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Since fibers can only sustain tension, the materials listed here are not stable on their own. They must be combined with a stable material that acts as a ground matrix, using a “uncoupled solid mixture” container as described in Section 4.1.2.14↑, within a <solid> tag. These fiber models may also be used in continuous fiber distribution models as described in Section 4.3.2↓, using a <fibers> tag.