Mechanical properties of material are one of the most important basic concepts in a well design and this section will briefly discuss about key mechanical properties and their applications. Furthermore, there is a discussion about effect of corrosion on the mechanical performance of load carrying components.
Mechanical Properties of Material
Basic mechanical properties are as follows;
Hardness is a resistance of materials to permanent deformation and is sometimes referred to a resistance to abrasion or scratching. The greater of the hardness, the harder it is for the materials to deform. The application of hardness is to inspect if materials have been properly treated during a heat treatment process. The comparison between the actual hardness and the standard hardness of materials will show whether the current batch of material is proper and suitable for use or not.
Strength of material is an ability to work within a load without failure of the material. Tensile and yield strength are critical properties in terms of material strength.
Tensile strength or ultimate tensile strength is the maximum stress on an engineering stress-strain curve. At this point, materials are plastically deformed but they may not be broken apart yet depending on types of materials.
Yield strength is a stress where a material starts to be in a plastic deformation region. Load applied to a material within yield strength will not permanently change a shape of material. Once the load is released, a material will come back to its original shape. However, if stress is more than yield strength, a material’s shape will be plastically deformed.
Determining yield strength from a stress-stain curve is not easy in many cases. Therefore, an offset yield point is usually defined as a practical standard. In order to find an offset yield point, it typically starts with 0.2% strain offset and then draws a parallel line to a stress-strain curve in the plastic region. An intercept point in a stress–strain curve with 0.2% strain offset is the offset yield point (Figure 1).
Elasticity & Plasticity
Elasticity is a material property to recover back to the original shape after stress is removed. On the other hand, plasticity happens when the applied stress exceeds yield stress of materials; thus, materials deform from the original shape permanently. When plotting a stress-strain curve, an elastic region is shown as a straight line and the ratio between stress and strain is called the Young Modulus of material. When stress is applied more than a yield point, the relationship between stress-strain does not behave as a straight line.
Toughness is the ability of a material to absorb energy before it is fractured. A tough material such as mild steel requires a huge amount of energy to break it apart, whereas a brittle material such as glass cannot absorb a lot of energy. Additionally, temperature has a big impact on the toughness of material. Some materials are very tough in standard temperature conditions but become brittle when operating in very cold conditions.
Ductility & Brittleness
Ductility is the ability of materials to plastically deform before they fracture when tensile force is applied. Ductile materials have a large scale of deformation prior to breaking apart. Conversely, materials that can withstand little or no plastic deformation before being parted are called brittle materials. Temperature has a big effect on ductility of material. A high temperature will increase ductility of material, while a cold temperature will decrease ductility so a material will be more brittle.
Malleability, which is a similar property to ductility, is the ability of solid materials to plastically deform by compressive force before they are fractured.
Effect of Corrosion on the Mechanical Performance
Corrosion damages both physical and mechanical properties of material. The following are effect of corrosion on mechanical properties.
Thickness reduction due to corrosion directly affects strength of materials. For example, 5” S-135 drill pipes premium class should have tensile strength of 436 klb; however, excessive corrosion damages internal and external surface area of drill pipes. The smaller surface area will result in reduction of tensile strength. Furthermore, it is very difficult to predict the strength of materials when localized corrosion occurs because a surface area of cracking don’t evenly distribute. Moreover, some of corrosive environments such as high temperature, high CO2&H2S, high chloride content, etc can dramatically degrade material properties.
Corrosion reduces toughness of materials because it can physically and chemically change properties of materials and tough material can be brittle. Additionally, low temperature environment can dramatically decrease toughness. Therefore, equipment used in low temperature conditions as subsea pipeline, subsea BOP, riser, etc must be designed to be able to work in very low temperature environment.
Corrosion can change ductile materials into brittle material and this causes failure of structure. Several situations leading to ductility reduction are low temperature, H2S & CO2 gas, cyclic load, etc.
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