Nonlinear matrices employed in asphalt-based composites exhibit a prominent nonlinear elastic-viscoplasticviscodamageable mechanical response. The constitutive model for this type of matrices requires a significant experimental effort to identify the material constants. To alleviate the characterization effort without losing reliability in the mechanical prediction, this paper presents a fast procedure decoupling the elasto-viscoplastic response from the viscodamageable one and facilitating the identification of the material constants via efficient optimization. This procedure relies on a combined experimental-numerical also applicable to other rheologically complex materials like polyurea, thermoplastics or elasto-viscoplastic polycrystals. The experimental part is deliberately designed to only conduct cost-and time-effective monotonic loads at different strain rates in tension and compression. These pure monotonic results are used in the constitutive model to predict more complex loading conditions such as multiple load-unload-reload (LUR) cycles. To do this, an elasto-viscoplastic constitutive model suitable to describe creep-like phenomena and large irreversible deformations is proposed. This model incorporates pressure and strain rate sensitivity, which is essential to capture the compression- tension asymmetry in asphalt-based composites. A rate-dependent damage model is proposed to describe the degradation rate of the elasto-viscoplastic part. The stress and the consistent tangent modulus are derived. The model implementation for user-defined finite element subroutines are given for explicit or implicit solvers. The proposed decoupling facilitates an efficient identification of the constant parameters via an in-house Nelder-Mead-based optimization method. To accelerate the identification, all the stress-strain curves, either in tension or compression, are simultaneously used with applying physically-based constrains. The framework is validated with asphalt matrix at room temperature (24 C) and verified under LUR conditions at different rates. The identified model predicts correctly the experimental observations, proving its applicability to investigate particle-based composites with highly nonlinear matrices. ... Read more

The fracture process in self-healing concrete with embedded brittle capsules entails challenges in terms of understanding how and when these capsules break to release the agent. This paper presents a combined experimental–numerical investigation in which a versatile three-dimensional simulation model is developed to investigate the fracture of this type of beams under three-point bending load. The model allows for correlating the overall strength with different damage events occurring in every constituent: the concrete, the capsules and the capsule-concrete interface. The constitutive concrete damage model uses a pressure-dependent failure initiation criterion followed by a bilinear softening law, whose parameters are validated by using a modified Nelder-Mead optimization algorithm enriched with user-defined constraints aimed at increasing the convergence ratio. The validated virtual model demonstrates that the ratio of capsule slenderness to concrete-capsule interface strength is the key parameter for an effective self-healing process as it gives full control to break the capsule at the right moment. The shorter the capsule is, the longer the breakage process can be. Additionally, it has been found that by increasing the length of the capsules can help to enhance the overall fracture energy of the beam even after fully broken. ... Read more

The constitutive modelling of semi-crystalline polymers (SCP) has to consider several aspects as rate- and temperature-dependence, self-heating, and in particular, the double yield (DY) phenomenon. A full characterization of all these complex features involves prominent efforts in terms of material testing and parameter identification (PI). The contribution of the crystalline phase plays an important role in the evolution of the plastic yield in the SCPs. In this work, a constitutive model, named Unified SCP (USCP), is proposed by modifying the physically-based Boyce-Parks-Argon (BPA) glassy model. The contribution of the crystalline phase is introduced in the strain softening/hardening evolution law of the strength, providing an alternative interpretation of the underlying morphological changes caused by the crystalline phase embedded in the amorphous phase. The proposed formulation extends the BPA model with a new contribution to capture the crystalline phase. A full thermo-mechanical coupled numerical framework is developed for the USCP model validation. The DY phenomenon at different strain rates with self-heating and thermal softening effects is investigated and predicted. The proposed model extension needs three material constants with clear physical meaning. To identify them, a fast in-house optimization process based on Nelder-Mead is used, in which only a single element test is required. The model accurately predicts the experimental results for both thermosets and thermoplastics such as epoxy, nylon 101, PA6 and LDPE under monotonic loadings reported by different authors. ... Read more

The double yield (DY) phenomenon observed in a wide variety of semi-crystalline polymers (SCP) adds difficulties in the material characterization. In this paper, a constitutive model, termed as explicit semi-crystalline polymer (ESCP) model, is proposed to study DY phenomenon as well as the rate- and temperature-dependent thermomechanical response below the glass transition temperature. The underlying yield kinetics due to the morphological changes of the spherulite micro-structure is represented by a rheological analogue described by a physically-based amorphous intermolecular resistance and a rate-independent crystalline interlamellar resistance. Independently-identified viscoelastic response and network resistance are also implemented to complete the model description. The activation and disclosure of the crystalline component depend on the saturated state of amorphous phase. The proposed model is validated against ex- perimental data obtained from different authors for three commonly used SCPs: nylon 101, LDPE and PA6. A straightforward parameter identification procedure, requiring a minimum number of calibration tests, is presented to illustrate the model usage. The thermomechanical-coupled analyses provide satisfactory predictions using simulated models of a cylinder compression and dogbone tensile tests at different rates, where the self-heating and thermal softening effects are naturally captured by the model. ... Read more

This research presents an experimental approach to study the mechanics of asphalt matrix, a crucial component in asphalt concrete, consisting of asphalt binder, fine aggregates and filler. Understanding its response is essential to guide and improve the new designs in advanced asphalt concrete. The proposed approach makes use of the dynamic mechanical analysis (DMA) to execute a variety of quasi-static and dynamic tests under tension and compression. Uniaxial tests reveal a remarkable compression-tension asymmetry of the asphalt matrix in terms of stiffness and strength. The peak stress and stiffness can be 5 and 3 times, respectively, larger under compression than in tension. Both properties strongly depend on the strain rate, albeit stiffness and peak stress in compression are more sensitive than in tension. A straightforward equation for the creep rupture envelop is derived by performing creep tests at different stress levels. Amplitude and frequency sweep tests and fatigue-recovery test are performed to explore the inherent self-healing capabilities of asphalt matrix at room temperature. Doubling the regularity of resting periods significantly helps to regain the stiffness and it leads to a 4-times extension of the fatigue life. ... Read more