Are you an engineer in the industrial or commercial sector? If you are part of the design team, you must know that selecting the best engineering materials is a critical part of your operations. Incorrect material selection has led to numerous disasters including the Boston Molasses Disaster, De Havilland Comet Plane Crashes and the Eschede Train Disaster of 1998.
In mechanical engineering, selecting a material can be a difficult undertaking considering the many factors the engineer must consider before settling on a material. The key to this process is to ensure the material not only supports design integrity, but also guarantees structural integrity in the harsh actual field application. As you select the best material for your design, you should be wary about some options.
What choices does the engineer have?
There are four common mechanical engineering materials. First, there are ferrous metals, which include stainless steels, carbon steels, alloy steels, cast iron and tool steels. The second category is plastics, which comprise of thermosets and thermoplastics. The nonferrous metals category covers titanium, magnesium, copper, aluminium and tungsten. Finally, we have adhesives, whose long list includes silicones, epoxies, anaerobics and hot melts.
Historically, and through research, some of these materials have gone into the bad books with engineers for some reasons. Here is a breakdown of 10 engineering materials you should never use.
Lead is one of the most interesting non-ferrous engineering materials. It features excellent malleability and a low melting point, making it perfect for casting. The metal can be easily recycled, and probably that is why it is abundant. Considering that lead is also low-priced, one might expect it to be the perfect metal for engineering design applications.
As a mechanical design engineer, one of your chief concerns is the strength of your product/component. Lead does not fit the bill on this consideration. The metal is extremely weak, worsened by its fatigue and creep characteristics. Even for designs that require minimal strength, lead may not fit. Unless you are adding alloys to the metal, you should NEVER use it. Lead is also an extremely toxic material, and is extremely harmful to the human body if inhaled or swallowed.
Cadmium is also on our list of 10 engineering materials you should never use. This toxic metal is greyish-white, or blue-white in elemental form. This material may be present in industrial paints.
The metal presents both short term and long-term ill effects. Some short-term effects from acute exposure include muscular pain, headache, weakness and sweating. Within 24 hours, the victim may have developed acute pulmonary oedema.
In the longer-term, one may contract cancer of the prostrate or lungs. Generally, the victim will initially suffer kidney damage. Experts also believe that this metal could cause osteoporosis, pulmonary emphysema, anosmia and anaemia.
Exposure may come from grinding, cutting, brazing and any other activity related to structure demolition/renovation. It is important to assess cadmium exposure in such maintenance or construction activities.
Asbestos was a popular engineering material owing to its heat resistance and strength. Most of its applications were in the 19th century. While companies knew the dangers of this material since the 1930s, it was not until the truth came out in 1970 that this material was banned.
Asbestos was mostly found in the building industry, where it was used for the construction of shingles, cement products and floor tiles.
What are the dangers of using this engineering material? You may have heard about mesothelioma cases in relation to asbestos exposure. Mesothelioma is a type of cancer that affects the lungs, heart or abdomen. Exposure to the material can also lead to these other diseases:
- Ovarian cancer
- Lung cancer
4. Crystalline Silica
Many minerals, including granite and sand, contain crystalline silica. The most common form of this material is quartz. Exposure to this material is rampant among high risk job workers in sectors such as rock drilling, tunnelling and foundry work.
Exposure to this material occurs mostly in the construction industry. Mixing concrete is one of activities that exposes workers to this dangerous material.
How dangerous is crystalline silica? The material is a serious human lung carcinogen. It can also lead to the sometimes-fatal silicosis condition. This material could also cause scar tissue in the lungs. The incurable silicosis can even lead to tuberculosis because it tampers with lung function.
The solution: avoid crystalline silica at all costs.
5. Halogenated Flame Retardants
These chemicals have been used together with construction materials at the manufacturing stage. They either inhibit chemical reactions, or form a protective film to stop the spread of fire. The problem is that when these retardants are heated, they change into toxic materials. When the gaseous fumes formed mix with dust, they can be inhaled or ingested. Effects of these toxins include cancer, foetal problems and immunotoxicity.
6. Hexavalent Chromium
Hexavalent chromium is a chemical compound that contains chromium in the 6th oxidation state. This is typically produced during stainless steel welding where the chrome that is present is not initially hexavalent but becomes so due to the high temperatures created during the welding process. Some other typical uses of chromium VI include anti corrosion additives for paint, electro plating and paint pigment. Chromium VI is a carcinogen which means that exposure to it increases the risk of cancer. It also causes damage to the eyes, nasal passages and respiratory system.
7. Depleted Uranium
Depleted uranium (DU) is produced as a by-product of the manufacture of enriched uranium. It should be noted that DU tends to be only 60% as radioactive as naturally occurring uranium.
It is most often used as armour and armour piercing rounds in military applications. Its high density is one of the reasons it is effective as a weapon (it is denser than lead) as it has more penetrating power. Depleted uranium is radioactive and as such is seen as a controversial choice for weapons. When a DU shell impacts a target, it tends to contaminate the area around the target and as such increases the possibilities of inhalation by civilians. This can increase their risk of cancer.
Mercury is a fascinating metal. It melts at -38.9oC and only boils at 356.6oC. Therefore, it saw widespread use in thermometers, barometers and level switches in the past. More recently mercury can be found in lighting and some electronics. But there is a reason it is always kept in a sealed glass tube. This is because it is a dangerous material and extremely toxic. Mercury can be fatal if ingested and causes damage to the central nervous, immune and digestive systems. Mercury is currently in the process of being phased out and soon we will no longer see it being used in any capacity except for the odd niche application.
9. Polyvinyl Chloride (PVC)
PVC is a thermoplastic. A thermoplastic is a plastic that can be melted and reformed without significant loss of mechanical properties. PVC is used almost everywhere and is supplied both as a rigid and soft plastic. It is used in piping, food packaging and in some places as a substitute for rubber. Although testing on animals has shown no serious health effects, there has been no conclusive testing on humans and as such there is a lot of confusion as to whether PVC is a health hazard during normal use. The end of the lifecycle of a PVC product is a different matter as it cannot be incinerated since hazardous fumes are released during burning. Furthermore, if left in land fill sites harmful chemicals used during the manufacture of the PVC begin to leach into the ground. This in turn contaminates drinking water.
10. Polychlorinated biphenyls (PCB’s)
PCB’s refer to a group of manmade chemicals that were widely used in industry. Some of the more typical applications include, but are not limited to; plasticizers, heat transfer, adhesives and plastics. PCB’s have been proven to cause cancer in animals and as such the EPA banned numerous uses. However, this does not mean that these chemicals are completely out of the picture. Care must be taken when using PCB’s as they are a suspected carcinogen and must be treated with respect.
Engineers take on massive responsibility, a single misstep during the design process can cause fatal accidents and exposure to toxic materials. There are many dangerous materials on the market that can cause serious harm to the public if misused and misunderstood. If there is no choice, then every step must be taken to ensure that the risks are reduced to acceptable levels. Always make sure that you understand the limits and dangers of the materials you use.