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Crack Description and Growth Mechanism
Chemical stress cracks are typical brittle cracks
which occur at or below the tensile strength of the plastic
or material. Specifically, the name "chemical stress cracking"
is given to the phenomenon where chemical agents adhere to,
or come into contact with, areas where tensile stress is present
(i.e., where the part is being loaded), and where cracking
occurs over time as a result of the combined effect of the
chemical agent and the stress. The surfaces of this type of
crack are smooth, and in certain cases, they may take on a
mirrored appearance.
In terms of growth mechanism, it is said that
gaps open between molecules when the part is loaded or in
some other condition of stress, that the chemical agent
penetrates into these gaps, and that the cohesive power
(or strong binding) between the molecules is reduced. The
exact mechanism has not yet, however, been completely understood.
Chemical stress cracks can be prevented from occurring by
eliminating either the chemical agent or the stress.
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Synonyms for Chemical Stress Cracking
Regularly used synonyms include the following:
Environmental stress cracking (ESC) or environmental stress fracture
Stress cracking or stress fracture
Solvent cracking or solvent fracture |
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Crack Examples and Countermeasures
Contact with plasticized polyvinylchloride and flexible packing
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Plasticized polyvinylchloride and flexible
packing contain added plasticizers, which are typical examples of
chemicals that lead to chemical stress cracking. Products using
plasticized polyvinylchloride include plastic hoses and electrical
cords. |
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In electrical cords and the like, materials
such as polyethylene, silicon rubber, and ethylene-propylene rubber
are used to cover or shield the wires. Similarly, packing materials
contain silicon rubber, ethylene-propylene rubber, or fluoroplastics.
If contact with plasticized polyvinylchloride cannot be avoided,
the part's material should be changed to a chemical-resistant grade. |
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Metal-insert moldings and press-fit metal components |
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Metal components are often coated in press
oil, cutting oil, anti-rust oil, and other similar types of oil
used in the manufacturing process. Accordingly, if degreasing has
not been carried out to a sufficient degree, these machining oils
may act as a chemical agent that leads to chemical stress cracking.
Plastic sections which come into contact with metal components often
contain residual stresses from molding and compressive stresses
caused by press-fitting components, and these can account for the
stress factors leading to chemical stress cracking. |
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Thorough degreasing should
be carried out for metal components; furthermore, these components
should be preheated before being used. |
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Screwed assembly |
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Relatively large stresses exist around
assembly screws, and particularly in the case of countersunk-head
screws, these stresses can be excessive. Accordingly, they may constitute
the stress factor leading to chemical stress cracking, and if chemical
agents are also present in these areas, cracking will occur. |
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Component shapes should be designed in
such a way that countersunk-head screws are not needed. Where usage
is unavoidable, washers should also be used and screws should not
be tightened beyond the appropriate torque. |
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Molded component thread shapes |
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Molded components to be connected to plastic
piping contain screw heads in certain areas for this purpose. It
is highly probably that large stresses will result from over-tightening
at the screw section, and this can lead to the generation of stress
concentrations, particularly at the bottom of screw threads. Cracking
will result if chemical agents come into contact with these areas
of stress. |
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Components should be assembled with the
correct fixing torques, and if cracking occurs even at the correct
torque, the material should be changed to a chemical-resistant grade. |
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Usage of detergents to clean molded parts |
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A wide range of different
detergents are used to remove dirt and contamination from molded
parts. Generally speaking, detergents contain a volatile chemical
agent such as alcohol, and it is highly probable that these substances
will constitute the chemical factor for the occurrence of chemical
stress cracking. Residual molding stress is present particularly
in areas with thin walls or non-uniform cross sections, and if detergent
should come into contact with these areas, cracking will result. |
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Detergents should be diluted with water before use. |
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Kitchen fittings |
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It is highly probable that
fittings in domestic kitchens, restaurants, and the like will come
into contact with vegetable oil, and this substance is a notable
cause of chemical stress cracking. |
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Change the material to a chemical-resistant grade.
Click to see a list of chemical-resistant grades |
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Toilet and bath products |
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By and large, the soap products used
in toilets and baths often constitute chemical agents that result
in chemical stress cracking. |
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Change the material to a chemical-resistant grade.
Click to see a list of chemical-resistant grades |
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Oil and grease for sliding sections of metal components |
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Certain types of oil and grease as used
to promote sliding of sections of metal components can provide the
chemical factors required for chemical stress cracking. Cracking
will result whenever these substances come into contact with areas
of plastic where stress is present. |
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Switch to the usage of fluoride-type oils and greases |
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Chemical Resistance Testing (or testing to confirm resistance
to chemical stress cracks)
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As shown in the photograph, the test pieces
are mounted on a jig which maintains constant curvature, and
the chemical agent is applied.
After allowing the test pieces to stand in the prescribed
environmental conditions for a specific period of time, the
condition of physical degradation is checked. This method
is referred to as the bending-form fixed strain test.
Test conditions Strain range:
0.2 to 1.0% (in 0.1% steps)
1.2 to 1.6% (in 0.2% steps)
Test environment:
23 deg.C, 50 to 60% RH
Standing time: 48 hours
Test piece shape: 2 (t) x 10 (w) x 150 mm (l) |
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Note: Chemical resistance testing should be carried
out in advance of usage for any chemicals which may make contact
with plastic products, and it is crucial that chemical stress crack
conditions be checked using these tests. |
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