Fig. 1. The average unit shear stress (circles) and base metal tensile strength (triangles) as a function of overlap distance. The open symbols represent failure in the brazed joint, whereas the dark symbols represent failure in the base metals. Overlap distance was shown in inches, mm, and in overlap “T” distances.
A half century ago (back in the early 1960’s) 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 (the preferred type of joint design for assemblies requiring the ability to withstand high pressure in service, such as gas bottles, etc.). The results were published in their committee report: AWS C3.1 in 1963, one of the recommendations of which 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 ten different laboratories around the country, using two different shear-type joint designs, four different base metals, and three different types of brazing filler metals (BFMs), for a total of about 1200 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 that was “realistic” to the real-life world of brazed components in industry and which could become a “standard” that everyone could (and would) use to evaluate joint strength.
The data generated by the ten labs was plotted on graphs such as that shown in Fig. 1.
Much of their testing showed that with overlaps exceeding 2T overlap, failure in shear-testing always occurred in the base metal, and not in the brazed-joint, whereas most of the brazed joints with overlaps less than 2T failed in the brazed joint itself.
Based on the results of that extensive study, the brazing industry has adopted the guideline that lap-joints should be designed with an overlap of at least 3T (as shown in Figure 2) where “T” is the thickness of the thinner of the two sheet metal pieces being brazing together.
For lap-joints in brazing, I always recommend the “3T-to-6T Rule,” the 6T being the limit beyond which, in my experience and opinion, overlaps merely become wasteful of BFM and base metals, since no additional strength or leak-tightness benefit is obtained by such long overlaps.