Creep of Polymeric Materials
Unlike metals and concrete which exhibit creep over decades, plastics
(polymers) can exhibit significant creep (time-dependent deformation under
constant load) even at room temperature over short periods. This severely
limits their use as primary structural members.
Example
A High-Density Polyethylene (HDPE) pipe is suspended horizontally and used to carry water across a small gap. The pipe acts as a simply supported beam carrying a constant uniform dead load (). The initial elastic deflection immediately after installation is mm.
The manufacturer specifies that for this specific HDPE compound at , the creep compliance function can be modeled by a creep coefficient such that the total deflection at time is .
Given , where is time in years under load, calculate the total deflection of the pipe after 5 years and after 20 years.
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Glass Brittleness and Safety Factor
Glass is a perfectly brittle material. It does not yield before failure. Its
strength is highly dependent on microscopic surface flaws (Griffith cracks),
making its actual breaking stress highly variable. Therefore, glass design
relies on probabilistic failure models rather than simple deterministic yield
limits.
Example
A glass canopy is being designed. The typical nominal tensile strength of annealed glass is MPa. However, due to the critical overhead nature of the canopy, the local building code requires that the probability of failure must not exceed 8 in 1000 ().
The manufacturer provides a Weibull statistical strength distribution for their glass panels, where the probability of failure is given by:
Where the characteristic strength MPa and the Weibull modulus (a measure of flaw consistency; higher means less variation).
Determine the maximum allowable design stress () for the canopy to meet the safety requirement. Compare this allowable stress to the nominal strength to find the effective safety factor.
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