Mechanical failure refers to when a mechanical component or system stops functioning as intended due to damage, wear, imperfect design, fatigue, or other causes. It is sometimes referred to as structural failure or fracture failure. Mechanical failure occurs when the applied stresses exceed the strength of the material or component.
Understanding mechanical failure is important for engineers, manufacturers, and operators of mechanical systems. Identifying and preventing failure modes improves safety and reliability. Some critical applications like aircraft and medical devices have very low tolerance for failure. Mechanical failure analysis also informs material selection, design improvements, quality control, and maintenance programs.
This article provides an overview of the common types, causes, and prevention of mechanical failure. Real-world examples help illustrate the impact for engineered systems and products. The goal is to explain mechanical failure modes in an accessible way for non-experts interested in materials, design, and reliability.
Types of Mechanical Failure
There are several common types of mechanical failure in materials:
Brittle vs Ductile Failure
Brittle failure occurs suddenly without plastic deformation and is characterized by rapid crack propagation. Ductile failure involves significant plastic deformation and energy absorption before fracture (Failure Modes: Understand the 5 most common failure types).
Fatigue Failure
Fatigue failure occurs when a material is subjected to cyclic loading and unloading. Microcracks initiate and grow, eventually reaching a critical size to propagate suddenly (Failures and failure modes in materials).
Creep Failure
Creep failure occurs when materials are subjected to a constant load level for an extended period of time, causing slow deformation until fracture. Creep failure depends on temperature, time under load, and material properties.
Corrosion Failure
Corrosion failure occurs when a material reacts with its environment, often due to electrochemical oxidation, and degrades over time leading to loss of function.
Causes of Mechanical Failure
Mechanical failure of materials can occur due to a variety of causes. Some of the most common causes include:
Flaws in the material itself: All materials contain some flaws such as voids, impurities, and variations in grain size and orientation. These flaws weaken the material and make it more prone to failure under stress. Manufacturing defects: Mistakes during manufacturing such as inadequate heat treatment, surface cracks, improper processing methods, etc. can introduce flaws that lead to premature failure. https://www.infinitiaresearch.com/en/news/failures-and-failure-modes-in-materials/
Improper design: Failure to account for operating stresses, material properties, safety factors, and environmental conditions in the design can lead to failure. The component may be too thin or small to withstand expected stresses.
Improper use: Using a material or component outside its designated operating conditions, overloading it beyond its capacity, or misaligning/misassembling it can induce stresses beyond its strength leading to failure.
Wear and tear: Prolonged use causes surface damage and material degradation leading to crack initiation and eventual failure. Vibration, corrosion, and fatigue loads over time contribute to wear and tear.
Factors Affecting Failure
There are several key factors that can lead to mechanical failure in materials:
Temperature – Materials become weaker at high temperatures and stronger at low temperatures. Exceeding the maximum service temperature can cause components to fail. According to [1], extreme high and low temperatures can lead to “thermal shock” failures.
Loading rate – The rate at which a load is applied greatly affects the point at which a material fails. Rapid loading typically lowers the failure point. According to [2], fast loading rates can cause brittle failure in materials that would otherwise fail ductilely under slow loading.
Corrosion – Corrosive environments weaken materials over time by degrading their chemical bonds. Stress corrosion cracking is a dangerous form of corrosion-assisted failure. According to [3], corrosion damage is a leading cause of mechanical failure in metals.
Fatigue cycling – Fluctuating loads and cyclic stresses eventually cause fatigue failure. Cracks initiate and grow with each stress cycle until final sudden fracture. According to [2], fatigue is responsible for 50-90% of all mechanical failures.
[1] https://injury.arnoldsmithlaw.com/five-types-of-mechanical-failures-that-cause-car-accidents-in-th.html
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9266056/
[3] https://www.lowmanlawfirm.com/blog/bid/63142/top-5-mechanical-failures-that-cause-car-accidents
Brittle vs Ductile Failure
Brittle and ductile are two distinct failure modes that materials can experience under an applied stress. The main difference between brittle and ductile failure is in the ability of the material to undergo plastic deformation before fracture.
Brittle fracture occurs without any appreciable plastic deformation. It happens suddenly when the applied stress reaches the ultimate tensile strength of the material, causing it to break apart. Brittle fracture surfaces tend to be flat with no plastic deformation. Materials like glass, ceramics, and some metals like cast iron show brittle failure.
Ductile failure, on the other hand, involves significant plastic deformation before the material fractures. The material elongates and deforms plastically first before breaking. Ductile fracture surfaces have a fibrous appearance and necking down of the material. Materials like structural steel and aluminum alloys undergo ductile failure.
The main difference arises from the atomic structure. Brittle materials have strong interatomic bonds but limited ability for dislocations to move. Ductile materials have more free space between atoms and weaker bonds, allowing dislocations to move easier. Temperature also affects – brittle materials become more ductile at higher temperatures.
Understanding the failure mode helps predict behavior and prevent catastrophic failure. Brittle materials fail suddenly without warning, while ductile materials deform visibly first. Design and material selection can promote ductile over brittle failure.
Source: Chapter 11 Fracture of Materials
Fatigue Failure
Fatigue failure occurs when a material fractures after repeated cyclic loading at stresses below the material’s ultimate tensile strength. According to Fatigue Failure – an overview | ScienceDirect Topics, fatigue failure is caused by the initiation and propagation of cracks that develop due to repeated cyclic stresses. There are three stages of fatigue failure:
1. Crack initiation – Repeated cyclic loading causes slip deformation and induces plastic strain at the microscopic level, initiating microcracks at the surface.
2. Crack propagation – With continued cyclic loading, the microcracks grow and coalesce into dominant cracks that propagate through the cross section.
3. Sudden fracture – As the dominant crack reaches a critical size, the remaining uncracked ligament becomes overloaded, resulting in sudden brittle fracture.
Some key factors that affect fatigue failure include stress range, mean stress, surface finish, temperature, corrosion, overloads, etc. Materials can undergo brittle or ductile fatigue fracture depending on whether plastic deformation accompanies crack propagation.
Creep Failure
Creep failure occurs when a material is subjected to stress over an extended period at high temperature https://www.xometry.com/resources/materials/creep-failure/. This causes the material to slowly and permanently deform until it fails. Creep is characterized by three stages:
- Primary creep – during the initial period, the strain rate decreases as deformation mechanisms activate
- Secondary creep – the strain rate is relatively constant due to a balance between strain hardening and thermal softening
- Tertiary creep – the strain rate accelerates as internal voids and cracks develop until final failure
The main mechanisms of creep deformation are dislocation glide and climb at low stresses, and diffusion flow at higher stresses and temperatures https://industrialphysics.com/knowledgebase/articles/creep-failure-vs-fatigue-in-metals/. Creep is more likely to occur in materials that are subjected to high temperatures for a long time, such as turbine blades, reactor cladding, and rocket engine nozzles. Preventing creep involves designing components to minimize stress, operate at lower temperatures, and allow for material creep.
Corrosion Failure
Corrosion failure occurs when a material deteriorates due to chemical reactions with its environment. Corrosion leads to loss of material, which can compromise the integrity and load-bearing capacity of components (Corrosion Failure, 2022).
The most common types of corrosion failures are uniform corrosion, galvanic corrosion, crevice corrosion, pitting corrosion, intergranular corrosion, and stress corrosion cracking. Uniform corrosion causes general thinning of the material, while localized corrosion like pitting leads to penetration at weak spots. Galvanic corrosion happens when two dissimilar metals are in contact in an electrolyte. Crevice corrosion occurs in gaps and interfaces due to concentration effects. Intergranular corrosion progresses along grain boundaries. Stress corrosion cracking is caused by the combined effects of tension stress and corrosive environment (Recognition and Analysis of Corrosion Failure Mechanisms, n.d.).
Corrosion degrades the mechanical properties of materials like strength, ductility and fracture toughness. Loss of material leads to reduced load bearing capacity. Cracks initiated by corrosion concentrate stress and can lead to sudden fracture. Pits, perforations and increased surface roughness due to corrosion also create stress concentrations. Preventing corrosion through design, coatings, cathodic protection, inhibitors etc. is key to avoiding corrosion-induced failures (Recognition and Analysis of Corrosion Failure Mechanisms, n.d.).
Preventing Mechanical Failure
There are several ways to help prevent mechanical failure in equipment and machinery:
Material Selection
Choosing the right materials for the application is crucial. Materials should have adequate strength, toughness, and durability for the expected stresses and operating conditions. Using overly brittle materials prone to fatigue or creep failure should be avoided. Consulting material selection guides and standards can help identify appropriate materials (https://www.linkedin.com/advice/3/how-do-you-prevent-mechanical-failures-skills-mechanics).
Design
The design process must account for potential failure modes and incorporate safety factors. Critical components should be over-designed and redundancies can be built-in. Proper modeling, simulation, and analysis verifies the design and uncovers weaknesses. Design principles that improve reliability include avoiding stress concentrations, managing fatigue loads, facilitating maintenance, and minimizing wear (https://tractian.com/en/blog/mechanical-failures-prevention).
Maintenance
Preventative maintenance examines equipment regularly to spot issues before failure occurs. Maintenance tasks include lubricating, cleaning, tightening fasteners, replacing worn components, and checking alignment. Nondestructive testing methods help detect flaws like cracks. Monitoring equipment condition and performance data can reveal problems.
Quality Control
Strict quality control ensures components meet specifications. Inspection validates proper fabrication, assembly, installation, and acceptable dimensional tolerances. Testing qualifications, certifications, and standards compliance reduces risk of premature failures.
Real-World Examples
Case studies of major mechanical failures can provide important lessons on how to prevent similar incidents in the future. Some notable examples include:
The collapse of the Hyatt Regency walkways in Kansas City in 1981, which killed 114 people and injured over 200 more. This incident was caused by a last-minute design change that doubled the load on the walkways without providing adequate load-bearing capacity (https://monolith.asee.org/documents/sections/2012/Midwest/ASEE_Midwest_2012_Thomas.pdf). Key lessons were to carefully review design changes and ensure adequate safety factors.
Failure of the O-rings in the Space Shuttle Challenger’s solid rocket booster in 1986, causing the shuttle to break apart shortly after launch. The failure was attributed to the O-rings becoming brittle in cold launch temperatures. This highlighted the need to fully test critical components under the most extreme expected operating conditions (https://enterpriseis.com.au/mechanical-failure-investigation-case-study/).
The Deepwater Horizon oil rig explosion and spill in 2010, caused by a blowout of the well the rig was drilling. The blowout preventer failed to activate properly due to a faulty design and lack of redundancy. This emphasized the importance of designing critical safety systems with multiple backup measures (https://www.synthx.com/mechanical-failure-forensic-analysis/).
Studying past failures helps engineers design more robust systems, choose higher safety factors, implement more testing, and install redundant safety features where needed. Thorough investigation provides invaluable data to prevent recurrence of similar events.
