Lab4

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University of Central Oklahoma Fracture Test - Experiment 4 Dylan Robinson, Joshua Jackson, and Kaleb Paddock Dr. Adnan Al-Ibadi Thursday 1:00 PM 10/20/22
Abstract Experiment 4 for the Strength of Materials laboratory covers a fracture test.[1] In this experiment, the slender column specimen is loaded into the testing apparatus (Universal Testing Machine or ZPM) and an axial force is applied to the specimen. In this particular experiment, we are asked to calculate a theoretical value for the fracture toughness that a brittle material can withstand, and then compare the experimental value with a finite element model.[2] Theory Equations associated with this experiment are located in the appendix. In aerospace sustainability engineering, structural members in aircraft develop internal stresses throughout their lifespan. Over time these members may develop small fractures, reducing the yield stress of the material. If left unabated, these members may fail. Engineers must use a variety of methods to discover these fractures. The inspection of structural members without disassembling a system oftentimes is the only viable method of determining the true condition of a metal. This type of inspection is called non-destructive inspecting or NDI. The most common type of NDI is a visual test method, where an engineer or technician visually determines the state of the material. Of course this method is least effective. Although there are many methods to perform this type of inspection, one of the most effective for fracture failures specifically, is crack testing with a liquid penetrant. In this test, a spray is applied to a test area that makes fractures more apparent. This test s’ effectiveness is determined by the experience of the inspector. These types of tests are also utilized in other fields of engineering, most notably civil engineering. Here engineers might use these methods to see if a structural design is meeting the outlined constraints throughout the lifespan of the structure.[3] Machines and Instruments ‘Figure 2’ shows the main machine used in this experiment. This machine applies an accurate and measured force onto our test specimen, as well as measures the displacement of the specimen due to the force applied. This ZPM is different from the standard equipment and is the same test stand utilized in the previous experiment, the Tension Test. Using this information, we are able to test our theoretical value for the critical force applied before the specimen that will cause it to break, or more specifically fracture. Experimental Method In this experiment, we rely heavily on the concepts of stress, strain, modulus of elasticity, and a few other key concepts specific to strength of materials. In this particular experiment, we employ some of the concepts
associated with determining the critical stress intensity factor, Kc. The critical stress intensity factor of materials has a multitude of applications in the engineering world, as one could imagine. Using the ZPM universal testing machine, and an acrylic test specimen, as well as some provided equations, we are able to calculate values for our fracture toughness. Figure 1: CT Test Specimen Diagram. Test Procedure For this experiment, our acrylic test specimen is loaded into the ZPM universal testing machine. The specimen is secured via three dowels as per the diagram above in figure 1, and shown below in figure 2. After the specimen is secured and the ZPM is calibrated, we begin applying force pulling the specimen apart. The displacement and the applied force is recorded to be employed in our equations for calculating the fracture toughness.
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Figure 2: Acrylic test specimen loaded into the ZPM.
Specimen Specifications In this experiment, we are performing a critical fracture test on an acrylic specimen. The specimen used was one of the compact tension specimen (CT) variety. In accordance with ASTM standard No. E399, the CT specimen is loaded into the ZPM universal testing machine and connected to it via a couple of dowel pins. The precise dimensions needed for the calculations included in our appendix section are listed below in table 1, and the figure below depicts the specimen loaded into the universal testing machine. Table 1: Specimen Dimensions
Results With the results from the ZPM aggregated in combination with the equation listed below, the value of fracture toughness is able to be calculated. The main equation employed to calculate the fracture toughness is: EQ. 1:Kic=(Pmax)(f)/(B)(W0.5) Here, ‘Pmax’ is the maximum load applied to the specimen before it breaks, ‘f’ is the shape factor for the crack, ‘B’ is the thickness of the specimen, and ‘W’ is the width. With the data from the ZPM provided (shown below in figure 3) we can calculate a value for our fracture toughness. The ZPM gave a lot of values for a lot of increments, so for the sake of saving space, we will only include the portion that gives us our maximum load applied. Figure 3: Table of Values from ZPM universal testing machine. From the table listed above, we are able to see that the maximum applied load before the specimen broke was of value 261.745 N. As shown in our appendix section, we have a value of Kic being equal to 4102974.4. Figure 4 shows the acrylic specimen fractured.
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Figure 4: Specimen after testing in ZPM. Validation and Discussion Given the data shown above in Figure 3, we were able to calculate the fracture toughness (Kic) from the maximum load value. This calculation is shown below in the appendix section. The maximum load value used for the calculation was 261.745 N. This value gave the specimen a fracture toughness of 4102974.4 Pa/m or 4.103 MPa/m.
Conclusion This test is important for understanding a material’ s characteristics. The methods shown in this experiment are the same methods used when determining which materials to use when building aircraft, automobiles, and building structures. This test allows engineers to obtain precise measurements while keeping parts cost low due to the small specimen size. In using these testing methods engineers are able to predict a material s performance characteristics with a high degree of certainty.
References [1] Khandaker, M. (2021). Laboratory Manual for Strength of Materials Lab. Department of Engineering and Physics (UCO) [2] Hibbeler, R. C. Mechanics of Materials . Pearson, 2022. [3] The 6 Most Important Methods for Crack Testing in Non-Destructive ... https://blog.foerstergroup.com/en/component-testing/the-6-most-important-methods-for- crack-testing-in-non-destructive-material-testing.
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Appendix