Bioinspired microstructure design simultaneously enhances strain-rate stiffening and toughening of composites, Qi, et al., Engineering Fracture Mechanics, 2024
Novelty:
Propose and prove that microstructure significantly affects strain-rate dependent mechanical properties of composites, expanding the current knowledge that the strain-rate dependence is mostly due to material attributes.
The research/modeling method differs from classical micromechanical modeling in which a microstructure is well-defined and stress analyses, geometrical analysis, boundary conditions, force balance analysis, etc., at the microscale are conducted to establish equations, from which the target parameter is solved as an expression of other parameters. Instead, one typical engineering structure model and three baleen-inspired microstructure models having the same effective composite properties are designed, from one to the next a new microstructure feature is added, thus decoupling effects of chemical constituents and geometric microstructure and explicitly showing mechanisms of important microstructure features.
How microstructure affects the stress and energy behaviors which modulate the macroscale strain-rate dependent properties is studied by finite element analysis simulations; further, a new three-parameter constitutive model is established, which can capture, quantify, and evaluate the microstructure-induced strain rate dependent properties.
Scientific questions:
Why keratinous materials, having similar chemical constituents of keratins yet different microstructures, show different strain-rate dependent properties?
Can this be used as a new route, optimizing the microstructure rather than complicating the chemical compositions, to enhance the strain-rate stiffening and toughening properties of composite materials which actually service under varying loading rates in practical engineering? If so, how to understand and assess the microstructure effects on the strain-rate dependent properties, and further what microstructures are good ones for strain rate stiffening and toughening?
Key of how:
(i) To hypothesize that the microstructure affects strain-rate dependent properties.
(ii) The exhibited strain-rate dependent mechanical properties of composites are usually results of coupled effects of material attributes and microstructure. Meanwhile, what microstructures are possible “good” ones for high strain-rate stiffening and high strain-rate toughening, and what should be control microstructure? To address these, the above-mentioned research/modeling method is developed.
Microstructure means the geometry and the spatial arrangement/organization of the reinforcement and the matrix, and their interfacial properties. Different microstructures indicate that at least two different phases (e.g., the reinforcement and the matrix) compose the composite material.
(iii) For the ideally designed microstructures, current manufacturing or 3D printing are hardly capable to produce at the micrometer scale accuracy. Thus, numerical modeling using finite element is used to conduct “virtual” compressive experiments under different strain rates (10^-1, 10^2, 10^4/s). This (a) generate stress-strain curves of all microstructure models and strain-rate dependent property parameters and (b) allows to investigate the mechanisms/to screen important influencing mechanisms, the microscale stress/strain and energy behaviors of the constituent phases in relation to the macroscale strain-rate dependent properties due to effects of adding certain microstructure features.
(iv) To establish the constitutive model, the strain rate effects are segmented into elastic and inelastic components (to obtain elastic and inelastic strain rate sensitivity indexes me and mie, respectively) and an idea to correlate the rate-dependent plastic stress with the average strain energy density is essential to find the microstructure parameter function of Km, as it is found that the rate-dependent plastic stress through plastic hardening closely relates to the rate-dependent plastic dissipation energy which dominates the rate-dependent toughening (overall toughness at certain strain rates)).
Set the stage:
Despite with the similar keratin constituents, keratinous materials having different microstructures show different strain rate sensitivities.
→ This observed phenomenon cannot be directly explained by current knowledge, as the strain rate dependence has been ascribed mostly to viscoelasticity and/or viscoplasticity, the organic polymer constituent nature (material attribute).
→ Considering that the microstructure affects microscale deformation behavior and damage characteristics which can be loading-process dependent, it may influence the macroscale strain-rate dependent responses of composites. But this is not yet clear.
→ Considering further that the microstructure-induced strain-rate stiffening and toughening potentially represents a new approach in optimizing composites’ mechanical performance in real-word applications that feature varying loading rates, effects of the microstructure design on strain-rate dependent behaviors deserve an in-depth, systematic study.
→ The extensive research on strain-rate dependent properties of biological and engineering composites could not address the effects of different microstructures on the different strain-rate dependent behaviors, since these usually set one target material (microstructure fixed) and investigate the overall strain-rate dependent properties. Similar scenario applies to the mechanical modeling on the rate-dependent behaviors of engineering composites.
→ This work consists of microstructure design, numerical simulation, constitutive modeling, results and discussions, and conclusions, which hopefully lays a foundation for the novel scheme of bioinspired microstructure design for high strain-rate stiffening and toughening composites.
Major points:
1. Background and motivation (bioinspired microstructure design has been used to enhance mechanical properties under constant loading rates; composite materials in practice are used under varying loading rates; extensive studies on strain-rate dependent properties of biological and engineering composites mostly set the material and microstructure first and ascribe the strain-rate dependency to material attributes, while how microstructure affects strain-rate dependent properties remain unclear.)
2. Numerical model and simulation: (i) microstructure design (ensuring the same equivalent volume fracture of the reinforcing phases, a successive adding of one microstructure feature, potential effective microstructures that lead to high strain-rate stiffening and toughening), (ii) constitutive models for constituent materials (considering rate-dependent material attributes and damage behavior), (iii) numerical implementation and FEA simulation, (iv) results and discussion: (1) stress and strain responses (all microstructure models show similar trend but the Microstructure MLTF shows highest stiffness and maximum strength, and largest strain rate stiffening and toughening, which is due to that microstructure affects local stress and deformation behaviors of the constituent phases), (2) energy and damage behavior (all microstructure models show similar trend but the Microstructure MLTF shows largest strain-rate stiffening and toughening due to including the soft-and-hard tubular lamellae and the mineralized lamellae structure features, which promotes nonlinear plastic deformation over larger volumes at higher stress levels, enlarging plastic hardening; the inelastic dissipation energy dominates the entire strain-rate toughening at higher strain rates).
3. Constitutive model: (i) derivation (considering elastic and inelastic stress components and connecting rate dependent plastic stress to average strain energy density to obtain the three parameters of me, mie, and Km), (ii) verification and analysis (me quantifies the strain-rate stiffening, mie and Km more represent strain-rate toughening by capturing rate-dependent plastic hardening).
4. Conclusions (in addition to material attributes, the microstructure does affect strain-rate dependent properties of composites, which represents a new route to enhance synergistically strain-rate stiffening and toughening by microstructure design; different microstructure features affect strain-rate stiffening and toughening differently, while the baleen-inspired lamellar-tubular microstructure increases both strain rate stiffening and toughening; the three-parameter model captures and assesses the microstructure-induced strain-rate dependent properties well, serving as an analytical tool for design and performance evaluation).
Here is the link of the article: https://www.sciencedirect.com/science/article/abs/pii/S0013794424005526