Fracture Mechanics

© Photo Fraunhofer IWM
© Photo Fraunhofer IWM
© Photo Fraunhofer IWM

We provide proof for safety, fitness of purpose and endurance of complex and highly stressed components by the use of modern fracture mechanics concepts. On this basis we develop solutions for improved component safety, optimised component design or inspection intervals together with our customers.

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

 
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 optimisation and a safety-related material selection. Moreover, we offer support for the preparation of expert reports and represent investigation results in front of technical regulatory authorities.

© Photo Fraunhofer IWM

IWM Verb

IWM Verb is a software tool for the assessment of components containing crack-like defects.

 

 

 

 

 

 

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Services

Our portfolio ranges from materials characterization to safety evaluation and lifetime assessment of components with defects.
 
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-standardised 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). Companies all over the world use the failure assessment software IWM VERB to solve fracture-mechanical problems. The optimization and the development of the software is performed in a close cooperation with our customers. Additionally, we provide training courses in fracture mechanics methods.

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© Photo Fraunhofer IWM

Simulation of a deep drawing test: circular plate with a surface crack; material Ti-6Al-4V

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 was mainly validated for ferritic steels. Therefore the use of this concept for other metals involves uncertainties. Within the framework of the program ESA-TRP “Structural Integrity of Pressurized Structures − Advanced Non-Linear Methodology” the failure of thin-walled components made of aluminium and titanium alloys and their welds was in the focus of the investigations.

Realistic fracture mechanic parameters for crack initiation and stable crack growth, so called crack resistance curves, could be determined for plate materials Ti-6Al-4V and Al2219-T87 with thicknesses between approximately 1mm to 6mm, by a test series of pre-cracked specimens applying accompanying optical strain measurements and their numerical simulation. A subsequent analytical failure assessment on basis of the FAD concept in combination with experimentally determined material parameters provides conservative results. Especially for the ductile aluminium alloy Al2219-T87 and their friction stir welding joints the failure load will be significantly underestimated. To improve the accuracy of the analytical assessment methods a correction of the plastic limit load for multiaxial loading, an alternative definition of the cut-off criterion and a strain-based method as an alternative to the mismatch option of the FAD have been proposed.

© Photo Fraunhofer IWM

Flat tensile specimen with a surface crack: optical strain measurement at the backside of the specimen to determine the crack initiation and the stable crack growth after the crack has reached the specimen thickness;

© Photo Fraunhofer IWM
© Photo Fraunhofer IWM

© Photo Fraunhofer IWM

Axial stresses in a railway axle with mounted wheel caused by superimposed interference fit and bending load

© Photo Fraunhofer IWM

Fracture surface of a round-shaped sample with a fatigue crack

© Photo Fraunhofer IWM

Crack propagation rates of two railway axle materials

Fatigue Crack Growth inside Railway Axles

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

Representative crack geometry

Variable amplitude loading

Stress concentration if the crack is located in a geometric transition region

Presence of residual stresses due to press fit, mechanical machining, heat treatment

Comprehensive material characterization because of variable stress intensity ratios

Most of these aspects have been addressed in the research project »Eisenbahnfahrwerke-2« funded by the BMWi and German Railway industry. Beside of the experimental determination of mandatory material parameters also the methodical assessment concept as well as solutions and software for the simulation of the fatigue crack growth in railway axles were developed.

The results of this research project have been successfully implemented in several railway axles dimensioning projects and the derivation of inspection intervals.

 

 

 

 

 

 

 

 

 

 

 

 

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© Photo Fraunhofer IWM

Welding residual stresses in a longitudinal stiffener and their degradation due to cyclic loading

© Photo Fraunhofer IWM

Fracture mechanics-based estimation of the fracture-Wöhler-curve on basis of the crack-initiation-Wöhler-curve

Fatigue and Fracture Mechanics Assessment of Weld Joints

By coupling of 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 stabilised state were determined by fatigue tests. In corresponding experiments (Fraunhofer LBF) a significant difference between the crack-initiation- and fracture-Wöhler-curves was found which can be very well described by means of fracture mechanics calculations. The calculations were based on crack propagation curves of the investigated material S460NL, an initial crack with a depth of 0.2 mm and a suitable fracture mechanics model implemented in IWM VERB. 

 

 

 

 

 

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Publications

Varfolomeev, I.; Luke, M.; Consideration of fatigue crack growth aspects in the design and assessment of railway axles, Recent Trends in Fracture and Damage Mechanics; Hütter, G.; Zybell, L. (Eds Springer International Publishing AG; Cham, Schweiz (2016) 103-124 Link

Varfolomeev, I.; Luke, M.; Burdack, M.; Effect of specimen geometry on fatigue crack growth rates for the railway axle material EA4T; Engineering Fracture Mechanics 78/5 (2011) 742-753 Link

Varfolomeev, I.; Burdack, M.; Moroz, S.; Siegele, D.; Kadau, K.; Fatigue crack growth rates and paths in two planar specimens under mixed mode loading; International Journal of Fatigue 58 (2014) 12-19 Link

Varfolomeev, I.; Windisch, M.; Sinnema, G.; Application of the strain-based FAD to failure assessment of surface cracked components; International Journal of Structural Integrity 6/6 (2015) 689-703 Link

Varfolomeev, I.; Luke, M.; Consideration of fatigue crack growth aspects in the design and assessment of railway axles, Recent Trends in Fracture and Damage Mechanics; Hütter, G.; Zybell, L. (Eds Springer International Publishing AG; Cham, Schweiz (2016) 103-124 Link

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