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The concept of integrating sensing into a material was birthed from the
fields of biomimetics, multifunctional materials and structural health
monitoring. Biomimetics is "the application of methods and systems
found in nature to the study and design of engineering systems and modern
technology." [1] Meanwhile, the field of multifunctional materials
seeks to give an engineering material additional functionalities; examples
include structural materials with energy harvesting , or permittivity
tuning capabilities. Finally, structural health monitoring looks monitor
the health of a structure or vehicle through the use of not only sensors,
but also computation and analysis.
In designing composites with integrated sensing we look to incorporate
both the sensors and the computation inside the composite in a distributed
addressable network analogous to that which can be found in an animal
nervous system. This demands investigation into several fields of engineering:
mechanical, electrical and computational.
Adding sensing to a composite material such that the end product acts
as a material with a nervous system inherently involves placing a "nervous
system" inside the host composit. In this case our "nervous
system" is - in fact - sensors and microcontrollers which will act
as inclusions and stress concentrators in the host composite. Damage induced
by the peak values of stress concentration around the embedded devices
is, in fact, one of the main concerns. To assess this and related issues,
we have fabricated a series of samples with and without embedded (dummy)
sensors and micro-processors in S2 glass fiber/epoxy laminates, and systematically
tested the samples while continuously monitoring the response by the acoustic
emission technique (for tensile tests only). In this manner we have sought
to address the process of damage initiation and evolution within the material.
Ideally, embedding geometries and practices can be optimized to limit
the detrimental impact on the composites structural properties.
For the purpose of our research we are using a unidirectional S2 glass
- epoxy resin matrix prepreg for our host material. The reasons for the
choice as opposed to a carbon fiber composite stem from cost and processing
considerations and the fact the failure modes and acoustic wave propagation
are similar enough between the two for corresponding geometries and loading
conditions.
Short beam shear tests are performed in accordance with ASTM D 2344 standards
and tested in a MTS model 309.20 load frame with 22.2 kN load cell.
Tests are performed in displacement control with crosshead rate of 1.25
mm/min with failure consisting of a 30% load drop-off, two-piece specimen
failure or excessive travel. Six specimen of each configuration (shown
below) were used.
Fatigue was performed using a 10 Hz sinusoidal input function with a
stress ratio of R = 0.1. Failure determined by excessive travel ( travel
> 2.54 mm).
Fatigue short beam shear tests indicate that the host composite material
is very sensitive to the local stress concentration introduced by the
embedment as it results in a decreased fatigue life. Additionally, it
has been found that the geometries involved have a profound effect on
the fatigue life of our host composite.
Fatigue results for various geometrical inclusion considerations.
Tensile tests were performed under quasi-static and fatigue characterization
using MTS load frame with hydraulic grips on dog-bone shaped samples.
An acoustic emission system - Physical Acoustic Corporation's PCI-2, R50
and micro30 sensors - is used to monitor matrix cracking, delaminations
and fiber breakage events and their locations. The material properties
were assessed testing unidirectional [0]n, [90]n, [±45]n laminates
instrumented with strain gages to measure the transversal and longitudinal
strains. For a better understanding of the acoustic emission results,
as well as for assessing the early damage initiation in laminates with
integrated dummy sensors we dedicated part of our efforts in exploring
the material microstructure. Standard optical metallographic techniques
allowed the characterization of void size, percentage and distribution
of flaws as well as the analysis of the sensor-matrix resin interface.
Typical failure of [45]5s samples with integrated
dummy sensors.
Micro-crack initiation at sensor and resin pocket
eyelet in [0] to 5% strain.
The outcome of this research highlights a significant difference of the
mechanical behavior, damage growth and location in samples with integrated
implants and different lay ups. Overall, the data acquired by testing
different laminates is consistent and indicates similar trends in all
the experiments. Some of the major results follow.
The failure mechanism initiates at the dummy sensor location in [0]n ,
[90]n and [45]n laminates. Stress concentration due to the presence of
material and geometrical discontinuities is certainly responsible for
early micro-cracks events around the embedded device. In [0]n the micro-damage
propagates freely along the fiber orientation within the resin matrix.
Nevertheless, the material tensile strength seems not to be compromised.
A net cross section failure at the sensor location was instead observed
in [90]n laminates with a significant reduction of the material overall
strength (24%). Finally, [45]n laminates with embedded chip resistors
showed a different failure behavior. Although subjected to shear loading
and in presence of stress concentration areas at the dummy sensor-resin
matrix interfaces, the failure of the sample never occurs at the sensor
location.
The aforementioned conclusions demonstrate that non-zero fiber orientations
seem more suitable for the embedment of sensors and devices within glass/fiber
laminates made by prepreg material, although the laminate lay up optimization
for bearing the desired loads is, of course, needed. Furthermore, the
damage initiation detected in samples with embedded devices has been always
identified at the sensor location and characterized by high values of
the signals amplitude so that the enhancement of the embedment and manufacture
techniques will be of paramount importance for the final material performance.
This aspect of our research can be clearly divided into three camps:
mechanical modeling, network design & modeling, and algorithms. Mechanical
modeling is necessary because without it we have no real understanding
of the physical principals at play in our system and experiments. Network
design and modeling must be considered carefully to ensure that the distributed
network is robust, efficient and performs its functions as expected. Finally,
algorithms are what takes sensor data and turns it into a quantification
of the health of the composite material.
Embedding micro-sensors in composite laminates produces material discontinuity
around the inclusions. This in turn produces stress concentrations at
or near the inclusions. Both 2D plane strain and 3D FEM models have been
developed to analyze the stress/strain state surrounding the embedded
micro-sensors within a unidirectional composite laminate. The objective
of the present numerical effort is to take into account the observed resin-rich
areas caused by embedment, and to determine their effects on the local
stress field around the embedment and the corresponding potential failure
modes.
Micrograph of a section of S-glass/epoxy composite
laminate with embedded simulated micro-sensor
with local finite element mesh of 2D FEM model overlayed.
From the output strain field, it appears that the maximum longitudinal
and shear strains occur at the sensor corners within the resin-coating
interface (resin rich region). Among the three strain components, the
shear strain has the maximum value. Since the neat epoxy resin can carry
the least strain among all the material components in the composite, shear
debonding at the resin-sensor coating interface is expected to be the
main cause of failure initiation.
The three stress components along the resin-composite and resin-sensor
coating interfaces generally have the same trend. However, in the latter
case, the magnitude of stress is higher. The maximum longitudinal and
transverse stresses occur at the sensor corner area. At the end of the
resin pocket, the transverse stress increases. However, this small increase
is not enough to cause fiber-matrix splitting. The shear stresses along
both interfaces show sharp rises at the sensor corners, decreasing monotonically
away from the singular point.
Based on the stress distribution results from, the values of the stress
applied at the far end that cause the initiation of the failure were calculated
both for the composite area and the neat resin area separately.
For the composite domain, maximum stress criterion is applied. Failure
is expected to occur when at least one stress component along one of the
principal material axes reach its corresponding strength. The criterion
is defined as follows:
- Tension failure
- Compression failure
- Shear failure
Here, the subscripts 1 and 2 refer to principal material axes of the composite
material. For the neat resin area (including the transition resin layer
and the resin pocket), von Mises criterion is applied. Failure is expected
to occur when the stress components satisfy the chosen failure criterion.
The von Mises criterion is defined as follows:
Under tensile loads, the initial failure is expected to be matrix cracking at the sensor corners in the resin-sensor coating interface at an
early stage of the loading process.
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[1] Bionics, Wikipedia
Last Date Modified: October 3, 2007
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