Table of Contents
Equipment & Materials
Sources of Error
Table of Figures
Figure 1: Stress Strain Diagram for Structural Steel
Figure 2: Fractured specimens
In this lab, students will be testing A-36 Steel, 1018 Steel, and 6061 Aluminum undergoing Tensional loads. All tests will be performed on the Instron machine. This experiment will show students how stress-strain graphs correlate to the material properties of each specimen. Students can observe the properties of failure due to tension and further understand the ASTM standards and procedures. Theoretical Background
Strain may be defined as a normalized measure of deformation representing the displacement between particles in a sample relative to a known reference length. As a load is applied to the sample (generally a round bar) its change in length can be recorded and compared to its initial length to determine normal strain. In this case, strain is found through the following equation:
Strain is commonly referred to as either tension or compression depending on the direction of the force applied to the sample. Tensile strain generally refers to elongation of a material as a result of a tensile test. Compressive strain refers to the compressing, shortening of a material.
In this particular laboratory, tensile strain is to be measured through the application of an Instron and extensometer. In this case, the extensometer is mounted to the different samples in order to determine the extension (L) the sample undergoes during testing. This value is used to directly determine the normal strain (). Since the initial length (Lo) of the extensometer is one inch, the extensometer gives the strain value directly. Normal Stress:
In this laboratory, the Instron is used to apply a tensile load (P) to the sample. By measuring the sample’s diameter using calipers, one may calculate the sample’s cross-sectional area. Knowing these values allows one to calculate stress through the following equation:
After obtaining values for stress and strain, a plot may be created to obtain a visual representation of the stress-strain relationship. By looking at these plots one is able to see certain material characteristics that are imperative in the comparison of different materials. The first characteristic to be noted is the proportional limit. The proportional limit may be defined as the maximum stress for which the stress-strain plot remains straight [Figure 1], or in other words, the maximum stress for which Hooke’s Law applies. The next characteristic worth considering is the elastic limit [Figure 1]. The elastic limit marks the maximum stress a material can undergo from which the strains encountered will be recoverable. In this case, the ability of the material to recover to its original shape is referred to as resilience. Resilience generally only refers to materials that have not yet encountered plastic behavior.
Certain materials may have no proportional limit, but presumably all materials will have an elastic limit that in some cases will be quite small. In certain instances, the proportional limit must be estimated in order to calculate desired parameters. This estimation marks the point at which the stress –strain plot shows a definite change in shape. For most metallic materials, the elastic limit occurs slightly above the proportional limit. This implies that there is a non-linear segment between the proportional and elastic limits.
The next point of importance on the stress-strain plot is the yield stress. The yield stress is the point at which the...
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