Creating reproducible signals from defects is of great importance for quantitative studies of acoustic emission (AE). A method of doing so is presented here which consists of indentation of hardened steel plates. A survey was made of a variety of steels, including A533B pressure vessel steel with an embrittled weld as well as embrittled Wl, 01 and 02 tool steels. Regions in these steels with a Rockwell C hardness greater than about 50 produced detectable AE during indentation. In many cases the signals were reproducible over an appreciable range and appeared to be the same for epicenter measurements as those produced by sudden unloading. However, monitoring on the same surface as the crack in some cases produced sets of two different but reproducible signals which gradually evolved with repeated loading which could be due to crack branching or zigzag motion. Regions in the steels with a hardness less than about 40 on the Rockwell C scale produced no detectable AE, even with appreciable plastic deformation. Examination with scanning electron microscopy indicated that the AE signals were produced by the nucleation and incremental growth of subsurface cracks, generally penny-shaped, less than a millimeter in size. A typical fracture toughness value for such cracks in an embrittled 02 tool steel was calculated to be about 19MNm-3/2. AE was also produced in as-received A533B steel by indentation fatique. The method suggests itself for materials studies of AE as well as a nondestructive method for in situ examination of structures for embrittlement
