Residual lifetime assessment of uPVC gas pipes

The Dutch gas distribution network consists of about 20% (22,500 km) of unplasticised poly(vinyl chloride) (uPVC) pipes, most of which have been installed from the mid-sixties up to the mid-seventies of the previous century and have been in service ever since. In the next decade the specified service lifetime of 50 years will be reached for these pipes. Replacing the uPVC gas pipes exactly after this specified service lifetime will lead to a costly and extremely labour intensive project. Postponing the replacement is only an option when this can be done without compromising the integrity of the network. It is therefore of great value for the network operators to have full knowledge on the condition of the pipes in their network. In this thesis the framework for a method that can determine the condition, and therewith the residual lifetime, of uPVC gas pipes is developed. Recent failure data shows that the majority of the failures in uPVC gas pipes is caused by excavation activities (third-party damage). The risk of life threatening situations after such a failure is considerably higher for a brittle fracture than for ductile failure behaviour of the pipe. Brittle uPVC gas pipes should therefore be replaced, which makes the impact behaviour the limiting factor for the service lifetime of these pipes. A review of the degradation mechanisms occurring during the lifetime of uPVC pipes shows that physical ageing is expected to be the most important mechanism that causes embrittlement. During physical ageing the polymer chains move towards their thermodynamically favoured positions, causing an increase in resistance against plastic deformation. Moreover the deformation behaviour localises, causing embrittlement on a macroscopic scale. The focus of this thesis is therefore on the influence of physical ageing on the mechanical behaviour of uPVC gas pipes. The procedure of determining the residual lifetime is based on these findings and is split into four aspects: the choice of a measure for the condition of the pipe material, characterisation of the change of the condition in time (its ageing kinetics), determining the critical condition and development of amethod ofmeasuring the current condition. Each of these aspects is individually described in consecutive chapters. The yield behaviour is selected as the measure for the condition of uPVC gas pipes, as the yield stress is a direct measure for the thermodynamic state (i.e. the age) and can also be linked to the impact behaviour of the material. The yield stress behaviour of uPVC is characterised using short-term tensile tests (at a wide range of strain rates and temperatures) in Chapter 1. The yield behaviour is accurately described by a pressure-modified Eyring relation that links the applied deformation rate to the yield stress and, vice versa, the applied stress to the plastic deformation rate. By hypothesising that failure occurs at a constant value of the accumulated plastic strain, the pressure-modified Eyring relation can be used to predict the failure time of loaded glassy polymers. This engineering approach is successfully applied to predict the time-to-failure of both polycarbonate and uPVC specimens. The predicted influence of stress level, temperature, loading geometry and thermal history of the specimens on the timeto- failure is in excellent quantitative agreement with experimentally obtained failure data. Furthermore, it is shown that the engineering approach can also be employed to predict the failure time for a pipe subjected to a constant internal pressure. This approach makes it possible to determine the long-term hydrostatic strength (LTHS) based on short-term tests only, and eliminates the necessity to carry out expensive long-term pressurised pipe tests (under the assumption that slow crack growth failure does not limit the LTHS). As already stated, physical ageing is expected to be the most important ageing process during the lifetime of uPVC gas pipes. The influence of physical ageing on the yield behaviour of uPVC is characterised and modelled in Chapter 2. The engineering approach presented in Chapter 1 is extended to include this ageing behaviour. The resulting approach is employed to predict experimentally obtained long-term failure data for tensile specimens and pipe segments subjected to a constant load. Some of these data sets reveal a so-called endurance limit caused by the ageing induced change in deformation behaviour. The good quantitative agreement between predictions and the experimental data is a strong indication that the physical ageing kinetics of the yield stress is described successfully. Moreover, the engineering approach is applied to the failure of tensile specimens subjected to a dynamic stress signal. The influence of both the frequency and the stress ratio of the signal has been proven to be correctly accounted for. The predictions of the failure times are rather conservative as the influence of physical ageing is somewhat underestimated for dynamic stress signals. At lower levels of dynamic stresses a second type of failure kinetics becomes apparent: fatigue crack growth failure. Preliminary results on fatigue crack growth failure show that slow crack growth failure can be ruled out as a limiting factor during the service life of uPVC gas pipes. These results confirm that physical ageing can indeed be expected to be the critical embrittlement process for uPVC gas pipes. The residual lifetime of the uPVC pipes can only be determined when the critical thermodynamic state at which the pipe should be taken out of service is known. As mentioned before, the fracture behaviour upon an impact load, such as those encountered during excavation activities, is a limiting factor for safe deployment of uPVC gas pipes. A direct relation between a critical yield stress and the ductile-to-brittle transition temperature (Td�¨b, a measure for the impact behaviour of the material) is hypothesised in Chapter 3. The influence of physical ageing on the Td�¨b of uPVC pipe material is determined employing instrumented falling weight tests on specimens taken from a uPVC water pipe at a range of thermodynamic states. The uPVC water pipe grade used in the experiments shows only a small increase in Td�¨b for the range of thermodynamic states investigated. The measured increase is in reasonable agreement with the prediction that follows from the proposed relation between the constant critical yield stress, Td�¨b and the ageing kinetics of this water pipe grade. Applying the ageing kinetics of the gas pipe grade, which differs significantly from that of the water pipe grade, shows that a more pronounced increase in Td�¨b can be expected from the gas pipe grade during its service life. This indicates that uPVC gas pipes can be expected to embrittle during their service life, as a result of physical ageing. The last aspect that is studied is in which way the condition of the pipes can be determined in a non-destructive way. Micro-indentation measurements were performed to probe the condition of uPVC pipes by relating the hardness that follows from the indentation curve with the yield stress of the material. The hardness proves to behave similarly as a function of time and temperature as the yield stress. In fact, a linear relation between hardness and yield stress is found within the range of thermodynamic states investigated. The rather low resolution of the lifetime assessment procedure is mainly caused by the scatter of the experimental data around the linear relation between the hardness and the yield stress. Decreasing the influence of local effects on the indentation measurement might decrease the scatter and improve the resolution of the procedure. With all four aspects of the lifetime assessment method considered, important steps towards a non-destructive condition measurement procedure for uPVC gas pipes are taken.