Residual Strain Research Articles

Facile eco-friendly process for upcycled sustainable perovskite solar cells

Developing a green technology to eliminate the impact of the toxic element Pb and harmful solvent is a challenge for sustainable perovskite solar cells (PSCs). Herein, we report a facile eco-friendly process for upcycled sustainable PSC by screening the polar Pb solubilizers (N,N-dimethylformamide, DMF, dimethyl sulfoxide, DMSO and butylamine, BA) coupling with multi-function potassium iodide (KI) precipitator. We find that DMF with strong Pb-dissolution capacity and weak coupled C = O − Pb adhesion exhibits superb separation and recovery of Pb from obsolete PSC devices. Subsequent closed-loop management of the DMF completely eliminates its potential harm to the ecological environment. Interestingly, the KI precipitator added to the Pb-containing DMF solution not only induces the formation of high-quality PbI2 and thus high-crystallized defect-less perovskite matrix, but also relieves the residual tensile strain stemming from lattice expansion leading to orderly lattice orientation for perovskite crystallization. As a result, the re-fabricated PSC based on recovered components (PbI2, ITO and Ag) shows a PCE of 22.78 % higher than that (20.76 %) of PSC based on fresh components, also exceeding the reported record efficiency of PSCs based on one or more recovered components. Such a stable, environment-friendly, upcycled process of retired PSCs offers a sustainable pathway for eliminating the adverse factors such as toxic Pb leakage and harmful solvent effluence, and thus boost their competitiveness in future energy market.

Transformation pathway of NiTi shape memory alloy and its modulation based on grain size gradient: A molecular dynamics study

The reduction of grain size to nanoscale is a common approach to enhance material properties, such as fatigue resistance and overall strength of Shape Memory Alloys (SMAs), but it normally decreases the volume fraction of the material undergoing reversible martensitic transformation (MT), leading to a reduction in the reversible strain amplitude. This paper explores the atomic relationship between specific transformation pathways and grain size/gradient of NiTi polycrystal based on molecular dynamics simulation. It is found that during a tensile loading-unloading cycle, four different transformation pathways can be observed, with their volume fractions being dependent on grain size. Importantly, these transformation pathways serve as the medium of the grain size effects on the global mechanical properties of NiTi. The gradient nanograined (GNG) NiTi with large gradients can maintain a high stress level while reducing the residual strain by adjusting the fraction of different transformation pathways, thus achieving both advantages of small residual strain and high strength during cyclic loading with large strain amplitude. The results provide atomic insights into the grain size effects on the transformation pathways of SMAs and the development of high-performance SMAs based on the grain size gradient.

Artificial neural network and ANFIS approaches for mechanical properties prediction and optimization of a turbine blade treated by laser shock peening

This research article covers a comprehensive investigation on the prediction and optimization of residual stresses, plastic deformations and damage induced by laser shock peening (LSP) on a thin Ti-6Al-4 V leading edge sample of a turbine blade. The study is based on a two-part approach: the first part involves a numerical simulation of the LSP process and the characterization of its effects on the material. The finite element analysis (FEA) is used to simulate the complex interactions between laser parameters, material properties and mechanical response. This numerical analysis generates valuable insights into the residual stress distribution, the plastic deformation and the potential damage within the sample being treated. Based on the numerical results, the second part highlights the novel application of the artificial neural network (ANN) and the adaptive neuro-fuzzy inference system (ANFIS) methods. By taking advantage of the strength of ANN and ANFIS, the models are able to learn accurately from all the data generated by numerical simulations, allowing them to precisely predict and optimize the effects induced by LSP: residual stresses, plastic strains and damage. This paper provides a significant contribution to the field of materials science and engineering, by offering an in-depth comprehension of the LSP process impact on Ti-6Al-4 V leading-edge turbine blade specimens. The combined utilization of numerical analysis and artificial intelligence AI-driven techniques offers a comprehensive approach to optimize the LSP process, leading to the fatigue resistance improvement and service life extension of turbine blades and other critical aerospace components. The proven capability of ANN and ANFIS enables innovative solutions in aerospace engineering and surface treatment technologies, promoting the AI implementation in materials processing and design.

Intergranular stress and plastic strain formation during laser scanning of additively manufactured stainless steel: An experimentally-driven thermomechanical simulation study

A novel experiment-driven modelling and simulation approach is proposed to study the formation of intergranular stresses and plastic strains during laser scanning of additively manufactured 316L stainless steel. Recently, laser scanning experiments were performed using a unique coupling between a continuous-wave laser and a scanning electron microscope. These experiments form the basis for the development of a thermo-elasto-viscoplastic polycrystal model endowed with the minimum necessary physics of the problem in order to facilitate simulating sufficiently large domains to obtain reasonable stress magnitudes. The case of a single laser line scan performed in vacuum is analyzed in detail. Polar dislocation density magnitudes and distribution predicted from the simulation are compared with those measured experimentally. Results reveal that for 93% of the grains in the lasered zone, a statistical measure of the predicted polar dislocation density (Nye’s) tensor lies within a factor of 2 (higher or lower) of the experimental one; this result sets a benchmark for future experiment-simulation comparisons. Subsequent investigation studies the origin of the local residual stresses and plastic strains. On the lasered surface, the grain surface-averaged normal residual stress component and plastic strain component along the scan direction reaches a value ∼1.7GPa and 0.04, respectively. The contribution of elastic anisotropy, plastic heterogeneity and strengthening due to microstructure refinement after laser scanning on the formation of stresses and plastic strains is studied in detail.

Experimental Investigation of the Mechanical Behavior of the Strain Isolation Pad in Thermal Protection Systems under Tension

A strain isolation pad is a critical connection mechanism that enables deformation coordination between the rigid thermal insulation tile and the primary structure in the thermal protection system of a reusable hypersonic vehicle. An experimental investigation has been conducted to determine the static, loading–unloading, and high-cycle fatigue (HCF) responses of the SIP with 0.2 mm adhesive under through-thickness tension at room temperature. The contributions of the rigid thermal insulation tile and metallic substructure have not been considered so far. The results indicate that the tensile behavior of the SIP joint is highly nonlinear. The static and fatigue tensile failures both initiate from the corner close to the adhesive/SIP interface due to the stress concentration and the edge effect. The uniform breakage of the aramid fiber can be seen on the cross-section. A novel method is proposed to quantify the residual strain due to the short-time ratcheting effect of the SIP joint in the initial loading–unloading tensile response. As the number of fatigue cycles increases, the thickness of the SIP joint continues to increase until failure. An explicit expression associated with the growth of SIP joint thickness, fatigue cycle number, and peak cyclic stress is established. The turning point of the thickness growth rate with the fatigue cycle number is proposed as a new fatigue failure index for the SIP joint under tensile fatigue, and a fatigue life prediction model is developed.

Cross‐layer all‐interface defect passivation with pre‐buried additive toward efficient all‐inorganic perovskite solar cells

AbstractThe buried interface in the perovskite solar cell (PSC) has been regarded as a breakthrough to boost the power conversion efficiency and stability. However, a comprehensive manipulation of the buried interface in terms of the transport layer, buried interlayer, and perovskite layer has been largely overlooked. Herein, we propose the use of a volatile heterocyclic compound called 2‐thiopheneacetic acid (TPA) as a pre‐buried additive in the buried interface to achieve cross‐layer all‐interface defect passivation through an in situ bottom‐up infiltration diffusion strategy. TPA not only suppresses the serious interfacial nonradiative recombination losses by precisely healing the interfacial and underlying defects but also effectively enhances the quality of perovskite film and releases the residual strain of perovskite film. Owing to this versatility, TPA‐tailored CsPbBr3 PSCs deliver a record efficiency of 11.23% with enhanced long‐term stability. This breakthrough in manipulating the buried interface using TPA opens new avenues for further improving the performance and reliability of PSC.

Fiber Bragg grating detection of gel time and residual strain of curing in epoxy resins

AbstractThe curing shrinkage of epoxy resin can generate residual strains and internal stresses that reduce the material's strength and life. In this paper, a thermocouple‐compensated fiber grating detection method for the gel time and residual strain in epoxy resin is proposed. The strain evolution and glass transition temperature of E51/W93 epoxy systems with different Al2O3 contents cured at 30°C are detected by fiber Bragg gratings and DSC tests. The generation mechanism of residual strain in epoxy resin filled with Al2O3 fillers is studied. The gel time, gel temperature and residual strain of epoxy resin can be measured by this method. The results show that the cure shrinkage of epoxy is affected by chemical shrinkage, thermal expansion or shrinkage, and demolding, resulting in residual strain of the material after curing. Moreover, the glass mold has a great interference with the curing shrinkage of epoxy. The Al2O3 filler can prolong the gel time of E51/W93 epoxy systems, and reduce the exothermic peak temperature of curing. When the content of Al2O3 filler exceeds 44.44 wt.%, the low proportion of epoxy and the thermal conductivity of the filler make the curing shrinkage of epoxy during the heating stage mainly affected by chemical shrinkage. The glass transition temperature of the epoxy with Al2O3 content of 70.59 wt.% is the maximum, and the residual strain is at a low level. This method is helpful to analyze the curing shrinkage characteristics of materials, and is of great significance for evaluating the properties of epoxy cured products and guiding the optimization of material parameters.Highlights A method for measuring the gel time and residual strain of curing in epoxy resin is proposed. The study focuses on the detection of key parameters during the curing of epoxy resins with different Al2O3 contents. The aim is to analyze the curing shrinkage and residual strain generation mechanism of epoxy resin filled with different contents of Al2O3.

Low Cycle Fatigue and Damage Evaluation on 16MnCr5 Alloy Steel

The 16MnCr5 alloy steel samples are evaluated in a normalized condition and put through low cycle fatigue (LCF) under constant amplitude strains of εa = 0.2%, 0.25%, 0.4%, and 0.6%. Mechanical properties are reported based on the LCF results. Fatigue damage is checked in specimens that have reached 80% of their nominal fatigue life by two methods that are different from traditional electronic microscopy: measuring the roughness of the surface and looking at it through full‐field optical microscopy. In addition, the electric resistivity is determined for the test specimens. It presents an increment from the blank value of 6.5 × 10−8 Ωm (free of fatigue damage) to 1.25 × 10−7 Ωm for the fatigue test specimen (εa = 0.6%). Finally, X‐ray diffraction patterns are determined for analyzing the residual equivalent strain at the microscale for the fatigued samples. The residual equivalent strain exhibits a trend toward increasing. For instance, it increases from 18.5 (blank value) to 34.0 με for the fatigue test specimen with εa = 0.6%.

Role of the microstructure and the residual strains on the mechanical properties of cast tungsten carbide produced by different methods

Cast tungsten carbide (CTC) is a biphasic, pearlitic-like structure composed of WC lamellae in a matrix of W2C. Besides excellent flowability, spherical CTC powders exhibit superior hardness and wear resistance. Nevertheless, the available literature generally fails to explain the physical mechanisms behind such a phenomenon. In the present work, the microstructure and the mechanical properties of the novel centrifugally-atomized spherical CTC have been extensively investigated. This material exhibited an extremely fine microstructure, with WC lamellae of 27–29 nm in thickness and bulk lattice strains of 1.0–1.4 %, resulting in a microindentation hardness of 31.4 ± 1.6 GPa. The results of this study clearly show that centrifugally-atomized CTC is mechanically superior to both spheroidized CTC and conventional cast-and-crushed CTC. In addition, the effect of a series of heat treatments on the bulk fracture toughness and the fatigue life of entire CTC particles was also investigated. The reduction of residual stresses in the bulk of particles upon annealing dramatically increased the indentation fracture toughness, whereas the bulk microindentation hardness remained essentially unaffected. Regarding the fatigue life of entire particles under uniaxial cyclic compressive loading, local phase transformation phenomena at the surface of the particles upon heat treatment were concluded to play the most critical role. Indeed, the cumulative fatigue damage was minimized in surface-carburized CTC powders, where compressive stresses were induced at the outermost surface.

Interpreting compression resistance and resilience of spacer fabrics with morphological and mechanical changes of different polymeric spacer monofilaments

Customization of spacer fabrics to have the desired compression resistance and resilience is critical to promote their technical applications. The compression properties are mainly determined by the mechanical features of monofilaments used as spacer yarns. This study investigates the compression properties of spacer fabrics knitted with different polymeric monofilaments by analyzing their compression stress at 40% strain, residual strain, and energy absorption from cyclic compression tests. The monofilaments in the fabrics before and after cyclic compression were unraveled to test their tensile properties and morphologies for interpreting their different fabric compression properties. It was found that knitting monofilaments substantially alters their macroscopic and/or microscopic structures, thereby affecting the mechanical behavior of spacer monofilaments, and having different fabric structures and compression properties. Knitting polyethylene terephthalate monofilament damages the mesophase and induces a two-thirds decline in Young's modulus, while knitting polyamide 6 and polybutylene terephthalate monofilaments largely alters the macrostructures but maintains their good resilience. Spacer fabrics knitted with stiffer polyethylene terephthalate monofilaments have superior compression resistance but inferior compression resilience, while those knitted with softer and more resilient polyamide 6 and polybutylene terephthalate monofilaments have better compression resilience. Polyethylene monofilament is not suitable for spacer fabrics because of its low Young's modulus and poor tensile resilience. Polyethylene terephthalate spacer fabrics have the best compression resistance–resilience balance in terms of compression stress at 40% strain and residual strain in the fourth cycle required by the standard EN ISO 3386-1, while polyamide 6 and polybutylene terephthalate monofilaments are more suitable for long-term service due to their high compression resistance and resilience.

Second-order deformation behaviors of free-rotational tension after free-end torsion for rolled AZ31B magnesium alloy

The accompanying dimensional change caused by the second-order deformation behaviors severely limits the accuracy of the size and shape control of the structural components. The effect of the microstructure evolution mechanism on the second-order deformation behaviors (e.g., Swift effect, inverse Swift effect) of magnesium (Mg) alloys has not been systematically reported. Therefore, in this study, in order to promote the precision forming , the deformation mechanisms of the second-order mechanical behaviors of rolled AZ31B Mg alloy were systematically investigated by employing the specifically designed multi-degree of freedom and multi-axial force-field coupling loading mode. The free-end torsion (FET) experiments were carried out in the range of 30°–90° along the transverse direction (TD) at room temperature and the free-rotatial tension was performed after FET. The Swift effect of axial shortening was observed during FET, and the cumulative effect of misfit strain caused by tensile twinning is the main reason for the Swift effect. Meanwhile, obvious inverse Swift effect was detected in the free-rotatial tension. It is found that the external reverse residual shear stress dominates the rapid reverse rotation of the first stage in the inverse Swift effect, and the nonlinear distribution of the residual shear strain increases this reverse rotation. The competitive effect of detwinning caused by reverse rotation and prismatic slip leads to a slow reverse rotation. The prismatic slip provides the driving force for the inverse Swift effect in the positive rotation stage until the residual shear stress is completely released.

Investigation of radiation tolerance of yttria stabilized zirconia in the ballistic collision regime: Effect of grain size and environmental temperature

The irradiation response of yttria-stabilized zirconia, having different grain sizes, subjected to 900 keV iodine ions at room temperature and 1000 K is reported. GIXRD and Raman spectroscopy indicate superior radiation tolerance of the fine-grained samples when compared to their coarse-grained counterparts which is attributed to the abundance of grain boundaries at relatively shorter distances. A reduction in the residual strain is observed in the samples irradiated at 1000 K. This is attributed to the clustering of the irradiation induced point defects thereby resulting in a decrease in their density, but with a corresponding increase in the extended defects density.

Effect of helium ion irradiation on the microstructure, mechanical properties and surface morphology of Inconel 625 alloy

Inconel 625 alloy is mainly used for reactor pressure vessel, fuel cladding, nuclear fuel elements and cryogenic channels for cooling high temperature superconductors in Tokamaks. This study focuses on deciphering the impact of helium ion irradiation on the properties of Inconel 625 alloy. Helium-ions interaction with material leads to blistering, void formation, swelling, and degradation of mechanical properties. To understand these effects, the samples of Inconel 625 alloy were irradiated by helium-ion fluence beams at three different helium-ion beam fluence (1 × 1013, 1 × 1014, 1 × 1015 He/cm2). A decrease in grain size was observed from 34.4 to 19.0 nm with an increase in the ion fluence. According to the results, the residual strain is increased, from 1.8 to 43.0 micro-strain with the increase in helium-ion fluence, however structure is relaxed with decrease in micro-strain at higher helium-ion fluence (1 × 1015 He/cm2) and also there is a prominent increase in grain size. The surface morphology as observed by scanning electron microscope (SEM) illustrates the fuzz formation at low fluence (1 × 1013 He/cm2), while samples irradiated with high helium ion fluence (1 × 1015 He/cm2) showed grain growth, voids and surface blistering (deep valleys). An increase in hardness value at low helium-ion beam fluence is observed due to the presence of large population of defects. However, at higher helium-ion beam fluence, a decrease in hardness value is observed. This decrease might be attributed to the dislocation clustering and grain growth.

Strain Relaxation Characterization on Lithium‐ion Batteries at Different States of Charge and Charging Rates

The strain on the cell casing can serve as an indicator of the internal state of the cell. Monitoring strain during the charging and discharging processes aids in determining optimal charging and discharging operations, thereby maximizing the lifespan and performance of the battery. Herein, strain dynamic curves are obtained for lithium‐ion batteries at low, medium, and high charge and discharge rates by affixing strain gauges to individual cells. The investigation delves into parameters such as strain relaxation time, maximum strain, and residual strain at various charge rates and states of charge. The experimental findings reveal distinctive patterns, indicating that the strain curve during high‐rate charging resembles a second‐order function, exhibiting more pronounced fluctuations as the rate increases. This stands in stark contrast to the strain exponential decay observed during conventional medium and low‐rate charging. Notably, the strain residual resulting from high‐rate charging proves to be several times higher than that observed in low‐rate charging, hinting at potential differences in the dynamic distribution of lithium ions within batteries during high‐rate versus medium‐low‐rate charging modes.

Characterization of Mechanical Heterogeneity and Study of the Mechanical Field at the Tip of the Stationary-Growing Crack in Dissimilar Metal Welded Joints

Abstract The mechanical heterogeneity in local areas of dissimilar metal welded joints and the micro-area mechanical state at the crack tip are key factors in determining Environment-Assisted Cracking (EAC). Traditional methods for acquiring material mechanical properties often result in destructive damage to specimens, while conventional “sandwich” models exhibit abrupt changes in interfacial mechanical properties and a lack of research into the mechanical field at the tip of the stationary or growing crack. In light of these challenges, this study, based on the analysis of microstructures in localized regions of the welded joint and the acquisition of material mechanical properties through indentation tests, developed a user-defined material subroutine (UMAT) to characterize the mechanical properties of non-uniform local areas within the welded joint. Additionally, it investigated the mechanical field at the tip of the stationary—growing crack using an integral method and a de-bond technique. The results indicate that non-destructive indentation tests can accurately acquire the material mechanical properties of local areas in the welded joint. Notably, significant changes in mechanical properties typically occur in the material interface regions, making them vulnerable points for potential failure. Furthermore, under the same load, mechanical heterogeneity significantly influences the distribution of the mechanical field at the crack tip. Crack propagation induces alterations in crack tip stresses, resulting in noticeable residual stresses and strains along the propagation path.

Biomimetic design and fabrication of thermally induced radial gradient shape memory scaffolds using fused deposition modeling (FDM) for bone tissue engineering

Bone defects pose a significant challenge, often exceeding natural healing capabilities. This study explores the potential of thermally induced radial gradient shape memory (RGSM) scaffolds for minimally invasive bone repair. Inspired by the natural porosity gradient of bone, these scaffolds feature a high-porosity inner zone that mimics cancellous bone and a low-porosity outer zone that resembles cortical bone. When the relationship between porosity and key properties was investigated, it was found that lower-porosity RGSM scaffolds exhibited higher compressive strength but experienced higher residual strain and lower shape recovery ratio compared to their higher-porosity counterparts. Despite this trade-off, the gradient design successfully mimicked the natural bone structure, potentially enhancing osseointegration and bone regeneration. These results demonstrate the feasibility of RGSM scaffolds for bone tissue engineering. This holds promise for advancing minimally invasive surgical techniques and improving the treatment of bone defects.

Deformation and Failure Characteristics of Strong-Weak Coupling Structures with Different Height Ratios Under Cyclic Loading and Unloading.

Strong-weak coupling outburst prevention technology can reduce the hazard of coal and gas outburst in mines based on hydraulic punching and grouting reinforcement. In this study, the mechanism of outburst hazards in the strong-weak coupling structure under mining disturbance was explored, and then cyclic loading and unloading experiments were performed on samples with different strong-weak height ratios (HRs) using the noncontact full-field strain testing (DIC) system and the acoustic emission (AE) system. The results show that the failure strength of the sample gradually increases with the increase in HR. The residual strain of the strong and weak structures undergoes three stages, i.e., the decelerated deformation, the constant-velocity deformation, and the accelerated deformation. Deformation mainly occurs in the weak structure and starts at the strong-weak interface. The AE signals present strong regional distribution characteristics and the Felicity effect, and the damage is concentrated near 70% of each stage in the cyclic loading process. As the HR rises, the weak structure transitions from brittle damage to ductile damage and from shear damage to tensile damage. In addition, due to the difference in Poisson effects of strong and weak structures, the strong structure transitions from a unidirectional stress state to a triaxial tensile-compressive stress state. When the HR increases to 85:15, the strong structure undergoes tensile damage.

Chloride transport in high-cycle fatigue-damaged concrete

This paper explored the penetration of chloride into concrete damaged by high-cycle compressive fatigue loading. Two environmental conditions including immersion and wetting-drying cycles were prepared for experimentally investigating the transport performance of chloride in fatigue-damaged concrete. Additionally, a prediction model was developed with the damage index of residual strain. Test results revealed that both apparent diffusion coefficients and contents of chloride at each depth increased in fatigue-damaged concrete, indicating that the fatigue damage significantly degraded the resistance of concrete to chloride penetration. Furthermore, for fatigue damaged concretes with extremely different fatigue cycles and load levels but the same residual strain, the chloride ion content profiles were highly overlapped, which demonstrated the reasonability of selecting residual strains as fatigue damage indexes. The applicability of the transport model was firstly validated with the measured chloride contents and then the chloride transport behavior in gradient fatigue-damaged concrete was investigated numerically using the validated model. The numerical results indicated that the chloride transport was more sensitive to the residual strain at the exposed edge than to the residual curvature and the effect of the later can be neglected.

Low residual stress SiC coatings on C/CA composites by a strategy of matching their expansion behaviors to improve thermal shock resistance

The poor thermal shock resistance of coated C/CA composites limits the applications with rapid temperature fluctuations due to the huge thermal mismatch between the coating and the substrate. A facile strategy of tailoring coefficient of thermal expansion (CTE) of C/CA is proposed to match their thermal deformation behaviors. The optimized C/CA by heat treatment has a positive correlation between CTE and temperature, and thus the as-prepared SiC coating has a decreased macroscopic residual strain of 80.7% due to their coordinated thermal response compared to the original C/CA. Meanwhile, benefited from the structural improvement of larger pore size and higher degree of graphitization of C/CA, a significantly decreased tensile stress of 60.3% in the coating is achieved due to the formation of small SiC spheres in the carbon network. Resultantly, the coated C/CA shows remarkably improved thermal shock resistance with narrower and fewer microcracks, and lower mass and linear cyclic ablation rates of 0.252 mg/s and 0.607 μm/s, respectively.

Comprehensive material study of Ge grown by aspect ratio trapping on Si substrate

High-quality monolithic Ge-on-Si is sought for CMOS-compatible optoelectronic devices. We examine the structural characteristics of Ge-on-Si grown by the aspect ratio trapping (ART) method on a SiO2/Si(001) template in pre-patterned holes. Transmission electron microscopy and surface topography analysis revealed high-quality Ge islands overgrown from the ART holes in SiO2. The superior crystal quality of Ge ART growth was also confirmed by comparing x-ray diffraction (XRD) data of Ge ART and Ge planar epilayer samples. The XRD and micro-Raman data additionally show a small residual strain in the islands which vanishes by reducing the hole diameter from 280 nm to 180 nm, while leading to only a minor increase in the crystallographic inclinations of the Ge islands from 0.34 deg to 0.54 deg. With finite element method simulations, we find that the small residual strain in Ge originates during the cool-down from growth to room temperature because of thermal expansion coefficient mismatch between Ge and SiO2. A tensile force develops along the [001] axis of the Ge pillar whose faster shrinkage to the room temperature volume is restricted by the oversized surface island.