A half century ago (back in the early 1960s) a lot of research work was done by The American Welding Society (AWS) Committee on Brazing and Soldering to determine appropriate criteria for brazing lap joints. These are the preferred type of joint design for assemblies requiring the ability to withstand high pressure in service, such as gas bottles. The results were published in their committee report AWS C3.1 in 1963. One of the recommendations was that joints should have an overlap of 3T or more, where “T” is the thickness of the thinner of the two sheet-metal pieces being brazing together.
Here’s how that recommendation came about. The AWS C3 committee arranged to conduct a series of round-robin testing in 10 different laboratories around the country. They used two different shear-type joint designs, four different base metals and three different types of brazing filler metals (BFMs) for a total of about 1,200 brazed shear-test specimens. Their intent was not only to find out what constituted a satisfactory joint overlap design for brazing but also to develop an easily reproducible test specimen. The specimen had to be “realistic” to the real-life world of brazed components in industry and be able to become a “standard” that everyone could (and would) use to evaluate joint strength.
The data generated by the 10 labs was plotted on graphs such as that shown in Fig. 1.
Much of their testing showed that with overlaps exceeding 2T, failure in shear testing always occurred in the base metal and not in the brazed joint. In contrast, most of the brazed joints with overlaps less than 2T failed in the joint itself.
We will conclude this discussion next time in Part 2.