Ductile Failure Research Articles

Static and dynamic damping mechanical performance of architected metal-epoxy interpenetrating phase composites

The present contribution investigates the static and dynamic damping mechanical response of architected aluminum-epoxy interpenetrating phase composites (IPCs), engineered with strut, triply periodic minimal surfaces (TPMS), and stochastic spinodal AlSi10Mg reinforcement phases. Both single-phase metamaterials and co-continuous, multi-phase composites are analyzed, assessing the role of the reinforcement phase design and the addition of silicon carbide (SiC) nano-whisker epoxy enhancements in the effective mechanical performance. Aluminum-epoxy IPCs yield a constitutive response with peak, plateau stress, and overall energy absorptions up to 25 times higher than the ones recorded for the underlying single-phase metamaterials. Inner plastic strains, probed through dedicated finite element analysis, provide insights into the inner damage evolution, leading to characteristic ductile failure patterns, as revealed by computer tomography analysis. The exceptional specific energy absorption attributes are complemented by outstanding dynamic performance characteristics, with significant loss moduli over a broad range of frequencies and damping ratios up to 0.29.

(FeCoNi)75Cu25-xSix high entropy alloys prepared by mechanical alloying and spark plasma sintering: Microstructure, deformation behavior and dynamic recrystallization kinetics

The present research aimed to produce (FeCoNi)75Cu25-xSix (x = 0, 5, 10, 15, 20, 25 at.%) high-entropy alloys (HEAs) using mechanical alloying (MA) and spark plasma sintering (SPS). The HEAs containing 0 to 25 at.% of Si are designated as Si0, Si5, Si10, Si15, Si20, and Si25 alloys. The microstructure, phase evolution, and deformation behavior of the HEAs were investigated before and after SPS. Compression tests were conducted on SPSed alloys at room temperature and 800 °C. XRD examination revealed a single-phase FCC structure in Si0 alloy, with Si5 and Si10 alloys consisting of distinct FCC phases. Si15 alloy contained BCC phase and intermetallic compound Ni74Si26. Si20 alloy had BCC and two intermetallics Ni74Si26 and FeSi, while Si25 alloy consisted of intermetallic Ni31Si12 and BCC phases. For Si0, Si5, Si10, and Si15 alloys, cold pressing resulted in ductile failure; for Si20 and Si25 alloys, it caused brittle failure. The Si20 alloy had the highest strength at 1520 MPa, while the Si25 alloy had the lowest strength at 810 MPa. Dynamic recrystallization (DRX) occurs at 800 °C in all alloys with a peak in flow stress followed by a gradual decrease to a steady state stress. Stress-strain curves at 800 °C were analyzed using DRX kinetics, and the Avrami exponent n was between 1 and 2 for all alloys.

Effects of Thermo-Mechanical Processing on the Microstructure and Mechanical Properties of β -Type Titanium Alloys

This study investigates the microstructural evolution and mechanical behavior of severe cold-rolled β-type Ti-29Nb-13Ta-4.6Zr (TNTZ) alloys under systematic solution heat treatments (ST) at 1063 K for durations ranging from 5 to 60 minutes. This comprehensive analysis provides valuable insights into the microstructural and mechanical characteristics of TNTZ alloys under varying solution heat treatment durations, offering a foundational understanding for optimizing their application in engineering contexts. Microstructural analysis reveals that both solution-treated (ST) and cold-rolled (CR) samples exhibit a predominant single body-centered cubic (BCC) β phase, while cold-rolled and solution-treated (CST-Q) samples display a combination of β and martensite orthorhombic α'' phases. ST samples demonstrate equiaxed grains with an average diameter of ~72 μm, albeit with limited clarity. In contrast, CST-Q samples treated for over 10 minutes exhibit finer equiaxed grains within the 7-14 μm range. Hardness values increase with prolonged solution heat treatment, reaching approximately ~183 HV for ST and ~234 HV for CR. Moreover, hardness continues to rise with increasing treatment duration, reaching ~204 HV for CST10Q, ~229 HV for CST30Q, and ~242 HV for CST60Q. Mechanical properties, including tensile strength, yield strength, and elongation, vary across samples. ST shows values of ~710 MPa, ~610 MPa, and ~25%, CR with ~1305 MPa, ~395 MPa, and ~17.5%, CST5Q with ~1042 MPa, ~440 MPa, and 17.5%, CST10Q with ~1010 MPa, ~650 MPa, and 21%, and CST60Q with ~930 MPa, ~660 MPa, and ~21%. Fracture surfaces of all samples exhibit dimple structures and microvoid nucleation, indicative of ductile failure.

Bond-slip model of headed bar and its application to component-based model for precast concrete joints under accidental loads

This study focused on investigating bond-slip behaviour of headed bars embedded in well-confined concrete. Test results based on eight pull-out specimens under displacement-controlled incremental loading showed that recommended development lengths from ACI318–19 are conservative in developing ultimate strength of headed bars associated with smaller slips than straight bars. Moreover, it was found that different (but sufficiently long) embedment lengths had a significant effect on the free-end slip but negligible effect on the loaded-end slip of headed bars. Additionally, the study highlighted the significance of head bearing in anchorage formation, especially during inelastic stage when bond strength had deteriorated. To address inaccuracy in predicting structural behaviour caused by inadequate anchorage strength of headed bars, a macro bond-slip model was proposed based on correlation in strain distribution obtained from the pull-out tests. Subsequently, the proposed model was incorporated into a component-based model (CBM) and validated against test results of PC joints under seismic loading. A numerical investigation using the validated CBM was conducted to study behaviour of exterior PC joints with headed bars and special detailing. These included Type X joint with X-bent bars and plastic hinge relocation (PHR), and Type A joint with an additional bar layer (ABL) in the beam-joint region, subjected to extreme loads such as seismic action or progressive collapse. The results revealed that seismic behaviour of exterior joints was governed by ductile failure of the beam at the PHR and vertical joint interface in Type X and Type A joints, respectively. Progressive collapse was primarily caused by column failure, with flexural and catenary action effectively preventing collapse. Notably, under both loading conditions, presence of ABL in Type A joint and X-bent bars with PHR in Type X joint improves flexural strength of beams at critical sections and joint shear strength, ultimately enhancing structural performance of joints. Design recommendations include replacing hooked bars with headed bars as well as implementing X-bent bars with PHR and ABL in PC joints. The authors advocate the strong-column-weak-beam design approach as an effective measure to prevent progressive collapse.

A comprehensive investigation of thermal management strategies in dissimilar micro-friction stir welding (dissimilar µFSW): Insights from numerical simulation and experimental observations

The increased heat dissipation rate from thin sheets and differences in material properties lead to difficulties in controlling the heat input in dissimilar micro-friction stir welding (dissimilar µFSW). As a result, improper material mixing and defect formation significantly increase, which deteriorates weld performance and escalates its rejection rate. This issue of thermal mismanagement is mitigated in the present work by utilizing suitable material positioning and tool offset strategies during the dissimilar µFSW of 0.5 mm thick AA 2024-T3 and AA 6061-T6. To date, post-weld analysis was predominantly utilized to derive the physics for explaining these strategies in dissimilar µFSW. However, for proper utilization of these strategies, it is important to comprehend the underlying process physics by investigating the parameters that evolved during the process, such as temperature distribution, material flow velocity, and material intermixing. The literature currently available lacks a systematic investigation in this direction. For this purpose, a 3-D thermo-mechanical model based on coupled Eulerian-Lagrangian formulation was employed in this study. The numerical results showed that an optimal temperature difference between the advancing and retreating sides facilitates achieving a higher degree of velocity symmetry with a uniform higher magnitude of velocity vectors. Importantly, this degree of velocity symmetry showed a good agreement with experimentally determined weld characteristics, namely macro and microstructures, and mechanical performance. By appropriate thermal management, a joint efficiency of 77.3 % (weld strength = 243.58 MPa) was obtained upon tensile loading, and the weld fractured from the heat-affected zone of the AA 6061-T6 side, exhibiting characteristics of ductile failure.

Coupled thermo-mechanical phase field modeling for shear ductile fracture at high strain rate and temperature

The objective of the current study is to introduce a coupled thermo-mechanical phase field model to elucidate shear ductile fracture under varying strain rates and temperatures. Both the plastic work threshold and fracture energy exhibit dependence on the strain rate and temperature. To explore this in detail, hat-shaped specimens are adopted to systematically investigate shear ductile fracture under distinct strain rates and temperatures by the split Hopkinson pressure bar apparatus. The simulated and experimental strain waveforms are then compared to calibrate and validate the damage parameters of the presented phase field model. The results demonstrate that the numerical simulations are in agreement with the experiments. Moreover, the fracture process of the hat-shaped specimen is also examined. The numerical results furnish a reliable description of the hat-shaped specimen from damage initiation to complete fracture. It's found that the influences of strain rate on shear stress and fracture initiation are less pronounced than that of temperature for U75V steel. In summary, the developed phase field model can reproduce shear ductile failure of the hat-shaped specimen under diverse strain rates and temperatures.

Dynamic mechanical response and deformation mechanism of poly (ε-caprolactone) (PCL) / epoxy resin(EP) shape memory polymer composites

The dynamic mechanical response and deformation mechanism of poly (ε-caprolactone) (PCL) and epoxy resins shape memory polymer (SMP) composites were investigated in this paper. The SMP composites were a blend of PCL and epoxy resins by using the facile melt-mixing method. The distribution of PCL in the composites was observed by SEM, which was in from microparticle morphology to continuous morphology with the increasing PCL content. And the dynamic mechanical analyzer (DMA) tests were performed to demonstrate the viscoelastic properties and shape memory effect of the PCL/EP composites. Based on the above characterization of PCL/EP composites, the split Hopkinson pressure bar (SHPB) tests were performed to study the dynamic mechanical properties at the strain rates of 4000/s to 7000/s. The results showed that the flow stress increased with the increasing strain rate, which exhibited an obvious sensitivity to strain rate. However, the flow stress of PCL/EP decreases with the increasing PCL content due to the low strength of PCL. And PCL/EP composites failed in a typical ductile failure mode under quasi-static loading, whereas an excellent plastic deformation ability was observed at high strain rates, which was attributed to the heat generated by the plastic work during the adiabatic compression. Combining with the experimental results, the corresponding deformation mechanism was also discussed.

Utilizing ensemble learning in the classifications of ductile and brittle failure modes of UHPC strengthened RC members

This study aims to achieve the swift and precise classification of ductile and brittle failure modes in flexural reinforced concrete (RC) members, specifically those with tension sides strengthened by ultrahigh performance concrete (UHPC). Employing six ensemble learning techniques—Bagging, Random Forest, AdaBoost, Gradient Boosting, XGBoost, and LightGBM—the authors utilize a comprehensive dataset comprising 14 features, which include manually labeled failure modes obtain from load–deflection curves. The model training spans four scenarios, varying in the inclusion or exclusion of features describing the cross-sectional area of RC members and moment resistance. XGBoost emerges as the most effective classifier, achieving an impressive 84% accuracy with high confidence. Additionally, the study employs the Shapley Additive Explanation (SHAP) technique on the best-performing model to illuminate the significance and impacts of various features in UHPC-strengthened flexural members’ failure modes. Notably, moment resistance and UHPC tensile strength surface as the most influential factors in predicting failure modes. Increased rebar yield strength, UHPC compressive strength, UHPC reinforcement ratio, and steel fiber volume in UHPC contribute to enhanced ductility in flexural members, while heightened moment resistance and UHPC layer thickness, along with a robust RC-UHPC interface, tend to induce brittleness. The introduction of such an effective failure modes classification model, coupled with the model’s explainability, instills trust in its predictions and facilitates seamless integration into real-world applications, particularly in seismic areas. The model’s ability to operate without the need for pre-experimental tests marks a significant advancement in the field.

Mechanical properties of a novel high nitrogen and low nickel stainless steel S35657

Stainless steel exhibits excellent ductility and corrosion resistance, making it highly promising for applications in civil engineering. In order to investigate the mechanical properties of a novel stainless steel S35657, a total of 150 coupons including 125 plates and 25 rods were tested. The failure modes, stress-strain curves, and mechanical parameters of coupons were obtained. Fractographic analysis using scanning electron microscopy was also performed to investigate the failure mechanisms. It was found that all coupons exhibited ductile failure mode with elongation up to 52.08%. The nominal yield strength standard value reached 370.50 MPa, while the ultimate tensile strength standard value reached 736.07 MPa. The various existing typical constitutive models were assessed, indicating that these models provide a good fit in the elastic stage but are not applicable in the hardening stage. Based on the Ramberg-Osgood model and test results, a new constitutive model applicable to stainless steel S35657 was proposed. Moreover, a design reliability analysis was carried out using the Equivalent Normalization Method (JC method), and partial factors for resistance of stainless steel S35657 were calibrated, which were 1.05 under windless load combinations and 1.15 under windy combinations.

Relation of craze to crack length during slow crack growth phenomena in high‐density polyethylene

AbstractThe craze‐crack mechanism occurring in high‐density polyethylene (HDPE) causing slow crack growth and environmental stress cracking is investigated in detail with respect to the relation of crack length and the related craze zone. This is essential for the understanding of the resulting features of the formed fracture surface and their interpretation in the context of the transition from crack propagation to ductile shear deformation. It turns out that an already formed craze zone does not inevitably result in formation of a propagating crack, but could also undergo ductile failure. For the examination, the full notch creep test (FNCT) was employed with a subsequent advanced fracture surface analysis that was performed using various imaging techniques: light microscopy, laser scanning microscopy, scanning electron microscopy, and X‐ray micro computed tomography scan. FNCT specimens were progressively damaged for increasing durations under standard test conditions applying Arkopal, the standard surfactant solution, and biodiesel as test media were used to analyze the stepwise growth of cracks and crazes. From considerations based on well‐established fracture mechanics approaches, a theoretical correlation between the length of the actual crack and the length of the preceding craze zone was established that could be evidenced and affirmed by FNCT fracture surface analysis. Moreover, the yield strength of a HDPE material exposed to a certain medium as detected by a classic tensile test was found to be the crucial value of true stress to induce the transition from crack propagation due to the craze‐crack mechanism to shear deformation during FNCT measurements.Highlights Progress of crack formation in high‐density polyethylene is analyzed by different imaging techniques Determined growth rates depend on distinction between craze zone and crack The ratio of the present crack to the anteceding craze zone is validated theoretically The transition from crack propagation to ductile shear deformation is identified An already formed craze zone may still fail by ductile mechanisms

An Improved Anchorage System for L-Shaped FRP Composites to Enhance the Seismic Response of Beam-Column Joints in a Low-Strength Substandard Reinforced Concrete (RC) Frame

Reinforced concrete buildings are prone to collapse during seismic events due to the brittle shear failure of non-seismic beam-column joints (BCJ). In this study, two one-third scale reinforced concrete (RC) frames incorporating various non-seismic details were tested under lateral cyclic loading. One of the RC frames was used as control, while the other was strengthened using externally bonded carbon-fiber-reinforced polymer (CFRP) sheets in a L-Shaped configuration with particular attention to anchorage to evade debonding. For the strengthening process, L-shaped CFRP sheets were bonded to the inner face of columns, extended on beams both above and below the joint up to a hinge length. To avert debonding, the L-shaped CFRP sheets were fully wrapped with CFRP sheets around the column, both near the joint and at the end of the sheet. The sheets were also wrapped around the beam, through two slots in the slab that were adjacent to the beam-column interface and at the far end of the sheet. Test results confirmed that the installation of CFRP sheets in an L-shaped configuration altered the brittle-shear failure mechanism of the beam-column joints to a ductile failure by repositioning the hinges away from the columns. Additionally, the proposed anchorage method successfully eradicated the debonding and peel-off of the CFRP sheets. Moreover, strengthening with the CFRP sheets in the L-shaped configuration enhanced the strength and ductility of the RC frame by 45% and 43%, respectively. According to the findings of this study, the application of L-shaped CFRP sheets proved effective in strengthening RC frame structures.

Seismic performance and design of an innovative dual vertical diaphragms joint between flat CFST column and H-beam

Steel-concrete composite structure is one of the research hotspots in the structural engineering field in recent years. Among the various types of composite structures, flat concrete-filled steel tube (CFST) column structures are preferred for residential buildings owing to the smooth elevation to coordinate with the indoor decoration. The seismic performance of beam-to-column joints is momentous as a key part of flat CFST column structures. This study provided a new type of joint between flat CFST column and H-beam with dual vertical diaphragms through the column. The design procedure for the vertical diaphragms was proposed to design the test specimens. An experimental study on the seismic behavior of the innovative joint under cyclic load was performed on three full-scale specimens. The seismic performance of the joint, including the failure mode, hysteretic and skeleton curves, ductility and energy dissipation capacity, was observed and discussed in detail. Subsequently, nonlinear finite element models (FEMs), based on the specimens, were developed using the ABAQUS/Standard program and validated by comparing the numerical results with the test ones. The experimental and numerical results demonstrated that the proposed joint exhibited satisfactory seismic performance and all the specimens exhibited a ductile failure mode. The failure process and damage distribution of the joint under earthquake were revealed based on the validated FEM. Furthermore, a mechanical model was established to predict the load-bearing capacity of the shear force and bending moment of the proposed joint. The mechanical model was validated to be accurate and reliable by comparing the numerical simulation results with the theoretical ones. This study provided a practical joint between flat CFST column and H-beam as well as its design method, which was beneficial for the promotion of the flat CFST column structures.

Ductile fracture prediction of additively manufactured Ti-6Al-4 V alloy based on void growth and coalescence of a unit-cell model

In this work, we have studied the fracture mechanism and proposed a micromechanical method with a unit cell model to predict the ductile fracture of additively manufactured (AMed) Ti-6Al-4 V alloy. The tensile experiments with smooth round bars and notched round bars were conducted to investigate the mechanical properties and ductile fracture behavior of materials. The proposed unit-cell model was validated by quasi-static tensile fracture tests to predict the ductile failure of titanium alloy manufactured by laser power bed fusion (L-PBF) under different stress states, which include a wide range of stress triaxialities. An equivalent initial void fraction was introduced to evaluate the ductile behavior of the titanium alloy. Our investigation shows that the stress states significantly influence the void coalescence behavior of AMed Ti-6Al-4 V alloy, and the fracture locus of L-PBF fabricated Ti-6Al-4 V alloy under different stress states was successfully predicted by the proposed unit-cell model under proportional loading. Our study provides useful guidance for the future design and application of metal materials for additive manufacturing.

Carbon Fiber Reinforced Polymer Adoption for Structural Rehabilitation

ABSTRACT Flexural strengthening is essential when structures face increased loads or usage changes, requiring localized reinforcement. Carbon Fiber Reinforced Polymers (CFRP) are widely used for repairing deteriorating concrete elements, notably boosting their flexural and shear strength and prolonging their lifespan. However, CFRP applications may encounter challenges like delamination. Therefore, it is important to select a good product and adhesive to enhance the performance of the fiber. The purpose of this study is to propose proper composite strengthening for a reinforced concrete structure to increase its flexural and shear capacity. A case study of a small office building in Italy is selected and one sectional frame consisting of six beams is studied. The first step in this process is to assess the moment and shear forces due to the initial loads based on the existing longitudinal and shear reinforcement. Then, the moment and shear forces due to the upgraded loads are calculated using SAP 2000 to determine if flexural strengthening is needed for any of the six beams being analyzed to avoid ductile failure of the steel rebars and brittle failure of the concrete. The use of CFRP showed positive results in terms of structural rehabilitation for the building, particularly when subjected to elevated loads. This not only contributes to the building’s overall safety but also extends its service life, making it a valuable and sustainable solution for structural enhancement in the face of varying load conditions.

Synergistic enhancement in mechanical properties of graphene/MWCNT reinforced Polyaryletherketone – carbon fiber multi-scale composites: Experimental studies and finite element analysis

This investigation focuses on the synergistic performance improvement in graphene/MWCNT reinforced Polyaryletherketone (PAEK) - carbon fiber (CF) multi-scale composites. FTIR revealed the chemical interactions while HRTEM, XRD and 3D X-ray microscopy gave insight into nanofiller dispersion and microstructural features. The functional groups on nanofillers along with structural features integrated various components of the multi-scale composites by formation of graphene/MWCNT/CF complex network that provided larger interfacial area, bridging effect and physico-chemical interaction with PAEK while restricting its segmental mobility. Multi-scale composites displayed significantly improved strength, fracture toughness, interlaminar shear strength, glass transition temperature and tribological performance. Under dynamic load, graphene/MWCNT reinforcement of matrix and CF synergistically increases the storage modulus and energy absorption characteristics. Wear and fracture surface morphology of nano and multi-scale composites showed ductile failure confirming interfacial adhesion. The failure behavior in experimental studies was supported by Abaqus/Explicit-based FEM models of fracture toughness response. This work provides a promising avenue to develop next generation high performance thermoplastic composites for structural applications.

Retrofitting of Damaged Crumb Rubber Concrete Frame Using Fiber Reinforced Polymers

Abstract: The research work is focused on the retrofitting of damaged crumb rubber concrete frame. The retrofitting of these frames was carried out using Carbon Fiber Reinforced Polymers (CFRP) wrapping technique accompanied with beam weakening technique, carried out at certain locations to achieve the desired failure mechanism. Two previously tested 1/3rd reduced scale models Crumb rubber concrete (CRC) and Normal concrete (NC) were CFRP retrofitted along with beam weakening and then tested dynamically. From the as-built testing strong-beam weak-column effect was observed, for that purpose beams weakening through cores removal away from the face of the column equal to two times depth of the beam, so as to shift the damage from the brittle failure mechanism of column and joint damage to more ductile damage in the beam. In addition to beam weakening the damaged beam column joints were CFRP retrofitted. A comparative study between the retrofitted and as-built models was done. The results obtained from testing has recovered the ductility up to 82% and response modification factor up to 90% for normal concrete while in case of crumb rubber concrete frame the ductility is recovered up to 76% and its response modification factor up to 81%. As intended, a much-distributed damage behavior/cracking pattern as compared to the reference models, especially in the case of normal concrete model. The intended purpose of the retrofitting in recovering seismic properties and a comparatively distributed damage/cracking pattern was achieved. The purpose of beamweakening through core removal for shifting the damage from columns and joints towards the beams for achieving a more ductile failure mechanism was achieved to some extent.

Enhancing joint performance in friction stir welding through tailored double-butt-lap geometry

The study investigates the effectiveness of a novel joint configuration, the double-butt-lap (DBL), in friction stir welding (FSW) to join 6-mm-thick AA 6061-T6 plates. Four welds, including a simple square butt (SSB) configuration and three DBL variations (with UB to LB ratios of 1:2, 1:1, and 2:1), were executed. Using consistent parameters, the investigation evaluates the force distribution, macrostructure, and mechanical properties of the joints. Results show varied force profiles; spindle torque and Z-force decrease notably in DBL configurations compared to SSB, while X-force decreases overall for all DBL variations. Macrostructural analysis indicates increased weld area and finer microstructures in DBL joints. A correlation between peak temperature, grain size, and hardness highlights larger grain sizes at higher temperatures, influencing joint hardness. The DBL configuration with a 2:1 ratio exhibits the highest joint efficiency (83.76%). Fractographic analysis indicates ductile failure in all samples. Bend testing reveals increased flexural strength and bend angles in two DBL-configured joints compared to SSB. The results indicate that using a 2:1 ratio for joining 6-mm AA6061-T6 plates through FSW is viable, providing valuable insights into enhancing mechanical properties by optimizing joint geometry.

Experimental Study on the Flexural Properties of Steel-Fibre-Reinforced Concrete Specimens with Different Heights

Flexural strength is an important mechanical property of steel-fibre-reinforced concrete. By designing three-point bending tests of concrete with five specimen heights, three steel fibre types, and two steel fibre mixing methods, the effects of the specimen height, steel fibre mixing method, and steel fibre type on the peak load, effect of size, section characteristics, strain characteristics, and characteristics of the load–displacement curve of concrete specimens were studied. The results show that the peak load of the control group is basically linear with the height of the specimen. After adding three kinds of steel fibres, the peak load of the specimen is greater than that of the control group in the same case. The peak load of the specimen increases by adding three kinds of steel fibres, and the increase is closely related to the height of the specimen. The residual stage of the load–displacement curve of the milling steel fibre and the end hook steel fibre are relatively flat, while the residual stage of the load–displacement curve of the shear steel fibre is relatively large, and the residual load is also greater than the residual load of the shear steel fibre. The specimens in the control group show brittle failure characteristics. As the height of the specimens increases, the failed section of the specimens is smoother. The development of cracks in the steel fibre specimens is more tortuous than that of the control group, showing ductile failure characteristics. Some tensile failure zones are still present where the fibres are densely distributed, and the failure characteristics of the specimens are further explained and proven by the strain characteristics.

Optimization of weldability and joint strength of Al-Mg-Si with additional Al-Si cladding based on a design of experiments investigation

Some aluminum alloys, such as those alloyed with Si and Mg, are difficult to weld due to their susceptibility to hot cracking. Previous research has investigated this challenge in resistance spot welding (RSW). To improve the weldability of the AW-6111 aluminum alloy, the roll cladding process was employed, combining it with AW-4045 as the cladding material. This resulted in a wider weld lobe, improved electrode wear, and enhanced joint quality. However, it is evident that the cladded material could benefit from improved welding conditions. Despite a larger nugget diameter, similar mechanical properties were observed for both the minimum and maximum boundary conditions of the weld lobe, determined by Inom and Imax.This study employed a design of experiments approach to optimize the joint strength and improve the weld quality of the AW-6111 + AW-4045 cladded sheets. The tensile-shear test results demonstrated an improvement in lap-shear strength of 38 % and 44 % compared with the previous study. Furthermore, the test samples predominantly exhibited a ductile partial thickness failure mode, and a significant improvement in the weld nugget quality of the cladded AW-6111 + AW-4045 was observed. The cladded joints also exhibited a Si alloying content of approximately 5 % in the heat-affected zone and fusion zone, thereby reducing the risk of crack formation and propagation, thus improving significantly the weld quality. The results of this study contribute with the continuous validation of Al alloys for the transport industry and with the use of cladding technology to enhance the aluminum sheet properties and respective weld quality.

A study on the fracture pattern change of high-burnup fuel cladding failed by pellet-cladding mechanical interaction failure under reactivity-initiated accident conditions

ABSTRACT In a part of the historical reactivity-initiated accident (RIA) simulated tests on high-burnup fuels performed at the Nuclear Safety Research Reactor, the fuel failure caused by pellet-cladding mechanical interaction (PCMI) led to splitting into upper- and lower-part pieces or even fragmentation of the cladding tube. A massive release of fuel fragments accompanied this fracture pattern change from previously known axial cracks and thus identified as a potential concern in safety evaluation regarding core coolability. Dedicated out-of-pile mechanical tests were performed with unirradiated Zircaloy-4 cladding specimens to clarify the condition of such fracture pattern change. The specimens were pre-hydrided and subjected to loading with axial-to-hoop strain ratios of ℇz/ℇθ = 0.5–1.25, simulating the effects of hydrogen embrittlement and pellet-cladding mechanical bonding of high-burnup fuels, respectively. The results indicate that higher biaxiality of the loading and lower ductility (failure strain level) assist the fracture pattern change. This study proposes a conservative criterion that a PCMI failure splits the cladding tube into more than two pieces when strain ratio ℇz/ℇθ >0.75 and a concurrent hoop strain < 1.7% at the failure instant.