scholarly journals Determination of crack initiation and crack growth stress-life curves by fracture mechanics experiments and statistical analysis

2016 ◽  
Vol 2 ◽  
pp. 3026-3039 ◽  
Author(s):  
Stefan Kolitsch ◽  
Hans-Peter Gänser ◽  
Reinhard Pippan
Keyword(s):  
Statistical Analysis ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Growth ◽  
Growth Stress

Assessment of Power Plant Components With Flaws and Defects Operating in the Long-Term Creep Range

2019 ◽  
Author(s):  
Magdalena Speicher ◽  
Thorben Bender ◽  
Andreas Klenk ◽  
Falk Mueller ◽  
Christian Kontermann ◽  
...  
Keyword(s):  
High Temperature ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Growth ◽  
Creep Crack Growth ◽  
Material Behavior ◽  
Critical Crack Length ◽  
Creep Crack ◽  
Combining Data ◽  
Crack Problems

Abstract Originating from defects and flaws in high temperature components crack initiation and crack propagation under service conditions can occur. Fracture mechanics data and procedures are needed to study crack problems and to support an advanced remnant life evaluation. During subsequent research in the past 35 years, data were determined for different high temperature materials. Methodologies and concepts taking into account the specific material behavior were developed in order to be able to describe crack initiation and crack growth and have appropriate assessment methods available. For creep crack initiation two criteria principles were used and for creep crack growth assessment based on the integral C* parameter were applied. Furthermore, a method for determination of critical crack length was developed allowing decisions whether modified stress analysis methods are sufficient or more complicated fracture mechanics methods are needed. To provide data and methodologies in a user-friendly way, a program system combining data and methods was implemented. The paper describes developed features and shows comparisons to other methods. The methods can be applied for design purposes as well as remnant life assessments.

Crack Initiation and Propagation in Static Loaded Fracture Mechanics Tests in Steels Containing Atomic Hydrogen

2020 ◽  
Author(s):  
Philippa L. Moore ◽  
Menno Hoekstra ◽  
Alex Pargeter
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Plastic Zone ◽  
Crack Growth ◽  
Crack Extension ◽  
Low Alloy Steels ◽  
Ductile Tearing ◽  
Initiation Fracture Toughness ◽  
Stable Tearing

Abstract Hydrogen is well known to have a detrimental influence on the ductility of low alloy steels, reducing the fracture toughness. Standard test methods to characterize fracture toughness of steels in terms of ductile tearing resistance curves have not been developed to account for any hydrogen-driven contribution to the crack extension, Δa. Simply plotting J or CTOD against Δa is not necessarily appropriate for defining the initiation fracture toughness for tests performed in a hydrogen-charging environment. This paper explores a method to further analyse experimental data collected during fracture toughness tests, which allows the contribution of plasticity (i.e. when blunting precedes ductile tearing) to be considered separately from the initiation of crack extension (which could be by stable tearing and/or by hydrogen-driven crack extension). The principle is based on the assumption that a crack growing by a hydrogen-driven mechanism in a quasi-static fracture mechanics test performed in environment may not be associated with significant ductility in the plastic zone (which would accompany crack growth by stable tearing). The analytical method presented in this paper compares the different points of deviation from linear behavior of the components of J, to isolate the effects of ductility within the plastic zone from pure crack extension. In this way, the point of crack initiation can be defined in order to determine the relevant initiation fracture toughness; whether by blunting and stable tearing, or by hydrogen-driven crack growth. This approach offers a screening method which is illustrated using examples of fracture mechanics specimens tested in environments of varying severity (air, seawater with cathodic protection, and sour service). This method can be used to identify the relevant definition of initiation fracture toughness while allowing for a combination of ductile tearing, hydrogen-driven crack extension, or both, to be present during the test.

Application of an Advanced Creep–Fatigue Procedure for Flexible Design of Steam Turbine Rotors Based on Fracture Mechanics Methods

2014 ◽  
Vol 137 (3) ◽  
Author(s):  
Shilun Sheng ◽  
Henning Almstedt
Keyword(s):  
High Temperature ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Steam Turbine ◽  
Crack Growth ◽  
Ductile Material ◽  
Creep Crack ◽  
Creep Fatigue ◽  
Turbine Rotors ◽  
Material Ductility

The demand for steam turbine components is driven not only by high efficiency but also by high plant operational flexibility. Steam turbine rotors are therefore exposed to increased temperatures and increased number of stress cycles. These aspects should be considered for life-time prediction. Fracture mechanics methods are usually applied when crack like defects are detected not only for new rotors but also for rotor components in service. Based on the findings, a decision has to be made with respect to acceptability considering high temperature effects as well as the expected future operating regime. For defect analysis in the high temperature range, crack initiation and crack propagation under combined creep and fatigue loading need to be taken into account. Based on fracture mechanics methods and long-term testing data, an advanced creep–fatigue procedure for the evaluation of crack initiation and crack growth has been developed within the German Creep Group W14 for creep crack growth (CCG) behavior. Furthermore, recent studies show that the crack size for creep crack initiation (CCI) depends on material ductility and creep strain in the ligament. This paper demonstrates the industrial application of the abovementioned method for steam turbine rotor assessment, which has a focus on crack initiation and crack growth under creep–fatigue conditions. For crack initiation, a simplified approach based on defect size and material ductility is compared to a standard approach—two-criteria-diagram (2CD). For the advanced evaluation concept, the CCI criterion is combined for analysis with a creep–fatigue crack growth (CFCG) procedure. The benefit of the method especially for ductile material will be highlighted.

Initiation and Growth of Small Fatigue Cracks of Steels Used for Nuclear Power Plants Under Low Cycle Regime

2014 ◽  
Author(s):  
Shota Hasunuma ◽  
Takeshi Ogawa
Keyword(s):  
Fatigue Crack ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Growth ◽  
Nuclear Power ◽  
Power Plants ◽  
Nuclear Power Plants ◽  
Low Cycle Fatigue ◽  
Fatigue Cracks ◽  
Cycle Fatigue

Low cycle fatigue tests were conducted for carbon steel, STS410, low alloy steel, SFVQ1A, and austenitic stainless steel, SUS316NG, which were used for nuclear power plants, in order to investigate the mechanism of fatigue damage when the plants were subjected to huge seismic loads. In these tests, the surface behavior of fatigue crack initiation and growth was observed in detail using cellulose acetate replicas, while the interior behavior was detected in terms of fracture surface morphology developed by multiple two-step strain amplitude variations with periodical surface removals. Fatigue crack growth rates were evaluated by elasto-plastic fracture mechanics approach. For SFVQ1A and SUS316NG, the fracture mechanics approach is available in order to predict the crack growth life from the metallurgical crack initiation size to the final crack length of the specimens. For STS410, numerous small cracks initiated, grew and coalesced each other on the specimen surface under low cycle fatigue regime.

Application of an Advanced Creep-Fatigue Procedure for Flexible Design of Steam Turbine Rotors Based on Fracture Mechanics Methods

2014 ◽  
Author(s):  
Shilun Sheng ◽  
Henning Almstedt
Keyword(s):  
High Temperature ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Steam Turbine ◽  
Crack Growth ◽  
Ductile Material ◽  
Creep Crack ◽  
Creep Fatigue ◽  
Turbine Rotors ◽  
Material Ductility

The demand for steam turbine components is driven by high efficiency but also by high plant operational flexibility. Steam turbine rotors are therefore exposed to increased temperatures and increased number of stress cycles. These aspects should be considered for life-time prediction. Fracture mechanics methods are usually applied when crack like defects are detected for new rotors but also for rotor components in service. Based on the findings a decision has to be made with respect to acceptability considering high temperature effects as well as the expected future operating regime. For defect analysis in the high temperature range, crack initiation and crack propagation under combined creep and fatigue loading need to be taken into account. Based on fracture mechanics methods and long-term testing data, an advanced creep-fatigue procedure for the evaluation of crack initiation and crack growth has been developed within the German Creep Group W14 for creep crack growth behavior. Furthermore, recent studies show that the crack size for creep crack initiation depends on material ductility and creep strain in the ligament. This paper demonstrates the industrial application of the abovementioned method for steam turbine rotor assessment, which has a focus on crack initiation and crack growth under creep-fatigue conditions. For crack initiation, a simplified approach based on defect size and material ductility is compared to a standard approach — Two-Criteria-Diagram (2CD). For the advanced evaluation concept, the creep crack initiation criterion is combined for analysis with a creep-fatigue crack growth procedure. The benefit of the method especially for ductile material will be highlighted.

Design of a PYTHON-Based Plug-In for Benchmarking Probabilistic Fracture Mechanics Computer Codes With Failure Event Data

2009 ◽  
Author(s):  
Jeffrey T. Fong ◽  
Roland deWit ◽  
Pedro V. Marcal ◽  
James J. Filliben ◽  
N. Alan Heckert ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Growth ◽  
Growth Rates ◽  
Field Data ◽  
Multiple Cracks ◽  
Event Data ◽  
Probabilistic Fracture Mechanics ◽  
Failure Event ◽  
Computer Codes

In a 2007 paper entitled “Application of Failure Event Data to Benchmark Probabilistic Fracture Mechanics (PFM) Computer Codes” (Simonen, F. A., Gosselin, S. R., Lydell, B. O. Y., Rudland, D. L., & Wikowski, G. M. Proc. ASME PVP Conf., San Antonio, TX, Paper PVP2007-26373), it was reported that the two benchmarked PFM models, PRO-LOCA and PRAISE, predicted significantly higher failure probabilities of cracking than those derived from field data in three PWR and one BWR cases by a factor ranging from 30 to 10,000. To explain the reasons for having such a large discrepancy, the authors listed ten sources of uncertainties: (1) Welding Residual Stresses. (2) Crack Initiation Predictions. (3) Crack Growth Rates. (4) Circumferential Stress Variation. (5) Operating temperatures different from design temperatures. (6) Temperature factor in actual activation energy vs. assumed. (7) Under reporting of field data due to NDE limitations. (8) Uncertainty in modeling initiation, growth, and linking of multiple cracks around the circumference of a weld. (9) Correlation of crack initiation times and growth rates. (10) Insights from NUREG/CR-6674 (2000) fatigue crack growth models using conservative inputs for cyclic strain rates and environmental parameters such as oxygen content. In this paper we design a Python-based plug-in that allows a user to address those ten sources of uncertainties. This approach is based on the statistical theory of design of experiments with a 2-level factorial design, where a small number of runs is enough to estimate the uncertainties in the predictions of PFM models due to some combination of the source uncertainties listed by Simonen et al (PVP2007-26373).

Microstructure and Strain-Based Fatigue Life Approach for High-Performance Welds

2014 ◽  
Vol 891-892 ◽  
pp. 1500-1506 ◽  
Author(s):  
Heikki Remes ◽  
Pauli Lehto ◽  
Jani Romanoff
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Growth ◽  
Welded Joints ◽  
Welded Joint ◽  
Damage Zone ◽  
Stress Gradient ◽  
Fatigue Crack Initiation ◽  
Initiation Period

Microstructure and pre-existing surface flaws in smooth notch geometries significantly affect the fatigue life of welded joints. Traditionally, a welded joint is assumed to incorporate crack-like defects and the crack propagation dominates the total fatigue life. For a smooth weld notch geometry, the macro crack initiation period becomes more significant, and this difference cannot be modelled with the existing stress or fracture mechanics ‑based approaches. In this paper, a microstructure and strain ‑based fatigue life approach is presented. In the approach, the fatigue damage process is modelled as a repeated crack initiation process within a material volume related to the microstructure. The novelty of the developed approach is that the size of the damage zone is defined from the grain size statistics without using fracture mechanics. The approach is able to consider the changes in the stress gradient, stress triaxiality and plasticity during the fatigue crack initiation and growth. The developed approach has been validated with experiments on submerged-arc and laser-hybrid welded joints. The predicted fatigue life, crack growth path and rate showed good agreement with the experiments. For a welded joint with smooth and favourable notch shape, the short crack growth, i.e. macro crack initiation period is dominant and it has a significant influence on the fatigue life.

Fracture and Fatigue Testing of Micro-Sized Materials for MEMS Applications

2005 ◽  
Author(s):  
Kazuki Takashima ◽  
Timothy P. Halford ◽  
Yakichi Higo
Keyword(s):  
Fracture Toughness ◽  
Fatigue Crack ◽  
Fatigue Life ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Amorphous Alloy ◽  
Crack Initiation ◽  
Crack Growth ◽  
Testing Machine ◽  
Fatigue And Fracture

We have developed a new type of mechanical testing machine for micro-sized specimens, which can apply a small static or cyclic load, and have investigated fracture and fatigue crack growth behavior of micro-sized specimens. Cantilever beam type specimens (10 μm × 10 μm × 50 μm), with notches were prepared from thin films of a Ni-P amorphous alloy by focused ion beam machining. Fatigue and fracture toughness tests were carried out in air at room temperature using the mechanical testing machine. Fatigue and fracture testing was completed successfully for micro-sized cantilever specimens. Once fatigue crack growth occurs, rapid sample failure was observed in these micro-sized specimens. This indicates that the fatigue life of micro-sized specimens is mainly dominated by crack initiation. This also suggests that even a micro-sized surface flaw can be a fatigue crack initiation site which will shorten the fatigue life of micro-sized specimens. As a result of fracture toughness tests, plane strain criteria for small scale yielding were not achieved for this amorphous alloy. Plane stress and plane strain dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This suggests that fracture mechanics is still valid for such micro-sized specimens.

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