Nondestructive Evaluation of Natural Fiber Reinforced ... · widespread acceptance in the North America automotive industry ... There are several factors that affect composite mechanical
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Nondestructive Evaluation of Natural Fiber Reinforced Composites: Material Properties and their Deterioration Industrial Research Chair in Applied Solid State Physics and Material Characterization Physics Department University of Windsor 8/4/2011 1 Elena Maeva, Inna Seviaryna, Dmitry Gavrilov, Jeff Sadler, Ganesh Venukadasula
Nondestructive Evaluation of Natural Fiber Reinforced Composites:
Material Properties and their Deterioration
Industrial Research Chair
in Applied Solid State Physics and Material Characterization
Physics Department
University of Windsor8/4/2011 1
Elena Maeva,Inna Seviaryna,Dmitry
Gavrilov, Jeff Sadler,
Ganesh
Venukadasula
Biocomposites
in Automotive Industry
Biofibre
reinforced composites gain widespread acceptance in the North America automotive industry.
Jute, hemp, flax and other biofibresAdvantages of biocomposites:
– Renewable resources for row materilas– Biodegradable or recyclable– Shift from petroleum‐based to bio‐based
materials– Reduced weight compared to synthetic
composites
8/4/2011 2
Defects in Biocomposites
Porosities Microcracks
Structural delaminationsInhomogenities
8/4/2011 3
Presenter
Presentation Notes
Throughout their life cycle, composites are susceptible to the formation of many possible defects, primarily due to their multiple-step production process, nonhomogeneous nature, and brittle matrix. Composite mechanical damage is typically in the form of delaminations or disbonds , broken fibers due to impact, fatigue damage that affects the zone of composite material via micro cracking, fiber delaminations, fiber breaks and overall loss of mechanical modulus, or can be caused by thermal damage from prolonged exposure to heat above resin cure temperatures as well as combination of effects due to extreme operational conditions. Because of composite materials complexity, complexity of the part geometry and often a limited part access, materials damage and materials condition sensing cannot be achieved via conventional NDT methodology. Of all nondestructive methods, only ultrasonic methods are directly sensitive to mechanical changes and can be used to directly assess the mechanical condition and integrity of the composite structure. Majority of NDT methods are based on and originated from metals experience. Many current test procedures are inadequately developed to properly and directly tackle the composite structural issues.
Biocomposite Degradation
Solar irradiation Temperature Humidity
Biodegradation
Stress Fatigue
The deterioration of a
material depends on its
resistance to all parameters.
The deterioration of a
material depends on its
resistance to all parameters. 8/4/2011 4
Presenter
Presentation Notes
There are several factors that affect composite mechanical performance. Environmental-solar irradiation (UV), temperature, water and bio-organisms. Also stress and fatigue cause some material deterioration as well.
Depth sensing indentation technique consists of printing an impression on the material surface by applying a known load with an indenter of known geometry and subsequently analyzing the load vs. displacement data. Hardness is defined as the resistance of a material to penetration by another, harder material. Modulus is a direct property of a material (unlike hardness), so data may be easily compared between samples. Ability of method responses of microscopic regions can be a key to understanding mechanical behavior of material systems.
BiocompositesBiocomposites
Degradation Degradation Material’s Properties
Tensile Test
Dynamic Microhardness
Presenter
Presentation Notes
During aging, materials strength decreases while elastic modulus and microhardness increases.
Matrix or Fibres?Matrix or Fibres?PP matrix fibr
e
Side 1 (UV, temperature,
moisture)
Side 2 (temperature, moisture)
PP matrix Fibre
Presenter
Presentation Notes
To evaluate how fibers and polymer matrix were affected by UV, we have measured microhardness (MH) of both of them. Micro hardness of polypropylene matrix and biofiber (switchgrass) were measured. Right graph shows results for fiber. MH and modulus have not changed during the exposure (300 hrs). Left graph shows results for matrix. As we can see, properties of polymer matrix have changes. Thus, reduction in mechanical stress is mostly due to matrix degradation. Side 1 of the sample exposed to UV and moisture has more significant damage and change in properties than another side. As we can see from the graph, the contribution of UV/Temp/moisture to change on property is more significant than temp/ moisture only.
Thermography
The main idea of thermographic
methods is extracting data from
the dynamics of heating and cooling down processes for the
sample excited with external heat impact.
FLIR SC4000 thermal imager(up to tens kHz frame grabbing
speed thermal sensitivity ~tenths of C).
Presenter
Presentation Notes
Since the processing methods (e.g. injection, extrusion, compression) for natural fiber–thermoplastic composites are all based on heating, understanding the thermal properties of composites at processing temperature is very important for future studies on controlling and optimizing the manufacturing process. One thermal property used in the study of heat transfer in injection molding process is the thermal diffusivity.
Thermography
Heat pulse
Flash methodthermal diffusivity (cm2/s) is a
function of its thermal
conductivity (k), specific heat
capacity (Cp
) and density ρ.
L is sample thicknesst1/2
is time required for the sample
back surface to reach half of the
maximum temperature rise.
Presenter
Presentation Notes
The main idea of all thermographic methods is extracting data from the dynamics of cooling down process for the sample excited with external heat impact. The dynamics is to be studied on the sequence of snapshots, captured by thermal imager with necessary frame rate. The keystone of the method is that the cooling rate of sound area is faster than those of defective areas. The difference between these dynamics (thermal contrast) can give the information on location and shape of the defect.
Thermal Diffusivity
Problems in Acoustic NDE of Biocomposites
Nonhomogeneous characteristics of biocomposites
pose significant challenges for defect detection:
Uneven surface of fibres cause ultrasound
scattering
High concentration of fibres leads to high
acoustic attenuation coefficient
Close acoustic properties of fibres and matrix
result in low reflection coefficient and low image
contrast
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fiber
matri
x
2
1ln21
AA
h
thv 2
Basic acoustic scaling factor
to determines thickness of
material, location of flaws,
location of interfaces
Relates to material's stiffness
Energy Loss (heat, scattering)
Relates to acoustic parameters and size of
small scatterers
like fibres in biomaterials
Relates to damping properties of the
material
Sound velocity Attenuation
Acoustic Method
vht
velocity of ultrasonic sound
Thickness of sample
Time of flight
Ahα
Amplitude of the signal
Thickness of the sample
Attenuation
time
amplitude
transducer sampleelectronics
tA1 A2
12
mechanical scanner PC
pulser-receiver
C-scan plane B-scan plane
sample
acoustical lens
Acoustic Images
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Internal Structure of BioInternal Structure of Bio‐‐ CompositesComposites
0 %
30%
40%
50%
Vertical cross‐section Horizontal cross‐section
Non‐uniform internal
structure.
Darker areas indicate
regions with fiber clusters or
trapped air (microvoids).
Increase of filler content in
material composition
corresponds to an increase of
darker areas on the acoustic
images.
Non‐uniform internal
structure.
Darker areas indicate
regions with fiber clusters or
trapped air (microvoids).
Increase of filler content in
material composition
corresponds to an increase of
darker areas on the acoustic
images.
Short FiberShort Fiber‐‐Reinforced Composites. Reinforced Composites. Wheat straw filler in PP/ 5% clay/2% MAPP
10 MHz frequency.
5 mm
Presenter
Presentation Notes
The images of composites with wheat straw filler have more complex and non-uniform internal structure. Some darker areas are present; they indicate regions with higher ultrasound attenuation. That may be caused by fiber clusters, presence of the fiber particles with larger dimensions or presence of trapped air (microvoids). The frequency used for scanning does not allow us to distinguish between these factors. Increase of filler content in material composition corresponds to an increase of darker areas on the acoustic images. This observation correlates with the increase in attenuation with more filler content in composite shown on graphs. The longitudinal sound velocity gradually decreases with adding the clay into PP and increasing the wheat straw fiber content. This can be attributed to an increase of the material’s heterogeneity and a decrease of its elasticity and/or crystallinity. Adding 30% of wheat straw to the PP increases composite’s attenuation more than two times. Further increase in fiber content up to 40% leads to a slight rise in attenuation and no differences are shown in attenuation containing 40 and 50% of wheat straw.
Acoustic microscopy allow to visualize the morphology on the surface of the sample and internal structure of the composite. Left column-fibers and clay inclusions are visible. Middle column- cracks at the interface between fiber and matrix are formed after mechanical damage of the material. Right column- sufrface of the sample is damaged but no significant changes inside.
Monitoring of Biocomposite Degradation
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Biocomposite
was weathered in an environmental chamber
for 0 hrs. 100hrs.
300 hrs.
3D reconstruction of the microcracks
distribution in the volume of
the biocomposite
exposed to accelerated weathering.
10x10 mm scanning area. 10 MHz frequency.
surface
Presenter
Presentation Notes
UV has the strongest influence on changes in weathered material. Images show change in structure of the composite during exposure to UV. Number of pores and micro cracks grows (White spots) .
0 hr
Changes in Biocomposites
during water absorption
24 hr 120hr 312hr
Brightness of the image pixel depends on the how the properties
of the
material (density and elastic modulus) differ from water properties (the more
difference the brighter image)Plain surface of the biocompsite
at the beginningFibers starts to expand out of matrix with increase in immersion
time
200 μm
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Acoustic images of the surface
Time of exposure
Presenter
Presentation Notes
Presence of water lead to swelling of reinforcements, which may develop stress at the interface and cause micro-cracking around swollen fibers. Other effect may also occur in the matrix structure like chain reorientation an d shrinkage. Water absorption also contribute to the loss of compatibilization between the fibers and the matrix as new hydrogen bonds are formed between water molecules and hydrophilic composites of the fibers. Smooth surface of the composite at the beginning Fibers begin to expand with time out of matrix
Water Absorption
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Biocomposites
with different fiber content
0%
20%
30%
40%
50%Increase in weight, %
Time, days
Presenter
Presentation Notes
Water absorption behavior of natural fiber reinforce composites receives attention because of its practical and theoretical importance. Water absorption behavior has effect on the physical and mechanical properties. As well as the fiber/matrix interference resulting in change of dimensional stability. The sensitivity to water uptake is a well known weakness that hinders the performance of biocomposite materials due to the hydrophilic nature of the polymeric matrix and/or the natural reinforcement. The analysis of the water absorption phenomenon and its degradative effect are one of the main focus areas of research in preparation of biocomposites. It is known that the performance of natural fiber composites is dependent on factors such as property of the individual components and their interfacial compatibility. One of the main drawbacks in using natural fibers is the high affinity of the fibers towards water absorption. Arbelaiz et al. reported that amorphous cellulose and hemicellulose are mostly responsible for the high water uptake of natural fibers, since they contain numerous easy accessible hydroxyl groups which give strong hydrophilic character to natural fiber. The moisture absorption by composites containing natural fibers has several adverse effects on their properties and thus affects their long-term performance. Sain et al. concluded that increased moisture in NF composites not only deteriorate mechanical properties but also provides necessary conditions for biodegradation and also change in dimensions. The water absorption behaviour of composites is measured by the immersion test according to ASTM D570-98. The increase in the weight of the composites according to their immersion time can identify the affinity of the composite towards moisture and further, it can be used to investigate the kinetics of water absorption
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Water Absorption
Increase in weight, %
Decrease in
sou
nd
velocity, %
Presenter
Presentation Notes
Change in sound velocity of the composite correlates with the increase in weight of the sample due to due to water absorption .
Theoretical Modeling. Randomly Oriented Short Fibres
