Senior Lecturer In Materials Engineering at Sheffield Hallam Univ
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Author
Area of Expertise:
Non-destructive methodologies for real-time monitoring of damage ,
Mechanical behaviour (fatigue, tension, bending etc.) of advanced materials ,
Life prediction and quantification of damage of materials and structures
Publications (21)
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Fused filament fabrication (FFF) is the most widely used additive manufacturing (AM) technique to produce fibre-reinforced polymer matrix composites, due to their low wastage, geometric flexibility and ease of use. Composite materials generally have superior properties such as being stiffer and more robust than conventional materials at a reduced weight leading to their application in a wide variety of sectors (aerospace, automotive etc). However, composites manufactured in this way are highly susceptible to defects such as high void content and poor bond quality at the fibre and matrix interfaces. These defects stop fibre-reinforced composite materials manufactured this way meeting industry standards and being used for structural applications. In the present work, a combination methodology of acoustic emission (AE) alongside tensile testing has been developed to investigate the structural integrity and mechanical performance of AM fibre-reinforced composites. Pure polymer samples and short carbon fibre reinforced composites were manufactured, and their mechanical properties were observed.
Fused filament fabrication (FFF) is the most widely used additive manufacturing (AM) technique to produce fibre-reinforced polymer matrix composites, due to their low wastage, geometric flexibility and ease of use. However, composites manufactured in this way are highly susceptible to defects such as high void content and poor bond quality at the fibre and matrix interfaces. In the present work, a combination method of Infrared Thermography, Acoustic Emission and micro-computerised tomography was developed for the monitoring of the FFF AM process. Both pure plastic and fibre-reinforced composites were manufactured, and the detection and development of defects created during the printing process were monitored. This combination of techniques allows for detection of defects such as porosity, voids and poor fibre-matrix bonding during printing and the verification of their presence after the printing without the need for destructive testing.
In the present work, a novel combination method of in-line monitoring and offline non-destructive evaluation was developed for the detection and monitoring of defects in additively manufactured specimen. The new methodology includes Infrared Thermography, Acoustic Emission and Micro-computerised Tomography to allow for the detection of anomalies during the printing process and the verification of their presence after the printing process without the need for destructive testing. It was found that the in-line monitoring can provide information on the efficacy of the printing process which is substantiated by the offline assessment.
Smart materials are an effective method for increasing safety and reliability across a range of applications. Intrinsic selfsensing mortar is one such material that could greatly improve cementitious infrastructure through the use of real time sensing and monitoring capabilities. This research aims to investigate the self-sensing behavior of mortar, when varying the volume of a stainless-steel functional filler. The results demonstrated a direct correlation between the applied stress and measured electrical resistivity of a sample. Infrared thermography has been also applied for the monitoring of the fracture behavior under monotonic flexure load.
Infrared thermography (IRT) is a well-established and well-documented nondestructive evaluation (NDE) technique which has been proved as one of the critical assessment tools providing not only qualitative but also quantitative results useful for various applications. Even though many post processing methodologies have been used for thermal imaging analysis, there is still a need for a methodology that could possibly reduce the noise, improve the Signal to Noise Ratio (SNR) and focus on a specific area of interest reconstructing automatically the thermal image. This work deals with fine-tuning the IRT method in order to assess the detectability of damage in composite materials.
This work deals with the study of the fatigue behavior of metallic materials for aerospace applications which have undergone erosion. Particularly, an innovative non-destructive methodology based on infrared lock-in thermography was applied on aluminum samples for the rapid determination of their fatigue limit. The effect of erosion on the structural integrity of materials can lead to a catastrophic failure and therefore an efficient assessment of the fatigue behavior is of high importance. Infrared thermography (IRT) as a non-destructive, non-contact, real time and full field method can be employed in order the fatigue limit to be rapidly determined. The basic principle of this method is the detection and monitoring of the intrinsically dissipated energy due to the cyclic fatigue loading. This methodology was successfully applied on both eroded and non-eroded aluminum specimens in order the severity of erosion to be evaluated.
Every year millions of people seek dental treatment to either repair damaged, unaesthetic and dysfunctional teeth or replace missing natural teeth. Several dental materials have been developed to meet the stringent requirements in terms of mechanical properties, aesthetics and chemical durability in the oral environment. Glass-ceramics exhibit a suitable combination of these properties for dental restorations. This research is focused on the assessment of the thermomechanical behavior of bio-ceramics and particularly lithium aluminosilicate glass-ceramics (LAS glass-ceramics). Specifically, methodologies based on Infrared Thermography (IRT) have been applied in order the structure – property relationship to be evaluated. Non-crystallized, partially crystallized and fully crystallized glass-ceramic samples have been non-destructively assessed in order their thermo-mechanical behavior to be associated with their micro-structural features.
The present work deals with the nondestructive assessment of the metallic materials’ mechanical damage. An innovative Nondestructive Evaluation (NDE) methodology based on two thermographic approaches was developed in order the state of fatigue damage to be assessed. The first approach allows the detection of heat waves generated by the thermomechanical coupling during the fatigue loading (online method). Specifically, both the thermo-elastic and intrinsic dissipated energy was correlated with the mechanical degradation and the remaining fatigue life. The second approach involves the monitoring of the materials’ thermal behavior using a Peltier device for accurate thermal excitation (offline method). The correlation of the thermal behavior and the state of damage was achieved by the determination of the material’s thermal response. The combination of these two approaches enables the rapid and accurate assessment of the cumulative damage.
Current work deals with the non-destructive evaluation (NDE) of the fatigue behavior of metal matrix composites (MMCs) materials using Infrared Thermography (IRT) and Acoustic Emission (AE). AE monitoring was employed to record a wide spectrum of cracking events enabling the characterization of the severity of fracture in relation to the applied load. IR thermography as a non-destructive, real-time and non-contact technique, allows the detection of heat waves generated by the thermo-mechanical coupling during mechanical loading of the sample. In this study an IR methodology, based on the monitoring of the intrinsically dissipated energy, was applied for the determination of the fatigue limit of A359/SiCp composites. The thermographic monitoring is in agreement with the AE results enabling the reliable monitoring of the MMCs’ fatigue behavior.
In the present work, a novel method of infrared (IR) thermography called Thermo - Electrical
Lockin Thermography (TELT) was developed for the characterization of subsurface defects in
materials and structures. This new IR thermography method is based on the thermal excitation of
materials under testing using a Peltier device and appropriate electronics allowing for accurate
thermal cycling. Results from using this method were compared with different IR methodologies
(i.e. Pulsed Phase thermography). It was found that Thermo - Electrical Lockin Thermography
provides not only qualitative but also quantitative results.
In this work an innovative methodology was employed for monitoring the fracture behavior in silicon carbide fiberreinforced ceramic matrix composites. This new methodology was based on the combined use of IR thermography and acoustic emission. Compact tension SiC/BMAS specimens were tested with unloading/reloading loops and the thermal dissipation due to crack propagation and other damage mechanisms was monitored by IR thermography. The accuracy of this technique was benchmarked by optical measurements of crack length. In addition, using acoustic emission descriptors, such as activity during the unloading part of the cycles, provided the critical level of damage accumulation in the material. Acoustic emission allowed to closely follow the actual crack growth monitored by IR thermography, enabling quantitative measurements.
Current work deals with the nondestructive evaluation (NDE) of the fracture behavior of ceramic matrix composite (CMCs) materials using combined infrared (IR) thermographic and acoustic emission (AE) characterization. IR thermography as a non-destructive, real-time and non-contact technique, allows the detection of heat waves generated by the thermo-mechanical coupling and the intrinsic energy dissipated during mechanical cyclic loading of the sample. Two different thermographic methodologies, based on the measurement of the surface temperature and on the intrinsically dissipated energy respectively, were applied in order to monitor the crack initiation and propagation and to rapidly assess the fatigue limit of cross-ply SiC/BMAS composites. Simultaneously, AE monitoring was employed to record a wide spectrum of cracking events ranging from matrix cracking to fiber fracture and pull-out. AE event rate, as well as qualitative indices like the rise time and peak frequency reveal crucial information allowing the characterization of the severity of fracture in relation to the applied load. Additionally, rapid assessment of the fatigue limit of CMCs composites was also attempted by AE. Testing a specimen at different load levels for predetermined blocks of cycles shows that the AE acquisition rate remains low for loads below the fatigue limit, while it increases abruptly for higher levels. The thermographic assesment of fatigue limit is in total agreement with the AE results enabling the reliable evaluation of the fatigue limit of the material by testing just one specimen. The application of combined NDE techniques proved very valuable for benchmarking purposes while the sensitivities of the methods act complementarily to each other providing a very detailed assessment of the damage status of the material in real time.
Infrared Thermography (IrT) has been shown to be capable of detecting and monitoring service induced damage of
repair composite structures. Full-field imaging, along with portability are the primary benefits of the thermographic
technique. On-line lock-in thermography has been reported to successfully monitor damage propagation or/and stress
concentration in composite coupons, as mechanical stresses in structures induce heat concentration phenomena around
flaws. During mechanical fatigue, cyclic loading plays the role of the heating source and this allows for critical and
subcritical damage identification and monitoring using thermography. The Electrical Potential Change Technique
(EPCT) is a new method for damage identification and monitoring during loading. The measurement of electrical
potential changes at specific points of Carbon Fiber Reinforced Polymers (CFRPs) under load are reported to enable the
monitoring of strain or/and damage accumulation. Along with the aforementioned techniques Finally, Acoustic Emission
(AE) method is well known to provide information about the location and type of damage. Damage accumulation due to
cyclic loading imposes differentiation of certain parameters of AE like duration and energy. Within the scope of this
study, infrared thermography is employed along with AE and EPCT methods in order to assess the integrity of bonded
repair patches on composite substrates and to monitor critical and subcritical damage induced by the mechanical loading.
The combined methodologies were effective in identifying damage initiation and propagation of bonded composite
repairs.
Infrared thermography is one of several non-destructive testing techniques which can be used for detection of damage in
materials such as ceramic matrix composites. The purpose of this study is to apply a non-destructive methodology for
analyzing the thermal effects in ceramic matrix composites caused by cyclic loading. Mechanical stresses induced by
cyclic loading cause heat release in the composite due to failure of the interface, which results in increasing the
material's temperature. The heat wave, generated by the thermo-mechanical coupling, and the intrinsic energy dissipated
during mechanical cyclic loading of the sample were detected by an infrared camera. The results were correlated with
acoustic emission events.
Composite materials are widely used especially in the aerospace structures and systems. Therefore, inexpensive and
efficient damage identification is crucial for the safe use and function of these structures. In these structures low-velocity
impact is frequently the cause of damage, as it may even be induced during scheduled repair. Flaws caused by lowvelocity
impact are dangerous as they may further develop to extended delaminations. For that purpose an effective
inspection of defects and delaminations is necessary during the service life of the aerospace structures. Within the scope
of this work, an innovative technique is developed based on current stimulating thermography. Electric current is
injected to Carbon Fiber Reinforced Composite and aluminium (Al) plates with concurrent thermographic monitoring.
For reference, both damaged and undamaged plates are inspected. Low-velocity impact damaged composite laminates at
different energy levels are interrogated employing the novel technique. Live and pulse phase infrared thermography is
employed for the identification of low-velocity impact damage at various energy levels while the electric current induces
the transient thermal field in the vicinity of the defect. In all cases conventional ultrasonics (C-scan) were performed for
the validation and assessment of the results of the innovative thermographic method.
Thermographic techniques offer distinct advantages over other techniques usually employed to assess damage
accumulation and propagation. Among the advantages of these techniques are the fully remote-non contact monitoring
and their ability for full field imaging. Due to the transient nature of the heat transfer phenomenon, phase and lock-in
techniques are of particular interest in order to increase the resolution of the signal or provide depth discrimination. Last
but not least, when a structure is subjected to load, these techniques can be used in order to monitor the irreversible
damage phenomena, as manifested by the local heat accumulation in the vicinity of the defect. This eliminates the need
for external heat source, as any cyclic loading can induce the heat gradient necessary to pinpoint the defect accumulation
and propagation.
In the aforementioned context, lock-in thermography has been employed to monitor the delamination propagation in
composites and the critical failure of bonded repairs when the materials are subjected to fatigue loading. Lock-in
thermography proved successful in identifying debonding initiation and propagation as well in depicting the
thermoelastic stress field around purposely induced discontinuities.
Acoustic Emission (AE) supplies information on the fracturing behavior of different materials. In this study, AE activity
was recorded during fatigue experiments in metal CT specimens with a V-shape notch which were loaded in fatigue until
final failure. AE parameters exhibit a sharp increase approximately 1000 cycles before than final failure. Therefore, the
use of acoustic emission parameters is discussed both in terms of characterization of the damage mechanisms, as well as
a tool for the prediction of ultimate life of the material under fatigue. Additionally, an innovative nondestructive
methodology based on lock-in thermography is developed to determine the crack growth rate using thermographic
mapping of the material undergoing fatigue. The thermographic results on the crack growth rate of aluminium alloys
were then correlated with measurements obtained by the conventional compliance method, and found to be in agreement.
One of the most frequent problems in concrete structures is corrosion of metal reinforcement. It occurs when the steel
reinforcement is exposed to environmental agents. The corrosion products occupy greater volume than the steel
consumed, leading to internal expansion stresses. When the stresses exceed concrete strength, eventually lead to
corrosion-induced cracking beneath the surface. These cracks do not show any visual sign until they break the surface,
exposing the structure to more accelerated deterioration. In order to develop a methodology for sub-surface damage
characterization, a combination of non destructive testing (NDT) techniques was applied. Thermography is specialized in
subsurface damage identification due to anomalies that inhomogeneities impose on the temperature field. Additionally,
ultrasonic surface waves are constrained near the surface and therefore, are ideal for characterization of near-surface
damage. In this study, an infrared camera scans the specimen in order to indicate the position of potential damage. For
cases of small cracks, the specimens are allowed to cool and the cooling-off curve is monitored for more precise results.
Consequently, ultrasonic sensors are placed on the specified part of the surface in order to make a more detailed
assessment for the depth of the crack. Although there is no visual sign of damage, surface waves are influenced in terms
of velocity and attenuation. The combination of the NDT techniques seems promising for real structures assessment.
The purpose of this study is to develop an innovative non-destructive methodology for analyzing the thermal
effects in metallic materials caused by fatigue. Mechanical stresses induced by cyclic loading in the material cause heat
release due to microstructural changes, which results in an increase of the material's temperature. The heat release was
quantified as a function of fatigue cycles in carbon steel samples. Mechanical hysteresis phenomena were analyzed to
identify the metrics of damage, which relates to thermal parameters characterizing the level of damage of the material as
a function of fatigue cycles.
Bonded repair offers significant advantages over mechanically fastened repair schemes as it eliminates
local stress concentrations and seals the interface between the mother structure and the patch.
However, it is particularly difficult to assess the efficiency of the bonded repair as well as its
performance during service loads. Thermography is a particularly attractive technique for the particular
application as it is a non-contact, wide field non destructive method. Phase thermography is also
offering the advantage of depth discrimination in layered structures such as in typical patch repairs particularly in the case where composites are used. Lock-in thermography offers the additional advantage of on line monitoring of the loaded structure and subsequently the real time evolution of any progressive debonding which may lead to critical failure of the patched repair. In this study composite systems (CFRP plates) with artificially introduced defects (PTFE) were manufactured. The aforementioned methods were employed in order to assess the efficiency of the thermographic technique. The obtained results were compared with typical C-scans.
his work deals with the study of fracture behavior of silicon carbide particle-reinforced (SiCp) A359 aluminum
alloy matrix composites using an innovative nondestructive method based on lock-in thermography. The heat wave,
generated by the thermo-mechanical coupling and the intrinsic energy dissipated during mechanical cyclic loading of the
sample, was detected by an infrared camera. The coefficient of
thermo-elasticity allows for the transformation of the
temperature profiles into stresses. A new procedure was developed to determine the crack growth rate using
thermographic mapping of the material undergoing fatigue: (a) The distribution of temperature and stresses at the surface
of the specimen was monitored during the test. To this end, thermal images were obtained as a function of time and
saved in the form of a movie. (b) The stresses were evaluated in a post-processing mode, along a series of equally spaced
reference lines of the same length, set in front of the crack-starting notch. The idea was that the stress monitored at the
location of a line versus time (or fatigue cycles) would exhibit an increase while the crack approaches the line, then attain
a maximum when the crack tip was on the line. Due to the fact that the crack growth path could not be predicted and was
not expected to follow a straight line in front of the notch, the stresses were monitored along a series of lines of a certain
length, instead of a series of equally spaced points in front of the notch. The exact path of the crack could be easily
determined by looking at the stress maxima along each of these reference lines. The thermographic results on the crack
growth rate of the metal matrix composite (MMC) samples with three different heat treatments were correlated with
measurements obtained by the conventional compliance method, and found to be in agreement.
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