Loading Process Research Articles

Microstructure evolution and mechanical properties of additively manufactured Ni-based GH4099 superalloy via hot isostatic pressing and heat treatment

In this study, GH4099 alloys with over 99.99% density were produced by laser powder bed fusion (LPBF) and followed by hot isostatic pressing (HIP) and heat treatment (HT). The effects of HIP and HT on the microstructure evolution and mechanical properties of the additively manufactured GH4099 superalloy were examined systematically. The as-deposited columnar crystals were effectively transformed into equiaxed crystals via HIP and HT. During HIP, the elimination of defects and elemental segregation in the deposited samples occurs. However, uneven precipitation of the γ′ phase within grains causes the emergence of minor gaps in the γ matrix, which adversely affects mechanical properties. The microstructure resulting from HIP+solution treatment (ST) consists entirely of equiaxed recrystallized grains. The presence of twin and long-period stacking-ordered (LPSO) phases at twin boundaries strongly hinders dislocation behavior, and the remelting of precipitates and carbides greatly improves plasticity. Moreover, during the loading process at 900°C, γ′ phase and carbides were re-precipitated, and the strain concentration was minimized during the precipitation process, resulting in the highest high-temperature strength. Conversely, in the HIP+solution and aging treatment (SAT) process, the progression of γ′ phase precipitation, condensation, annexation, and growth contributes to a reduction in strength. Coupled with an increase in the plasticity of the LPBF GH4099 alloy.

Experimental study and numerical simulation verification of the macro- and micromechanical properties of the sandstone–concrete interface under freeze–thaw cycles

In cold-region tunnel engineering, the bonding surface between concrete and surrounding rock is highly susceptible to engineering disasters such as the cracking of support structures under the influence of freeze-thaw cycles, which severely affects the stability of tunnel engineering. This study investigates the macroscopic and microscopic mechanical properties of the sandstone–concrete interface under freeze–thaw cycles and reveals the microdamage and fracture evolutionary laws during the freeze–thaw loading process via the coupled expansion of water–ice particle phase changes via particle flow numerical simulation methods, aiming to provide theoretical support for the design and construction of rock–soil and tunnel engineering in high-altitude frozen soil areas. The results indicate that the interface strength characteristics of the sandstone–concrete specimens exhibit a stable decreasing trend with an increase in the number of freeze-thaw cycles and a decrease in roughness. Taking the most unfavorable condition as an example, the exacerbation of freeze-thaw deterioration resulted in shear strength reductions of 16.53%, 27.01%, and 37.17% for specimens (JRC=3.727), while an increase in roughness led to shear strength increases of 11.00% and 15.14% for specimens (NT=60 cycles). The acoustic emission characteristics during the loading process of the sandstone–concrete interface specimens reflect the microcracking evolutionary activity of the specimens quite well, and setting 1.0 as the recommended value for the precursor determination of the b-value and CV(k) index is most reasonable. Under freeze-thaw cycles, cracks first initiate partial microcracks at the interface and on the outer side of the specimen, primarily dominated by tensile cracks and evolving with a "slow–fast" trend toward both sides of the interface. Under shear stress, particles at the interface first undergo slip dislocation due to their lower bonding strength, generating cracks, subsequently inducing significant displacement of particles on both sides of the interface, resulting in crack propagation toward both sides of the interface, ultimately penetrating and forming shear bands leading to macroscopic failure.

Rebar-concrete bond behaviour in grouted sleeve connectors

Grouted sleeve connectors (GSCs) are commonly used in precast/prefabricated concrete structures. To ensure force transfer, connectors tend to be overdesigned, and this has a detrimental effect on the plastic deformation capacity of the connection region and the displacement ductility of the precast structure. To enable improvements in the design of precast RC connections involving GSCs, an accurate analysis of the bond behaviour of GSCs is required. This paper presents an analytical model for the local bond behaviour under the particular sleeve confinement. A representative cross-section of a GSC is treated as interconnected hollow circular tubes. Three stages are identified according to the state of the grout, namely elastic, partially cracked, and fully cracked. The confining pressure is formulated within the cross-section, and the bond stress is subsequently evaluated by a modified bond stress-slip law accounting explicitly for the confinement. Compared with traditional bond-slip laws, the model is capable of representing the evolution of confining stress in the loading process. Comparison with relevant experimental results show good agreement. A subsequent parametric study demonstrates the trends of influence of various parameters, and notably, within allowable limits, a reduction of the grout thickness tends to increase the bond strength in the connectors.

Failure Mechanism of Rear Drive Shaft in a Modified Pickup Truck

This paper investigates the failure mechanism of the rear drive shaft in a modified pickup truck which had operated for about 3000 km. The investigation included macroscopic and microscopic evaluation to document the morphologies of the fracture surface, measurement of the material composition, metallographic preparation and examination, mechanical testing, and finite element modelling and calculations. The results obtained suggest that rotation-bending fatigue was the primary cause of the drive shaft failure. The crack initiation is located in the root of the machined threads on the drive shaft surface and expanded along the side of the machining line surface. The main cause of fatigue cracks is attributable to a high stress concentration owing to a large unilateral bending impact under overload. Meanwhile, the bidirectional torsional force also produces a higher stress concentration and thus accelerates the fatigue crack to expand radially along the surface. Furthermore, the hardness of the central section of the drive shaft was marginally below standard. This deficiency results in harm to the bearings and other mechanical components, as well as expediting the enlargement of cracks. Finite element analysis revealed significant contact stress between the bearing and drive shaft, with stress levels exceeding the fatigue limit stress of the parent material. This highlights the need for reevaluation of the heat treatment process and vehicle loading quality to enhance the drive shaft’s longevity.

Optimization of mixed cargo allocation and stowage with heterogeneous vehicles

In order to solve the problem of low efficiency and high loading costs in business logistics and distribution operations, this paper classifies non-standard cargoes based on their spatial and temporal characteristics. Based on this, a dual-layer planning model for multi-vehicle cargoes loading and transportation is constructed to jointly optimize the loading process. This paper designs a space-inspired heuristic algorithm to solve the model and verifies the feasibility of the model. The results show that compared to a single vehicle type, the joint optimization of multi-vehicle combinations for loading and transportation has stronger adaptability and can better accommodate changes in cargo volume. Compared to the single vehicle type, not only did the loading rate increase by 18%, but also effectively reduced the transportation cost by 3%. Different proportions of spatial and temporal characteristic cargoes have different effects on the loading and transportation. Adjusting the proportion of cargoes that can be placed on both upper and lower layers to 50% can better utilize transportation resources such as vehicles. Increasing the proportion of urgent cargoes can improve the economic efficiency of loading and transportation.

Experimental study on axial compressive behavior of concrete-filled corrugated steel tubular columns

Concrete-filled corrugated steel tubular (CFCST) column is a novel type of steel-concrete composite column, comprising four corrugated steel plates, four square-section steel bars at the corners, and infilled concrete. The transversely placed corrugated steel plates can effectively prevent their premature local buckling, release the axial load, and provide lateral confinement effects on the infilled concrete. This paper investigated the axial compressive behavior of CFCST columns through experimental, numerical, and theoretical approaches. Experiments were conducted on 20 specimens, focusing on their axial load-resistant behavior. The specimens demonstrated excellent axial bearing capacity, ductility, and residual bearing capacity. Besides, the mechanical behavior of corrugated steel plates throughout the loading process was analyzed through the development of the horizontal and vertical strains. Furthermore, a finite element (FE) model was developed to simulate the axial compressive behavior of CFCST columns, which was validated against the experimental data. The FE model was used to analyze the compressive behavior of each part in CFCST columns, revealing that the infilled concrete and steel bars primarily bear the axial load, while the corrugated steel plates mainly provide lateral supports to the infilled concrete. Finally, a simplified method for calculating the critical width of corrugated steel plates and a formula for the axial bearing capacity of CFCST columns were proposed. It was indicated that the limiting width-to-thickness ratio of the corrugated steel plates was larger than that of the steel elements in conventional CFST columns. These formulas were demonstrated to be accurate enough and suitable for the engineering designs.

A life cycle assessment of the laboratory—scale oxidative liquefaction as the chemical recycling method of the end-of-life wind turbine blades

According to the latest reports, estimated values of 50,000–66 000 t of end-of-life wind turbine blades (WTB) are expected to be decommissioned in Europe in 2025–2030, posing a significant threat from the environmental and waste management perspectives. This study aims to present the preliminary Life Cycle Assessment (LCA) with sensitivity and uncertainty analysis of the lab-scale oxidative liquefaction process of the WTB, as the original method to recover the high-quality glass fibers with simultaneous production of the secondary chemicals: phenols, ketones, acids, and fatty acids, from the oxidation of the epoxy resin from the polymer matrix. The LCA is based on the experimental results of the oxidative liquefaction process carried out on a laboratory scale using a Parr 500 ml batch reactor, at two different conditions sets for the functional unit (FU) of 1 kg of treated WTB. Each of the analyzed scenarios resulted in higher impact indicators compared to the landfilling. The highest quality fibers were obtained at 350 °C and 40 wt % H2O2 content resulted in 5.52 ± 1.20 kgCO2 eq Climate change impact and 97.8 ± 20.6 MJ of Resource use, fossil per kg of recycled WTB. The lowest quality fiber recovered in char, yet well separated from the matrix obtained at 250 °C and the lowest H2O2 content resulted in 0.0953 ± 0.487 kgCO2 eq Climate change impact and 8.84 ± 7.90 MJ of Resource use, fossil per kg of recycled WTB. The hot spot and sensitivity analysis indicated, that the oxidizer for the process – hydrogen peroxide, when acquired as a shelf product causes a significant burden on the whole process, with sensitivity ratios on the total impact indicators varying across the categories from 0.56 to 0.99. Substitution of H2O2 with theoretical 0-input oxidizer allowed to significantly lower environmental load of the recycling process, which in all of the analyzed scenarios presented environmental benefits compared to landfilling with recovery of the glass fiber and secondary chemicals.

Twinning–Detwinning-Dominated Cyclic Deformation Behavior of a High-Strength Mg-Al-Sn-Zn Alloy during Loading Reversals: Experiment and Modeling

The deformation behavior of a high-strength Mg-Al-Sn-Zn alloy under loading reversals has been thoroughly examined through a combination of experimental measurements and crystal plasticity modeling. We focused on an age-treated alloy fortified by distributed Mg2Sn particles and Mg17Al12 precipitates, which underwent two distinct loading cycles: tension-compression-tension (TCT) and compression-tension-compression (CTC), aligned with the extrusion direction (ED). The initial and deformed microstructures of the alloy were analyzed using the electron backscattering diffraction (EBSD) technique. Notably, the alloy displays tensile and compressive yield strengths (YS) of 215 MPa and 160 MPa, respectively, with pronounced anelastic behavior observed during unloading and reverse loading phases. Utilizing the elasto-viscoplastic self-consistent model incorporating a twinning–detwinning scheme (EVPSC-TDT), the cyclic stress–strain responses and resultant textures of the alloy were accurately captured. The predicted alternation between various slip and twinning modes during plastic deformation was used to interpret the observed behaviors. It was found that prismatic <a> slip plays an important role during the plastic deformation of the studied alloy, and its relative activity in tensile loading processes accounts for up to ~66% and ~67% in the TCT and CTC cases, respectively. Moreover, it was discerned that detwinning and twinning behaviors are predominantly governed by stresses within the parent grain, and they can concurrently manifest during the reverse tensile loading phase in the TCT case. After cyclic deformation, the area fractions of residual twins were determined to be 7.51% and 0.93% in the TCT and CTC cases, respectively, which is a result of the varied twinning–detwinning behavior of the alloy in different loading paths.

Evolution of the damage precursor based on the felicity effect in shale

Damage precursors during hydraulic fracturing in shale gas reservoirs may be better understood if the deformation, failure, and acoustic emission (AE) characteristics under cyclic loading are known. Therefore, the purpose of this paper is to investigate the quantitative damage based on the Felicity effect under constant stress lower limit uniaxial cyclic loading-unloading rates (0.5, 1.0, 1.5, 2.0, and 2.5 kN/s). Variations in the b-value and the spatiotemporal evolution of cumulative AE were also used to observe how shale fractures formed. The findings reveal that during the unloading stage, there are many cumulative AE events when the stress level is low (≤1.50 kN/s) but that this number drops significantly when the stress level increases above (>2.0 kN/s). The AE amplitude, AE counts, and cumulative AE energy of each cycle in a loading-unloading test show an increasing trend, but the rate increases in the last cycle. During the whole process of loading and unloading, the Kaiser effects were present in the 3rd cycle at stress levels (≤1.5 kN/s). Still, the Felicity effect appeared in the 2nd and 1st cycles during 2.0 and 2.5 kN/s cyclic loading. The Kaiser effect occurs in the linear elastic stage, while the Felicity effect occurs in the crack initiation and crack damage stage. Furthermore, the Felicity ratio (FR) variations during shale deformation and failure can be divided into four phases: (Phase I = 1.01 ≥ FR > 0.89), (Phase II = 0.89 ≥ FR > 0.48), (Phase III = 0.48 ≥ FR > 0.23), and (Phase IV = FR ≤ 0.23). The b-value is relatively higher under the loading rate below (≤1.50 kN/s), indicating an increase in the number of small AE events. In contrast, the fact that the b-value is relatively smaller under the loading rate above (>2.0 kN/s) indicates that, the number of large AE events increases the number of cracks and fractures. These findings provide important design references for damaged precursors during hydraulic fracturing in shale gas reservoirs.

Meta‐heuristic optimization techniques for scheduling in multiple‐input multiple‐output multiple‐access channel system

SummaryIn a highly dense multiple‐input multiple‐output (MIMO) communication system, proper resource allocation is exceedingly challenging. The simultaneous selection of antennas at the base‐station (BS) and scheduling of users in an uplink multiple access channel (MAC) system add up to the signal processing load in the MIMO wireless system. The method chosen for combining the user scheduling and antenna selection must have a very low computational complexity. However, the exhaustive search algorithm (ESA), which achieves the best sum‐rate capacity in a MIMO‐MAC system, has a very high level of complexity. In this paper, we use a metaheuristic‐based optimization algorithms to optimize the sum‐rate capacity in a MAC system with significantly less computational complexity. First, we use the beluga whale optimization (BWO) metaheuristic method to solve the optimization problem. The BWO is a swarm‐based algorithm that draws inspiration from beluga whale behavior. An improved BWO (iBWO) is also presented and is utilized for the same optimization challenge. The proposed iBWO scheme improves the search capability of the algorithm and thereby ensures better exploration and exploitation capabilities of the beluga whales. We compare the performance of these algorithms with a standard differential evolution (DE) algorithm. The simulation results demonstrate that the proposed iBWO is a very competitive meta‐heuristic algorithm for solving optimization problems. The proposed iBWO algorithm achieves near optimal throughput in a MIMO‐MAC system. Also, the computational complexity related to the suggested optimization technique in a MIMO‐MAC system is found to be very less compared with the complexity related to ESA.

Experimental investigation on flexural performance of concrete beams strengthened with SMA-GFRP-ECC smart composite materials

The structural performance improvement of concrete members is by far a crucial theoretical issue for engineers, and the development of modern smart and composite materials makes it possible to gradually enhance the durability design of the concrete structure. In this study, six beams of the same size and reinforcement ratio, the proposed composite beam (SMA-GFRP-ECC) and five comparative beams (RC, R-ECC, SS-ECC, GFRP-ECC, SMA-ECC), were designed and tested under low-cycle unidirectional cyclic loading and unloading conditions. The energy dissipation capacity, displacement ductility, residual deformation, and self-repairing performance of each concrete beam were evaluated. Afterward, a concise calculation model for the studied composite beam is deduced and developed based on the existing relevant constitutive models and concrete assumptions. The test results indicate that compared with RC beams, the composite reinforced ECC beams show obvious multi-cracking and smaller crack width during the loading process and have good bending ductility. The innovative SMA-GFRP-ECC beam is capable of a high bearing capacity, ductility, and damage self-repairing. The new proposed beam has more than 80% of the maximum crack width recovery capacity during unloading. Hence, the proposed SMA-GFRP-ECC beam is a rather good first attempt of strengthened beams, combining the advantages of SMA, GFRP, and ECC.

Perception of the question tune in Guanzhong Mandarin

This study investigates the perception of the question tune in Guanzhong Mandarin and the interaction between tone and tune using the AX paradigm. Our findings reveal a complex interplay between lexical tones and question tunes, in which when the lexical tone is rising, it is more difficult to perceive question tunes. Furthermore, the research argues that the question tune retrieval requires extra working memory load and cognitive processing, because of the tune information brought by a high register and a high boundary tone.

Permeability evolution of fractured coal subject to confining stress and true triaxial stress loading: experiment and mathematical model

The accurate elucidation and prediction of coal permeability evolution under stress loading conditions are crucial for coalbed methane production. In this study, flow experiments were conducted on six cylindrical coal samples and four cubic coal samples under both confining and true triaxial stress loading conditions, respectively. The structure and characteristic parameters of the fractures inside each coal sample were obtained using the computed tomography scanning system and image processing technologies. The coal permeability under both types of loading processes was calculated through the transient pulse method. A mathematical model was developed to assess the evolution of coal permeability under true triaxial loading based on the current true triaxial permeability model and fractal theory. The results revealed that during the confining pressure loading, the coal permeability decreased exponentially with effective stress and was effectively described using the SD model. Additionally, the coal permeability initially rapidly decreased, followed by a gradual decrease, and eventually stabilized at a constant value. Particularly, during the first three loading steps, the fracture aperture and corresponding permeability of the six cylindrical coal samples decreased by ∼51.79%–57.83% and ∼38.06%–42.12%, respectively. However, during the final three loading steps, the fracture aperture and corresponding permeability of the six coal samples decreased by ∼18.26%–23.08% and ∼22.15%–26.93%, respectively. Moreover, owing to the various crossing angles of complex fracture networks with each principal stress, the effect of each principal stress on the coal permeability evolution was highly anisotropic during triaxial stress loading. Particularly, the permeability of the ST1 sample decreased by 43.08%, 14.84%, and 42.08% during the loading of each principal stress. Similarly, the permeability of the ST2 sample decreased by ∼65.74%, 14.29%, and 19.97%. The permeability reductions for the ST3 sample were ∼34.03%, 55.85%, and 10.12%, while those for the ST4 sample were ∼35.97%, 46.51%, and 17.52%. The SD model failed to describe these anisotropic effects. Compared with the SD model, the improved model, based on the current true triaxial permeability model and fractal theory, effectively described the anisotropic effect of each principal stress on the permeability of coal samples with complex fracture networks under triaxial stress conditions.

Low-carbon economic scheduling for AIES considering flexible load coordination and multiple uncertainties

The disordered energy use in traditional agricultural production has resulted in high energy purchase costs and carbon emissions. The coupling of integrated energy systems with agriculture offers the potential for flexible coordination of agricultural loads while agricultural loads are highly influenced by changes in the natural environment. In this context, this paper proposes a low-carbon and economic scheduling optimization method for agricultural integrated energy system (AIES), which comprehensively takes temperature, light intensity, and the coordination of agricultural loads into consideration. Firstly, solar and biomass resources are integrated through energy conversion equipment to support energy supply from both the electrical distribution network (EDN) and the gas network. At the same time, the modelling process of agricultural load is refined, which is divided into crop irrigation, greenhouse temperature control and residential consumption. The models of agricultural multi-energy complementary and energy storage equipment are established to achieve flexible conversion between different energy sources. In addition, the distributionally robust optimization (DRO) method based on multiple discrete scenarios is adopted to deal with the uncertainties arising from environmental condition fluctuations during the optimization process. Finally, the proposed model is tested on 4 various regions and 2 different seasons. Numerical results show that the proposed model can effectively enhance the economic and environmental performance of the AIES.

Influence of fractal-based contact friction coefficient on the stiffness of disc springs: Experimental and numerical studies

In order to study the influence of friction coefficient on the stiffness of disc spring, A series of disc springs ( D = 71 mm) were selected in this paper. The tension and compression experiments of disc spring components were carried out by universal testing machine. The load-displacement characteristic curves of disc springs with different friction surfaces were obtained. The results show that in the loading process, the friction effect hinders the increase of the deformation of the disc spring, so that the actual stiffness of the disc spring is improved and the actual compensation amount is reduced, so this result is worthy of attention. In order to further study the influence of friction coefficient on the loading process of disc spring, according to the load-deformation formula of disc spring in A-L method, the tangential stress caused by friction is added, and the stiffness calculation model of corresponding disc spring is established by Workbench finite element software. The coefficients of different friction surfaces are used as variables for numerical calculation. The results show that the theoretical calculation, numerical calculation and experimental results of stiffness increase with the increase of friction coefficient under 0.55 h (the compensation displacement of A71 disc spring under the working displacement of corrugated tube ±10 mm). Among them, the numerical calculation results are close to the experimental results, and the error value is less than 1%. The numerical calculation results are corrected by using the experimental data, and the corrected model calculation results are in good agreement with the measured results.

“Click” amphotericin B in prodrug nanoformulations for enhanced systemic fungemia treatment

Severe nephrotoxicity and infusion-related side effects pose significant obstacles to the clinical application of Amphotericin B (AmB) in life-threatening systemic fungal infections. In pursuit of a cost-effective and safe formulation, we have introduced multiple phenylboronic acid (PBA) moieties onto a linear dendritic telodendrimer (TD) scaffold, enabling effective AmB conjugation via boronate chemistry through a rapid, high yield, catalysis-free and dialysis-free “Click” drug loading process. Optimized AmB-TD prodrugs self-assemble into monodispersed micelles characterized by small particle sizes and neutral surface charges. AmB prodrugs sustain drug release in circulation, which is accelerated in response to the acidic pH and Reactive Oxygen Species (ROS) in the infection and inflammation. Prodrugs mitigate the AmB aggregation status, reduce cytotoxicity and hemolytic activity compared to Fungizone®, and demonstrate superior antifungal activity to AmBisome®. AmB-PEG5kBA4 has a comparable maximum tolerated dose (MTD) to AmBisome®, while over 20-fold increase than Fungizone®. A single dose of AmB-PEG5kBA4 demonstrates superior efficacy to Fungizone® and AmBisome® in treating systemic fungal infections in both immunocompetent and immunocompromised mice.

Delay of Transmitted Data in the Remote Patient Monitoring System through AMQP and CoAP

Remote Patient Monitoring (RPM) is a healthcare solution that uses technology to monitor patients outside conventional healthcare settings. It is especially useful for people with chronic conditions or needing regular monitoring. One of the main reasons for the increase in the number of deaths each year is the increase in cardiovascular diseases, including hypertension. Online blood pressure monitoring offers many advantages but also potential challenges. This work reviews the key communication technologies and research challenges in the real-time transmission of measured blood pressure data. Delay in these systems is not tolerated as it involves human lives. To conduct the experimental studies, a prototype of an experimental intelligent system was created to study the delay and processor load of the RPI4 gateway. The measured blood pressure data is sent to the Things Board cloud using the AMQP and CoAP protocols. The experimental results are particularly useful for RPM system designers. The results of this research facilitate an informed decision on the choice of protocol that transmits the data from the gateway to the cloud in the process of designing remote patient monitoring systems.

Experiment and analysis on dynamic characteristics of marine soft clay

A series of cyclic triaxial tests were conducted on marine soft clay deposits to establish and validate a predictive model for cumulative plastic strain. Additionally, a numerical model of particle flow code in cyclic triaxial tests was developed. The effects of confining pressure, moisture content, and dynamic stress ratio on the dynamic properties of marine soft clay were examined, considering factors such as volume deformation and microscopic failure patterns. The results indicated that both the predictive model and numerical model showed strong consistency with the experimental data. The plastic strain of marine soft clay was influenced by moisture content, stress ratio, and confining pressure in a consistent manner, with moisture content being the primary factor. A predictive model for the cumulative plastic strain of marine soft clay was successfully established, allowing for the evaluation of dynamic properties from the perspective of cumulative plastic strain. During the loading process in the numerical model, microcracks within the soil structure gradually compacted, and the main displacement of the specimen extended from the vertical center axis to the sides, ultimately resulting in shear damage.

INVESTIGATING FIRE-INDUCED DAMAGE ON FIBER-REINFORCED CONCRETE SAMPLES USING NONDESTRUCTIVE EVALUATION METHODS

This research article investigates the effect of severe thermal exposure on the mechanical behavior of fiber-reinforced concrete specimens. Having been exposed to direct fire, the specimens undergo the fracture experiment to determine their mechanical properties. Fire-damage examination is performed with nondestructive techniques such as acoustic emissions (AE) and ultrasonic pulse velocity (UPV). The main purpose is to demonstrate that AE parameters, such as amplitude, cumulative hits, and energy, have a strong correlation with mechanical parameters and temperature. Besides that, acoustic emission monitoring is also an assessment tool of the structural integrity throughout the entire loading process. The distribution of AE events and the variation of key AE parameters across the load process give very useful hints to differentiate between intact and fired-damaged specimens. The fact that the AE can be used on-site to determine cracks means that it may be used as an alternative method to describe concrete's mechanical properties after fire exposures.

Stress, damage, and fatigue performance analysis of CFRP/Al double-sided countersunk riveted joints with variable rivet-hole clearance

The CFRP/Al countersunk rivet joints are one of the most fatigued dangerous points of the aircraft fuselage. To deeply investigate the failure mechanisms of the riveted joint structures, this paper utilizes electromagnetic riveting technology to prepare single-lap joint specimens and investigates the quasi-static tensile and fatigue failure behaviours of the joints under different rivet-hole clearances and stress levels. In addition, to assist this investigation, a three-dimensional finite element model, combining riveting and fatigue loading, is developed. This model takes into account the strain rate effects during riveting process and progressive failure behavior of various typical damage modes in CFRP laminates. The results indicate that Aluminum alloy sheets fracture dominates the fatigue failure of the joints. Increasing the clearance can reduce the stress amplitude on the shear plane and further enhance the fatigue life of the joints. Furthermore, all additional CFRP damage during the loading process is the growth of the initial riveting damage, and the damage-induced fibre bearing capacity decrease is considered the reason for the increase in stress amplitude of the Al sheet. Last, excessive clearance enlargement does not appreciably diminish the initial riveting damage, instead, it could result in a joint strength reduction.