This paper presents a novel apparatus, combined with application of the Virtual Fields Method (VFM) for the high frequency characterisation of the mechanical response of ultra‐soft materials. The viscoelastic response is characterised under harmonic deviatoric loading at a range of frequencies using the dynamic VFM in the frequency domain. Obtaining useful high rate data on soft materials is challenging using typical test methodologies, as the low speed of sound in the material makes stress equilibrium difficult to obtain; additionally, the low stiffness results in small stresses and weak measurement signals. This novel apparatus and analysis method have been shown capable of obtaining important material characterisation data that would be impossible to access using currently existing test techniques.
{"title":"Characterising the frequency‐response of ultra‐soft polymers with the Virtual Fields Method","authors":"Aaron Graham, C. Siviour","doi":"10.1111/str.12386","DOIUrl":"https://doi.org/10.1111/str.12386","url":null,"abstract":"This paper presents a novel apparatus, combined with application of the Virtual Fields Method (VFM) for the high frequency characterisation of the mechanical response of ultra‐soft materials. The viscoelastic response is characterised under harmonic deviatoric loading at a range of frequencies using the dynamic VFM in the frequency domain. Obtaining useful high rate data on soft materials is challenging using typical test methodologies, as the low speed of sound in the material makes stress equilibrium difficult to obtain; additionally, the low stiffness results in small stresses and weak measurement signals. This novel apparatus and analysis method have been shown capable of obtaining important material characterisation data that would be impossible to access using currently existing test techniques.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-05-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12386","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42117018","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. M. García, Marie-Thérèse Auclair, T. Morgeneyer
Stereo‐digital image correlation (SDIC) is used for the analysis and quantification of parasite bending stresses in a fatigue specimen due to its mounting in a misaligned load frame. The aim of this work is to provide an easy way to estimate the induced parasite bending stresses due to the load frame misalignment, using the out‐of‐plane displacement measurements obtained by SDIC and beam theory. Analytical solutions of parasite stresses as a function of out‐of‐plane misalignments are provided for flat fatigue specimens having (a) tangentially blending fillets between the uniform test section and the ends and (b) continuous radius between the ends. Simple procedures for aligning a commercial load frame in accordance with international standards are also provided. The bending stresses profile along the specimen main axis obtained by numerical simulations have shown a very good agreement with the results obtained by the analytical models. The proposed methodology could complement standard alignment techniques based on samples instrumented with strain gauges or, in their absence, even replace established procedures.
{"title":"On the use of stereo‐digital image correlation for the alignment of a fatigue testing machine in accordance with international standards: A feasibility study","authors":"J. M. García, Marie-Thérèse Auclair, T. Morgeneyer","doi":"10.1111/str.12382","DOIUrl":"https://doi.org/10.1111/str.12382","url":null,"abstract":"Stereo‐digital image correlation (SDIC) is used for the analysis and quantification of parasite bending stresses in a fatigue specimen due to its mounting in a misaligned load frame. The aim of this work is to provide an easy way to estimate the induced parasite bending stresses due to the load frame misalignment, using the out‐of‐plane displacement measurements obtained by SDIC and beam theory. Analytical solutions of parasite stresses as a function of out‐of‐plane misalignments are provided for flat fatigue specimens having (a) tangentially blending fillets between the uniform test section and the ends and (b) continuous radius between the ends. Simple procedures for aligning a commercial load frame in accordance with international standards are also provided. The bending stresses profile along the specimen main axis obtained by numerical simulations have shown a very good agreement with the results obtained by the analytical models. The proposed methodology could complement standard alignment techniques based on samples instrumented with strain gauges or, in their absence, even replace established procedures.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12382","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46245677","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}
A. Bragov, L. Igumnov, A. Konstantinov, L. Kruszka, D. A. Lamzin, A. Lomunov
This paper presents the features of a method for determining the dynamic properties of brittle materials using the split Hopkinson bar technique on mortar as an example. Experiments were conducted both with and without a loading pulse shaper, a deformable copper disc (pad) placed on the impacted end of the input bar. Experimental data from the experiments without the pad were processed with and without dispersion correction based on a direct solution of the Pochhammer–Chree frequency equation of pulses in the measuring bars. There were compared both the deformation graphs and mechanical characteristics of the tested material for three variants of the split Hopkinson pressure bar experiments: with and without a loading pulse shaper, and with the dispersion correction procedure for pulses in the measuring bars. The phenomena observed during these experimental schemes and the processing of obtained data are described and discussed; further, the effect of the nature of the load increase is revealed.
{"title":"Methodological aspects of testing brittle materials using the split Hopkinson bar technique","authors":"A. Bragov, L. Igumnov, A. Konstantinov, L. Kruszka, D. A. Lamzin, A. Lomunov","doi":"10.1111/str.12389","DOIUrl":"https://doi.org/10.1111/str.12389","url":null,"abstract":"This paper presents the features of a method for determining the dynamic properties of brittle materials using the split Hopkinson bar technique on mortar as an example. Experiments were conducted both with and without a loading pulse shaper, a deformable copper disc (pad) placed on the impacted end of the input bar. Experimental data from the experiments without the pad were processed with and without dispersion correction based on a direct solution of the Pochhammer–Chree frequency equation of pulses in the measuring bars. There were compared both the deformation graphs and mechanical characteristics of the tested material for three variants of the split Hopkinson pressure bar experiments: with and without a loading pulse shaper, and with the dispersion correction procedure for pulses in the measuring bars. The phenomena observed during these experimental schemes and the processing of obtained data are described and discussed; further, the effect of the nature of the load increase is revealed.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12389","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44111755","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}
Fatigue cracks at rivet holes occur at advanced service life of aircrafts due to cyclic loading. As a repair method, adhesively bonded stiffener patches enhance the fatigue life of the structure by delaying crack initiation and reducing crack growth. Combining a crack sensor with a repair patch to a sensor‐based stiffener patch allows crack growth reduction and monitoring at the same time. This paper presents a feasibility study on the integration of eddy current sensors into carbon fibre reinforced plastic (CFRP) repair patches. To this end, a specific patch design is developed, and samples are manufactured by ultrasonic fabrication. The performance of the patches is investigated in fatigue tests in terms of ‘crack reinforcement’ and ‘crack detection.’ Both requirements are met. The undertaking of future efforts to tailor the sensor‐based repair patch concept to an individual application in aircraft maintenance seems reasonable.
{"title":"Integration of eddy current sensors into repair patches for fatigue reinforcement at rivet holes","authors":"S. Schmid, U. Martens, W. Schomburg, K. Schröder","doi":"10.1111/str.12387","DOIUrl":"https://doi.org/10.1111/str.12387","url":null,"abstract":"Fatigue cracks at rivet holes occur at advanced service life of aircrafts due to cyclic loading. As a repair method, adhesively bonded stiffener patches enhance the fatigue life of the structure by delaying crack initiation and reducing crack growth. Combining a crack sensor with a repair patch to a sensor‐based stiffener patch allows crack growth reduction and monitoring at the same time. This paper presents a feasibility study on the integration of eddy current sensors into carbon fibre reinforced plastic (CFRP) repair patches. To this end, a specific patch design is developed, and samples are manufactured by ultrasonic fabrication. The performance of the patches is investigated in fatigue tests in terms of ‘crack reinforcement’ and ‘crack detection.’ Both requirements are met. The undertaking of future efforts to tailor the sensor‐based repair patch concept to an individual application in aircraft maintenance seems reasonable.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12387","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42369486","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 present study experimentally investigates the mechanical response of T700/M21 Carbon Fibre Reinforced Polymers subjected to coupled shear and transverse tension at intermediate strain rates ranging from 10−3 to 15 s−1. Off‐axis tension specimens with transverse and oblique tabs are used for the various tests. Moreover, the specimen aspect ratio is quite low, since dynamic tests are performed in this study. The axial stress–strain curves are successfully simulated using finite element simulations with a viscoelastic model. Based on these numerical simulations, specific stress correction factors are computed to take the effect of end constraints into account for such low aspect ratio specimens. Finally, an increase in the material strength with the strain rate is observed.
{"title":"Experimental investigation of the strain‐rate effects on the failure of composite materials with off‐axis tensile tests on unidirectional plies","authors":"T. Fourest, J. Berthe","doi":"10.1111/str.12385","DOIUrl":"https://doi.org/10.1111/str.12385","url":null,"abstract":"The present study experimentally investigates the mechanical response of T700/M21 Carbon Fibre Reinforced Polymers subjected to coupled shear and transverse tension at intermediate strain rates ranging from 10−3 to 15 s−1. Off‐axis tension specimens with transverse and oblique tabs are used for the various tests. Moreover, the specimen aspect ratio is quite low, since dynamic tests are performed in this study. The axial stress–strain curves are successfully simulated using finite element simulations with a viscoelastic model. Based on these numerical simulations, specific stress correction factors are computed to take the effect of end constraints into account for such low aspect ratio specimens. Finally, an increase in the material strength with the strain rate is observed.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-04-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12385","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43376235","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.12356","DOIUrl":"https://doi.org/10.1111/str.12356","url":null,"abstract":"No abstract is available for this article.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12356","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45157078","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}
H. Al‐Karawi, Rüdiger U. Franz Bock und Polach, M. Al‐Emrani
Fatigue cracks have appeared as a significant issue for joints and connections in existing steel structures in the last decades. Therefore, those are a major inspection and maintenance matter for any steel structure's operator. This emphasises the importance of using a reliable detection method to determine the crack size and assessing the severity of such a crack on the structural integrity of a structure. In this article, the effectiveness of strain measurement in detecting fatigue cracks in transversal non‐load carrying welded attachment subjected to out of plane axial loading is studied. Numerical analysis and experimental investigations allowed to correlate the decrease in strain measured by attached gauges to the crack depth at the weld toe. In addition, different strain evolution patterns were found during fatigue testing, and the fracture surfaces of the specimens were observed to interpret these patterns. Moreover, the crack position with respect to the weld toe surface was predicted via strain measurements.
{"title":"Crack detection via strain measurements in fatigue testing","authors":"H. Al‐Karawi, Rüdiger U. Franz Bock und Polach, M. Al‐Emrani","doi":"10.1111/str.12384","DOIUrl":"https://doi.org/10.1111/str.12384","url":null,"abstract":"Fatigue cracks have appeared as a significant issue for joints and connections in existing steel structures in the last decades. Therefore, those are a major inspection and maintenance matter for any steel structure's operator. This emphasises the importance of using a reliable detection method to determine the crack size and assessing the severity of such a crack on the structural integrity of a structure. In this article, the effectiveness of strain measurement in detecting fatigue cracks in transversal non‐load carrying welded attachment subjected to out of plane axial loading is studied. Numerical analysis and experimental investigations allowed to correlate the decrease in strain measured by attached gauges to the crack depth at the weld toe. In addition, different strain evolution patterns were found during fatigue testing, and the fracture surfaces of the specimens were observed to interpret these patterns. Moreover, the crack position with respect to the weld toe surface was predicted via strain measurements.","PeriodicalId":51176,"journal":{"name":"Strain","volume":"4 2","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12384","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41256716","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}
D. Millar, M. Mennu, K. Upadhyay, C. Morley, P. Ifju
Soft materials such as hydrogels and elastomers exhibit very low stiffness and strength as well as large deformations, which makes their mechanical characterisation extremely difficult through conventional methods. This paper presents a novel experimental technique for the mechanical shear property characterisation of these materials. Agarose hydrogels were chosen as a model material for this study. The new in‐plane shear test method incorporates 3‐D‐printed acrylonitrile butadiene styrene (ABS) grips for specimen mounting and digital image correlation (DIC) for full‐field strain measurement on both sides of the specimen gauge section. These grips utilise barb‐like pegs to secure the specimen while load is applied. In order to evaluate the methodology, four concentrations of agarose hydrogel (4.0%, 2.5%, 1.5%, and 0.5% wt./solvent volume) were tested. Results for the agarose hydrogel demonstrated excellent repeatability. The obtained shear moduli show a monotonic increase with gel concentration. Furthermore, the range of shear moduli applicable to the novel testing method was determined.
{"title":"An improved direct shear characterisation technique for soft gelatinous and elastomeric materials","authors":"D. Millar, M. Mennu, K. Upadhyay, C. Morley, P. Ifju","doi":"10.1111/str.12383","DOIUrl":"https://doi.org/10.1111/str.12383","url":null,"abstract":"Soft materials such as hydrogels and elastomers exhibit very low stiffness and strength as well as large deformations, which makes their mechanical characterisation extremely difficult through conventional methods. This paper presents a novel experimental technique for the mechanical shear property characterisation of these materials. Agarose hydrogels were chosen as a model material for this study. The new in‐plane shear test method incorporates 3‐D‐printed acrylonitrile butadiene styrene (ABS) grips for specimen mounting and digital image correlation (DIC) for full‐field strain measurement on both sides of the specimen gauge section. These grips utilise barb‐like pegs to secure the specimen while load is applied. In order to evaluate the methodology, four concentrations of agarose hydrogel (4.0%, 2.5%, 1.5%, and 0.5% wt./solvent volume) were tested. Results for the agarose hydrogel demonstrated excellent repeatability. The obtained shear moduli show a monotonic increase with gel concentration. Furthermore, the range of shear moduli applicable to the novel testing method was determined.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-03-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12383","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47385930","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}
L. Fletcher, F. Davis, S. Dreuilhe, A. Marek, F. Pierron
Current high strain rate testing procedures generally rely on the split Hopkinson bar (SHB). In order to gain accurate material data with this technique, it is necessary to assume the test sample is in a state of quasi‐static equilibrium so that inertial effects can be neglected. During the early portion of an SHB test, it is difficult to satisfy this assumption making it challenging to investigate the elastic–plastic transition for metals. With the development of ultra‐high speed imaging technology, the image‐based inertial impact (IBII) test has emerged as an alternative to the SHB. This technique uses full‐field measurements coupled with the virtual fields method to identify material properties without requiring the assumption of quasi‐static equilibrium.
{"title":"High strain rate elasto‐plasticity identification using the image‐based inertial impact (IBII) test part 1: Error quantification","authors":"L. Fletcher, F. Davis, S. Dreuilhe, A. Marek, F. Pierron","doi":"10.1111/str.12375","DOIUrl":"https://doi.org/10.1111/str.12375","url":null,"abstract":"Current high strain rate testing procedures generally rely on the split Hopkinson bar (SHB). In order to gain accurate material data with this technique, it is necessary to assume the test sample is in a state of quasi‐static equilibrium so that inertial effects can be neglected. During the early portion of an SHB test, it is difficult to satisfy this assumption making it challenging to investigate the elastic–plastic transition for metals. With the development of ultra‐high speed imaging technology, the image‐based inertial impact (IBII) test has emerged as an alternative to the SHB. This technique uses full‐field measurements coupled with the virtual fields method to identify material properties without requiring the assumption of quasi‐static equilibrium.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-02-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12375","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41946114","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}
L. Fletcher, F. Davis, S. Dreuilhe, A. Marek, F. Pierron
Current high strain rate testing techniques typically rely on the split‐Hopkinson bar (SHB). The early response in an SHB test is corrupted by inertia making it difficult to accurately characterise the transition from elasticity to plasticity for metals. Therefore, a new test method is required. This article is the second in a two‐part series which aims at developing a new high strain rate test for elasto‐plasticity identification using the image‐based inertial impact (IBII) method. The goal of this article is to validate the new method experimentally using IBII tests on aluminium 6082‐T6 (minimal rate sensitivity) and stainless steel 316L (rate sensitive). Comparison of the quasi‐static and dynamic stress–strain curves for the aluminium case showed minimal difference providing experimental validation of the method. The same comparison for the steel showed that the method was able to detect rate sensitivity.
{"title":"High strain rate elasto‐plasticity identification using the image‐based inertial impact (IBII) test part 2: Experimental validation","authors":"L. Fletcher, F. Davis, S. Dreuilhe, A. Marek, F. Pierron","doi":"10.1111/str.12374","DOIUrl":"https://doi.org/10.1111/str.12374","url":null,"abstract":"Current high strain rate testing techniques typically rely on the split‐Hopkinson bar (SHB). The early response in an SHB test is corrupted by inertia making it difficult to accurately characterise the transition from elasticity to plasticity for metals. Therefore, a new test method is required. This article is the second in a two‐part series which aims at developing a new high strain rate test for elasto‐plasticity identification using the image‐based inertial impact (IBII) method. The goal of this article is to validate the new method experimentally using IBII tests on aluminium 6082‐T6 (minimal rate sensitivity) and stainless steel 316L (rate sensitive). Comparison of the quasi‐static and dynamic stress–strain curves for the aluminium case showed minimal difference providing experimental validation of the method. The same comparison for the steel showed that the method was able to detect rate sensitivity.","PeriodicalId":51176,"journal":{"name":"Strain","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2021-02-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/str.12374","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43133629","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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