Lab Report 2 - This experiment aims to provide a practical demonstration of fatigue testing PDF

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Summary

This experiment aims to provide a practical demonstration of fatigue testing of materials. The
specimen used for the experiment is mild steel. This report summarizes the objectives of the
experiment, the apparatus used throughout the experiment, followed by an introduction and
theo...

Description

INSTITUTE OF SPACE TECHNOLOGY

March 8

Experiment# 2 Fatigue testing of materials

2018

GROUP A 1 (AERO 15-A)

SUBMITTED TO:

SIR AHMED HAFEEZ SUBMITTED BY: SAFI UL HAQ (160101001) FARRUKH JAVED (160101006) FAIZAN ANJUM (160101007) ASAD AKHLAQ (160101011) HAMZA MASOOD (160101013) MOHSIN ALI (160101036)

MECHANICS OF MATERIALS LAB REPORT

Table of Contents Abstract: .................................................................................................................................................. 3 Objectives: .............................................................................................................................................. 4 Apparatus ................................................................................................................................................ 4 Fatigue Testing Machine: .................................................................................................................... 4 Lathe Machine: ................................................................................................................................... 4 Introduction ............................................................................................................................................ 5 Theory ..................................................................................................................................................... 5 Fatigue failure: .................................................................................................................................... 5 Fatigue testing: ................................................................................................................................... 6 Types of fatigue testing:...................................................................................................................... 7 Experimental procedure ......................................................................................................................... 8 Readings and Graphical Visualization ..................................................................................................... 9 Conclusion: ............................................................................................................................................ 10 References ............................................................................................................................................ 10

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Abstract: This experiment aims to provide a practical demonstration of fatigue testing of materials. The specimen used for the experiment is mild steel. This report summarizes the objectives of the experiment, the apparatus used throughout the experiment, followed by an introduction and theory regarding the experiment. Furthermore the experimental procedure is mentioned along with the results and graphical visualization.

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Objectives: • •

To demonstrate the use of the fatigue testing machine for testing specimens subjected to cyclic (fatigue) loadings. To analytically approximate the fatigue damage accumulated in a part which is subjected to a known fatigue spectrum.

Apparatus Fatigue Testing Machine: Using this bench top unit, the basic principles of fatigue testing can be conveyed. A rotating metal test bar, clamped at one end, is loaded using a spring balance with a point force. This results in a cyclic bending load on the cylindrical specimen. The amplitude of the cyclic loading can be continuously adjusted using a threaded spindle with hand wheel. After a certain number of load cycles, the specimen breaks as a result of material fatigue. In this case the machine is shut down automatically by the stop switch. The number of load cycles is counted by an electrical counter and displayed digitally.

Figure 1 Fatigue Testing Apparatus

Lathe Machine: A lathe is a tool that rotates the work piece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, turning, with tools that are applied to the work piece to create an object with symmetry about that axis.

Figure 2 Lathe Machine

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Introduction Fatigue of metals is a well-known situation where yielding (and then rupture) can be caused by a large number of stress variations (magnitude and direction) at a point even though the max stress is less than the yield stress and respectively the ultimate stress.

Theory Fatigue failure:

Figure 3: Fatigue Fracture with Benchmarks

A fatigue failure begins with a small crack resulting from a tensile stress at a macro or microscopic flaw. Once started, the crack will develop at a point of discontinuity in the material, such as change in cross section, a keyway or a hole. Less obvious points at which fatigue failure is likely to begin are internal cracks or even irregularities caused by machining processes. In other words, when a load below yield strength of a material is applied repeatedly to a metallic specimen, localized hardening occurs. Then a small crack appears, this crack is a line of stress concentration which causes it to grow. As the crack grows, the cross sectional area of the material gets smaller until it can no longer support the load. This loading on which the fracture takes place is called fatigue loading and the fracture is called fatigue failure.

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Figure 4 Appearance of typical fatigue fracture surface

Cracks generally start at the surface of the material. As the crack grows, the two surfaces rub against each other, polishing both surfaces to a dull metallic finish. The fractured surface shows the sign of plastic deformation as well as crystalline finish.

Fatigue testing:

Figure 5: Fatigue Testing Machine

To determine the strength of materials under the action of fatigue loads, specimens are subjected to repeated or varying forces of specified magnitudes. For the rotating test a constant bending load is applied and the number of revolutions of the specimen required for failure is recorded. The first test is made at a stress which is somewhat under the ultimate strength of the material. The second test is performed at a stress less than that used in the first. The process is continued and the results are plotted as an S-N graph.

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Figure 6: SN Graph

A prediction of failure for various stress levels can be made by studying a material’s S-N curve. The most important part of the curve is often the portion to the right of the bend (or knee) in the curve that identifies what is termed the Endurance Limit or the Fatigue Limit. The Endurance Limit defines the stress level below which the material will theoretically withstand an infinite number (~108) of stress cycles without fracture.

Types of fatigue testing: 1. The Rotating Bending Testing Machine is similar to the original railroad axle-type Wohler used where the bending moment is constant along the beam length. Each point on the Surface of the Rotating Bend Specimen is subjected to fully-reversed cycling (σm = 0) and the tests are generally Constant Amplitude. 2. Reciprocating Bending Testing Machines utilize a rotating crank to achieve a non-zero mean stress through positioning of the specimen with respect to the motor. 3. Similar to the Reciprocating Bending set-up, the Direct Force Fatigue Testing Machines apply axial loads. 4. None-the-less, computer controlled "Servo hydraulic Systems" are the most common in operation today. Computerized Servo hydraulics is capable of both monitoring and controlling a desired cyclic pattern in any of the following: • Load-Time • Strain-Time • Displacement-Time 5. Component Testing or Full-Scale Prototype Tests are still carried out on Fatigue-Critical structures. Most fatigue testing is conducted at the basic material level; differences between laboratory and service conditions must be considered by mechanical designers.

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Experimental procedure 1. First of all we performed turning on the specimen to reduce it to the size (7.8 mm in diameter) we needed for this experiment. 2. Then we used sand paper to make its surface smooth so it can perform more efficiently during the experiment. 3. Then we clamped one end of the specimen into the adapter at the shaft end and the other end into the adapter at the load end. 4. After that we applied approximately 240 N loads to have a precise bending loading condition. Any load above 250 N was highly prohibited as it would have damaged the specimen. 5. Then we set the revolution counter to zero and started the motor. The revolution counter noted the number of cycles after which the failure occurred. 6. The test terminated itself through the fracture of the specimen, opening the micro switch and hence stopping the motor. 7. We noted the time it took for the fracture to occur from the computer connected with the apparatus. 8. From the time taken for fatigue failure, we calculated the number of cycles to failure [N = RPM × time for failure (min)]. 9. Then we noted the appearance of the fractured surface. Two different colors appeared at the fractured area, one darker area and one brighter one. The darker area for striations, slow crack growth and brighter ones for sudden fracture. Using results which the computer displayed along with the Force (N)-Revolutions graph for this experiment we obtained the endurance limit.

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Readings and Graphical Visualization The image below shows the force exerted on the specimen and the number of revolutions it took for fracture to occur. It also contains a graph of the force acting on the specimen at a certain number of revolutions.

Figure 7

The next two images give us a table of the force acting on the specimen taken at various numbers of revolutions

Figure 8

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Figure 9

At the end of the experiment, the image below shows the appearance of the specimen.

Figure 10: Specimen after experiment

Conclusion: From this experiment it is concluded that fatigue is a material property which varies from material to material. Initially the material starts breaking slowly and then breaks faster with increasing cycling load.

References http://www.mutiaranata.com/product/detail/wp-140-fatigue-testing-machine https://en.wikipedia.org/wiki/Lathe http://www.materials.unsw.edu.au/tutorials/online-tutorials/1-fatigue-testing

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