This non-uniform distribution of stresses means that the thickness need not be uniform to address the higher stresses. By applying the additional thickness only where the stresses are higher, it is possible to manufacture a more cost-effective elbow. This approach would allow us to avoid wasting material by applying the additional thickness only where it’s required.
Like the pipe, we still need to know the strength of the laminate before we can determine what the thickness needs to be. Very few fittings are tested under long term pressure testing (e.g., ASTM D2992). It is more common to conduct short term burst testing of fittings to determine laminate strength (e.g., ASTM D1599), but testing 5 samples (as required by ASTM D1599) of any fitting can become very expensive. It is more cost effective and much more common to conduct short term testing of coupons cut from typical laminates (e.g., ASTM D638). This latter method is possible in fitting laminates as the materials are such that they can usually be applied on a flat sheet (and absence of coupon curvature is usually a requirement for the relevant tensile test methods). The same can’t be said for the filament wound laminates used in pipe. Once the laminate strength is known, a design factor is applied to the laminate strength to calculate an allowable stress, and the required fitting thickness is calculated using the modified equation above.
This approach of applying a pressure stress multiplier to the basic pipe equations can be used for almost any fitting. We just have to use appropriate values for the pressure stress multiplier (m). As we’ve already seen, “m” for an elbow is 1.25. But some fittings, tees for example, have a much larger “m”. A typical value for “m” of a tee is about 2.5, so a tee has to be significantly thicker than a pipe to provide the same factor of safety.
This same approach can also be applied to butt joints. Although butt joints are similar in shape to that of pipe (i.e. cylindrical), the discontinuity in the geometry where the pipe ends meet results in stress concentrations within the joint. Applying a pressure stress multiplier “m” of 1.67 will result in an acceptable factor of safety to the joint design.
The design approach just described is an appropriate method for designing fittings and joints, but we haven’t yet actually proven that the component is adequate. Recall that the approach utilized strength properties from flat sheet laminates and application of a “fudge factor” (i.e. the pressure stress multiplier). For example, we haven’t yet demonstrated that the fudge factor was adequate, or that we applied the reinforcements appropriately. To provide evidence that the component actually achieves the factor of safety required, testing of the actual component is necessary. This is the purpose of proof testing (also called proof of design testing). There are several methods that can be used for proof testing. The first of these is to actually burst the component in accordance with ASTM D1599. While this is the most direct approach, it requires specialized equipment – particularly for larger diameter components, and it carries with it some safety concerns. Unlike filament wound pipes, which typically fail by weeping, fittings often fail by actually bursting. A lot of energy can be released during these sometimes “exciting” events, so appropriate precautions need to be taken to avoid damaging equipment and facilities, and more importantly, for ensuring the safety of the personnel involved.
