Circumferential Notch Tensile Specimens Research Articles

Effect of retrogression and reaging (RRA) on pitting and stress corrosion cracking (SCC) resistance of stir zone of high strength AA7075-T651 alloy joined by friction stir welding

The main purpose of this study is to investigate the influence of retrogression and re-aging post weld heat treatment (RRA-PWHT) on potentiodynamic corrosion (PC) and stress corrosion cracking resistance (SCC) of stir zone (SZ) of AA7075-T651 alloy butt joints developed using friction stir welding (FSW). FSW was employed to overcome the problems in fusion welding of AA7075-T651 alloy such as solidification cracking, heat affected zone (HAZ) softening, grain coarsening, dissolution of strengthening precipitates and inferior mechanical performance. The SZ of joints were subjected to potentiodynamic corrosion test in 3.56 wt% NaCl solution. The circumferential notch tensile (CNT) specimens were subjected to SCC test at various conditions of axial stress level in 3.56 wt% NaCl solution. The microstructural features of butt joints SZ were characterized using optical (OM), scanning electron (SEM) and transmission electron microscope (TEM). The results disclosed that RRA-PWHT joints exhibited greater PC and SCC resistance compared to as-welded joints. The threshold stress level for the failure of CNT specimens of parent metal, as-welded joints and RRA-PWHT joints was 242 MPa, 175 MPa and 198 MPa, respectively. The threshold stress level of RRA-PWHT joints and as-welded joints is 81.81% and 72.31% of parent metal threshold stress level. It is correlated to increase in vol% and coarsening of grain boundary precipitates with enrichment of copper at the grain boundaries.

Estimating fracture toughness of various matrix structured ductile iron using circumferentially notched tensile bars

In this experimental study, circumferentially notched tensile (CNT) specimens with various notch root radii of 0.05, 0.1, 0.16, 0.25, 0.4, and 0.8 mm were employed to investigate the influence of matrix structure on the fracture toughness of 1.03% Cu, 1.25% Ni, and 0.18% Mo alloyed ductile iron. The assayed matrix structures in this study were ferritic, pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic. The microstructures were obtained by several heat treatment processes. The microstructures and morphology of broken surfaces of CNT specimens were also examined by the optical and scanning electron microscopes. It was observed that for the steady-phased (ferritic, pearlitic/ferritic, and pearlitic) matrixes, the increasing rates of fracture toughness under the applications of different notch tip radii were lower than that of the tempered martensitic, lower and upper ausferritic matrixes. It was also inferred that lower values of fracture toughness were obtained by the applied rapid method.

Efficient approach for cohesive zone based three-dimensional analysis of hydrogen-assisted cracking of a circumferentially notched round tensile specimen

An efficient ABAQUS based finite element (FE) analysis for hydrogen-assisted cracking is presented. Analysis of two dimensional (2-D) diffusion of hydrogen is coupled with three dimensional (3-D) stress analysis incorporating cohesive elements. The deleterious effect of hydrogen on crack growth is introduced through the hydrogen enhanced decohesion (HEDE) mechanism where modification to the cohesive energy is related to local hydrogen concentration. The proposed method is applied to hydrogen-assisted cracking of a circumferentially notched tensile (CNT) specimen under three different conditions of hydrogen supply and its performance is demonstrated through the close agreement between the corresponding computed critical stress intensity factors (SIFs) and the experimental results reported in the literature. The modelling duly accounts for the off-centre precracked ligament usually resulting from fatigue precracking of CNT specimens. The results clearly bring out the non-axisymmetric nature of concentration distribution and stress field, and reveal enhanced hydrogen diffusion at a point on the outer edge of the ligament profile where crack depth is the highest and wherefrom the crack extension is likely to initiate.

Investigation of role of alloy microstructure in hydrogen-assisted fracture of AISI 4340 steel using circumferentially notched cylindrical specimens

The present study investigates the role of three different microstructures of a high strength steel AISI 4340 in relation to its susceptibility to hydrogen-assisted cracking (HAC), and demonstrates the ability of circumferential notch tensile (CNT) testing in distinguishing the role of the three microstructures. Critical stress intensity factors (SIFs) of CNT specimens of the steel with different microstructures (viz., the spheroidite (the as-received steel), the untempered martensite (the oil-quenched steel) and the tempered martensite (the oil-quenched + tempered steel)) were determined under two conditions at the room temperature, i.e., in air and under in-situ cathodic charging in 3.5% NaCl solution. The CNT tests under the two conditions show the as-received steel (with the spheroidite microstructure) to have a considerably high critical SIF (~58MPam0.5) in air, and an insignificant decrease in critical SIF data when the test condition was changed to the cathodic charging, suggesting an insignificant susceptibility of the steel with the spheroidite microstructure to HAC. In contrast, the oil-quenching of the steel dramatically decreased the critical SIF to ~17MPam0.5 in air, suggesting the highly strained martensitic microstructure to be more susceptible to cracking in air. The low critical SIF of the oil-quenched steel became even lower (~10MPam0.5) under the cathodic charging condition, suggesting increased susceptibility of the martensitic microstructure to HAC. However, tempering of the martensitic microstructure at 300°C for 2h raised the critical SIF in air (by ~3 times), i.e., fracture resistance increased dramatically, but no significant improvement in the resistance to HAC was noticed. The hardness data of steels with the different microstructures are consistent with the trend in the corresponding SIF data. The study also provides two practical information: (a) an SIF below 10MPam0.5 under operating load would avoid HAC in AISI 4340 steel in applications, and (b) the relatively new CNT technique is a viable low-cost testing option in assessing the role of microstructural variations in the context of HAC of high strength structural steels.

Validity requirements of circumferentially notched tensile specimens for the determination of the interfacial fracture toughness of coatings

A simple method was developed for evaluating the interfacial fracture toughness of coatings on substrates using circumferentially notched tensile (CNT) specimens. Mild steel cylindrical substrates of 0°, 15°, 30°, 45° and 60° notch angles with electroplated nickel were tensile tested. A well defined pre-crack was introduced at the interface for the quantitative evaluation of adhesion. In situ acoustic signals and scanning electron microscope were used to analyze the crack initiation and propagation. Finite element analyses were used to evaluate the critical interface energy release rate. The size of the plastic zone was determined for different notch angles to validate application of the linear elastic approach in determining the interfacial fracture toughness. The validity requirements have been proposed for this specimen, considering the yield strength of the coating and substrate, pre-crack position, notch angle and plastic zone size. The obtained interfacial fracture toughness values using CNT specimens was found to be very close to the values obtained by others using standard specimens.

Quantitative evaluation of the adhesion of metallic coatings using cylindrical substrate

This paper introduces an effective interfacial fracture toughness test based on interface fracture mechanics theory. This testing method uses a circumferentially notched tensile (CNT) specimen, which is ideally suited for determining the interfacial fracture resistance of coatings. Unlike other interfacial fracture tests, this test is simple to prepare, requires minimum test setup and is easy to model. An interfacial pre-crack was generated between a nickel coating and mild steel cylindrical substrate to evaluate adhesion strength. In situ acoustic and SEM analyses were used to determine the crack initiation or the critical load of failure. The critical energy release rate, critical stress intensity factors and phase angle were determined using the J integral which was determined by applying the critical load to the finite element model. A detailed finite element analysis was carried out to study the effect of different interface pre-crack positions and mode mixity on energy release rate for different notch angles and elastic modulus ratios. The cracking resistance of the interface was characterised by the notch angle of CNT specimens. The analysis showed an increase in interfacial fracture toughness as phase angle increases and was significant when the phase angle was large. The combined results of computational and experimental analysis showed that any defect or stress concentration at the interface could significantly weaken the adhesion of coating.

Threshold stress intensity factor and crack growth rate for stress corrosion cracking of simulated heat affected zone in caustic solution

The threshold stress intensity factor for stress corrosion cracking (KIscc) of the simulated heat affected zone (HAZ) of mild steel in caustic solution has been determined using circumferential notch tensile (CNT) technique. The HAZ microstructure produced upon manual metal arc welding of grade 250 steel was simulated over a length of 35 mm of CNT specimens, using a thermo-mechanical simulator. Inter- and trans-granular stress corrosion cracking has been confirmed using a scanning electron microscope. The results presented here validate the ability of CNT technique for the determination of KIscc of HAZ and Base metal. Crack growth rates have also been determined using CNT technique. Further, the effect of microstructures on KIscc and crack growth rate is discussed in the present study. The determined KIscc of Base metal and simulated heat affected zone in 30% caustic solution is 24 and 45 MPa m1/2, respectively.

Determination of threshold stress intensity factor for stress corrosion cracking (K ISCC) of steel heat affected zone

Threshold stress intensity factor for stress corrosion cracking of heat affected zone (HAZ) of mild steel in caustic solution has been determined using circumferential notch tensile (CNT) technique. HAZ microstructure produced during manual metal arc welding of grade 250 steel was simulated over a length of 35 mm of CNT specimens, using a thermo-mechanical simulator. Inter-granular stress corrosion cracking has been confirmed with scanning electron microscope. The results presented here validate the ability of CNT technique for determination of K ISCC of HAZ and base metal.

Fracture toughness for CNT specimens from numerically obtained critical CTOD values

Many approaches for estimating mode I fracture toughness ( K IC ) using circumferentially notched tensile (CNT) specimen have been demonstrated in the literature. In this paper, an effective approach for estimating fracture toughness from the numerical solution of critical crack tip opening displacement (CTOD) is proposed. An elasto-plastic finite element analysis is used to estimate critical CTOD values for CNT specimens. A number of materials are analysed, and the relationship between K IC and critical CTOD for CNT specimens is obtained. The proposed relationship is validated by comparing the fracture toughness values obtained from the relationship with those obtained experimentally using CNT specimens. The fracture toughness ( K IC ) calculated according to this relationship from numerically obtained critical CTOD is found to be in close agreement with the experimentally obtained fracture toughness for the respective materials.

Circumferential Notch Tensile Testing: Role of Imposed Electrochemical Potentials in Susceptibility of Steel to Caustic Cracking

A fracture mechanics based novel approach, i.e., circumferential notch tensile (CNT) testing has been employed for determination of threshold stress intensity factor for susceptibility of engineering materials to stress corrosion cracking . Using CNT technique, of a mild steel at an open circuit potential in NaOH at was determined to be . CNT tests have also been performed under imposed electrochemical potentials to understand the mechanistic aspects of caustic cracking as well as to develop guidelines for mitigation of caustic cracking. An imposed potential in the active-passive potential regime caused much more rapid failure (than was observed at ). The fractography of the CNT specimens tested at and showed evidence of stress corrosion cracking. At an imposed potential in the passive region , the specimen did not fail even after an order of magnitude longer testing, indicating a considerable improvement in resistance to caustic cracking as a result of this application of . The resulting fractograph confirmed the absence of caustic cracking. The study has established the use of CNT testing as a simple, relatively fast and economical approach for generating the data, and showed that the data are consistent with an electrochemical mechanism for caustic cracking.

Role of imposed potentials in threshold for caustic cracking susceptibility ( KISCC): Investigations using circumferential notch tensile (CNT) testing

A fracture mechanics-based novel approach, i.e. circumferential notch tensile (CNT) testing has been employed for determination of threshold stress intensity factor for susceptibility of engineering materials to stress corrosion cracking ( K ISCC) using small specimens. Using CNT technique, K ISCC of a carbon steel at an open circuit potential ( E corr) in 500 g L −1 NaOH at 100 °C was determined to be 42.9 MPa m 1/2. In order to establish the application of the CNT technique in understanding the mechanistic aspects of caustic cracking as well as for developing guidelines for mitigation, tests have also been performed under the imposed electrochemical potentials. An imposed potential in the active–passive potential regime ( E a–p) caused an extremely rapid failure (than observed at E corr) whereas, at an imposed potential in the passive region ( E p), the specimen did not fail even after relatively very long exposure time. The fractography of the CNT specimens tested at E corr and E a–p presented evidence of SCC. The study has established the use of experimental CNT testing as a simple, relatively fast and cost-advantageous approach for generating the K ISCC data, which are also consistent with the electrochemical mechanism for caustic cracking.

Validation of a novel approach to determination of threshold for stress corrosion cracking ( KISCC)

Determination of threshold stress intensity for stress corrosion cracking ( K ISCC), using circumferential notch tensile (CNT) specimens has been developed recently. This has also been claimed to be a novel and considerably cost-advantageous approach in comparison with the traditional methods, viz., compact tension (CT) and double cantilever beam (DCB) techniques for determination of K ISCC. However, the limited first K ISCC data, generated using CNT specimens have been queried for the lack of their comparison with the data generated by other techniques. This work presented here provides the first comparative results and validation for use of the novel CNT testing for generation of K ISCC data.

A novel approach to the determination of the threshold for stress corrosion cracking (K ISCC) using round tensile specimens

This article discusses determination of threshold stress intensity for propagation of stress corrosion cracking (KISCC), using circumferential notch tensile (CNT) specimens. Use of round tensile specimens is a novel and cost-advantageous approach to determination of KISCC. However, compliance of this specimen geometry to the constraints for application of linear elastic fracture mechanics (LEFM) has traditionally been argued, and hence this aspect is addressed in detail. The LEFM suits best the materials that undergo brittle cracking, and hence a highly brittle material, cast iron, has been selected as the test material. However, susceptibility of this material to caustic embrittlement has been established employing another technique, viz. slow strain rate testing and fractography of the specimens. Using CNT specimens, KISCC has been determined for the cast iron in hot caustic solutions, and the features of intergranular caustic cracking and secondary cracking have been established using scanning electron microscopy.