Some biological materials, such as the skeleton of certain deep-sea sponges, exhibit a very high fracture toughness, although they consist mainly of glass, which is known to be very brittle. The high fracture toughness is mainly caused by the so-called “material inhomogeneity effect”, i.e. the pronounced variation of the material properties between glass layers and the very thin, soft protein layers in between.
Each rod of the skeleton of a deep-sea sponge consists of glass layers with very thin protein interlayers in between.
The crack driving force, measured in terms of the near-tip J-integral Jtip, decreases when a crack comes close to a region with a higher Young's modulus E or yield stress σy and increases when it comes close to or enters a region with a lower E or σy. If a soft interlayer is inserted into a matrix material parallel to the applied load, i.e. perpendicular to the crack path, the crack driving force increases near the first interface of the interlayer, IF1, and decreases near IF2. Therefore, the crack easily grows into the interlayer, but stops there and is not able to re-initiate growth, if Jtip becomes smaller than the critical J-integral Jc of the matrix material. This leads to an improvement of the fracture toughness or the fatigue life of the component due to the soft interlayer.
Variation of the relative crack driving force Jtip/Jfar near a soft interlayer. The crack driving force Jtip increases, if the crack tip is located in front of the interlayer, and strongly decreases near the second interface IF2. Jfar denotes the far-field J-integral, i.e. the crack driving force of a specimen made of homogeneous matrix material at the same load.
We have investigated the utilization of the material inhomogeneity effect in matrix materials consisting of metals, polymers, ceramics, as well as in thin film materials. In order to improve the fracture toughness and/or fatigue resistance of components, thin soft interlayers are inserted into the matrix material parallel to the applied load. A computational tool based on the configurational force concept has been generated in order to work out the optimal architectural parameters of these composites, e.g. the optimum interlayer thickness for a given load. In the two videos below, the effect of a thin, soft interlayer on crack growth is demonstrated. A tremendous increase in fracture toughness is noticed in the specimen with the soft interlayer. This increase in fracture toughness appears although the interlayer material fractures in a cleavage mode, i.e. behaves very brittle.
Crack growth in a specimen made of steel CK45 (yield stress σy = 1660 MPa) with a single interface, but without a soft interlayer.
Crack growth in a specimen made of steel CK45 with a single, 90µm thick interlayer made of interstitial-free steel DC04 (yield stress σy = 390 MPa).
The matrix material CK45 fractures in a micro-ductile mode, whereas the interlayer material exhibits cleavage fracture, i.e. behaves very brittle.
The situation is more complicated, if the component is biaxially loaded, since a high tensile load perpendicular to the soft interlayer would result in immediate material failure. Currently, we investigate the ability of arrangements of elliptic inhomogeneities to deflect and arrest cracks in order to enhance the failure stress and the fracture toughness of intrinsically brittle materials. The crack trajectories near the material inhomogeneities are evaluated by applying a computationally very efficient approach, the crack trajectory interpolation (CTI) method. The aim is to find optimum arrangements of inhomogeneities that are able to catch and trap all possible cracks that originate from a free surface. In doing so, the void volume shall be as small as possible.
Staggered arrangement of elliptical voids with aspect ratio ry/rx = 2 and the optimum distances between the voids, Dy = 2.45ry and Dx = 10rx, so that the trapping efficiency reaches 100%. The green lines are the crack trajectories determined by the CTI-method.
Parts of these investigations have been funded by the COMET program within the K2 Center “Integrated Computational Material, Process and Product Engineering, IC-MPPE” (strategic projects A4.20 and P1.3) and the Austrian Research Promotion Agency (FFG).