I'll take a stab at trying to explain the difference between glass and metals, and also try to keep it as brief as possible (although brief is a relative term.) In order to keep it brief, it cannot be comprehensive. So please, no berry punches for leaving out special cases or second/third order effects. For those of you who get bored reading this, feel free to move on to the next post. There will not be a test at the end
To understand what fatigue is, we must understand the difference between ductile and brittle behavior. In general, metals are ductile, and glass and ceramics are brittle (I'll leave polymers and composites out of the discussion.)
All materials deform when stressed. As long as the deformation is limited to the stretching/compressing of the microstructure, the deformations are reversible, and will disappear when the stress is removed. Fully reversible deformation is known as "elastic" deformation. Elastic deformation does not damage the material in any way.
When stress levels get high enough, the limits of microstructure stretching/compressing are exceeded. For brittle materials, once this stretching/compressing limit is exceeded, the atomic bonds rupture, and the material fails catastrophically (ie it shatters.) For ductile materials on the other hand, the microstructure can undergo additional deformation by altering the microstructure. These deformations are usually irreversible, and are known as "plastic" deformation. Plastic deformation represents damage to the microstructure of the material, and generally results in a weakening of the material. Over time and multiple stress cycles, the plastic damage will build up in the material, continuing to weaken the material. This build up of plastic damage and weakening is what we call "fatigue." Fatigued materials will fail at lower stresses than undamaged materials, and these lower stress failures are called "fatigue fails." If enough plastic deformation stress cycles are accumulated, the material can fail without additional stress.
In some cases the altered microstructure leads to a stronger/harder material. This is known as work hardening. This work hardened material is usually less ductile than the original material and more prone to cracking when the (higher) elastic stress limit is reached.
Now to the punch line: Since brittle materials fail before undergoing any plastic deformation, they don't build up plastic damage in the microstructure. Therefore they cannot "fatigue" in the classical sense.
Now some more on brittle failure:
Brittle materials normally fail at macroscopic stresses much lower than what is required to rupture the atomic bonds. IE their strength is much lower than would be predicted based on the strength of the bonds between the constituents. Why is this? The answer is cracks, most often microscopic cracks.
Stress is load divided by the cross sectional area supporting the load. In the presence of a crack, the cross sectional area is reduced, but the load remains constant, thus the stress is increased wherever there are cracks. And, this stress increase is not uniform. The stress is concentrated at the tip of the crack. The amount of stress concentration is determined by the length of the crack and the radius of the crack tip. Longer cracks and smaller radii lead to higher stress concentrations. The atomic bonds start to rupture when the stress at a crack tip exceeds the bond strength. Once the material starts to rupture, the crack gets longer, which causes the stress at the crack tip to increase even more. The result is sudden and catastrophic fail.
In practice, all brittle materials contain microscopic cracks which determine the engineering (usable) strength of the material. These are taken into account when designing objects made from brittle materials. The objects will continue to function as designed unless a much larger crack develops, and the stress concentration at the tip of that crack exceeds the working stress.
Now, there is a phenomenon known as "stress corrosion" that can affect glass and other brittle materials. Stress corrosion is often referred to as "static fatigue," but that is just lazy and confusing terminology. (The term "static fatigue" was coined before the mechanism of failure under static load was understood.) Stress corrosion is a different mechanism than cyclic stress fatigue or creep fatigue (which hasn't been discussed.)
Stress corrosion is the result of corrosion that is accelerated by the stress concentration at the tip of microcracks in brittle materials. The corrosion at the crack tip allows the crack to grow in length over time. If the crack ever grows to a length where the stress concentration at the tip exceeds the atomic bond strength, then rapid catastrophic failure can occur. Rates of stress corrosion are affected by availability of moisture at the crack tip and pH of the crack tip environment, as well as the particular material.
In theory, beverage bottles can be subject to stress corrosion. However, if the mechanism were operative in practice, we should see older bottles of carbonated beverages exploding on a regular basis, especially bottles with higher internal pressures. Since exploding bottles are rarely observed in the absence of massive overpressure. It is safe to assume that stress corrosion is not a significant factor in the life expectancy of beverage bottles. I did do a limited amount of searching for reports of stress corrosion bottle failures, but was unable to locate any.
As to observable stretching of beer bottles, I really doubt that you have ever seen this. Soda lime glass has a typical Young's modulus of 72 GPa or 10.4 million psi. This means that for every 1000 psi tensile stress in the glass, the glass would stretch ~1/10,000 th of an inch along each 1 inch of length (or 0.01%.) I know I could never detect this without special instruments.
Single use bottles are designed to be cheaper and lighter than multiuse bottles, so they are manufactured with significantly thinner glass. Because of this they are less able to withstand the rigors of being handled thru recycling and refilling. They are by design, crappier bottles.
Hope the above helps, and sorry it's so long. Feel free to ask questions.
Brew on
