HARDNESS TESTING

Figure 1-24.—Guerin process. easily; precautions must be taken to avoid this if possible. If they are ignited, you should extinguish them immediate y with an extinguishing powder such as powdered soapstone, clean, dry, unrusted cast iron chips, or graphite powder. Water or any CAUTION standard liquid or foam extinguisher causes magnesium to burn more rapidly and may cause small explosions. SUBSTITUTION AND INTERCHANGEABILITY OF AIRCRAFT METALS In selecting interchangeable or substitute materials for the repair and maintenance of naval aircraft, it is important that you check the appropriate aeronautic technical publications when specified materials are not in stock or not obtainable from another source. It is impossible to determine if another material is as strong as the original by mere observation. There are four requirements that you must keep in mind in this selection. The first and most important of these is maintaining the original strength of the structure. The other three are maintaining contour or aerodynamic smoothness, maintaining original weight, if possible, or keeping added weight to a minimum, and maintaining the original corrosive-resistant properties of the metal. Because different manufacturers design structural members to meet various load requirements, you can appreciate the importance of checking the specific technical publication. Structural repair of these members, apparently similar in construction, will thus vary in their load-carrying design with different aircraft. Structural repair instructions, including tables of interchangeability and substitution for ferrous and nonferrous metals and their specifications for all types of aircraft used by the Navy, are normally prepared by the contractor. Such instructions are usually contained in the NA 01-XXX-3 manual covering structural repair instructions for specific models of aircraft. Similar information is also contained in General Manual for Structural Repair, NA 01-1A-1. Aerospace Metals-General Data and Usage Factors, NA 01-1A-9, provides precise data on specific metals to assist in selection, usage, and processing for fabrication and repair. Always consult these publications and the NA 01-XXX-3 aircraft manual for the specific type of aircraft when confronted with a problem concerning maintenance and repair involving substitution and interchangeability of aircraft structural metals. Be sure you have the most recent issue of the aeronautic technical publication. HARDNESS TESTING Learning Objective: Recognize hardness testing methods, related equipments, and their operation Hardness testing is a method of determining the results of heat treatment as well as the state of a metal prior to heat treatment. Since hardness values can be tied in with tensile strength values and, in part, with wear resistance, hardness tests are an invaluable check of heat-treatment control and of material properties. Practically all hardness testing equipments now in service use the resistance to penetration as a measure of hardness. Included among the better known bench-type hardness testers are the Brinell and the Rockwell, both of which are described and illustrated in this section. Also included are three portable type hardness testers now being used by maintenance activities. 1-35