| 1. |
- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
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ECCl/EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels
- 2017
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Ingår i: International Journal of Fatigue. - London : Elsevier. - 0142-1123 .- 1879-3452. ; 100, s. 251-262
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Tidskriftsartikel (refereegranskat)abstract
- Fatigue test data of duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue (VHCF) regime is presented. Fatigue testing was conducted using ultrasonic fatigue test equipment operating at 20 kHz under fully reversed tension-compression load condition. Scanning Electron Microscope (SEM) analysis of the fracture surfaces and external surfaces of failed specimens was conducted. Electron Channelling Contrast Imaging (ECCI) and Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens was done to analyse the accumulation of plastic fatigue damage and fatigue crack growth in the grains adjacent to the external surface and crack initiation site. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 was carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the Crystallographic Growth Region (CGR) showed features like grain boundaries and fatigue striations on the fracture surfaces. SEM analysis of the external surfaces of fatigue loaded specimens showed inhomogeneous accumulation of plastic fatigue damage. ECCl/EBSD analysis showed Persistent Slip Bands (PSBs) in ferrite grains in LDX 2101 grade but no PSBs were observed in any grains of 2304 SRG specimens. The barrier effect of grain and phase boundaries on short fatigue crack propagation was observed. TEM analysis of thin foils cut from the failed specimens of LDX 2101 showed stacking faults in austenite grains and they were seen to stop at the grain and phase boundaries. (C) 2017 Elsevier Ltd. All rights reserved.
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| 2. |
- Hallbäck, Nils, et al.
(författare)
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Development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half-plane
- 2014
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 51, s. 2878-2892
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Tidskriftsartikel (refereegranskat)abstract
- A distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half plane has been developed. The dipole density distribution is represented with a weighted Jacobi polynomial expansion where the weight function captures the asymptotic behaviour at each end of the crack. To allow for opening and sliding at crack kinking and branching the dipole density representation contains conditional extra terms which fulfil the asymptotic behaviour at each endpoint. Several test cases involving straight, kinked and branched cracks have been analysed, and the results suggest that the accuracy of the method is within 1% provided that Jacobi polynomial expansions up to at least the sixth order are used. Adopting even higher order Jacobi polynomials yields improved accuracy. The method is compared to a simplified procedure suggested in the literature where stress singularities associated with corners at kinking or branching are neglected in the representation for the dipole density distribution. The comparison suggests that both procedures work, but that the current procedure is superior, in as much as the same accuracy is reached using substantially lower order polynomial expansions.
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| 3. |
- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
-
Development of the distributed dislocation dipole technique for the analysis of closure of complex fractures involving kinks and branches
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents the development of the distributed dislocation dipole technique (DDDT) for the analysis of crack surface closure of crack cases involving kinks and branches. Crack cases in which closure occurs are analyzed by reformulating the Bueckner's principle taking the contact stresses at the contacting portions of the crack surfaces into account. Stress intensity factors corresponding to opening and sliding mode of deformation at the crack tips are computed. Three test cases involving kinked and/or branched cracks with at least one of the crack segments undergoing crack surface closure when subjected to remote tensile loading are analyzed. The results obtained from the DDDT are compared to those obtained from the Finite Element Method (FEM) analysis of the same crack cases. This comparison shows that the computation of stress intensity factors for the crack cases involving crack surface closure are less acurate compared to fully open crack cases. However, the stress intensity factors are still computed to an accuracy of within 2 percent if the Jacobi polynomial expansions of at least the sixth order are used to represent the crack surface opening and sliding displacements. Higher order Jacobi polynomials lead to increased accuracy.
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| 4. |
- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
-
Development of the distributed dislocation dipole technique for the analysis of closure of complex fractures involving kinks and branches
- 2018
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Ingår i: European journal of mechanics. A, Solids. - : Elsevier. - 0997-7538 .- 1873-7285. ; 69, s. 168-178
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the development of the distributed dislocation dipole technique (DDDT) for the analysis of straight, kinked and branched cracks where parts of the cracks may close during loading. The method has been developed for plane problems. Crack cases in which closure occurs are analyzed by reformulating the Buecicner's principle, taking into account the contact stresses at the contacting portions of the crack surfaces. Stress intensity factors corresponding to opening and the in-plane sliding mode of deformation at the crack tips are computed. Several test cases involving straight, kinked and/or branched cracks where parts of the cracks undergoes crack surface closure when subjected to the outer loading are analyzed. The results obtained from the DDDT are compared to those obtained from a Finite Element Method (FEM) analysis of the same crack cases. This comparison shows that the computation of stress intensity factors for the cases involving crack surface closure are less accurate than those for fully open crack cases. However, for the cases under consideration, the stress intensity factors were still computed with a maximum difference of approximately 2 per cent compared to the FEM calculations if Jacobi polynomial expansions of at least the twelfth order were used to represent the crack surface opening and sliding displacements. In most cases under consideration, sixth order Jacobi polynomial expansions were sufficient to obtain results within that margin of deviation.
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| 5. |
- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
-
EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Fatigue test data (stress-life) of two duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue regime (VHCF) is presented. The fatigue testing is conducted using an ultrasonic fatigue testing equipment operating at 20 kHz under fully reversed tension-compression condition (R=-1). Scanning Electron Microscope (SEM) studies of the fracture surfaces and external surfaces of failed specimens is conducted. Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens is done to analyse grains near the external surface and crack initiation site. Analysis of accumulation of plastic fatigue damage and growth of cracks in the Crystallographic Growth Region (CGR) is carried out. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 is carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the CGR show features like grain boundaries and fatigue striations. The barrier effect of grain and phase boundaries on short fatigue crack propagation is observed. ECCI images and EBSD analysis show that Persistent Slip Bands (PSBs) are observed in ferrite grains in LDX 2101 grade. On the other hand, no PSBs are observed in any of the grains in 2304 SRG. The TEM observations in thin foils cut from the failed specimen of LDX 2101 show stacking faults in austenite grains. Stacking faults were observed to stop at the grain and phase boundaries.
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| 6. |
- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
-
Fatigue initiation and strength of duplex stainless steel strip specimens in the very high cycle fatigue regime
- 2014
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Ingår i: Very high cycle fatigue 6 (VHCF6).
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Konferensbidrag (refereegranskat)abstract
- Fatigue studies of cold-rolled duplex stainless strip steel were performed in the very high cycle fatigue life region. The duplex austenitic-ferritic microstructure gives this grade a combination of high mechanical strength and high corrosion resistance. Fatigue properties of thin steel strips are particular due to cold rolling introducing a very fine microstructure. Crack initiation and fatigue strength are controlled by steel microstructure and alloying. The initiation and growth of the very short initial fatigue crack in very high cycle fatigue are unclear and subject to different descriptions. Fatigue test data of thin strip specimens at very high fatigue lives are scarce due to testing difficulties. For practical reasons testing must be performed at ultrasound test frequencies which involves fixturing problems. A test setup including the load chain ultrasonic horn, fixture and specimen was designed for resonance with a horse-shoe design of a screw fixture. The design of the horse-shoe fixture and the specimens along with FEM calculation of eigenfrequency are presented. Fatigue testing was performed at 20 kHz in R=-1 conditions up to fatigue life of 107 to 5*109 cycles. Fatigue strength was tested and crack initiation was studied on the fracture surface using FEG-SEM at the initiation site.
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| 7. |
- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
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Fatigue strength, crack initiation, and localized plastic fatigue damage in VHCF of duplex stainless steels
- 2016
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Ingår i: Steel Research International. - : Wiley-VCH Verlagsgesellschaft. - 1611-3683 .- 1869-344X. ; 87:7, s. 899-910
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Tidskriftsartikel (refereegranskat)abstract
- The fatigue strength of two-duplex stainless steel grades, 2304 SRG and LDX 2101, with austenitic–ferritic microstructure is tested using ultrasonic fatigue testing equipment operating at 20 kHz. The testing is conducted in tension-compression mode with the load ratio R=-1. The fatigue strength is evaluated at 107, 108, and 109 load cycles and the estimates of fatigue strength are higher for the LDX 2101 grade. The fatigue crack initiation mechanisms are analyzed using a scanning electron microscope. The fatigue cracks, in all cases, appear to initiate due to accumulation of plastic fatigue damage at the surface. In the 2304 SRG grade, accumulation of fatigue damage occurs at the external surface of fatigued specimens in the form of extrusions at the grain/phase boundaries and in the form of individual slip lines in the austenite phase. Meanwhile, in the LDX 2101 grade accumulation of plastic fatigue damage in the form of extrusions and intrusions occurs mainly within the ferrite grain. When the crack is microstructurally short, the crack growth appears to be crystallographic in nature and the crack appears to change its direction propagating from one grain into another.
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| 8. |
- Tofique, Muhammad Waqas, 1986-
(författare)
-
Initiation and early crack growth in VHCF of stainless steels : Experimental and theoretical analysis
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mechanical fatigue is a failure phenomenon that occurs due to repeated application of mechanical loads. Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Increasing numbers of structural components have service life in the VHCF regime, for instance in automotive and high speed train transportation, gas turbine disks, and components of paper production machinery. Safe and reliable operation of these components depends on the knowledge of their VHCF properties. In this thesis both experimental tools and theoretical modelling were utilized to develop better understanding of the VHCF phenomena.In the experimental part, ultrasonic fatigue testing at 20 kHz of cold rolled and hot rolled stainless steel grades was conducted and fatigue strengths in the VHCF regime were obtained. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. For the cold rolled stainless steels crack initiation and early growth occurred through the formation of the Fine Granular Area (FGA) observed on the fracture surface and in TEM observations of cross-sections. The crack growth in the FGA seems to control more than 90% of the total fatigue life. For the hot rolled duplex stainless steels fatigue crack initiation occurred due to accumulation of plastic fatigue damage at the external surface, and early crack growth proceeded through a crystallographic growth mechanism.Theoretical modelling of complex cracks involving kinks and branches in an elastic half-plane under static loading was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The technique was implemented for 2D crack problems. Both fully open and partially closed crack cases were analyzed. The main aim of the development of the DDDT was to compute the stress intensity factors. Accuracy of 2% in the computations was attainable compared to the solutions obtained by the Finite Element Method.
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| 9. |
- Tofique, Muhammad Waqas, 1986-
(författare)
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Review of VHCF studies, short crack models and theory of vibrations
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This literature review loosely connects different concepts of very high cycle fatigue which are of significance within the scope of this thesis work. The ultrasonic fatigue testing equipment and its basic principle of operation is discussed in detail. The phenomenon of damping in the light of theory of vibrations is outlined. The significance of the understanding of damping phenomenon in the ultrasonic fatigue testing is highlighted. On the other hand, SN fatigue data obtained by several researchers for different materials are reviewed. In addition, the fundamental mechanisms responsible for fatigue crack initiation are also summarized. Considerable attention is focussed on the mathematical models developed for the prediction of growth behaviour of short fatigue cracks. The basic concepts, and models, which led to the development of the distributed dislocation dipole technique, have also been reviewed.
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| 10. |
- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
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Very high cycle fatigue crack initiation mechanisms in different engineering alloys
- 2016
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Ingår i: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21). - : Elsevier. ; , s. 1181-1190
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Konferensbidrag (refereegranskat)abstract
- The fatigue crack initiation mechanisms prevalent in high strength martensitic steel grades, hot rolled plate duplex stainless steels, cold rolled strip duplex stainless steel and a super alloy grade were compared. The fatigue testing of all the grades was conducted in the VHCF regime using an ultrasonic fatigue testing equipment operating at 20 kHz. Scanning electron microscope (SEM) observations of the fracture surfaces revealed the presence of a microstructure controlled initial growth of short fatigue cracks in all the tested grades. Fracture surfaces of the failed specimens of a high strength martensitic steel grade revealed the typical fine granular area (FGA) within the fish-eye area around the internal inclusions. Fatigue crack initiation in the cold rolled strip duplex stainless steel grade occurred at surface defects left over by the cold rolling process of this grade. However, the presence of FGA around the surface crack initiating defect was observed similar to the internal crack initiations in the high strength martensitic steels. By mapping the FGA size development during VHCF loading, as obtained from fracture surfaces, FGA growth results were obtained. A similar study on hot rolled plate duplex stainless steel grades, 2304 SRG and LDX 2101, revealed the presence of an initial crystallographic growth region (CGR) in which crack growth direction is changed by microstructural barriers such as phase and grain boundaries. The early plastic fatigue damage accumulation occurred predominantly in one phase or at the austenite-ferrite phase boundaries. On the other hand, an initial transcrystalline fatigue crack growth was observed in the Ni-based super alloy grade Inconel 718.
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