Deju Zhu

A novel biomimetic material duplicating the structure and mechanics of natural nacre

Nacre from mollusk shell is a high-performance natural composite composed of microscopic mineral tablets bonded by a tough biopolymer. Under tensile stress, the tablets slide on one another in a highly controlled fashion, which makes nacre 3000 times tougher than the mineral it is made of. Significant efforts have led to nacre-like materials, but none can yet match this amount of toughness amplification. This article presents the first synthetic material that successfully duplicates the mechanism of tablet sliding observed in nacre. Made of millimeter-size wavy poly-methylmethacrylate tablets held by fasteners, this “model material” undergoes massive tablet sliding under tensile loading, accompanied by strain hardening. Analytical and finite element models successfully captured the salient deformation mechanisms in this material, enabling further design refinements and optimization. In addition, two new mechanisms were identified: the effect of free surfaces and “unzipping.” Both mechanisms may be relevant to natural materials such as nacre or bone.

Puncture resistance of the scaled skin from striped bass: collective mechanisms and inspiration for new flexible armor designs.

The structure and mechanics of fish scales display unusual and attractive features which could inspire new protective materials and systems. This natural material is therefore attracting attention over the past few years, and recent work demonstrated the remarkable performance of individual fish scales. A puncture event as would occur from a predators attack however involves more than one scale, and in this article we therefore investigate collective mechanisms occurring within the scaled skin of a fish in the event of a predators attack. We first demonstrate that in striped bass (Morone saxatilis), the scales increase by four to five times the force required to puncture the skin. We show that individual scales from striped bass provide a remarkable barrier against sharp puncture, regardless of the stiffness of the substrate. The scalation pattern in striped bass is such that three scales overlap at any point on the surface of the fish, which we show effectively multiplies the puncture force by three. We determined that the friction between scales is negligible and therefore it does not contribute to increasing puncture force. Likewise, we found that the local arrangement of the scales had little effect on the puncture performance. Interestingly, because the scales are several orders of magnitude stiffer than the substrate, indenting a few isolated scales results in sinking of the scales into the substrate. The high local deflections and strain within the soft tissue may then result in blunt injury before the sharp indenter penetrates the scales. Stereo-imaging and image correlation performed around a puncture site in fish reveal that the surrounding scales collectively contribute to redistributing the puncture force over large volume, limiting local deflections and strains in the soft tissues. The structure and mechanisms of natural fish scales therefore offer an effective protection against several types of threat, and may inspire novel versatile protective systems with attractive flexural properties.

Biomimetic Hard Materials

Materials such as bone, teeth, and seashells possess remarkable combinations of properties despite the poor structural quality of their ingredients (brittle minerals and soft proteins). Nacre from mollusk shells is 3,000 times tougher than the brittle mineral it is made of, a level of toughness amplification currently unmatched by any engineering material. For this reason, nacre has become the model for bio-inspiration for novel structural materials. The structure of nacre is organized over several length scales, but the microscopic brick-and-mortar arrangement of the mineral tablets is prominent. This staggered structure provides a universal approach to arranging hard building blocks in nature and is also found in bone and teeth. Recent models have demonstrated how an attractive combination of stiffness, strength, and toughness can be achieved through the staggered structure. The fabrication of engineering materials that duplicate the structure, mechanics, and properties of natural nacre still present formidable challenges to this day.

The Mechanical Performance of Teleost Fish Scales

High-performance natural materials and system are now serving as models for new engineering designs. In this work we have investigated the structure and mechanics of a single teleost fish scale from striped bass Morone saxatilis as part of larger project on novel flexible protective systems inspired from fish skin. These scales are about 300 microns thick and consist of a hard outer bony layer supported by a cross ply of collagen fibrils. Basic properties were obtained from tensile tests on single scales. While the bony layer is brittle, the collagen layer can undergo large deformations, eventually failing by extensive fiber pullout. Perforation tests with a sharp needle on a single scale resting on a soft substrate were also used to assess the performance and mechanics of a scale under a predators bite. We found that multiple small circumferential cracks developed on the top surface of the bony layer near the penetration site, while four major radial cracks formed through the thickness of bony layer.

The nonlinear flexural response of a whole teleost fish: Contribution of scales and skin

The scaled skin of fish is an intricate system that provides mechanical protection against hard and sharp puncture, while maintaining the high flexural compliance required for unhindered locomotion. This unusual combination of local hardness and global compliance makes fish skin an interesting model for bioinspired protective systems. In this work we investigate the flexural response of whole teleost fish, and how scales may affect global flexural stiffness. A bending moment is imposed on the entire body of a striped bass (Morone saxatilis). Imaging is used to measure local curvature, to generate moment-curvature curves as function of position along the entire axis of the fish. We find that the flexural stiffness is the highest in the thick middle portion of the fish, and lowest in the caudal and rostral ends. The flexural response is nonlinear, with an initial soft response followed by significant stiffening at larger flexural deformations. Low flexural stiffness at low curvatures promotes efficient swimming, while higher stiffness at high curvatures enables a possible tendon effect, where the mechanical energy at the end of a stroke is stored in the form of strain energy in the fish skin. To assess the contribution of the scales to stiffening we performed flexural tests with and without scales, following a careful protocol to take in account tissue degradation and the effects of temperature. Our findings suggest that scales do not substantially increase the whole body flexural stiffness of teleost fish over ranges of deformations which are typical of swimming and maneuvering. Teleost scales are thin and relatively flexible, so they can accommodate large flexural deformations. This finding is in contrast to the bulkier ganoid scales which were shown in previous reports to have a profound impact of global flexural deformations and swimming in fish like gar or Polypterus.

Biomimetic Tapered Fibers for Enhanced Composite Toughness

Nacre from mollusc shells is well known for its high toughness and strength. A key-mechanism for its mechanical performance is the progressive locking generated by the waviness of the mineral tablets it is made of. This allows nacre to generate strain-hardening, arrest cracks and spread inelastic deformations over large volumes before failure. Here we have incorporated a similar feature to short fibers, by machining tapered ends with well defined opening angles on steel pins used to reinforce composites. We performed single-fiber pullout tests on a tapered steel fiber in an epoxy matrix, which showed an improvement in work of pullout (WOP) of up to 27 times for tapered fibers compared to straight fibers. We expect similar increase of toughness for a composite reinforced with tapered fibers. The experimental results indicated the existence of an optimum taper angle to maximize WOP while preventing the brittle fracture of the matrix. An analytical model was derived to understand the interaction between fiber and matrix and to analyze the contributing factors to the WOP. The analytical model captures the trends of tapered fiber pullout and provides useful predictions of the influence of different parameters on WOP.