What is Creep in Materials?

Engineering, as a discipline, relies on tight tolerances, repeatability and precision components conforming to exacting specifications. This allows engineers to accomplish extraordinary things, from constructing vast bridges that span rivers to building medical prosthetics that help someone live a more dignified and comfortable life. But despite the precision and quality control that go into manufacturing and installing components, failures still occur over time, often with no discernible cause. One of the primary reasons this happens is creep.

Creep is the slow, time-dependent, permanent deformation of a material under constant stress, occurring below its yield strength and accelerated by elevated temperature. Unlike elastic deformation, it does not recover when the load is removed.

Creep is a failure mode that must be accounted for in engineering, regardless of the application. It affects any component where polymers or softer metals carry sustained load and is one of the primary failure modes across chemical, aerospace, automotive and construction engineering.

This article gives an engineering-focused explanation of what creep is, how it affects assemblies and how to spot it, as well as offering practical strategies for managing and mitigating the risk. Whether you are specifying fasteners for a high-temperature assembly or selecting a polymer screw for a clamped joint, understanding creep will offer you a better engineering outcome.

 

Contents:

• Defining Creep In Materials

• The Three Stages of Creep: Primary, Secondary and Tertiary

• Creep Behaviour Across Engineering Materials

• Creep In Fasteners: Bolt Relaxation, Preload Loss & Clamped Joints

• How to Prevent Creep in Materials: Strategies for Engineers

• How to Detect and Monitor Creep Before It Becomes Failure

• Creep Has Been Detected. What Now?

• Wrapping Up: Understanding Creep In Materials

• FAQs

Defining Creep In Materials

In engineering, most failure modes announce themselves visibly. Overload produces visible deformation, fatigue leaves a fracture surface with clear crack propagation marks and corrosion is rarely subtle. 

Creep, by contrast, is patient, which is precisely what makes it dangerous.

Creep is the slow, time-dependent, permanent deformation of a material under constant stress. What makes it particularly unpredictable is that creep occurs below the material's yield strength, meaning a component can be perfectly designed against static overload, correctly specified, properly installed and carrying a load well within its rated capacity and still deform progressively until it fails. 

Given enough time, creep will find a way.

The Two Governing Factors: Stress and Temperature

Creep is driven by two variables working in combination: sustained mechanical stress and elevated temperature.

• Stress: provides the driving force, which in most cases comes from sustained mechanical load. Unlike a sudden overload that causes immediate deformation, sustained stress below the yield strength creates a constant incentive for the material's internal structure to change over time.

• Temperature: provides the mobility, the thermal energy that allows the structure of a material to change over time on a molecular level. At low temperatures, atomic movement is largely frozen out and the material resists change. As the temperature rises, that resistance drops. 

Like stress, the effect is non-linear, and the two variables amplify each other. A process running slightly hotter than designed, a thermal shield missing from a fastener or a component relocated closer to a heat source can all have consequences that appear disproportionate to the change that caused them. 

Creep vs. Related Phenomena: Getting the Terminology Right

Creep is frequently confused with two related but distinct phenomena which are frequently confused with each other.

Stress relaxation is particularly relevant in bolted joints and is covered in detail later in this article. The simplest way to understand the difference is this: creep is what happens when a material is free to deform and it slowly changes shape under the load. Stress relaxation is what happens when it is not free to deform, so is held at a fixed length, the material instead gradually loses the internal stress it was carrying. Same underlying cause, different outcome depending on whether the material can move or not.

The Three Stages of Creep: Primary, Secondary and Tertiary

Creep does not happen all at once or at a constant rate. From the moment a material begins to creep through to the point of eventual failure, it passes through three distinct stages. Each has its own characteristics, its own underlying mechanism and its own implications for how an engineer should respond. Understanding which stage of creep a component is in can mean the difference between scheduling a routine inspection and issuing a stop-work order.

Primary Creep: The Initial Response

When a load is first applied and sustained, the creep rate starts at its highest point and then slows down. This might seem counterintuitive, as you would assume a component fixed in place for years would be more at risk than one installed recently, but the reason it occurs makes sense once you understand what’s happening inside the material.

As the material begins to deform, its internal structure is effectively working against the load. Microscopic imperfections within the crystal structure, called dislocations, begin to move in response to the stress. As they do, they quickly run into obstacles like grain boundaries, other dislocations and small particles within the microstructure. These cause them to start to pile up. The more they pile up, the harder it becomes for further movement to occur. The material is, in a sense, stiffening itself against the load it is carrying.

This process is known as work hardening. It’s why the creep rate decelerates throughout the primary stage rather than continuing at its initial pace.

For engineers, primary creep is most visible in new installations. The initial relaxation of a bolted joint after tightening, the early settling of a gasket under bolt load and the bedding-in of a bearing under preload are all manifestations of primary creep. In many cases, this is expected and accounted for. A re-torque of a gasketed joint after initial pressurisation exists precisely because this behaviour is predictable. 

If you've ever re-torqued a gasketed joint after first running it up to temperature, you've already worked around primary creep without calling it that.

Secondary Creep: The Long Game

After the initial work hardening that occurs during primary creep, the creep rate stabilises at a low, roughly constant value and stays there, potentially for months or decades depending on the material and the conditions. This is secondary creep, also called steady-state creep. It typically accounts for the vast majority of a component's working life.

What is happening inside the material is a tug of war. Work hardening is still trying to slow deformation down, while elevated temperature is continuously undoing some of that hardening in a process called thermal recovery. When those two effects reach a balance, the creep rate stabilises. It is not zero, but it is steady and predictable.

That steady rate matters enormously in situ. It is the number that engineers use to calculate how long a component will last and it’s acutely sensitive to both stress and temperature. A modest increase in either doesn’t just accelerate the creep rate, it can multiply it many times over, dramatically shortening the lifespan of a component before maintenance becomes critical.

Tertiary Creep: The Point of No Return

At some point, the balance that defined secondary creep breaks down. The creep rate begins to climb and doesn’t stop. If you’ve ever pulled the studs from an old flange and found them measurably longer than they went in, you’ve seen the end state of this process up close.

This is tertiary creep and, without intervention, it ends in one outcome: creep rupture. Creep rupture is the outright failure of a fastener under a load that it was originally well capable of carrying.

Several things drive this acceleration, usually in combination. Tiny voids begin to open up inside the material, typically at grain boundaries. These gradually grow and link together into cracks. In components under tension, the cross-section begins to narrow, which increases the local stress even though the applied load has not changed, accelerating the process further.

The warning signs are worth knowing: 

• Dimensional change that is accelerating after a long period of stability.

• Surface cracking at stress concentrations or grain boundaries. 

• A visible narrowing of cross-section in loaded members and in bolted joints.

• A sudden increase in preload loss after months or years of steady behaviour.

If tertiary creep is allowed to run its course, the resulting fracture tends to look quite different from other failure modes. The crack follows grain boundaries rather than cutting through the grains themselves, leaving a rough, granular fracture surface with visible evidence of the voids that formed along the way. 

Unlike fast fracture there is rarely dramatic deformation at the moment of failure, because the damage has been accumulating quietly throughout the component's service life. A fast fracture is a sudden, catastrophic failure that happens in materials that have developed cracks from strain, material fatigue or manufacturing defects, which are often undetectable. When the strain acting on the loaded material exceeds what it can support, focusing on the tip of the crack within it, it will suddenly and violently fracture without prior deformation or warning. 

For anyone involved in failure analysis, a creep rupture misidentified as a fatigue-related failure, like a fast fracture, leads to the wrong corrective action and the same failure will most likely repeat.

Below, you’ll find a brief table summarising the three different stages of creep, along with other useful information to help manage and anticipate each:

 

Creep Behaviour Across Engineering Materials

Creep affects every engineering material. What varies between materials is the temperature and stress at which it becomes a practical concern. It is strongly associated with high-temperature applications such as jet engines and chemical process reactors, but it’s a mistake to think of it as exclusively a high-temperature phenomenon. 

Creep in plastic materials is a particular design concern because polymers can show measurable deformation at room temperature under modest loads. A nylon bracket carrying a sustained load subject to ambient temperature will still creep. 

The temperature threshold that matters is not absolute temperature in degrees, but temperature relative to the material's melting or softening point. This concept is known as homologous temperature.

Homologous temperature is defined as the ratio of operating temperature to melting temperature, both expressed in Kelvin:

Homologous Temperature = Operating Temperature (K) / Melting Temperature (K) for metals or Glass Transition Temperature (K) for polymers

For most metals, creep becomes an engineering concern at a homologous temperature of around 0.4:1, when the operating temperature reaches roughly 40% of the absolute melting point. For carbon steel, this corresponds to approximately 370 °C to 400 °C, a range that includes many industrial process environments. For aluminium, the threshold falls at just 100 °C to 190 °C, well within reach of engine bay automotive applications. For polymers, whose relevant threshold is the glass transition temperature rather than the melting point, the concern is often right at or just below room temperature, which is given as 25 °C.

The practical implication is not to assume that only hot components are at risk.

The table below gives a working reference for the materials most commonly encountered in engineering practice.

 

One important note that applies across every row: the onset temperatures above indicate when creep becomes a meaningful engineering concern, not an absolute threshold below which the material is immune. Creep is a continuous phenomenon, so the rate simply becomes negligible at lower temperatures for most materials. 

Creep In Fasteners: Bolt Relaxation, Preload Loss and Clamped Joints

Bolt relaxation can be difficult to understand and differentiate from creep. The mechanisms that cause it are very similar, but unlike creep, bolt relaxation’s failure state is not a creep rupture or a breakage within an assembly. Instead, it’s a loose bolt that no longer holds a joint together, potentially causing leaks and other issues.

Why Bolts Lose Preload

When you tighten a bolt, you are stretching it. Not visibly, but at a molecular level it has elongated slightly under the tension. That stretch is what creates the clamping force holding the joint together. The bolt wants to spring back to its original length, but the joint won't let it, so instead it sits in a state of constant tension. So far, so good.

The problem is that over time, getting worse at elevated temperatures, the bolt's internal structure slowly adjusts to being in that stretched state. It does not get any longer, because the joint is holding it at a fixed length. Instead, the force it is pushing back with gradually drops. The bolt ends up the stuck at this new length and no longer gripping the joint as tightly as it was when first assembled. The clamping force has quietly bled away and, from the outside, there’s no discernible cause as to why.

This matters because joints are designed around a minimum clamping force. A precision assembly needs consistent clamping to stay in alignment. In each case, the bolt does not need to fail outright for something to go wrong, it just needs to relax past a threshold. The consequences show up as a leak, a loosened connection or a loss of alignment that nobody can immediately explain.

Where the Risk Concentrates:

• High-temperature joints: The combination of elevated temperature, sustained bolt load and a gasket that is itself creeping makes these the highest-risk bolted assembly in common engineering practice. The gasket cold-flows, the bolt relaxes and the joint gradually loses the seating stress it was assembled with.

• Joints with polymer or soft metal components: Any assembly that includes a PTFE-faced gasket, nylon washer, aluminium flange or copper washers introduces a material that will creep under bolt load. The soft element deforms progressively, reducing the effective grip length of the bolt and dropping preload.

• Nylon insert locking nuts in sustained-load applications: The nylon element works by gripping the bolt thread under elastic deformation. Under sustained load and elevated temperature, it creeps, reducing grip and degrading locking performance over time. An all-metal locking mechanism is more reliable in these conditions.

• Tapped holes in a softer material under load from harder components: The thread flanks carry the full bolt load in bearing and creep under it over time, reducing thread engagement and dropping preload. The risk scales with temperature and load. It’s negligible in a cool, lightly loaded assembly and a real concern in a warm, highly loaded one.

Practical Design Responses:

• Match fastener material to service temperature. Do not assume a material rated for a given temperature under short-term loading will maintain its preload under sustained long-term loading at that same temperature.

• Use hardened steel washers. In joints with soft or creep-prone materials, a hardened washer distributes bearing load, reduces contact stress on the soft element and slows the rate of creep deformation.

• Prefer all-metal locking mechanisms. Where prevailing torque is required in elevated-temperature or sustained-load applications, all-metal locknuts maintain their locking performance where nylon insert nuts become unreliable. Be mindful of both galvanic corrosion and thread galling when specifying and assembling all-metal components, however.

• Apply a re-torque schedule. Initial gasket creep and bolt relaxation after first pressurisation are predictable. A planned re-torque after the joint has first seen operating conditions is standard good practice, not a sign that something has gone wrong.

• Design for joint stiffness. A stiff joint with short, large-diameter bolts, rigid flanges and minimal soft elements in the load path loses a smaller percentage of preload for a given creep deformation than a long, flexible one.

How to Prevent Creep in Materials: Strategies for Engineers

Prevention is considerably cheaper than remediation. Most creep failures can be traced back to a design or specification decision made before the component ever saw a load.

• Start with the right material: A material with adequate creep resistance for the service conditions will perform indefinitely; one without it will fail on a schedule determined by the laws of physics. 

• Control operating temperature: Since temperature is one of the two primary drivers of creep, keeping it as low as practically possible is always worthwhile. This can be achieved through insulation, local cooling, routing components away from heat sources or simply being accurate about the actual service temperature rather than defaulting to a worst-case figure.

• Reduce sustained stress: Lower stress means slower creep, and stress is often something the designer has direct control over. Upsizing a component, increasing bolt diameter, adding a support bracket or redistributing load across more fasteners all reduce the sustained stress on individual elements and extend creep life. Pay particular attention to stress concentrations like notches, sharp section changes and poorly toleranced fits.

• Specify heat treatment alongside alloy: For creep-critical components, the material specification should include the required heat treatment condition, not just the alloy designation. The same alloy processed differently can have substantially different creep behaviour.

• Inspect and maintain on a schedule: Set dimensional inspection checkpoints for creep-critical components, define acceptance criteria before inspection rather than after and have a clear plan for when a measurement approaches its limit. For polymer components under sustained load, replace on a time basis rather than waiting for visible signs of distress. By the time a creep-loaded polymer looks wrong, it may already be in tertiary behaviour.

How to Detect and Monitor Creep Before It Becomes Failure

Creep rarely arrives without warning. Unlike brittle fracture, it progresses over time and leaves traces a prepared engineer can find.

• Visual and dimensional inspection: Visible elongation or bowing, surface cracking at grain boundaries and visible reduction in cross-section are all indicators of meaningful creep progression. For bolted joints, look for fasteners that turn with less resistance than expected, uneven gasket compression, or joints that have begun to weep despite correct assembly. Baseline dimensional measurements taken at commissioning and repeated at defined intervals are the most rigorous and practical monitoring tool. A component whose dimensions are stable is behaving predictably; one whose rate of change is accelerating has likely entered tertiary creep.

• Non-destructive testing: Ultrasonic testing detects internal voids and grain boundary damage before surface signs appear. Replica metallography allows direct assessment of grain boundary damage and void density and is widely used for power plant remaining life assessment. Acoustic emission monitoring provides real-time warning of crack initiation in high-value or safety-critical applications. These techniques require specialist equipment but are well established where the cost of failure justifies the investment.

• Strain monitoring: Strain gauges bonded to the surface provide a direct record of deformation over time, giving a creep curve for the actual component in actual operating conditions. Digital image correlation maps full-field strain across a surface and is increasingly accessible. For simpler applications, reference marks scribed onto a component at manufacture provide a low-cost baseline for periodic manual measurement.

Creep Has Been Detected. What Now?

Finding evidence of creep is not automatically a crisis, but the response needs to be proportionate to the stage.

• Step 1: Establish which stage you are in. Primary creep in a new installation is normal. Secondary creep in an established component is a managed situation. Tertiary creep requires immediate action. If you cannot determine the stage from existing data, treat it as tertiary until you can prove otherwise.

• Step 2: Reduce the load if tertiary creep is suspected. Tertiary creep accelerates the longer a component remains under full load. Take the system offline, redistribute load to alternative paths, or add temporary support. Disruptive, but considerably less disruptive than an uncontrolled failure.

• Step 3: Assess remaining life. The Larson-Miller parameter is the most widely used method for metals in secondary creep, combining time and temperature into a single parameter for extrapolating long-term behaviour from shorter-term data. The Omega method is better suited to components already showing tertiary damage. Both require material-specific data and are best carried out in consultation with a materials engineer where the stakes are high.

• Step 4: Replace, remediate or redesign. Options include replacing the component on a planned basis, modifying operating conditions to slow the creep rate, or redesigning with a more appropriate material or lower stress level. Replacement is usually the most straightforward answer for small components; life extension through condition modification may be more practical for large, expensive or difficult-to-access ones, provided the assessment supports it.

• Step 5: Learn from it. A creep finding signals that something in the original design, specification or operating assumption was not quite right. Check whether similar components elsewhere in the system are in the same position. Update the design basis. Revise the inspection schedule. The goal is not just to fix the component in front of you, but to prevent the same failure from repeating elsewhere.

Wrapping Up: Understanding Creep In Materials

By reading this guide, you should now have a solid understanding of what creep is, why it happens and how to design against it, detect it, and respond when it is found. We've broken down the key points to remember below.

Key Takeaways:

• What Creep Is: The slow, permanent deformation of a material under sustained stress below its yield strength, driven by the combination of mechanical load and elevated temperature.

• The Three Stages: Creep progresses through primary (decelerating), secondary (steady-state) and tertiary (accelerating) stages. Tertiary creep is a stop-work condition until properly assessed.

• Every Material Is At Risk: Metals, polymers and soft materials all creep. The threshold varies dramatically, from above 600 °C for creep-resistant alloy steels down to room temperature for some polymers such as PTFE and nylon.

• Use The Right Data: Short-term tensile properties do not describe long-term creep behaviour. For sustained-load applications, always seek out creep-specific material data.

• Bolted Joints Need Attention: Stress relaxation silently reduces clamping force over time. Material selection, joint stiffness, hardened washers and re-torque schedules are your practical countermeasures.

Further Reading:

• Ultimate Screw Buying Guide: Types & Materials: Check out the different materials and fasteners available to ensure you choose correctly for your application.

• What Does Strength Mean?: A practical guide to what strength means in engineering terms.

So, there we have it. Whether you’re specifying components for a high-temperature industrial assembly or simply want to understand why a bolted joint has started to leak, understanding creep is an invaluable piece of engineering knowledge. We hope this guide has given you both the theory and the practical tools to approach it with confidence.

For precision fasteners, polymer components and engineering hardware suited to demanding applications, Accu has the range, the specifications and the expertise to support your project at every stage.

FAQs

Q: What causes creep failure in materials?

A: Creep failure happens when a material is left under load for long enough, at a high enough temperature, that it slowly deforms past the point of being able to do its job. The cause is always the same underlying mechanism: the material's internal structure rearranging itself over time, but in practice a few specific things tend to trigger it. The most common are running a component hotter than the original design assumed, leaving it under load near its long-term creep limit (which is much lower than the strength figure on the datasheet), choosing the wrong material for the conditions, or missing stress concentrations like thread roots, sharp corners or areas of higher temperature on an assembly.

Once a component reaches tertiary creep, it can fail at stresses it once handled comfortably, which is why creep is treated as a planned-life issue rather than an indefinite one. Creep failure in materials is almost always preventable with the right material choice, sensible operating conditions and a realistic view of how long the part actually needs to last.

Q: At what temperature does creep become significant in steel? 

A: For different grades and types of steel, the temperature at which creep becomes significant varies. For carbon steel, creep becomes a practical engineering concern above approximately 350 °C. Austenitic stainless steels such as 304 and 316 enter the creep range from around 425 °C, but retain useful creep resistance up to around 600 °C.

For higher temperature applications, titanium is an excellent choice. There are numerous grades and types specifically formulated for both high temperature performance and creep resistance.

Q: What is the difference between creep and stress relaxation? 

A: Both are driven by the same underlying mechanism, which is time-dependent material behaviour at the microstructural level. The difference is in the boundary condition. Creep occurs under constant stress, producing increasing strain over time. Stress relaxation occurs under constant strain. It’s a fixed displacement, producing decreasing stress over time. In bolted joints, stress relaxation is the dominant effect: the bolt length is constrained by the joint, so the material's creep tendency manifests as a drop in clamping force rather than further elongation.

Q: How do you prevent creep in bolted joints? 

A: The most effective measures to resist creep in a bolted joint are:

• Select a fastener material with adequate creep resistance for the service temperature. 

• Use washers for even load distribution.

• Specify all-metal locking mechanisms over nylon insert nuts in elevated-temperature applications.

• Apply a re-torque schedule after initial pressurisation of gasketed joints.

Q: What is the Larson-Miller parameter? 

A: The Larson-Miller parameter is a widely used tool for estimating the creep and stress rupture life of metals. It combines operating temperature and time into a single value that can be used to extrapolate long-term material behaviour from shorter-term test data or to estimate remaining service life, given known operating conditions. It requires material-specific constants and is most reliably applied by or in consultation with a materials engineer.

Q: What materials have the best creep resistance? 

A: Among metals, nickel superalloys offer the highest creep resistance, followed by purpose-developed creep-resistant steels and titanium alloys in the intermediate temperature range. Among engineering polymers, PEEK offers the best creep resistance.