Damage is inevitable in plastic pipes that are used for long periods of time. The type of damage can be divided into brittle damage and ductile damage. Brittle damage is the absence of significant large plastic deformation near or in the broken section of the pipe. The deformation of the pipe before fracture is uniform, resulting in the pipe fracture quickly through the plane perpendicular to the direction of stress, the strain is not large, and the energy required to fracture is not large. In contrast, ductile failure has irreversible plastic deformation accompanied by much larger deformation. Fracture surface shape and fracture energy is the most important indicator to distinguish between brittle and ductile damage.
1. The theoretical basis of brittle damage:
Since it is impossible for all physically present material surfaces to be free of cracks and defects (e.g., surface scratches, internal inclusions, micropores, grain boundaries, phase interfaces, etc.), the fracture theory of fracture suggests that these cracks and defects cause stresses to concentrate at the tip of the crack well above the average stress to which the pipe is subjected. When it reaches and exceeds a critical condition, the crack will lose stability and expand, eventually causing the material to fracture at the lowest nominal stress.
2. Molecular theory:
The molecular theory of brittle damage considers the damage of a material as a relaxation process, and macroscopic fracture as a process of breaking of microscopic chemical bonds and thermal activation. That is, when the energy of the thermal movement of atoms exceeds the barriers between bound atoms, it causes the chemical bonds to dissociate. When the number of ruptured chemical bonds reaches a certain number, the material loses its load-bearing capacity and fracture occurs.
3. Brittle damage during the use of PVC- U pipes:
The main reasons for the brittle damage of PVC-U pipes in the process of use can be summarized as the following four points:
①Impact of air hammer:
When PVC-U pipe is used, it fails to exhaust the air in the pipeline or the exhaust valve is not set properly, when there is gas in the pipeline, it is easy to make the air pocket in the pipeline, although the air pocket in the pipeline can not directly cause water hammer, but it can cause harm with the help of water hammer.
Under normal conditions, the water flow in the pipeline can be approximated as a constant flow (pressure, flow rate, temperature remains unchanged). In this state, the gas in the water to be gradually precipitated, the formation of bubbles of varying sizes up to the pipe wall, bubbles according to the flow of water forward movement. In the uphill section, due to the role of buoyancy, the bubble flow rate may be greater than the water flow rate. Because the wall has a certain roughness, the bubble movement direction is the same, it is difficult to gather into a large bubble. Small bubbles along the wall of a certain width forward flow, after the highest point of the exhaust valve, the exhaust pipe diameter of the bubble has the conditions to discharge, while other bubbles by the thrust of the water flow downstream. Due to the turbulent flow at the pipe wall and flow speed and tangential characteristics, so that some small bubbles through the exhaust pipe across the exhaust hole also flow downstream.
Crossing the exhaust valve bubble downhill, the direction of movement and bubbles by the direction of the buoyancy force opposed to the direction of the combined force of this buoyancy force generated by the resistance, it is bound to slow down the bubble movement, the latter bubble is easy to hit the front bubble and all into a large bubble. Air bag can cause a pipe burst, is due to the rapid switch valve or pump start and stop, so that the pipeline has a large pressure increase, the compressible properties of gas, so that the stress is concentrated to the air bag to generate high pressure and burst.
②Effect of scratches:
The installation and transportation process causes scratches, knocks and other damage to the PVC-U pipe surface.
According to the crack theory of brittle damage, these scratches will make the stress concentrated at the tip of the scratch, and much higher than the average stress to which the pipe is subjected, when this stress reaches and exceeds a critical value, the crack will expand, and because PVC-U is a brittle material, the crack will expand quickly, causing brittle damage.
③Impact of water strike:
Water strike refers to the pressure transient process, is a special important phenomenon caused by the unstable flow in the pipeline. When for some reason caused by a sudden change in the flow rate in the pipeline, such as switching the valve too fast, a sudden power failure and stopping the pump, will cause a sudden change in pressure in the pipe, resulting in water strikes. Water strike is also known as water hammer. Usually, water hammer is caused by the following reasons in the water piping system:
(1) the shutdown of the pump
(2) water piping system pressure is too high, the valve, faucet suddenly closed. The physical reasons for the occurrence of water hammer phenomenon, mainly due to the liquid has inertia and compressibility. Valve closure generates a booster wave, booster wave transmitted to the pipeline upstream reflected back to the decompression wave and transmitted to the valve until the time required for 2L / a. L = pipe length; a = pressure wave speed.
When the valve closing time Tc < 2L / a, the earliest generated by the valve to the pipeline upstream propagation, and reflected back to the decompression wave, in the valve all closed when it has not reached the valve, then the valve may produce the maximum water impact pressure, known as the direct water impact.
When the valve closure time Tc>2L/a, reflected back from the upstream of the pipeline decompression wave will partially offset the water strike boost, so that the valve at the water strike pressure does not reach the direct water strike boost value, known as indirect water strike.
When the water strike pressure is large enough to cause brittle damage to PVC-U pipe, even if the pipe is not damaged, when the water strike occurs repeatedly, especially when the frequency is high, a large internal pressure is generated in the pipe at the same time, the fatigue load is also applied to the pipe.
④Effects of Dropping
When storing, transporting and installing PVC-U pipes, the throwing of the pipes can cause small cracks or defects in the pipes, where stress concentration can easily occur and lead to brittle damage of the pipes according to the crack theory of brittle damage.