Stress Concentration Sites Research Articles

The deformation mechanism of low symmetric Ti–Pt intermetallic compounds containing high density of planar defects

Intermetallic compounds typically exhibit limited plastic deformation capacity due to challenges in activating dislocation slip and deformation twinning, coupled with a lack of alternative deformation mechanisms. Ti–Pt alloys are a prevalent type of intermetallic compound utilized in high-temperature shape memory alloys and as materials for energy applications in electric fields. However, they often exhibit poor deformation capability. Here, we prepared a low-symmetry intermetallic phase, Ti4Pt3, which demonstrates significant plastic deformation capability. This phase features a high density of parallel planar defects, resulting in an exceptionally large lattice periodicity perpendicular to these defects. Through in-situ scanning electron microscope compression tests, we observed substantial plastic deformation in this new phase. Analysis of the deformed Ti4Pt3 phase revealed that the dense planar defects create uniformly distributed sites of internal stress concentration, enabling a rapid increase in back stress within crystals. This phenomenon leads to notable lattice rotation and localized order-disorder transitions, both crucial mechanisms facilitating plastic deformation and enhancing deformation capacity. Our research underscores the potential of leveraging structural asymmetry to enable unconventional deformation mechanisms, thereby enhancing the plasticity of intermetallic materials.

On the Mechanical Behavior of LP-DED C103 Thin-Wall Structures

Laser Powder Directed Energy Deposition (LP-DED) can produce thin-wall features on the order of 1 mm. These features are essential for large structures operating in extreme environments such as regeneratively cooled nozzles and heat exchangers, which often make use of refractory metals. In this work, the mechanical behavior of LP-DED C103 was investigated via quasi-static tensile testing and low cycle fatigue (LCF) testing. The effects of vacuum stress relief (SR) and hot isostatic pressing (HIP) heat treatments were investigated for specimens in the vertical and horizontal build orientations during tensile testing. The AB and SR properties were lower than literature values for wrought and laser powder bed fusion (L-PBF) bulk components but higher than electron beam powder bed fusion (EB-PBF). The application of a HIP cycle improved strength by 7% and ductility by 27% past the initial as-built condition. Fracture images reveal that interlayer stress concentration sites are responsible for fracture in specimens in the vertical orientation. Meanwhile, fracture in the horizontal specimens mainly propagates at a slanted angle typical of plane stress conditions. The LCF results show cycles to failure ranging from 100 cycles to 8000 cycles for max strain levels of 2% and 0.5%, respectively. Fractography on the fatigue specimens reveals an increasing propagation zone as max strain levels are increased. The impact of these findings and future work are discussed in detail.

Experimental study on the effect of filament-extrusion rate on the structural, mechanical and thermal properties of material extrusion 3D-printed polylactic acid (PLA) products

AbstractMaterial extrusion (MEX), also commonly referred to as fused deposition modeling (FDM) or fused filament fabrication (FFF) is currently one of the most commonly used additive manufacturing techniques. The quality of the 3D-printed objects fabricated by MEX methods highly relies on various printing parameters, one of which is the so-called filament extrusion multiplier (k). In this study, 3D-printed parts were prepared by MEX technique during which the material feeding rate was adjusted by varying the extrusion multiplier in the range of 97–105% (k = 0.97–1.05). The fabricated parts were tested for their geometrical, structural, mechanical, and thermal conductivity properties. Based on computed tomographic analysis and scanning electron microscopic images, increasing the k parameter resulted in smaller voids, along with gradually decreasing porosity (from 5.82 to 0.05%). Parallel to the decreasing defects, the thermal conductivity of the parts improved from 0.157 to 0.188 W/mK as determined by light-flash analysis technique. On the other hand, when k was set to ≥ 1.03 the geometrical accuracy declined, the size of the specimens considerably increased relative to the nominal values, especially in the X–Y directions due to excess material getting “squeezed” on the sides of the specimens. This latter phenomenon also resulted in the formation of a number of stress concentration sites, which manifested in the decrease of mechanical properties. Accordingly, the tensile, flexural, and impact strength of the samples improved up to k = 1.03; however, above that it dropped considerably.

Reactive extrusion fabrication of thermoplastic starch with Ca2+ heterodentate coordination structure for harvesting multiple-reusable PBAT/TPS films

Creating multiple-reusable PBAT/TPS (PT) films presents a novel solution to reduce carbon emissions from disposable packaging, addressing challenges like the high creep of PBAT and the glycerol migration of TPS. Consequently, adopting reactive extrusion to fabricate reversible cross-linking TPS with high shape memory performance, low migration, and homogeneous dispersion in PBAT matrix was a fascinating strategy. Herein, starch, glycerol and CaCl2 (calcium chloride) were extruded to fabricate TPS-Ca with Ca2+ heterodentate coordination structure and confirmed by XPS, 1H NMR and temperature-dependent FTIR. The results of DMA, dynamic rheology, flow activation energy and SEM revealed that TPS-Ca exhibited significant temperature-sensitive reversible properties and robust melt flow capability, enabling micro-nano scale dispersion in PBAT. Noteworthy, PBAT/TPS-Ca (PT-Ca) would recover 100 % length within 20 s by microwave heating after being loaded under the hygrothermal environment. Meanwhile, the migration weight of glycerol decreased from 2.5 % to 1.2 % for the heat-moisture-treated PBAT/TPS (HPT) and PBAT/TPS-Ca (HPTCa). Remarkably, the tensile strength and elongation at the break of HPT-Ca increased to 20.0 MPa and 924 %, respectively, due to reduced stress concentration sites in the phase interface. In summary, our study provides a streamlined strategy for fabricating multiple-reusable PT, offering a sustainable solution to eliminate carbon emissions linked to disposable plastic.

Surface morphology of API 5L X65 pipeline steel processed by ultrasonic impact peening: An integrated experimental and computational study

Ultrasonic impact peening (UIP) is a highly effective surface engineering technique to improve material durability through peening-induced surface plastic deformation. These improvements are determined by not only the generation of surface compressive residual stresses but also the alteration of surface morphological features. In literature, researchers have focused primarily on the average roughness (Sa) to understand the morphology changes induced by UIP. From the surface engineering perspective, in addition to Sa, other surface morphological characteristics play key roles in determining material durability, particularly surface skewness (Ssk) and surface kurtosis (Sku), which describe the asymmetrical distribution of surface profiles and the sharpness of peak features, respectively. In this study, surface morphological characteristics of API 5L X65 pipeline steel processed by UIP are investigated with a focus on the effects of processing parameters on Sa, Ssk, and Sku. The results indicate that the optimized UIP conditions lead to the preparation of surfaces with valley structures as the dominant feature accompanied by blunt peaks, resulting in the elimination of stress concentration sites towards enhanced material durability. In addition, surface hardness, residual stresses, and peening-induced deformation depth are measured. Furthermore, to gain a comprehensive understanding of the material response and the underlying mechanisms driving surface morphological alterations by UIP, a finite element method (FEM)-based model is developed. By investigating the effect of process parameters (peening amplitude, peening cycles, and pin size) on the surface deformation depth and stress/strain distribution, a correlation between these process variables with surface characteristics is established.

Towards Preventing Anastomotic Leakage through the “No-perforation Virtual Staples”

Anastomotic leakage (AL) is a severe postoperative complication after gastrointestinal surgery, potentially leading to morbidity and mortality. The factors contributing to AL include poor blood flow to the suture line, tension, infection, underlying conditions, and surgeon-related factors. However, the stress concentration, which is a localized increase in stress due to discontinuities caused by sutures or staples, has often been overlooked. The working hypothesis is that by removing problematic stresses and simulating mechanical loadings on the anastomosis, improved recommendations for stapler design, surgical technique, and anastomotic construction can be derived to reduce the occurrence of leaks. The objective of this study is to develop and optimize "no-perforation" magnetic staples that can effectively minimize stress and prevent AL. In this study, magnet-metal bar couples coated with a biocompatible adhesive hydrogel were utilized to replace existing surgical staples. By employing magnetic forces, the tissue was deformed into a curved shape, which provided additional security for the new "magnetic staples". This technique aimed to eliminate complications such as high tension and poor blood flow often associated with traditional staples. To ensure safety, a double protection system consisting of the pectin adhesive (previously patented at BWH) and the "lock-in forces" of the magnetic staples was implemented. It was believed that relying solely on adhesion would not effectively prevent leaks. Preliminary data indicated that applying virtual staples for anastomoses was feasible. The primary innovation of this approach is the substantial reduction in stress concentration when the anastomosis is under tension. The conventional surgical teaching concerning bowel anastomoses has centered on the dangers posed by excessive tension and compromised blood supply. In contrast, virtual staples offer the advantage of adjustable shape and force, allowing for optimization of tension and blood supply. The study successfully identified the biomechanical forces and stress concentrations that contribute to anastomotic leaks and developed new "virtual staples" to address this issue. The new "virtual staples" are made of two magnetic compression rings crafted from medical-grade silicone and feature 8 strategically embedded magnets. These rings are engineered with a specific surface curvature to ensure optimal axial alignment and elimination of stress concentrations. Cases for delivery have also been designed and fabricated, which are compatible with standard commercial delivery devices used in surgical settings. This ensures a seamless integration into existing medical procedures and surgical workflows. Initial in-vitro experiments were conducted on sections of the porcine intestine to test the effectiveness of the Virtual Staples. These preliminary tests have shown promising results, including an absence of leakage at the joined or sealed tissue sites. Our research aims to predict the stress concentration sites in tissues and evaluate the effects of applying adhesive virtual staples under new conditions. Through a combination of experiments and simulations, we designed non-perforation staples and optimized the design by considering various factors such as staple size, shape, magnet strength, and stress distribution. Initial in-vitro experiments indicated the effectiveness of the design. With further optimization of the design, along with animal testing and clinical trials, this new technique has the potential to revolutionize surgical procedures. The MA Acorn Innovation Grant 2023. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Characteristic fatigue damage near the Σ3(111) coherent twin boundary in micron-sized copper specimen

To investigate the effect of a Σ3(111) coherent twin boundary on the fatigue damage of micron-sized Cu, a tension–compression cyclic-deformation test was performed on a bicrystal specimen. The fatigue damage (extrusion/intrusion) occurred preferentially around the twin boundary, induced by stress concentration on one side of the specimen due to the deformation constraint between the crystals. Even though stress relaxation occurred on the opposite side, the fatigue damage along the twin boundary penetrated the specimen. The shapes of the extrusion and intrusion of the opposing sides were coincident, and the dislocation density in the damaged region was lower than that in the far region. These results indicate that the dislocations generated at the stress-concentration sites overcame the stress-relaxation effects and traveled along the twin boundary to escape from the opposite side because of the close distance between the surfaces in the specimen. The ease of dislocation ejection promoted the formation of extrusion/intrusion and decreased the fatigue strength of the specimen. The findings will facilitate the development of materials with improved resistance to fatigue damage.

Анализ напряженно-деформированного состояния узловых соединений высотных зданий при учете климатических воздействий

Purpose: to analyze the design solutions of the junctions of floors with external walls in buildings erected using monolithic technology. To identify defects and damages that occur on facades and overlaps with perforations for thermal pads during construction and operation. To investigate by numerical methods the stress-strain state of the overlap disk at the junction with the outer wall, taking into account temperature influences. Materials and methods: the standard version of the nodal connection of a monolithic building of various storeys is considered. The fragment of the junction joint of the overlap disk with the wall was modeled in the Ansys and SCAD software complexes. The model consisted of an anisotropic material — concrete and reinforcement rods included in it. The behavior of the stress-strain state under various temperature influences was studied. Results: numerically, taking into account geometric parameters and temperature loads, the patterns affecting the nature of changes in the values of normal stresses in the characteristic sections of the perforated plate are revealed and the existing types of design solutions are evaluated. The locations of stress concentration zones and their impact on performance are determined. Practical significance: it is shown that the most dangerous cross-section in perforated discs of overlaps takes place at the key joints. It is determined that vulnerability in such compounds is caused by the influence of alternating cyclic temperatures. Patterns leading to an increase in normal stresses at stress concentration sites have been established. Fundamentally new design solutions have been proposed, protected by utility model patents, which allow to reduce stress values and thereby ensure higher performance in monolithic buildings, including high-rise ones.

Novel strategy for automated quality enhancement of up-facing inclined surfaces by incremental dual laser powder bed fusion

The low quality of up-facing inclined surfaces, combined with a lack of in-situ treatments, is a crucial challenge in Laser Powder Bed Fusion of metals. A recently proposed strategy, so-called Incremental Dual Laser Powder Bed Fusion (IDLPBF), aims to enhance the quality of up-facing inclined surfaces of metal parts directly during building. This is achieved through automatically alternating between laser remelting and building in height steps of 1.5 mm. Laser remelting involves selectively removing powder that is covering the inclined surface through nanosecond pulsed laser-induced shock waves and subsequently remelting the uncovered surface using a continuous wave laser. Treated surfaces are hence composed of laser remolten zones separated by elevated ridges in the overlap region, referred to as stitching zones. These stitching zones can compromise the geometrical accuracy and, consequently, pose a serious risk to fatigue performance. This work focuses on the enhancement of stitching zones and resulting quality of inclined surfaces through IDLPBF of a novel tool steel (M789). The paper covers the effect of various scan strategies for laser remelting on the stitching zone size. By optimizing the orientation of consecutive remelting passes and the position of the remelting fields, the stitching zone width could be reduced from 830 µm to 70 µm. Moreover, the optimized IDLPBF process was successfully applied to a large range of surface inclinations with respect to the horizontal build plate (5°-60°). For the first time, a significant quality improvement for 60° inclined surfaces was achieved, reducing the stress concentration factor by 35 %. The significantly reduced maximum stress concentrations were largely observed at stitching zones, in contrast to multiple high stress concentration sites randomly scattered across as-built surfaces. As a proof of concept, 60 mm high demonstrator parts including typically inaccessible surfaces located in the shadow zone were fully automated manufactured by IDLPBF.

Effect of δ–ferrite and strain-induced martensite on hydrogen embrittlement of STS 308 L and STS 316 L all-weld metals

The susceptibility of multipass flux-cored arc welds of STSs 308 L and 316 L to hydrogen embrittlement (HE) was investigated at various temperatures. Cracks predominantly initiated and propagated along the δ-ferrite in the interdendritic region at 25 °C. δ-ferrite served as preferred hydrogen trap sites, and the δ/γ boundary became the stress concentration site. At −100 °C, STSs 308 L and 316 L exhibited high susceptibility to HE owing to strain-induced martensite formation, providing additional hydrogen trap sites and dislocation accumulation. Despite strain-induced martensite actively transformed at −196 °C, the HE disappeared owing to the sluggish diffusion of hydrogen to stress-concentrated areas.

Quantitative Evaluation of the Relationship between Strain and Color Change in Opal Photonic Crystal Films and Application into Complex Specimen Geometries

The color change of opal photonic crystal films (OPCFs) due to deformation was quantitatively evaluated using digital image correlation (DIC) analysis. OPCFs were pasted on specimens of three different gauge geometries, and random patterns were formed on the opposite side of each specimen for DIC analysis. To assess the applicability of using OPCFs-based strain characterization for analyzing steel structural components and associated metallurgical analyses, smooth, width-gradient, and holed specimens were prepared in this study. As deformation increased in the smooth specimen, the color of the OPCFs changed significantly. The color change in the OPCFs could be quantitatively converted into strain values through Hue value analysis. Heterogeneous strain distributions could also be quantitatively analyzed using OPCFs-based analysis at the submillimeter or millimeter scale. When the strain gradient is too high, for example, near a stress concentration site such as a hole, local peeling of the OPCFs away from the specimen surface can occur. Consequently, for quantitative characterization, we must take proper care when measuring this upper limit of the “strain gradient” as well as strain, which would depend on the adhesion and surface condition of the specimen.

Innovative Stress-responsive Liner Design for Lower Limb Disability

After lower limb amputation or disability, prosthetic limbs are essential for regaining mobility, but skin integrity often degrades due to mechanical stresses at the stump–prosthesis interface. In this study, the finite element method was utilized to predict the areas at the stump–prosthetic interface experiencing the highest levels and distributions of mechanical stresses. The empirical results of the finite element analysis (FEA) delineated the stress concentration sites. Consequently, we devised an innovative liner design tailored to these stress patterns, which was subsequently implemented. The simulation outcomes validated the effectiveness of the liner, demonstrating significant reductions in stress levels at the junction of the residual limb and the prosthetic socket. The results demonstrated that the designed liner was highly effective in reducing stress, particularly in three critical areas: the lateral area of the tibia head (LTH), the tibia end area (TE), and the fibular end (FE). Stress values at the LTH were reduced from 55 to 10 kPa, at the TE from 60 to 28 kPa, and at the FE from 72.3 to 14 kPa. In essence, this liner represents a bespoke intervention, with the FEA serving as a predictive tool that not only identifies critical stress points but also guides the design of personalized prosthetic liners. These findings will improve the structural and functional compatibility of the prosthesis, thereby enhancing wearer comfort and reducing the risk of skin-related complications for persons with disabilities.

Influence of single point incremental forming on the low-cycle fatigue performance of AA6061-T6

The current study focuses on the influence of single point incremental forming (SPIF) process on the low-cycle fatigue (LCF) performance of AA6061-T6 sheet. The base material (BM) displays a saturated cyclic response for all strain amplitudes. The SPIF-formed specimens show similar behavior, especially at lower strain amplitudes; however, they exhibit significant cyclic softening at higher strain amplitudes. The LCF life of SPIF specimens (considered in rolling and transverse directions) for all strain amplitudes is slightly reduced due to overlapping/repeated sheet deformation and scallops formed during tool movement. Overlapping affects strain accumulation during SPIF, and scallops affect strain accumulation during cyclic loading and act as a stress concentration site. The texture alteration during the incremental forming also influences the ductility and the subsequent LCF life of the SPIF components.

Achieving high mechanical and wear properties in the AZ31/(CeO2+ZrO2)p surface composite using friction stir processing: Application of vibration

In this study, the friction stir vibration processing (FSVP) have been devoted to the study the impact of the vibration frequency on the metallurgical and mechanical behavior of the AZ31/CeO2–ZrO2 surface composite. The structure, local shear punching test (SPT), hardness, and wear performance of the produced surface composite were studied. The results showed that with the use of vibration during FSP, the stir zone area broadened and resulted in eradication of the flow voids and enhanced the materials flow and the (ZrO2+CeO2)p powders dispersion in the matrix. It was also found that vibrating motion acted as an auxiliary energy and resulted in the enhancement of the shear punching strength and wear resistance by about 29% and 48% in comparison to that of the FSPed sample. This is attributed to the absence of stress concentration sites and better particle dispersion-strengthening.

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.

Fatigue performance of cracked diaphragm cutouts in steel bridge reinforced employing CFRP/SMA

Under long-term service conditions, fatigue cracking occurs at the diaphragm arc-shape cutouts in the orthotropic steel deck (OSD) bridges subjected to vehicle-induced vibration and cyclical wheel load. To repair cracks of the diaphragm cutouts, this study proposes two repair methods of the carbon fiber reinforced polymer (CFRP) sheets and shape memory alloys (SMA)/CFRP composite patches on the basis of crack-stop holes. The crack-stop hole eliminates the obvious source of fatigue propagation at the crack tip, but it weakens the section and harms the entirety of the diaphragm. The edge of the crack-stop hole in the cracked section becomes a new stress concentration site and the potential crack initiation point. The CFRP sheets can effectively improve the stiffness and reduce the stress amplitude of the cracked part. Moreover, the SMA/CFRP composite patches can further introduce precompression by activating SMA to reduce the stress level. Relative to the reinforcement by the crack-stop hole only, the fatigue notch factor is reduced by 12.28% and 30.76% after bonding CFRP sheets and SMA/CFRP composite patches, respectively. The fatigue tests show that the bonding of CFRP sheets and SMA/CFRP composite patches can effectively postpone the initiation of fatigue cracks and inhibit crack propagation, so they are ideal repair methods for strengthening fatigue cracks of the diaphragm cutouts in the OSD bridges.

Evaluation of mechanical properties of triple-junction-free polycrystalline graphene

Although chemical vapor deposition (CVD) has emerged as an important method for producing large-scale and relatively high-quality graphene, CVD-grown graphene inherently contains grain boundaries (GBs), which degrade its mechanical properties. To compensate for these characteristics, various studies have been conducted to maintain the mechanically superior properties by controlling the density of defects and GBs. In this study, the mechanical properties of triple junction (TJ)-free polycrystalline graphene, which is expected to exhibit excellent properties, were investigated through molecular dynamics simulations because TJ is well-known as a crack nucleation site due to stress concentration. We adopted the phase-field crystal method to model CVD-grown graphene-containing TJ-free polycrystalline materials. From a series of numerical simulations, we found that the fracture strength increases as the density of the GB increases. This trend is consistent with that presented in a previous experimental study measured by nanoindentation. It was determined that the variation in the fracture strength is related to the discontinuous density of 5–7 pairs, which act as stress-concentration sites. Additionally, we observed that the fracture strength was higher than that of polycrystalline graphene with TJ. We believe that these results have a higher mechanical advantage compared to the low strength of TJs shown in previous studies and will be important for future structural reliability-based graphene applications.

Stiffening surface lowers ice adhesion strength by stress concentration sites

Mechanically softening materials have been intensively investigated to reduce ice adhesion strength based on the adhesion failure mechanism. Achieving icephobic surfaces with both low ice adhesion strength and mechanical robustness is challenging. The surface stiffening-induced stress concentration sites on elastomer should be possible to improve the mechanical robustness and lower the ice adhesion strength simultaneously, as it can build material inhomogeneity (stiffness mismatch) and geometry irregularity-induced cracks on the surface layer of elastic materials. To reveal this hypothesis, the stress concentration sites are introduced to obtain the robust low ice adhesion surface (RLAS) by implementing rigid nanocomposites with the elastic modulus of GPa scales (e.g. silica nanoparticles, SiO2NPs, ca. 68.9 GPa) into the surface layer of elastomers (e.g. polydimethylsiloxane, PDMS, ca. 2.7 MPa). The effects of the material inhomogeneity (stiffness mismatch) and geometry irregularity-induced stress concentration sites on the reduction of ice adhesion strength are revealed. The minimum ice adhesion strength of the RLAS is down to 10.5 kPa, and its ice adhesion strength remains at 13.8 kPa after 25 icing/deicing cycles. Originating from abundant potential crack initiation sites and stiff nanoparticles, our designed nano-composite materials remarkably lower the ice adhesion strength and are over 2-fold mechanically stiffer than pure elastomer. This strategy offers an unconventionally hardening surface approach for designing icephobic materials with combined durability and robustness.

Improvement in deformation degree of Zr surface-layered Bi-2223 ceramics by diffusion annealing temperature

This study investigated the effects of different annealing temperatures (650 °C ≤ T ≤ 840 °C) on the surface morphological and mechanical performance properties of Zr surface-layered Bi-2223 materials with scanning electron microscopy (SEM) images, Vickers microhardness (Hv) measurements, and semi-empirical mechanical approaches. It was observed that the ceramic compound exposed to 650 °C annealing temperature exhibited the superior performance features due to the enhancement in the deformation degree. This is because the Zr ions behaved as the nucleation centers to prevent the propagations of cracks and dislocations throughout the main matrix depending on the decrease in the degree of granularity and distributions of crystal structure problems over a wider area. Similarly, the SEM pictures indicated that the diffusion mechanism increased the random distributions of the thinner plate-like granular structures (serving as nucleation centers), leading the decrease in the coupling problems between the grains. Among the materials, the highest surface densification was observed for the compound exposed to 650 °C. Namely, surface morphological analysis showed a strong correlation between microstructure and mechanical performances. Further, the zirconium ions were found to decrease in the non-recoverable stress concentration sites, crack-initiating defects, and dislocations in the ceramic system. Accordingly, the sensitivity to the applied test load was noted to decrease dramatically. Shortly, crack growth size and velocity were observed to be more easily under control. Correspondingly, the Zr ions delayed considerably the beginning points of saturation limit (load-independent) regions for the bulk Bi-2223 superconducting materials. Additionally, the Zr ions led to the change in the mechanical characteristic behavior from typical indentation size effect to reverse indentation size effect. Lastly, the microindentation hardness measurements were semi-empirically analyzed by the different models. According to the comparison, Hays-Kendall mechanical model was noted to provide the closest parameters to the load-independent microhardness results.

Investigation of failure and development of mitigation techniques of a cracked aircraft wing spar cap

The research is aimed to identify the failure mechanism which leads to cracks development in the wing spar cap of a transport aircraft and subsequently devise a mitigation strategy to prevent such defects. The research objective was accomplished in two stages. In the first stage, failure analysis of the spar cap was carried out through a hierarchal experimental approach where a combination of various experimental techniques (i.e., compositional analysis, stereoscopy for dimensional measurement of crack, and fractographic analysis through SEM/EDX) was employed to ascertain the cause of failure. Microscopy revealed multiple cracks originating from the hole edge which were studied in detail to ascertain the cause of failure. The failure analysis revealed the presence of corrosion products inside the hole due to crevice corrosion as a prime contributor to crack initiation. Further, cyclic loading caused the crack to propagate, while the presence of undiffused lead particles in the cracked surface confirms the occurence of deposition corrosion and crack branches at various places suggested Stress Corrosion Cracking (SCC) . The numerical analysis helped to identify that although the hole was a stress concentration site yet it was material weakening due to corrosion which primarily caused the failure . The second stage entails the development of a mitigation strategy to avoid corrosion-assisted crack development. For this purpose, two different types of coatings, i.e., Sulphuric Acid Anodizing (SAA) and Chromic Acid Anodizing (CAA) were applied to the specimens and then subjected to salt spray testing as well as exposed to environmental conditions. SAA coating appeared to be more corrosion resistant as it showed little signs of corrosion as compared to original (uncoated) and CAA-coated samples.