The fillet weld demonstration is usually performed with carbon steel. The demonstration is a simple T-joint fillet break test as described in D1.1. It usually comes about after asking the ago old question, "Which is "stronger" a single pass weave bead or multiple stringer beads that produce the same size weld?"
There are two samples welded. One sample is made with a single pass 5/16 inch fillet weld (i.e., both legs are 5/16 inch). I find that most welders are forced to use a weave in order to make the proper sized weld. There is also some technique involved so as not to have incomplete fusion in the root (i.e., bridging the root).
The second sample is welded and the resulting weld size is 5/16 inch. The welders are directed to use three stringers on the second sample.
The test position isn't a big variable. 5/16 inch fillets can be made in all positions by welders with adequate skill, but typically the samples are deposited in either the flat or vertical positions.
After the welds have cooled to room temperature the fun begins. Each welder has the opportunity to break the weld either with repeated blows from a sledge hammer or with a hicky and long handle. Over the years the results is always the same. Once again it is important to compare apples to apples. So the welds have to pass the visual examination before being fractured and they have to be fused to the root in order to be a valid test. Likewise, the comparison is only valid if the weld sizes are comparable. A 3/8 fillet compared to a 5/16 fillet that is on the small size isn't a good comparison.
I'm not going to give you the results. That takes all the fun out of the comparison, but let me warn you that the difference is clearly evident and 99 out of 100 welders will draw the same conclusion once the comparison is done.
I've done the test with aluminum GMAW. The results are not comparable to carbon steel due to the fact the metallurgy is different and heat treatable aluminums are time at temperature sensitive.
I'm not sure how to answer the last question you ask. I guess the best reply is to say that all my observations are written in my reports. Questions that I have about the project are always in writing and rarely do I do something in an informal manner. All questions directed to the engineer are also included in the reports that are distributed to the owner, general contractor, building official, etc. Those questions require a written response. I don't tell other parties "how to do" anything unless I'm asked. As the third party inspector, I am the eyes and ears of the engineer of record and the owner. I am not the fabricator's consultant unless they are the party that retained my services. I found out long ago that I'm not nearly as smart as I would like to be. Whether working with a fabricator, welder, or engineer, the secret is knowing how to ask the right question so that the response produces the desired outcome.
Unless someone asks for help or an opinion, unsolicited help is usually perceived as offensive. It's as if you are saying, "Hey stupid, you're doing it wrong! This is how you should do it!" Unsolicited help is not usually well received even if your motives are well intended. Whether you are working with welders, mechanics, or engineers, you'll get a better response if you can make your point in the form of a question rather than a statement of fact.
Best regards - Al