Work Hardening

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Work Hardening

Often in structural design, structural members are designed to be in service below the yield stress. The reason being that once the load exceeds the yield limit, the structural members will exhibit large deformations (imagine for instance a roof sagging) that are undesirable. Thus materials with larger yield strength are preferable.

**Figure 9:** After work hardening, the stress-strain curve of a mild steel (left) resembles that of high-strength steel (right).
$\begin{figure}\hfil \epsfxsize =4.0in \epsfbox{Fig/work_hardening.eps} \hfil \end{figure}$

When work hardening occurs, the hardness of the material increases considerably as the material is deformed.

We will for now concentrate on steel, a commonly used structural material. Mild steels have a yield strength somewhere between 36 and 50 ksi. When work-hardened, the yield strength of this steel increases. Work hardening is the process of loading mild steel beyond its yield point and unloading as shown in Figure 9. When the material is loaded again, the linear elastic behavior now extends up to point A as shown. The negative aspect of work hardening is some loss in ductility of the material. It is noteworthy that mild steel is usually recycled, for instance the tensile specimens tested to failure in the EGR 75 lab. Because of this, the yield strength may be a little higher than expected for the mild steel specimens tested in the laboratory.

**Figure 10:** State of stress for the tensile test.
$\begin{figure}\hfil \epsfxsize =5.0in \epsfbox{Fig/Mohrs_circle.eps} \hfil \end{figure}$

Next: Failure Analysis
Up: Tutorial
Previous: Brittle and Ductile Behavior

2003-06-27