If that seems a little dense, here is a short overview of the concept. (A version of this material has appeared in Desktop Engineering magazine.)
Material Basis Values
A-Basis and B-Basis values (sometimes called allowables or allowable values) are statistically computed confidence bounds on estimates of a material property, usually strength.
Composite material properties can vary. That variance makes definitions of material properties somewhat … fuzzy. That’s where basis values come in. We can say with 95% confidence that 99% of the material will meet or exceed the A basis value, and 90% of the material will meet or exceed the less conservative B basis value. Or can we?
Computing Basis Values
Material property basis values can be computed from tests using three batches of material, all from the same manufacturer. It’s like throwing three darts at a wall, then drawing two concentric circles around them. The circles are your acceptance region (inner) and basis values (outer).
When subsequent manufacturers step up and throw, their darts must hit the inner circle or risk their materials being rejected.
Within the acceptance region, we cannot tell any significant difference between samples; if your sample lands in that region, it’s considered identical and accepted. But that circle is rather small and hard to hit. In fact, these computed acceptance criteria are frequently unrealistic for a large proportion of composite part manufacturers (CPMs). These unrealistic expectations have significant and costly repercussions for the industry at large.
You might think the problem stems from the small sample size (three batches) initially used to compute the basis values, but it goes deeper. Because of the way basis values are computed, adding more samples actually makes matters worse. With more samples, we become better at telling differences between batches and our acceptance region – where we cannot tell the difference between two materials – shrinks.
This could, theoretically, result in many, most or even all material samples being rejected – including those from the original batches!
Generic Basis Values
With generic basis values, the acceptance region is defined first, from a much larger set of samples from multiple manufacturers, such that at least 90% of CPM’s will produce acceptable product by simply following the documented procedures. Basis values are then computed for this acceptance region.
This gives us a much larger acceptance region, and lower basis values, than traditional methods. But, and this is key, it gives us basis values that CPMs can actually meet. In fact, we can conclude with 95% confidence that their materials will support the generic basis values. This is a stronger claim than currently used certify materials, and can result in clear and unequivocal certification of composite parts for the majority of CPM’s using a material’s generic basis values.