Testing
When American Handforge produces a forging, there are numerous quality assurance tests the material is subjected to. Some tests are what we call "destructive," and others are called "indicators," which do not harm or consume any material for the testing. The indicators are useful in predicting how the material will perform in an actual destructive test.
Thunking a watermelon before purchase, and listening to the sound it gives, to gauge its ripeness.
We won't find out the true quality of the melon until later, but for the moment we rely on our senses to evaluate it. This would be considered an indicator test — and for most of us, it hasn't been too useful. We do it anyway before we buy the melon, because we have no other way of determining its quality.
Another example is the common medical tests — blood pressure and cholesterol measurements — that doctors use to gauge our health. There's general disagreement in the medical community about setting absolute limits for either measurement, though there's agreement that high values for both are generally considered unhealthy. The same dilemma applies to aluminum products, though the "number limits" have been a bit more clearly defined in material specifications — just how they were arrived at is still somewhat contentious. Conductivity measurements are another complex indicator test.
Hardness Testing
There are two common indicator tests performed on hand forgings supplied in tempered conditions and reported on a test report. Hardness is a great indicator test for strength. Hardness testing is performed on the surface of the parts and yields a number (examples: HRB 85, BHN 126) for evaluation purposes. There are two hardness indicator tests useful for estimating the strength of hardened aluminum — Brinell and Rockwell — named for their respective inventors.
To simplify the process: each tester makes a small dent in the surface of the aluminum by striking it with a controlled, rounded "hammer," then measures how big the dent was. For both methods, at a constant force, a bigger dent is reported as a lower number — harder material resists the hammer more, so the dent is smaller and the hardness value is higher.
Rockwell Hardness
Produces a very small surface round dent in the material.
Brinell Hardness
Produces a somewhat larger dent than the Rockwell method.
What the Numbers Mean
The respective machines produce numbers that are indicators of strength — and that's where the evaluation process begins. It always relies on past correlation of destructive mechanical property tensile testing to be useful. A history of testing thousands of aluminum products — recording both hardness and tensile results at the same time — has produced a record of expected hardness values for various alloys and temper conditions. These expected values are useful for quality assurance purposes, but they provide no real usable data for engineering purposes, and are never the final word on actual strength.
Because of this inherent vagueness, no aluminum specification uses hardness minimums as a quality control requirement. Hardness testing is done on a routine basis, and values are reported for information only.
Final Acceptance
The final acceptance of the lot is based not on an indicator, but on an actual destructive test cut from the center of the forging. Just like the watermelon test, the only real way to know its ripeness is to cut into it — the same is true for aluminum hand forgings. Each piece is tested for surface hardness indicators, but tensile properties are determined destructively on a piece cut from the center of a forging, on a lot basis.
The tensile values, the facts, rule and are the final decision in any dispute involving strength issues. The hardness indicators are just opinions.
Everyone must rely on facts in the final analysis — and that's why tensile strength is a requirement in material specifications.