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Common types of defects

 

Common defects can typically be identified within two main categories, manufacturing flaws and in - service damage. Using Non Destructive Inspection methods it is usually straightforward to determine the diference between something like a manufacturing void versus an impact delamination.

 

Typical Manufacturing flaws include:

 

Inclusions (Also known as FO), Foreign material  within the laminate, e.g. pre-preg backing paper, peel ply or other foreign object, accidentally included in material during manufacture, can promote delamination/unbonds and significantly reduce mechanical properties.

Improper fibre splicing/ply joining - Insufficient ply overlap at ply joins, creates non uniform load paths and internal or external cracks

Fibre wrinkling/kinking/defects - Individual fibers not aligned with the load path, can form during layup i.e. tight radii corners, poor layup techniques, uneven consolidation, material inability to drape etc.

Voids - An inclusion of air between the laminate layers, can be caused by poor layup and compaction as well as contamination during mixing of resin or the cure cycle. Can act as stress concentrations and will have an effect on some of the mechanical properties i.e. lower transverse and through-thickness tensile, flexural, shear and compression strengths.

Porosity -  Similar to voids, except being very small in size and often more dispersed. Void content considered negligible if less than 1-2%.

Resin rich areas  Excess resin trapped within the plies, can cause stress concentrations and reduce mechanical properties. Caused by inadequate resin bleed and/or non uniform compaction.

Fibre misalignment, distortion - As with fibre wrinkling, especially prevalent at sudden section change, such as inserts.

Fibre breakage - Individual fibers broken, either due to cutting or can be caused by excessive fibre curvature at sharp radii corners.

Unbonds  Plies or components not bonded, often due to contamination, poor surface treatment or poor consolidation or as a result of an inclusion. 

Imperfections due to machining  Delamination and fibre breakage caused by operations such as hole drilling and trimming. Can also introduce defects such interlaminar cracking and unbonds if not performed correctly.

 

Common in service damage includes:

 

Delaminations Separation of the individual plies within the laminate. This type of defect typically occurs in-service due to impacts or crush type loads, and can have a severe detrimental effect on mechanical properties, particularly in compression.

Fibre breakage - Fibres broken from overload, more common with higher modulus fibre grades.

Cracks - Typically caused by fibre breakage or manufacturing flaws such as wrinkling as well as resin rich area.

Disbonds - A bonding failure due to overload or reduced strength join from a manufacturing flaw such as a void, poor surface treatment, poor joint design etc.

Resin Microcracking - Overload of the resin or thermal overload leading to cracking within the resin matrix.

BVID - Barely Visible Impact Damage is a low energy impact leading to delamination without significant surface indication of damage.