{"id":31,"date":"2018-03-31T19:45:14","date_gmt":"2018-03-31T23:45:14","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/engineeringinsociety\/?post_type=part&p=31"},"modified":"2019-05-28T20:06:51","modified_gmt":"2019-05-29T00:06:51","slug":"chapter-2-engineering-disasters","status":"publish","type":"part","link":"https:\/\/pressbooks.bccampus.ca\/engineeringinsociety\/part\/chapter-2-engineering-disasters\/","title":{"raw":"Chapter 3 Engineering Disasters","rendered":"Chapter 3 Engineering Disasters"},"content":{"raw":"https:\/\/en.wikipedia.org\/wiki\/Engineering_disasters<\/a>\r\n\r\nShortcuts in engineering design can lead to\u00a0<\/span>engineering disasters<\/b>. Engineering is the science and technology used to meet the needs and demands of society.<\/span>[1]<\/a><\/sup>\u00a0These demands include\u00a0<\/span>buildings<\/a>,\u00a0<\/span>aircraft<\/a>,\u00a0<\/span>vessels<\/a>, and computer software. In order to meet society\u2019s demands, the creation of newer technology and infrastructure must be met efficiently and cost-effectively. To accomplish this, managers and engineers have to have a mutual approach to the specified demand at hand. This can lead to shortcuts in engineering design to reduce costs of construction and fabrication. Occasionally, these shortcuts can lead to unexpected design failures.<\/span>\r\n\r\n \r\n

Importance of safety<\/span><\/h2>\r\nIn the field of engineering, the importance of safety is emphasized. Learning from past engineering failures and infamous disasters such as the Challenger explosion brings the sense of reality to what can happen when appropriate safety precautions are not taken. Safety tests such as\u00a0<\/span>tensile testing<\/a>,\u00a0<\/span>finite element analysis<\/a>\u00a0<\/span>(FEA), and failure theories help provide information to design engineers about what maximum forces and stresses can be applied to a certain region of a design. These precautionary measures help prevent failures due to overloading and deformation.[2]<\/a><\/sup>\r\n

Background of failure<\/span><\/h2>\r\nFailure occurs when a structure or device has been used past the limits of design that inhibits proper function.[3]<\/a><\/sup>\u00a0<\/span>If a structure is designed to only support a certain amount of\u00a0<\/span>stress<\/a>,\u00a0<\/span>strain<\/a>, or loading and the user applies greater amounts, the structure will begin to deform and eventually fail. Several factors contribute to failure including a flawed design, improper use, financial costs, and miscommunication.\r\n

Failure due to static loading<\/span><\/h2>\r\n
\r\n
\r\n\r\n\"\"<\/a>\r\n
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<\/a><\/div>\r\nStress\u2013strain curve showing typical yield behavior for ductile metals. Stress (\u03c3) is shown as a function of strain (\u03f5). Stress and strain are correlated through Young's Modulus: \u03c3=E\u03f5 where E is the slope of the linear section of the plot.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nStatic loading is when a force is applied slowly to an object or structure. Static load tests such as tensile testing, bending tests, and torsion tests help determine the maximum loads that a design can withstand without permanent deformation or failure. Tensile testing is common when calculating a stress-strain curve which can determine the\u00a0<\/span>yield strength<\/a>\u00a0<\/span>and\u00a0<\/span>ultimate strength<\/a>\u00a0<\/span>of a specific test specimen.\r\n\r\nThe specimen is stretched slowly in tension until it breaks, while the load and the distance across the gage length are continuously monitored. A sample subjected to a tensile test can typically withstand stresses higher than its yield stress without breaking. At a certain point, however, the sample will break into two pieces. This happens because the microscopic cracks that resulted from yielding will spread to large scales. The stress at the point of complete breakage is called a material\u2019s ultimate tensile strength.[4]<\/a><\/sup>\u00a0<\/span>The result is a\u00a0<\/span>stress-strain curve<\/a>\u00a0<\/span>of the material's behavior under static loading. Through this tensile testing, the yield strength is found at the point where the material begins to yield more readily to the applied stress, and its rate of deformation increases.[5]<\/a><\/sup>\r\n

Failure due to fatigue<\/span><\/h2>\r\nWhen a material undergoes permanent deformation from exposure to radical temperatures or constant loading, the functionality of the material can become impaired.[6]<\/a><\/sup>[7]<\/a><\/sup>\u00a0<\/span>This time\u2013dependent plastic distortion of material is known as\u00a0<\/span>creep<\/a>. Stress and temperature are both major factors of the rate of creep. In order for a design to be considered safe, the deformation due to creep must be much less than the strain at which failure occurs. Once the static loading causes the specimen to surpass this point the specimen will begin permanent, or plastic, deformation.[7]<\/a><\/sup>\r\n\r\nIn mechanical design, most failures are due to time-varying, or dynamic, loads that are applied to a system. This phenomenon is known as fatigue failure.\u00a0<\/span>Fatigue<\/a>\u00a0<\/span>is known as the weakness in a material due to variations of stress that are repeatedly applied to said material.[8]<\/a><\/sup>\u00a0<\/span>For example, when stretching a rubber band to a certain length without breaking it (i.e. not surpassing the yield stress of the rubber band) the rubber band will return to its original form after release; however, repeatedly stretching the rubber band with the same amount of force thousands of times would create micro-cracks in the band which would lead to the rubber band being snapped. The same principle is applied to mechanical materials such as metals.[5]<\/a><\/sup>\r\n\r\nFatigue failure always begins at a crack that may form over time or due to the manufacturing process used. The three stages of fatigue failure are:\r\n