Anisotropic materials formed by living organisms possess remarkable mechanical properties due to their intricate microstructure and directional freedom. In contrast, human-made materials face challenges in achieving similar levels of directionality due to material and manufacturability constraints. To overcome these limitations, an approach using 3D printing of self-assembling thermotropic liquid crystal polymers (LCPs) is presented. Their high stiffness and strength is granted by nematic domains aligning during the extrusion process. Here, a remarkably wide range of Young's modulus from 3 to 40 GPa is obtained during by utilizing directionality of the nematic flow during the printing process. By determining a relationship between stiffness, nozzle diameter, and line width, a design space where shaping and mechanical performance can be combined is identified. The ability to print LCPs with on-the-fly width changes to accommodate arbitrary spatially varying directions is demonstrated. This unlocks the possibility to manufacture exquisite patterns inspired by fluid dynamics with steep curvature variations. Utilizing the synergy between this path-planning method and LCPs, functional objects with stiffness and curvature gradients can be 3D-printed, offering potential applications in lightweight sustainable structures embedding crack-mitigation strategies. This method also opens avenues for studying and replicating intricate patterns observed in nature, such as wood or turbulent flow using 3D printing.

Adhesive bonding is increasingly being used for composite structures, especially in aerospace and automotive industries. One common joint configuration used to test adhesive strength is the single-lap shear joint, which has been widely studied and shown to produce significant normal (peeling) stresses. When bonding composite structures, the normal stresses are capable of causing delamination before the adhesive bond fails, providing inconclusive engineering data regarding the bonding strength. An alternative test is the block shear joint, which uses a shorter sample geometry and a compressive-shear loading to reduce normal stresses. Analytical models proposed by Goland and Reissner and Hart-Smith are used to compare the edge-bending moment for the two joint configurations. The stress distributions along the bondline are also compared using finite element analysis. Experimental tests are conducted to evaluate these analyses and the failure modes of each configuration are recorded. Block shear samples demonstrate a joint strength over 100% higher than single-lap shear specimen bonded with the same adhesive material. The lower joint strength measured in single-lap shear is found to be potentially misleading due to delamination of the composite adherend.

In this paper, we propose a novel design and fabrication strategy to produce architected structures for use as the core in composite sandwich structures. A traditional foam core or honeycomb structure is lightweight and stiff, but susceptible to permanent deformation when subjected to excessive loading. Here we propose the use of an architected structure composed of arrays of hollow truncated cone unit cells that dissipate energy and exhibit structural recovery. These structures, printed with a viscoelastic material, rely on buckling of their sidewalls to dissipate energy and snap-back to prevent permanent deformation. We explore the mechanical response of these conical unit cells in terms of their buckling strength and post-buckling stability condition and develop design maps for the same by relating them to non-dimensional geometric parameters α,β,γ where α represents the slenderness of the curved sidewalls, β is the angle of the sidewall to the base, and γ represents the curvature of the sidewall. A validated finite element model is developed and used to investigate the effect of these parameters. We show that the volume normalized peak buckling load is directly proportional to both α & β, but is independent of γ. The post-buckling stability is influenced by γ as a large radius of curvature makes the structure less likely to exhibit structural bistability. Due to viscoelastic dissipation, for certain geometric parameters, the structures exhibit pseudo-bistability which allows them to recover to their original configurations without the need for external stimuli or energy.

Interfaces in polymer-based carbon fiber composites are critical regions that are most susceptible to delamination under static and impact loads. The interface strength and fracture toughness can be improved significantly by structural micro-reinforcements using polymer additive manufacturing (PAM). A micromechanics embedded macro-model is developed for the evaluation and extraction of interface properties of laminated coupons with printed polymer reinforcements (PPRs) at ply interfaces to efficiently tailor the interface properties. Given the microstructure of the printed reinforcements, an automatic cohesive insertion module is developed to simulate the improved interface fracture toughness resulting from a wavy crack path steering around PPRs. The applicability and accuracy of the developed modeling approach are demonstrated using ENF coupons with PPRs fabricated and tested. The validated modeling approach is then applied to explore the effect of geometric configurations of PPRs through double layer printing on the enhanced Mode II energy release rate. The developed numerical model provides a digital testing tool for tailoring interface properties, optimization of PPRs, and extraction of enhanced interface properties for delamination failure prediction of a large scale composite structure.

Sandwich composites and syntactic foams independently have been used in many engineering applications. However, there has been minimal effort towards taking advantage of the weight saving ability of syntactic foams in the cores of sandwich composites, especially with respect to the impact response of structures. To that end, the goal of this study is to investigate the mechanical response and damage mechanisms associated with syntactic foam core sandwich composites subjected to dynamic impact loading. In particular, this study investigates the influence of varying cenosphere volume fraction in syntactic foam core sandwich composites subjected to varying dynamic impact loading and further elucidates the extent and diversity of corresponding damage mechanisms. The syntactic foam cores are first fabricated using epoxy resin as the matrix and cenospheres as the reinforcement with four cenosphere volume fractions of 0% (pure epoxy), 20%, 40%, and 60%. The sandwich composite panels are then manufactured using the vacuum assisted resin transfer molding process with carbon fiber/vinyl ester facesheets. Dynamic impact tests are performed on the sandwich composite specimens at two energy levels of 80 J and 160 J, upon which the data are post-processed to gain a quantitative understanding of the impact response and damage mechanisms incurred by the specimens. A qualitative understanding is obtained through micro-computed tomography scanning of the impacted specimens. In addition, a finite element model is developed to investigate the causes for different damage mechanisms observed in specimens with different volume fractions.

The adverse effects of sea water environment on the fatigue life of woven carbon fiber/vinyl ester composites are established at room temperature in view of long-term survivability of offshore structures. It is observed that the influence of sea water saturation on the fatigue life is more pronounced when the maximum cyclic displacement approaches maximum quasi-static deflection, that is, the reduction in the number of cycles to failure are comparable between dry and sea water saturated samples at lower strain ranges (37% at 0.46% strain), but are drastically different at higher strain ranges (90% at 0.62% strain). Key damage modes that manifest during the fatigue loading is also identified, and a non-linear model is established for predicting low cycle fatigue life of these composites in dry and sea water saturated conditions.

Fiber reinforced ceramic matrix composites (FRCMCs) have been shown to retain exceptional thermal and mechanical properties at high temperatures in inert atmosphere. However, FRCMCs like carbon fiber reinforced carbon ceramic matrix composites (C/C CMCs) degrade rapidly at temperatures as low as 450^oC in oxidizing environments due to the conversion of solid carbon to gaseous oxides of carbon. With a rising demand for composite material structures to perform in extreme environmental conditions involving high temperature gradients, the prediction of oxidation damage is crucial in designing such structures. In this paper, a multi-scale finite element analysis framework is presented to study the oxidation behavior of ceramic matrix composites.

Delamination is a common failure mode observed in fiber reinforced polymeric composites subjected to thermo-mechanical loads. Reinforcements at the interlaminar regions or in the through-thickness direction have been previously shown to improve the damage resistance and durability of these composites. In particular additive manufacturing (AM), also called 3D printing, has been shown to increase the interlaminar shear strength when used for custom printing of polymer reinforcements over carbon fiber prepreg prior to the layup process of layered composites. In this paper, printing of interlaminar polymer reinforcements using fused deposition modeling (FDM) technique within AM is proposed for improving the interlaminar fracture toughness of carbon fiber laminates. The rationale behind this improvement in the interlaminar fracture toughness values due to printed polymer reinforcements (PPR) is investigated by elucidating the influence of varying the process parameters, such as print speed, spacing, and dimensions. An in-depth understanding of the bond quality between the prepregs and the printed polymers as well as the interlaminar fracture surfaces formed on the Mode-II fracture toughness values are established by developing an experimentally validated thermo-chemical finite element modeling framework.