Cyclic loads can lead to the failure of a component at much lower levels of tension and distortion as equivalent static loads. The 'fatigue behaviour', as it is termed, of many materials is extensively well known and is applied in the corresponding areas of construction.
Cyclic loads can be brought about, among other things, by sudden pressure changes conveyed through the medium to be transported, a change in the load on buried cables due to traffic (rail traffic, heavy road traffic), earthquake, construction works, alternating push-pull loads or by thermal expansion etc. The intention with a research project completed at the German Plastic Centre (SKZ) was to define the basic interrelationship between static and cyclic loads affecting heated tool butt welds and the like for plastic pipes.
Milled recesses on test pieces (in this case, a sample from a PP pipe)
Materials analysed
Table 1: Materials analysed
Table 1 shows the materials analysed. In addition to the influence exerted by the material to be welded, consideration was also be given here to the effect of variations in welding parameters and the removal of welding bead.
As a test piece, "Shape 2" was selected in accordance with DVS Guideline 2203-2 (German Welding Society) as shown in fig 1.
Fig 1: Sample shape 2 according to DVS 2203-2
It was already evident from preliminary tests that even welded samples failed in the tensile test and such failures occurred predominantly outside the welded area and were of a ductile nature. Since, in this research project, the intention was to specifically analyse the weld characteristics, an additional stage in the machining was deployed in which the test pieces were provided with two lateral milled recesses in the welded zone in order to bring about a failure in the weld seam. So as to be able to make a precise comparison between non-welded and welded samples, the non-welded samples (base material) also received these lateral milled recesses (see fig 2).
Fig 2: Milled recesses on test pieces (in this case, a sample from a PP pipe)
Both the external and the internal bead were removed on the "PE3" samples (pipe, PE100, dia. 110mm, SDR11) so as to enable identification of any possible influence of the weld bead on fatigue behaviour. In the interests of characterising the long-term behaviour during cyclic loading, fatigue tests were carried out in the form of Wöhler fatigue tests as well as load increase tests. Quasi-static tensile tests according to DIN EN ISO 527 at the same time served as a reference point for the short-term behaviour.
Test procedures
Wöhler curves based on the string-of-pearls process were determined for the fatigue strength range in relation to the various base materials and welded joints. In addition to the Wöhler tests, load increase tests were also carried out. The load levels were increased in steps of DF = approx. 100N after 1,000 load cycles and evaluated together with the changes in position. Fig 3 shows the evaluation of the load increase test taking a welded PE sample as an example. Whereas the cyclic force (strain amplitude) is subject to a linear stage increase in the test, the resulting elongation (strain amplitude) will change only up to a pre-determined load limit in proportion to the strain applied. If the elongation that occurs is out of proportion to the strain applied, irreversible distortion processes will come about initially (start of damage). This level of damage enables a rapid assessment to be made of the level of endurance strength.
Fig 3: Load increase test on a welded PE sample
By way of an equivalent to the short-term tensile welding factor fz, it is also possible to use this to determine "long-term welding factors" fLSV for load increase tests in cases involving fracture failures for long-term cyclic loads. In addition, it is also possible to determine a long-term welding factor fSB at the start of damage.
Using a comparison of these weld factors and fatigue strength values, it is thus possible to answer the question of the extent to which a weld seam exposed to cyclic long-term loading will affect fatigue strength in a different way to short-term load.
Results
In the following a consideration of the fatigue strength values (determined on the basis of the load increase test) is carried out by comparison to the short-term strength values. The results of the Wöhler tests revealed a good correlation with the results of the load increase tests.
Fig 4 first of all shows the level of fracture determined by means of load increase tests in relation to the strength level derived from the short-term test for the welded pipes analysed. Set out in the right hand section of fig 4 is a table showing the weld factors derived from the short-term tests (fz) and the load increase tests (fLSV).
Fig 4: Comparison of short-term strength with strength levels established at the point of fracture during the load increase tests
Generally-speaking, the strength values derived from the load increase test are lower than the short-term strength values. In the case of the PE- and PP-welds, the weld seam does not function as a weak point either during short-term or long-term loading. This situation is different as regards PVC-U welds. As far as short-term loading is concerned, no reduction in strength occurs that may be imputed to the weld. On the other hand, the effect of cyclic long-term loading is definitely to reduce strength. This also means that a reduction in strength contingent on the weld as a result of cyclic long-term loading cannot be detected by means of short-term tests.
Summary
It is evident that the effect of cyclic long-term loading is, generally-speaking, to reduce strength, regardless of whether the item in question is a welded joint or a homogeneous component. This must be taken into account in the interests of designing a component that is reliable in operation and the fatigue tests supply important characteristic values in relation to this which would as a rule not be available. In addition, it has emerged that any weld seam, depending on the material, will display a marked variation in terms of its effect on short-term and long-term strength. As far as the materials PE and PP are concerned, it was not possible, either during application of short-term or long-term loads, to observe any weakening imputable to the weld seam. In the case of the PVC-U pipes examined, the effect of the weld was clearly to reduce strength during the application of cyclic long-term loads but not while short-term loads were being applied. Therefore static short-term tests do not deliver reliable strength values that will be of use for component design in all cases, especially not when the component is exposed to cyclic and/or dynamic load occurrences during its life cycle. Thus, for reliable long-term layout, long-term tests with the corresponding instances of load application are necessary. The variation in welding parameters did not have an effect on any of the materials examined. The removal of the weld bead on the PE pipe displayed no effect on fatigue behaviour.
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Authors: Dr Benjamin Baudrit, Dipl.-Ing. Frank Dorbath, Dr-Ing. Kurt Engelsing, Dr-Ing. Peter Heidemeyer, Prof. Dr-Ing. Martin Bastian
