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How to prevent stress cracking from breaking your product?

Within the field of plastic products, stress cracking is a common, if not the most common, quality problem. Its consequences are often severe since problems tend to occur only after the products have already hit the market. In most cases, however, customer dissatisfaction could be avoided in the design phase. Here is how, but first let’s look at the phenomenon itself briefly (for more in-depth information please see the link at the end of the article).

A short description of Stress-Cracking as a phenomenon

When a plastic component is subjected to stress, force is transmitted along the polymer chains. The molecules need to struggle in order to stay in contact with each other. Under stress the polymer chains start to orientate towards the direction of the pull and some soon some permanent deformation can be measured on the component geometry. This is what we know as creeping. Depending on the level of the load and time, at some point we may notice crazing on the component as small cracks on the component surface. Shortly after this the part tends to break off. Chemical substances and elevated temperature – separately but especially as a combination – catalyze the process. In such cases the phenomenon is known as environmental stress cracking (ESC).

How to prevent Stress-Cracking from happening?

Preventing Stress-Cracking requires competent plastic product design? The following aspects should reduce the risk considerably:

TIP: Stress cracking is affected by time, load and temperature. You should define the expected lifetime for the product and recognize the occurring forces, surrounding temperature and the chemical service environment. Occasional peaks on load or temperature are not dangerous – constant, long term conditions are the determining factors behind stress cracking. Chemical exposure also easily triggers the effect.

TIP: Amorphous materials like ABS, PS, PMMA or PC are more sensitive to continuous load as well as to chemicals than semi-crystalline plastics/polymers. If transparency is not a key issue, steer away from them in applications that involve continuous load. When selecting a material, it is wisest to pay attention to the mechanical values at the upper end of the service temperature, not at room temperature.

TIP: Determine the highest stress facing the component as that is the spot where the crack propagation most likely gets started. For example, if your design needs to withstand pull of 1000 Newtons with a cross-section of 20 mm2 leading to stress of 50 MPa, and the tensile strength of the selected material is 60 MPa (in expected long term temperature), you are not safe. It would perhaps be acceptable with a sudden, temporary load, but in order to avoid stress cracking, you should aim to double the strength of the spot and thus cut the stress level to half. This combination of load-time-temperature is case-dependent, but in principle the stress levels under continuous load should be remarkably lower than the yield or tensile strength of the material is in that temperature.

TIP: Request you supplier for a stress-rupture-curve for the potential materials. They give invaluable information of material behavior under constant load in alternative temperatures. Unfortunately, however, they are rarely available.

TIP: Use reinforced materials. Glass increases the strength of the material and is not equally sensitive to temperature or continuous load.

TIP: Turn the stress into compression instead of tension, if possible.

TIP: Remember that internal stress can also break the component. Screws, metal inserts and poorly designed snap-fits are all features that typically fail under stress cracking. Considerable changes in material thickness may also, in time, cause internal stress and cracking.

TIP: Test. Testing against stress corrosion is difficult since it may take thousands of service hours before the results appear in actual conditions, and you most likely do not have the time at your disposal. It is therefore wise to set the first of your parts from production under stress in an elevated, controlled temperature. If you catalyze the process by adjusting both stress and temperature slightly above the expected values you have at least some indication of the sensitivity of the component (to stress cracking). Do not exaggerate, however, because too high a stress and temperature will inevitably lead to breakage in a short period of time but doesn’t yield results that would have tangible information.

Stress cracking failures are probably so common because designers in general are simply not aware of its existence. If you take the phenomenon and aspects above in consideration as part of your plastic part design, problems are much less likely to occur.

Recommended link: Michael Sepe, The effects of stress

If you have good links, photos or cases on the topic, I’d be happy to add those to this article. Please comment or contact markus(at)plasticprop.com.

Comments

George Hill
Dear Your comments are very interesting , and I would like to know if you have some comments with this phenomena in PC bottles for water ? Thanks
Lawrence
Will you suggest selection of plastic material to enhance the strength ?
Markus
Lawrence, If stress cracking is your concern... Try to keep away from amorphous materials. Adding some fibers should carry part of the load. Make the construction considerably stronger than what temporary load would require. Check the service temperature in comparison to the material selection. Best Regards,
Shahrin
Sometimes I had experienced component (using pmma material) crack at same area (inwards bend r) after I applied acrylic cement (methylene chloride) to fuse it with another part (also using pmma material). Any advise to prevent this crack from happening?
Markus
Shahrin, I believe the bondage is not very strong when the PMMA components are glued together. My experience on that is limited. You could try welding with heat, but I have my doubts on that as well. Good luck, Markus
Shahrin
Hi Markus, Can you tell me any recommended interference fit for pmma material to pmma material? I use a pmma material cover to act as a lid for a pmma casing part. It need to be press fit condition due to it functionality. Very greatfull if you could help on this.
Markus
Shahrin, Is this an assembly fit or is the lid opened constantly by the user? It all depends very much on the scale of the full system, the dimensions of the surrounding areas. I suggest you make a good guess and order SLS 3D print of the system. Stereolitography material is a little brittle and you can probably get a tangible feeling how it works. In case of PMMA it is essential that you don't leave any permanent tension on the press fit element. If so, stress corrosion will most likely occur on the product. Markus
Shahrin
Hi Markus, It is assembly fitting. Currently I having crack issue due to stress. So I would like to know any recommendation for press fit tolerance for PMMA material. I can't find any reference in the web.
Markus
Shahrin, It sounds like the locking is based on tight fitting and friction rather than a mechanical snap fit where some hook element is locked behind an other element. In case of PMMA I would prefer the latter solution. It is important that there is no permanent tension on the hook element. I'm arfaid the reference table you are looking for does not exist. Can you take the existing components, use a file to make the fitting as loose as possible but still working and then test if how it tends to crack in comparison to the current samples . You can try catalyzing the effect somehow (elevated temperature, CRC spray..) Markus
Syed Afsar
We are manufacturing Lead Acid Battery Containers and Covers from ABS material by injection molding.. The covers have threaded Vent Plug holes in which the vent plugs (also of ABS ) are tightened after filling of electrolyte. We have observed that after storage of the finished batteries (still without electrolyte) for about 3 months at our customer's warehouse , the battery top covers develop cracks initiated from the vent plug holes. Pleas note that the vent plugs are screwed on to the top cover. Please let us know why this is happening.
Nilo
I have some PET bottle which cracked middle of the bottle like direct line! can you help me? When I see it, I think that this bottle cuted with a knife! But I see this failure in the market too.
Markus
Nilo, I would assume that this is due to stress-cracking, but on the other hand one could expect there to be some crazing around the crack. I'm not so sure if it looks like a knife cut. It seems like you are not the only one with this problem. Here is an article about the subject: http://www.britishsoftdrinks.com/write/MediaUploads/Publications/StressCrackingofPET.pdf Could your bottle supplier help on this? Markus
nilo
Thanks a lot for your comment.
Markus
Syed, sorry for the delay ABS is very prone to stress-cracking even when not in harsh chemical environment. Is it possible to switch to some semi-crystalline grade? Markus
Nagesh U R
Dear Markus We make caps of aerospace connectors by LDPE material with 2% colorant. During usage the part is cracking( at customer area) I understand this may be due to presence of stress int he part which leads to Environmental Stress cracking. What chemical I should used to do aging test so that I can ensure part is good.? Since the part is opaque we are not able check the Polarized light stress checking method. Pls help me regarding this.
Markus
Nagesh, LDPE should not be very sensitive to the stress cracking phenomenon. Here is a link to check the chemical environment. http://www.cdf1.com/technical%20bulletins/Polyethylene_Chemical_Resistance_Chart.pdf Could it be that the design simply is not strong enough? It is very typical that plastic structure can withstand some stress easily for a short period of time, but fail after some hours or days. In case of continuous load, you should make it at least three times stronger. I hope this helped, Markus
Jothi.N
Hi Markus, We are facing crack issues in Juicer Jar ( Mixer Grinder) in SAN material, if any additives available with elastomer properties to avoid crack issues, / any other materials available in same spec ( we are using SAN-2300 Water clear Material - Styrolution material) .
Markus
Jothi.N SAN is britte. All the boxes in my refridgerator are SAN and more or less broken. I don't have experience of the said grade, but perhaps you could try some other product, impact modified perhaps. Typically toughness is increased with EPDM particles, but this makes the product very similar to ABS and you loose transparency. Or does the component crack without mechanical impact? If so, you might check if there is internal stress or if something is causing permanent stress in the system. Avoiding that would help. Or could you switch to PMMA or PC? Or ff the case is not very cost sensitive, even amorphous PA, like EMS Grilamid TR? Markus
Derick
I have a thermoplastic part which has been over-molded with a thermoset material. Due to their differences in coefficient of thermal expansion, a crack is produced on thermoset due to high temperatures. One of possible solutions is to have all the part itself over-molded with a thermoset material as well. But what is a geometrical design solution that can help compensate for the expansion and contraction of both plastic material types on each other?
Markus
Derick, My experience on thermosets is limited. In case of thermoplastics, you can avoid stress-cracking by avoiding amorphous materials and possibly sharp corners on insert. It is also important to check there are no chemicals or cleaning substances on the surface of the insert. You might also try to preheat the insert before overmolding. I Good luck, Markus Markus
Masum billah
We can't prevent cracking problem of a chair stooper(L type). We make it HDPE&LDPE. Plz give your guide line how prevent it. This stopper is attached at steel frame.
Markus
Masum, Are you overmolding it or is it a snap-fit of some sort. It it is overmolded and very long, shrinkage will cause considerabe stress on the piece. If it is a snap-fit, it sounds like the fitting is too tight. PE should not be very sensitive to stress-cracking. Perhaps you could try with PP instead. Markus
Andrew
Is there any way to find which chemical caused ESC if chemical residue is very samll?
Markus
Andrew, I would set several components under same stress (bend them between similar jaws) and then pour the possible chemicals to the stressed area, one by one. The chemical on the product that fails first would be the most critical. Markus
Ken Stone
In solar project, we are using thousands of acrylic Fresnel Lens. The Fresnel lens is mounted on a steel frame. The lens have a lot of small cracks along the edge. Some are growing into large cracks. Can this be stopped?
michael
markus, is there a way to stop my hd cap from ESCR problem? all because my customer complain that my cap got crack after some month of usage, they pack sodium chlorine product in the bottle and cap with my hd material of hrv140
Markus
Ken, Sorry for late reply. I missed your message during my holiday. If the fitting is tight, there is continuous compression that will easily lead to stress cracking. And I assume the lenses do not crack before mounted? It might also be that changes on temperature increase stress. In ideal situation the lens would kind of float inside the steelframe. Is it possible to have a gap between them and lock the lens in place with some other method? Markus
Markus
Michael, To my experience HRV140 is PP homopolymer. If you check this link, you can see that PP and chlorine make a bad combination. https://www.ineos.com/globalassets/ineos-group/businesses/ineos-olefins-and-polymers-usa/products/technical-information--patents/ineos-pp-chemical-resistance-guide.pdf I recommend you try some other material. Compatibility can be found on corresponding sources. Good luck, Markus
Jan Venema
Dear Markus, We have a part that shows stress cracking, it's a ABS bush with 3/8" thread. A brass part is screw in the ABS part. Can the thightning torque be the cause of the cracking? Can the collor of the ABS have an effect on the strenght? Best regards Jan
Markus
Jan, Yes, I would say that the tightening can easily cause the cracking. One can generate quite a force with a thread without noticing it. Is it the thread area that is cracking or the opening of the hole? Very common cause for cracking is use of countersunk screws. You might try what torque is required for breaking the thread. The torque that is used in practise should be roughly only 15-25% of this in order to avoid long term problems. This is the rule of thumb is use in general, I don't see why it could not be applied on this. If there is long term elevated temperature involved, this will increase the problem. Markus

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