Fatigue Behavior and Fracture Mechanics

© Fraunhofer IWM
Fatigue Fracture

Load-bearing components and structures in all industrial sectors such as automotive, vehicle, railway, aerospace, steel and bridge constructions are exposed to complex stresses. Many technical cases of damage can be attributed to material fatigue due to cyclic mechanical loading. In order to ensure the reliable use of critical components, the assessment of material fatigue is of decisive importance.

On the basis of our material and damage models for components and structures subjected to fatigue loading, we perform reliable service lifetime calculations so our customers are able to fully benefit from the potential of the strength reserves of their construction materials under extreme loads.

© Fraunhofer IWM
© Fraunhofer IWM

Modern Fracture Mechanics


For the design and assessment of safety-relevant components, fracture mechanics methodologies are state-of-the-art. In the presence of crack-like defects (detected or postulated), the methods of fracture mechanics provide information about the fitness of purpose of components, the remaining service life and the safety regarding an uncontrolled crack growth. Well-founded decisions about commissioning, lifetime extension, periodic inspections or the exchange of a component can be provided on this basis. Typical industrial areas for the application of fracture mechanics are:

 

  • Aerospace
  • Plant construction
  • Pressure vessels and piping systems of power plants
  • Gas and steam turbines
  • Pipelines
  • Railway vehicles
  • Steel structures, i.e. for crane and bridge construction
  • Welded components
© Fraunhofer IWM

We support our customers with our competencies in material testing, numerical simulation, analytical failure assessment and failure analysis related to fracture mechanics questions. Together with our customers, we develop solutions for the assessment of the safety and the remaining service lifetime of machines and plants, the extension of their service lifetime, the determination of inspection intervals, the design optimization and a safety-related material selection. Moreover, we offer support for the preparation of expert reports and represent investigation results before technical regulatory authorities. 

Fraunhofer IWM video series: Materials deformation and failure behavior

Dr. Michael Luke

Material deformation, damage and crack formation

Application of Fracture Mechanics Assessments: IWM VERB

© Fraunhofer IWM

IWM VERB is a software tool which is used for the assessment of components with crack-like defects. The numerical basis of the software consists of elastic and elastic-plastic fracture mechanics methods and solutions, which are in accordance with internationally accepted guidelines and recommendations. Its application mainly refers to metallic components that are statically and cyclically loaded. However, non-metallic materials can also be assessed as long as their behavior can be described with common fracture mechanics concepts.

The failure assessment software IWM VERB is applied by many companies around the world to solve their specific fracture mechanics issues. Therefore, the development and adaptation of the software is accomplished in close collaboration with the client. Moreover, we offer training courses for the efficient application of fracture mechanics methods.

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What we offer

Our portfolio ranges from materials characterization to safety evaluation and lifetime assessment of components with defects.

  • Material qualification
  • Microstructure and failure analysis
  • Numerical and experimental stress and strain analysis of components under static and cyclic loading
  • Numerical component assessment under static and cyclic loading experimentally supported by component tests
  • Numerical and experimental strength and safety analysis
  • Welding simulation and strength analysis of welded joints under consideration of residual stress and distortion
  • Numerical simulation of shot-peening, deep rolling and high frequency mechanical impact and resulting residual compressive stresses in the surface-near layer of metallic structural materials
  • Calculation of service lifetime of mechanical surface-treated (such as shot-peened, deep rolled and high frequency mechanical impact) components including the quantification of the lifetime improvement and the estimation of the cost-benefit ratio

Experimental determination of fracture mechanics parameters:

  • Fracture toughness, crack resistance curves, fatigue crack growth curves
  • Crack resistance tests in a temperature range of about -196 °C to 600 °C in compliance with test standards such as ASTM E399, E1820, E1921, E647
  • Non-standardized tests under custom specified conditions or with non-standard specimen geometries

Stress analyses: 

  • Of specimens and components with cracks or other defects
  • Under complex thermo-mechanical constraints
  • Using state-of-the-art and advanced material models
  • With consideration of elastic-plastic deformation, creep and progressing damage
  • Under static and cyclic loading
  • Considering welding stresses

Fracture mechanics assessment:

  • In accordance with specific standards such as R6, SINTAP, FITNET, BS 7910, API 579 or FKM guideline
  • Using special software, for example, IWM VERB or ERWIN
  • Including custom-specific software implementation (component and crack geometry, stress conditions, material properties)
  • Using deterministic and probabilistic methods

Furthermore, we develop advanced solutions and evaluation concepts and consistently implement them into our calculation programs and the FKM guideline "Fracture mechanics strength assessment" co-developed by us (FKM = Forschungskuratorium Maschinenbau).

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Selection of measurement and equipment highlights

  • Electromechanical, servo-hydraulic and resonance testing machines (N to MN) to carry out static and cyclic material and component tests
  • Multi-axial loading devices (tension-torsion; span) for multiaxial component testing
  • Air-conditioned shock and vibrations tests
  • Metallography and fracture surface analysis
  • Equipment to perform fracture mechanical tests (i.e. ASTM E399, E1820, E1921 and E647) over large force and temperature ranges
  • Determination of strains by use of clip-gauges or digital image correlation (DIC) for areal strain measurements

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Topics

 

Structural integrity of pressurized structures: Advanced non-linear methodology

 

Fracture mechanics assessment according to the concept of the Failure Assessment Diagram FAD requires precise calculation methods for the determination of fracture mechanics load parameters and also experimental and numerical validation of the assessment concept. The FAD concept as a part of different technical codes...

 

Digital methods for lifetime assessment

 

At Fraunhofer IWM, digital methods are developed which focus on a systematical use of material data for accelerated product development. Material data stored in structured data spaces and semantic knowledge graphs enable an interoperable connection to analyzation tools that cover the specific steps along the product development chain. In the frame of the internal Fraunhofer research project...

 

Increasing lifetime by means of mechanical surface layer strengthening

 

The surface layer of metallic components and structures is of decisive importance for their lifetime. In all industrial sectors, where components are exposed to high stress such as automotive, vehicle and railway construction, aerospace, steel and bridge construction, drive technology and gear manufacturing, combustion engines, steam and gas turbines, compressor and pump manufacturing, tool and...

 

Assessment of manufacturing defects

 

The evaluation of faults in cast steel components is a vital part of strength calculation. Using the examples of materials G20Mn5 and G22NiMoCr5-6, a validated assessment concept was provided in the course of an Industrial Collective Research (IGF) project. Critical to this project were the validation of different non-destructive test methods, strength tests of samples with defects, as well as damage and fracture mechanical analyses of the experiments. The samples...

 

Fatigue and fracture mechanics assessment of weld joints

 

By combining cyclic plasticity models, numerical welding process simulation, fatigue and fracture mechanics models the lifetime of a welded component can be described realistically. In the following example welding residual stresses and their stabilized state were determined by fatigue tests. In corresponding experiments (Fraunhofer LBF) a significant difference between between the crack-initiation- and fracture-Wöhler-curves...

 

Calculation of residual stress and warping during welding

 

The heterogeneous nature of welding in which the material is heated up locally and cools down induces internal stresses and leads to component distortion. This has a negative effect on component quality and functionality and can also cause damage, e.g cracks, and/or reduce the service life. Internal stress and distortion are closely related, and many factors lead to their generation. The material properties, clamping conditions as well as welding procedure and...

 

Damage analysis
 

We answer questions that occur during the quality assurance phase of industrial production or due to failure during service. The scientific investigation of damage to and failures of metals and metallic components is well established within the Fraunhofer IWM. We will build an expert project team customized for your individual task, which will assess the situation and discuss the next steps with you to reach an effective solution. Because time is such an extremely important factor...

 

Fatigue crack growth of railway axles

 

If a crack is detected in a cyclic loaded component or its presence cannot be excluded by non-destructive testing, the remaining lifetime of this component can be estimated by the use of the fracture mechanics method. This occurs during the assessment or the dimensioning of railway axles of vehicles in service and newly designed vehicles. Several material and operating aspects have to be taken into account...

 

Probabilistic material and component assessment


In engineering practice, the design of a component is normally ascertained based on deterministic material data and component loading scenarios. The considered loads are based on experiences, measured data and load scenarios covering a whole variety of possible service load cases. Minimum values from standards or measurements are generally used as material properties. Such a procedure delivers information concerning whether...

Fatigue Behavior and Fracture Mechanics Publications

 

Fatigue Behavior

Contributions to scientific journals, books and conferences as well as dissertations and project reports...

Fracture Mechanics

Contributions to scientific journals, books and conferences as well as dissertations and project reports...