high number of times-even though the metal is not highly stressed.