The effect of strain rate and temperature on the mechanical behaviour of mild steel

Steel is a widely used material in various applications, such as bridges, buildings, ships, and automotive. These applications are subjected to different types of loads, so it is crucial to understand the mechanical properties of steel under various loading conditions.

Understanding how strain rate and temperature affect the mechanical properties of steel is essential for designing and manufacturing high-quality products that can withstand various loading conditions.

This blog post will discuss how steel's strain rate and temperature impact its mechanical properties.

What is Strain Rate? How does it affect the mechanical properties of steel?

Stain rate is the rate at which steel deforms per unit time, usually expressed in units per second (s^-1). The strain rate is an important parameter affecting steel's mechanical properties. When steel is subjected to an external force or load, it deforms or changes shape. The extent of this deformation depends on the magnitude and duration of the applied load and the properties of the steel itself. 

For example, the strain rate experienced by steel during a slow bending process would be very low, while the strain rate during an explosive impact would be very high. The strain rate of steel can be affected by several factors, such as the speed of the applied load, temperature, and the type of loading. It can also vary depending on the specific type of steel, as different steel alloys can have different strain rate sensitivities.

Steel is ductile at low strain rates, it can withstand more stress without breaking, and can deform easily without breaking.

However, at high strain rates, steel loses its ductility and becomes brittle, which means it can fracture more easily without deforming. This impacts steel’s ability to absorb energy and increase its propensity to to fail catastrophically at high temperatures.

In general applications, the strain rate of steel is often an important consideration, particularly in areas such as impact testing, crashworthiness, and high-speed machining. Knowing the strain rate of steel materials can help engineers better understand and predict its behaviour under different loading conditions, which can be useful in optimizing designs and improving safety.

The Effect of Temperature on Steel's Mechanical Properties

The temperature also has a significant impact on the mechanical properties of steel. Steel becomes stronger but more brittle at low temperatures due to decreased ductility. 

On the other hand, steels ductility increases at high temperatures and can withstand additional stresses before breaking. However, at very high temperatures, steel can become soft and weak, leading to excessive deformation or failure.

Examples of strain rate and temperature affecting some of the common applications of steel:-

1. Automotive Industry: In the automotive industry, mild steel is widely used to manufacture structural components, such as car frames and chassis. High strain rate steel is typically used for safety-critical such as the car's safety cage, which is designed to absorb energy in the event of a crash. It can also help reduce the weight of the vehicle without sacrificing safety. On the other hand, lower strain rate steel is typically used for non-critical parts where fatigue resistance is more critical, such as the car's chassis, suspension system, and other non-critical parts. 

Steel is also subject to high temperatures in applications such as exhaust systems and engine components. High temperatures can cause steel to undergo thermal expansion, leading to dimensional changes and stress concentrations, potentially resulting in premature failure. Therefore, steel components used in high-temperature applications must be designed with proper allowances for thermal expansion and made of materials that can withstand the expected temperature range.

2. Construction Industry:  In the construction industry, the choice of strain rate of steel depends on the specific application and the desired mechanical properties. High-strain rate steel is often preferred for safety-critical parts of a structure, and where the steel components are subjected to dynamic loads such as wind and seismic forces such as the support beams in a bridge. In contrast, lower strain rate steel is more suitable for parts that are subjected to static loads, such as the building's frame. 

At high temperatures, steel can experience a loss of strength and ductility due to thermal softening, which can compromise the structure's integrity. On the other hand, steel structures exposed to low temperatures can experience reduced toughness and increased brittleness, making them susceptible to brittle fracture.

 3. Manufacturing Industry: In the manufacturing industry, mild steel is used to produce a wide range of products, such as pipes, fittings, and machinery components. The strain rate and temperature can affect the machinability and weldability of mild steel in these applications. For example, mild steel can exhibit improved machinability at low strain rates, making it easier to manufacture components with complex shapes. At high temperatures, however, mild steel can exhibit increased susceptibility to cracking during welding, reducing the quality of these components.

Understanding how strain rate and temperature affect the mechanical properties of steel is crucial for designing and manufacturing high-quality products that can withstand various loading conditions. The strain rate and temperature can significantly impact the ductility, strength, toughness, and load-carrying capacity of steel, which are essential for its various applications, including construction, automotive, and manufacturing industries. 

Therefore, engineers and designers should consider the effect of strain rate and temperature on steel's mechanical properties while selecting the appropriate material and designing structures or components to ensure their reliability, durability, and safety.