Fatigue fracture of tough hydrogels

Tough hydrogels of many chemical compositions have been developed in recent years, but their fatigue fracture has not been studied. The lack of study hinders further development of hydrogels for applications that require long lifetimes under cyclic loads. Examples include tissue engineering, soft robots, and stretchable electronics. Here we study the fatigue fracture of a polyacrylamide-alginate tough hydrogel. We find that the stress-stretch curve changes cycle by cycle, and reaches a steady state after thousands of cycles. The threshold for fatigue fracture is about 53 J/m2, much below the fracture energy (~10,000 J/m2) measured under monotonic load. Nonetheless, the extension of crack per cycle in the polyacrylamide-alginate tough hydrogel is much smaller than that in a single-network polyacrylamide hydrogel.Engineering and Applied Science

Rheological investigation and modeling of healing properties during extrusion-based 3D printing of poly(lactic-acid)

The focus of the present paper is the rheological study of poly(D,L-lactic-acid) (PDLLA) towards a modeling of their healing properties during 3D direct pellet printing extrusion (DPPE). The viscoelastic properties of PDLLA and the filament temperature during deposition are first characterized. The influence of DPPE processing conditions is investigated in terms of temperature, time, and printing speed. For this, we propose a modeling of the process-induced interphase thickness between two deposited layers considering the non-isothermal polymer relaxation and accounting for the contribution of entanglement rate through the Convective constraint release model. Hence, taking into account the induced chain orienta-tion and mobility coming from filament deposition, this model quantifies the degree of healing between 3D-printed layers. Eventually, the proposed model is validated by comparing the theoretically calculated degree of healing with experimental tensile properties and lap shear results

Micromechanics of cross-linked thermosets and application to composite multiscale modelling

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Micromechanics of deformation and fracture in highly cross-linked thermosets

Advanced constitutive models for polymers have been essentially developed for thermoplastics with relatively limited applications/extensions to thermosets. Thermosets differ from thermoplastics by the cross-linking. Recent extensions of these constitutive models provide accurate predictions over a wide range of loading configurations, strain rates and temperature, encompassing below and above transition temperature regimes, although at the prize of a very large number of parameters, often larger than 30. Still, these models, mixing phenomenological and micromechanics ingredients, are often not rich enough to capture complex behaviors such as for instance severe non-linearity upon unloading, while missing also micromechanical connection to the failure process. Based on an extensive experimental test program on the highly cross-linked RTM6 epoxy, the viscoplastic response is found very similar to thermoplastics, with hardening-softening-re-hardening, large back stress upon unloading and existence of shear band patterns at very small scale. A molecular physics-based model of the deformation process occurring through the activation of nanometer scale shear transformation zones (STZ) has been developed, borrowed from the metallic glass field. The viscoplastic deformation is the result of the cooperative activation of STZ’s, sensitive to rate, temperature, stress state and stress level. This model involves only 5 parameters to identify, all with physical meaning. The model quantitatively captures all the experimental trends, even some complicated responses during creep tests performed after plastic deformation at intermediate stress levels showing backward followed by forward creep. Such as model will never replace closed form constitutive models for the treatment of large scale components but can be used to understand small scale mechanics as well as for identifying macroscopic models. In this research, a new micromechanics-based fracture model based on the attainment of a local maximum principal stress within a given volume is also proposed and validated for a wide range of stress states

Micromechanics of deformation and fracture in highly cross-linked thermosets and size effects

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Why is Mechanical Fatigue Different from Toughness in Elastomers? The Role of Damage by Polymer Chain Scission

Although elastomers often experience 10-100 million cycles prior to failure, there is currently a limited understanding of their resistance to fatigue crack propagation. We use soft and tough double-network elastomers tagged with mechanofluorescent probes to understand the role of damage by sacrificial bond scission on their mechanical durability and fracture toughness. Damage accumulation and localization ahead of the crack tip depend on the areal density of sacrificial bonds, as well as on the applied load (i.e., cyclic or monotonic). This information serves to engineer fatigue resistant elastomers, understand fracture mechanisms, and reduce the environmental footprint of the polymer industry.</p

Micromechanics of deformation and fracture in highly cross-linked thermosets – impact on composite modelling

Advanced constitutive models for polymers have been essentially developed for thermoplastics with relatively limited applications/extensions to thermosets. Recent extensions of these constitutive models provide accurate predictions over a wide range of loading configurations, strain rates and temperature, encompassing below and above transition temperature regimes, although at the prize of a very large number of parameters, often larger than 30. Still, these models, mixing phenomenological and micromechanics ingredients, are often not rich enough to capture complex behaviors such as for instance severe non-linearity upon unloading or possible size effects, while missing also micromechanical connection to the failure process. This directly impacts the development of predictive multiscale models for polymer based composites. Based on extensive experimental test program on the highly cross-linked RTM6 epoxy, the viscoplastic response is found very similar to thermoplastics, with hardening-softening-re-hardening, large back stress upon unloading and existence of shear band patterns at very small scale [1]. Furthermore, size effects are revealed when looking at nanoindentation data as well as indirectly when looking at the response of unidirectional composites. A molecular physics-based model of the deformation process occurring through the activation of nanometer scale shear transformation zones (STZ) has been worked out [2]. The viscoplastic deformation is the result of the cooperative activation of STZ’s, sensitive to rate, temperature, stress state and stress level. This model involves only 7 parameters to identify, all with physical meaning. The model quantitatively captures the experimental trends, even some complicated responses during creep tests performed after plastic deformation at intermediate stress levels showing backward followed by forward creep. It also captures the size dependent strength resulting from large strain gradients putting a constraint on the development of the micro-shear banding process. In addition, a new micromechanics-based fracture model based on the attainment of a local maximum principal stress at the tip of microdefects is proposed and validated for a wide range of stress states [3]. The implication of these results and micromechanical models on composite modelling is not straightforward. This is addressed through the analysis of in situ SEM compression tests on thick UD carbon fiber reinforced RTM6 matrix composite, involving the determination of digital image correlation strain fields [4]. Difficulties remain to quantitatively capture the experimental response with the models identified on bulk resin data

Micromechanics of deformation and fracture in highly cross-linked thermosets – Impact on composite modelling

Advanced constitutive models for polymers have been essentially developed for thermoplastics with relatively limited applications/extensions to thermosets. Recent extensions of these constitutive models provide accurate predictions over a wide range of loading configurations, strain rates and temperature, encompassing below and above transition temperature regimes, although at the prize of a very large number of parameters, often larger than 30. Still, these models, mixing phenomenological and micromechanics ingredients, are often not rich enough to capture complex behaviors such as for instance severe non-linearity upon unloading or possible size effects, while missing also micromechanical connection to the failure process. This directly impacts the development of predictive multiscale models for polymer based composites. Based on extensive experimental test program on the highly cross-linked RTM6 epoxy, the viscoplastic response is found very similar to thermoplastics, with hardening-softening-rehardening, large back stress upon unloading and existence of shear band patterns at very small scale [1]. Furthermore, size effects are revealed when looking at nanoindentation data as well as indirectly when looking at the response of unidirectional composites. A molecular physics-based model of the deformation process occurring through the activation of nanometer scale shear transformation zones (STZ) has been worked out [2]. The viscoplastic deformation is the result of the cooperative activation of STZ’s, sensitive to rate, temperature, stress state and stress level. This model involves only 7 parameters to identify, all with physical meaning. The model quantitatively captures the experimental trends, even some complicated responses during creep tests performed after plastic deformation at intermediate stress levels showing backward followed by forward creep. It also captures the size dependent strength resulting from large strain gradients putting a constraint on the development of the micro-shear banding process. In addition, a new micromechanics-based fracture model based on the attainment of a local maximum principal stress at the tip of microdefects is proposed and validated for a wide range of stress states [3]. The implication of these results and micromechanical models on composite modelling is not straightforward. This is addressed through the analysis of in situ SEM compression tests on thick UD carbon fiber reinforced RTM6 matrix composite, involving the determination of digital image correlation strain fields [4]. Difficulties remain to quantitatively capture the experimental response with the models identified on bulk resin data

Towards more predictive composite models

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