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Etching Test

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Figure 7-69.—Rupturing fillet weld test plate. (fig. 7-69) until a break occurs in the joint. This force may be applied by hydraulics or hammer blows. In  addition  to  checking  the  fractured  weld  for soundness, now is a good time to etch the weld to check for cracks. Etching Test The  ETCHING  TEST  is  used  to  determine  the soundness of a weld and also make visible the boundary between the base metal and the weld metal. To accomplish the test, you must cut a test piece from the welded joint so it shows a complete transverse section of the weld. You can make the cut by either sawing or flame cutting. File the face of the cut and then polish it with grade 00 abrasive cloth. Now place the test piece in the etching solution. The etching solutions generally used are hydrochlo- ric  acid,  ammonium  persulfate,  iodine  and  potassium iodide, or nitric acid. Each solution highlights different defects and areas of the weld. The hydrochloric acid dissolves  slag  inclusions  and  enlarges  gas  pockets, while nitric acid is used to show the refined zone as well as the metal zone. Tensile  Strength  Test The term TENSILE  STRENGTH  may be defined as the resistance to longitudinal stress or pull and is meas- ured in pounds per square inch of cross section. Testing for tensile strength involves placing a weld sample in a tensile testing machine and pulling on the test sample until it breaks. Figure  7-70.—Standard  tensile  test  specimen. The essential features of a tensile testing machine are the parts that pull the test specimen and the devices that  measure  the  resistance  of  the  test  specimen.  Another instrument,  known  as  an  extensometer  or  strain  gauge, is also used to measure the strain in the test piece. Some equipment comes with a device that records and plots the stress-strain curve for a permanent record. The tensile test is classified as a destructive test because  the  test  specimen  must  be  loaded  or  stressed until it fails. Because of the design of the test machine, weld  samples  must  be  machined  to  specific  dimensions. This explains why the test is made on a standard speci- men, rather than on the part itself. It is important that the test  specimen  represents  the  part.  Not  only  must  the specimen be given the same heat treatment as the part but it also must be heat-treated at the same time. There are many standard types of tensile test speci- mens, and figure 7-70 shows one standard type of speci- men commonly used. The standard test piece is an accurately machined specimen. Overall length is not a critical item, but the diameter and gauge length are. The 0.505-inch-diameter  (0.2  square  inch  area)  cross  section of the reduced portion provides an easy factor to ma- nipulate arithmetically. The 2-inch gauge length is the distance  between  strain-measuring  points.  This  is  the portion of the specimen where you attach the exten- someter. In addition, you can use the gauge length to determine  percent  elongation. The tensile test amounts to applying a smooth, steadily increasing load (or pull) on a test specimen and measuring the resistance of the specimen until it breaks. Even if recording equipment is not available, the testis not difficult to perform. During the test, you observe the behavior of the specimen and record the extensometer and gauge readings at regular intervals. After the speci- men breaks and the fracturing load is recorded, you measure  the  specimen  with  calipers  to  determine  the percent of elongation and the percent reduction in area. In  addition,  you  should  plot  a  stress-strain  curve.  From the data obtained, you can determine tensile strength, 7-42



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