Bridges are declared unsafe after a long use'. Why?

Explanation

**Step-by-Step Solution:**

1. **Understanding Elastic Fatigue:**
- Bridges are subjected to various loads over time, including heavy vehicles passing over them. This repeated application of stress leads to a phenomenon known as elastic fatigue.
- Elastic fatigue refers to the gradual loss of a material's ability to return to its original shape after being deformed.

2. **Impact of Repeated Strains:**
- When heavy trucks pass over a bridge, they exert significant strain on the materials (like steel or concrete) that make up the bridge.
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