Notched Tensile Specimens Research Articles

Microstructural Engineering of Mn-Alloyed Austenitic Steel for Hydrogen Storage and Delivery

Austenitic stainless steels are commonly used for hydrogen storage and transportation. These alloys have a high nickel (Ni) content, which increases alloy cost. In this study, high manganese (Mn) austenitic alloys were evaluated as potential lower cost alternatives. Two heats of high Mn alloys with different stacking fault energies (SFE) of ~29 mJ·m-2 and 49 mJ·m-2 were acquired. Additionally, a new vanadium (V)-microalloyed high Mn alloy was designed to achieve a SFE of 47 mJ·m-2 to minimize planar slip deformation mechanisms. Post-processing via cold working in conjunction with aging was also performed on the V-microalloyed high Mn steel. Hydrogen embrittlement sensitivity was investigated using circumferential notch tensile specimens cathodically charged with hydrogen in a 0.05M NaOH electrolytic solution. The alloys were compared to a cold-worked 316L stainless steel, which exhibited no strength loss due to hydrogen. The high Mn alloys with SFE of ~29 mJ·m2 and 49 mJ·m-2 had notch strength losses of 11 and 6 pct, respectively. The V-microalloyed high Mn steel in the as-hot-rolled condition had a notch strength loss of 17 pct. The V-microalloyed high Mn steel in the cold worked and aged condition indicated no notch strength loss in hydrogen, which was comparable to the performance of the 316L stainless steel.

Stress state analysis of root notch for X80 girth welds with variable wall thickness and misalignment geometric features

Notches formed due to weld geometric features seriously affect the structural safety of the pipeline. In this paper, the stress states of girth welds under various forming conditions and internal pressures are thoroughly evaluated via numerical simulation analysis. Controlled studies are carried out between crack-free and cracked welds. The susceptibilities of stress state to variations in internal pressure under different forming conditions are elucidated. Also, newly modified single-edge notch tensile specimens (MSENT) are manufactured, and the strain characteristic and fracture behavior analysis are performed. The calculation results reveal that the stress states of either the root notch or the crack tip are significantly intensified by welds' geometric features, and the increased brittle fracture sensitivity can be qualitatively demonstrated by MSENT analysis. Simultaneously, the misalignment continuously enhances the sensitivity of stress states to internal pressure variation. However, opposite patterns are observed between crack-free and cracked welds characterizing variable-wall-thickness features. The research results can provide a strong reference for the structural design, service evaluation, and related research of pipelines.

A shear modified GTN model based on stress degradation method for predicting ductile fracture

The Gurson–Tvergaard–Needleman (GTN) model has provided a powerful description of the nucleation growth and coalescence of micro voids, but it has limitations in simulating shear fracture due to the absence of a description of shear localization behavior. A shear improvement method is proposed for simulating the ductile fracture of materials under different stress states. The modified model not only allows for strain hardening of the matrix material, but also accounts for the stress degradation caused by shear. The strength equation of the material is described by both the shear stress state function and a decay function, making it easier for materials under shear stress state to experience material softening and further inducing shear fracture. The modified GTN model is developed by incorporating the shear stress degradation factor into the yield function, while taking into account both void growth and shear failure mechanisms. By carefully calibrating the model’s parameters, the deformation and fracture processes of tensile, plane strain, notch tensile, and compression specimens in the 7A52 aluminum alloy are simulated. The damage evolution behavior of the material under different stress states is analyzed. The results indicate that the damage include void growth mechanism and void shear mechanism. The proportions of these two mechanisms vary under different levels of stress triaxiality. Upon localizing material deformation, the shear stress state intensifies, and the shear damage mechanism assumes a critical role in fracture. The modified GTN model accurately predicts the load-displacement response and fracture path of the 7A52 aluminum alloy under a wide range of stress states.

Local strain evolution and microstructural characterisation of hydrogen-induced damage at different strain rates in dual phase (DP 780) steel

Effect of strain rate on the damage behaviour of hydrogen (H)-charged dual phase (DP 780) steel via the in-situ digital image correlation (DIC) technique is investigated in this work. Since stress concentration sites like notches are common in engineering practice, two types of uniaxial tensile tests have been carried out using smooth and notch tensile specimens for detailed analysis. The study reveals the significance of hydrogen embrittlement in DP 780 steel, as no strain rate effect is observed on the mechanical property in the case of an uncharged smooth tensile specimen. However, a significant effect of strain rate is detected after the H-charging. Hydrogen showed a lesser ability to aid the failure process when the applied strain rate is raised, as it could diffuse over a limited distance during the tensile test. The local axial and width strains, along with necking and fracture strains, are quantified for each specimen to understand the strain rate effect better. A centre-line crack is observed in every H-charged specimen's fracture surface owing to the presence of MnS inclusion in DP steel along the central line and its interaction with the atomic hydrogen. Moreover, the degree of hydrogen embrittlement is substantially higher in the notch tensile specimens than in the smooth ones.

Identification of micro‐failure processes of HDPE‐henequen fiber composite material by using acoustic emission monitoring: Effect of fiber surface modification

AbstractIt is well known that fiber‐matrix interface region is crucial to ensure an efficient stress transfer between matrix and reinforcement and that it controls the composite behavior when subjected to external loads. In this experimental study, micro‐damage mechanisms of HDPE/surface modified henequen fiber reinforced composites are investigated in tensile quasi‐static mode. Two essential parameters of the materials composition were taken as main indicators: fiber length and quality of the fiber matrix adhesion. Essential Work of Fracture theory (EWF) was applied on double edge notch tensile (DENT) specimen geometry. Acoustic Emission (AE) technique was coupled to composite samples to obtain the characteristics of stress waves signals to monitor in real time damage evolution and failure mechanisms detection. Results demonstrated the substantial effect of chemical bonds interaction in the interface area on the composite mechanical properties. The total fracture work (Wf) exhibited an increasing value in both components, We and Wp, in chemically treated fiber composites. Likewise, mechanical properties are reported to be higher by 37% in maximum load in such materials. Elastic waves detected by AE were identified and correlated to micromechanical events during composite damage sequence. Most of the signals corresponded to microcracks (signals of low amplitude, short duration), propagation of microcracks (signals of medium amplitude), fracture of fibers and matrix (signals of long duration and high amplitude). AE signals detected exhibiting higher energy were associated to an enhanced fiber/matrix interface.

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.

Microstructure, tensile properties and fracture toughness of friction stir welded AA7075-T651 aluminium alloy joints

Abstract The main objective of this investigation is to study the microstructure, tensile properties and fracture toughness of friction stir welded (FSW) butt joints of 10 mm thick AA7075-T651 plates. The microstructural features of stir zone (SZ), thermos-mechanically affected zone (TMAZ), heat affected zone (HAZ) were analyzed using optical microscopy technique. The tensile properties were evaluated using smooth and notch tensile specimens and compared to base metal properties. The microhardness survey was done across the weld cross section and correlated to the failure of tensile specimens. Compact tension (CT) specimens were used to evaluate the fracture toughness of welded joints. The fractured tensile and CT specimens were analyzed using scanning electron microscopy (SEM). Results showed that the FSW AA7075-T651 specimens welded using axial load of 12 kN, tool rotation speed of 750 rpm and welding speed of 30 mm/min exhibited 412 MPa tensile strength and 9% elongation. It showed 88 and 89% of base metal strength elongation. The joints showed fracture toughness of 23 MPa m1/2 which is 80% of base metal fracture toughness. The superior tensile and fracture toughness properties of joints are mainly attributed to the evolution of finer grains in SZ due to the stirring action of FSW tool.

Experimental validation of a topological derivative-based crack growth control method using digital image correlation

PurposeThe purpose of this paper is to experimentally validate the crack growth control based on the topological derivative of the famous Rice's integral.Design/methodology/approachSingle edge notch tensile specimens with two configurations were tested. Displacement fields near notch were experimentally obtained using the digital image correlation method. These displacements were used to verify the minimization of the associated shape functional, which is defined in terms of the Rice's integral, when a set of controls (holes) positioned according to the topological derivative information, is inserted. Based on the Griffth's energy criterion, this minimization represents an improvement in the fracture toughness of cracked bodies.FindingsThe experimental tests confirmed that a decrease around 27% in the value of the associated shape functional can be obtained following this approach. Therefore, the results allow us to conclude that the predictive methodology for crack growth control based on the topological derivative is feasible.Originality/valueThis is the first work concerning experimental validation of crack growth control based on the topological derivative method.

Fatigue Crack Growth of Natural Rubber/Butadiene Rubber Blend Containing Waste Tyre Rubber Powders

Fatigue crack growth in NR/BR compound and the effect of two different types of recycled rubber powder (RRP) i.e. micronized cryo-ground 74 μm and ambient-ground 400 μm were studied using fracture mechanics approach. Absolute and relative hysteresis losses using single-edge notch tensile (SENT) specimens were determined with a displacement-controlled strain compensating for permanent set of the samples throughout the Fatigue Crack Growth (FCG) experiments. Results indicated a correlation between absolute/relative hysteresis loss and fatigue crack growth rate under specific dynamic strain amplitudes. Differences in relative hysteresis loss showed that additional energy dissipation, due to multiple new crack surfaces at the crack tip, contributes to the FCG of the RRP compounds. At higher tearing energy, beside other factors affecting the FCG performance of the RRP compounds, both higher absolute and relative hysteresis loss are slightly detrimental to the crack growth rates.

Ageing condition of tensile specimens: fracture behavior of notched Al2024 sheet under tensile loading

The effect of different (T6 and T8) heat treatments on tensile properties and fracture behavior of Al2024 in presence and absence of notch by means of microhardness, tensile tests and scanning electron microscopy were investigated. Tensile and hardness specimens were subjected to two different artificial ageing (T6 and T8 heat treatment) for various times. These included under ageing (UA), peak ageing (PA) and over ageing (OA). T8 heat treatment, which has a cold rolling between solutionizing and ageing in its steps, showed a higher value of hardness and yield strength in comparison with common artificial ageing of T6 heat treatment. In notched-tensile specimens, yield stress was found to increase up to the peak ageing condition with a simultaneously decrease in elongation at fracture. This behavior was converse at OA condition. Although introducing of notch increase the yield stress of samples under T6 and T8 conditions in comparison with un-notched samples, the notch strengthening phenomenon was observed only under T8 treatment. Despite of an enhancement in strengthening by applying notch on tensile samples, the elongation to failure was notably lessen in both notched-heat treated samples in comparison with un-notches ones. Also, it was confirmed that the toughness of notched samples of both heat treatments at PA condition were significantly lower than un-notched ones. Consequently, toughness decrement was considerably dominated by the role of deformability compared to strengthening factor, however, the presence of cold rolling in the process of heat treatment (T8) could reduce the harmful effects of notch by increasing the stress bearing capacity in contrary with T6 heat treatment. Moreover, inserting the mechanical properties of peak aged samples from the un-notched tensile test in Abaqus finite element software; the V-shaped notch tensile test was simulated and confirmed the experimental results. It was shown in SEM results that the presence of notch enhanced the contribution of cracked particles, compared to particle/matrix deboning and matrix deformation, therefore, the non-homogeneous distribution of fracture features confirmed the harmful effect of notch. In the following, the distribution of three fracture micro-mechanisms were homogeneous in un-notched samples, which demonstrated the superior values of toughness in smooth samples. The present finding sheds light on development of processing techniques to optimize the mechanical properties of Al 2024 alloy.

FATIGUE CRACK GROWTH AND FATIGUE FRACTURE MORPHOLOGY OF RECYCLED RUBBER POWDER–FILLED NR/BR BLEND COMPOUND

ABSTRACTThe effect of two different types and particle sizes (micronized cryo-ground 74 μm or ambient-ground 400 μm) of recycled rubber powder (RRP) was studied during fatigue crack growth (FCG) in a natural rubber/butadiene rubber (NR/BR) compound using a fracture mechanics approach. Absolute and relative hysteresis losses using single-edge notch tensile specimens were determined with a displacement-controlled strain compensating for the permanent set of the samples throughout the FCG experiments. Differences in relative hysteresis loss showed that additional energy dissipation, due to multiple new crack surfaces at the crack tip, contributes to the FCG of the RRP compounds. At higher tearing energy, beside other factors affecting the FCG performance of the RRP compounds, both higher absolute and relative hysteresis loss are slightly detrimental to the crack growth rates. At lower tearing energy, the larger RRP-filled compound showed slower, but not significant, different crack growth rates than the NR/BR control compound. Fracture morphologies for NR/BR and RRP-filled compound were associated with different fracture surface topographies at various tearing energies, which revealed the dependency of the crack growth microstructure on the tearing energies.

Mitigation of Intergranular Cracking in Al-Mg Alloys via Zn-Based Electrode Potential Control in Sodium Chloride Solution

Mitigation of the initiation and propagation of intergranular stress corrosion cracking (IG-SCC) in a single edge notch tensile specimen is achieved through use of various Zn-based anodes on highly...

Analysis of grain refining and subsequent coarsening along on adjacent zone of friction stir welded armour grade aluminium alloy joints

This research article presents the metallography characteristics and mechanical behavior of eco-friendly friction stir welded heavy thick rolled aluminium alloy. The travel speed and pin rotational speeds were 25 mm min−1 and 250 rpm. For this investigation tapered threaded pin profile were used for this metal joining with dimensions of (12 × 36 × 11.6) mm. Evaluated mechanical properties of tensile and microhardness values of nugget zone has been compared with parent aluminium alloy also correlated with microstructure. The metallurgical characteristic of microstructural topographies was examined optical microscopy. The fracture surface morphology has revealed of smooth and notch tensile specimen was using a secondary electron detector by SEM. The elemental analysis has been carried out in stir zone to analyze the constitutions aid of energy dispersive spectroscopy. TEM analysis has carried out qualify and quantify the strengthening precipitates of armour grade aluminium alloy joints. The SAD patterns also used to represent the polycrystalline nature of metal as well as correlate the semi- coherent and incoherent phase formations. From the results of the armour aluminium alloy joints were an appreciably lower value than the parent metal. The transverse tensile strength value of FSW joints was found to be 70% than the base aluminium alloy.

Influence of grain size on the small fatigue crack initiation and propagation behaviors of a nickel-based superalloy at 650 °C

GH4169 at 650 °C in atmosphere was investigated by using single edge notch tensile specimens. The number of main cracks and crack initiation mechanisms at the notch surface strongly depended on the grain size. The crack initiation life accounted for more percentages of the total fatigue life for the alloy with smaller grain size. The fatigue life generally increased with increasing crack initiation life. The small crack transited to long crack when its length reached ˜10 times the grain size.

Progressive Failure Simulation of Notched Tensile Specimen for Triaxially-Braided Composites.

The mechanical characterization of textile composites is a challenging task, due to their nonuniform deformation and complicated failure phenomena. This article introduces a three-dimensional mesoscale finite element model to investigate the progressive damage behavior of a notched single-layer triaxially-braided composite subjected to axial tension. The damage initiation and propagation in fiber bundles are simulated using three-dimensional failure criteria and damage evolution law. A traction–separation law has been applied to predict the interfacial damage of fiber bundles. The proposed model is correlated and validated by the experimentally measured full field strain distributions and effective strength of the notched specimen. The progressive damage behavior of the fiber bundles is studied by examining the damage and stress contours at different loading stages. Parametric numerical studies are conducted to explore the role of modeling parameters and geometric characteristics on the internal damage behavior and global measured properties of the notched specimen. Moreover, the correlations of damage behavior, global stress–strain response, and the efficiency of the notched specimen are discussed in detail. The results of this paper deliver a throughout understanding of the damage behavior of braided composites and can help the specimen design of textile composites.

Experimental and finite element analysis of progressive failure in friction stir welded AA6061-AA7075 joints

In the present study, an in-situ analysis of crack initiation and propagation during tensile loading was captured using high speed camera for dissimilar friction stir weld joint of AA6061-T6 and AA7075-T651 prepared with plain cylindrical (CYL) and threaded with three intermittent flat faces (TIF) tool pins. To study notch sensitivity, experiment was also carried out for notch tensile sample creating notch at weld center. Joint failures were further predicted by finite element (FE) method. Results indicate that for joints obtained with CYL tool pin, crack initiated from vicinity of tunnel region however, propagated through the boundary of stir zone (SZ) and thermo-mechanically affected zone. FE results also indicate that the stresses and strain were confined near tunnel region which act as crack initiation site. For joints obtained with TIF tool pin, no tunnel/ void was noticed in the SZ and the sample failed from the heat affected zone on the advancing side (AA6061-T6) of the joints. For notch tensile specimen, the specimen exhibited minimum elongation when compared with tensile specimen and notch strength ratio > 1 indicated that the joint was notch insensitive or in other words, notch strengthened. FE result indicated that the higher stresses were confined near the notch root from where crack initiated and propagated through the SZ. Fractographic analysis showed the presence of dimples, micro-voids with some shear failure marks for all joints indicating the ductile fracture.

The susceptibility of P110 downhole tubular steel to sulfide stress cracking in H2S and NaCl

The study aims to investigate the effect of heat treatment of the P110 downhole tubular steel on its resistance to sulfide stress cracking (SSC), with an aim of producing P110 steel with higher strength which in turn reduces the cost via the reduction of the tubular wall thickness. The reduction of the tubular wall thickness maximizes the diameter of the wellbore, which increase the production rate per well. The susceptibility of P110 downhole tubular steel to SSC has been investigated using small circumferential notch tensile (CNT) specimens. The P110 steel was tested with and without heat treatment in a deaerated 3.5% NaCl solution containing 0.01 M Na2S2O3 (as a source for H2S) at room temperature. The experimental results show the heat-treated P110 steel to be susceptible to SSC whereas the un-heat-treated steel was not susceptible to SSC. Scanning electron microscopy suggested intergranular propagation of stress corrosion cracks.

A Strength Property Study of Notched Tensile Specimen by Material due to the Existence or Non-existence of Crack and Hole

A Strength Property Study of Notched Tensile Specimen by Material due to the Existence or Non-existence of Crack and Hole

Comparison of microstructure, mechanical properties, and residual stresses in tungsten inert gas, laser, and electron beam welding of Ti–5Al–2.5Sn titanium alloy

Electron beam welding (EBW), pulsed Nd:YAG laser beam welding (P-LBW), and pulsed tungsten inert gas (P-TIG) welding of Ti–5Al–2.5Sn alloy were performed in order to prepare full penetration weldments. Owing to relatively high power density of EBW and LBW, the fusion zone width of EBW weldment was approximately equal to P-LBW weldment. The absence of shielding gas due to vacuum environment in EBW was beneficial to the joint quality (low oxide contents). However, less cooling rates were achieved compared to P-LBW as an increase in heat-affected zone width and partial α′ martensitic transformation in fusion zone were observed in EBW weldments. The microstructure in fusion zone in both the EBW and P-TIG weldments comprised of both acicular α and α′ martensite within the prior β grains. Hardness of the fusion zone in EBW was higher than the fusion zone of P-TIG but less than the fusion zone of P-LBW weldments due to the observed microstructural differences. Notch tensile specimen of P-LBW showed higher load capacity, ductility and absorbed energy as compared to P-TIG and EBW specimens due to the presence of high strength α′ martensite phase. Maximum sheet distortions and tensile residual stresses were observed in P-TIG weldments due to high overall heat input. The lowest residual stresses were found in P-LBW weldments, which were tensile in nature. This was owing to high power density and higher cooling rates in P-LBW operation. EBW weldment exhibited the highest compressive residual stresses due to which the service life of EBW weldment is expected to improve.

Experimental investigation on fracture toughness of Al6061–graphite by using Circumferential Notched Tensile Specimens

This paper presents the experimental work carried out on the fracture behavior of aluminium 6061 (Al6061) and graphite particulate composites. The required specimens are prepared using stir casting method with graphite proportions ranging from 3 to 12 % by weight. The fracture behavior of Al6061-graphite particulate composites produced using stir casting method, was investigated by conducting experiments on Universal Testing Machine (UTM). Circumferential Notched Tensile (CNT) specimens were utilized to evaluate the fracture toughness of the composites. From the experiment the fracture toughness KIC= 15.85MPa m1/2 is obtained for Al6061-9% Graphite. Further, the experimental results revealed that, except 12% graphite, the fracture toughness of Al6061-graphite was observed to increases with an increase in weight percentage of graphite. The experimental results reinforce that Al6061-graphite particulate MMCs are suitable for automotive and aerospace applications requiring high fracture toughness apart from good wear resistance.