Mechanical Properties: Steel Outperforms FRP

Steel and fiberglass have fundamentally different properties. Steel is an elastic material with excellent ductility and impact strength, while fiberglass exhibits poor ductility and impact strength. Simply put, when steel only dents, fiberglass (composite) will break.

An elastic material like steel expands instantly when a significant load is applied, and just as quickly returns to its original state once the load is removed. Steel remains fully elastic as long as the stress remains at or below the yield stress of the steel.

In comparison, a rigid, viscoelastic material like FRP is subject to time-dependent strain and weakens over time as load is applied.

Mechanical Properties of Steel

Mechanical properties pertain to reaction of the material to an applied load. The mechanical properties of metals determine their range of usefulness and establish expected service life. Mechanical properties also help classify and identify materials.

The most common mechanical properties considered for materials are ductility, strength, and hardness. For all three properties, steel tanks outperform tanks made of FRP.

Ductility allows a material to bend rather than break. The higher the ductility, the more the material can change shape without cracking or rupturing. One measure of ductility is the amount a material can stretch without brittle fracture.

Steel stretches about 15 times more than fiberglass reinforced plastic. Its higher ductility means buried steel tanks can resist loads from high water tables and heavy trucks better than more brittle materials, such as FRP.

For the same reason, steel tanks are safer than brittle FRP tanks under severe conditions such as lightning strikes, tornadoes, mud slides, extreme temperature changes, and earthquakes.

Strength is necessary to resist the loads imposed on the tank. The flexural strength of a material is its ability to resist deformation under load, that is, how much stress an object can endure before it breaks.

For underground tanks, loads include traffic and soil loads pushing down, groundwater pushing up on the tank bottom, and pressures created when filling and withdrawing product. Steel endures high stresses without cracking and is the high-performance choice for underground tanks.

Hardness characterizes a material’s resistance to indentation. The hardness level of an FRP tank must be regularly measured to ensure that it remains consistent over time. Inadequate hardness may be an indicator of incompatibility with products stored, leading to leaking or evaporation.

However, the steel used to fabricate tanks is inherently hard enough to resist penetration from sharp objects. Further, steel tanks are compatible with all types of fuels, chemicals, and liquids normally stored in underground tanks.

Steel is Your Strongest Choice

Specify steel when choosing the material for your tanks or pipelines. Remember that only steel has long term material strength that’s not time-dependent, as well as superior mechanical properties.

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