JEC COMPOSITES MAGAZINE - Issue #107 - August/September 2016 - 64

TECHNOLOGY software
Tab. 1: Definition of the damage indicators associated to the NDT techniques used

NDT technique
Acoustic emission AE)

Associated damage indicator
NEA(σi) is the number of acoustic events accumulated for each stress ramp to σi

Passive IR thermography
(IRT-p)

Km(σi) is the thermoelastic coefficient calculated for each stress ramp to σi

Active IR thermography
(TIR-a)

T(σi) is the cooling rate calculated at each stress level σi

Bulk pulse-echo
longitudinal waves (LW)

Guided waves
(GW)

A(σi) is the maximum amplitude of the wave at each stress level σi
E33(σi) is the dynamic elastic modulus in the thickness direction at each stress level σi
SE(σi) is the spectral energy of the S0 mode at each stress level σi
E11(σi) is the dynamic elastic modulus in the axial direction at each stress level σi

ing in multiple scatters that certainly weaken the signals.
A global characterization of the material damage and its quantitative
monitoring under different mechanical loading further requires the
calculation of certain damage indicators.

Definition and analysis of damage indicators
Table 1 defines seven damage indicators (dimensionless variables normalized between 0 and 1) representing a relative variation of certain
physical characteristics of the material obtained from the five NDT
techniques.
Figure 6 shows the evolution of the previously defined damage indicators versus applied stresses. Each damage indicator value corresponds
to the mean of five measurements obtained on five samples with a
maximum standard deviation of 1%. These damage indicator evolutions indicate the damage initiation from the same stress value (50
MPa), which can be considered as a damage threshold of the tested
material. They also indicate that damage increases as the stress increases, but with different sensitivities, which are due to the anisotropic and
multi-scale features of the induced damage.
Actually, the GW used in the axial direction is more sensitive to the
damage that occurs parallel to the loading direction. This axial dam-

age was evaluated by measuring both the variations of the spectrum
energy (DGW-SE) and the axial elastic modulus (DGW-E11), which reach
respectively 100% and 60% at the material failure. The LW propagating in the through-thickness direction examines the material perpendicularly to the loading direction, where the variation due to the elastic
constant (E33) versus the applied stress is very small [12]. Therefore,
this small variation may not reflect the real extent of the induced damage. However, the measured attenuation of the LW in the thickness
direction becomes increasingly higher as the stress increases. This attenuation could be due to multiple reflections of the ultrasonic beam
with the discontinuities created in the material, which grow more and
more as the stress increases. Therefore, the defined damage indicator,
DLW-A associated with the maximum back-wall amplitude variation of
the LW, allows a more realistic evaluation of the damage induced in
the thickness direction.
On the other hand, the AE sensors can detect signals coming from any
direction and take into account all damage mechanisms from the microscopic (matrix cracking, matrix/fibre debonding, fibre breakage)
to the macroscopic (delamination) scale. These are the reasons why
the associated damage indicator (DAE) is higher.
At last, the thermography measurements are exploited differently
when compared with previous conventional analyses (based on thermal images or temperature variation curves). Concerning passive
thermography, the thermoelastic coefficient (K) is measured as well as
its variation for each applied stress interval of 50 MPa up to failure. The
associated damage indicator (DIRT-p) increases continuously when the
stress increases. Concerning the active infrared thermography measurement, the associated damage indicator (DIRT-a) also increases continuously when the stress level increases. This means that the material
cools more and more quickly when the damage increases.

Conclusion

Fig. 6: Evolution of the damage indicators versus applied stresses

The proposed hybrid NDT approach makes it possible to accurately
characterize the mechanical damage of composite materials. Its benefit is a more efficient interpretation of the damage characteristics and
their cross-validation in order to increase the reliability of the monitoring strategy, based on an original definition of damage indicators
that provides a quantitative evaluation of the material's damage state.

jec composites magazine / No107 August - September 2016

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #107 - August/September 2016