Chapter 3 Engineering Disasters
Shortcuts in engineering design can lead to engineering disasters. Engineering is the science and technology used to meet the needs and demands of society. These demands include buildings, aircraft, vessels, and computer software. In order to meet society’s 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.
Importance of safety
In 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 tensile testing, finite element analysis (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.
Background of failure
Failure occurs when a structure or device has been used past the limits of design that inhibits proper function. If a structure is designed to only support a certain amount of stress, strain, 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.
Failure due to static loading
Static 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 yield strength and ultimate strength of a specific test specimen.
The 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’s ultimate tensile strength. The result is a stress-strain curve 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.
Failure due to fatigue
When a material undergoes permanent deformation from exposure to radical temperatures or constant loading, the functionality of the material can become impaired. This time–dependent plastic distortion of material is known as creep. 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.
In 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. Fatigue is known as the weakness in a material due to variations of stress that are repeatedly applied to said material. 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.
Fatigue 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:
- 1.) Crack initiation- when repeated stress creates a fracture in the material being used
- 2.) Crack propagation- when the initiated crack develops in the material to a larger scale due to tensile stress.
- 3.) Sudden fracture failure- caused by unstable crack growth to the point where the material will fail
Note that fatigue does not imply that the strength of the material is lessened after failure. This notion was originally referred to a material becoming “tired” after cyclic loading.
Failure due to miscommunication
Engineering is a precise discipline and in order to be precise, communication among project developers is pertinent for a successful product. There are several forms of miscommunication that can lead to a flawed design in a system. There are various fields of engineering that have to intercommunicate when working toward a mutual goal. These fields include civil, electrical, mechanical, industrial, chemical, biological, and environmental engineering. When creating a modern automobile, electrical engineers, mechanical engineers, and environmental engineers are required to work together to produce a fuel-efficient, durable product for consumers. If engineers do not adequately communicate among one another, a potential design could have flaws and be unsafe for consumer purchase. Engineering disasters can be a result of such miscommunication. Such disasters include the 2005 levee failures in Greater New Orleans, Louisiana during Hurricane Katrina, the Space Shuttle Columbia disaster, and the Hyatt Regency walkway collapse.
Failure due to software
Software has played a role in many high-profile disasters.
- Ariane 5 Flight 501
- Mars probe
- Denver International Airport – Automated baggage system
- TAURUS — UK share settlement system and dematerialised central share depository.
- Therac-25 — A radiation therapy machine responsible for six overdoses due to faulty software.
- Airbus A320 — The Airbus A320, while was controversial in software engineering circles, being the first civilian Fly-by-wire aircraft. In the Airbus flight control systems, the computer has the final say on all decisions, meaning the safety of passengers depends upon the accuracy of the software specification, and the competence of the engineering teams producing the (multiple, independent) software stacks. The Strasbourg A320 crash of Jan 21, 1992is partially related to software in that poor user interface design was a contributing factor.
- Failure at Dharan — Patriot Missile clock issue.
Infamous disasters in engineering
Engineering products and inventions are utilized everyday including computers, microwaves, and elevators. A broken microwave can have limited consequences; however, when larger projects such as infrastructures and airplanes fail, multiple people can be affected which leads to an engineering disaster. A disaster is defined as a calamity that results in significant damage which may include the loss of life. Large-scale engineering disasters are recorded in the history books. Just like any other mistake, these disasters become reminders and guidelines of how to improve and not repeat the same mistakes. In-depth observations and post-disaster analysis have been documented to a large extent to help prevent similar disasters from occurri