Tough natural materials such as nacre, bone, and silk exhibit multiscale hierarchical structures where distinct toughening mechanisms occur at each level of the hierarchy, ranging from molecular uncoiling to microscale fibrillar sliding to macroscale crack deflection. An open question is whether and how the multiscale design motifs of natural materials can be translated to the development of next-generation biomimetic hydrogels. Here, we will discuss a recent work [1] on fabricating strong and tough hydrogel with architected multiscale hierarchical structures using a freeze-casting–assisted solution substitution strategy. The underlying multiscale multimechanisms are attributed to the gel’s hierarchical structures; hydrogen bond–enhanced fibers with nanocrystalline domains; and cross-linked strong polyvinyl alcohol chains with chain-connecting ionic bonds. This study establishes a blueprint of structure-performance mechanisms in tough hierarchically structured hydrogels and can inspire advanced design strategies for other promising hierarchical materials.
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Themes: Failure Mechanisms in Advanced Materials and Structures
MAXWELL STRESS AND ELECTROSTRICTION IN DIELECTRICS AND THEIR IMPLICATIONS FOR FRACTURE MECHANICS
In fracture mechanics of smart materials, the influence of electric fields on the propagation of cracks plays a key role. While the piezoelectric effect has been thoroughly investigated in this regard, nonlinear electrodynamic phenomena are oftentimes
disregarded.
As an example, stemming from the microscopic Lorentz force, electrostatic actions manifest themselves macroscopically in terms of surface tractions at discontinuities, body forces caused by graded fields and body couples due to local non-collinearity of electric field and polarization. All three of these manifestations are derived from the Maxwell stress tensor, whose formulation in polarizable matter is still being debated to date [1]. By contrast, electrostriction represents a constitutive effect only inherent to dielectric materials, interlinking mechanical strains with the square of the electric field and polarization, respectively. Due to identical mathematical structures of electrostrictive and Maxwell stresses in isotropic materials, both effects are sometimes treated equivalently.
In this work, these nonlinearities are studied with respect to an elliptic cavity in an infinite dielectric, providing a Griffith crack in the limiting case of a vanishing semi-minor axis. In this context, predominant models of the Maxwell stress tensor are compared and precisely distinguished from electrostriction, ultimately evaluating their individual contributions to crack tip loading.
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ENHANCING THE POST-CRACK TENSILE STRAIN CAPACITY OF CEMENT-BASED COMPOSITES USING FIBRILLAR WASTE BYPRODUCTS
The objective of this study is to assess the positive effect of fibrillar waste byproducts, such as biochar, on enhancing the tensile strain capacity and ductile behavior of cementitious composites by evaluating their fracture energy and fracture process zone length through the Work of Fracture Method (WFM). Cementitious mortars enriched using a low amount of biochar of 1 wt% exhibit 100% higher fracture energy over the OPC mortar indicating that the incorporation of the byproduct significantly increases the composite’s ability to absorb strain energy at the post-peak/strain softening area. As indicated by the 1.9x higher characteristic length of fracture process zone, lch, of biochar-mortar, the effective incorporation of the fibrillar byproduct holds a great potential to increase the post-crack tensile strain capacity that leads to a significantly improved ductile behavior of the cementitious composite.
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FAILURE MECHANISMS AND STATISTICAL METHOD FOR THE FATIGUE LIFE PREDICTION OF COKE DRUMS
Coke drums are major production equipment in petroleum refineries. In this presentation, failure mechanisms of the coke drums are analyzed. A statistical fatigue life estimation method is then proposed for the coke drums.
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NUCLEATION AND GROWTH OF CRACKS IN ELASTOMERS [Keynote]
We explore fracture nucleation and propagation within a transparent polydimenthylsiloxane elastomer using the “poker-chip” specimen. Global measurements are correlated with optical visualization at high spatial and adequate temporal resolution to identify the sequence of events; this is augmented with interrupted tests and x-ray computed tomography scans to probe the three dimensional geometry of the nucleation and growth of cracks. The experimental results are used to identify the different types of response, ranging from growth of surface cracks, to interior nucleation and growth of a single crack, to a completely nucleation dominated response. The dependence of the response on the specimen constraint, characterized by the specimen thickness, is explored through simulations within a finite deformation framework; a preliminary criterion for nucleation of cracks under multiaxial loading is proposed.
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BRITTLE FAILURE IN HYBRID STEEL-GLASS BEAM-COLUMN JOINT PROTOTYPE. EXPERIMENTAL INVESTIGATION AND NUMERICAL MODELLING.
A small-scale hybrid glass beam-column connection prototype is tested in order to assess its rotational characteristics and post-fracture performance. To simulate fracture process in glass, possibility of using two numerical Finite Element (FE) approaches is explored and the results are compared to the experimental findings focusing on the connections failing in a brittle manner.
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A MODE-III CRACK WITH SURFACE EFFECT IN A MAGNETOELECTROELASTIC MEDIUM
In this paper, the contribution of surface effect to the anti-plane deformation of a magnetoelectroelastic medium weakened by a crack is investigated. The surface magnetoelectroelasticity is incorporated by using the extended surface/interface model of Gurtin and Murdoch. The mixed boundary value problem of the mode-III crack is formulated by using a continuous distribution of screw dislocations and the dislocations of electric potential and magnetic potential on the crack, and the problem is finally reduced to solving a system of coupled Cauchy singular integro-differential equations, which can be numerically solved by the decoupling and collocation methods. The results show that the stresses, eldctric displacements and magnetic induction near the crack tips exhibit the logarithmic singularity when the surface effect is considered. When there is no surface effect on the crack face, the classical square-root singularity of the near crack-tip fields can be observed.
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ROLE OF WIRE ASPECT RATIO AND CRACK ASPECT RATIO ON FRACTURE BEHAVIOR OF WIRE SPECIMEN
Accurate stress intensity factor (SIF) solutions for cylindrical specimens with different wire aspect ratios and crack aspect ratios are required to determine the fracture toughness of rods and wires. The mode I geometric factor solutions of various crack configurations in a cylindrical fracture specimen in tension have been determined using liner elastic fracture mechanics. Finite element analysis (FEA) is applied to compute this as a function of wire aspect ratio (𝐻/𝐷), crack aspect ratio (𝑎/𝑏), and relative crack depth (𝑎/𝐷). It is found that the geometric factor is independent of wire aspect ratio for shallow cracks but has a major influence for deeper cracks. Also, the geometric factor is higher for concave cracks which facilitates the crack propagation. The mechanistic causes of the same are explained. Fracture toughness measurements on polymethylmethacrylate (PMMA) were carried out for the experimental validation of the solutions. The application of these solutions to fracture toughness measurements at the micro- and nanoscale, particularly in ceramic fibers and high strength metallic wires, is discussed.
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MULTISCALE TOUGHENING MECHANISM IN HYBRID FIBER REINFORCED CEMENT-BASED NANOCOMPOSITES
In this study a thorough evaluation of the toughening mechanism in cement-based nanocomposites reinforced with hybrid networks of carbon nanofibers (CNFs) and polypropylene microfibers (PPs) took place. The critical values of fracture toughness/stress intensity factor, KIC, were experimentally determined on prismatic notched specimens of nano and micro scale fiber reinforced cementitious composites using the two parameter fracture model (TPFM). The post-crack energy absorption capacity of the hybrid-composites was assessed by evaluating the dimensionless toughness index, I20, calculated through linear elastic fracture mechanics (LEFM) tests. The addition of CNF/PP networks at low volume fractions of about 0.1 vol% in cementitious matrix results in a significant improvement in the KIC (85-240%) and 1.6 – 10x higher I20 compared to the CNF or PP reinforced materials. Relative to the single-scale fiber reinforcement, the synergy between the nano- and micro- scale fibers results in a multi-scale crack arresting distinctively increasing the toughening effect in the hybrid fiber-cementitious mortar nanocomposites.
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THEORETICAL, EXPERIMENTAL AND COMPUTATIONAL STUDY THE OFF-AXIS ELASTIC CONSTANTS, FRACTURE AND STRENGTH OF UNIDIRECTIONAL FIBER COMPOSITE [Keynote]
In this work a theoretical/analytical, computational and experimental study of unidirectional glass-fiber reinforced epoxy composites is carried out. The concept of boundary interphase is used in order to determine the elastic constants of the composite. A finite element analysis is adopted in order to correlate with the derived theoretical values of the elastic constants. Finally, these results are compared with experimental findings obtained from tensile experiments performed on composites of the material used in order to predict the fracture of composites.
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