Fracture Mechanics - A Definitive Guide

What is Fracture Mechanics?

Fracture mechanics is a methodology that uses the study of material properties to predict and diagnose failures of an asset, component or structure which may have an existing crack or flaw. It looks at two factors of a material; the strength and the toughness. The analysis of the strength of a material involves analysing how good the material is at handling a load. The analysis of the toughness of a material involves understanding how good the material is at stopping a crack propagating. 

Fracture mechanics looks at the material properties and the structural integrity of a structure, component or asset, using an engineering critical assessment (ECA). An ECA is based on fracture mechanics theory and looks at whether a flaw is safe from brittle fracture, fatigue, creep or plastic collapse under specified load conditions. TWI Ldt puts these processes into practice at all life-stages including design, fabrication, operation and decommissioning. TWI offers fracture mechanics services in a variety of industry sectors, using its experience of over 40 years to do this.

When are ECAs Carried out? 

ECAs are carried out at various points of an asset’s life and can be required as part of a fabrication specification. They are routinely carried out by the oil and gas, nuclear, aerospace, petrochemical and power industries. ECAs are used to evaluate the integrity of structures including pipelines, pressure vessels, platforms, mooring chains and risers, rigs and wind turbines.  

Examples of TWI using Fracture Mechanics
Pipelines

TWI carried out an engineering critical assessment to advise on the fracture resistant properties and maximum allowable flaw size of an iced up gas lift and kill line which was susceptible to brittle fracture. They carried out fractography on the tested tensile and fracture toughness specimens.

TWI asset integrity engineers inspected the possible damage mechanisms of a stainless steel pipeline and assessed the affected integrity performance of the pipe where it had experienced low temperature excursions. The damage mechanisms comprised of a possible reduction in toughness and external pitting to cracking resulting from fatigue. TWI also carried out an ECA to determine the critical tolerable flaw size, against which a detected crack-like flaw could be assessed.

A one year study was carried out by TWI and ISIS Neutron and Muon Source looking at the relaxation of residual stresses on a pipe spool subjected to 3% strain. The test was performed on pipes used in the offshore industry to make hydrocarbon flowlines and pipelines. The results have impacted the nuclear power and oil and gas industry through the improvement of fracture integrity assessments. 

Pressure Vessels 

TWI investigated the fracture of a large pressure vessel being manufactured during a hydraulic test. The failure occurred at the flange end of the vessel with a crack through two locations. They looked at the two fracture initiation sites and found that they were pre-existing cracks in the heat affected zone of the submerged arc weld that joined the flange end forging to the vessel shell.

Mooring Chains and Risers

An engineering critical assessment was carried out by TWI on the structural integrity of a mooring system for a floating production, storage and offloading facility after the discovery of heat affected zone cracking. TWI carried out fracture toughness tests and used fracture mechanics analysis to calculate the limiting flaw sizes that would not lead to failure of the structure and to determine fatigue crack growth.

Rigs

The TWI Structural Integrity Department worked on the life extension of offshore rigs. They carried out structural analysis and fracture mechanics based assessments of fatigue and fracture behaviour.

Wind Turbines

The demand for renewable energy has increased and therefore driven offshore wind farms to be extended and the design of individual wind turbines to be developed to deliver higher energy per unit. This means that the support structures of the wind turbine need to be modified to withstand the additional loading. TWI produced a report, during a TWI exploratory project, to present a general survey into standards used for design and manufacture of these wind turbines. The selection criteria of the material is now based heavily on its strength and toughness. TWI used a fracture mechanics model and an engineering critical assessment to estimate the minimum toughness requirements which compared to the recommended standards.

TWI's Fracture and Mechanical Testing Laboratory
TWI’s Fracture and Mechanical Testing Laboratory offer services in fracture toughness testing and tensile/large-scale pressure testing. They also offer mechanical testing in a seawater environment. The fracture and mechanical testing laboratory looks at the fracture performance of a structural component under a load.

TWI Training and Examinations

Structural Integrity Assessment and Practical Application of BS 7910 Procedures for the Assessment of Flaws in Metallic Structures.

TWI Training and Examinations offers the Structural Integrity Assessment and Practical Application course and covers fitness-for-purpose and engineering critical assessment. It is an ideal course for fracture engineers as it gives an in-depth insight into applications and industry standards of fracture mechanics.

TWI Software

As a fracture engineer, you can benefit from the software products and services that TWI offers. They are designed to help make fracture mechanics analysis simple and accurate.

TWI Software services relevant to fracture mechanics include:

CrackWISE

CrackWISE is a fitness-for-service (also called engineering critical assessment) software that automates assessments based on the evaluation of flaws using fracture mechanics principles. Click here to find out more about CrackWISE software.

IntegriWISE

IntegriWISE software is a fitness-for-service (FFS) assessment software for evaluating the integrity of ageing assets, components and structures. The software calculates critical operational conditions, assesses the different damage mechanisms, conducts multiple ‘what if’ scenario assessment and includes a material database. Click here to find out more about IntegriWISE.

Professional Development as a Fracture Mechanics Engineer

The Welding Institute can support you through your membership process as a fracture mechanics engineer. We have mentors, comprised of Welding Institute volunteers, which specialise in various industry sectors.

One of the main benefits that you will gain as a member is access to TWI information. This is especially relevant to you, as you will gain access to TWI’s technical knowledge of fracture mechanics.

You will gain access to:

• TWI’s technical library – this includes books, reports, journals, standards, conference proceedings and publications.
  
• TWI’s e-library – as a member, you can download information that you access on our online library resource. This makes access to relevant information easily accessible to you.
  
• As a fracture mechanics engineer and Member of The Welding Institute, you qualify for a 5% discount on all TWI Training and Examination courses. These can aid your career development by enabling you to gain a deeper insight and knowledge into your area of expertise. You can also gain CPD points through attendance of these courses. 
  
• Another member benefit includes The Welding Institute's Technical Group Meetings. They cover the topics; manufacturing, welding processes, materials, offshore oil and gas, pressure and process plants, structural integrity, polymers and NDT Condition Monitoring. 
  

Click here to view all member benefits.