Today's KNOWLEDGE Share:Plastic Fatigue

Today's KNOWLEDGE Share:

Plastic Fatigue

I recently completed a failure analysis where the component cracked through fatigue. Fatigue is one of the primary plastic component failure mechanisms, along with impact, creep, environmental stress cracking, and molecular degradation.

Fatigue failure is the formation and propagation of cracks due to intermittent, repetitive or cyclic loading. Most fatigue failures are associated with cyclic loads that are significantly below the yield strength of the plastic material. Fatigue is sometimes referred to as dynamic fatigue in order to distinguish it from creep, which is at times called static fatigue.

“The failure occurs due to the cyclic nature of the load which causes microscopic material imperfections (flaws) to grow into a macroscopic crack (initiation phase). The crack can then propagate to a critical size that results in structural or pressure boundary failure of the component.” George Antaki, Ramiz Gilada, in Nuclear Power Plant Safety and Mechanical Integrity, 2015

Fatigue cracks commonly initiate at stress concentrations, structural discontinuities, or flaws within the molded component. Fatigue cracks can also propagate from existing macroscopic cracks, initiated through another failure mechanism, such as impact or molecular degradation.

Fatigue results in the progressive weakening of a component with increasing time under dynamic loading conditions, such that loads to be supported satisfactorily for short durations produce failure after long durations. Essentially, the intermittent, repetitive or cyclic loading results in a decay in the strength of the component. The fatigue strength of a plastic material depends on the period of action of the applied force. The applied stress can be compressive or tensile. This can result in various situations where the maximum and minimum stress of the dynamic loading are:

·   Tension - Tension

·   Tension - Neutral

·   Tension - Compression

·   Compression - Neutral

·   Compression - Compression

This is illustrated in the graphic below from “Fatigue and Tribological Properties of Plastics and Elastomers” Second Edition by Laurence McKeen.

In general, the higher the mean applied stress, the faster that cracking will initiate and propagate.

Source:Jeffrey A.Jansen

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#polymers #plastic #polymerscience #materialsscience #fatigue #failure #cracking #failureanalysis #stress #force #tension #compression #testing #mechanical #degradation

Today's KNOWLEDGE Share: Beautiful FRP art sculpture brought to shape by BFG International!

Today's KNOWLEDGE Share:

Check out this beautiful FRP art sculpture brought to shape by BFG International!

The company is proud to have recently completed the stunning sculptural 'Athar' monument in Bahrain. The monument was commissioned by the Supreme Council For Women and was unveiled on 30th November. This striking sculpture celebrates the achievements of women in Bahrain society, and its beautiful form was expertly realized by BFG using 3-dimensional design engineering skills and advanced composite materials.

The monument comprises two independent structures’ one of which integrates a 12m cantilever. The entire sculpture is self-supporting, with no steel structure. Manufactured from custom engineered Glass Fiber Reinforced Polymer, the precision molded shapes perfectly represent the complex three-dimensional forms envisaged by the original design. The unique properties of this advanced material ensure the monument will retain its form and finish for decades to come even in the harshest environment and conditions.

The entire sculpture was erected in less than 7 days, a testament to expert design and to the skills of the BFG installation teams!

Source: #managingcomposites #thenativelab

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#composites #compositestructures #design #gfrp #engineering #3d #materials #polymerscience #advancedmaterials #bahrain #bfg #monument #art #environment

Today's KNOWLEDGE Share: Loss of Molecular Weight

Today's KNOWLEDGE Share:

Loss of Molecular Weight

It is correct that the degradation of plastic parts can lead to a decrease in their mechanical performance due to a loss of molecular weight.

However, the explanations provided in some posts I see on this platform may not always be entirely accurate.

The resistance to cavitation, which is the initiation of brittle failure, is controlled by the molecular weight between entanglements (Me) rather than the overall molecular weight (Mw). This means that cavitation will not be greatly affected by a loss of Mw as long as it remains well above Me (which is true in most commercial plastic grades except the very high flow grades).

However, many other mechanical properties, starting already from tensile strength, will sharply decrease as Mw decreases due to progressive disentanglement of the shorter polymer chains. This is especially confirmed by testing polymers at cryogenic temperatures where reputation and plasticity are suppressed.

The above explains why molecular weight does not influence the stress-strain response (measured in compression to avoid failure) while most tensile data will start collapsing with a decrease in Mw.

Source:VITO LEO

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#polymers #polymerscience #stress #brittle #tensile #mechanical #testing #partfailure #molecularweight #entanglements 

Today's KNOWLEDGE Share:Polycarbonate Thermoforming Skylights System

Today's KNOWLEDGE Share:

Polycarbonate Thermoforming Skylights System

advantage is variability in sizes with maximum unit size of 2 m x 2 m,

based on the requirements, and the manufactruers will do the die for it.

The Sky domes is made from polycarbonate solid UV sheets which achieve many advantages compared to other materials like;

Up to 250 times more impact resistant than glass

Light transmission value of up to 90 %, depending on the color and thickness of the sheets

Absorb dangerous UV radiation

Can be used at temperatures between – 100 °C up to +115 °C.

Fire performance according to EN 13501-1 is B-s1-d0 or B-s2-d0, depending on thickness and color

Reduce sound levels depending on sheet thickness

Designed against the harsh weathering of GCC countries

Up to 10 years of limited warranty against yellowing available in the market.

Thermoforming Skylights Design

1.The best cost-effective choice for the skylight with a high level of safety and security is Polycarbonate Thermoforming System, providing the same insulating quality as the glass skylight and is much stronger as well.

2.Polycarbonate Thermoforming Skylights System's advantage is variability in sizes with a maximum unit size of 2 m x 2 m.

3.For better thermal insulation, we can do single, double, or triple-layer glazing, as you can choose Dome or Pyramid shape.

4.For any span larger than 2 meters, this system can be used as a multi unit of domes or pyramids by using special extruded aluminium profile gutters and chairs.

Source:Al Mamary

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#skylights #thermoforming #polycarbonate #plastics #impact #uv

Today's KNOWLEDGE Share:FROZEN SKIN

Today's KNOWLEDGE Share:

The frozen skin that develops during fill will barely subsequently shrink, since it is already solid !

As a consequence, thin parts, where the frozen skin represents a significant fraction of the thickness, will shrink way less than thick parts where frozen skin is a negligible fraction of total thickness!

Note that since frozen skin is highly fill-rate dependent, when filling slowly you will observe less shrinkage, hence less need for pack.

The IMFLUX process (low pressure molding) does indeed claim essentially no need for a pack.

While this is true, please note that filling as slowly as possible (near 100% frozen skin at end of fill, just before risking a short-shot) means that you will produce VERY ORIENTED plastic parts with :

- very high birefringence (amorphous)

- very anisotropic mechanical properties (semi-crystalline).

Many technical parts would rather benefit from low molecular orientation and maximum isotropy of performance, hence a faster fill followed by the appropriate packing is often a better choice.

source:VITO LEO

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#injectionmolding #injectionmoldingmachine #frozenskin

#fillrate #shrinkage