Distributed Strain Measurement Research Articles

Contact-free measurement and non-linear finite element analyses of strain distribution along wood adhesive bonds

Abstract The strain distribution along wood adhesive bonds was studied using a contact-free measurement system based on a white-light digital image correlation (DIC) technique. Two different specimen geometries and three different adhesives were investigated. The specimen geometries were according to the standards EN302-1 and ASTM D905. The adhesives tested were a phenolic resorcinol (PRF), a one-component polyurethane (PUR) and an epoxy (EPX). In addition to the experimental investigation, a finite element study using a non-linear fracture mechanics model for the adhesive bond line was carried out, aimed at investigating whether deformation measurements could predict differences in the mechanical behaviour of the adhesives. The measurement technique was found to be capable of distinguishing, in terms of their strain distributions at a given load, adhesives that differed markedly from one another. For example, the brittle PRF adhesive showed more localised strains than the more ductile EPX and PUR adhesives did at the same load level. Another conclusion from this study is that the measurement technique used is applicable to situations in which large strains occur. Thus, the technique used here is of great interest for use in the calibration of finite element models and constitutive theories and for the design of test set-ups.

Electromagnetic forming of aluminum alloy sheet: Free-form and cavity fill experiments and model

A series of high-rate electromagnetic-forming experiments are presented that consider free-forming and two configurations of cavity fill operations, one with a flat-bottomed die and the other with a hemispherical protrusion on the bottom of the die cavity. The experiments are performed on 1 and 1.6 mm AA5754 and 1 mm AA5182 aluminum alloy sheet; all of which are candidates for lightweight automotive structural applications. Increasing energy levels of discharge resulted in increased cavity fill and strain level in the formed parts. The effect of die geometry on formability, strain state and location of failure is examined. Numerical simulations of the high-rate deformation and structural impact that occur during electromagnetic forming are presented, and provide insight into the physics of the problem and the transient nature of this high-rate deformation process. A transient electromagnetic finite-element code is used to model the time-varying currents that are discharged through the coil in order to obtain the transient magnetic forces that are imparted to the workpiece. The body forces generated by electromagnetic induction are then used as the loading condition to model the high-rate deformation of the workpiece using an explicit dynamic finite-element code. A “two-way, loose coupling” of the electromagnetic analysis with the elastic-plastic structural analysis is utilized to account for the effect of changing workpiece geometry on the transient body forces. Validation of the numerical model is performed through comparison between predicted and measured strain distributions within the formed parts.

Stress Concentrations and Notch Sensitivity in Woven Ceramic Matrix Composites Containing a Circular Hole—An Experimental, Analytical, and Finite Element Study

In this paper, the stress concentrations associated with a circular hole and subjected to uniform tensile loading in woven ceramic matrix composites (CMCs) have been determined experimentally through strain gauge measurements. The stress concentrations were calculated from the measured strain distributions surrounding the holes during the initial loading period. Then, the load‐carrying capacity of notched CMCs with various hole radius to width ratios (a/w) has been determined. The objective is to study the stress concentrations due to circular notch, and then evaluate the notch sensitivity in woven CMCs. Both quasi‐isotropic [0/90/+45/−45]s and cross‐ply [0/90/0/90]s laminates of woven SiC/SiNC composites were considered. The experimental data of stress concentrations were compared with the results obtained from a complex variable method and finite element analysis. The notched strength data were also analyzed on the basis of some existing theories. The results show significant stress concentrations in the range of 3.75–4.75 near the notch edges during the early stage of loading. But in most cases, the notched strength data show notch insensitivity in CMCs, particularly with small hole diameters. This occurs mostly due to stress redistribution effects during inelastic deformation in ceramic matrix composites. The degree of notch insensitivity in CMCs is observed to decrease with comparatively larger diameter holes. For notch insensitivity, the limiting value of a dimensionless parameter n, comprising of fracture toughness, un‐notched strength, and width of the specimen, has been determined on the basis of existing notch sensitivity theories. The stress concentration factors are seen not to be influenced by the lay‐up sequence of the laminate. Reasonable agreement was observed between the theory and experimentally determined stress concentration factors, particularly for small hole diameters.

Simultaneous temperature and strain measurement with combined spontaneous Raman and Brillouin scattering

We report on a novel method for simultaneous distributed measurement of temperature and strain based on spatially resolving both spontaneous Raman and Brillouin backscattered anti-Stokes signals. The magnitude of the intensity of the anti-Stokes Raman signal permits the determination of the temperature. The Brillouin frequency shift is dependent on both the temperature and the strain of the fiber; once the temperature has been determined from the Raman signal, the strain can then be computed from the frequency measurement of the Brillouin signal.

Analysis of strain uniformity during multi-pressing in equal channel angular extrusion

Abstract Effective strain distribution within a sample subjected to equal channel angular extrusion up to 4 passes was examined in full 3-D using a commercial FVM code. Strain uniformity was evaluated by using particle tracking method and measuring strain distribution on the transverse and longitudinal planes. Strain uniformity was achieved after 4 passes when BC route was used.

Schemes of a fiber-optic multiplexing sensor array based on a 3×3 star coupler

A Michelson-based fiber-optic low-coherence interferometric quasi-distributed sensing system permitting absolute length measurement in a sensor array is proposed. The main part of the sensing system is a fiberoptic 3 x 3 star coupler. The architecture of fiber-optic sensors can be easily realized as a linear sensor array, twin sensor arrays, or a loop sensor array. The proposed sensing scheme will be useful for the measurement of strain distribution. An important application could be deformation sensing in smart structures. A six-sensor array is demonstrated experimentally.

Direct measurement of strain distribution along a wood bond line. Part 1: Shear strain concentration in a lap joint specimen by means of electronic speckle pattern interferometry

Abstract Phenol-resorcinol-formaldehyde (PRF) and one-component polyurethane (PUR) resins were used to manufacture single lap joint samples corresponding to EN 302-1. 3D electronic speckle pattern interferometry (ESPI), which enables measurement of spatial displacement of less than 0.1 μm, was used during tensile shear experiments to observe full-field in-plane and out-of-plane deformation of the lap joint samples and to detect strain concentrations in the vicinity of glue lines. Finite element analysis was performed to validate ESPI measurements. In general, ESPI measurements showed that in a lap joint experiment a very small volume of material close to the ends of the overlapping area is highly strained. ESPI and finite element analysis pointed out that PUR glue lines are characterised by much higher shear deformations than PRF glue lines, especially at the ends of the overlapping area. However, due to the lower Young's modulus of PUR resin compared to PRF, higher shear strains but lower shear stress concentrations can be expected in PUR, which was confirmed by the FE model.

Direct measurement of strain distribution along a wood bond line. Part 2: Effects of adhesive penetration on strain distribution

Abstract The effect of adhesive penetration into wood on the distribution of strain along a mechanically strained bond line was evaluated. With this goal, spruce wood specimens were bonded with phenol-resorcinol-formaldehyde (PRF) and one-component polyurethane resin (PUR). Penetration of the adhesives into cell cavities was observed in a zone extending over several 100 μm from the bond line. Electronic speckle pattern interferometry was used to record the elastic deformations caused by mechanical tests in transverse compression. By differentiating 2D displacement maps, the strain distribution was calculated. Both adhesives provided an increase in the stiffness of the adhesive-wood interface due to penetration into cell cavities. Compression strain was much lower in the bond line area than in the wood. Shear strain was also low in the bond line and increased gradually with increasing distance. No change was observed in wood unaffected by the adhesive. The effects were more pro-nounced for PRF resin than for PUR.

1706 FBGセンサによる高空間分解能ひずみ分布測定に関する研究(J08-2 光ファイバセンサとその応用,J08 知的材料・構造システム)

1706 FBGセンサによる高空間分解能ひずみ分布測定に関する研究(J08-2 光ファイバセンサとその応用,J08 知的材料・構造システム)

519 皮膜散乱光法によるひずみ分布測定(GS15 計測23)

519 皮膜散乱光法によるひずみ分布測定(GS15 計測23)

223 移動窓関数を用いた位相シフトデジタルホログラフィ干渉法によるひずみ分布計測(OS4-2 相関,ホログラフィ,スペックル,OS4 実験力学手法の新展開3)

223 移動窓関数を用いた位相シフトデジタルホログラフィ干渉法によるひずみ分布計測(OS4-2 相関,ホログラフィ,スペックル,OS4 実験力学手法の新展開3)

圧電材料と配列型センサプローブによる材料表面のひずみ分布計測に関する研究

圧電材料と配列型センサプローブによる材料表面のひずみ分布計測に関する研究

Measuring strain distributions in amorphous materials

A number of properties of amorphous materials including fatigue, fracture and component performance are governed by the magnitude of strain fields around inhomogeneities such as inclusions, voids and cracks. At present, localized strain information is only available from surface probes such as optical or electron microscopy1,2. This is unfortunate because surface and bulk characteristics in general differ. Hence, to a large extent, the assessment of strain distributions relies on untested models. Here we present a universal diffraction method for characterizing bulk stress and strain fields in amorphous materials and demonstrate its efficacy by work on a material of current interest in materials engineering: a bulk metallic glass3,4,5. The macroscopic response is shown to be less stiff than the atomic next-neighbour bonds because of structural rearrangements at the scale of 4–10 A. The method is also applicable to composites comprising an amorphous matrix and crystalline inclusions.

On a direct determination of non-uniform internal strain fields using fibre Bragggratings

A new method for measuring non-uniform strain fields using fibre Bragg grating sensors isproposed and demonstrated. The method is based on the measurement of the compleximpulse response of the grating by optical low coherence reflectometry and theretrieval of the local Bragg wavelength by the layer-peeling technique. This methodis applied to measurements of non-uniform strain distributions generated in adouble-edge-notched thin plate made of epoxy, where a 24.1 mm long grating is embeddedat the specimen centre along the loading direction and orthogonally to the notches. Theaxial strain distribution along the grating is determined with an absolute error of30 με. The results are compared with finite element simulations and a good agreement isobserved.

Tensile and bending deformation of Ni 3Al heavily cold-rolled foil

Local deformation characteristics of 95% cold-rolled Ni 3Al foils were examined in tensile and bending tests along the rolling direction. The foils fracture without appreciable elongation in the tensile tests, while deform to 93° without cracking in the bending tests. However, the local deformation and fracture appear similarly in both of the tests: plastic deformation occurs in local regions by slip on the {111} planes where the slip deformation dominantly occurred during the prior cold rolling; cracks initiate along the shear bands which were formed during the cold rolling. The similarity of the local deformation was quantitatively confirmed by strain distribution measurements, i.e. the peak strain amounts to ∼10% just after crack initiation in both of the tests. These measurements verified that the extent of the local deformation is large enough to yield the good ductility in bending tests and not in tensile tests.

Correlation of strain, wing tilt, dislocation density, and photoluminescence in epitaxial lateral overgrown GaN on SiC substrates

Epitaxial lateral overgrown (ELOG) gallium nitride (GaN) on SiC is being studied as a possible substrate for blue laser diodes. A defect density below 2.2×107cm−2 in the wings, compared to 2×109cm−2 in the windows, was achieved. Interaction of the overgrown GaN with the SiO2 mask causes a few degree wing tilt and a transition region of high defect density between windows and wings. Diminished PL, strong tensile stress, and a defect correlated line at around 3.4eV emerge in this up to two-micron-wide transition region. By changing the mask material from SiO2 to SiN we were able to reduce the wing tilt drastically to below 0.7°. This eliminates the defective transition region and extends the low strain and the low defect density area of the ELOG wings. The methods used to study strain, wing tilt, and threading dislocations in the ELOG samples are microphotoluminescence (μPL), transmission electron microscopy, x–ray diffraction, and scanning electron microscope. We also demonstrate the use of the first momentum of the μPL spectra as an effective means to measure strain distribution.

High spatial resolution microwave detection system for Brillouin-based distributed temperature and strain sensors

We present a microwave detection system for coherent detection of spontaneous Brillouin-based distributed temperature and strain measurements. The system was designed to overcome the existing bandwidth limitations of a previously used commercial spectrum analyser and provide a commercially practical solution to the detection of the Brillouin frequency shift and intensity for long range high spatial resolution measurements. The detection system bandwidth corresponds to a potential spatial resolution of ∼60 cm. The system was demonstrated as a temperature sensor over a range of 30 km, with a temperature resolution of 1.6 °C and spatial resolution of ∼2 m. It was also demonstrated as a combined temperature and strain sensor over a range of 6.3 km with ∼1.3 m spatial resolution; temperature and strain resolutions of 3 °C and 80 µε respectively were achieved.

Arbitrary strain distribution measurement using a genetic algorithm approach and two fiber Bragg grating intensity spectra

This paper proposes and verifies a simple, convenient, and low cost method to inversely measure arbitrary strain distributions by applying a genetic algorithm approach to analyze the reflection intensity spectra of two fiber Bragg gratings (FBGs). The proposed method involves bonding one uniform FBG and one chirped FBG to the same location of the structure of interest such that they both encounter the same strain field. The arbitrary strain distribution within the fiber gratings is then determined inversely from the two Bragg intensity spectra by means of a genetic algorithm population-based optimization process. The proposed measurement method is suitable for many smart structure-monitoring applications.

Low-coherence Michelson interferometric fiber-optic multiplexed strain sensor array: a minimum configuration

A minimum configuration Michelson fiber-optic low-coherence interferometric quasi-distributed sensing system is proposed that permits absolute length measurement in remote reflective sensor arrays. The sensor's reflective signal characteristics have been analyzed, and the relationship between intensities of light and number of sensors is given for evaluation of multiplexing potential. The proposed sensing scheme will be useful for the measurement of strain distribution. An important application may be strain monitoring in smart structures. Experimentally, four-sensor array has been demonstrated.

Fatigue behaviour of mechanically fastened aluminium joints tested in spectrum loading

The objective of the present work was to investigate the fatigue behaviour of single overlap mechanically fastened aluminium joints tested in FALSTAFF spectrum. In order to induce varying degrees of fatigue damage, three joints were loaded with different number of spectrum blocks. Strain gauges were used to measure strain distribution between bolt rows and to determine the degree of secondary bending at certain points. Two instrumented bolts were employed to measure their axial and shear response during loading. The load transfer between the bolt rows was calculated using the measured strain distributions. The achieved results were then compared with the ones obtained from measurements done on the instrumented bolts. The overall test results indicated that the friction wear between the joint parts significantly affected the fatigue behaviour of the inspected joints.