Kristoffer A. Brekken, R. Kaufmann, V. Aune, M. Langseth, T. Børvik
Deformable components such as sandwich structures possess promising properties for use in protection systems. Detailed studies on energy absorption and fluid–structure interaction effects are necessary for the application of deformable sandwich structures in blast resistant design. In this paper, an existing shock tube facility has been extended with a transparent section to observe and measure fluid flow and the structural response of deformable components during transient dynamic loading. The extension was instrumented with pressure sensors and load cells to measure the pressure and force transmitted through the component during testing. The transparent design allows the use of optical measurement techniques. Here, high‐speed cameras were used both for digital image correlation and background‐oriented schlieren imaging. Tests with free‐standing plates and sandwich components were performed. A strong dependency was observed between the plate mass, and thus the velocity of the plates, and the pressure measured upstream and downstream of the components. The tests were simulated with a one‐dimensional numerical model for compressible shock flow with fluid–structure interaction. The numerical model accurately reproduced the shock flow and component displacements measured experimentally. Overall, the experimental set‐up presented in this study proved to be suitable for the detailed examination of deformable components subjected to airblast loading.
{"title":"An experimental facility for detailed studies on energy absorbing components subjected to blast loading","authors":"Kristoffer A. Brekken, R. Kaufmann, V. Aune, M. Langseth, T. Børvik","doi":"10.1111/str.12452","DOIUrl":"https://doi.org/10.1111/str.12452","url":null,"abstract":"Deformable components such as sandwich structures possess promising properties for use in protection systems. Detailed studies on energy absorption and fluid–structure interaction effects are necessary for the application of deformable sandwich structures in blast resistant design. In this paper, an existing shock tube facility has been extended with a transparent section to observe and measure fluid flow and the structural response of deformable components during transient dynamic loading. The extension was instrumented with pressure sensors and load cells to measure the pressure and force transmitted through the component during testing. The transparent design allows the use of optical measurement techniques. Here, high‐speed cameras were used both for digital image correlation and background‐oriented schlieren imaging. Tests with free‐standing plates and sandwich components were performed. A strong dependency was observed between the plate mass, and thus the velocity of the plates, and the pressure measured upstream and downstream of the components. The tests were simulated with a one‐dimensional numerical model for compressible shock flow with fluid–structure interaction. The numerical model accurately reproduced the shock flow and component displacements measured experimentally. Overall, the experimental set‐up presented in this study proved to be suitable for the detailed examination of deformable components subjected to airblast loading.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-06-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42485058","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Shot peen forming is a process widely used to shape aircraft components such as wing skins, yet its fundamental working is still crudely understood. It is understood that a light conventional peen forming treatment applied uniformly over an initially flat plate will induce isotropic in‐plane stretching of the surface layer and will thus lead to a panel curving with identical curvatures in all directions. However, [1] made the startling observation that uniformly peen formed aluminium plates of different aspect ratios all bent along their laminating direction irrespective of the peening direction. This experimental result is counterintuitive because the residual stresses due to the lamination process are 1 order to 2 orders of magnitude smaller than those induced by the shot peen forming treatment. In the present study, we apply the eigenstrain theory to estimate the effect of the different sources of anisotropy on uniformly peen formed aluminium plates. Potential sources of anisotropy included the plastic anisotropy of rolled aluminium, nonequibiaxial initial stresses that redistribute when their equilibrium is disturbed by peening, the geometry of the specimens and externally applied prestress. For the alloy and peening conditions considered, we show that plastic anisotropy had no discernible influence on the resulting shape of the peen formed specimens. Initial residual stresses, on the other hand, caused slightly larger bending loads in the rolling direction of the alloy. Although the magnitude of these loads was approximately 30 times smaller than peening‐induced loads, it was sufficient to overcome the geometric preference for rectangular sheets to bend along their long side and cause all unconstrained specimens to bend along the rolling direction instead. Our analysis highlights the importance of the history of the material that is being peened. Residual stresses already present in the part before peening must be considered to ensure good simulation predictions.
{"title":"Eigenstrain‐based analysis of why uniformly shot peened aluminium plates bend more in the rolling direction","authors":"Hong Yan Miao, M. Lévesque, F. Gosselin","doi":"10.1111/str.12451","DOIUrl":"https://doi.org/10.1111/str.12451","url":null,"abstract":"Shot peen forming is a process widely used to shape aircraft components such as wing skins, yet its fundamental working is still crudely understood. It is understood that a light conventional peen forming treatment applied uniformly over an initially flat plate will induce isotropic in‐plane stretching of the surface layer and will thus lead to a panel curving with identical curvatures in all directions. However, [1] made the startling observation that uniformly peen formed aluminium plates of different aspect ratios all bent along their laminating direction irrespective of the peening direction. This experimental result is counterintuitive because the residual stresses due to the lamination process are 1 order to 2 orders of magnitude smaller than those induced by the shot peen forming treatment. In the present study, we apply the eigenstrain theory to estimate the effect of the different sources of anisotropy on uniformly peen formed aluminium plates. Potential sources of anisotropy included the plastic anisotropy of rolled aluminium, nonequibiaxial initial stresses that redistribute when their equilibrium is disturbed by peening, the geometry of the specimens and externally applied prestress. For the alloy and peening conditions considered, we show that plastic anisotropy had no discernible influence on the resulting shape of the peen formed specimens. Initial residual stresses, on the other hand, caused slightly larger bending loads in the rolling direction of the alloy. Although the magnitude of these loads was approximately 30 times smaller than peening‐induced loads, it was sufficient to overcome the geometric preference for rectangular sheets to bend along their long side and cause all unconstrained specimens to bend along the rolling direction instead. Our analysis highlights the importance of the history of the material that is being peened. Residual stresses already present in the part before peening must be considered to ensure good simulation predictions.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49441028","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The main aim of this work is to comparatively reveal the effect of fibre type, length and content on compressive strength and microstructure of structural geopolymer concrete (GPC) produced under constant mixture and curing parameters in order to address the significant gap in present literature. Firstly, GPCs with different NaOH concentrations (i.e., 6, 9, 12 and 15 M) and activator solution/binder (a/b) ratios (i.e., 0.45 and 0.55) were produced in ambient curing condition, and optimum production parameters were determined based on the preliminary evaluations. Then, glass and polypropylene fibres in 6‐mm length (GS6 and PP6) and polyamide and polypropylene fibres in 12‐mm length (PY12 and PP12) were included in GPCs at ratio of 0.4%, 0.8% and 1.2% (by volume). Compressive strength, apparent porosity, bulk density, ultrasonic pulse velocity (UPV), X‐ray diffraction (XRD) and scanning electron microscope (SEM) analysis of GPC samples were carried out comparatively. The inclusion of GS6 fibre enhanced the compressive strength thanks to fibre surface being covered by geopolymer gel and the strong adhesion between GS fibre and geopolymer matrix. SEM images of fibre reinforced GPC (FRGPC) also confirmed the experimental findings, which were attributed to improvement in compressive strength. Regardless of the fibre type, the maximum compressive value strength was obtained from GPC specimens with 0.4% fibre and then decreased. Higher fibre inclusions led to poor compaction, workability issues and inhomogeneous fibre dispersions. A very good relation (R2 = 0.98) was acquired between UPV and compressive strength values of GPC/FRGPC samples.
{"title":"Effect of fibre characteristics on physical, mechanical and microstructural properties of geopolymer concrete: A comparative experimental investigation","authors":"Fatih Kantarcı","doi":"10.1111/str.12453","DOIUrl":"https://doi.org/10.1111/str.12453","url":null,"abstract":"The main aim of this work is to comparatively reveal the effect of fibre type, length and content on compressive strength and microstructure of structural geopolymer concrete (GPC) produced under constant mixture and curing parameters in order to address the significant gap in present literature. Firstly, GPCs with different NaOH concentrations (i.e., 6, 9, 12 and 15 M) and activator solution/binder (a/b) ratios (i.e., 0.45 and 0.55) were produced in ambient curing condition, and optimum production parameters were determined based on the preliminary evaluations. Then, glass and polypropylene fibres in 6‐mm length (GS6 and PP6) and polyamide and polypropylene fibres in 12‐mm length (PY12 and PP12) were included in GPCs at ratio of 0.4%, 0.8% and 1.2% (by volume). Compressive strength, apparent porosity, bulk density, ultrasonic pulse velocity (UPV), X‐ray diffraction (XRD) and scanning electron microscope (SEM) analysis of GPC samples were carried out comparatively. The inclusion of GS6 fibre enhanced the compressive strength thanks to fibre surface being covered by geopolymer gel and the strong adhesion between GS fibre and geopolymer matrix. SEM images of fibre reinforced GPC (FRGPC) also confirmed the experimental findings, which were attributed to improvement in compressive strength. Regardless of the fibre type, the maximum compressive value strength was obtained from GPC specimens with 0.4% fibre and then decreased. Higher fibre inclusions led to poor compaction, workability issues and inhomogeneous fibre dispersions. A very good relation (R2 = 0.98) was acquired between UPV and compressive strength values of GPC/FRGPC samples.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-06-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45526617","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The objective of this work is to analyse the influence of adhesive fillets on the fatigue/fracture behaviour of single‐strap adhesive repairs of carbon‐epoxy composites. A cohesive zone model (CZM) appropriate for high‐cycle fatigue analysis was employed. A preliminary model validation was performed using results ensuing from experimental testing of single‐strap adhesive repairs without fillets. Subsequently, the numerical model was used to investigate the effect of outer, inner and both (outer and inner) fillets on the quasi‐static strengths and on the fatigue lives of these repairs. It was concluded that inner fillets provide the best option concerning the increase of fatigue life or a maximum fatigue load for a given planned service‐life of the repaired component.
{"title":"Influence of adhesive fillets on fatigue behaviour of single‐strap composite repairs","authors":"F. Ramírez, R. Moreira, M. D. de Moura","doi":"10.1111/str.12454","DOIUrl":"https://doi.org/10.1111/str.12454","url":null,"abstract":"The objective of this work is to analyse the influence of adhesive fillets on the fatigue/fracture behaviour of single‐strap adhesive repairs of carbon‐epoxy composites. A cohesive zone model (CZM) appropriate for high‐cycle fatigue analysis was employed. A preliminary model validation was performed using results ensuing from experimental testing of single‐strap adhesive repairs without fillets. Subsequently, the numerical model was used to investigate the effect of outer, inner and both (outer and inner) fillets on the quasi‐static strengths and on the fatigue lives of these repairs. It was concluded that inner fillets provide the best option concerning the increase of fatigue life or a maximum fatigue load for a given planned service‐life of the repaired component.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" 11","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41253674","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
N. Vonk, S. Van Weele, G. Slokker, M. van Maris, J. Hoefnagels
Most methodologies to measure the moisture‐induced deformation (hygro‐expansion) of paper microconstituents, including fibres and interfibre bonds, are low resolution or time‐consuming. Hence, here, a novel method is proposed and validated to measure high‐resolution full‐field strain maps of paper microconstituents during hygro‐expansion, based on environmental scanning electron microscopy (ESEM). To this end, a novel climate stage enables accurate control of the relative humidity (RH) near the specimen in the ESEM from 0%–100%. The fibre surface, which is decorated a priori with a microparticle pattern, is captured during RH change. Subsequently, correlating the fibre surface using a dedicated global digital image correlation algorithm enables high‐resolution hygro‐expansion strain maps. Method optimisation involved performing contrast enhancement, scan‐correction to reduce ESEM artefacts and a background correction, resulting in a strain resolution of 6·10−4 . Method validation revealed that the fibres' crystallinity is affected by the electron beam, even for minimal invasive electron beam settings. Interestingly, however, the fibres consistently exhibit conventional hygro‐expansion behaviour during the drying slopes. Using the optimised procedure, hygro‐expansion characterisation of two interfibre bonds and four interfibre bond cross‐sections revealed the competition between the low longitudinal and large transverse fibre hygro‐expansion in the bonded area.
{"title":"Challenges and solutions of environmental scanning electron microscopy characterisation of biomaterials: Application to hygro‐expansion of paper","authors":"N. Vonk, S. Van Weele, G. Slokker, M. van Maris, J. Hoefnagels","doi":"10.1111/str.12440","DOIUrl":"https://doi.org/10.1111/str.12440","url":null,"abstract":"Most methodologies to measure the moisture‐induced deformation (hygro‐expansion) of paper microconstituents, including fibres and interfibre bonds, are low resolution or time‐consuming. Hence, here, a novel method is proposed and validated to measure high‐resolution full‐field strain maps of paper microconstituents during hygro‐expansion, based on environmental scanning electron microscopy (ESEM). To this end, a novel climate stage enables accurate control of the relative humidity (RH) near the specimen in the ESEM from 0%–100%. The fibre surface, which is decorated a priori with a microparticle pattern, is captured during RH change. Subsequently, correlating the fibre surface using a dedicated global digital image correlation algorithm enables high‐resolution hygro‐expansion strain maps. Method optimisation involved performing contrast enhancement, scan‐correction to reduce ESEM artefacts and a background correction, resulting in a strain resolution of 6·10−4 . Method validation revealed that the fibres' crystallinity is affected by the electron beam, even for minimal invasive electron beam settings. Interestingly, however, the fibres consistently exhibit conventional hygro‐expansion behaviour during the drying slopes. Using the optimised procedure, hygro‐expansion characterisation of two interfibre bonds and four interfibre bond cross‐sections revealed the competition between the low longitudinal and large transverse fibre hygro‐expansion in the bonded area.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-06-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49530752","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Dufour, G. Colantonio, C. Bouvet, J. Perie, J. Passieux, J. Serra, Institut John-Eric Dufour, Clément Ader
Even though the simulations used to describe the failure of laminates are becoming more and more predictive, complex testing under multiaxial loadings is still required to validate the design of structural parts in a wide range of industrial domains. It is thus essential to assess the actual boundary conditions to allow for an objective comparison between testing and calculations, in particular since the structural tests are complex and often leads to buckling. Therefore, accurate estimation of force and moment fluxes applied to the specimen is critical. In this context, stereo digital image correlation (SDIC) has proven to be an important measurement tool and provides very well‐resolved surface displacement fields, but the exploitation of such measurements to calculate fluxes remains problematic when testing composites. The first objective of this study is both to reduce the uncertainty associated with fluxes determination on a complex test and to simplify the extraction process with respect to existing procedures. The second objective is to make this methodology robust to geometrically non‐linear deformations. In this paper, we propose a new methodology that extracts minimal boundary conditions in the form of 3D mechanically admissible displacements fields. The approach developed uses a finite element SDIC (FE‐SDIC) method regularized by means of mechanical behaviour admissibility equations. Results show that the new methodology outputs much more accurate fluxes than classical data generated from multiple differentiations of the displacement fields. Excellent noise robustness is obtained and quantified. Numerical predictions have been satisfactorily compared with experimental data from one structural‐scale composite specimen under complex testing.
{"title":"Monitoring structural scale composite specimens in a post‐buckling regime: The integrated finite element stereo digital image correlation approach with geometrically non‐linear regularization","authors":"J. Dufour, G. Colantonio, C. Bouvet, J. Perie, J. Passieux, J. Serra, Institut John-Eric Dufour, Clément Ader","doi":"10.1111/str.12450","DOIUrl":"https://doi.org/10.1111/str.12450","url":null,"abstract":"Even though the simulations used to describe the failure of laminates are becoming more and more predictive, complex testing under multiaxial loadings is still required to validate the design of structural parts in a wide range of industrial domains. It is thus essential to assess the actual boundary conditions to allow for an objective comparison between testing and calculations, in particular since the structural tests are complex and often leads to buckling. Therefore, accurate estimation of force and moment fluxes applied to the specimen is critical. In this context, stereo digital image correlation (SDIC) has proven to be an important measurement tool and provides very well‐resolved surface displacement fields, but the exploitation of such measurements to calculate fluxes remains problematic when testing composites. The first objective of this study is both to reduce the uncertainty associated with fluxes determination on a complex test and to simplify the extraction process with respect to existing procedures. The second objective is to make this methodology robust to geometrically non‐linear deformations. In this paper, we propose a new methodology that extracts minimal boundary conditions in the form of 3D mechanically admissible displacements fields. The approach developed uses a finite element SDIC (FE‐SDIC) method regularized by means of mechanical behaviour admissibility equations. Results show that the new methodology outputs much more accurate fluxes than classical data generated from multiple differentiations of the displacement fields. Excellent noise robustness is obtained and quantified. Numerical predictions have been satisfactorily compared with experimental data from one structural‐scale composite specimen under complex testing.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-06-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47741021","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Kunnoth Sriram, P. Mahajan, S. Ahmad, N. Bhatnagar
Compressive behaviour of open and closed cell polyurethane foam samples under large deformation is studied using micro‐Computed Tomography (micro‐CT), Digital Volume Correlation (DVC) technique and micro‐Finite Element (micro‐FE) modelling. The micro‐CT images of the foam samples at different compression strains are used to determine anisotropy in the foams, to obtain qualitative information on deformation mechanisms, to quantify the deformation and strains using a local DVC approach and to generate images for micro‐FE modelling of the foam samples. Micro‐FE modelling predicts the deformation using an elastoplastic material model coupled with continuum damage mechanics. Two different types of boundary conditions, experimentally derived (ExBC) and interpolated from DVC (IPBC), were implemented to evaluate the displacements in the micro‐FE models. A reduced integration scheme in micro‐FE analysis resulted in high artificial energy and was discarded in favour of full integration. The displacement predicted by IPBC matched with DVC displacement contours for closed cell foam. The ExBC‐predicted axial displacement (W) showed a better agreement with DVC than transverse displacements (U, V) contours. However, a significant statistical comparison (R2 > 0.70) of all displacements was obtained for both IPBC and ExBC. For open cell foam, both boundary conditions predicted a significant difference in the displacement contours with respect to DVC measurements. Still, the axial displacements of ExBC and IPBC showed a better statistical significance (R2 > 0.70).
{"title":"Evaluation of compressive behaviour of porous structures under large deformation using micro‐CT, DVC and micro‐FE","authors":"Kunnoth Sriram, P. Mahajan, S. Ahmad, N. Bhatnagar","doi":"10.1111/str.12441","DOIUrl":"https://doi.org/10.1111/str.12441","url":null,"abstract":"Compressive behaviour of open and closed cell polyurethane foam samples under large deformation is studied using micro‐Computed Tomography (micro‐CT), Digital Volume Correlation (DVC) technique and micro‐Finite Element (micro‐FE) modelling. The micro‐CT images of the foam samples at different compression strains are used to determine anisotropy in the foams, to obtain qualitative information on deformation mechanisms, to quantify the deformation and strains using a local DVC approach and to generate images for micro‐FE modelling of the foam samples. Micro‐FE modelling predicts the deformation using an elastoplastic material model coupled with continuum damage mechanics. Two different types of boundary conditions, experimentally derived (ExBC) and interpolated from DVC (IPBC), were implemented to evaluate the displacements in the micro‐FE models. A reduced integration scheme in micro‐FE analysis resulted in high artificial energy and was discarded in favour of full integration. The displacement predicted by IPBC matched with DVC displacement contours for closed cell foam. The ExBC‐predicted axial displacement (W) showed a better agreement with DVC than transverse displacements (U, V) contours. However, a significant statistical comparison (R2 > 0.70) of all displacements was obtained for both IPBC and ExBC. For open cell foam, both boundary conditions predicted a significant difference in the displacement contours with respect to DVC measurements. Still, the axial displacements of ExBC and IPBC showed a better statistical significance (R2 > 0.70).","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46748791","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Afshar, J. Stjärnesund, E. Gamstedt, O. Girlanda, F. Sahlén, D. Tjahjanto
As a non‐destructive inspection method, micro‐computed tomography has been employed for determining local properties of a cellulose‐based product, specifically pressboard. Furthermore, by utilizing the determined properties in a detailed numerical model, by means of a finite element analysis, we demonstrate a continuum anisotropic viscoelastic‐viscoplastic model. Through such a combination of non‐invasive experiments with accurate computations in mechanics, we attain a better understanding of materials and its structural integrity at a pre‐production stage increasing the success of the first prototype. In detail, this combination of micro‐computed tomography and finite element analysis improves accuracy in predicting materials response by taking into account the local material variations. Specifically, we have performed indentation tests and scanned the internal structure of the specimen for analysing the densification patterns within the material. Subsequently, we have used a developed material model for predicting the response of material to indentation. We have computed the indentation test itself by simulating the mechanical response of high‐density cellulose‐based materials. In the end, we have observed that pressboard, having initially a heterogeneous density distribution through the thickness, shows a shift in the densification to the more porous part after indentation. The densification maps of the simulated results are presented by comparing with the experimental results. A reasonable agreement is observed between the experimental and the simulated densifications patterns, which suggests that the proposed methodology can be used to predict densification also for other fibre‐based materials during manufacturing or in service loading.
{"title":"A micro‐CT investigation of densification in pressboard due to compression","authors":"R. Afshar, J. Stjärnesund, E. Gamstedt, O. Girlanda, F. Sahlén, D. Tjahjanto","doi":"10.1111/str.12442","DOIUrl":"https://doi.org/10.1111/str.12442","url":null,"abstract":"As a non‐destructive inspection method, micro‐computed tomography has been employed for determining local properties of a cellulose‐based product, specifically pressboard. Furthermore, by utilizing the determined properties in a detailed numerical model, by means of a finite element analysis, we demonstrate a continuum anisotropic viscoelastic‐viscoplastic model. Through such a combination of non‐invasive experiments with accurate computations in mechanics, we attain a better understanding of materials and its structural integrity at a pre‐production stage increasing the success of the first prototype. In detail, this combination of micro‐computed tomography and finite element analysis improves accuracy in predicting materials response by taking into account the local material variations. Specifically, we have performed indentation tests and scanned the internal structure of the specimen for analysing the densification patterns within the material. Subsequently, we have used a developed material model for predicting the response of material to indentation. We have computed the indentation test itself by simulating the mechanical response of high‐density cellulose‐based materials. In the end, we have observed that pressboard, having initially a heterogeneous density distribution through the thickness, shows a shift in the densification to the more porous part after indentation. The densification maps of the simulated results are presented by comparing with the experimental results. A reasonable agreement is observed between the experimental and the simulated densifications patterns, which suggests that the proposed methodology can be used to predict densification also for other fibre‐based materials during manufacturing or in service loading.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46726019","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Utilising a series of mechanically over‐excavated cavities along borehole is a novel technique for enhancing the permeability of soft coal seams and, consequently, gas drainage. The evolution of cracks induced by a wide range of pressure‐relief around an over‐excavated hole is intrinsically complex. In this study, the mechanical behaviour and crack evolution of the specimens containing an over‐excavated hole under uniaxial compression loading were studied by experimental and 3D numerical simulation. The results indicated that the peak strength and elastic modulus of the specimens gradually decrease with increasing cavity diameter and length, which is also verified by the numerical simulation. The inclusion of cylindrical cavities in over‐excavated holes results in reduced crack initiation stress and a greater degradation of peak stress and elastic modulus, despite having an equivalent volume to the ellipsoidal cavity. This is likely attributed to the difference in stress concentration between the cylindrical and ellipsoidal cavities. The crack propagation process can be classified into four stages based on the acoustic emission (AE) event counts, initial crack compaction, stable crack propagation, unstable crack propagation and post‐peak failure stage. The two AE indices, rise angle and average frequency value, demonstrated that the failure is dominated by tensile crack and gradually transformed to shear crack. Stress redistribution is essential in the initiation and propagation of cracks. Tensile stress concentration leads to cracks forming at the top and bottom of the hole, which propagate in the direction of loading. Compressive stress concentration results in shear cracks forming at the left and right sides of the hole, which propagate diagonally. The failure pattern of the specimen is ultimately determined by a combination of tensile and mixed crack propagation. The experimental and numerical results contribute to a deeper understanding of the crack evolution mechanism of coal seams with over‐excavated holes.
{"title":"Mechanical behaviour and fracture evolution of coal specimens containing an over‐excavated hole: Experimental study and numerical modelling","authors":"Zhongyi Man, Liu Chun, Mingyao Wei, Yonglong Wang","doi":"10.1111/str.12443","DOIUrl":"https://doi.org/10.1111/str.12443","url":null,"abstract":"Utilising a series of mechanically over‐excavated cavities along borehole is a novel technique for enhancing the permeability of soft coal seams and, consequently, gas drainage. The evolution of cracks induced by a wide range of pressure‐relief around an over‐excavated hole is intrinsically complex. In this study, the mechanical behaviour and crack evolution of the specimens containing an over‐excavated hole under uniaxial compression loading were studied by experimental and 3D numerical simulation. The results indicated that the peak strength and elastic modulus of the specimens gradually decrease with increasing cavity diameter and length, which is also verified by the numerical simulation. The inclusion of cylindrical cavities in over‐excavated holes results in reduced crack initiation stress and a greater degradation of peak stress and elastic modulus, despite having an equivalent volume to the ellipsoidal cavity. This is likely attributed to the difference in stress concentration between the cylindrical and ellipsoidal cavities. The crack propagation process can be classified into four stages based on the acoustic emission (AE) event counts, initial crack compaction, stable crack propagation, unstable crack propagation and post‐peak failure stage. The two AE indices, rise angle and average frequency value, demonstrated that the failure is dominated by tensile crack and gradually transformed to shear crack. Stress redistribution is essential in the initiation and propagation of cracks. Tensile stress concentration leads to cracks forming at the top and bottom of the hole, which propagate in the direction of loading. Compressive stress concentration results in shear cracks forming at the left and right sides of the hole, which propagate diagonally. The failure pattern of the specimen is ultimately determined by a combination of tensile and mixed crack propagation. The experimental and numerical results contribute to a deeper understanding of the crack evolution mechanism of coal seams with over‐excavated holes.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41849206","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Issue Information","authors":"","doi":"10.1111/str.12419","DOIUrl":"https://doi.org/10.1111/str.12419","url":null,"abstract":"No abstract is available for this article.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-05-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42993460","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"材料科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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