Failure Analysis and Fractography of composite materials

Causes of failure(Root cause)

Fracture Modes

Fractography

General procedure
(key steps to be done in failure analysis)

1)Design Failures


-Design is not able to sustain some test performed due to the poor design

2)Manufacturing Defects


-Properties that do not meet requirements due to the defects

3)In-Service anomalies


-Service conditions that is outside design parameters

Intralaminar Failure
-Cracks that happens within the laminates

Translaminar Failure
-cut(across) through the ply

Interlaminar Failure (Delamination)
-Cracks formed in-between the ply

1)Collate background information


2)Visual inspection(provides information about fracture modes)


3) Nondestructive Testing( determined to prevent the destruction of important information during dissection)


4)Photography(visual evidence of the defect)


5)Dissection(cutting small samples to look under the microscope and determine the defects)


6)Verification of materials and construction


7)Optical Microscopy


8)Electron Microscopy


Microscopy

1)Optical microscopy (using light)
-can only see the surface of the material

2)Scanning electron microscopy (SEM) (using electrons)
-Can view the material in-depth
-More powerful
-Higher magnification

-Examination and interpretation of fracture surfaces.
-Provides information/location about the causes of failure


Crack Path Sequencing
-Determines the crack path sequence.(example which cracks appears first and why the cracks stops)

Fiber dominated failures

1)Tension failure

  • Can be identified by the radial lines (spread outlines)
    -Rougher surface is at he endpoint
    -Can be used to determine the crack growth direction

2)Compression failure
-Caused by micro bulking of fibers

3)Flexural failures
-Are characterized by the presence of zones of tensile and compressive failures.

Fracture morphology


-If fiber/matrix interface strength is weak, the degree of debonding will be high and broom-like failures result.


-If fiber/matrix interface strength is high, the degree of debonding will be low and a relatively smooth fracture surface is produced.

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Delamination dominated failures

Microflow & Scraps are formed when cracks meet

To determine the crack growth direction:
Microflow--> scrap

Scraps:
-Oriented parallel with the overall crack growth direction.

Microflow:
-Caused by alignment of the fracture features.

River lines

-Determines crack growth directions in delamination
-Small lines that converge intro one determines the direction of crack growth