Themes: Failure Mechanisms in Advanced Materials and Structures 

USING A HIERARCHY OF POROSITY TO IMPROVE THE FRACTURE TOUGHNESS OF METAMATERIALS

Mechanical metamaterials have been quickly growing in popularity based on their lightweight, multifunctional properties. One of the factors limiting their widespread adoption in weight baring applications, however, is their poor fracture toughness compared to bulk materials. Arrestor planes have been added to gyroid surface metamaterials and solid beams to manipulate the path of a propagating crack and improve the fracture toughness. The arrestor planes used a hierarchy of porosity interacting with the features inherent in the gyroid topology to direct propagating cracks into natural features that served to arrest the crack. This methodology was tested in both brittle polymer and stainless steel with toughening ranging from 22% to 300% depending on material.
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THE IMPACT OF MULTIAXIALITY ON THE STATIC AND FATIGUE FRACTURE OF CARBON/EPOXY POLYMER COMPOSITES

Polymer composites can be used in a plethora of applications, creating lightweight and durable structures. Their anisotropy together with the complexity of the laminate structure can lead to multiaxial stress states within the material, which can significantly affect the fracture process. In this work, carbon/epoxy laminates with different stacking sequences, and consequently different stress states, are tested under static and fatigue conditions. It is demonstrated that multiaxiality plays a crucial role in the fracture process and that shear stresses create severe damage conditions within the material.
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STRONG AND TOUGH FIBROUS HYDROGELS REINFORCED BY MULTISCALE HIERARCHICAL STRUCTURES WITH MULTIMECHANISMS [Keynote]

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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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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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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