| 1. |
- Ekh, Johan, et al.
(författare)
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Secondary Bending in Multi Fastener, Composite-to-Aluminium Single Shear Lap Joints
- 2005
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Ingår i: Composites Part B. - : Elsevier BV. - 1359-8368 .- 1879-1069. ; 36, s. 195-208
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Tidskriftsartikel (refereegranskat)abstract
- A novel technique to study secondary bending due to the eccentric load path in single shear, composite-to-aluminium lap joints has been developed and used in a parametric study. The method is based on the specimen curvature, which is derived from lateral displacements of the surface. The lateral displacements are measured with an optical measurement system or calculated with the finite element method. It was found that the specimen curvature is equivalent to the secondary bending which is usually measured with strain gauges in the vicinity of the bolt holes. The two most important parameters affecting specimen curvature is the overlap length and the thickness of the aluminium plate in the joints.
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| 2. |
- Huntley, Jonathan M., et al.
(författare)
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Microstructural strain analysis by high magnification moiré interferometry
- 1995
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Ingår i: Interferometry VII. - Bellingham, Wash : SPIE - International Society for Optical Engineering. - 9780819419040
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Konferensbidrag (refereegranskat)abstract
- A high magnification phase-stepping interferometer has been constructed which has a spatial resolution of the order of one micrometers and a sub-mm field of view. Laser illumination is delivered to the microscope head by polarization-preserving single-mode optical fibers. The head itself is a compact unit consisting of collimating optics, objective lens, CCD camera, and a separate white light source. Phase gratings are cast on the polished specimen surface by replication from a master grating, in either silicone rubber or epoxy resin. Subsequent evaporation of a thin layer of gold onto the grating increases the reflectivity and reduces the speckle noise in the images. By switching between the laser illumination and the white light unit, it is possible to view the underlying microstructure in exact registration with the measured displacement fields. The instrument is illustrated with several applications including the visualization of delamination cracks in graphite-epoxy composites and measurement of the strain-to-failure of polymer-bonded-explosives.
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| 3. |
- Kajberg, Jörgen, et al.
(författare)
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High strain-rate tensile testing and viscoplastic parameter identification using microscopic high-speed photography
- 2004
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Ingår i: International journal of plasticity. - 0749-6419 .- 1879-2154. ; 20:4-5, s. 561-575
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Tidskriftsartikel (refereegranskat)abstract
- A combined experimental/numerical method for determination of constitutive parameters in high strain-rate material models is presented. Impact loading, using moderate projectile velocities in combination with small specimens (sub mm) facilitate tensional strain rates in the order of 104-105 s-1. Loading force is measured from one-dimensional wave propagation in a rod using strain gauges and deformation is monitored with a high-speed camera equipped with a microscope lens. A sequence of digital photographs is taken during the impact loading and the plastic deformation history of the specimen is quantified from the photographic record. Estimation of material parameters is performed through so called inverse modelling in which results from repeated FE-simulations are compared with experimental results and a best choice of constitutive parameters is extracted through an iterative optimisation procedure using the simplex method. Results are presented from a preliminary tension test of a mild steel (A533B) at a strain rate well over 104 s-1. The sensitivity of the evaluated material parameters to errors in measured quantities is studied. The method, especially the optical technique for measurement of deformation will be further developed.
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| 4. |
- Melin, L. Gunnar, et al.
(författare)
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A study of Mode-I delamination cracks by high-magnification moiré interferometry
- 1998
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 58:3-4, s. 515-525
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Tidskriftsartikel (refereegranskat)abstract
- Delamination cracks in carbon-fibre/epoxy composites loaded in Mode-I have been studied by high-magnification moiré interferometry. The moiré interferometer has the capability of switching between laser illumination for acquiring the displacement field and white-light illumination for visualisation of the underlying microstructure. Four specimens with different stacking geometries have been investigated. The delamination cracks have been simulated by using Teflon films. The resulting displacement fields have been compared to numerical solutions calculated by the finite-element method. The comparisons indicate that the Teflon films for some of the specimens did not delaminate fully. Otherwise the agreement is good, confirming that the displacement field around such cracks is satisfactorily described by linear-elastic fracture mechanics. The agreement was improved when a matrix-rich region in front of the crack tip was included in the finite-element model.
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| 5. |
- Melin, L. Gunnar, et al.
(författare)
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A study of the displacement field around embedded fibre optic sensors
- 1999
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 30:11, s. 1267-1275
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Tidskriftsartikel (refereegranskat)abstract
- Deformation fields around optical fibres embedded in carbon fibre/epoxy composite specimens have been measured using moiré interferometry. The inclusion of the optical fibre resulted in large strain gradients. Calculated displacements from finite element analysis were compared to the experimental results. The numerical analysis showed that the displacement field on the specimen surface is smoothed out through the moiré grating thickness, an effect which is most pronounced at the material interfaces. With this influence taken into consideration a reasonable good quantitative agreement between the experiments and the finite element analysis was obtained. The finite element analysis also showed that the grating stiffness did not affect the measured displacements as long as the grating had a lower stiffness than the specimen
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| 6. |
- Melin, L. Gunnar
(författare)
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Composites studied by moiré methods and high speed photography
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fibre composites are today used as construction material in a growing number of applications. Carbon fibres/epoxy matrix composites are employed in high performance applications as aircrafts where a high stiffness/weight ratio is important. The structures are made of laminates built by stacking thin sheets of the composite with unidirectional ordered fibres. Such laminates are vulnerable to impact damages caused by for example a dropped tool. It is desired to develop computational methods to model such impacts and predict the resulting reduction in the performance. Then accurate models of the mechanical behaviour of the material are needed, models based on measurements not only at quasi-static loading but also at high strain rates. In this thesis two versions of the optical moiré technique are applied to investigate the mechanical behaviour of carbon fibre/epoxy matrix composite laminates. The work is made in co-operation with the Aeronautical Research Institute of Sweden (FFA). With moiré methods the in-plane displacement field on a specimen surface can be measured with high accuracy and with high spatial resolution. Moiré photography is combined with high speed photography to study specimens loaded at high strain rates. Dynamically loaded delamination cracks are studied and numerical finite element models show good agreement to the measured displacement fields. Measurements of the mechanical properties transverse to the fibres show that the composite has a weak dependence of strain rate. Apparent crack speeds of more than 2000 m/s are observed. The dynamic loading is made with a tensile split Hopkinson bar apparatus. Moiré interferometry is used to measure the displacement fields at high magnification. Here mode I loaded delamination cracks and strain concentrations at fibre optics sensors embedded in composites are studied, in both cases at static load. Finite element models have acceptable agreement with the measured results.
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| 7. |
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| 8. |
- Melin, L. Gunnar, et al.
(författare)
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Effects of strain rate on transverse tension properties of a carbon/epoxy composite: studied by moiré photography
- 1999
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 30:3, s. 305-316
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Tidskriftsartikel (refereegranskat)abstract
- The dependence on strain rate of the mechanical properties of a high performance carbon fibre/epoxy composite loaded in transverse tension has been investigated. Dog-bone shaped specimens have been tested in quasi-static and dynamic loading conditions. The dynamic tests were performed in a split Hopkinson bar at strain rates between 100 and 800 s-1. A moiré technique combined with high-speed photography, at framing rates of 0.25-1 MHz, was used for extraction of the local strain fields. The transverse mechanical properties were found to have weak or no dependence on strain rate. The average transverse modulus did not depend on strain rate, whereas the strain to and stress at failure were found to increase slightly with increased strain rate. For these dog-bone shaped specimens the strain evaluated by conventional Hopkinson bar technique was found to underestimate the true strain field measured by moiré technique. Finally, the moiré technique facilitated crack-propagation monitoring in real time. Crack speeds up to 2300 m s-1 were measured at transverse crack propagation.
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| 9. |
- Melin, L. Gunnar
(författare)
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High-speed moire photography for studying dynamic properties in carbon fiber composites
- 1998
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Ingår i: Optical Engineering. - : SPIE-Intl Soc Optical Eng. - 0091-3286 .- 1560-2303. ; 37:2, s. 642-649
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Tidskriftsartikel (refereegranskat)abstract
- In the experiments reported here, a moire technique for measuring in-plane displacements was applied to the study of carbon/epoxy composite specimens deformed at high strain rates. The moire fringes were analyzed using the Fourier transform method. The measurements show that the material behaves in a linearly elastic manner up to fracture. Crack propagation rates of about 2000 m/s in the grating plane were observed. The optical aspects of the experiment are discussed
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| 10. |
- Melin, L.Gunnar, et al.
(författare)
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High strain rate tensile testing using microscopic high speed photography
- 1998
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Ingår i: Experimental mechanics : advances in design, testing and analysis. - : Taylor and Francis Group. ; , s. 175-179
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Konferensbidrag (refereegranskat)abstract
- A method is presented that combines advanced experimental and numerical techniques in the determination of constitutive parameters. The general principle is that a specimen, producing a non-homogeneous stress state, is deformed rapidly. Load and deformation is recorded during the deformation and the event is also simulated using FEM. For the numerical calculation a set of constitutive parameters is used which can be systematically adapted so that a good agreement with experimental results is achieved. Strain rates of 10 exp 4 - 10 exp 5 s exp -1 are possible because the specimen can be made small. Microscopic high-speed photography is used to record the deformation of specimens of sub millimetre size. The method allows both tensile and compressive testing.
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| 11. |
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| 12. |
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| 13. |
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| 14. |
- Melin, L. Gunnar, et al.
(författare)
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Tensile Impact Delamination of A Cross-Ply Interface Studied By Moiré Photography
- 1995
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Ingår i: Fatigue & Fracture of Engineering Materials & Structures. - : Wiley. - 8756-758X .- 1460-2695. ; 18:10, s. 1101-1114
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Tidskriftsartikel (refereegranskat)abstract
- Impact induced delamination along a cross-ply interface in carbon fiber/epoxy laminates is studied by high resolution moire photography. The specimens were loaded in a tensile split-Hopkinson bar giving mode I dominated fracture, and a high speed camera captures images during loading and delamination. The resulting moire fringes are analysed to produce full field displacement maps of the area around the loaded and propagating crack tips. The displacement map prior to failure shows good agreement with numerical solutions, calculated using a 3D self-adaptive p-version of the finite element method. The calibrated finite element solutions are then used to give further information about the matrix cracking zone size around the crack tip and the energy release rate. In comparison to quasi-static loading, tensile impact loading was found to increase the failure load and the resulting energy release rates; some physical explanations for this behaviour are discussed. It was concluded that the procedures presented have good potential for further determination of rate dependent material properties in carbon/fiber epoxy composites
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