Plastics Failure & Rubber Failure, Litigation & The Need For Long Term Durability Studies

In an age of consumer champions, regulatory agencies and alert attorneys the plastics designer, manufacturer, fabricator and ultimate retailer are under drastic pressure to assure themselves, their customers, and the general public that their product can do what it is supposed to do throughout a prolonged life span and furthermore, do it in a safe and trouble-free manner. Whilst it is accepted that nothing lasts forever, the key to performance of plastics products is that it must remain serviceable for a reasonable life cycle, and failure must not occur in a manner that could jeopardise the equipment or individual it services. At the end of a useful service life it should ideally expire peacefully having no detrimental effect on its surrounding environment.

Plastics failure can cause economic and legal problems, as well as contributing to personal injury and death. The ‘owners’ of plastic products that have failed are, for obvious reasons, generally reluctant to publicise the fact. Failure diagnosticians tend to be restricted from doing so by confidentiality agreements and for this reason the activity is predominately covert. As a consequence the potential benefits such as learning from the mistakes and misfortunes of others, and identifying priorities for research and critical issues in product development are far from being fully exploited.

It is clear from the extent of plastic failures received by Rapra that this limited dissemination of plastic failure knowledge within the public domain has resulted in a continual cycle of plastic failure incidents from all industrial sectors. The lessons of good plastic product design are not being learnt even in light of the enormous growth in product liability cases that have imposed an entirely new dimension on the consumer product environment. It is now well established in law that manufacturers are liable for injuries resulting from defective product; for injuries from a hazard associated with a product against which the user should have been warned; or for damages caused by misapplication of a product which could have been foreseen by the manufacturer.

It is a practical necessity to understand why plastics fail in order to minimise the failure scenario. Rapra Technology has acquired this knowledge due to 80 years dealing with a diverse clientele providing technical services aimed at problem solving and in particular failure diagnosis.

Failure is a practical problem with a product and implies that the component no longer fulfils its function. Frequently, the ability to withstand mechanical stress or strain (and thereby store or absorb mechanical energy) is the most important criterion in service and consequently mechanical failure is usually a primary concern. However failure may be attributed to loss of attractive appearance or shrinkage etc.

The two main forms of mechanical failure are ductile and brittle failure. Ductile failure is, by definition, failure at high strain. It is relatively straightforward to design plastic components to avoid ductile failure. However, in practice, ductile materials often fail in a brittle manner, which becomes much more difficult to predict from a theoretical standpoint. Brittle fracture is a low energy process characterised by failure at low strain, with little or no deformation. Components contain small, crack like defects which can act as stress concentration features; these micro-cracks grow under load and may eventually lead to rapid catastrophic failure.

When considering the design and development of a plastic product it is imperative that a designer fully understands the fundamental limitations of plastics. A designer must be aware that plastics are:-

· Non-linear, visco-elastic materials

· Temperature dependent

· Materials that physically age

· Susceptible to chemical attack and environmental stress cracking

· They will, under the action of a tensile stress, eventually fail

· The time to fail will diminish as the stress increases

· The time to fail will diminish as the temperature increases

· The time to fail will diminish in the presence of certain environments

· The time to fail will diminish under the action of cyclic loading

· The moulding process can result in significant levels of residual stress in components

· Weld lines are planes of weakness, particularly in fibre filled materials

· Most plastics are highly notch sensitive.

· Mechanical anisotropy due to the alignment of fibre reinforcement

· Moulded articles rarely achieve theoretical material properties

Rapra’s experience has shown that many designers do not consider and / or are aware of these issues when considering the use of plastic materials. We have designers who can design but have no real appreciation for the material they are proposing to use. Rapra has found that Poor product design is endemic to all plastic sectors including the medical, pharmaceutical, automotive, rail, aerospace, packaging, oil / gas, energy, engineering and construction industries.

More than 5,000 failures have been the subject of study at Rapra. Interestingly the number of failures evaluated has increased significantly during the past five years.

Rapra Technology Plastic’s consultancy provides a range of services which allow engineers to prove designs at an early stage, ensuring that their product will be right first time inclusive of finite elemental analysis (FEA – ABAQUS), material selection (Plascams), evaluation of design aspects, injection mould simulation (3D Sigma), long term durability studies (creep, creep rupture, fatigue and actual product endurance testing.

A key to good plastic product validation is the generation of durability long term performance data. It is essential that designers and manufacturers of plastic products understand that short-term data provided by material manufacturers is useful only as a comparative guide between generic and sub-generic groups and cannot be used to gauge material performance in the long term.

In order to provide confidence that a plastic component will perform in the long term a prediction of failure stress in time under simulated in-service conditions i.e. temperature, environment is strongly recommended. Predicted long term data can then be correlated with 3D Sigma / FEA calculated residual and operational in-service stresses to determine a safe working life time for the product. Ideally testing of actual components is preferred so that the effects of possible moulding defects, moulded-in stresses etc can be assessed. However, due to complex component geometries and sizes this is not always feasible and subsequently material test specimens are tested.

Typical long term mechanical failure mechanisms resulting in catastrophic brittle cracking include creep rupture, fatigue i.e. cyclic stressing and environmental stress cracking which are discussed as follows:

Creep Rupture

Over a long period of time at constant load, most polymers will creep, causing failure. An aggressive environment accelerates failure. Creep rupture analysis generates a time to failure data for different constant stress levels. This data can be used to predict the life of a component and can be used in design calculations.

This method generates a time to failure curve for static creep at different stress. The data can be used to predict the effective life of a component where it is continually loaded under static conditions. The test can be carried out in aggressive environments to simulate operating conditions. Each test at an individual stress level is run for a maximum of 1000hrs.

For longer-term predictions, tests are carried out at elevated temperature. Then, data is predicted using time-temperature superposition techniques. Time temperature superposition is a well-established technique that is used extensively in the assessment of the long term (50 year) design stress of plastic pipes ISO 1167, BS EN ISO 9080.

These curves are then shifted to fit “by eye”, T5 to T4 and T5 + T4 to T3, etc. and a common shift factor found that can be applied to all of the data generated to produce a long term master curve at the required temperature.

The master curve can then be used to establish the failure stress (sf) of the material in the environment at the service temperature and at the desired life of the component.

Dynamic Fatigue Testing

If the component is subjected to any form of cyclic loading, then fatigue failure will be prominent, especially when there is an aggressive environment. Most plastics undergo a ductile to brittle transition during fatigue. Therefore, after a number of cycles, the fatigue strength dramatically drops. An ESC agent can make the transition more dramatic or occur after less cycles.

Fatigue testing is carried out at a relatively slow cycle rate (typically 0.5-1Hz). The number of cycles to failure (to a maximum of 106 cycles) is determined for different stress levels. The resulting curve can be used to predict the life of a component if the cyclic stress can be measured or calculated. In many cases the maximum permitted stress in a plastic component is significantly lower than expected from FEA calculations. High frequency cyclic loading measurements are erroneous since heat is generated causing the material to be plasticised.

Fatigue testing can be carried out at elevated temperatures and in aggressive environments. For longer-term predictions, tests are carried out at elevated temperature. Then, data is predicted using time-temperature superposition techniques. Samples can be tested in tension or compression and also with a cyclic load on top of a baseline load or a cycle through compression and tension.

Summary

The causation of plastics failure has many forms, most of which would be pre-empted by undertaking a thorough plastic feasibility study to ensure attainment of at least adequate product quality.

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Car Transport by Train for More Safety

As the train exists, it is considered as one of the most beneficial means of transportation. Nowadays, train is used for shipping goods such as food, large and heavy cargos and even cars. They usually use this because of its capability to transport large and heavy items, that is why, it is the best choice of people to transport freight all over the world. Car transport by train is also common, since it is more cost-effective and energy efficient compared to road transportation that travels in long distances.

Car transport by train is less supple than the road transport; it is because they run on a definite track and it can also be transferred from rail to road or sea. The car transport by train is usually in lower cost compared into air freight or truck freight. Freight train has two types; Through Freight Train and Local Freight train. When the train travels in one or more terminals before reaching the final stop, it is called Through Freight train, since it goes through the terminals without loading or unloading the freight. This type of train will only stop to its destined location and it can travel over long distances.

The Local Freight train is composed with collection and distribution process where the cargo cars are left with the shippers. Local Freight train at almost every town and it will just stop to pick up or set out cars along the way. Compared to Through Freight, local freight will only travel in short distances.

Because there are a vast number of trucking, shipping and flying transportation, there are many rail freight companies, add another ways of operation in order to compete with those shipment companies. There are several companies now that already includes freight train or car transport by train. By this means, you are ensured that your products are being transported with close protection. Because of the climate change, shipment companies realize that transport by train is very energy efficient in terms of transporting goods. The train transportation is an old method of transportation yet it is an innovative idea and also you are not risk of danger rather that road transportation.

There are a few things you should look for whilst choosing an auto transport broker. First is the area they provide. Constantly, the automobile shipping firms will supply services nationwide, so chances are high that they will be able to assist you out.

Regularly automotive is transported by the use of automobile delivery trailers or by train. However when you possess high-priced automobile then it’s larger to opt for enclosed automotive shipping as on this approach of transportation your car won’t experience from any sort debris, rain, and wind etc. Excluding this, your vehicle may also no longer experience certain damages in all probability to occur at some stage in shipment. On a regular basis, it calls for about one specific week to deliver your automobile in your location. Therefore, choose the company that will guarantee you with quality service using car transport by train or other means.

Preventing the Incidence of Falls Among the Workers

Speaking about the matters of public safety, the relevance of safety rails and guardrails can’t be denied at any point of time. Whether at rooftops, balconies, and roads or in industrial danger zones, these equipments always give extra support to the people in the mentioned areas. These devices are very essential indeed in preventing falls and other kinds of physical injuries. The boundaries at various places are guarded by these devices that provide ample support to the cause of preventing accidents. In a hoard of public utility places these things can be seen, and also in the areas within the realm of private property.

The rails of any type are generally placed in places with high elevation and obviously with greater probability of inflicting a fall. The rails are generally maid up of metals, especially iron and copper. But certain alloys like brass are also used for the construction of these rails. These are being strategically placed in order to give protection to the people from being injured in certain premises. To a considerable extent, large and strong rails with delicate designs also prevent suicides, thus saving precious lives. These particular equipments have become indispensable in the modern urban and even rural settings.

The importance of guards can be observed in the matters of traffic and automotive safety. There are multiple usages of rails that are placed for the purpose of providing varying degrees of guard. The vehicles are prevented to go astray, that is losing control and move in an unintended direction. In addition to this, the vehicles are prevented against ramming to other vehicles by going to wrong lanes. They are also restricted to stay on their paths and not to fall in a river or trench, if any, by the side of the road. The drivers too drive in a more responsible manner in presence of proper guarding facilities.

A very significant point must be noted at the context of ongoing discussion. The main job of the posts in the guards is not fully resisting the impact of a speedy vehicle. They, although built of metals, are not strong enough to handle the effects of the impact. Their job is to transfer, rather spread the force of the impact in different posts so that a singular point or spot is not severely damaged and the probability of saving the people inside the vehicle including the driver increases to an extent.

The engineers who work in the field of transportation engineering always properly evaluate the areas that need the placing of guards. In zones where construction works are regularly being undertaken, the labourers feel protected in a better manner if the area is guarded by rails. In places where the traffic density is low, like those in the vicinity of rural areas, cheap rails are used so that the cost of maintenance and repair is low. The case is just the opposite when the area is urban with heavy traffic density. Hence, the quality of rails is somewhat dependent on the traffic density.