Crack Tip Shielding from a Plastic ‘Inclusion’

2011 ◽  
Vol 465 ◽  
pp. 1-8
Author(s):  
M. Neil James ◽  
C.J. Christopher ◽  
Yan Wei Lu ◽  
K.F. Tee ◽  
Eann A Patterson
Keyword(s):  
Plastic Deformation ◽  
Fracture Mechanics ◽  
Plastic Zone ◽  
Crack Growth ◽  
Displacement Field ◽  
Driving Force ◽  
Applied Load ◽  
Elastic Stress ◽  
Ductile Material ◽  
Crack Tip Shielding

This paper presents a very brief overview of the philosophy underlying a plastic inclusion approach to defining the boundary stresses imposed on the applied elastic stress or displacement field by the plastic deformation attendant on crack growth in a ductile material. It leads to two new fracture mechanics parameters, KR and KS. KR defines a retardation component arising from wake contact and the Poisson’s contraction associated with the plastic zone, whilst KS describes a compatibility-induced component arising from shear at the elastic-plastic interface. These additional components imply that KF is not directly comparable with KI, as it describes the net driving force on the crack from the applied load.

scholarly journals Initial stages of yield in nanoindentation

1999 ◽  
Vol 14 (6) ◽  
pp. 2219-2227 ◽  
Author(s):  
J. D. Kiely ◽  
K. F. Jarausch ◽  
J. E. Houston ◽  
P. E. Russell
Keyword(s):  
Plastic Deformation ◽  
Plastic Zone ◽  
Applied Load ◽  
Elastic Behavior ◽  
Load Relaxation ◽  
Crystal Surfaces ◽  
Small Deviations ◽  
Single Crystal Surfaces ◽  
Complete Relaxation ◽  
Interfacial Force

We have used the interfacial force microscope to perform nanoindentations on Au single-crystal surfaces. We have observed two distinct regimes of plastic deformation, which are distinguished by the magnitude of discontinuities in load relaxation. At lower stresses, relaxation occurs in small deviations from elastic behavior, while at the higher stresses they take the form of large load drops, often resulting in complete relaxation of the applied load. These major events create a relatively wide plastic zone that subsequently deepens more rapidly than it widens. We discuss these findings in terms of contrasting models of dislocation processes in the two regimes.

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.

Fatigue crack growth modelling by successive blocking of dislocations

1992 ◽  
Vol 437 (1900) ◽  
pp. 375-390 ◽  
Keyword(s):  
Plastic Zone ◽  
Crack Growth ◽  
Driving Force ◽  
Transition Point ◽  
Growth Modelling ◽  
Microstructural Parameter ◽  
Critical Position ◽  
Crack Driving Force ◽  
Short Cracks ◽  
The Moment

Work hardening and the study of instability is incorporated into the description of the growth of a crack in terms of the successive blocking of the plastic zone by slip barriers, such as grain boundaries, and the subsequent initiation of the slip in neighbouring grains. A simple equation is derived to determine the critical position of the crack tip in relation to the grain boundary where the plastic zone is blocked at the moment of slip transmission. The intermittent pattern of decelerating and accelerating behaviour of short cracks and the existence of non-propagating cracks is explained. Instability in crack growth is seen to occur when the rate of hardening is insufficient to compensate for the increase in crack driving force in relation to the increase in crack length. This is associated with fracture toughness. The transition point between the short and long crack régimes is seen to occur when the size of the plastic zone is of the order of the microstructural parameter.

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.

scholarly journals A Finite Element Analysis of a Crack Penetrating or Deflecting into an Interface in a Thin Laminate

2006 ◽  
Vol 312 ◽  
pp. 173-178 ◽  
Author(s):  
Sharon Kao-Walter ◽  
Per Ståhle ◽  
Shao Hua Chen
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Driving Force ◽  
Crack Tip ◽  
Element Analysis ◽  
Linear Fracture ◽  
Finite Element Calculations ◽  
Straight Ahead

The crack tip driving force of a crack growing from a pre-crack that is perpendicular to and terminating at an interface between two materials is investigated using a linear fracture mechanics theory. The analysis is performed both for a crack penetrating the interface, growing straight ahead, and for a crack deflecting into the interface. The results from finite element calculations are compared with asymptotic solutions for infinitesimally small crack extensions. The solution is found to be accurate even for fairly large amounts of crack growth. Further, by comparing the crack tip driving force of the deflected crack with that of the penetrating crack, it is shown how to control the path of the crack by choosing the adhesion of the interface relative to the material toughness.

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.

scholarly journals Experimental and Numerical Study on Plastic Zone Variation ahead of Fatigue Crack Tip

2018 ◽  
Vol 165 ◽  
pp. 06004
Author(s):  
Zhang Wei ◽  
Zhou Daoqing ◽  
Cai Liang
Keyword(s):  
Plastic Deformation ◽  
Fatigue Crack ◽  
Plastic Zone ◽  
Crack Growth ◽  
Crack Tip ◽  
Image Correlation ◽  
In Situ Observation ◽  
Crack Growth Behaviour ◽  
Single Overload

The plastic deformation ahead of crack tip is of great significance to analysis of the fatigue crack growth behaviour. Using the in-situ microscopy experiment technique, the variation of strain field in the vicinity of crack tip is investigated within load cycles at the small time scale. The contours of plastic zones are measured through the in-situ observation and digital image correlation (DIC). Finite element method (FEM) is also used to simulate the plasticity ahead of the crack tip. Furthermore, the numerical studies are extended to the single overload case to analyse the effect of large plastic zone on the subsequent crack growth. The evolution of residual stress is extracted by FEM simulation to explore the influence of plastic deformation before, during and after the single overload applied on the following crack propagation. Based on the FEM analysis, a model is proposed to approximate the size of the overload effect zone. Finally, some experimental data and numerical simulations are employed to validate this model.

Progressive Debonding of Multilayer Interconnect Structures

1997 ◽  
Vol 473 ◽  
Author(s):  
Michael Lane ◽  
Robert Ware ◽  
Steven Voss ◽  
Qing Ma ◽  
Harry Fujimoto ◽  
...  
Keyword(s):  
Thin Films ◽  
Crack Growth ◽  
Driving Force ◽  
Adhesion Energy ◽  
Time Dependent ◽  
Multilayer Thin Films ◽  
Processing Conditions ◽  
Interface Morphology ◽  
Entire Sample ◽  
Crack Growth Velocity

ABSTRACTProgressive (or time dependent) debonding of interfaces poses serious problems in interconnect structures involving multilayer thin films stacks. The existence of such subcriticai debonding associated with environmentally assisted crack-growth processes is examined for a TiN/SiO2 interface commonly encountered in interconnect structures. The rate of debond extension is found to be sensitive to the mechanical driving force as well as the interface morphology, chemistry, and yielding of adjacent ductile layers. In order to investigate the effect of interconnect structure, particularly the effect of an adjacent ductile Al-Cu layer, on subcriticai debonding along the TiN/SiO2 interface, a set of samples was prepared with Al-Cu layer thicknesses varying from 0.2–4.0 μm. All other processing conditions remained the same over the entire sample run. Results showed that for a given crack growth velocity, the debond driving force scaled with Al-Cu layer thickness. Normalizing the data by the critical adhesion energy allowed a universal subcriticai debond rate curve to be derived.

Finite Element Analysis of Creep Damage Behavior Near the Cavity on Bimaterial Interface

2006 ◽  
Vol 324-325 ◽  
pp. 951-954 ◽  
Author(s):  
Qing Min Yu ◽  
Zhu Feng Yue ◽  
Yong Shou Liu
Keyword(s):  
Finite Element ◽  
Elastic Moduli ◽  
Elastic Stress ◽  
Creep Damage ◽  
Ductile Material ◽  
Bimaterial Interface ◽  
Modulus Ratio ◽  
Element Analysis ◽  
Damage Law ◽  
The Difference

Fracture along an interface between materials plays a major role in failure of material. In this investigation, finite element calculations with Kachanov–Rabotnov damage law were carried out to study the creep damage distribution near the interface cavity in bimaterial specimens. The specimens with central hole were divided into three types. The material parameters of K-R law used in this paper were chosen for a brittle material and ductile material. All calculations were performed under four load cases. Due to the difference between elastic moduli of the bounded materials, the elastic stress field as a function of the Young’s modulus ratio (R=E1/E2) was determined. At the same time, the influence of model type on elastic stress distribution near the cavity was considered. Under the same conditions, the material with larger modulus is subjected to larger stress. The creep damage calculations show that the location of the maximum damage is different for each model. The distributions of creep damage for all three models are dependent on the material properties and load cases.

Fracture mechanics in design and service: ‘living with defects’ - Application of fracture mechanics to rubber articles, including tyres

1981 ◽  
Vol 299 (1446) ◽  
pp. 189-202 ◽  
Keyword(s):  
Fracture Mechanics ◽  
Crack Growth ◽  
Strain Energy ◽  
Tensile Failure ◽  
Practical Application ◽  
Complicated Fracture ◽  
Fracture Work

The use of a fracture mechanics approach, based on the rate of release of strain energy, to account for various features of the failure of vulcanized rubbers is outlined. The properties considered include those to which fracture mechanics is often applied — tear, tensile failure, crack growth and fatigue — and others to which its application is less usual — abrasion, ozone attack and cutting by sharp objects. The relation of macroscopically observed properties to the basic molecular strength of the material is also discussed. An example of a quantitative practical application of the rubber fracture work, to groove cracking in tyres, is then considered. Finally, the rather more complicated fracture that can occur in rubber—cord laminates is discussed and it is shown that the energetics approach can be applied to some features, at least, of this.

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