Tech Briefs

Mechanistic-Based Multiaxial- Stochastic-Strength Model for Transversely- Isotropic Brittle Materials

The methodology is applicable to a wide variety of graphite, coatings, and composite materials.

A methodology has been developed and the software written to predict the probability of failure of transversely isotropic (a type of anisotropy) materials under generalized thermomechanical loading. This methodology is mechanistic in that it is based on the physical characteristics of brittle fracture, and morphological in that it considers the size, shape, and orientation distribution of strength controlling defects or flaws. On that basis, it can also account for a material’s failure modes and direction of damage initiation from loading. It is capable of predicting an anisotropic material’s probability of failure under transient and cyclic loading. This innovation can be applied to materials such as graphite, coatings, or the individual brittle constituents of composite materials.

The strength of an isotropic brittle material is independent of the direction of the applied load. However, for many brittle materials, the strength of the material changes with the direction of the applied load. These are termed anisotropic materials. The most common type of material strength anisotropy is transverse isotropy, where material strength is isotropic within a plane but orthogonal to that plane, the strength is greater than or less than the in-plane strength.

The unit sphere methodology is an attempt to provide an improved mechanistic basis to the problem of predicting strength response of an anisotropic and composite material under multiaxial loading as compared to polynomial interaction equation formulations. It has multiple unique features including flaw orientation and fracture toughness anisotropy. These physically based anisotropy functions are general and can model tightly defined or more diffuse material anisotropy textures describing flaw populations. Innovative equations were developed in order to achieve this capability. The methodology also includes consideration of strength scatter to predict material probability of failure, shear sensitivity of flaws, and accounting for multiple failure modes regarding overall failure response. One novel feature of this methodology is the ability to predict the orientation of critical flaws under multiaxial loading.

With this capability, the model can be tuned to the physical attributes of the material by, for example, mapping the toughness response of the material with respect to orientation for indentation testing, or mapping the observed orientation anisotropy of the pre-existing flaws in the material. In principle, by this methodology, the physical attributes of the anisotropic material can also be used to predict the multiaxial strength response of the material.

It is intended for aerospace applications where trade-offs must be performed regarding safety, durability, and weight. It is anticipated that this software will be used with finite element or micromechanics-based codes describing the behavior of composite materials. This incorporation would allow the full exercise of the new methodology, including incremental time/load steps, and fatigue of composite laminates and woven composite structures.

This work was done by Noel N. Nemeth of Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. LEW-19018-1