Fundmentals common trend of a steel. The yield
Fundmentals of Material Science
October 24, 2018
Cleofas, Kim Lester J.
Oreiro, Christer V.
Experimental Study on the Mechanical Behavior of EN08 Steel at Different Temperatures and Strain Rates
Steels are used in industries such as manufacturing, gas and oil, automotive, packaging, shipbuilding, transporting, and mining. EN08 carbon steel is a common medium carbon and medium tensile steel and known for its tensile strength and toughness. EN08 steel is also known because of its machinability quality condition. It is commonly used in manufacturing mechanical parts such as general-purpose axels, gears, studs, bolts, and shafts. The microstructure of EN08 steel is dependent on the heat that is being applied to it.
The graph shows the behavior of the steel at room temperature under three different quasi-static strain rates of 0.15s-1, 0.015s-1, and 0.0015s-1 respectively. It showed that the yield and ultimate stress increase as the strain rate increases. It is noted that it is common trend of a steel. The yield stress of the steel cannot be identified because of unclear yield region. The trend of strain hardening is independent of the strain rate and the main difference is because of the initial yield. This simply shows that quasi static response of the steel at room temperature is similar to the response of BCC metals where the flow stress is controlled by the yield stress.
Under the strain rate of 0.0015s-1 and 0.15s-1, the graph shows, as the temperature increases, the yield stress decreases. The stiffness of the yield stress were almost the same at the temperatures 523K and 723K regardless on the strain rate. On the other hand, it decreases on the temperature 923K. As far as we have concerned on the plastic region, strain hardening lessen completely at temperatures above 523K where the softening phenomenon happens immediately after the yield strength and where it becomes steeper due to damage-significant evolution. This shows that there is no strain hardening and on thermal stresses remain.
On the two tables shown above, we can clearly see that on the strain rate of 0.0015s-1 and 0.15s-1 at different temperatures. The graph becomes steeper at around 500K on lower strain rate and around 700K on the higher strain rate. The dynamic strain aging was present on the lower strain rate graph causing abnormal jumps of the graph. This phenomenon happens when diffusing solute atoms and mobile dislocations interact in the material. In most metals, the yield strength and ultimate tensile strength decreases as the temperature increases. This is because of rapid increase in atoms mobility, the diffusion-controlled processes impact at high temperatures, a greater mobility of dislocations, and increase of equilibrium concentration of vacancy.
The graph showed three different initial height, and also resulted to a different strain rates but with a slight increase in ultimate stress when the strain rate increases. It is also observed that the graph gave a close results on the energy that is being applied.
To fully understand the behavior of metals, its microstructure should be studied at failure. SEM or Scanning Electron Microscopy is a current analysis tool that can measure damaged areas that is interpreted in micro-graphs. It showed that void density is more noticeable as the temperature increases.
The graph shows the effect of temperature on the fracture mechanism by cavitation of the two given strain rates. The damage increases as the strain rate and temperature increases.
Regardless of the temperature, as the strain increases, the damage also increases. It is basically a characteristic same as most alloys. For each loading combination, the damage effect is lower on the initial stages and starts to increase on a higher scale as the plastic strain and, accordingly, the dissipated energy evolves which leads to degradation of the metal.