Laboratory 6. Metals Tested in Tension. Jeremy Wheeless. Group Members: entire laboratory section. Submittal date: 4/10/2014

Transcription

1 Laboratory 6 Metals Tested in Tension Jeremy Wheeless Group Members: entire laboratory section Submittal date: 4/10/2014 Section Information: Thursday, 9:30a-1230p Laboratory TA: Wenyu Cai

2 Results and Discussion Contents Introduction... 1 Procedures... 1 Results and Discussion... 2 General Expectations and Information... 2 Comparative Analysis of Ultimate Strength... 4 Comparative Analysis of Young s Modulus... 4 Comparative Analysis of 0.2% offset yield strength... 5 Comparative Analysis of Ductility... 6 Conclusion... 8 References... 9 Appendix A... A1-A6 Appendix B... B-1 Appendix C... C-1

3 Introduction & Procedures Introduction Materials science is one of the most important and relevant industries with respect to engineering and manufacturing of safe and reliable products. Materials scientists test materials to define desirable and undesirable attributes and provide this information to the engineering community, which allows for the proper utilization of materials in specific applications. Typically these types of tests are performed to demonstrate the conformance of the material to the product specification" that has been created by an engineer (Ferron-Folliard). The primary focus of this report is to discuss an experiment that is done on metals that are placed in tension to the point at which they fail. The three metals to be tested and analyzed, steel, aluminum, and cast iron, are heavily used in the civil and architectural engineering industries. On the outset of this experiment, detailed information was provided in supplementary lab documentation about all three specimen with respect to their chemical compositions. This information allowed for assumptions to be made about how the metals would be predicted to perform when tested in tension. However, the information provided in the lab documentation was somewhat incomplete and required further testing and analysis of data. This report concludes with the results of this testing and provides for a more thorough understanding of each material that will allow a professional engineer to properly make use of the specific materials tested in specific applications. Procedures The execution of the procedures of this experiment were governed by ASTM E8/E8M Standard Test Methods for Tension Testing of Metallic Materials (Ferron-Folliard). Mild steel, aluminum, and cast iron were the specimens analyzed, and all three were tested using the 810 Material Test System, which is a product manufactured by MTS Systems Corporation. The testing machine applied tensile loads up to the point of failure for all specimens. Tests such as these are ordinarily, the only way to determine how materials behave when they are subjected to loads" (Gere-Goodno). The test was computer automated, generating a high level of accuracy in the deployment, and the data was recorded on 1/2 second intervals using a computer-controlled data acquisition system (Ferron-Folliard). During this test, both an imposed "constant rate of displacement" and "constant loading rate" were implemented to measure the data that is analyzed herein (Ferron-Folliard). All three specimens had approximately similar dimensions, which were machined such that failure would occur in a specific region of the specimen s volume. By machining the specimens in this way it allowed for the close study necessary to present findings with the high level of accuracy as those provided in this report. 1

4 Results and Discussion Results and Discussion General Expectations and Information In an effort to reduce redundancy in this report, general properties and expectations of the material response will be provided here. All three specimens differ in chemical composition, which greatly effects their response to the tensile loads that will be applied. Mild steel, and cast iron differ chemically in subtle ways, but the carbon content is the overwhelming governing factor that allows them to withstand tensile loading. Aluminum is chemically different than steel and cast iron in many ways. However, aluminum alloys can be formulated with strengths similar to steel products and exhibit many qualities similar to that of steel when placed under tensile loads. Given the specific chemical compositions, as well as the parameters of this experiment, it is expected that the steel will always outperform the aluminum and cast iron in terms of desirable attributes. These attributes include, ductility, modulus of elasticity, yield strength and ultimate strength, and will be discussed in detail in the body of the report. Further general expectations were extracted from external resources. The textbook titled Mechanics of Materials contains information about commonly used engineering materials. Close study of table H-3 in the textbook shows that a decreasing trend can be observed amongst the materials with respect to the performance in offset yield stress, ultimate strength, and ductility. From the text it can be deduced that the order of performance generally exhibited will be steel will as the highest, aluminum in the middle, and cast iron as the lowest. This table offers very general information, and lacks specifics about the exact components of the material tested in this experiment and is therefore only used as a way to check that the testing was properly administered. For illustrative purposes a stress-strain diagram (figure 1), which models the material response of all three specimens has been included in this report. A majority of the data presented in this report came from this figure by empirical methods. The diagram will include labels that illustrate regions in which values were empirically derived, as well as general regions of the diagram that are discussed. The diagram is very complex due to constraining all three materials to one plot. This constraint decreases fineness needed to properly represent the data. Due to this constraint, the diagram will not be referenced throughout the report but instead acts as an illustrative guide that shows where the values were found. It is worth noting that when all three material responses are placed on the same diagram it gives the observer a broad sense of how the materials differ, it however lacks detail that is needed to accurately determine the values provided in this report. Therefore, separate diagrams are also provided for each material, which will be referenced and can be found in the Appendix A: Engineering Calculations & Models. For all of the calculations and measurements provided in the following discussion, a calibrated measuring device was used for each procedure, and the figures conveyed herein reflect the significant figures provided by each measuring device. 2

5 Results and Discussion Figure 1: graphical modeling of stress and strain of the steel, aluminum, and cast iron specimens tested. This diagram displays the ultimate strengths of all three specimens, which are designated by a large X. The diagram also shows the elongation of each specimen. 3

6 Results and Discussion Comparative Analysis of Ultimate Strength A clear definition of stress, and ultimate strength helps in understanding the information provided in the following analysis. A stress is an internal force that acts on the molecular level which holds the material together. Stress is a measure of a force that acts over a given area of material. The ultimate strength, or often called ultimate stress, of a material is related to the maximum amount of load that can be imposed upon a given area of the material. With any material, after the ultimate stresses have been achieved through loading, the material will deform without any further increase of the load. In engineering practice, knowing the ultimate stress values associated with a given material allows for safe, appropriate, and affordable designs to meet specific applications of use. Some expectations with respect to the ultimate strength can be provided. It is common knowledge in civil engineering practice that cast iron is" weak and brittle in tension, but strong and less brittle in compression" (Ferron-Folliard). Therefore it is expected that cast iron will underperform when compared to steel. From Table 1 in Lab 6 Material Response to Tensile Loading, we can expect to observe the aluminum to have a minimum ultimate strength of 62 ksi. Table 1: Provided in this table are the tensile strengths of steel, aluminum, and cast iron. These values were determined empirically by carefully analyzing the data generated by the computer controlled acquisition system employed during the experiment. Steel Aluminum Cast Iron Ultimate Tensile Strength (ksi) The values of this table conform to the expectations previously listed. The ultimate stress of steel is greater than aluminum, which is greater than that of cast iron. The value determined for the aluminum shows that this specimen performed higher than the minimum value provided in the laboratory documentation. These results show that the test was performed correctly, and also offer information about these materials that would allow for proper implementation in an engineering application. Values reported in Table 3 were empirically determined from Figures A.1, A.2, and A.3 in Appendix A: Engineering Calculations & Models. Comparative Analysis of Young s Modulus When a force is applied to an object, the object physically responds. The response may be bending, buckling, stretching, or all of the above. In engineering practice the response is typically not something that should be observable in an engineered product, like a beam or column in a building. Most materials used in civil engineering practice are typically elastic up to a point, which means that if deformations occur, due to loading, the materials bounce back after the load is removed. When a material is loaded with tensile forces the material stretches. When the load is removed and was low in magnitude (i.e., below the proportional limit), the material returns to its original shape. Materials scientists have found that most materials respond proportionally to imposed loads. Which essentially means, if you apply more load you get more response. "For most metals, when a uniaxial load is applied, the metal deforms immediately and the stress [or internal forces] is proportional to the strain 4

7 Results and Discussion up to a certain load level" (Ferron-Folliard). This proportionality is quantified by what is called modulus of elasticity, which is often referred to as Young's Modulus, after [an] English scientist, Thomas Young ( ), in connection with an investigation [he performed] of tension and compression of prismatic bars (Gere-Goodno). Some expectations can be presented with respect to the comparison of Young s modulus amongst the specimens tested in this experiment. Due to a higher carbon content, "cast iron is significantly weaker and much less ductile compared to mild steel when tested in tension" which leads one to expect that the Young s modulus of the cast iron specimen will be lower than that of the mild steel (Mamlouk-Zaniewski). It is common knowledge in the civil engineering industry that aluminum alloys can be formulated with strengths similar to steel products, [however] the modulus of elasticity of aluminum is only about one-third that of steel (Mamlouk-Zaniewski). This knowledge allows one to surmise that steel will outperform both the aluminum and cast iron, with respect to their Moduli of elasticity, but do not imply any relevant information about the differences that will be noticed between the cast iron and aluminum. The testing methods used will allow for further discussion in this regard and no assumptions can be made without purely speculating, or retrieving information from the manufacturer of these specific specimens, which at this time is unavailable. Table 2: Young s Moduli for steel, aluminum, and cast iron specimens tested in this laboratory. Sample calculations for the values tabulated in this table can be found in Appendix A: Engineering Calculations. Steel Aluminum Cast Iron Young s Modulus (ksi) The values reported in Table 1 meet the expectations stated above, and provide insight into the differences between cast iron and aluminum. As stated, Young s modulus of steel is approximately three times larger than that of aluminum, and is also greater than that of the cast iron. From the results above it can be observed that the cast iron has a higher Young s modulus than the aluminum, which implies that this specimen exhibits more of a propensity to resist deformation than that of the aluminum specimen. Given these properties, if these materials were to be selected for a structural application in which a tensile load were to be applied, the first choice would be steel. This analysis alone does not however offer any insight as to how cast iron or aluminum should be selected. Sample calculations for the values found in Table 1 can be found in Appendix A: Engineering Calculations & Models. Comparative Analysis of 0.2% offset yield strength When considering the offset yield strength it is important to understand why it is used. The yield strength, often referred to as the yield stress or elastic limit is the transition between elastic and plastic behavior that can be observed when a material is undergoing tensile loading (Mamlouk- Zaniewski). When materials scientists, and engineers create graphs and models (stress strain diagrams) that represent this behavior for materials with differing chemical compositions, the models often differ. In some materials, such as cast iron, it is difficult to observe when the material actually moves into the inelastic response phase, and the determination of the yield strength becomes highly subjective. Due to 5

8 Results and Discussion the error that is involved with this subjectivity, the materials testing industry as a whole, has created a standardized method by which an arbitrary yield stress may be determined by the offset method, in which a straight line is drawn on the stress-strain diagram parallel to the initial linear part of the curve [i.e., Young s modulus] but offset by some standard strain such as (Gere-Goodno). The intersection of the offset line and the stress-strain curve defines the offset yield strength, affectively reduces ambiguity in modeling material response, and unifies the material testing industry. The.2% offset yield cannot be observed in Figure 1, because the resolution of this figure is too course to display an appropriate representation. Table 1 in Lab 6 Material Response to Tensile Loading, provides detailed information about the chemical composition of the aluminum specimen tested in this experiment and allows for a more exact expectation, which is that the minimum yield strength of the aluminum will be 45 ksi (Ferron-Folliard). Table 3: This table includes the offset yield strength values determined for all specimens tested. The determination of these values is through a graphical method and is therefore subjective, which produces the risk of error. The graphs that show this determination can be found in Appendix B: Modeling data. Steel Aluminum Cast Iron 0.2% Offset yield strength (ksi) The values in table 2 conform to the preliminary expectations as well as those outlined above. It can be observed that there is a decreasing trend between the offset yield strengths of the specimens, and that the aluminum specimen exceeds the minimum value expected. The offset yield strength of steel also conforms to the general expectation that steel will outperform the other materials. The values in Table 2 were empirically determined from figures A.1a, A.2, and A.3a found in Appendix A: Engineering Calculations & Models. Comparative Analysis of Ductility A proper definition of ductility will help in the interpretation of the results of this analysis. A very succinct and appropriate interpretation is as follows: Ductility is the property that enables a bar of steel to be bent into a circular arc or drawn into a wire without breaking. A desirable feature of ductile materials is that visible distortions occur if the loads become too large, thus providing an opportunity to take remedial action before an actual fracture occurs. Gere-Goodno 6

9 Results and Discussion Ductility is a characterization that allows for a comparison amongst engineering materials. This comparison may be established by measuring how much elongation, and reduction in cross sectional area which has occurred in a specimen when it has fractured due to tensile loading. Some expectations may be presented with respect to the ductility of the materials tested in this experiment. It is common knowledge that high strength, ductility, [and] the ability to carry tensile loads are the primary positive attributes of steel as a structural material (Mamlouk-Zaniewski). From table 1 provided in Lab 6 Material Response to Tensile Loading it can be expected that approximately 10% elongation will be observed from the aluminum specimen (Ferron-Folliard). It is also expect that due to a higher carbon content, "cast iron is significantly weaker and much less ductile compared to mild steel when tested in tension" (Mamlouk-Zaniewski). Given these expectations it is assumed that the materials will conform to the general expectations laid out previously. Table 4: presents the percentages of reduction in cross-sectional area and elongation of all specimens tested in this experiment. Steel Aluminum Cast Iron Percent Elongation (%) Percent Reduction in Area (%) From the values found in this table in can be again observed that the amounts conform to the general expectations given. The figures in the table also show that steel will show approximately 33% elongation prior to failure. This amount is amazing! What is observed is that this specimen will stretch by almost a third of its own length prior to failure. This shows why steel is the material of choice in structural applications. If the steel is overloaded, building inspectors may notice deformations that might occur and would be able to take remedial actions if found in time. Sample calculations for the values shown in Table 4 can be found in Appendix A: Engineering Calculations & Models. 7

10 Conclusion Conclusion This experiment has given many insights into the typical behavior of metals loaded in tension. It has shown that iron rich metals such as cast iron and mild steel can have varying attributes that depend upon their chemical composition. The results and discussion of Young s modulus, ductility, and ultimate strength show that the experiment, calculations, and measurements employed therein, all yielded outcomes that show that steel performs better than both aluminum and cast iron in terms of desirable attributes for engineering applications. These results also help to explain why steel is one of the most commonly used civil engineering materials used in tensile loading applications. The discussion of offset yield strengths shed light on a common engineering practice that unifies the material science field of study, by reducing ambiguity in a commonly needed and used value. The offset yield discussion also helped to further prove that steel is more desirable for tensile applications than cast iron or aluminum. With the results from the tests performed on the cast iron and steel, and the analysis performed in this document a more complete, and robust list of properties is now available that can now accompany the detailed information previously provided for the aluminum specimen. Given this detailed information, the specific materials tested in this experiment can now be used in specific applications, with a higher level of confidence in their performance. Higher levels of confidence correlate to safer products, and peace of mind. 8

11 References References Ferron-Folliard. Laboratory 6, Tensile Testing of Aluminum, Mild Steel, and Cast Iron. Austin: Department of Civil, Architectural and Environmental Engineering, University of Texas at Austin, print. Gere-Goodno. Mechanics of Materials. Toronto: Cengage, Book. Mamlouk-Zaniewski. Materials for Civil and Construction Engineers. New Dehli: Pearson, R-1

12 Appendix A: Engineering Calculations & Models A-1

13 Appendix A: Engineering Calculations & Models A-2

14 Appendix A: Engineering Calculations & Models Appendix A Figure A.1: Stress-Strain diagram illustrating the entire material response to loading. This figure demonstrates the ductility and ultimate stress level of the steel specimen tested in the experiment. A-3

15 Appendix A: Engineering Calculations & Models Figure A.1a: Stress-Strain diagram illustrating the entire material response to loading. This figure demonstrates the.2% offset yield strength of the steel specimen tested in this laboratory. Other labels on this graph are for reference only. A-4

16 Appendix A: Engineering Calculations & Models Figure A.2: Stress-Strain diagram illustrating the entire material response to loading. This figure demonstrates the ductility, ultimate stress level, and.2% offset of the cast iron specimen tested in the experiment. A-5

17 Appendix A: Engineering Calculations & Models Figure A.3: Stress-Strain diagram illustrating the entire material response to loading. This figure demonstrates the ductility, and ultimate stress level of the aluminum specimen tested in the experiment. Figure A.3a: Stress-Strain diagram illustrating the entire material response to loading. This figure demonstrates the ductility, and ultimate stress level of the aluminum specimen tested in the experiment. A-6

18 Appendix B: Measured Data Appendix B Steel Specimen Data Before Testing Diameter1 (in) Diameter2 (in) Diameter3(in) Average (in) Average Area (in.²) After Testing Diameter1 (in) Diameter2 (in) Average (in) Average Area (in.²) Maximum Load (kips) Aluminum Data Before Testing Diameter1 (in) Diameter2 (in) Diameter3(in) Average (in) Average Area (in.²) After Testing Diameter1 (in) Diameter2 (in) Average (in) Average Area (in.²) Maximum Load (kips) Cast Iron Data Before Testing Diameter1 (in) Diameter2 (in) Diameter3(in) Average (in) Average Area (in.²) After Testing Diameter1 (in) Diameter2 (in) Average (in) Average Area (in.²) Maximum Load (kips) 7.94 B-1

19 Appendix C: Notations Appendix C Stress: σ = F A i Stress: σ Force: F Initial cross-sectional area: A i Strain: ε = σ E Strain: ε Young s Modulus: E Instantaneous cross-sectional area: A Percent Elongation: %E Length at fracture: L f Original length: L 0 Percent area reduction: %RA Original area: A 0 Area at fracture: A f Young s Modulus: E = σ ε True Stress: σ T = F A %E = L f L 0 L %RA = A 0 A f A C-1