A self-constraint strengthening mechanism and its application to seashells
- PDF / 607,934 Bytes
- 6 Pages / 576 x 792 pts Page_size
- 14 Downloads / 216 Views
A self-constraint strengthening mechanism for multilayered brittle materials is proposed. The strengthening is a result of the self-constraint of the individual layers on each other, and no additional reinforcements are needed. The proposed model predicts that when individual brittle layers are stacked and properly "glued" together with a weak interphase, each layer will be ensured a minimum tensile strength, regardless of the flaw size in the individual layers. Estimation of the minimum strength using this model yields an apparently close agreement with the measured values for one type of nacreous structure reported in the literature. It is also predicted that low-strength ceramic sheets, which might be produced by some low-cost fabrication techniques, can be used to construct high strength man-made nacreous ceramics.
I. INTRODUCTION The coverings of mollusks, a group of marine animals, are known as seashells. There are more than 30,000 species of marine mollusks, most of which belong to the three major subgroups: the Gastropoda (20,000 species), the bivalvia (10,000 species), and the Cephalopoda (1000 species).1 Nacre, a type of structure that is composed of flat sheets of aragonite (CaCO3), is commonly found in all the three major subgroups.2"6 Each aragonite sheet in the nacreous structure is separated from the next by a very thin layer of organic interphase, which is mainly protein.7"10 Typically, the aragonite sheet is about 250-1000 nm thick, 910 and the thickness is fairly constant in any given specie.7 The interphase layer is about 10-50 nm thick and occupies only about 2 - 4 % of the total volume. 7 ' 10 Generally, the nacreous structure can be viewed as a stack of many hard aragonite sheets "glued" together with a weak organic interphase. The monolithic aragonite itself is weak and brittle, with a KK of less than 0.25 MPa • Vm 10 and a Young's modulus of 70 GPa. 7 1 0 However, once stacked and glued together to form a layered structure, the aragonite exhibits a strength in the range of 220 MPa and a toughness of 10 ± 6 MPay/m.9 Its specific strength becomes comparable to and its toughness becomes higher than those of many monolithic ceramics such as ZrO2 and S13N4.9 More impressive is that the nacreous structure overcomes the main drawback of ceramics 7 ' 8 —the lack of tolerance to defects. The unusual behavior of the nacreous structure has been well documented, and its toughening mechanisms have also been well recognized, which include crack blunting/branching, microcracking formation, plate pullout, crack bridging, and sliding of aragonite layers.9'11 The strengthening mechanism of aragonite, however, is less clear.9 Sarikaya and Aksay proposed that a J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
Downloaded: 01 Apr 2015
tensile stress applied to the nacre can somehow be transferred to a compressive stress during loading, and the load-carrying ability of the nacre is thus greatly increased.9 To better understand the unique strengthening effect of the layered nacreous structu
Data Loading...
