Combining ultrasonic vibration (UV) with metal forming processes is investigated as a novel techn... more Combining ultrasonic vibration (UV) with metal forming processes is investigated as a novel technology that has been able to reduce the forming force and enhance this process. This paper attempts to elucidate the effect of Ultrasonically Assisted Deep Drawing (UADD) process on the forming force and thickness distribution of the formed sample. Therefore, a Finite Element (FE) model is developed to simulate this process and further investigate the ultrasonic micro-hammer mechanism in UADD process. Experimental tests were conducted to validate the established numerical model. Accordingly, a robust technological equipment was designed and fabricated, so that by application of ultrasonic vibration, the drawing die will be stimulated in longitudinal mode at the frequency of 20 kHz and thus, remain in the resonant condition. A reasonable congruence was observed when the forming force results and cup configurations from experimental tests and numerical solutions were compared. Therefore, the numerical model was used to evaluate the deformation behavior of the sheet at different amplitudes and frequencies. The results confirmed continuous vibration and ultrasonic micro-hammer conditions exhibit different behavior during the UADD process, and the latter occurs when the ultrasonic die separates from the workpiece surface. Although the UV application under micro-hammer condition significantly reduces the forming force, it has a destructive effect on the thickness distribution of the sheet and causes severe thinning. The current study provides a better understanding of the ultrasonic micro-hammer and its effects on the sheet metal forming process, which is the fundamental step in exploring this process.
The International Journal of Advanced Manufacturing Technology, 2019
This paper focuses on the system frequency variation effect on the ultrasonically assisted deep d... more This paper focuses on the system frequency variation effect on the ultrasonically assisted deep drawing (UADD) process, based on experimental and numerical approaches. A special ultrasonic die is precisely designed and fabricated to improve draw-ability and reduce the forming force. The longitudinal resonance frequency of the vibratory system is established as 20 kHz prior to forming load application on the system. Consequently, a three-dimensional finite element model of the deformation process is developed. The output of this simulation shows good agreement with the experimental test results. The UADD test with 5 μm amplitude demonstrated 5.6% decrease in maximum average force and 12.2% increase in the drawing ratio when compared with the conventional deep drawing (CDD). In addition, the resonant frequency variation effect on the forming force is revealed during the experimental tests. Since the excitation frequency is tuned out with the natural frequency of the system at the beginning of the test, no forming force reduction was observed at this stage. To further investigate this effect, two complementary steps are implemented in this work. In the first phase, the natural frequency of the system is determined by numerical modal analysis and reshaping the blank at various deformation stages. Consequently, forced vibrations with frequencies very close to the natural frequencies, calculated from the first phase, are applied to the system in the second phase, to simulate the forming process. Based on the validated numerical model, the forming force, friction force, and material flow stress distributions are disclosed for UADD in various excitation frequencies.
In this study, two types of specimens, woven [0, 90]s and unidirectional [0]s glass/epoxy composi... more In this study, two types of specimens, woven [0, 90]s and unidirectional [0]s glass/epoxy composites, were studied. Acoustic emission (AE) monitoring with wavelet-based signal processing technique and Fuzzy C-Means (FCM) clustering method was used to detect the different fracture mechanisms during delamination in quasi-static three-point bending test. The three-point bending test simulates thrust force, the most effective factor in delamination, throughout the process of drilling without backup plate. The obtained AE signals were decomposed into various wavelet levels and classified using FCM. Scanning Electron Microscopy (SEM) observation was used to determine the different fracture mechanisms. The results show that the energy of AE signals has been concentrated in two significant components for both types of specimens, and the dependency percentage of damages in the two specimens is different.
Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stress... more Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stresses coexist. In the objective to monitor online the SCC on real structures, it seems reasonable to characterize and recognize acoustic emission during static U-bend tests. The present study is concerned with static tests on 304 stainless steels in two different media (5% HCl and 5% H2SO4) in order to find a criterion to distinguish, the two different mechanism of SCC (anodic dissolution and hydrogen embrittlement) by characteristic parameters and waveform analysis of AE signals. In the next stage, effect of residual stress on stress corrosion cracking in chloride solution has been studied. Three samples have been prepared: one of them was stress relieved; the second one was made sensitive to SCC by heat treatment and the last one was cold worked by rolling. Cumulative count, amplitude distribution and waveform analysis were selected as AE-parameters. AE with amplitudes ranging from 39 to 65 dB with different counts and energy occurred during SCC of SS-304 at room temperature. According to the results, acoustic emission is able to be used as robust technique for mechanism detection of SCC. In addition it can be used to measure the effect of residual stress in manufacturing process on SCC.
In using acoustic emissions (AEs) for mechanical diagnostics, one major problem is the discrimina... more In using acoustic emissions (AEs) for mechanical diagnostics, one major problem is the discrimination of events due to different types of damage occurring during loading of composite materials. Unsupervised pattern recognition analyses (fuzzy c-means clustering) associated with a principal component analysis (PCA) are the tools that are used for the classification of the monitored AE events. Composites at different layups are used with the acoustic emission technique. A cluster analysis of AE data is achieved and the resulting clusters are correlated to the damage mechanisms of the material under investigation. Time domain methods are used to determine new relevant descriptors to be introduced in the classification process to improve the characterization and the discrimination of the damage mechanisms. The results show that there is a good fit between clustering groups and damage mechanisms. Additionally, AE with a clustering procedure are effective tools that provide a better discrimination of damage mechanisms in glass/polyester composite materials.
In using acoustic emissions (AEs) for mechanical diagnostics, one major problem is the discrimina... more In using acoustic emissions (AEs) for mechanical diagnostics, one major problem is the discrimination of events due to different types of damage occurring during loading of composite materials. Unsupervised pattern recognition analyses (fuzzy c-means clustering) associated with a principal component analysis (PCA) are the tools that are used for the classification of the monitored AE events. Composites at different layups are used with the acoustic emission technique. A cluster analysis of AE data is achieved and the resulting clusters are correlated to the damage mechanisms of the material under investigation. Time domain methods are used to determine new relevant descriptors to be introduced in the classification process to improve the characterization and the discrimination of the damage mechanisms. The results show that there is a good fit between clustering groups and damage mechanisms. Additionally, AE with a clustering procedure are effective tools that provide a better discrimination of damage mechanisms in glass/polyester composite materials.
Volume 3: Advanced Composite Materials and Processing; Robotics; Information Management and PLM; Design Engineering, 2012
Drilling is a dynamic process which causes some defects in composite materials such as delaminati... more Drilling is a dynamic process which causes some defects in composite materials such as delamination, fiber pull out and matrix cracking. Because of non-stationary behavior of drilling process, using online method to monitor these damage mechanisms is inevitable. In this paper, acoustic emission signals and wavelet analysis are applied to monitor drilling action from entry to exit. The results show that the selected monitoring indices from the time domain parameters and wavelet packet coefficients are capable of detecting the drilling stages and damage mechanisms during the process effectively.
Abstract Drilling composite materials is one of the secondary processes of manufacturing industri... more Abstract Drilling composite materials is one of the secondary processes of manufacturing industrial structures. However, drilling composite materials presents a number of problems such as degradation of mechanical behavior. In this study, effects of spindle speed, feed rate, and drill point geometry on residual tensile strength are studied. Acoustic emission technique with a wavelet-based signal processing method is developed to monitor the residual tensile strength of drilled laminates. Cumulative count, amplitude, and energy are ...
Composite materials have been widely used in various industries due to their superior mechanical ... more Composite materials have been widely used in various industries due to their superior mechanical properties. Drilling is a very common machining operation to install fasteners for assembly of laminates. Delamination, however, is a serious concern in the drilling of fiber reinforced composite materials, because it reduces their compressive residual strength. This paper studies the effects of drilling parameters on the thrust force, adjusted delamination factor and compressive residual strength of uni-directional glass/epoxy resin. The design of the experiment was based on the Taguchi method. The results highlight the importance of the feed rate for maximizing the compressive residual strength of drilled laminates. The Acoustic Emission (AE) technique was also used to monitor both drilling process and compression test. The objective was to establish a correlation between AE parameters and mechanical characteristics. The results reveal that root mean square (RMS) can be used for monitoring thrust force and AE energy for compression force.
In this study, acoustic emission (AE) monitoring with a Fuzzy C-Means (FCM) clustering is develop... more In this study, acoustic emission (AE) monitoring with a Fuzzy C-Means (FCM) clustering is developed to detect the delamination process during quasi-static 3-point bending test on glass/epoxy composite materials. The main fracture mode that should be emphasized and has an effect on the residual strength of composite materials is delamination. The 3-point bending test simulates thrust force due to drilling process without backup plate. In this work, two types of specimen at different layups, woven [0,90]s and unidirectional [0]s, leading to different levels of damage evolution, were studied. Using acoustic emission monitoring can help to detect these fracture mechanisms. The obtained AE signals were classified using FCM. Dependency percentage of damages in each class is different in two specimens. Three parameters (Peak Amplitude, Count, and Average Frequency) were used to validate the FCM based classification. The results show that there is a good agreement with the FCM classification and microscopic observation by SEM. Introduction Delamination is one of the most common failure modes of composite materials. It may be caused by service life conditions or, may result from production process such as drilling. As the fields of application expand, the use of types of machining such as turning, drilling, milling, and cutting-off has increased in GFRP fabrication. However, the glass fibre constituent often renders the machining of GFRP difficult. Machining of composite materials requires a better understanding of cutting processes to achieve accuracy and efficiency [1]. Among the problems caused by drilling, delamination is an important defect in composite structure. delamination drastically reduces assembly tolerance and strengthagainst fatigue, thus degrading the long-term performance of composites[2]. Thrust force is the most affective factor in delamination induced by drilling[3]. The 3-point bending test simulates thrust force due to drilling process without backup plate and regardless of other parameters like friction and spindle speed with specific geometry of the tool. In order to identify the type of damage in composites, which involves fiber failure, the transverse failure and delamination, the acoustic emission (AE) technique is also promising for the detection of the damage type [4]. The acoustic emission phenomenon is the result of transient elastic wave propagation generated by a rapid release of energy within a material due to changes in local stress and strain fields. These elastic waves have a wide range of frequencies distribution including audible frequencies and frequencies in MHz range [5-6]. This technique is based on the detection of elastic surface stress waves caused by the dissipation of elastic energy due to the opening of a crack or plastic deformation. Several studies were already conducted on the mode I fracture to determine the surface energies of composite materials by creating a crack propagation effect [7-8]. To find correlation between AE parameters and damage mechanism several studies have been carried out [9,10]. So far most studies have used AE descriptors such as counts, amplitude, energy, wavelet and also multiparameters method to characterize the development of failure mechanisms [11,12].
Composite materials have been widely used in various industries due to their superior mechanical ... more Composite materials have been widely used in various industries due to their superior mechanical properties. Drilling is a very common machining operation to install fasteners for assembly of laminates. Delamination, however, is a serious concern in the drilling of fiber reinforced composite materials, because it reduces their compressive residual strength. This paper studies the effects of drilling parameters on the thrust force, adjusted delamination factor and compressive residual strength of uni-directional glass/epoxy resin. The design of the experiment was based on the Taguchi method. The results highlight the importance of the feed rate for maximizing the compressive residual strength of drilled laminates. The Acoustic Emission (AE) technique was also used to monitor both drilling process and compression test. The objective was to establish a correlation between AE parameters and mechanical characteristics. The results reveal that root mean square (RMS) can be used for monitoring thrust force and AE energy for compression force.
Drilling results in damage such as delamination and matrix cracking around the hole and might ult... more Drilling results in damage such as delamination and matrix cracking around the hole and might ultimately causes degradation in the residual tensile strength of the drilled components. The damage induced during the drilling of composites can be detrimental to the mechanical behavior of the composite products. In this work, the effects of machining parameters (feed rate and cutting speed) and drill point angle on thrust force, the adjusted delamination factor and residual tensile strength are investigated. The Taguchi technique for the design of experiments was employed to analyze the thrust force, adjusted delamination factor and residual tensile strength of woven glass/resin epoxy. The results show that feed rate and drill point angle are the most important parameters. During tensile testing of drilled laminates, acoustic emission (AE) events were recorded. By feature extraction of AE time domain parameters, the suitable parameter for detecting the characteristics of thrust force and tensile force were determined. The AE mean power (MP) and cumulative count correlated well with thrust force and tensile force, respectively.
Conventional methods of analysis for drilling of composite materials usually study the amount of ... more Conventional methods of analysis for drilling of composite materials usually study the amount of damaged area, thrust force, and effective parameters. However, these methods do not provide the investigator with sufficient information about drilling mechanisms. In the current investigation, a procedure for diagnosing different drilling mechanisms based on the analysis of the signals of acoustic emission is presented. According to the number of time domain acoustic emission parameters, using multi-variable methods of analysis is unavoidable. In this work, unsupervised pattern recognition analyses (fuzzy C-means clustering) associated with a principal component analysis are the tools that are used for the classification of the recorded acoustic emission data. After classification of acoustic emission events, the resulting classes are correlated with the different drilling stages and mechanisms. Acoustic emission signal analysis provides a better discrimination of drilling stages than mechanic-based analyses.
Journal of The Acoustical Society of America, 2008
Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stress... more Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stresses coexist. In the objective to monitor online the SCC on real structures, it seems reasonable to characterize and recognize acoustic emission during static U-bend tests. The present study is concerned with static tests on 304 stainless steels in two different media (5% HCl and 5% H2SO4) in order to find a criterion to distinguish, the two different mechanism of SCC (anodic dissolution and hydrogen embrittlement) by characteristic parameters and waveform analysis of AE signals. In the next stage, effect of residual stress on stress corrosion cracking in chloride solution has been studied. Three samples have been prepared: one of them was stress relieved; the second one was made sensitive to SCC by heat treatment and the last one was cold worked by rolling. Cumulative count, amplitude distribution and waveform analysis were selected as AE-parameters. AE with amplitudes ranging from 39 to 65 dB with different counts and energy occurred during SCC of SS-304 at room temperature. According to the results, acoustic emission is able to be used as robust technique for mechanism detection of SCC. In addition it can be used to measure the effect of residual stress in manufacturing process on SCC.
In this study, two types of specimens, woven [0, 90] s and unidirectional [0] s glass/epoxy compo... more In this study, two types of specimens, woven [0, 90] s and unidirectional [0] s glass/epoxy composites, were studied. Acoustic emission (AE) monitoring with wavelet-based signal processing technique and Fuzzy C-Means (FCM) clustering method was used to detect the different fracture mechanisms during delamination in quasi-static three-point bending test. The three-point bending test simulates thrust force, the most effective factor in delamination, throughout the process of drilling without backup plate. The obtained AE signals were decomposed into various wavelet levels and classified using FCM. Scanning Electron Microscopy (SEM) observation was used to determine the different fracture mechanisms. The results show that the energy of AE signals has been concentrated in two significant components for both types of specimens, and the dependency percentage of damages in the two specimens is different.
Structural parts made of composites have frequently to be drilled in the industry. However, littl... more Structural parts made of composites have frequently to be drilled in the industry. However, little is now about the interacting conditions between the drill tool and material, which may be multi-type and multi-size. Delamination free in drilling different fiber reinforced composites is the main objective of present paper. Therefore the influence of drilling and materials variables thrust force and delamination of GFRP composite was investigated experimentally. Drilling variables are cutting speed and feed; material variable is fiber orientation. Acoustic Emission sensing was employed for online detection of composite damage induced by drilling .This paper addresses an application of wavelet-based signal processing technique on a composite during drilling. The wavelet methodology is introduced and procedure of waveletbased acoustic emission (AE) analysis methods is demonstrated. Result shows Acoustic Emission analysis by wavelet method can monitor damage mechanism in drilling of composites.
Combining ultrasonic vibration (UV) with metal forming processes is investigated as a novel techn... more Combining ultrasonic vibration (UV) with metal forming processes is investigated as a novel technology that has been able to reduce the forming force and enhance this process. This paper attempts to elucidate the effect of Ultrasonically Assisted Deep Drawing (UADD) process on the forming force and thickness distribution of the formed sample. Therefore, a Finite Element (FE) model is developed to simulate this process and further investigate the ultrasonic micro-hammer mechanism in UADD process. Experimental tests were conducted to validate the established numerical model. Accordingly, a robust technological equipment was designed and fabricated, so that by application of ultrasonic vibration, the drawing die will be stimulated in longitudinal mode at the frequency of 20 kHz and thus, remain in the resonant condition. A reasonable congruence was observed when the forming force results and cup configurations from experimental tests and numerical solutions were compared. Therefore, the numerical model was used to evaluate the deformation behavior of the sheet at different amplitudes and frequencies. The results confirmed continuous vibration and ultrasonic micro-hammer conditions exhibit different behavior during the UADD process, and the latter occurs when the ultrasonic die separates from the workpiece surface. Although the UV application under micro-hammer condition significantly reduces the forming force, it has a destructive effect on the thickness distribution of the sheet and causes severe thinning. The current study provides a better understanding of the ultrasonic micro-hammer and its effects on the sheet metal forming process, which is the fundamental step in exploring this process.
The International Journal of Advanced Manufacturing Technology, 2019
This paper focuses on the system frequency variation effect on the ultrasonically assisted deep d... more This paper focuses on the system frequency variation effect on the ultrasonically assisted deep drawing (UADD) process, based on experimental and numerical approaches. A special ultrasonic die is precisely designed and fabricated to improve draw-ability and reduce the forming force. The longitudinal resonance frequency of the vibratory system is established as 20 kHz prior to forming load application on the system. Consequently, a three-dimensional finite element model of the deformation process is developed. The output of this simulation shows good agreement with the experimental test results. The UADD test with 5 μm amplitude demonstrated 5.6% decrease in maximum average force and 12.2% increase in the drawing ratio when compared with the conventional deep drawing (CDD). In addition, the resonant frequency variation effect on the forming force is revealed during the experimental tests. Since the excitation frequency is tuned out with the natural frequency of the system at the beginning of the test, no forming force reduction was observed at this stage. To further investigate this effect, two complementary steps are implemented in this work. In the first phase, the natural frequency of the system is determined by numerical modal analysis and reshaping the blank at various deformation stages. Consequently, forced vibrations with frequencies very close to the natural frequencies, calculated from the first phase, are applied to the system in the second phase, to simulate the forming process. Based on the validated numerical model, the forming force, friction force, and material flow stress distributions are disclosed for UADD in various excitation frequencies.
In this study, two types of specimens, woven [0, 90]s and unidirectional [0]s glass/epoxy composi... more In this study, two types of specimens, woven [0, 90]s and unidirectional [0]s glass/epoxy composites, were studied. Acoustic emission (AE) monitoring with wavelet-based signal processing technique and Fuzzy C-Means (FCM) clustering method was used to detect the different fracture mechanisms during delamination in quasi-static three-point bending test. The three-point bending test simulates thrust force, the most effective factor in delamination, throughout the process of drilling without backup plate. The obtained AE signals were decomposed into various wavelet levels and classified using FCM. Scanning Electron Microscopy (SEM) observation was used to determine the different fracture mechanisms. The results show that the energy of AE signals has been concentrated in two significant components for both types of specimens, and the dependency percentage of damages in the two specimens is different.
Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stress... more Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stresses coexist. In the objective to monitor online the SCC on real structures, it seems reasonable to characterize and recognize acoustic emission during static U-bend tests. The present study is concerned with static tests on 304 stainless steels in two different media (5% HCl and 5% H2SO4) in order to find a criterion to distinguish, the two different mechanism of SCC (anodic dissolution and hydrogen embrittlement) by characteristic parameters and waveform analysis of AE signals. In the next stage, effect of residual stress on stress corrosion cracking in chloride solution has been studied. Three samples have been prepared: one of them was stress relieved; the second one was made sensitive to SCC by heat treatment and the last one was cold worked by rolling. Cumulative count, amplitude distribution and waveform analysis were selected as AE-parameters. AE with amplitudes ranging from 39 to 65 dB with different counts and energy occurred during SCC of SS-304 at room temperature. According to the results, acoustic emission is able to be used as robust technique for mechanism detection of SCC. In addition it can be used to measure the effect of residual stress in manufacturing process on SCC.
In using acoustic emissions (AEs) for mechanical diagnostics, one major problem is the discrimina... more In using acoustic emissions (AEs) for mechanical diagnostics, one major problem is the discrimination of events due to different types of damage occurring during loading of composite materials. Unsupervised pattern recognition analyses (fuzzy c-means clustering) associated with a principal component analysis (PCA) are the tools that are used for the classification of the monitored AE events. Composites at different layups are used with the acoustic emission technique. A cluster analysis of AE data is achieved and the resulting clusters are correlated to the damage mechanisms of the material under investigation. Time domain methods are used to determine new relevant descriptors to be introduced in the classification process to improve the characterization and the discrimination of the damage mechanisms. The results show that there is a good fit between clustering groups and damage mechanisms. Additionally, AE with a clustering procedure are effective tools that provide a better discrimination of damage mechanisms in glass/polyester composite materials.
In using acoustic emissions (AEs) for mechanical diagnostics, one major problem is the discrimina... more In using acoustic emissions (AEs) for mechanical diagnostics, one major problem is the discrimination of events due to different types of damage occurring during loading of composite materials. Unsupervised pattern recognition analyses (fuzzy c-means clustering) associated with a principal component analysis (PCA) are the tools that are used for the classification of the monitored AE events. Composites at different layups are used with the acoustic emission technique. A cluster analysis of AE data is achieved and the resulting clusters are correlated to the damage mechanisms of the material under investigation. Time domain methods are used to determine new relevant descriptors to be introduced in the classification process to improve the characterization and the discrimination of the damage mechanisms. The results show that there is a good fit between clustering groups and damage mechanisms. Additionally, AE with a clustering procedure are effective tools that provide a better discrimination of damage mechanisms in glass/polyester composite materials.
Volume 3: Advanced Composite Materials and Processing; Robotics; Information Management and PLM; Design Engineering, 2012
Drilling is a dynamic process which causes some defects in composite materials such as delaminati... more Drilling is a dynamic process which causes some defects in composite materials such as delamination, fiber pull out and matrix cracking. Because of non-stationary behavior of drilling process, using online method to monitor these damage mechanisms is inevitable. In this paper, acoustic emission signals and wavelet analysis are applied to monitor drilling action from entry to exit. The results show that the selected monitoring indices from the time domain parameters and wavelet packet coefficients are capable of detecting the drilling stages and damage mechanisms during the process effectively.
Abstract Drilling composite materials is one of the secondary processes of manufacturing industri... more Abstract Drilling composite materials is one of the secondary processes of manufacturing industrial structures. However, drilling composite materials presents a number of problems such as degradation of mechanical behavior. In this study, effects of spindle speed, feed rate, and drill point geometry on residual tensile strength are studied. Acoustic emission technique with a wavelet-based signal processing method is developed to monitor the residual tensile strength of drilled laminates. Cumulative count, amplitude, and energy are ...
Composite materials have been widely used in various industries due to their superior mechanical ... more Composite materials have been widely used in various industries due to their superior mechanical properties. Drilling is a very common machining operation to install fasteners for assembly of laminates. Delamination, however, is a serious concern in the drilling of fiber reinforced composite materials, because it reduces their compressive residual strength. This paper studies the effects of drilling parameters on the thrust force, adjusted delamination factor and compressive residual strength of uni-directional glass/epoxy resin. The design of the experiment was based on the Taguchi method. The results highlight the importance of the feed rate for maximizing the compressive residual strength of drilled laminates. The Acoustic Emission (AE) technique was also used to monitor both drilling process and compression test. The objective was to establish a correlation between AE parameters and mechanical characteristics. The results reveal that root mean square (RMS) can be used for monitoring thrust force and AE energy for compression force.
In this study, acoustic emission (AE) monitoring with a Fuzzy C-Means (FCM) clustering is develop... more In this study, acoustic emission (AE) monitoring with a Fuzzy C-Means (FCM) clustering is developed to detect the delamination process during quasi-static 3-point bending test on glass/epoxy composite materials. The main fracture mode that should be emphasized and has an effect on the residual strength of composite materials is delamination. The 3-point bending test simulates thrust force due to drilling process without backup plate. In this work, two types of specimen at different layups, woven [0,90]s and unidirectional [0]s, leading to different levels of damage evolution, were studied. Using acoustic emission monitoring can help to detect these fracture mechanisms. The obtained AE signals were classified using FCM. Dependency percentage of damages in each class is different in two specimens. Three parameters (Peak Amplitude, Count, and Average Frequency) were used to validate the FCM based classification. The results show that there is a good agreement with the FCM classification and microscopic observation by SEM. Introduction Delamination is one of the most common failure modes of composite materials. It may be caused by service life conditions or, may result from production process such as drilling. As the fields of application expand, the use of types of machining such as turning, drilling, milling, and cutting-off has increased in GFRP fabrication. However, the glass fibre constituent often renders the machining of GFRP difficult. Machining of composite materials requires a better understanding of cutting processes to achieve accuracy and efficiency [1]. Among the problems caused by drilling, delamination is an important defect in composite structure. delamination drastically reduces assembly tolerance and strengthagainst fatigue, thus degrading the long-term performance of composites[2]. Thrust force is the most affective factor in delamination induced by drilling[3]. The 3-point bending test simulates thrust force due to drilling process without backup plate and regardless of other parameters like friction and spindle speed with specific geometry of the tool. In order to identify the type of damage in composites, which involves fiber failure, the transverse failure and delamination, the acoustic emission (AE) technique is also promising for the detection of the damage type [4]. The acoustic emission phenomenon is the result of transient elastic wave propagation generated by a rapid release of energy within a material due to changes in local stress and strain fields. These elastic waves have a wide range of frequencies distribution including audible frequencies and frequencies in MHz range [5-6]. This technique is based on the detection of elastic surface stress waves caused by the dissipation of elastic energy due to the opening of a crack or plastic deformation. Several studies were already conducted on the mode I fracture to determine the surface energies of composite materials by creating a crack propagation effect [7-8]. To find correlation between AE parameters and damage mechanism several studies have been carried out [9,10]. So far most studies have used AE descriptors such as counts, amplitude, energy, wavelet and also multiparameters method to characterize the development of failure mechanisms [11,12].
Composite materials have been widely used in various industries due to their superior mechanical ... more Composite materials have been widely used in various industries due to their superior mechanical properties. Drilling is a very common machining operation to install fasteners for assembly of laminates. Delamination, however, is a serious concern in the drilling of fiber reinforced composite materials, because it reduces their compressive residual strength. This paper studies the effects of drilling parameters on the thrust force, adjusted delamination factor and compressive residual strength of uni-directional glass/epoxy resin. The design of the experiment was based on the Taguchi method. The results highlight the importance of the feed rate for maximizing the compressive residual strength of drilled laminates. The Acoustic Emission (AE) technique was also used to monitor both drilling process and compression test. The objective was to establish a correlation between AE parameters and mechanical characteristics. The results reveal that root mean square (RMS) can be used for monitoring thrust force and AE energy for compression force.
Drilling results in damage such as delamination and matrix cracking around the hole and might ult... more Drilling results in damage such as delamination and matrix cracking around the hole and might ultimately causes degradation in the residual tensile strength of the drilled components. The damage induced during the drilling of composites can be detrimental to the mechanical behavior of the composite products. In this work, the effects of machining parameters (feed rate and cutting speed) and drill point angle on thrust force, the adjusted delamination factor and residual tensile strength are investigated. The Taguchi technique for the design of experiments was employed to analyze the thrust force, adjusted delamination factor and residual tensile strength of woven glass/resin epoxy. The results show that feed rate and drill point angle are the most important parameters. During tensile testing of drilled laminates, acoustic emission (AE) events were recorded. By feature extraction of AE time domain parameters, the suitable parameter for detecting the characteristics of thrust force and tensile force were determined. The AE mean power (MP) and cumulative count correlated well with thrust force and tensile force, respectively.
Conventional methods of analysis for drilling of composite materials usually study the amount of ... more Conventional methods of analysis for drilling of composite materials usually study the amount of damaged area, thrust force, and effective parameters. However, these methods do not provide the investigator with sufficient information about drilling mechanisms. In the current investigation, a procedure for diagnosing different drilling mechanisms based on the analysis of the signals of acoustic emission is presented. According to the number of time domain acoustic emission parameters, using multi-variable methods of analysis is unavoidable. In this work, unsupervised pattern recognition analyses (fuzzy C-means clustering) associated with a principal component analysis are the tools that are used for the classification of the recorded acoustic emission data. After classification of acoustic emission events, the resulting classes are correlated with the different drilling stages and mechanisms. Acoustic emission signal analysis provides a better discrimination of drilling stages than mechanic-based analyses.
Journal of The Acoustical Society of America, 2008
Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stress... more Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stresses coexist. In the objective to monitor online the SCC on real structures, it seems reasonable to characterize and recognize acoustic emission during static U-bend tests. The present study is concerned with static tests on 304 stainless steels in two different media (5% HCl and 5% H2SO4) in order to find a criterion to distinguish, the two different mechanism of SCC (anodic dissolution and hydrogen embrittlement) by characteristic parameters and waveform analysis of AE signals. In the next stage, effect of residual stress on stress corrosion cracking in chloride solution has been studied. Three samples have been prepared: one of them was stress relieved; the second one was made sensitive to SCC by heat treatment and the last one was cold worked by rolling. Cumulative count, amplitude distribution and waveform analysis were selected as AE-parameters. AE with amplitudes ranging from 39 to 65 dB with different counts and energy occurred during SCC of SS-304 at room temperature. According to the results, acoustic emission is able to be used as robust technique for mechanism detection of SCC. In addition it can be used to measure the effect of residual stress in manufacturing process on SCC.
In this study, two types of specimens, woven [0, 90] s and unidirectional [0] s glass/epoxy compo... more In this study, two types of specimens, woven [0, 90] s and unidirectional [0] s glass/epoxy composites, were studied. Acoustic emission (AE) monitoring with wavelet-based signal processing technique and Fuzzy C-Means (FCM) clustering method was used to detect the different fracture mechanisms during delamination in quasi-static three-point bending test. The three-point bending test simulates thrust force, the most effective factor in delamination, throughout the process of drilling without backup plate. The obtained AE signals were decomposed into various wavelet levels and classified using FCM. Scanning Electron Microscopy (SEM) observation was used to determine the different fracture mechanisms. The results show that the energy of AE signals has been concentrated in two significant components for both types of specimens, and the dependency percentage of damages in the two specimens is different.
Structural parts made of composites have frequently to be drilled in the industry. However, littl... more Structural parts made of composites have frequently to be drilled in the industry. However, little is now about the interacting conditions between the drill tool and material, which may be multi-type and multi-size. Delamination free in drilling different fiber reinforced composites is the main objective of present paper. Therefore the influence of drilling and materials variables thrust force and delamination of GFRP composite was investigated experimentally. Drilling variables are cutting speed and feed; material variable is fiber orientation. Acoustic Emission sensing was employed for online detection of composite damage induced by drilling .This paper addresses an application of wavelet-based signal processing technique on a composite during drilling. The wavelet methodology is introduced and procedure of waveletbased acoustic emission (AE) analysis methods is demonstrated. Result shows Acoustic Emission analysis by wavelet method can monitor damage mechanism in drilling of composites.
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