求助！！！

我有两个问题
1. elongation in 2‘’ 是什么意思 难道是2英寸的伸长率？ 讲不通。
2.在硬度方面讲到含碳量时有这个要求。  DI of 2.50 using the caterpillar method of calulating DI. 其中的DI是什么意思？
When specified for localized hardening processes, the
    grade shall be designated by the suffix letters “A”,"H",
    or "L".  The suffix letter "H" shall contain 0.28-0.35%
    carbon and suffix letter "L" shall contain 0.38-0.45%
    carbon.  [color=#FF0000]Both will meet a min. DI of 2.50 using the
    Caterpillar method of calculating DI. The suffix letter
    “A” shall contain 0.28-0.35% carbon and meet a minimum
    DI of 3.00 using the Caterpillar method.

DI---dimension?