This invention relates generally to the quality testing of heat-fused plastic pipe joints and more particularly concerns a method and template for producing a coupon which can be used to test the properties, such as the tensile strength and ductility, of a pipe joint as the pipe is being laid in the field.
Plastic pipes, such as pipes made of medium or high density polyethylene, can be joined by a variety of methods, a common joining method being butt-fusion. The procedure for this method involves inserting pipes to be fused into a specially designed fusion machine which aligns and holds the pipes axially with respect to each other and with pipe ends adjacent to each other, clamping the pipes securely in the jaws of the fusion machine, cleaning of the pipe ends to be fused, facing the pipe ends to ensure clean and square pipe ends with material exposed that is suitable for heat fusion, heating the pipe ends for an appropriate amount of time, and then joining the heated pipe ends under pressure and allowing the fused pipes to cool.
The integrity and usefulness of a pipeline requires quality fused joints with acceptable mechanical performance qualities. Therefore, tensile test methods have been devised which are intended to assure that a pipeline is being constructed of such quality as can reasonably be expected to pass final testing of the pipeline for use as designed. Unfortunately, most of the tensile test methods now available require instrumentation and apparatus which are not suited for field use. Therefore, tensile test coupons are used in laboratory test methods and are typically produced on non-portable machine tools.
A few destructive field-testing methods have been devised for checking the mechanical performance of the fused joint during pipeline production. The most common in-production test method is the “bend back” test. In the “bend back” test, a strap of material is extracted from a fused joint and its adjacent sections of pipe. The extracted strap is bent in such a direction that the maximum tensile and compressive bending stresses are applied to the portions of the strap that originated on the outer and inner diameters of the pipe. According to this test, a “good” joint is one which shows good bond integrity after bending. The straps are typically air bent, if practical, but if more force is needed to bend the strap to the degree required, implements may be applied.
The required length of the “bend back” strap may vary and, for greater pipe wall thicknesses, the forces required to bend the strap become high, containment in the case of failure becomes more difficult and the method becomes safety and cost prohibitive. In addition, the levels of stress imposed upon the fused joint are heavily dependent on uncontrolled or unknown factors such as the bend radius and the types of tooling used to bend the strap. These variables result in uncertainty as to the significance of any passing grade resulting from this test. Furthermore, for larger pipe diameters which require longer straps of pipe for testing, the material cost for the strap of pipe required to apply an appropriate bend test load can be quite expensive.
It is, therefore, an object of this invention to provide a method and template which facilitate the efficient and precise extraction of high quality tensile test coupons from a fused joint. Another object of this invention is to provide a method and template which facilitate speedy field evaluation of the quality of a fusion joint. A further object of this invention is to provide a method and template which ensures that the failure of the coupon in the tensile test will occur at the joint. It is also an object of this invention to provide a method and template which produce tensile test results which are qualitatively comparable to both/either a sample made from the pipe material and/or against predetermined qualitative criteria for acceptability. Still another object of this invention is to provide a method and template which require extraction of less pipe material for destructive tests than the “bend back” test.