Brittle Fracture

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Brittle Fracture is the sudden, very rapid cracking of equipment under stress where the material exhibited little or no evidence of ductility or plastic degradation before the fracture occurs. Unlike most other tensile failures, where the material plastically strains under overload conditions and becomes thinner until the point of rupture, when a piece of equipment suffers a brittle fracture, there is no thinning or necking down. Rather, this damage mechanism often causes cracking without warning, sometimes fracturing equipment into many pieces.

Brittle fracture is often caused by low temperatures. If the steel temperature is at or below its ductile-to-brittle transition temperature (DBTT), then it will be susceptible to brittle fracture. Combine this with a critical sized flaw and high stress on that flaw (either applied or residual), and then you are likely to experience a brittle fracture.

Other factors that can increase the susceptibility to brittle fracture include:

Metallurgical Degradation

Metallurgical degradation can occur in some steels at higher temperatures and can include things like temper embrittlement, graphitization, sigma phase embrittlement, and 885 embrittlement.

Steel Cleanliness and Grain Structure

As for steel cleanliness and grain structure, large grain sizes and steel contaminants can reduce steel toughness, so it's important to be aware and mindful of this during material selection amd QA/QC.

High Material Thickness

When it comes to material thickness, thicker components have a higher degree of susceptibility to brittle fracture because they have higher tri-axial stresses. Also, thicker materials produce a state of higher constraint, and are therefore less likely to deform under stress as opposed to crack initiation and propagation.

Colder Operating Temperatures

Operating an equipment or piping at temperatures colder than its lower design temperature (LDT, also known as Minimum Design Metal Temperature or MDMT in ASME codes) at sufficient operating pressure can also cause brittle fracture. Hence it is always important to operate an equipment/piping within its design limits. API RP 579 part-3, "Assessment of Existing Equipment for Brittle Fracture" provides guidelines to check if an equipment can be operated at temperatures colder than LDT, without brittle fracture damage to the equipment.

Types of Brittle Fracture

There are two major types of brittle fractures: transgranular and intergranular. With transgranular fractures, the fracture travels through the grain of the material. It changes direction from grain to grain due to the different lattice orientation of atoms in each grain, following the path of least resistance. Intergranular fracture, on the other hand, occurs when the a crack travels along the grain boundaries, as opposed to through the grains themselves. Intergranular fracture usually occurs when the phase in the grain boundary is weak and brittle.

Brittle Fracture Prevention/Mitigation

In order to reduce the risk of brittle fracture, one must be sure to keep materials operating at or above their DBTT during both service and testing. Likewise while conducting repairs, taking steps to establish and find flaws that might weaken the material while in-service or during pressure testing will reduce the chances of brittle fracture. This topic is covered in more detail in API RP 571 - Damage Mechanisms Affecting Fixed Equipment in the Refining Industry.

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Articles about Brittle Fracture
November/December 2023 Inspectioneering Journal

Part 2 of this three-part series on brittle fracture focuses on methods for evaluating the risk in pressure equipment and the evolution of screening and assessment methods.

September/October 2023 Inspectioneering Journal

This article is Part 1 of a new three-part series on brittle fracture and focuses on the fundamental concepts and damage morphology associated with the unstable rupture of pressure equipment.

January/February 2023 Inspectioneering Journal

Ferritic materials change in toughness as a function of temperature, but the change is non-linear and happens at different temperatures for different materials.

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November/December 2022 Inspectioneering Journal

Discussion on the new Annex 9J procedure with examples using the Part 3 exemption curve, the 9J General Procedure, and the 9J Simple Procedure.

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May/June 2022 Inspectioneering Journal

Part 2 of the three-part series on stress corrosion cracking (SCC) that examines common engineering and FFS methods that can be leveraged to understand and evaluate the propensity for an existing form of SCC to lead to loss of containment.

Partner Content

Auto-refrigeration can impose low temperatures onto process vessels and piping causing them to be at risk of brittle fracture, the sudden break-before leak phenomena that can result in catastrophic rupture of the equipment.

May/June 2020 Inspectioneering Journal

This article provides an overview of brittle fracture, details on several industry failures, and a summary of deficiencies and concerns with current published methods for screening susceptibility of equipment to potential brittle fracture failures.

May/June 2020 Inspectioneering Journal

There are many places where brittle fracture risk can sneak into your plant and many reasons why a new or revised brittle fracture assessment may be required when reviewing your pressure systems. Don’t overlook this dangerous failure mechanism.

Authors: Greg Garic
March/April 2019 Inspectioneering Journal

Brittle fracture and material toughness issues are important concerns in equipment design and FFS. These issues increase when temporary start-up and shutdown conditions require more detailed assessments than provided for in vessel and piping codes.

Authors: Greg Garic
May/June 2018 Inspectioneering Journal

There is concern in the industry over recent findings of reduced toughness fittings and flanges at risk of brittle fracture. This article provides an overview; possible contributors; measures taken to address; and a proposed FFS approach to address...

January/February 2018 Inspectioneering Journal

Given the concern throughout industry regarding the potential for brittle fracture failures, PWHT guidance to address potential issues arising from the recent changes in PWHT code requirements for carbon steel is examined in this article, and...

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The OmniScan™ X3 64 phased array and TFM flaw detector is equipped with enhanced phased capabilities and even faster TFM. The rugged and portable instrument features powerful 64-element probes with a 128-aperture TFM.

November/December 2017 Inspectioneering Journal

In-service equipment failures present a considerable challenge to reliability engineers. This article presents a case study of a convection tube failure in a furnace and the analyses that were performed to understand the root cause and determine the...

Authors: James R. Widrig
January/February 2014 Inspectioneering Journal

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November/December 2013 Inspectioneering Journal

Service failures and safety incidents of machines, structures, and pressure equipment have been experienced in the oil and gas industry for many years without warning, with varying degrees of consequential damages to health, safety,...

Authors: Fernando Vicente
November/December 2013 Inspectioneering Journal

Auto-refrigeration is a process where an unintentional and/or uncontrolled phase change of a hydrocarbon from a liquid state to a vapor occurs, resulting in a very rapid chilling (refrigeration) of the liquid containing local equipment and/or...

January/February 2007 Inspectioneering Journal

In previous parts of this series, I have covered many corrosion and degradation issues, some environmental cracking diseases, metallurgical degradation mechanisms, issues associated with welding and some external corrosion problems. In part 14,...

Authors: John Reynolds
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September/October 2005 Inspectioneering Journal

Strain-aging problems are another form of metallurgical degradation and thankfully are not very common and becoming less so; but since strain-aging does still occasionally occur, it still makes the list of one of the “99 diseases of pressure...

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September/October 2005 Inspectioneering Journal

Another form of metallurgical degradation at higher temperatures is called sigma phase embrittlement. As the name implies, a metallurgical phase change occurs in some stainless steels when they are heated above about 1000F (540C).

Authors: John Reynolds
September/October 2005 Inspectioneering Journal

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Authors: John Reynolds
November/December 2003 Inspectioneering Journal

Hydrogen Embrittlement (HE) is an insidious form of degradation that can strike during equipment fabrication, cleaning, repairs or while in-service. It stems from the infusion of atomic hydrogen into some higher strength steels that then leads to...

Authors: John Reynolds
May/June 2000 Inspectioneering Journal

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