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Investigation of failure in different materials using acoustic emission technique
No full textME dissertationTo estimate the behaviour of different materials or structures regarding their inherent flaws and
defects, there are different methods available within the domain of science and engineering.
Nowadays, Non-destructive testing (NDT) is one of the most efficient method available for
testing of any structure or equipment or part without damaging actual body of the specimen.
Among the various NDT, wave based techniques are assuming importance since the propagation
characteristics of the wave change with damage and can be directly related to the damage.
Acoustic Emission (AE) technique is one of such passive wave based monitoring technique.
Acoustic Emission waves are the elastic waves, generated by rapid release of energy from
different sources like cracks initiation, propagation and growth. This technique is very much
remarkably sensitive to crack advancement, able to grab the source of damage involves real time
monitoring. Though with AE, various advantages exist for analyzing the true nature by
localization of cracks within the material, however, there are still some shortcomings associated
with AE data recording.
In this study, Acoustic emission technique was applied to track the damage progression under
varying loading rates in different types of materials such as mild steel, concrete and FRP
composites. Mild steel was loaded at 1.5, 4 & 8 mm/min, concrete was load at 0.5, 1 & 1.5
mm/min and FRP composites were loaded at 0.25, 0.5, 1, 1.5 & 2 mm/min. The AE data for mild
steel and concrete was recorded using resonant frequency sensors R15α and R3α. For
composites, Wideband frequency sensor WDI-AST sensor was used with a frequency range of
100-900 kHz.
The amplitude of AE hits and Ib-values analysis was used to quantify damage and to monitor the
cracking patterns in all three materials. The recorded value of amplitude of AE hits were in total
coherence with the load-time plot wherein major damage was indicated by higher values of
amplitude. Ib-value analysis clearly indicated the damage severity in all the materials at all
instants from moderate to severe. Ib-value also exhibited distinct curves for all the three
materials. For mild steel, a major drop was observed during the yielding of the specimen and Ibvalue
remained constant for the rest of loading time. For concrete, there was a continuous fall in
the Ib-value indicating that the damage was progressive leading to failure. For composites, the
fall was observed after half loading time indicating the damage progression only after certain
extent of applied loading
Damage Severity Assessment in Ductile and Brittle Media Using Improved b Value Analysis of AE Data
No full textME, MEDNon-destructive testing has evolved as one of the most preferred testing method for structural
health monitoring. Structural health monitoring in the field of aerospace, mechanical, nuclear
power, civil engineering, etc. requires techniques to detect damage at early stages of
development. The early detection of damage helps to take appropriate action required for
retrofitting. Risks to human lives can be decreased if the time of catastrophic failure is well
known. AE is preferred over other NDT techniques for its in-situ examination and for its
active nature. Damage quantification tools are used to analyse data to know the type, size and
severity of damage. This report presents the use of improved b value analysis to differentiate
Ib values for indication of failure mechanisms in ductile and brittle material.
In the present study, mild steel specimens were used as ductile material and concrete cubes
cured for different time period were used as brittle material. The mild steel specimens were
tested in tensile loading at different loading rate. The R15α sensors were mounted with the
help of tape using grease as couplant. In bending, a notch was deliberately machined on
tension side in mild steel specimens. In case of concrete, cubes cured for different time period
were tested under compressive load. The R3α sensors used for the study of brittle material
were mounted with the help of grease and tape. The AE data captured for both ductile and
brittle material was analysed using improved b value analysis. Amplitude, absolute energy,
improved b value, load and other parameters were plotted with time to understand failure
mechanisms in ductile and brittle materials. The Ib value for both ductile and brittle material
was obtained
Ultrasonic Guided Wave and Acoustic Emission Techniques for monitoring corrosion in Reinforced Concrete Structures
Get PDFCorrosion of reinforcement is one of the principal causes of premature failure of Reinforced Concrete (RC) structures. In India, the annual loss due to corrosion has been estimated about 4% of the country’s gross domestic product (GDP) i.e., about USD 40 billion in a year in both infrastructure and industry segments. Exposure to extreme environments and the continuous ingress of chlorides in concrete from various sources neutralises the protective passive layer of concrete which is alkaline with a pH ranging from 12 to 13. The presence of moisture and oxygen leads to the formation of oxides and the initiation of reinforcement corrosion takes place. Corrosion of reinforcement in concrete affects the durability of the RC structures in two ways: the formation of rust products with larger volume than steel leads to spalling and the cracking of the concrete cover; and the area of cross-section of the steel bars reduces drastically due to dissolution of the steel leading to pit formation and hence, loss in the tensile strength and load carrying capacity of the structure. Therefore, an early prognosis and diagnosis of corrosion-induced damage in RC structures could facilitate the engineers to adopt correct repair measures.
A large variety of non-destructive techniques have been reported by researchers worldwide for detecting corrosion-induced damage in RC structures like visual inspection, electrochemical, Optical methods, PZT and piezo ceramic patches etc. but they suffer from practical applications owing to large civil infrastructure. Recently, the advent of wave propagation technologies for damage detection in RC structures have given a new dimension to damage monitoring capabilities. In this work, an integrated approach using active Ultrasonic Guided Wave (UGW) and passive Acoustic Emission (AE) technique for corrosion monitoring in RC structures has been attempted. Active UGW monitors the deterioration of reinforcing bar while corrosion takes place in RC structure whereas the AE technique listens to the cracks and records the damage in the surrounding concrete.
Initiation of corrosion in RC structures before it can be observed visually is done using the two techniques. For this, AE sensors were mounted on the concrete surface and typical guided wave modes were propagated through the embedded rebar. The NDT results were co-related with the already established electrochemical techniques. Active monitoring using UGW was found to have limited significance for tracking the initiation of corrosion when only the outer surface of the reinforcing bar is affected. Surface modifications on the embedded rebar during the initial stages of corrosion is clearly indicated by the L(0,1) mode. On the other hand, passive monitoring using AE clearly identified the onset and the initiation of corrosion during the early stages. The origin and the location of micro-cracks were well identified with the help of AE event maps.
Further, the techniques were used to investigate the progression of corrosion-induced damage in RC structures. It was found that UGW clearly differentiated the surface corrosion from the pitting corrosion with the help of specific guided wave modes. On the other hand, AE parameters like AE hits and cumulative signal strength (CSS) successfully picked up the regions of initiation of steel depassivation and progression of corrosion in the form of micro- and macro-cracking in the surrounding concrete. From the AE event maps, the entire damage progression from initiation of corrosion damage to damage progression in the later stages could be clearly observed.
Further, the effect of varying rates and different corrosion environments were investigated using the two wave propagation techniques. It was confirmed that corrosion mechanism is unaltered by varying rates of accelerated corrosion though the damage progression is delayed and picked up by both UGW and AE. In case of RC beams subjected to accelerated corrosion in the absence of chlorides simulating slow corrosion, UGW confirmed that pitting is insignificant due to slow steel-concrete bond deterioration, by picking up only delamination. On the other hand, AE clearly indicated the corrosion initiation as well as progression stages by cumulative AE hits.
The efficacy of UGW and AE techniques was also investigated to monitor the corrosion impediment offered using Fiber Reinforced Polymer (FRP) wraps on corroding RC cylinders. UGW monitoring with surface-seeking and core-seeking modes established that pulse-echo method is suitable to assess corrosion delay in FRP wrapped samples. The recorded AE hits exhibited significant decrease after the wrapping was done indicating success in corrosion impediment. This was confirmed by drop in the values of amplitude of AE hits from 100 dB to 65 dB after wrapping. No peak/knee was observed in the CSS curve, indicating that FRP wraps effectively delayed the corrosion progression.
It is observed that corrosion-induced damage in RC structures can effectively be tracked right from initiation to progression to concrete cracking using an integrated health monitoring approach involving wave propagation technologies of active ultrasonic guided wave and passive acoustic emission techniques along with benchmark electrochemical measurements. This would go a long way in non-destructive evaluation of residual capacities of reinforcements, which would help a great deal in devising a post-corrosion maintenance strategy of RC structures
Finite Element Approach for Simulation of Powder Mixed Electric Discharge Machining Process and Experimental Validation
No full textM.E. (CAD/CAM and Robotics)Electrical Discharge Machining (EDM) is one of the most extensively used nontraditional
material removal process capable of machining tough, high strength, corrosion
resistant electrically conductive materials. Mixing of suitable material in powder form into
the dielectric fluid is a recent advancement in EDM process to improve its process
capabilities and is known as powder mixed EDM (PMEDM) process. Present work aimed
at studying the thermal aspects of Powder Mixed EDM process using ANSYS Workbench
12.0 (Transient Thermal Analysis module and Static Structural Analysis module) to
simulate temperature distribution, volume removed, cooling rate and stresses developed
into the workpiece. Study is related to temperature distribution due to a single spark for
one cycle varying different process parameters employing the Gaussian type of heat
distribution inside the spark channel. Temperature distribution simulated using Transient
Thermal analysis module is used to estimate volume removed and temperature variation in
radial and depth direction using a code developed with Visual C++ 6.0. Development of
different temperature zones (unaffected, heat affected, melting and evaporating zone),
shape and size of the craters have been studied. Temperature distribution during
simulation is found to be maximum at centre of spark region and decrease in radial
direction following Gaussian distribution. Cooling rate of the workpiece material have
been estimated using different discharge current, fraction of heat incident, pulse on and
pulse off duration for a one complete cycle. Coupled Thermal-Structural analysis has been
done to find stresses that develop into work material during PMEDM process. For
validation experiments have been conducted on the H11 (hot die steel) workpiece using
different set of process parameters. Experimental validation confirmed that 20 to 25%
heat is transferring to the workpiece as predicted volume using Cw ranging from 0.2 to
0.25 during simulation is found to match with experimental volume removed for different
pulse on and discharge current settings. Observing microstructure cracks appearing on the
machined surface validates higher cooling rate and equivalent stresses predicted during
simulation. Detailed study of the craters using microscope was completed to confirm the
shapes and sizes of the craters that formed during the experimentation
Modified Epoxy Coatings for Corrosion Inhibition in Reinforcing Bars in Concrete
No full textCorrosion of reinforced concrete structures is one of the biggest problems faced by the construction industry with billions of dollars spent annually on corrosion control and mitigation. Epoxy Coated Rebars (ECRs) have been reported to effectively mitigate corrosion in concrete structures. However, their utilization is constrained by the presence of micro-pores and the inherent brittleness of epoxy coatings. This study delves into the mechanisms of corrosion inhibition through the application of nano-modified epoxy-based coatings. Three distinct formulations of modified epoxy-based coatings were precisely evaluated: nano-clay-modified epoxy coatings with polyaniline, graphene-derivatives modified epoxy coatings with carbon nanotubes and self-healing tung-oil microcapsules-based nano-modified epoxy coatings for plain bars directly and when they were further embedded in concrete and exposed to accelerated chloride corrosion. A multifaceted evaluation approach of Non-Destructive Testing (NDT) and Destructive testing techniques were employed to ascertain coating corrosion efficacy comprehensively. NDT monitoring comprises of visual inspection, corrosion current measurements and ultrasonic guided waves monitoring techniques whereas destructive testing techniques involved evaluation of loss in mass, tensile strength and pull-out strength vis-à-vis corrosion exposure.
Nano-clay modified epoxy coatings examines the efficacy of nano-modified epoxy coatings using four different silane-treated nano-clays (Montmorillonite-MMT, Organic Montmorillonite-OMMT (Cloisite®15A and Cloisite®30B) and Halloysite-HNT) in a combination with polyaniline (PANI). PANI being good pigment material for anti-corrosion coatings is expected to provide enhanced corrosion resistance due to its unique ability to intercept electrons from the metal surface and transfer them to outside the coating. It was confirmed that nano-clay modified epoxy coatings showed superior performance in comparison with pure epoxy coatings, as a result of enhanced barrier properties by nano-clay and synergetic effect of polyaniline fragments closely stacked with nano-sized nano-clay platelets throughout the coating. Delay in corrosion initiation in pure epoxy is 7-8 days in comparison to 70-75 days for PANI with Cloisite®30B nano-clay. Further, ultrasonic guided waves transmitted signals dropped to zero for PE samples in 40 days whereas in PANI with Cloisite®30B after 90 days of exposure ultrasonic signals fall by only 10%. Notably, formulations featuring Cloisite®30B nano-clay with PANI showcased superior barrier properties, with the intercalation of nano-clay particles facilitating tortuous diffusion pathways, thereby impeding aggressive ion ingress and corrosion propagation. Additionally, hydroxyl group of Cloisite®30B promotes interfacial interaction between nano-clay and polymer matrix resulting in impermeable network. Characterization results of silane-treated clays demonstrated the successful grafting of the silane agent on the nano-clay structure, while FE-SEM of different coatings displays the dispersion of nano-fillers inside the coating matrix.
Incorporation of graphene-derivatives in the form of graphene oxide (GO) and reduced graphene oxide (rGO) with carbon nanotubes (CNT) and silane agents was studied further for improved ductility and corrosion inhibition properties. Hybrid mixture of CNTs and rGO/GO enhances the mechanical strength of the coating matrix whereas silane agents improve dispersion and bonding characteristics of nano-fillers. The corrosion initiates earlier in rGO/CNT coatings, occurring within 50 days, while no corrosion was observed in GO/CNT coatings even after 150 days of exposure to corrosion. Additionally, in ultrasonic guided wave monitoring, transmitted signals dropped to zero for rGO/CNT samples in 125 days whereas in GO/CNT coatings after 150 days of exposure ultrasonic signals are completely healthy. GO exhibited superior corrosion inhibition performance over rGO, due to its successful blocking of aggressive larger chloride ions, by providing a dense impenetrable network due to the presence of functional groups on basal planes of GO sheets.
Further the nano-clay and graphene derivatives-based coatings were further examined with self-healing tung-oil microcapsules to develop smart self-healing coatings. In smart nano-clay modified epoxy coating studies, Cloisite®15A nano-clay were incorporated with tung-oil microcapsules in epoxy coatings (MNC) and compared with pure epoxy. These coatings were pre-damaged to investigate the self-healing capabilities of coatings. Corrosion initiates in 15–17 days for NC coatings (only nano-clay) and 8–10 days for MC coatings (only tung-oil microencapsulated epoxy coating), whereas it initiates in 30–35 days with MNC coatings. Synergetic effect of nano-filler with self-healing microcapsules enhanced the overall performance of epoxy coatings. Smart graphene-derivatives (GO and rGO) modified epoxy coating was also studied with tung-oil microcapsules (MrGO and MGO). A dual layer coating combination (M+GO) with additional dual coating of MC coating as top layer and GO/CNT coating as base layer was also investigated. All three coatings exhibit encouraging outcomes after 160 days of accelerated corrosion exposure; however, in the case of the M+GO coatings, there is no corrosion initiation at all after 160 days. Smart coatings formulations exhibited pronounced self-healing attributes, facilitated by tung-oil polymerization triggered upon by coating damage or corrosion initiation. By using tung-oil in nano-modified epoxy coatings, significant reductions in mass loss alongside, noteworthy enhancements in residual tensile strength vis-à-vis pure epoxy coatings, with negligible compromise on bond strength was observed.
In essence, this research effort brings out the pivotal role of advanced nano-modified epoxy coatings in mitigating corrosion-induced structural degradation, thereby bolstering infrastructure resilience and longevity. The understanding obtained from this research offers invaluable insights into the development of robust, cost-effective, and environmentally sustainable corrosion mitigation strategies, vital for fostering resilient infrastructure in the face of evolving challenges
Study of Relationship of Mechanical Properties of Indian Wood Species with Cutting Forces and Surface Finish in Slot Milling using a High Speed 3-axis Vertical CNC Router System
No full textMaster of Engineering-Production and Industrial EngineeringWood is an important natural material and being used for various applications since ages. The larger percentage of the timber wood is used for furniture making as well as a construction material in addition to being used as an engineering material. The wood as an engineering material possess anisotropic properties and some time has random defects. During manual wood machining operations one can take care of anisotropic nature of wood properties as well as the random defects but it is obtained at the cost of a lot of time. The CNC machining of wood can help us save a lot of machining time while simultaneously offer better machining accuracy, but it is desired that the relationship of machining parameters to be selected and the desired machined properties of the wood samples be studied to explore the full potential of using CNC wood machining systems. The wood as an engineering material has several mechanical properties but those mechanical properties which directly influence the machinability of wood are required to be studied.
In this thesis work the previously published literature has been studied in detail to identify the mechanical properties of wood which effect the machinability of wood. Further a systematic approach has been used to determine the values of these mechanical properties as per standard procedure available from the Bureau of Indian Standards as well as standard published standard procedures. The six Indian wood species are used in the present study where we have tried to establish a correlation between the mechanical properties of these wood species with the cutting forces observed in slot milling and the surface finish obtained in the bottom of the slotted cut.
It has been observed that the wood grain orientation is a key factor for machinability of wood because different slot milling orientation gives different measurable output of surface finish and cutting forces. Procedures to find out different mechanical properties are discussed and their averaged values are considered for analysis. A high rpm 3-axis vertical CNC router is used in the present study for slot and plunge milling while a force dynamometer is used for recording the magnitude of cutting forces thus produced. A stylus type surface roughness tester was then used to measure surface roughness of various machined samples. The results of different mechanical properties, various cutting forces and surface finish are thus analysed to identify the relationship among them. The literature surveyed for the present study, the procedure followed for determining the values of various measurable parameters and hence observed results and discussions have been presented systematically in this thesis report
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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