Quantitative assessment of fatigue damage is a critical issue that needs to be addressed both in the mixtures and pavement designs [
4]. Many different approaches were mentioned in the literature to quantify the fatigue damage process of the asphalt mixture. Saboo et al. [
5] developed a new phenomenological model, which could accurately predict the fatigue life of mixtures by studying the existing phenomenological models. María Castro et al. [
6] combined continuous damage theory with the phenomenological method to establish a model based on continuous damage theory, which could estimate standard fatigue curves with fewer data. Shen S. et al. [
7] and Bhasin et al. [
8] used the energy dissipation method to evaluate the fatigue cracking of asphalt mixture and found that the energy method could accurately predict the cracking life of different materials. Liu G. et al. [
9] used the 4-point bending beam fatigue test under energy control mode to evaluate the fatigue damage characteristics of different content of recycled asphalt mixture. Lv S. et al. [
10] established a unified description model of fatigue equation of asphalt mixture under low temperature and low loading frequency, which means that the fatigue equations and curves under different loading frequencies could be characterized by one equation and one curve. Sadek et al. [
11] established a probabilistic framework that included the viscoelastic continuous damage method and considered the variability of input parameters. The analysis results under this framework were more consistent and reliable than those under deterministic viscoelastic continuous damage analysis. Babadopulos et al. [
12] coupled the aging model with the simplified viscoelastic continuous damage model based on the work potential theory and successfully incorporated aging into the mixture fatigue damage model. Cao W. et al. [
13] used the viscoelastic continuous damage method to analyze the results of LAS and TS tests. It was found that the relationship between damage characteristics and test methods and strain distribution was slightly dependent, and the degree of dependence was material specific. Mello, L.G.R. d et al. [
14] applied the continuous damage theory to the bending fatigue test to evaluate its applicability and proposed a practical method for predicting fatigue behavior by comparing the effects of different parameters and coefficients obtained in the research on the fatigue behavior of the asphalt mixture. Moreno-Navarro F. et al. [
15] proposed a new approach which combines the study of the changes produced in the geometry and the energy dissipated by the material in each load cycle and provided a more refined analysis of fatigue damage in asphalt mixtures. Among different approaches, phenomenological approaches, which associated stress or strain at the bottom of the asphalt layer with repeated loading times leading to fatigue, were widely used in structural design and fatigue performance analysis of conventional asphalt pavement because of their simplicity. Unfortunately, this method could hardly describe the damage process and structure degradation of asphalt materials. Fracture mechanics methods were used to study fatigue by monitoring the crack length development and propagation followed by fracture failure. However, the initiation and the propagation of microcracks or damage occupied most of the fatigue life, so it was critical to apply the damage approach to explain the fatigue damage evolution process of asphalt mixtures. When using the damage analysis method, the first and the most basic problem was to define an appropriate damage variable to describe the damage state of materials. The fatigue development process and critical damage value described by different damage variables were also different [
16]. Shen S. and Carpenter et al. [
17,
18] utilized the ratio of dissipated energy change (RDEC) as an indicator for assessing the resistance to fatigue damage of asphalt mixtures. Kim et al. [
19] used the viscoelastic continuous damage model to analyze the fatigue damage characteristics of asphalt mixtures. Jiang et al. [
20] and Ni et al. [
21] applied the damage variable defined by residual strength decay to the study of fatigue damage characteristics in asphalt mixtures. The fatigue damage model could reflect the damage state of asphalt mixtures more accurately. However, the definition of damage based on the dissipated energy method and continuous damage model method is too complex to be obtained directly from the test, and the measurement of residual strength is not continuous, therefore time- consuming. Considering that strength degradation is the most direct reason for structure failure, taking the residual strength as a fatigue damage variable is worth studied.
During the fatigue test, the dynamic modulus decreases with the continuous evolution and accumulation of internal damage, and the dynamic modulus decay is a damage variable that can be continuously measured. Therefore, the dynamic modulus is also a universal variable to describe fatigue damage of asphalt mixture. However, the existing models of residual strength and dynamic modulus are often proposed independently, and most of them lack a good consideration about the relationship between them, which results in a large number of tests when studying the fatigue behavior of asphalt mixture and describing its fatigue damage state. In addition, the residual strength test is destructive, and only one residual strength value can be obtained for each specimen. The test cost is very high. Moreover, there are individual differences among the specimens, and the comparability between the residual strength of different specimens is poor. Consequently, it is necessary to study the relationship between the two damage variables and propose a method for defining the damage variable with good applicability and high reliability.
Different test conditions mean different stress states [
22]. For complex stress states, the internal structure is affected by factors such as stress redistribution, residual stress, and difficulty in accurately capturing stress (strain). These factors therefore make the internal structure challenging to analyze. Compared with other fatigue test methods, the stress state of the specimen in direct tension test is relatively single. Moreover, the internal stress-strain relationship is more natural to analyze. In the United States, in recent years, new fatigue test methods have been developed. Viscoelastic continuum damage theory was applied to simulate the asphalt behavior in direct tension [
23]. The corresponding test method standard is AASHTO TP107. Behrooz K. et al. [
24] presented a new method for simulating the behavior of asphalt concrete also in uniaxial tension. Besides, Lv S. et al. [
25] simulated modulus decay modes under different loading conditions by using a non-linear fatigue damage model, which indicated that tensile failure was the main reason of fatigue damage for asphalt mixture. Therefore, in this paper, the analysis was conducted according to the flowchart, as shown in
Figure 1. This research presents direct tensile fatigue test and fatigue residual test of asphalt mixtures at different stress levels. The non-linear fatigue damage models of asphalt mixtures with dynamic modulus decay and residual strength decay as damage variables were established based on the dynamic modulus decay model and residual strength decay model, respectively. Under the assumption that the residual strength and dynamic modulus at the same time depend on the same damage state inside the material, the coupled relationship between the residual strength and dynamic modulus was given, and the coupled model of residual strength and dynamic modulus was established. The paper proposes a modified formula for calculating the damage variables associated with residual strength and dynamic modulus based on the relationship between two kinds of damage variables.