Materials Fatigue Failure - Lab Report Example

LABORATORY EXPERIMENT: FATIGUE TEST This laboratory study mainly focused on investigating the nature of materials’ fatigue failure through a practical experimental. An elaborate study of the underlying principles, stages and several factors that make fatigue failure such a complex engineering phenomenon was done. This was followed by thorough research works on the conventional fatigue test methods. The fatigue failure cycles and hardness data were recorded in a table for further experimental analysis. The data obtained from the experiment were then used to plot graphs. The data and graphs obtained were then analyzed with the help of engineering knowledge and principles. Fractographic machine and Optical microscope were used in studying the fatigue failures of each of the paper clips. The graphs plotted demonstrate the relationship between the changes in the number of cycles with respect to the paper size clip. Through careful analysis of the experimental results, mechanical properties of the material can be established. Table of Contents Abstract 1 Introduction 3 Experimental methodology 4 Fractography Tests 6 Results 6 Discussion of the results 11 References 13 Appendices: Experimental Data and Graphs 13 Appendix A: Hardness Measurement Results 13 Appendix B: Strain-life curve for sample 1 13 Appendix C: Strain-life curve for sample 2 14 Appendix D: Strain-life curve for sample 3 14 Introduction Materials such as ceramics (e.g., carbide, silicon, and porcelain), metals (zinc, aluminum, copper, iron, etc.); or polymers (milk jugs and other household utensils) are carefully tested by engineers and scientists through materials quality testing. Quality testing reveals certain materials’ mechanical properties, such the ultimate tensile stress, yield stress, etc. Ultimate tensile strength of a material is a measure of material’s strength. During use, material may degrade, causing it to fail at lower than its actual ultimate tensile stress. For instance, if a material is repeatedly loaded, material will undergo a failure known as fatigue. Fatigue failure normally occurs at much lower stresses than the material’s ultimate strength. Fatigues are common types of failures for materials and have been studied for decades. Fatigue failure occurs daily in the objects that we are familiar with. For instance, in airplanes, the wings undergo fatigue several cycles per flight. Bridges undergo fatigue each time a vehicle or people pass over them. However, the fact that a material undergoes a fatigue does not necessarily mean it will fail or break. Scientists and engineers conduct careful laboratory experiments to ensure that materials don’t fail when they are subjected to fatigues. They do so by designing with values way above the actual fatigue failure of the material. Fatigue properties of the various materials differ from one material to the other. Fatigue property of a material depends on the material’s source, quality, duration and type of stress. Given that paper clips are not high-quality materials, they are normally made from very cheap steels that have low tensile strengths and in cases where quality variability does not matter. This laboratory work that was conducted in groups of six people represents the measurements and evaluations of the paper clip hardness were measured with the use of hardness machine. A protractor was used to measure the maximum angle at which the paper clip fails. The main aim of this laboratory report is to illustrate how the different sizes of magnetic paper clips behave under different cyclic loads. Also, the report gives in-depth analysis of the results based on the paper clip hardness. Experimental methodology The laboratory was conducted using a protractor to measure the angle at which materials break, a hardness test machine to establish the clip hardness, and a Fractography machine to analyze the fractures of the magnetic paper clips. Three different types of magnetic paper clips were used, e.g. large medium and small. The large, medium and small were of thicknesses 4mm, 3mm, and 2mm respectively. Firstly, by the use of the wire cutter provided in the laboratory, a piece of steel wire was cut from each of the clips. Each piece was then mounted vertically with the help of a vice and a protractor as a reference point (datum). Using the provided angle on a protractor, the clip was bended to a certain predetermined angle then released back 0. This process forms a cycle of a fatigue. This procedure was repeated for all other paper clips. The range of angles chosen was between 0 to 90 degrees. Data was carefully recorded into a cycle- to -fail table. The photo shown below illustrates the first process of measurements. [Include the photo here] Secondly, the hardness of the samples was tested with the help of the hardness test machine (Vickers 432SVD). The hardness test machine was switched on. By looking straight in the optical microscope, two distant lines could be moved simultaneously by the left hand through the positioning knob. Thirdly, the right hand positioning knob was rotated until the two lines approached and almost got in touch. Care was taken to ensure that the lines were never overlapped. The ‘zero’ button was pressed so as to set the D1 and D2 values to 0.0um. Fourthly, the experimental sample was put on the anvil and then adjusted through the thumb wheel to the point when a clear image of the sample is formed. After this, the ‘Start’ button was pressed so that the indenter lowers onto sample and then the massage ‘Testing, Please wait’ is displayed on the screen. The sample was loaded in less than five seconds to avoid damages. The indenter was then allowed to move away from the sample, leaving a diamond-mark in the sample. Next, the filar-positioning knob was used to place the right and left line to the right and left-hand corners of the X D1 diagonal. Afterward, the X D1 measurements were entered by pressing the ‘enter’ key. This procedure was conducted for all other sample sizes. Lastly, all the samples were taken into the next laboratory for fractography tests. Fractography is defined as the scientific study of fracture surfaces in materials. The fractographic techniques are habitually employed in determining the possible causes of failures in engineering structures, more so in product failures and the practices of forensic engineering or failures analysis. In materials science, fractography is used in developing and evaluating theoretical models of cracks growth characteristics. Fractography Tests The sample was put into a Fractography Tests Machine. The camera was then adjusted in the X and Y plane to focus on the sample fracture. After this, the camera was zoomed and the photos of the fracture taken. Results The experimental data obtained is as presented in Appendix A. The strain-life curves for the three samples are shown in Appendices B, C and D. The photos of the samples fractures are as shown below: [I the photos here] $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Large clip bend angle 70 degrees $CM_DATE 2015-02-25 $CM_TIME 10:07:11 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 34.0 $$SM_MICRON_BAR 372.0 $$SM_MICRON_MARKER 1mm $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Large clip bend angle 70 degrees $CM_DATE 2015-02-25 $CM_TIME 10:07:11 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 34.0 $$SM_MICRON_BAR 372.0 $$SM_MICRON_MARKER 1mm $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Medium clip bend angle 70 degrees $CM_DATE 2015-02-25 $CM_TIME 10:02:39 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 40.0 $$SM_MICRON_BAR 219.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Medium clip bend angle 70 degrees $CM_DATE 2015-02-25 $CM_TIME 10:02:39 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 40.0 $$SM_MICRON_BAR 219.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Small clip bend angle 70 degrees $CM_DATE 2015-02-25 $CM_TIME 10:13:39 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 70.0 $$SM_MICRON_BAR 382.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Small clip bend angle 70 degrees $CM_DATE 2015-02-25 $CM_TIME 10:13:39 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 70.0 $$SM_MICRON_BAR 382.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Large clip bend angle 90 degrees $CM_DATE 2015-02-25 $CM_TIME 10:42:27 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 40.0 $$SM_MICRON_BAR 219.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Large clip bend angle 90 degrees $CM_DATE 2015-02-25 $CM_TIME 10:42:27 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 40.0 $$SM_MICRON_BAR 219.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Medium clip bend angle 90 degrees $CM_DATE 2015-02-25 $CM_TIME 10:44:28 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 44.0 $$SM_MICRON_BAR 240.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Medium clip bend angle 90 degrees $CM_DATE 2015-02-25 $CM_TIME 10:44:28 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 44.0 $$SM_MICRON_BAR 240.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Small clip bend angle 90 degrees $CM_DATE 2015-02-25 $CM_TIME 10:46:50 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 70.0 $$SM_MICRON_BAR 382.0 $$SM_MICRON_MARKER 500um $CM_FORMAT JEOL/EO $CM_VERSION 0.1 $CM_COMMENT Small clip bend angle 90 degrees $CM_DATE 2015-02-25 $CM_TIME 10:46:50 $CM_OPERATOR $CM_INSTRUMENT JCM-5000 $CM_ACCEL_VOLT 10.00 $CM_MAG 70.0 $$SM_MICRON_BAR 382.0 $$SM_MICRON_MARKER 500um Discussion of the results Materials hardness is one of the most significant mechanical properties of a material from science and engineering points. From the samples’ hardness tests, it can be noticed that the results are different. The results vary from one group to the other. For instance, in hardness test of small sample, Group 1got 288.8 as the result while Group 3 obtained 188.5 for the sample. This is a difference of 100. The question we ask is why the large variation in measurement. One possible reason is that Group 3 could have tested a sample that had already been tested. Mechanical strength of any material reduces after several tests. As can be observed, from the strain-life graphs of three paper clips, at smaller angles, there is a big difference in the number of cycles. As the chosen angles increase, there is a reduction in the number of cycles. The difference in number of cycles of the three samples is, however, very small. Bending the sample at an angle of 90 degrees causes much more deformations in the sample than when the sample bent by a small angle, say 10 degrees. Fatigue is defined as the weakening of a material due to cyclic loads. From the Fractography results, it can be observed that the fracture is coarse though the part cut by the wire cutter is smooth. Also, it can be noticed that the cracks start from one side but doesn’t reach the other end of the other end is torn off. Very small cracks can also be observed before the fracture. Conclusion From the above results and the previous discussions, it is clearly evident that increasing the angle of bending decreases the number of cycles. Also, another conclusion that can be drawn from this laboratory experiment is that the material’s initial mechanical conditions affect how it behaves when subjected to fatigue. For instance, material, which has been subjected to cyclic loading before is most likely to fail faster. References http://en.wikipedia.org/wiki/Fractography [Accessed: March 5. 2015] http://en.wikipedia.org/wiki/Fatigue_(material)#S-N_curve [Accessed: March 7. 2015 Appendices: Experimental Data and Graphs Appendix A: Hardness Measurement Results [Insert the results obtained here] Appendix B: Strain-life curve for sample 1 [Insert the Strain-life curve for sample 11here] Appendix C: Strain-life curve for sample 2 [Insert the Strain-life curve for sample 2 here] Appendix D: Strain-life curve for sample 3 [Insert the Strain-life curve for sample 3 here] Read More