A complete guide to Metal Failure Modes and Analysis

26 February 2025

What is a Failure Mode in metal?

A failure mode in metal is the specific way a metal part fails due to repeated mechanical, thermal, or chemical stress. These failure modes may be:

Fracture failures
Fatigue failures
Corrosion-induced cracking

Each failure mode is characterised by distinct physical, chemical, and microstructural changes in the material. Some failures, such as ductile fracture, involve significant plastic deformation before breaking, while others, like brittle fracture, occur suddenly without much warning. Other failure modes, such as fatigue failure or creep, develop over time due to prolonged exposure to specific conditions.

What is the difference between damage mechanism and failure mode?

While failure modes focus on the way a metal fails, damage mechanism looks at the process or events that lead to material degradation over time, due to factors such as:

Stress
Overloading
Temperature
Environmental exposure
Wear and tear

What are the different types of failure modes in metals?

There are many different reasons why metals fail. Mechanical stresses, environmental conditions, and operational processes can all play a role in metal failures. Failure Investigations can help to understand the root cause of failure through in-depth analysis.

Fracture failure

This occurs when a material (like metal) breaks because of cracks or fractures that propagate under stress. When the metal is overloaded with a force greater than its strength, the cracks begin to grow and eventually cause the material to separate. There are two types of fracture failures:

Ductile fracture: The material bends or stretches (known as plastic deformation) over time before it breaks. This usually happens in flexible metals such as steel or aluminium.
Brittle fracture: The material breaks suddenly, with no signs of plastic deformation. This usually happens in stiff materials like cast iron.

Example: A bridge that is designed to handle heavy traffic has weight limits. If a heavy truck passes over the bridge and exceeds those limitations, the bridge experiences overload. This stress causes a crack to form at a weak point like a weld joint, and over time, the crack grows, eventually leading to fracture and failure.

Fatigue failure

When metals are subjected to repeated cyclic loading, fatigue failure can occur, even if the stresses are within the metal’s ultimate tensile strength. Unlike a single large force, fatigue failure is caused by many small cycles of stress, which gradually cause cracks to form and grow until the metal breaks. It usually initiates at stress concentrators like notches, welds, or joints, and propagates over time.

Example: When you repeatedly bend a paperclip back and forth, over time it will eventually break. This is because each bend is a repeated stress cycle.

Creep failure

Creep failure is the gradual deformation of metals over a long period of time when it is exposed to constant stress at high temperatures. Under constant stress, the metal slowly stretches or changes shape over time. The high temperature exposure causes the material to become softer and weaker, and after gradual stretching, the metal reaches a point where it can no longer withstand the stress and breaks.

This type of failure is common in components such as turbines, boilers, or engines due to their exposure to high temperatures.

Example: A metal pipe carrying hot steam may slowly deform over time due to the constant high temperature and pressure. Eventually, this leads to the pipe breaking.

Corrosion failure

Corrosion-related failure occurs when materials are exposed to chemical reactions between metal and its environment, such as oxygen, water, or chemicals. This reaction weakens the material over a long period of time, eventually causing it to fail. Common types of corrosion include:

Rust: Oxidisation of iron and steel in moisture-rich environments.
Galvanic corrosion: When two different metals are in contact and react with each other.
Pitting corrosion: Localised deep pits or holes caused by accumulated chemicals or moisture.
Stress-corrosion cracking (SCC): When corrosion is combined with stress or overloading effects.
Intergranular corrosion: A corrosive attack along the material grain boundaries.
Uniform corrosion: A general surface corrosion leading to material loss

Example: A steel ship hull that sails through seawater is at risk of corrosion failure, as salt water accelerates the corrosion process by reacting with the metal, forming rust (iron oxide) on the surface. Over time this weakens the hull, especially around areas of high exposure like the waterline or beneath the ship where the metal remains in constant contact with the saltwater.

What impact do material defects have on metal failures?

Failure modes help us understand how a metal fails under certain conditions, but understanding material defects gives us early warning signs of potential failure. Defects can arise during manufacturing, material processing, or service conditions and often act as stress concentrators, leading to premature failure.

Casting Defects

Casting defects are imperfections formed during the solidification of molten metals during the casting process.

Porosity: Trapped gas forms small holes or voids in the metal.
Cold shut: Two streams of metal fail to fuse properly, causing rapid cooling.
Misruns: The metal cools and solidifies before filling the entire mould cavity.
Inclusions: Foreign materials become embedded in the casting.
Shrinkage cavities: Voids form as the metal contracts during cooling.

Welding Defects

These are irregularities that occur during the welding process which can affect the integrity, strength, or appearance of the welded joint.

Porosity: Gas bubbles trapped in the weld cause small holes or voids.
Cracks: Cracks form during cooling or due to hydrogen absorption.
Incomplete penetration: The weld does not penetrate through the full thickness of the base material.
Overlapping: Molten weld metal flows over the base metal without fusing to it.
Inconsistent weld beads: Small molten droplets scatter outside the weld pool, creating an uneven weld.
Distortion: Metal deforms due to uneven heating and cooling during the weld process.

Rolling & Forging Defects

During metalworking processes, metals are shaped by applying pressure, which can sometimes cause defects due to improper temperatures, processing errors, and stresses.

Laminations: Layered internal defects from trapped gas or inclusions.
Cracks: Surface or internal fractures due to improper forging temperature.
Seams: Long, thin defects caused by folding of metal during rolling.
Underfilling: Insufficient material in forged parts leading to incomplete shapes.

Heat Treatment Defects

Heat treatment helps to improve the strength and hardness of metals through controlled heating and cooling. However, there are risks of defects if the process is not properly controlled.

Decarburization: Loss of carbon from the metal surface, reducing hardness.
Quench cracks: Cracks due to rapid cooling and internal stresses.
Overheating: Excessive temperature weakening grain structure.
Soft spots: Uneven hardness in heat-treated metal.

Surface Defects

Surface defects occur on the outer layer or surface of a material, often affecting its appearance and functionality. These commonly occur during manufacturing processes, as well as wear and tear and regular use such as scratches and dents. Other defects include:

Scaling: Heat causes the metal surface to react with oxygen, forming an oxide layer.
Blistering: Trapped gas beneath the metal surface causes a bubble or blister to form during cooling or curing.
Mottling: Uneven colouration forms or surface patterns form due to improper cooling, variations in composition, or thermal gradients.
Peeling: Separation of coatings or layers from the base metal.

What are the failure modes and defects across different metals?

Failure modes and defects in metals vary depending on their composition, processing, and application.While some failure modes such as fatigue and corrosion are common across most metals, the specific causes and characteristics can differ.

Steel

Duplex alloys, austenitic and ferritic stainless steels are prone to brittle fractures, especially in high-carbon or low-temperature environments. Fatigue failure occurs in structures like bridges, aircraft, and pipelines due to cyclic stress. Hydrogen embrittlement can cause steel to weaken, leading to cracking. Common defects in steel include laminations from trapped gas, decarburization from heat treatment, and slag inclusions from welding or casting.

Aluminium

Aluminium is known for its corrosion-resistance but is prone to fatigue, stress corrosion cracking (SCC), and creep failure in high-strength aluminium alloys. Defects like porosity arise from trapped gas during casting, while cold shuts result from incomplete fusion in castings. Oxide inclusions, caused by aluminium oxide contamination, weaken welds.

Copper and Brass

Copper, brass, and bronze are highly conductive and corrosion-resistant, but are susceptible to stress corrosion cracking when exposed to ammonia. Dezincification leads to loss in brass, causing the metal to weaken. Defects include porosity in cast and welded copper components, oxidation that leads to the formation of a green patina, and hot cracking in welding when cooling is too rapid.

Titanium

Titanium is strong and corrosion-resistant, but can experience fatigue cracking due to cyclic stresses, creep failure in aircraft components, and hydrogen embrittlement. Common defects include alpha case formation, which creates a brittle surface layer during high-temperature exposure, porosity from casting, and oxygen contamination, which reduces ductility and toughness.

Nickel

Nickel-based alloys are used in extreme environments, which makes them susceptible to creep rupture, thermal fatigue, and oxidisation. Defects such as microstructural segregation, where uneven alloy composition weakens the material, porosity in cast superalloys, and carbide precipitation, which weakens grain boundaries at high temperatures, are critical concerns.

Understand Metal Failures with our expert analysis

A metal failure can be costly and dangerous, but understanding the root cause is the first step toward prevention. Whether it’s fatigue, corrosion, or material defects, analysing the failure helps improve design, material selection, and maintenance strategies.

Need expert insights or failure analysis services? Get in touch

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