JEC COMPOSITES MAGAZINE - Issue #98 - July/August 2015 - 48

fractography

Fig. 7: Vertical stabilizer after recovery from Jamaica Bay (Courtesy of NTSB/
NASA) [3]

Fig. 5: Mode II fracture morphology illustrating the formation and inclination of cusps

Fig. 6: Mixed Mode I/II fracture morphology illustrating riverlines and textured
microflow (Mode I) and cusps (Mode II) features

by contamination or by improper handling of the fractured
components.

Methodology
Fractographic analysis is a challenging and time-consuming
process given the large fracture surfaces generated and the
complexity of the mechanisms that occur during the failure of
polymer composites. The analyst is given a failed component
and the task to deduce the cause of failure. However, such a task
requires the analysis of a large amount of information to draw
scientifically sound conclusions. Considering the large area of
fractured surface generated during failure (much of which can
be secondary damage) and the fact that the task often entails
the analysis of large aircraft components, one can clearly see
the challenge faced by the investigator. Coupled with the fact
that one often has to provide results in a short time, it is of great
importance that the analyst follows a strict and highly efficient
procedure to minimize the time required and maximize the
collated information. Once the investigator has been presented
with the failed component, it is essential to identify the aim and
the objectives of the fractographic analysis and agree upon them
with the customer, who should supply the background information. Then the failed component(s) are documented and clearly
labelled (assembly diagrams, Figure 3) to identify the relative
location of the various parts within the component.
The component is visually inspected and photographed (usually

with a flatbed scanner or ordinary camera) to locate the damaged area and identify interesting features or anomalies. Prior to
dissection and exposure of the fractured surfaces, the investigator might employ non-destructive techniques (ultrasounds,
X-ray radiography, computed tomography) to glean information
regarding the extent and location of interior damage otherwise
invisible to the naked eye (Figure 4). Based on the visual and
non-destructive inspection, the component is carefully and
methodically dissected and then inspected using optical and
scanning electron microscopy to characterize the fractured
surfaces. The general approach in the fractographic analysis is
to characterize the various failure mechanisms in terms of morphology and direction, as well as their interactions, and make
observations regarding the composite microstructure.
The three types of failure in laminated composites (translaminar, intralaminar and interlaminar) exhibit different morphologies whilst the role the constituents (reinforcement and matrix)
play in the formation and morphology also differs. Whether
in a fibre-dominated or a matrix-dominated failure of the two
constituents in a polymer composite, it is usually the matrix that
reveals the direction of the crack growth and offers information
about the interaction between the failure mechanisms (Figure
5). Particularly in interlaminar failures (delaminations), the
morphology of the deformed matrix not only provides information about the delamination growth direction but also the
matrix toughness, crack speed, loading rate, moisture content
and processing, among others. In complex loading conditions,
for example such as Mixed Mode I/II, the analyst can approximate the mode-mixity based on the amount and morphology of
the Mode I and Mode II representative features evident at the
fractured surface; that is textured microflow and riverlines in
Mode I and cusps in Mode II (Figure 6).
Once the analyst has completed the microscopic inspection
of the fractured surfaces, the analysis of all the collated information (background, findings from visual/non-destructive
inspection and microscopy, etc.) commences with the aim to
deduce the sequence of events and identify the source of failure.
The results are finally discussed with the customer and a report
with the detailed fractographic analysis is delivered.

48 jec composites magazine / No98 July - August 2015
Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #98 - July/August 2015
