A. Moitra

Characterization of Crack Arrest Phenomena in a Modified 9Cr-1Mo Steel

Crack arrest toughness (K1A) is a measure of the material’s ability to stop a propagating cleavage crack. However, experimental evaluation of K1A from impact tests of small size specimens is often difficult due to the uncertainties involved in the determination of the two necessary parameters, namely, the crack arrest load (Parrest) and the crack arrest length (aarrest). In this study with Mod.9Cr-1Mo steel in normalized and tempered and in subsequent cold worked conditions, these two parameters and thus the K1A have been determined from instrumented impact tests carried out in the ductile-brittle transition temperature regime. While the Parrest has been directly determined from the load-displacement traces, the aarrest was inferred using an analytical technique, namely, the “key curve” method. The crack arrest phenomenon has been attributed to the cumulative effect of the blocking of the cleavage crack at the microstructural boundaries with high degree of misorientations as identified by scanning electron microscope and electron back scattered diffraction studies. The effect of cold work on the K1A has been found to be insignificant.

A Local Damage Approach to Predict Crack Initiation in Type AISI 316L(N) Stainless Steel

A local damage approach based on plastic strain equivalent to uniform strain and grain diameter of the material is proposed for prediction of crack initiation. Plane strain, plane stress, and 3D FEM simulations are carried out for compact tension (CT) geometry with blunt notch of different a/W ratios under mode-I loading. Elastic-plastic fracture parameters have been estimated based on certain assumptions on blunting at notch tip and micromechanisms of events leading to onset of crack. The various crack initiation parameters evaluated based on proposed local damage approach and initial assumptions have been verified by conducting experiments on CT specimens and subsequent scanning electron microscopy study on fracture surface. The laboratory scale experimental results of AISI 316L(N) stainless steel material are in good agreement with FEM-predicted fracture parameters for notch type of stress raisers. The local damage approach and FEM procedure established in the present study would be easily extendable to the analysis of stress raisers in components for the prediction of crack initiation under elastic-plastic condition.

Effect of specimen size in determining ductile to brittle transition temperature

Ductile to brittle transition temperature (DBTT) for 9Cr–1Mo steel has been determined from Charpy impact testing for full size and subsized specimens. DBTT was obtained at various percentage of upper shelf energy (USE). Assuming that most of the energy is spent in crack initiation, notch root volumes of subsized specimens (VNS) were normalised with full size specimen (VNF), and a power law relationship between DBTT and notch root volume has been established. From finite element method, it is observed that the sum of von Mises stress (σeq) and hydrostatic stress (σh) reaches ∼2400 MPa (fracture stress, σf*) as the specimen dimension decreases at a temperature corresponding to 33% USE. This corresponds to ∼68 J of full size specimen used in the determination of nil ductility transition temperature.

Coupled FEM and experimental analysis to characterize initial crack growth regime in AISI 316L(N) stainless steel

Purpose – The purpose of this paper is to present a methodology to predict initial crack growth behavior of crack or notch like stress raisers in AISI 316L(N) stainless steel material subjected to monotonic loading condition. Design/methodology/approach – The methodology for critical crack blunting corresponding to crack initiation in crack or notch like stress raisers is based on critical plastic strain (Epc) at a characteristic distance (lc), where uniform strain (Eu) is considered as Epc and two grain diameter is considered as lc. Further crack growth is based on parabolic crack tip opening displacement (CTOD) scheme established based on coupled experimental and FEM analysis of compact tension (CT) specimen subjected to mode-I loading condition. The FEM predicted load-displacement plots is compared with experimental result of CT specimens with different a/W ratios. It has shown that the proposed methodology could account initial crack blunting appropriately and predict the fracture load and load-displa...

A Study of Tensile Flow and Work-Hardening Behavior of Alloy 617

The simple power relationship σ = Κεpn satisfactorily expresses the tensile flow behavior of many metals and alloys in their uniform plastic strain regime. However, many FCC materials with low stacking fault energy have opposed such power law relationship. Alloy 617, an age-hardenable Ni-based superalloy is also observed not to obey the simple power law relationship neither in its solution-treated nor in its aged conditions. Various flow relationships were used to obtain the best fit for the tensile data, and different relationships were identified for the different aged conditions. The work-hardening rate (θ) demonstrates three distinct regions for all aged conditions, and there is an obvious change in the trend of θ versus σ. In the initial portion, θ decreases rapidly followed by a gradual increase in the second stage and again a decrease in its third stage is perceived in the Alloy 617. These three-stage characteristics are attributed to a commonly known precipitate, γ′: Ni3(Ti, Al) which evolves during aging treatment and well recognized under transmission electron microscopy (TEM) observation. TEM results also reveal a slight degree of coarsening in γ′ over aging. The tensile flow and the work-hardening behavior are well correlated with other microstructural evolution during the aging treatments.

Ductile damage parameters and far field J-integral for high hardening steel: numerical simulations and experimental Validation

Purpose The purpose of this paper is to present a methodology to assess material damage parameters for ductile crack initiation and growth ahead of a crack/notch tip in high hardening steel like AISI type 316L(N) stainless steel. Design/methodology/approach Ductile damage parameter and far field J-integral have been obtained from standard FEM analysis for a crack/notch tip undergoing large plastic deformation and resulting in crack initiation/growth. In conjunction with experimental results, the damage variable for low strength and high hardening material has been derived in terms of continuum parameters: equivalent plastic strain (eeq) and stress triaxiality (φ). The material parameters for damage initiation and growth in 316LN SS have been evaluated from tensile and fracture tests. With these material tensile/fracture parameters as input, elastic-plastic eXtended Finite Element Method (X-FEM) simulations were carried out on compact tension (CT) specimen geometry under varying initial stress triaxiality conditions. Findings The material parameters for damage initiation and growth have been assessed and calibrated by comparing the X-FEM predicted load-displacement responses with the experimental results. It is observed that the deviations in the predicted load values from the experimental data are within 6 percent for specimens with a/W=0.39, 0.55, 0.64, while for a/W=0.72, it is 17 percent. Originality/value The present study is a part of developing methods to obtain calibrated material damage parameters for crack growth simulation of components made of AISI 316L(N) stainless steel. This steel is used for fast breeder reactor-based power plant being built at Kalpakkam, India.

Evaluation of Ductile-to-Brittle Transition Temperature by Reference Temperature (T0) Approach at Higher Loading Rates for a Mod.9Cr-1Mo Steel

To evaluate the ductile-to-brittle transition temperature of ferritic–martensitic steels, the ASTM E1921-based reference temperature (T0) approach has now been widely recognized; however, until now, the standard restricts itself to static/quasi-static loading rates. It is well recognized that the flow stress of rate-sensitive material increases with the strain rate, and thus it is imperative that the increase in loading rate would lead to limited plasticity-induced brittleness, reflected in higher T0. There have been efforts in the literature for developing empirical correlations to derive T0 at higher loading rates from T0 at quasi-static loading rates or vice versa. However, there is a need to experimentally evaluate the T0 at higher loading rates, especially for the 9Cr-1Mo family of steels, proposed to be used as wrapper material in the upcoming commercial liquid-sodium-cooled fast breeder reactors in India. The present study is directed toward determining T0 for Mod.9Cr-1Mo steel at loading rates of 1.12, 3, and 5 m/s.