Engineering: Design Failure Analysis: Why Structures Break
Investigating loads, stresses, materials, and failure modes
Engineering: Design Failure Analysis: Why Structures Break
Investigating loads, stresses, materials, and failure modes
Engineering - Grade 9-12
- 1
A steel cable in a small pedestrian bridge can safely carry 12,000 N before permanent damage begins. During a test, the cable is loaded with 8,000 N. Calculate the factor of safety for the cable. Then explain what the result means.
Factor of safety equals failure load divided by working load.
The factor of safety is 12,000 N divided by 8,000 N, which equals 1.5. This means the cable can carry 1.5 times the test load before reaching the point where permanent damage begins. - 2
A beam has a small crack on its bottom surface near the center, where bending tension is highest. Explain why this crack is dangerous even if it is small.
Think about where tensile stress is largest in a bending beam.
The crack is dangerous because cracks concentrate stress at their tips. Since the bottom center of the beam is already in high tension during bending, the crack can grow and lead to sudden fracture. - 3
A column in a building is made longer without increasing its diameter or changing its material. Describe how this change affects the risk of buckling.
Making the column longer increases the risk of buckling. Slender columns are less stable under compression and can bend sideways even when the material itself has not crushed. - 4
A bracket made of aluminum is repeatedly loaded and unloaded 50,000 times. It eventually breaks even though the load was always below the strength measured in a one-time test. Identify the likely failure mode and explain why it happened.
Failure can depend on the number of load cycles, not only the size of one load.
The likely failure mode is fatigue failure. Repeated loading can cause microscopic cracks to form and grow over many cycles until the part breaks, even when each individual load is below the static strength. - 5
A roof truss is designed for snow, wind, and its own weight. During a severe storm, wet snow builds up unevenly on one side of the roof. Explain why uneven loading can be more dangerous than a uniform load with the same total weight.
Uneven loading can create larger bending moments, twisting, or unbalanced forces in certain members. Even if the total weight is the same, some parts of the truss may carry much more load than expected and could fail locally. - 6
A rectangular wooden beam is 10 cm wide and 20 cm tall. Another beam has the same material and length but is rotated so it is 20 cm wide and 10 cm tall. Which orientation is better for resisting vertical bending loads, and why?
For bending, height usually matters more than width.
The beam that is 10 cm wide and 20 cm tall is better for resisting vertical bending loads. A taller beam has more material farther from the neutral axis, which greatly increases bending stiffness and strength. - 7
During an investigation, engineers find rust on steel reinforcement bars inside a concrete beam. Explain how corrosion can lead to structural failure.
Consider both the steel reinforcement and the concrete around it.
Corrosion weakens steel by reducing its cross-sectional area and strength. Rust also expands compared with the original steel, which can crack the surrounding concrete and reduce the bond between steel and concrete. - 8
A support plate has a circular hole drilled through it for a bolt. A crack begins at the edge of the hole. Explain why holes are common locations for cracks to start.
Holes are common crack starting locations because they interrupt the flow of stress through the material. The stress becomes concentrated around the edge of the hole, especially under tension or repeated loading. - 9
A concrete wall fails during an earthquake. It did not fall because of vertical weight alone, but because it was shaken side to side. Identify the type of loading involved and explain why it can be severe.
Earthquake loads change direction and magnitude over time.
The wall experienced dynamic lateral loading. Earthquake shaking can create rapidly changing sideways forces that cause bending, shear, cracking, and repeated stress reversals in structural elements. - 10
A bridge component has a calculated working stress of 90 MPa. The material yield strength is 250 MPa. Calculate the factor of safety against yielding. State whether a factor of safety of 2.0 is met.
Use the same units in the numerator and denominator.
The factor of safety is 250 MPa divided by 90 MPa, which is about 2.78. Since 2.78 is greater than 2.0, the design meets the factor of safety requirement against yielding. - 11
A tall signpost fails on a windy day at the base rather than halfway up the pole. Explain why the base is often the most critical location for this kind of structure.
The base is often most critical because the bending moment from wind is largest there. The base must resist the accumulated effect of the wind force acting along the height of the post. - 12
In a failure analysis report, an engineer writes that the failed part showed ductile behavior. Describe two signs that would support this conclusion.
Ductile materials usually deform visibly before final fracture.
Two signs of ductile behavior are noticeable plastic deformation before fracture and a rough, stretched, or necked fracture surface. These signs show that the material absorbed energy and changed shape before breaking. - 13
A brittle ceramic support breaks suddenly with almost no visible bending or stretching. Explain why brittle materials can be risky in structures that may experience impact or sudden loads.
Brittle materials can be risky because they often fracture with little warning and absorb relatively little energy before breaking. Impact or sudden loads can create high local stresses that cause rapid crack growth. - 14
A team redesigns a metal part by replacing a sharp inside corner with a rounded fillet. Explain how this change can reduce the chance of failure.
Sharp corners tend to create high local stress.
A rounded fillet reduces stress concentration compared with a sharp corner. By spreading the load path more smoothly through the material, it lowers peak stress and makes crack initiation less likely. - 15
A structure failed after a contractor substituted a lower-grade material without updating the design calculations. Explain why material substitutions must be reviewed by engineers before construction.
Material substitutions must be reviewed because different materials can have different strength, stiffness, toughness, corrosion resistance, and fatigue behavior. A lower-grade material may not meet the loads or safety factor assumed in the original design.