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Fractographic Characterization of Pipe and Tubing Failures

This paper was originally published and presented at The Society of Plastics Engineers’ ANTEC in 2011.

Abstract

Plastic piping systems are an important commercial product used in a wide variety of applications.  Because of the diversity of applications and wide range of material used to produce pipes, many different types of failures can result in service.  Evaluating these failures through a systematic analysis program allows an assessment of how and why the pipes failed.  An essential portion of the failure analysis process is the fractographic examination, which provides information about the crack origin location, and the crack initiation and extension modes.  The focus of this investigation was to characterize the surfaces of intentionally cracked laboratory samples in order to gain a more thorough understanding of pipe fracture mechanisms. This paper will document some of the key fracture features associated with overload of various materials used to produce commercial piping systems.

Background

Failure within components can be defined a number of ways.  Most commonly, one of three classifications is used:

  • The part has become completely inoperable
  • The part is operable, but is not fully functional
  • The part is functional, but is unreliable or unsafe

For the purpose of this evaluation, failure has been defined as the presence of a crack or rupture in the pipe that would disrupt the flow of the contents or allow the contents to unexpectedly exit the pipe.  This most closely resembles the first definition above.

While each pipe failure is unique, cracking is simply a response to stresses placed on the part, both from internal and external sources.  A crack forms within the pipe as a response to relieve the stresses.  This can happen in one of two general ways, ductile or brittle fracture.  Ductile fracture is a bulk response of the polymer.  As part of the ductile failure mechanism, yielding takes place.  Yielding represents the large scale rearrangement of the molecular structure of the material as a response to relatively high stresses. Given sufficiently high stresses, cracking occurs through disentanglement.  Conversely, brittle fracture is a localized response of the polymer.  Like ductile failure, cracking takes place through disentanglement.  However, the cracking is related to molecular reorganization without yielding.

The exertion of mechanical loads, both internal and external, induces stresses within the pipe.  If these stresses are sufficient, cracking will occur.  The type of failure is a function of a number of parameters, including:

  • The pipe material
  • Source of the stress – internal vs. external
  • Magnitude of the force
  • Force loading angle
  • Pipe geometry
  • Pipe support

The principle stresses that act on a pipe are the result of tensile and compression loading, as illustrated in Figure 1. Often this loading results in bending, a mixed mode including both tensile and compressive stresses.  The third type of stress, shear, is usually not applicable to pipe, with the possible exception of twisting.

Types of pipe stress

Figure 1 – Pipe loading demonstrating tensile stress (a), compressive stress (b), and bending stress (c).

Considering simply the implications of internal pressure, pipe is adequately modeled as a thin wall cylinder.  In this case, there are three principal stresses on the pipe wall, the shell of the cylinder.  These are circumferential or hoop stress, longitudinal stress, and radial stress. The cylinder shown in Figure 2 has an internal diameter d and a wall thickness of t. If the applied internal pressure is p then the hoop stress is represented by F1, the longitudinal stress by F2, and the radial stress by F3.

Pipe axial hoop stress

Figure 2 – The three principal stresses on the pipe wall are illustrated.

Using the approximation of the pipe as a thin wall cylinder, the following equations define the hoop stress and the longitudinal stress:

F1 = p x d / 2t                                                      (1)

F2 = p x d / 4t                                                      (2)

Where F1 is the hoop stress, F2 is the longitudinal stress, p is the internal pressure, d is the internal pipe diameter, and t is the pipe wall thickness.   Under these conditions F3, the radial stress is generally considered negligible. (Ref. 1)

Based upon the stresses exerted on the pipe, the crack will have a longitudinal or circumferential orientation, as illustrated in Figure 3.Transverse longitudinal cracking pipe

Figure 3 – Longitudinal cracking (a) and  circumferential cracking (b) are illustrated within a pipe.Cracking represents the partial fracture of a solid material.  This results in the creation of two mating fracture surfaces.  The examination and interpretation of the features present on the fracture surfaces is a discipline known as fractography.  Fractographic studies involve a combination of visual, microscopic and scanning electron microscopic (SEM) examinations. A thorough understanding of the mechanisms of pipe failure is important.  Fractography is used to characterize the mode of the failure and can provide invaluable information regarding the stresses and conditions leading to the failure.  In this investigation, commercial pipe samples fabricated from different materials were stressed through overload to intentionally create laboratory failures.  A fractographic examination was subsequently conducted in order to understand and document the resulting fracture surface features. This study was conducted to further the understanding of the failure mechanisms routinely observed with plastic pipes.

Experimental

Commercially available pipe, tubing and hose samples representing a range of materials were selected for the investigation.  Each sample was mechanically tested by applying high pressure with water until failure.  High pressures were achieved by using a mechanical pump, which is capable of producing pressures up to approximately 3000 psi by hand. All failures occurred within 3 minutes of the initial application of pressure with the pump. The prepared fracture surfaces were examined using a Keyence digital microscope.  The materials examined as part of this study included:

  • Crosslinked Polyethylene (PEX) Pipe
  • Polyethylene (PE) Tubing
  • Chlorinated Poly(vinyl chloride) (CPVC) Pipe
  • Poly(vinyl chloride) (PVC) Pipe

Download the PDF of the complete original paper.

 

Jeffrey A. Jansen

Jeffrey A. Jansen is the Engineering Manager and a Partner at The Madison Group. He was elected as a Fellow of the Society by the Society of Plastics Engineers. Jeff is a proven plastic professional with more than 30 years of experience solving problems and addressing opportunities related to polymeric materials. He specializes in failure analysis, material identification and selection, as well as compatibility, aging, and lifetime prediction studies for thermoplastic materials. Jeff has performed over 5,000 investigations, both for industrial clients and as a part of litigation. He regularly presents seminars and webinars, covering a wide range of topics related to plastics failure, material performance, testing, and polymer technology. Jeff is a graduate of Carroll College and the Milwaukee School of Engineering.