| 1. |
- Nguyen, S., et al.
(författare)
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Experimental characterisation of tyre indentation by simulated runway debris
- 2011
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 47:4, s. 343-350
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Tidskriftsartikel (refereegranskat)abstract
- As part of an investigation to assess the impact threat posed to vehicles by tyre-lofted runway debris, it is important to gain a quantitative understanding of the interaction between inflated tyres and foreign objects. In this paper, experiments involving indentation of an inflated tyre to represent over-rolling of a foreign object were used to estimate the energy that could be transferred to the over-rolled object. The maximum kinetic energy that a 24-mm diameter spherical stone could potentially attain from such an indentation-related loft mechanism by a tyre inflated to 0.34 MPa (50 psi) was 20 J. © 2010 Blackwell Publishing Ltd.
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| 2. |
- Bader, Thomas K., et al.
(författare)
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Microstructure–Stiffness Relationships of Common Yew and Norway Spruce
- 2012
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 48:4, s. 306-316
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Tidskriftsartikel (refereegranskat)abstract
- Yew (Taxus baccata L.) exhibits among conifers a unique macroscopic elastic behaviour. For example, it shows a comparatively low longitudinal elastic modulus related to its comparatively high density. We herein explore the microstructural origin of these peculiarities, aiming at the derivation of microstructure–stiffness relationships. We measure stiffness properties of yew at different hierarchical levels and compare them to corresponding stiffnesses of Norway spruce (Picea abies [L.] Karsten). Cell wall stiffness is investigated experimentally by means of nanoindentation in combination with microscopy and thermogravimetric analysis. On the macroscopic level, we perform uniaxial tension and ultrasonic tests. Having at hand, together with previously reported stiffnesses, a consistent data set of mechanical, chemical and physical properties across hierarchical levels of wood, we discuss influences of microstructural characteristics at different scales of observation. Moreover, a micromechanical model is applied to predict trends of effects of the microstructure on the investigated stiffness properties. On the cell wall level, particularly, the amount of cellulose and its orientation – which was earlier reported to be distinctly different for yew and spruce – result in differences between the two considered species. On the macroscopic scale, model predicted effects of the annual ring structure on transverse stiffness and shear stiffness are found to be smaller than effects of the microfibril angle and mass density.
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| 3. |
- Forsberg, Fredrik, et al.
(författare)
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Full three-dimensional strain measurements on wood exposed to three-point-bending : analysis by use of digital volume correlation applied to synchrotron radiation micro-computed tomography image data
- 2010
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 46:1, s. 47-60
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Tidskriftsartikel (refereegranskat)abstract
- A microscale three-point bend experiment on wood has been carried out. The full 3D strain field of the microscale wood structure has been determined by use of digital volume correlation, based on reconstructed 3D image data acquired with synchrotron radiation micro-computed tomography. The wood specimen, which measures 1.57 × 3.42 × 0.75 mm3, was scanned in different load states along the three-point bend load cycle, from unloaded state to failure. The correlation algorithm is based on a Chebyshev polynomial description of the displacements, which allows a continuous representation of the 3D deformation fields. The methodology of the correlation algorithm is described thoroughly and its performance is tested for a 3D structure that is exposed to a virtual pre-defined deformation. The performance is tested both for noise free volume data as well as for structures with additive noise content. The performance test shows that the correlation algorithm resolves the applied deformation satisfyingly well. In the real experiment, on wood microstructure, the displacement fields show a structural behaviour that is consistent with what is expected for a specimen exposed to three-point bend. However, there are also anomalous effects present in the displacement fields that can be coupled to characteristic features in the cellular structure of the wood. Furthermore, 3D strain calculations based on the obtained displacement data shows a concentration of tensile strain in the region where the specimen eventually collapses. The experimental results show that the use of X-ray-based tomography with high spatial resolution in combination with digital volume correlation can successfully be used to perform 3D strain measurements on wood, at the microscale
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| 4. |
- Sjödahl, Mikael
(författare)
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Image and complex correlation near discontinuities
- 2010
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 46:1, s. 3-11
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the effect of imposing continuity constraints in real and complex image correlation has been investigated. It is found that in regions where continuity can be assumed a second-order spline expansion of the underlying deformation field provides a globally smooth estimate of the deformation field. In general, only an estimate of the deformation at the subinterval centres needs to be provided from local correlation. If the assumption of continuity is violated multiple peaks in the correlation function are introduced. If the discontinuity exceeds one speckle diameter the correlation peaks will be separated, but with reduced correlation values. Otherwise, the different peaks will blend resulting in a deformed correlation function and a correlation value that approaches unity for small discontinuities. In this case, the Hessian is shown to provide a good estimate of the position of a discontinuity. Furthermore, a methodology to perform automatic remeshing is sketched. A subinterval edge is placed at the position of the detected discontinuity, the surrounding subintervals are reduced in size and the continuity constraint is relaxed. The methodology has proven successful on one-dimensional images but it can be transferred with reasonable effort to multi-dimensional images.
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| 5. |
- Afshar, R., et al.
(författare)
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A micro-CT investigation of densification in pressboard due to compression
- 2023
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Ingår i: Strain. - : John Wiley & Sons. - 0039-2103 .- 1475-1305. ; 59:4
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Tidskriftsartikel (refereegranskat)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.
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| 6. |
- al-Karawi, Hassan, 1993, et al.
(författare)
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Crack detection via strain measurements in fatigue testing
- 2021
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 57:4
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Tidskriftsartikel (refereegranskat)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.
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| 7. |
- Hochreiner, G., et al.
(författare)
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Influence of Wooden Board Strength Class on the Performance of Cross-laminated Timber Plates Investigated by Means of Full-field Deformation Measurements
- 2014
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 50:2, s. 161-173
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Tidskriftsartikel (refereegranskat)abstract
- Although cross-laminated timber (CLT) plates are increasingly used in high-performance building structures, a tailored composition of them or, at least, a performance-based classification scheme is not available. Especially, the influence of the quality of the ‘raw’ material (wooden boards) on the load carrying capacity of CLT elements is hardly investigated yet. For this reason, within this work, bending tests on 24 CLT plates consisting of wooden boards from three different strength classes have been carried out. The global mechanical response as well as the formation of failure mechanisms were investigated, including a full-field deformation measurement system, which allowed for a qualitatively as well as quantitatively identification of board failure modes. Interestingly, no influence of the board strength class on the elastic limit load of the CLT plates was observed, but the situation was different for the load displacement history beyond the elastic regime, where basically, two different global failure mechanisms could be distinguished. The obtained knowledge about the ‘post-elastic’ behaviour of CLT plates may serve as a basis for the optimisation of CLT products and the development or improvement of design concepts, respectively. Moreover, the obtained large ‘post-elastic’ capacity reserve of CLT consisting of high quality boards could lead to a better utilisation of the raw material.
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| 8. |
- Johansson, Sara, et al.
(författare)
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Microscale deformation mechanisms in paperboard during continuous tensile loading and 4D synchrotron X-ray tomography
- 2022
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 58:5
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Tidskriftsartikel (refereegranskat)abstract
- A better physical understanding of mesoscale and microscale mechanisms behind deformation and failure of paperboard material is important to optimize industrial packaging converting processes and decrease waste. In this study, these mechanisms were investigated using synchrotron X-ray tomography during in situ continuous uniaxial tensile loading. High spatial and temporal data resolution enabled quantification of rapid changes in the material occurring before, during and after material failure. The evolution of 3D strain fields, fibre orientations and sample thickness showed that deformation and failure mechanisms differ significantly between samples tested in machine direction (MD), cross direction (CD) and 45° from the loading direction. In 45° and CD, gradual failure processes could be followed across several load steps. Immediately after failure, the in-plane fracture region was significantly larger in both 45° and CD compared to MD. Both fracture characteristics and strain field distributions differed between the three material directions. Significant fibre reorientation was an active deformation mechanism in 45° already from the beginning of the loading, also present in CD after peak load but absent in MD. The MD-dependent mechanisms interpreted and quantified at the scale of the fibre network in this study can help guide model development and likely have wider applicability to other paper-based materials.
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| 9. |
- Kajberg, Jörgen, et al.
(författare)
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High-temperature split-hopkinson pressure bar with a momentum trap for obtaining flow stress behaviour and dynamic recrystallisation
- 2014
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Ingår i: Strain. - : Wiley. - 0039-2103 .- 1475-1305. ; 50:6, s. 547-554
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Tidskriftsartikel (refereegranskat)abstract
- In hot forming processes at elevated temperatures like wire rolling, microstructural changes such as repeated dynamic recrystallisation and grain growth occur. An experimental method to obtain the flow stress behaviour and to capture the recrystallised microstructure for materials subjected to large deformations, high temperatures between 900 and 1200°C and high strain rates around 5000s-1 is presented. The method is based on the split-Hopkinson pressure bar arrangement complemented with an inductive heat source. Furthermore, a momentum trap is added to ensure that the specimen is loaded only once. By quenching the specimen directly after the single loading, the dynamically recrystallised microstructure is preserved. The quenching is performed within 0.1s of loading by dropping the specimen into a water bath. By applying the momentum trap technique, the compressive loading of the specimen could be interrupted at a strain level slightly above the strain level corresponding to the peak stress, which is a good estimation for the onset of dynamic recrystallisation. © 2014 Wiley Publishing Ltd.
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| 10. |
- Lederer, Wolfgang, et al.
(författare)
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Exploring a Multi-modal Experimental Approach to Investigation of Local Embedment Behaviour of Wood under Steel Dowels
- 2016
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Ingår i: Strain. - : John Wiley & Sons. - 0039-2103 .- 1475-1305. ; 52:6, s. 531-547
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Tidskriftsartikel (refereegranskat)abstract
- A multi-modal experimental approach for analysing the embedment behaviour of timber connections with steel dowels is proposed in this study. In this approach, a standard mechanical embedment test on single-dowel connections is combined with an optical measurement of surface deformations of the connection based on digital image correlation principle and an X-ray micro-computed tomography examination of the deformations in the dowel-wood interface. The latter is conducted on cylindrical cores including the dowel hole, physically extracted from the loaded specimen at three characteristic points of the load-deformation curves. The major challenge of this procedure is disrupted load transfer between the cylindrical core specimens and the external material they were plugged in for further analysis. Despite its challenges and limitations, the method revealed a potential for an unprecedented insight into the micromechanics of dowel connections and for effective correlation of the micro-level observations with the external macroscopic load-deformation characteristics.
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