Engineering Students Understand the Elastic Neutral Axis, but What About the Plastic Neutral Axis?

Starting in engineering statics, undergraduate engineering students are taught how to find and calculate the elastic neutral axis (ENA) for a cross sectional shape by finding the centroid. The concept of the ENA is fundamental in terms of understanding and determining the flexural bending stresses and deflections of beams where plane strains remain plane. However, the plastic neutral axis (PNA) is seldom discussed in undergraduate engineering mechanics courses even in mechanics of materials. Perhaps this is due to the amount of course content in mechanics of materials. However, for students first learning about the PNA in a senior level structural steel course, bridging the gap back to fundamental mechanics is challenging since engineering mechanics courses are taken early on. This paper presents a review of how to locate the ENA and PNA and their purpose in flexural behavior of beams. A new calculus based expression for calculating PNA is presented. Engineering examples are provided. All of this information was presented, discussed and assessed with two groups of senior civil engineering students in two structural steel design courses. The results of this assessment are also presented. Results indicate that the concept and purpose of the PNA needs to be introduced to students earlier in their engineering coursework. The work presented herein is of interest to civil and mechanical engineering students, engineering faculty, and professional practitioners who are analyzing and designing steel beams for various types of engineering projects where flexural strength and capacity are important.