Aged Specimens Research Articles

Experimental and numerical studies on mechanical properties of hot-stamped tailor welded AA7075-T6

AA7075-T6 sheets with 1, 1.5, and 2 mm thicknesses were tailor-welded using the gas tungsten arc welding process. The welded blanks were solution heat treated, hot stamped, and simultaneously quenched to produce U-shaped components, and finally artificially aged. A finite element model using the ABAQUS software was developed to simulate the forming process. The experimental and numerical results showed no rupture in the components. Tensile and hardness tests revealed that the maximum tensile strength, yield strength, and microhardness of the aged specimen were approximately 96%, 93%, and 107% of the initial material, respectively. Maximum and minimum microhardness of 196 and 145 HV were observed in the heat-affected and fusion zones of the specimens, respectively. Increasing the tailoring ratio increased the strengthening behavior of the components. With a tailoring ratio of 1, the maximum and minimum elongations at break were 70% and 40% of the original aluminum sheet for 2 mm and 1 mm thick sheets, respectively.

Effect of Solution Temperature and Quench Delay on the Microstructure and Mechanical Properties of an Extruded Al-Mg-Si-Cu-Mn alloy

The effect of solution temperature and quench delay on the microstructure and mechanical properties of an extruded Al-Mg-Si-Cu-Mn alloy was investigated by employing differential scanning calorimetry, transmission electron microscopy, hardness test, and tensile test. The extruded specimens were held at 500 and 540 <sup>o</sup>C, respectively, for 80 min for solutionizing and then immediately quenched in water or exposed to air for 30 sec before water quenching. Each specimen was further heat treated at 180 <sup>o</sup>C for 6 hr for artificial aging. Quantitative analysis of the precipitates in the artificially aged specimens was performed using high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and fast Fourier transforms (FFT) analysis. Solution treatment at 540 <sup>o</sup>C led to higher number density of precipitates and higher strengths after artificial aging, compared with solution treatment at 500 <sup>o</sup>C. Quench delay resulted in a lower number density of precipitates and lower strengths after artificial aging. The specimen solution treated at 540 <sup>o</sup>C with no quench delay showed the largest peak area of precipitate formation in the differential scanning calorimetry, and concurrently exhibited the highest strengths after artificial aging. Strength reduction by quench delay was higher in the specimen solution treated at 500 <sup>o</sup>C than that treated at 540 <sup>o</sup>C; this was consistent with the results of higher reduction in the number density of precipitates.

Tailoring functional properties of a FeMnSi shape memory alloy through thermo-mechanical processing

The influence of thermo-mechanical processing on the microstructure and functional properties of a Fe–17Mn–5Si–10Cr–4Ni-1(V–C) (wt%) shape memory alloy was systematically investigated. The as-received material was subjected to 25 % cold rolling followed by a recrystallization at 925 °C and single or double aging treatments. Transmission electron microscopy revealed the formation of the ε-martensite and annealing twin boundaries and Shoji-Nishiyama orientation relationships of ε-martensite and γ-austenite in double aged specimen. Cyclic tensile testing demonstrated that the recrystallized and double aged alloy exhibited excellent pseudoelasticity. In the incremental strain test, the alloy achieved the highest peak stress and pseudoelasticity at each cycle. In the constant stresses test, the alloy accumulated a minimal residual strain of only 0.12 % over 50 cycles. This stability was attributed to a strong precipitation strengthening and the interactions between the martensite and the refined microstructural features. In addition, the recrystallized and double aged sample resulted in the greatest recovery stress of 450 MPa upon heating after pre-straining, because of its high yield strength suppressing new martensite formation during cooling process. The results of high-resolution transmission electron microscopy identified a non-Shoji-Nishiyama orientation relationship between the stress-induced ε-martensite after the stress recovery test and γ-austenite matrix, inducing additional irrecoverable strain and raising the recovery stress. Overall, the study can demonstrate that the tailored thermo-mechanical processing enables optimizing the functional performance of FeMnSi alloys.

Material flow behavior and microstructural refinement of AA6061 alloy during friction extrusion

In this work we used friction extrusion (FE), a solid phase processing technique, to produce dense, fully consolidated 5 mm rods of aluminum alloy 6061 (AA6061). The combination of large shear stresses and high temperatures at the tool-billet interface during extrusion produced equiaxed, dynamically recrystallized grains and finely distributed precipitates. Texture and microstructure evolution during extrusion was investigated in detail using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Smoothed particle hydrodynamics simulation was performed to study complex material flow during extrusion. Simulation results suggest spiral material flow during extrusion which corroborated experimental results. Advantages of friction extruded microstructure over conventionally extruded counterparts are also explored using flash annealing for solution treatment followed by artificial aging. Mechanical properties of the as-friction extruded, and the artificially aged specimens were evaluated using tensile testing and compared with conventional extruded material properties.

Characteristics of Aged Silicone Rubber Insulators Used in Outdoor for 20 Years

To collect aged silicone rubber (SiR) insulators with different numbers of years in use, which is for developing a degradation diagnostic system of SiR, is difficult. Therefore, it is desired to make degraded SiR surface which has similar characteristics to the degraded surface in outdoor. To realize the fabrication, we characterized the surfaces of aged SiR specimens cut out from a 22 kV line post insulator used in the coastal area for 20 years. The results showed that to introduce fine particles into the surface is necessary for making degraded surface having the minute surface roughness. Additionally, it was found that to activate the dispersion of LMW with foreign matters is also necessary. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Effect of Alkaline Environment on the Bond Behavior of TRM Composites Through Single-lap Shear Tests

Textile Reinforced Mortars (TRM) are increasingly used for strengthening existing masonry structures, offering a sustainable solution. This system consists of textile fibers embedded in an inorganic matrix, typically cement or lime-based mortars, applied to the substrate’s surface. Although the mechanical properties of these composite systems have been extensively studied, there is still limited understanding of their long-term performance and durability. This study aims to enhance knowledge in this area by examining the bond behavior of TRM composites exposed to an alkaline environment using single-lap shear tests. For this purpose, single-lap shear specimens were constructed using clay brick as the substrate, a lime-based mortar, and two different fabrics—basalt and glass. These specimens were subjected to an alkaline solution to simulate accelerated aging conditions, with constant temperature and pH. The specimens were tested after 1000, 3000, and 5000 hours of exposure to evaluate their bond behavior compared to control specimens of the same age. Additionally, the effect of the alkaline environment on the material properties of the composite constituents was investigated. Elastic modulus tests were conducted on cylindrical mortar samples and compared to reference samples, displaying satisfactory resistance to alkaline conditions. The study identified and compared the strength, stiffness, and failure mode of each specimen to the reference stage, providing valuable design parameters for various field applications.

Glycerol-derived organic carbonates: environmentally friendly plasticizers for PLA.

Polylactic acid (PLA) stands as a promising material, sourced from renewables and exhibiting biodegradability-albeit under stringent industrial composting settings. A primary challenge impeding PLA's broad applications is its inherent brittleness, as it fractures with minimal elongation despite its commendable tensile strength. A well-established remedy involves blending PLA with plasticizers. In this study, a range of organic carbonates-namely, 4-ethoxycarbonyloximethyl-[1,3]dioxolan-2-one (1), 4-methoxycarbonyloximethyl-[1,3]dioxolan-2-one (2), glycerol carbonate (3), and glycerol 1-acetate 2,3-carbonate (4)-were synthesized on a preparative scale (∼100 g), using renewable glycerol and CO2-derived diethyl carbonate (DEC) or dimethyl carbonate (DMC). Significantly, 1-4 exhibited biodegradability under ambient conditions within a week, ascertained through soil exposure at 25 °C-outpacing the degradation of comparative cellulose. Further investigations revealed 1's efficacy as a PLA plasticizer. Compatibility with PLA, up to 30 phr (parts per hundred resin), was verified using an array of techniques, including DSC, DMA, SEM, and rotational rheometry. The resulting blends showcased enhanced ductility, evident from tensile property measurements. Notably, the novel plasticizer 1 displayed an advantage over conventional acetyltributylcitrate (ATBC) in terms of morphological stability. Slow crystallization, observed in PLA/ATBC blends over time at room temperature, was absent in PLA/1 blends, preserving amorphous domain dimensions and mitigating plasticizer migration-confirmed through DMA assessments of aged and unaged specimens. Nevertheless, biodegradation assessments of the blends revealed that the biodegradable organic carbonate plasticizers did not augment PLA's biodegradation. The PLA in the blends remained mostly unchanged under ambient soil conditions of 25 °C over a 6 month period. This work underscores the potential of organic carbonates as both eco-friendly plasticizers for PLA and as biodegradable compounds, contributing to the development of environmentally conscious polymer systems.

Influence of different long and short ageing protocols on the mechanical behaviour and damage mechanisms of a hemp fibre reinforced polypropylene

This paper deals with the study of the evolution of the durability of short hemp fibres reinforced composites subjected to long-term ageing. The compounds were stored for ten years in atmospheric conditions (natural ageing) prior to the study. Therefore, two kinds of specimen are studied. First, the aged compounds were used to moulded tensile specimens. The residual mechanical behaviour of natural aged hemp/PP composites (natural aged specimens) were compared to properties obtained 10 years ago. This storage showed that the hemp is more sensitive to time degradation than PP and was confirmed by the initial degradation of compounds pointed out by ATG. Then, injected natural aged polypropylene reinforced with several hemp fibre weight fractions compounds were subjected to two different protocols in order to characterize their durability (aged specimens). The specimens were subjected to water immersion at ambient temperature and storage in a climatic chamber maintained at 40 °C with a relative humidity of 98%. The fracture surfaces were observed with SEM to identify the physical phenomena leading to the observed loss of mechanical properties. A weight water sorption of 14% with respect to hemp weight fraction was, for the first time, determined as a critical water sorption for which the reinforcement effect becomes negligible. The physical phenomenon responsible for this critical absorption was finally investigated with Infrared Spectroscopy, showing the occurring of a dissolution of the fibres.

Indentation mechanics and native collagen content in the cartilaginous endplate: A comparison between porcine cervical and human lumbar spines

This study characterized the regional indentation mechanics and native collagen content in cartilaginous endplates (CEPs) from the porcine cervical spine, young human lumbar spine, and aged human lumbar spine. Seventeen endplates were included in this study: six porcine cervical, nine young human lumbar, and two aged human lumbar. Width and depth measurements were obtained using a digital caliper and used to size-normalize and identify the central, anterior, posterior, and lateral regions. Regional microindentation tests were performed using a serial robot, where surface locations were loaded/unloaded at 0.1 mm/s and held at a constant 10 N force for 30 s. Loading stiffness and creep displacement were obtained from force-displacement data. Immunofluorescence staining for type I and type II collagen was subsequently performed on sagittal sections of all endplate regions. 255 images were obtained from which fluorescence intensity, sub-surface void area, and cartilage thickness were measured. CEPs from the young human lumbar spine were, on average, 27% more compliant, 0.891 mm thicker, had a lower fluorescence intensity for native collagen proteins within the cartilage (−58%) and subchondral bone (−24%), and had a sub-surface void area that was 19.7 times greater than porcine cervical CEPs. Compared to aged human lumbar CEPs, young human lumbar CEPs were 57% stiffer, 0.568 mm thicker, had a higher fluorescence intensity for native collagen proteins within the cartilage (+30%) and subchondral bone (+46%), and had a sub-surface void area that was 10.6 times smaller. Although not a perfect mechanical and structural surrogate, porcine cervical CEPs provided initial conditions that may be more representative of the young and healthy human lumbar spine compared to aged human cadaveric specimens. The indentation properties presented may have further applications to finite element models of the human lumbar spine.

Generation and classification models of ultrasonic signals in aged cast austenitic stainless steel (CASS)

In this study, we attempted to generate new ultrasonic signals from thermally aged CASS specimens using a statistical-based model to augment the experimental data. A variational auto-encoder (VAE) model is adopted to generate the ultrasonic signals. To verify the quality of the generated data (signals), two ML algorithms (k-nearest neighbor and multi-layer perceptron algorithms) were used for classification with newly generated data for training and experimental data for testing. The results showed that new ultrasonic signals could be generated effectively with a small amount of experimental data, with the newly generated data showing features approximating those of the experimental data, indicating high classification accuracy rate for ultrasonic signals from aged CASS specimens. In addition, a sampling method for the generated model is suggested.

Чужеродные моллюски в прибрежной части дагестанского района Каспийского моря

The species Corbicula fluminea and C. fluminalis (O.F. Müller, 1774), alien to the ecosystems of the Russian sector of the Middle Caspian, penetrated into the estuaries, forming new ecotones. Mollusks of the genus Corbicula are included in one hundred of the most actively spreading universes around the world. In 2018-2022, live individuals of the bivalve mollusk C. fluminalis (O.F. Müller, 1774) were found in the waters of the northern part of the Agrakhan Bay, in the area of the Chased Gate, Kubyakinsky Bank, Kuznechonok. For the first time, finds of C. fluminalis and C. fluminea specimens of different ages were recorded in the Prisulak canal and the Yuzbash-Sulak reservoir. There is an expansion of the habitat of corbicules in the study area. Possible ways of invasion are assumed: introduction of corbicule larvae into the Caspian Sea basin with ballast waters of ships plying through the Volga-Don Canal; introduction of Asian fish species into the inland waters of Dagestan; transfer by waterfowl. High adaptive abilities and different breeding strategies of Corbicula mollusks increase the possibility of successful naturalization in estuarine and freshwater ecosystems of the Dagestan region of the Caspian Sea, as well as in desalinated areas near the mouths of the rivers of the Northern Caspian and the northern part of the Middle Caspian.

Modelling of the surface morphology and size effects on fatigue strength of L-PBF Inconel 718 by comparing different testing specimens

The aim of this study was to investigate the impact of surface roughness and size effect on the uniaxial fatigue strength of aged specimens made of Inconel 718, obtained by laser-powder bed fusion (L-PBF) both under as-built and machined surface conditions. The surface profiles were scanned with a 3D optical profilometer, and then modelled and filtered by the fast Fourier transform. This latter allowed to obtain the relevant parameters to calculate the term area in the Murakami model to account for the effect of surface roughness on fatigue strength prediction. Two additional terms were added to the standard Murakami formula: one to account for the size effect and the other to consider the nonzero value of the average stress. A cylindrical plain and a miniaturized plain specimens, in the as-built condition, were employed to calibrate the proposed model, while a machined plain cylindrical and a miniaturized notched specimens were used as validators with low prediction errors.

Effect of hygrothermal aging on compression behavior of CFRP material with different layups

This study investigates the mechanical degradation mechanism and statistical analysis of residual compressive strength of three lay-up laminates under more than one-year seawater environment. Fick and Langmuir model were used to describe the moisture absorption process with a maximum moisture equilibrium content of 3.78% and various characterization methods were used to reveal the microstructural evolution of CFRP during aging. Experimental results demonstrate that the multiple fiber directions in the MD specimens provide additional moisture absorption channels, leading to a stronger wicking effect and a higher equilibrium moisture content. The layups can significantly affect the compression failure mode after long-term aging. The compressive strength of the cross-ply and multidirectional laminates decreased by 50% while that of unidirectional laminates decreased by 28%, and surface cracking is easily observed in the aged specimens. An empirical prediction model based on the Langmuir model and experimental data was proposed and verified by the data from the literature, with at least a 10% improvement in prediction accuracy. Meanwhile the three-parameter Weibull distribution was used to describe the residual compressive strength after hygrothermal aging and tested by Kolmogorov-Smirnov test. It is expected to provide insight for the application of CFRP in seawater environment.

Effect of Process Condition and Test Environment on Tensile Properties in Cold-Rolled Al-Cu-Mg Alloys

Severe plastic deformation processing and subsequent aging treatment have been known to be effective for achieving higher strength than the conventional aging treatment in aluminum alloys. This study prepared the Al-Cu-Mg-based alloy sample, Al-5.3Cu-2.8Mg (mass%). The alloys were solution treated at 480, 495 and 505°C, and cold-rolled by 90%. The effect of process condition and test environment on tensile properties in cold-rolled Al-Cu-Mg alloys was investigated. Results confirm that strength and ductility were improved with increasing the solution heat treatment temperature regardless of test environment. 0.2% proof stress and ultimate tensile strength were higher than aging treatment specimens, but elongation to failure was lower than aged one. Hydrogen embrittlement susceptibility increased with increasing solution treatment temperature. Ductile fracture with many dimples is observed in both cold-rolled and aged specimens. Second-phase particles were observed at the bottom of the dimples. There was no significant difference in fracture surface between the different test environments.

Thermal Aging Characteristics of Silicone Rubber Exposed to Mineral Oil

Room temperature vulcanized (RTV) silicone rubber is extensively utilized as a coating material for power transformer bushings because of its high hydrophobicity and excellent anti-pollution flashover resistance. However, the performance of silicone rubber may suffer significantly when exposed to mineral oil. Till now, literature available on understanding the degradation behavior of silicone rubber exposed to mineral oil at different temperatures is very rare. Therefore, in the present work oil immersion test is performed. RTV silicone rubber was thermally aged in mineral oil at 40°C and 80°C for 200 hours. Post ageing, to examine the changes in the surface properties of the aged specimens 3D microscopy, scanning electron microscopy (SEM) analysis, contact angle, proof tracking index (PTI), and shore hardness measurements were made. The results indicate that the degree of surface degradation was proportional to the temperature. Further, thermogravimetric analysis (TGA), differential thermal analysis (DTA), and infrared thermography were employed to evaluate the thermal properties of aged and unaged specimens. TGA-DTA and infrared thermography indicated a remarkable reduction in thermal stability and heat dissipation capability post mineral oil ageing. The effect became more adverse with increasing ageing temperature. Also, ageing resulted in a decrease in alternating current (AC) breakdown voltage and PTI. Finally, all the experimental outcomes were correlated and the degradation dynamics of mineral oil-aged silicone rubber are explained in detail through possible chemical reactions, fourier transform infrared (FTIR) spectroscopy and energy dispersive X-ray spectroscopy (EDAX) investigations.

Prevalence and Molecular Identification of Cryptosporidium Species among Human Population in Zakho District, Duhok Province, Kurdistan Region, Iraq

This cross-sectional study was carried out to determine the prevalence and molecular identifications of Cryptosporidium species among the human population in Zakho district, Duhok Province, Kurdistan Region, Iraq. The study was performed during the period from August 2021 to March 2022 and involved 700 fecal specimens of both genders and different ages collected from Zakho General Hospital and pri¬mary schools located in the suburban area of Zakho City. The data were analyzed using statistical soft¬ware version SPSS (25). Cryptosporidium species oocyst was found in 87.57% (613/700) of the examined specimens using the Modified Ziehl Neelsen technique. Males showed a slightly higher infection rate than females (88.4% vs 85.5%) with a non-significant difference (P> 0.05). The highest infection rate (91.7%) was present in individuals between the ages of >30 to 40 years, with a statistically significant difference (P<0.05). Molecular identification of Cryptosporidium species was estimated using 18S-rRNA gene for Cryptosporidium genus using Nested PCR technique and genus-specific primers (Cry18 AS1 and Cry18 S2) that yielded 347 bps product indicating Cryptosporidium genus, then species-specific (Cry18 AS1 and Cry18 S1) were used for the detection of Cryptosporidium parvum and Cryptosporidium hominis. 45/50 samples were positive and revealed 240 bps. The acquired sequences were deposited into the GeneBank database under the accession numbers OP107010 for C. hominis, OP104444, OP153870, OP163188, OP163414, and OP164631 for C. parvum.

Monitoring the Durability Issues of Asphalt Concrete Mixtures

The asphalt concrete mixture is prone to environmental issues such as moisture damage and ageing. This may exhibit a great significance in the service performance of asphalt concrete pavement mixtures which may be more susceptible to many types of early distresses throughout its fatigue life. In the present investigation, asphalt concrete mixtures were prepared and compacted with the aid of laboratory roller compaction into a slab samples. optimum binder content was implemented. Extra samples were prepared at higher and lower binder content of 0.5 % (above and below the optimum). Asphalt concrete beam specimens were obtained from the prepared slab samples with the aid of a diamond saw. Part of the Asphalt concrete beam specimens were tested under four point’s repeated flexural stresses after practicing moisture damage while another part was subjected to long term ageing. The rate of change in the flexural strength was monitored and compared among the various testing conditions at 20 ºC environment and under constant micro-strain level of 750. It was observed that the lower flexural strength was observed for moisture damaged specimens while higher flexural strength could be detected for aged specimens as compared with the control mixtures. The binder content exhibits a significant influence on flexural strength of the asphalt concrete specimens since it declines significantly at higher or lower binder content as compared with that of specimens prepared at the optimum.

Structure-property relationships of differently heat-treated binder jet printed Co-Cr-Mo biomaterial

This investigation systematically examines the influence of sintering temperature and aging treatment on the density, microstructure evolution, phase formation, and mechanical properties of a binder jet printed Co-Cr-Mo biomedical alloy. Sintering at 1380 °C for 2 h yielded a near-fully dense part (99.1%) with favorable mechanical properties (up to 325 HV0.1 hardness and up to 693 MPa ultimate tensile strength). The grain size remained unchanged after aging at 800 °C for 24 h (89 ± 21 µm). Aging resulted in increased microhardness and tensile strength due to phase formation (Cr23C6, CrMo, and ε phase), but a significant decrease in ductility. Consequently, the sintered and aged specimen exhibited higher hardness (522 HV0.1), yield strength (641 MPa), and ultimate tensile strength (854 MPa) compared to cast Co-Cr-Mo alloy. Biocompatibility testing with fibroblasts showed a cell viability of 95 ± 2%, indicating that binder jet printing did not affect the biocompatibility of the Co-Cr-Mo alloy. Exemplary printed parts including hip-joint, partial denture, and small-scale knee joint were successfully demonstrated. This study highlights the comparable properties of binder jet Co-Cr-Mo alloy to the cast alloy, affirming its potential for biomedical applications.

Investigating the effects of Coating systems on the degradation behavior of 3D-Printed pressure vessels

This study investigates the long-term performance of 3D-printed pressure vessels subjected to accelerated aging through water mass infusion. Thermoplastic composites were used to construct the pressure vessels using Material extrusion 3D printing, and subsequent submersion in an ocean water solution at elevated temperatures was employed to simulate accelerated aging. The effectiveness of various coating systems in minimizing the long-term degradation of these 3D-printed pressure vessels was also examined. Implosion experiments were conducted on both aged specimens with and without coating systems using a high-pressure tank facility. The results demonstrate that the application of polyurethane coating systems significantly slows the degradation process of 3D-printed vessels, with aged specimens exhibiting a reduced critical operational depth by 10.3 % compared to those without coatings. The pressure differential between unweathered specimens with and without coatings was 3.61 %, increasing to 23.3 % for weathered specimens. Coatings proved effective in reducing mass absorption and increasing critical collapse pressure. The analytical collapse pressure prediction closely aligns with experimental results, enabling performance generalization for other coating systems. This research contributes insights into the use of coating systems to enhance the longevity and performance of 3D-printed pressure vessels, highlighting their potential for mitigating degradation and improving structural integrity in long-term underwater applications.

Bending behavior of 3D printed sandwich structures with different core geometries and thermal aging durations

In recent years, the Fused Deposition Modeling (FDM) method has been employed in the production of small-scale structural elements with moderate loads, such as small-unmanned aerial vehicles, sports equipment, dental implant molds, and similar applications. In these cases, the FDM technique can utilize polymers like Polyamide (PA), Polylactic Acid (PLA), and Acrylonitrile Butadiene Styrene (ABS) to manufacture structural components. Lightweight sandwich structures are utilized as structural elements in various industries due to their unique characteristics, high stiffness-to-weight ratio, and energy absorption capabilities. Although the number of studies on the mechanical properties of sandwich structures manufactured with FDM has increased in recent years, experimental data on the mechanical characteristics of sandwich structures manufactured with FDM under different thermal aging durations are still insufficient. Driven by this motivation, the energy absorption capabilities of sandwich structures with different four core geometries (i.e., circular, hexagonal, square, and triangular) were experimentally investigated under different thermal aging durations. The sandwich structures were manufactured from PLA material by the FDM method. Four different thermal aging durations, 0, 15, 30 and 45 days, were considered for environmental conditions. Quasi-static three-point bending experiments were conducted to assess the energy absorption capability of lightweight sandwich structures featuring diverse core topologies. The bending test results demonstrate that the core topology significantly affects the energy absorption abilities of sandwich structures. Moreover, fractographic analysis using scanning electron microscopy (SEM) was conducted to gain deeper insights into the impact of thermal aging on aged specimens. In addition, it was concluded that the energy absorption performances of all sandwich structures with different core structure topologies were adversely affected by increasing the thermal aging time.