Due to the poor thermal conductivity of thermoplastics, residual stresses (tensile and compressive residual stresses) are build up in the walls of the product during cooling processes after extrusion. Deformations and distortion of the products often result from the residual stresses, whereby the presence of the residual stresses becomes visible. In the case of plastic pipes, axial and tangential residual stresses are created. The following considerations de
al exclusively with tangential residual stresses in plastic pipes.
Development of residual stresses
Thermoplastic plastics, especially semi-crystalline plastics, undergo large volume changes in the range of 20% during cooling from the melt. Cooling of plastic pipes usually takes place from the outside. The outer skin of the plastic pipe is already frozen during calibration and can therefore no longer carry out any free deformations. The rest of the pipe still has almost melt temperature, as the example in the following figure shows.
This means that the volume of hot material inside the pipe wall will still shrink considerably, while the outer skin is already frozen and thus rigid. The effort of the hot material to shrink is hindered by the rigid outer skin, resulting in residual tensile stresses inside the pipe wall. The equilibrium of forces is maintained by residual compressive stresses simultaneously built up in the outer layer. At first, the residual stresses do not manifest themselves in larger deformations. Only when the pipe is cut in the axial direction and a strip is removed from the pipe wall can the “snapping in” of the pipe be observed with pipes that are primarily cooled from the outside:
The extent of deformation often, but not always, gives an impression of the level of residual stresses. However, information on the residual stress distribution cannot be derived solely from the deformation. For this purpose, for example, special test procedures or simulation methods can be used, as shown in the following list.
Measurement of residual stresses in plastic pipes
The so-called Janson method is often used to measure tangential residual stresses in plastic pipes. It is relatively widespread, but there are other methods. The following table shows a list of the most important procedures described in the literature:
In the Janson test procedure, the determination of the residual stresses is based on the deformation of a tube specimen caused by the free-cutting (cutting out strips in axial direction). The change in deformation is measured with the aid of two markings on a pipe section of a certain length. A maximum residual stress in the pipe wall is calculated from the deformation by means of formulaic relationships.
The small-scale measuring method also uses the deformation of a specimen caused by residual stress after free cutting. In this method, a longitudinal segment is cut out of the test specimen, disturbing the equilibrium of the residual stresses in the tube. A deformation reaction of the ring body occurs, so that the distance between the two cut edges changes. With the aid of a tensile testing machine, the specimen is reshaped and the force required to reach the initial state is determined. The tangential residual stresses are calculated on the assumption of a linear stress distribution over the pipe wall thickness.
The slitting method can be used to determine the maximum residual stresses in axial and tangential directions. To determine the tangential residual stresses, the tube is cut open lengthwise and the deformation (change in diameter) of the tube sample is observed. The outer diameter is measured after a specified period of time. The maximum tangential residual stresses are calculated using a formula. The method is similar to the Janson method.
The Layer Removal and Subsequent Slitting-Method (LRSS) is one of the most commonly used methods for plastic pipes. In this method, the tangential residual stresses are measured by means of deformation phenomena. With the LRSS method, material layers of different thicknesses are removed from a defined number of samples. The specimens are then cut open in the axial direction. The deformation caused by the cut-out segment is measured and recorded. The wall thickness can be changed either by turning off the outer surface or by turning off the inner surface of the pipe.
Drill hole method:
The borehole method is a widely used relaxation method for the determination of residual stresses. The application is reliable for a wide variety of sample types and leads quickly to results. Furthermore, the method causes only marginal damage to the sample. The procedure and the measuring procedure are standardized according to ASTM E837. Both axial and tangential residual stresses can be measured with the borehole method. The residual stresses are determined by drilling a hole (small diameter) into the pipe surface and the resulting changes in strain. The strain changes of the surrounding surface are measured with the aid of strain gauge rosettes. In recent years, however, more and more optical techniques have been used to record the elongation phenomena. Through a bore with an incremental depth increase, the stress distribution can be measured up to a depth in the order of the bore diameter. The residual stresses are calculated from the strains on the surface that increase with the depth of the hole.
Evaluation of the Janson measuring method
Statement about stress distribution
It must be distinguished whether a real residual stress distribution is measured or a linear residual stress distribution is assumed. The Janson method assumes a linear residual stress distribution. This means that residual compressive stresses are present on the outer surface, which decrease linearly within the pipe wall and then change into resi
dual tensile stresses up to the pipe
It is obvious that this linear residual stress profile represents a simplified assumption. In reality, residual stress profiles are formed which show a non-linear course, as shown, for example, in the following figure from a simulation:
The calculation of absolute values for the maximum residual stresses that occur with the Janson method is therefore highly error-prone.
Effort and application
Users describe the execu
tion of the Janson test as complicated and not very reproducible. This is often problematic because residual stresses are often used as a quality criterion.
1. saw cut
Residual stresses are reduced as soon as the saw cut is made in order to remove the axial strip. This effect can be more or less pronounced depending on how the saw cut is performed, which leads to falsified measurement results.
If the removed strip is not wide enough, both edges will move towards each other and may collide. This interrupts the deformation and distorts the measurement result. With thin-walled pipes, it is possible to guide the two butt edges past each other. Afterwards, however, the two pipe surfaces rub against each other, which also leads to a distorted measurement result. The alternative of cutting out a wider axial strip is not recommended, as it makes it more difficult to measure the distance between the marks to be moved, which also leads to measurement errors.
Deformations occur during the saw cut. However, these are undefined and cannot be included in the Janson test. This also makes it more difficult to reproduce the measurement results and leads to errors.Deformations occur during the saw cut. However, these are undefined and cannot be included in the Janson test. This also makes it more difficult to reproduce the measurement results and leads to errors.
The Janson test is widely used in practice, but has various problems:
- Assumption of a linear residual stress curve within the pipe wall
- No statement about the voltage distribution possible
- Difficult handling of the test procedure, resulting in poor reproducibility
Various efforts are currently being made to develop optimised test procedures.
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