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Zhiliang Pan Department of Mechanical Engineering, University of North Carolina at Charlotte, Charlotte, North Carolina 28223, USA

Haoze Li Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29208, USA

Qiuming Weia) Department of Mechanical Engineering, University of North Carolina at Charlotte, Charlotte, North Carolina 28223, USA

Xiaodong Lib) Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29208, USA; and Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA (Received 16 March 2014; accepted 1 July 2014)

The defense mechanism that nacre (mother-of-pearl) uses to protect its living organism against high-speed predatory attack can provide lessons for engineered armor design. However, the underlying physics responsible for nacre’s dynamic energy dissipation has hitherto remained a mystery to be uncovered. Here we demonstrate a new energy dissipation mechanism hidden in nacre and activated only upon dynamic loading, where the crack terminates its propagation along nacre’s biopolymer interlayers but straightly impinges the aragonite platelets (95 vol%) in a transgranular manner. This intergranular–transgranular transition promotes the fracture energy dissipation, far exceeding that of the currently-used engineered ceramics. The mechanistic origin accounting for the enhancement of fracture energy dissipation is attributed to the unique nanoparticle architectured aragonite platelets. The dynamic manifestation in nacre can inspire a new route to design stronger-and-tougher engineered ceramic armors.

I. INTRODUCTION

Ceramic materials typically possess high strength and hardness but poor fracture toughness, entailing a limit for wider engineering applications. It is widely accepted that scarce plastic deformation surrounding the crack tip is primarily attributed to the notorious brittleness of the material. Upon quasi-static loading, a crack often prefers to select the grain boundary as its propagation path, whereas the grain interior remains comparatively intact. When the loading rate is boosted up to dynamic/ballistic regime, crack propagation can be diverted to cleave grain interior.1,2 This change of crack trajectory is deemed as a so-called intergranular–transgranular transition. However, the resulting mechanical performance, i.e., compressive strength, exhibits only marginal improvement.2 The limited mechanical enhancement in polycrystalline ceramics is attributed to the lack of plastic

Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2014.179 J. Mater. Res., Vol. 29, No. 14, Jul 28, 2014

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deformation events inside the individual grains, as evidenced by microscopic observations. Only when lateral pressure is imposed, the crack-dominated failure process will give way to the dislocation-controlled plastic deformation, accompanied with a brittle–du