Understanding Damage mechanisms in FRP

October 2, 2017

Preventing failures and optimizing the service life of any structure is the result of a balance between the service loads applied and the actual capacity of the structure. This is the basis of Fitness for Service or Suitability for Service evaluation.

Failure modes relate only to the way that failure appears to the naked eye. They are observed through visual inspection.

Damage mechanisms are the underlying processes and changes that lead to the appearance of a failure mode. They cannot usually be detected with the naked eye and often require techniques such as non-destructive evaluation and analytical techniques involving destructive testing, including microscopy.

For fiberglass reinforced plastic (FRP), there are 3 damage mechanisms that can occur:

  • Damage to the matrix, or resin
  • Damage to the reinforcement fibers
  • Damage to the interface of the matrix

These can be expanded to identify that both mechanical and chemical forces play a role in the damage.

Creep Rupture

A common result of these damage mechanisms that leads to failure of FRP is known as “Creep Rupture”.

The damage mechanisms identified all relate to constituents of FRP that combine to give it strength and stiffness. For this reason, changes in stiffness, also known as creep, will generally result from damage.

The relationship of damage mechanisms, common failure modes and laminate flexural modulus are shown in Figure 1. Creep is also observed in some qualification tests, such as for developing the pressure rating of pipe. At this writing, it is not possible to a priori quantify the relationship of each damage mechanism to the changes in stiffness, and it is used as a proxy to indicate progression of bulk damage from accumulated damage mechanisms.  In some cases where the resin has been damaged, such as in reduction of the glass transition temperature, the damage does not immediately show as creep.

Laminate stiffness

Changes to the matrix or resin that appear to conventional, visual inspection as corrosion barrier damage might also affect the laminate stiffness.  Damage to the corrosion barrier can affect stiffness, usually in proportion to the thickness of the corrosion barrier. UltraAnalytix® non-destructive inspection has found that damage to corrosion barriers is often detected and quantified before visual inspection.

Sometimes, these detections serve to reduce the stiffness values calculated, thus providing conservative values to owners.

Adhesive bonds are commonly used to join FRP structures (secondary bonds). They are used for joining shell and pipe sections as well as most repairs. The post-processing for UltraAnalytix will reliably provide information on the condition of these bonds. Several third parties have verified UltraAnalytix analysis of several types of joints: FRP-to-FRP joints such as for pipe, reinforcements to FRP tanks and structures, FRP and carbon fiber applied to steel and repairs to FRP.

Learn more about UltraAnalytix non-destructive inspection for FRP assets.

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