2023 ASEE Annual Conference & Exposition

The global pandemic caused higher education materials laboratory instructors to think differently and become adaptable to sudden changes; for example, the need for classes and labs to abruptly switch to online learning. The study of material properties is important in mechanical engineering classes including Mechanics of Materials, Manufacturing, and Design. Hands-on learning is a key factor for mechanical engineering students to thoroughly understand material properties and their impact on physical behaviors, practical applications, and design principles. This raises the question of whether a stress-strain measurement apparatus can be designed more cost-effectively and on a smaller scale than brick-and-mortar scale instruments found in conventional teaching labs. Among the benefits of a small, inexpensive device is enabling students to 1) perform stress-strain experiments themselves, 2) better understand equipment and procedures, and 3) observe and measure properties in each material being studied. An inexpensive bench top stress-strain apparatus would also be useful in classes where material properties are important to know via direct measurement but this knowledge is tangential to the class’s core curriculum – for example, Capstone senior design. Previous work has revealed the existence of a stress-strain sample thickness threshold below which tests return material properties (e.g., Young’s Modulus) inconsistent with measured bulk properties. This empirical threshold must be identified to determine minimum sample thicknesses for a tabletop take-home stress-strain measurement apparatus that returns results consistent with tabulated bulk material properties. This sample thickness governs how much force the apparatus must produce to drive parts to failure. Hence, it is the central design parameter for a tabletop stress-strain tester. This paper’s goal is to show a process to determine what thickness samples undergoing stress-stain tensile testing produce material properties consistent with bulk materials for three common engineering metals (aluminum, brass, and steel).

The sample “thinness” issue was systematically studied to determine how tensile properties are affected by part thickness and to reveal the point at which experimental tensile test results produced using a benchtop stress-strain tensile tester agree with tabulated values. Waterjet cut specimens of identical shape but with increasing thickness were tensile tested to failure on an Instron 5967 Universal Testing Machine. Mechanical properties and the way properties vary with sample thickness and metal type were then studied to find the thickness threshold where bulk material properties emerge.