Unidirectional composite materials loaded in axial tension may exhibit very different failure patterns. Substantial fiber pull out or a very even fracture surface may be observed. The composite may fail catastrophically or gradually. The failure process is largely influenced by the load sharing between fibers and the distance over which stress is transferred back into the broken fiber due to fiber-matrix interaction. This load sharing and stress transfer in turn depend on the material used for fiber and matrix, as well as the manufacturing of the composite. The failure is also influenced by the statistical fiber strength distribution.
For the class project a fiber fracture simulation program was developed. This program allows the user to investigate the effects of statistical fiber strength distributions and the effect of fiber-fiber interaction on the failure process. For the simulation a model consisting of a number of parallel fibers each divided into a number of segments along its length is created. Each cell in this model is statistically assigned a strength value. Failure of a cell is governed by a failure criterion, which is a function of the stress in the cell, the strength of the cell, and some load sharing coefficients which govern the load sharing among neighboring fibers. Initially all cells in the simulation model are colored black to indicate that they are not failed. During the simulation the applied stress is slowly increased from a stress value at which no fiber failure can be observed. If the stress reaches a value for which the failure criterion for a cell is satisfied the color of this cell is changed to white to indicate that the cell is failed. This allows the user to observe the progress of fracture in the composite as stress is increased. Failure of the composite occurs if a continuous failure path through all fibers can be observed.
Below are Mpeg movies of three examples for different failure patterns:
So that you can see the final fracture path, here are still images of these.
This work was done by Andreas Obst. This project would not have been possible without the courses taught by Dr. Reifsnider on Strength and Life of Composite Material Systems, and by Dr. Kriz and Mr. Gordon on Scientific Visual Analysis and Multimedia.
Click image to return to Class 1994 page.