Evaluation of Selected Artificial Aging Protocols for Dental Composites Including Fatigue and Fracture Tests

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Our research is the second step toward both establishing a standard aging protocol as well as selecting methods useful for evaluating the degradation of dental materials.

Abstract

The durability and performance of dental composites are essential for ensuring long-lasting dental restorations. However, there is a lack of a standardized procedure for evaluating the lifespan of dental materials. Our proposal assumed that dental materials should be tested under aggressive aging conditions to accelerate materials’ degradation in vitro and such an approach should simulate prolonged material usage in the oral cavity. A comprehensive examination of the impacts of three aging methodologies on various mechanical properties, including the flexural strength (FS), diametral tensile strength (DTS), hardness (HV), fracture toughness (FT), flexural fatigue limit (FFL), and microstructure of selected dental materials (Resin F, Flow-Art and Arkon), was conducted. The findings revealed that preformed aging results in an average reduction of 30% in the mechanical strength properties of the dental composites when compared to the control. Notably, a strong correlation was identified between FS and FFL post-aging whereas no such relationship was observed between these parameters and FT. This paper highlights the significance of aging tests for new dental composites and recommends a focus on flexural strength and fracture toughness to optimize costs and time efficiency. Furthermore, the establishment of a standardized test for fracture toughness in dental composites is recommended. It is proposed that a minimum flexural strength of more than 32–48 MPa after aging should be maintained. A more extensive analysis of commercially available materials is suggested to refine the proper evaluation methods for composite materials.

1. Introduction

Data collected by the Global Burden of Disease Study (2017) show that untreated dental caries constitute the most common health condition [1,2]. In the European Union, dental health expenditures (EUR 90 bn) are comparable to those spent on diabetes (EUR 111 bn) and cardiovascular diseases (EUR 119 bn). Unfortunately, in most countries, dental procedures are not covered by health insurance and the payments are out-of-pocket for patients. The current global situation (high inflation, wars, and the recent COVID-19 pandemic) makes dental treatment unattainable for many people, even in more developed countries [3]. Therefore, research aimed at increasing the durability of dental restorations is very important.

A more complex evaluation including artificial aging should be carried out to aid in the development of new dental restorations. Unfortunately, ISO and ADA standards do not provide a methodology for this type of testing. Standardized tests in ISO 4049:2019 and ADA No. 27 primarily address the general characterization of dental composites. The evaluation of mechanical properties is limited to determining flexural strength and diametral tensile strength after 24 h in water (37 °C), which should be considered as output properties (control). In studies, aging under aqueous conditions did not significantly deteriorate the characteristics of dental composites. However, it is noteworthy that the outcomes were contingent upon the duration of water immersion [4,5]. The water sorption and solubility test included in the standards can be considered useful for a preliminary assessment of material behavior in a water environment. However, it should be noted that the sample immersion time is limited to 7 days, which is not sufficient to achieve the complete saturation of the sample. Considering the complex environment of the oral cavity, aging methods that rely solely on water immersion are insufficient for predicting the clinical effectiveness of biomaterials. Additionally, new research directions aimed at increasing the durability of dental restorations involve the use of new, often complex technologies such as polymer matrix modification (e.g., click chemistry) [6], filler modification (e.g., composite fillers with bactericidal properties), and the selection of coupling agents. In addition, another research direction attempts to develop self-healing material [7]. Considering the above aspects, our proposal assumed that dental materials should be tested under conditions of aggressive aging to accelerate the degradation of materials in vitro, which will be useful in predicting clinical effectiveness. Such methods are widely used in other industrial fields and can simulate long-term use [8].

Tests of selected basic aging methods were carried out [9]. Given that resin-based dental composites undergo hydrolysis upon interaction with OH⁻ ions, alkaline environments characterized by elevated hydroxyl ion concentrations serve as suitable aging media for in vitro evaluations. Notably, NaOH solution has emerged as a prevalent choice for expediting aging processes in dental composite assessments [10,11]. Additionally, exposing the materials to repeated temperature changes has a notable influence on the materials under investigation. Considering that thermal fluctuations occur approximately 20 to 50 times daily within the oral environment [12], it is imperative to take this into account during the material testing approach for appraising the service life of dental composites.

Based on the obtained results [9], a complex aging protocol consisting of thermocycles and an aggressive environment, 0.1 M NaOH solution, was selected. The thermocycling process engenders the heightened plasticization and swelling of the polymer matrix, rendering it more susceptible to infiltration by NaOH, which functions as a corrosive aging medium. As a simplification of this method, thermocycles have been replaced by aging at elevated temperatures. The proposed complex aging protocols should simulate prolonged—possibly several to even a dozen or more—years of material usage in the oral cavity, thereby furnishing valuable insights into its prospective clinical performance in vitro. The null hypothesis in our study was that the properties of the tested resin matrix and two dental composites would remain stable after hydrothermal accelerated aging.

2. Materials and Methods

The first tested material was Resin F (Arkona, Niemce, Poland), an unfilled material. Two filled materials, Flow-Art (Arkona, Niemce, Poland) and Arkon (Arkona, Niemce, Poland), had the same matrix composition as Resin F, but they contained different quantities of fillers (

Table 1. Detailed information about tested materials.

Material Name	Manufacturer	Type	Matrix	Filler	Filler Content [wt.%]
Resin F	Arkona (Niemce, Poland)	Unfilled resin	bis-GMA, TEGDMA, UDMA, bis-EMA	None	0
Flow-Art		Flowable composite		Al-Ba-B-Si glass, Ba-Al-B-F-Si glass, pyrogenic silica	62
Arkon		Universal composite			78

bis-GMA—Bisphenol A glycerolate dimethacrylate; TEGDMA—triethylene glycol dimethacrylate; UDMA—urethane dimethacrylate; bis-EMA—Bisphenol A ethoxylate dimethacrylate.

). Such material selection allows the examination of the durability of the polymer/resin-based material in relation to the durability of the filler system and the interface between the filler and matrix.

The flexural strength (FS), diametral tensile strength (DTS), Vickers hardness (HV), fracture toughness (FT), flexural fatigue limit (FFL), and microstructure were evaluated to determine the influence of each aging protocol on the material properties.

Table 2. Description of the selected aging protocols.

Aging Protocol	Description
Control	24 h, 37 °C, distilled water
7 d NaOH	7 days, 60 °C, 0.1 M NaOH
thermocycling + NaOH	7500 cycles, 5 °C and 55 °C, water and 7 days, 60 °C, 0.1 M NaOH
5 d water + NaOH	5 days, 55 °C, water and 7 days, 60 °C, 0.1 M NaOH

shows a description of the selected aging protocols used to evaluate the tested materials.

2.1. Flexural Strength

Rectangular samples with dimensions of 2 mm × 2 mm × 25 mm were used in the flexural strength (FS) measurements (n = 7 per group), which were assessed through a three-point bending test. Measurements were conducted using a Zwick Roell Z020 universal testing machine (Zwick–Roell, Ulm, Germany) with a traverse speed of 1 mm/min.

2.2. Diametral Tensile Strength

Cylindrical samples (diameter 6 mm; height 3 mm) were used in a diametral tensile strength test (n = 9 per group) utilizing a Zwick Roell Z020 universal testing machine (Zwick–Roell, Ulm, Germany) at a traverse speed of 2 mm/min.

2.3. Hardness

The hardness of the examined materials was assessed using the Vickers method with a Zwick ZHVµm hardness tester (Zwick–Roell, Ulm, Germany). A load of 1000 g was applied with a penetration time of 10 s. Nine measurements were conducted on three randomly selected samples out of the nine DTS samples for each study group.

2.4. Fracture Toughness

To measure fracture toughness (FT), the notched specimens (n = 6 per group) were subjected to a three-point flexural strength test in a universal testing machine (Zwick Roell Z005, Ulm, Germany) with a traverse speed of 1 mm/min. A metal mold was used to prepare rectangular samples (2 mm × 4 mm × 20 mm). A sharp central notch was created by inserting a blade into a carefully manufactured slot positioned at the midpoint of a mold.

The FT was calculated from Formula (1):

$$\mathrm{F}\mathrm{T}=\left[{\displaystyle \frac{\mathrm{P}·\mathrm{S}}{\mathrm{B}·{\mathrm{W}}^{3/2}}}\right]·\mathrm{f}\left({\displaystyle \frac{\mathrm{a}}{\mathrm{W}}}\right)$$

(1)

Here, P is the peak load at fracture, S is the span (14 mm), B is the specimen thickness, W is the specimen width, and $\mathrm{f}\left({\displaystyle \frac{\mathrm{a}}{\mathrm{w}}}\right)$ is the calibration function for a given geometry $\mathrm{f}\left({\displaystyle \frac{\mathrm{a}}{\mathrm{w}}}\right)=2.9{\left[{\displaystyle \frac{\mathrm{a}}{\mathrm{W}}}\right]}^{{\displaystyle \frac{1}{2}}}-4.6{\left[{\displaystyle \frac{\mathrm{a}}{\mathrm{W}}}\right]}^{{\displaystyle \frac{3}{2}}}+21.8{\left[{\displaystyle \frac{\mathrm{a}}{\mathrm{W}}}\right]}^{{\displaystyle \frac{5}{2}}}-37.6{\left[{\displaystyle \frac{\mathrm{a}}{\mathrm{W}}}\right]}^{{\displaystyle \frac{7}{2}}}+38.7{\left[{\displaystyle \frac{\mathrm{a}}{\mathrm{W}}}\right]}^{{\displaystyle \frac{9}{2}}}$ [13].

2.5. Flexural Fatigue Limit

The flexural fatigue limits (FFL) at 10⁵ cycles with a frequency of 0.5 Hz (n = 15 per group) were determined using the staircase method described in detail in [14,15,16]. Briefly, the stress applied in each subsequent test was adjusted by a fixed increment of 3.75 MPa, either increased or decreased, based on whether the preceding test led to failure. Approximately 50% of the initial flexural strength value was used for the first specimen. Rectangular samples with dimensions of 2 mm × 2 mm × 25 mm were used for this study. A universal testing machine (Zwick Roell Z020, Zwick–Roell, Ulm, Germany) was employed in this study.

Measurements were conducted using the mean fatigue limit ($\overline{\mathrm{F}\mathrm{F}\mathrm{L}}$) and its standard deviation (SD), which are given by Formulas (2) and (3).

$$\overline{\mathrm{F}\mathrm{F}\mathrm{L}}={\mathrm{X}}_0+\mathrm{d}\left({\displaystyle \frac{\sum {\mathrm{i}·\mathrm{n}}_{\mathrm{i}}}{\sum {\mathrm{n}}_{\mathrm{i}}}}-{\displaystyle \frac{1}{2}}\right)$$

(2)

$$\mathrm{S}\mathrm{D}=1.62\mathrm{d}({\displaystyle \frac{\sum {\mathrm{n}}_{\mathrm{i}}·\sum {{\mathrm{i}}^2·\mathrm{n}}_{\mathrm{i}}-{\left(\sum {\mathrm{i}·\mathrm{n}}_{\mathrm{i}}\right)}^2}{{\left(\sum {\mathrm{n}}_{\mathrm{i}}\right)}^2}}+0.029)$$

(3)

The lowest stress level at which a failure occurs is denoted by i = 0, followed by i = 1, etc. The number of failures at a given stress level is recorded and marked as n_i. In the formulas, X₀ is the lowest stress level considered in the analysis and d is the stress increment (3.75 MPa) employed in the sequential tests.

2.6. Microstructure Evaluation Based on Scanning Electron Microscopy

The microstructure of the materials was examined using a high-resolution scanning electron microscope (HR-SEM) (FEI Nova NanoSEM 450, FEI, Hillsboro, OR, USA), which was equipped with a high-sensitivity circular backscatter (CBS) detector. The chemical composition was assessed using an energy-dispersive spectrometer (EDS, EDAX/AMETEK, Materials Analysis Division, Model Octane Super, Mahwah, NJ, USA). Before the analysis, a 10 nm layer of gold was applied to coat the resin samples.

2.7. Statistical Analysis

Descriptive statistics were generated and the obtained results were statistically analyzed using Statistica version 13 software (StatSoft, Kraków, Poland). The normality of the variables was assessed using the Shapiro–Wilk test. Depending on the results, either the Kruskal–Wallis test with multiple comparisons of mean ranks or one-way ANOVA with a post hoc test (Fisher’s least significant difference) was used. The significance level was set at α = 0.05. Additionally, Spearman’s rank order correlations between analyzed research methods were determined.

3. Results

To evaluate the effect of selected aging protocols on the tested material properties, a wide range of research methods were employed. The obtained flexural strength, diametral tensile strength, hardness, fracture toughness, and flexural fatigue limit values of the tested materials are presented in

Table 3. The flexural strength (FS), diametral tensile strength (DTS), hardness (HV), fracture toughness (FT), and flexural fatigue limit (FFL) values of the tested materials after the selected aging protocols. Normally distributed variables are given as means with standard deviations (SDs) while non-normally distributed variables are given as medians with quartile deviations (QDs). The results with the same assigned letter within the same material (Resin F, Flow-Art, or Arkon) are significantly different (p ≤ 0.05).

Material	Aging Protocol	FS [MPa]	SD/QD *	DTS [MPa]	SD/QD *	HV	SD/QD *	FT [MPa]	SD	FFL [MPa]	SD
Resin F	Control	87.5 a,b,c	9.1	39.6 a	9.6	16	1 *	0.78 a,b,c	0.07	38.75 a,b	7.09
	7 d NaOH	72.5 a	8.2	31.3	10.3	15 a	0 *	0.62 a	0.05	22.77 a,c	8.61
	thermocycling + NaOH	65.3 b,d	9.8	34.5	12.9	16	1 *	0.56 b	0.06	25.63 b,d	3.55
	5 d water + NaOH	76.1 c,d	11.5	25.4 a	8.6	17 a	1 *	0.56 c	0.09	37.13 c,d	11.35
Flow-Art	Control	97.0 a,b,c	16.2	47.5 a	5.7 *	35 a	1 *	1.13 a,b	0.09	55.63 a,b	27.85
	7 d NaOH	58.4 a	10.8	38.6 b	6.8 *	29 a,b	3 *	1.05 c,d	0.12	26.63 a,c	4.06
	thermocycling + NaOH	63.5 b	5.5	36.5	13.5 *	33 b	1 *	0.86 a,c	0.05	23.13 b,d	3.55
	5 d water + NaOH	55.3 c	9.0	28.8 a,b	7.9 *	36	1 *	0.81 b,d	0.09	45.94 c,d	16.50
Arkon	Control	101.0 a,b	21.2 *	44.4 a,b	6.6	63 a,d	2	1.48 a,b,c	0.12	62.34 a,b,c	2.36
	7 d NaOH	54.9 a,c	13.5 *	44.0 c,d	6.5	48 a,b,c	2	0.96 a,d	0.14	18.75 a,d	5.74
	thermocycling + NaOH	58.3 b	5.1 *	33.5 a,c	4.1	58 b,d,e	2	0.98 b	0.09	21.38 b,e	3.58
	5 d water + NaOH	79.2 c	18.1 *	32.6 b,d	4.4	62 c,e	2	0.81 c,d	0.05	29.20 c,d,e	4.89

Aging protocols: control—water, 24 h, 37 °C; 7 d NaOH—0.1 M NaOH, 7 days, 60 °C; thermocycling + NaOH—water, 7500 thermocycles, 5/55 °C and 0.1 M NaOH, 7 days, 60 °C; 5 d water + NaOH—water, 5 days, 55 °C and 0.1 M NaOH, 7 days, 60 °C; *—median value with quartile deviation.

. Box-and-whisker plots of the obtained results and exact p values are shown in the Appendix A (Figure A1, Figure A2, Figure A3, Figure A4, Figure A5, Figure A6, Figure A7, Figure A8, Figure A9, Figure A10, Figure A11, Figure A12, Figure A13, Figure A14 and Figure A15).

Compared with those in the control group, the FS, DTS, FT, and FFL values of all the samples decreased after aging. No or little change in hardness was observed after almost all aging protocols except for 7 d of NaOH treatment.

Selected SEM micrographs showing surface degradation in Resin F, Flow-Art, and Arkon after aging are presented in Figure 1, Figure 2 and Figure 3. Visible areas show that the filler was washed out (plucking) or fractured, the bond between the matrix and the filler was broken (debonding), or the polymer matrix delaminated (peeling). Additional SEM images for all groups are provided in the Appendix A (Figure A16, Figure A17 and Figure A18).

4. Discussion

The evolution of dental materials requires thorough characterization to predict their clinical behavior effectively. Accelerated aging simulates the long-term effects of normal use in a shortened timeframe by exposing materials to extreme environmental conditions [8]. In the literature, there is an urgent need for a more comprehensive characterization of new materials, allowing for the determination of their stability during use [17,18]. Nevertheless, a crucial aspect is the systematization and selection of standardized methods used for this purpose.

Thus, this study evaluated the impact of various protocols as an initial effort to establish a standardized aging process for dental materials. These protocols were selected based on the evaluation of basic aging methods [9]. Dental composites are typically evaluated using standard tests such as three-point bending, diametral tensile strength, and hardness due to their procedural simplicity and reproducibility. However, it is worth emphasizing that not all material characteristics have the same value. Certain properties, such as flexural strength, modulus, fracture toughness, and fatigue resistance, are strongly correlated with clinical performance [19,20,21]. In this study, the fatigue flexural limit and fracture toughness were also selected for material characterization.

The obtained results (Table 3) indicated that aging significantly reduced flexural strength, fracture toughness, and fatigue flexural limits in the tested materials, leading to the rejection of the stated hypothesis.

The greatest differences in properties after aging could be observed for FFL, which may suggest that this method is the most sensitive for detecting changes occurring in materials due to aging [22]. However, this test is time-consuming, and a larger research group (at least 15 samples) is needed, making it cost-prohibitive. Additionally, there was a relatively high cycling range for the majority of the examined groups, contributing to high standard deviation values. This may indicate some internal flaws in the materials, potentially explaining the inconsistency in the scatter of the cycling loading range. Increasing the number of measurements could have an impact on achieving more homogeneous results. However, it would significantly prolong the durations of studies and their costs.

The fatigue strength of dental composites is typically approximately 55–65% of the static flexural strength [23], which agrees with our results. However, after aging, the ratio of FFL to FS was usually lower (37–45%) than that of the control in our study. The FS and FFL values are well correlated (

Table A1. Results of Spearman correlation coefficients between the analyzed research methods.

Analyzed Methods	Scatter Plot	Spearman Correlation Coefficient	p Value
Flexural fatigue limits vs. flexural strength		0.67832	0.01532
Flexural fatigue limits vs. fracture toughness		0.23077	0.47053
Flexural strength vs. fracture toughness		0.05594	0.86289

), and the observed changes in basic three-point bending tests can be used to predict how a material will behave under fatigue conditions [24].

Determining the flexural strength threshold values after aging for dental composites would be worthwhile. Standard 4049 specifies that restorative materials should have more than 80 MPa for occlusion-bearing areas. After aging, none of the tested materials met the ISO 4049 minimum requirement [25]. The functional chewing forces range between 5 and 20 N, and assuming a contact area of approximately 1 mm², the resulting stresses correspond to chewing forces [26]. However, it is important to emphasize that this is a great simplification. To simulate restoration behavior under clinical conditions, finite element analysis studies were performed. The maximum occlusal bite force that may occur in clinical conditions in the molar region averages between 300 and 600 N [27]. The stresses generated during the simulation of the composite material under vertical loading (simulation of functional occlusal loads) were greater than 400 MPa and those under oblique loading (simulation of lateral chewing forces) were approximately 8 MPa [28]. The flexural strength of modern dental materials, which additionally meet the higher requirements of ISO 4049 standards, is approximately 10 times greater [29] than the average chewing force (8–12 MPa), considering the stresses generated in the restoration. Therefore, if the fatigue flexural limit is on average approximately 40% of the flexural strength post-aging, the flexural strength limit for composite materials after aging should be within the range of 32–48 MPa.

Among the most common causes of failure in dental treatment are fractures and secondary caries. Flaws in dental materials may occur during restoration preparation or can develop during use [30]. Therefore, describing the composite resistance to the propagation of flaws under stress is very important [22]. In our study, there was no relationship between FT and FS or FFL (Table A1). Other studies reported a significant correlation between FT and FS [20] but this study was conducted for non-aged materials only. To the best of the authors’ knowledge, no study has evaluated the relationship between FT and FS after aging, and this requires further verification. Additionally, it is important to note that FT values depend on the chosen research method and pre-cracking conditions, which may affect the observed correlations [13,31,32]. The samples for FS and FFL had the same dimensions, which may further explain the strong correlation between these results. However, the samples for FT had a different geometry, which may also have contributed to the lack of observed correlation. Unfortunately, despite emphasizing the importance of conducting fracture toughness studies and comparing the most commonly used testing methods, a standardized approach for this test in dental composites is still lacking [13,32,33,34,35,36].

It is necessary to specify certain limits of FT that materials should meet, especially following accelerated aging. Studies have indicated that the fracture toughness of human enamel ranges from 0.7 to 1.3 MPa√m while that of dentine rangers from 1.7 to 3 MPa√m [37]. The literature shows that the fracture toughness of dental composites typically ranges from approximately 1 to approximately 2 MPa√m [31] while for unfilled materials, it is approximately 0.8 MPa√m [38], which is consistent with the obtained results. After aging, the FT values of the tested materials decreased by an average of approximately 30%. Other studies also show that fracture toughness decreases significantly after different techniques for material aging [38,39,40,41]. The FT values after aging can indicate the stability and longevity of materials in the oral cavity, suggesting that fracture toughness should be included in standard tests for dental composites. There is also a need to standardize the testing procedure due to varying existing methodologies in the literature.

According to our results, accelerated aging significantly reduced the strength properties (FS, FT, DTS, FFL) compared to those of the control condition (with decreases from 13 to 70%, on average, by approximately 31%, Figure 4). Our study revealed a decrease in hardness after aging (Figure 4). This was consistent with other findings wherein fluctuations in hardness changes were observed over a duration or used medium [42,43,44].

In general, numerous studies have agreed that artificial aging results in a notable decline in the mechanical properties of dental composite materials [45,46,47]. The underlying cause of these alterations stems from inherent degradation processes within the materials. This degradation is driven by medium absorption, leading to plasticization and chemical reactions that cause the leaching of components and the formation of voids, fractures, and cracks, compromising the structural integrity of the polymer matrix [48,49,50,51]. This occurrence aligns with Griffith’s fracture theory, which suggests that any imperfection in the microstructure of a material can behave like a crack in brittle materials such as dental composites [52]. Thermal and chemical aging in composites leads to noticeable microstructural changes, including macro filler fracturing and the surface delamination of the resin (Figure 1, Figure 2, Figure 3, Figure A16 and Figure A17). These results suggest that progressive degradation undermines the integrity of composite microstructures, particularly at the filler-matrix interface. Glass fillers are leached out, creating voids and releasing degradation products that may alter the pH within the polymer network structure, thereby accelerating hydrolysis [53]. The studied composite materials differ in filler content, which affects their performance. Key processes include resin matrix plasticization and degradation from environmental factors. Strength decreased by about 25% for Resin F. The resin matrix exhibited sorption values nearly 2.5 times higher than those of Arkon, which contained the most filler [9]. The extent of change in filled materials is influenced by the degradation of the filler and the coupling agent connecting the matrix and filler. High-filling materials require modified filler surfaces for stable interactions with the polymer matrix [54,55].

Considering the selected protocols, NaOH medium induces accelerated hydrolysis in the tested materials; however, it should be noted that substances present in the oral cavity, such as enzymes and bacteria, accelerate and cause hydrolysis [56]. Nevertheless, clinical conditions also involve physical parameters, such as variable temperature, which can additionally induce stresses at the interface of matrix–filler phases characterized by significantly different thermal conductivities [57]. Our findings suggest that this complex method accelerates aging processes in the tested material and can be valuable for assessing clinical performance in vitro [58,59]. However, an analysis should be conducted for a wide range of products available on the market. This may allow for identifying a protocol that will be an accessible method for researchers and will standardize the approach to evaluating new dental composites.

5. Conclusions

Based on the obtained results and within the study’s limitations, the following conclusions and comments can be drawn:

• The process of aging results in a reduction in materials’ durability by an average of 30% compared to the control values.
• There is a strong correlation between flexural strength and the fatigue flexural limit after aging.
• No significant relationships were identified between the material’s flexural strength or fatigue flexural limit and its fracture toughness after aging.
• A standard for fracture toughness testing for dental composites should be developed.
• The minimum value of FS after aging, postulated by the authors, should exceed 32–48 MPa.
• A comprehensive examination of commercially available materials is recommended to enhance the appropriate assessment methods for new composite materials, allowing for the determination of their stability during use.
• To minimize the costs and time of material characterization, testing the flexural strength and fracture toughness after aging should be prioritized.