Optical Strain Measurement Research Articles

Net change in periosteal strain during stance shift loading after surgery correlates to rapid de novo bone generation in critically sized defects.

In an ovine femur model, proliferative woven bone fills critically sized defects enveloped by periosteum within 2 weeks of treatment with the one-stage bone-transport surgery. We hypothesize that mechanical loading modulates this process. Using high-definition optical strain measurements we determined prevailing periosteal strains for normal and surgically treated ovine femora subjected ex vivo to compressive loads simulating in vivo stance shifting (n = 3 per group, normal vs. treated). We determined spatial distribution of calcein green, a label for bone apposition in first the 2 weeks after surgery, in 15°, 30°, and 45° sectors of histological cross sections through the middle of the defect zone (n = 6 bones, three to four sections per bone). Finally, we correlated early bone formation to either the maximal periosteal strain or the net change in maximal periosteal strain. We found that treatment with the one-stage bone-transport surgery profoundly changes the mechanical environment of cells within the periosteum during stance shift loading. The pattern of early bone formation is repeatable within and between animals and relates significantly to the actual strain magnitude prevailing in the periosteum during stance shift loading. Interestingly, early bone apposition after the surgery correlates well to the maximal net change in strain (above circa 2000-3000 με, in tension or compression) rather than strain magnitude per se, providing further evidence that changes in cell shape may drive mechanoadaptation by progenitor cells. These important insights regarding mechanobiological factors that enhance rapid bone generation in critically sized defects can be translated to the tissue and organ scale, providing a basis for the development of best practices for clinical implementation and the definition of movement protocols to enhance the regenerative effect.

An integrated study of die powder fill, transfer and compaction process using digital image correlation method

A major advantage of the powder metallurgical (P/M) manufacturing process is its ability to shape powder directly into a final component with a primary goal of a high quality, homogeneity of density and mechanical properties and productivity. In this research, powder die filling, powder transfer and powder compaction process have been studied in succession using a novel experimental set-up that utilizes a high strength transparent wall section to observe and record the particle movement and powder compaction during the entire sequence leading up to the formation of a green part. The natural powder pattern itself, as observed from the transparent wall section, is utilized for obtaining full-field displacement and strain measurement. The test set-up and the strain measurement technique offer a means of quickly obtaining density distribution data in select cases. In addition to the above, several powder flow characteristics during die filling, powder transfer and powder compaction under a range of test conditions have been noted through a series of high-speed photographic recordings. The observations reveal increased porosity in the die wall region due to friction and formation of shear bridges during powder transfer stages during suction filling. Spatial density data from optical strain measurements in the top, middle and bottom regions of the die are consistent with similar bulk density measurements from mass and volume of the 3 regions.

Long-range Brillouin optical time-domain analysis sensor employing pulse coding techniques

In this paper we describe and implement a long-range Brillouin optical time-domain analysis (BOTDA) sensor, for both temperature and strain measurements, using optical pulse coding techniques. A theoretical analysis of Simplex coding applied to BOTDA systems is presented and experimentally demonstrated for both Brillouin loss and Brillouin gain configurations. With the proposed technique, ∼7.1 dB and ∼10.3 dB of signal-to-noise ratio improvements are demonstrated in BOTDA measurements using 127-bit and 511-bit Simplex codes, respectively. This feature allows us to extend the dynamic range of the measurements, overcoming the limitations to the maximum usable optical power imposed by pump depletion and modulation instability; thus, the sensing range can be extended by several tens of kilometers while keeping meter-scale spatial resolution. Experimental results show the capabilities of optical pulse coding techniques to achieve 1 m spatial resolution over 50 km of standard single-mode fiber enabling temperature and strain resolutions equal to 2.2 °C and 44 µε, respectively.

Optical fibre strain measurement for tunnel lining monitoring

A trial strain-monitoring system using Brillouin optical time-domain reflectometry (BOTDR) technology was set up to monitor joint movements in the concrete tunnel lining in an existing London Underground tunnel. The BOTDR strain sensor system allows the measurement of strain distribution along an optical fibre using the reflective technique, requiring access to only one end of the fibre. Measurements were obtained by a strain-sensing optical fibre installed along the tunnel lining. The joint movements were captured by measuring the strain along the fibre across the segment joints. The results show that there is good agreement between the joint movements evaluated by the BOTDR strain sensor system and those by conventional vibrating-wire strain gauges. Whereas conventional strain measurement gauges monitor the strain variations at discrete locations, a BOTDR strain sensor can provide a continuous strain distribution of the tunnel lining. The results demonstrate the practicality of using the BOTDR strain-sensing system to monitor the movement of tunnel linings.

Assessment of Quasi-Static and Fatigue Loaded Notched GRP Laminates Using Digital Image Correlation

This paper presents the results of an experimental study carried out to assess the effects of cumulative damage on the strain response of open-hole (notched) quasi-isotropic [45/0/-45/90]4s E-glass/913 epoxy laminate specimens under quasi-static tension, and constant amplitude tension-tension, compression-compression and tension-compression cyclic loading conditions. In-plane surface strain was measured as a function of applied load and loading cycles for quasi-static and cyclic loading respectively using digital image correlation (DIC) and compared with strain gauge, fibre Bragg grating (FBG) optical sensor and contact extensometer strain measurements. DIC proved successful in monitoring local (near the hole) and global strains, providing critical information on the changes in strain distribution resulting from damage formation and growth incurred.

Finite element analysis and optimization of process parameters during stamp forming of composite materials

In the manufacture of parts for high performance structures using composite materials, the quality and robustness of the parts is of utmost importance. The quality of the produced parts depends largely on the process parameters and manufacturing methodologies. This study presents the use of a temperature dependant orthotropic material for a coupled structural-thermal analysis of the stamp forming process. The study investigated the effects of process parameters such as pre-heat temperature, blank holder force and process time on the formability of composite materials. Temperature was found to be the dominant factor governing the formability of the composite material while higher blank holder forces were deemed to be important for achieving high quality of the parts manufactured. Finally, an optimum set of parameters was used to compare the simulations with experimental results using an optical strain measurement system.

Applying finite element analysis to compression garment development

Finite element analysis (FEA) techniques have been successfully integrated into the development of a variety of sports products, such as athletic footwear, for many years. This paper reports on the possibility of using FEA to assist the development of technical apparel, specifically compression garments. Of particular interest to the pattern engineer and developer are the strain in the garment fabric and the compression given to the wearer. FEA of apparel presents some unique challenges due to the nature of the materials involved and the complex interaction between body and garment. Two dimensional fabric pieces are assembled into a three-dimensional garment in a process akin to sewing. A three dimensional scanned body provides the form around which the garment is virtually assembled and analyzed. Simulated distributions of material strain and pressure on the body are presented. Results of simple simulations are validated against experimental evidence from studies using an optical strain measurement system. Good correlation is evident between simulated and experimentally measured strain on the garment. The ability to simulate garment surface strain and pressure against the body prior to the manufacture of a physical garment is anticipated to be of significant benefit to pattern engineers and garment developers.

An investigation on the application of a non-destructive optical strain measurement system to fracture toughness testing

An investigation on the application of a non-destructive optical strain measurement system to fracture toughness testing

Experimental and numerical study of a milling machine-based dieless incremental sheet forming

In this paper, some relevant issues concerning an innovative sheet metal forming method, a milling machine-based Dieless Incremental Sheet Forming (DSF) will be analyzed. The experimental setup is based on the replacement of the commonly used dies by a second forming (Slave) tool where the movement is synchronized with the first forming (Master) tool, thus creating a flexible support. DSF can be used on both CNC machines [1] and industrial robots [2]. In this paper, the authors are only focusing on the experiments with a parallel kinematics 3-axis milling (PKM) machine with 3 additional linear actuators. The deformation state of the formed part will be analyzed using the Vialux-AutoGrid optical strain measurement system and will be compared to the results of 3D FEM modelling.

Experimentally Observed Strain Distributions Near Circular Discontinuities of AA6061-T6 Extrusions During Axial Crush

An experimental investigation to determine the strain distribution and collapse behaviour for AA6061-T6 square cross-sectional extrusions with and without circular discontinuities under quasi-static axial compressive loading was completed. Three-dimensional digital image correlation (DIC) was utilized for strain assessment. In order to validate the results of the optical strain measurement system, tensile tests were first conducted employing both the DIC technique and a traditional extensometer. Strain observations from both methods were found to be very consistent prior to strain localization in the test specimen. Quasi-static axial crushing tests were then conducted. Extrusions considered for the present research had a nominal side width, wall thickness and length of 38.1 mm, 3.15 mm, and 200 mm, respectively. A centrally-located circular hole with diameter of either 14.29 mm, 10.72 mm or 7.14 mm was incorporated into the extrusion. Square tubes without any discontinuities were also considered in the experimental testing program. Testing results showed that the collapse mode of the extrusion altered from global bending to a cutting and splitting deformation mode with the presence of the circular discontinuity. Strain localization occurred near the vicinity of the holes for all specimens. For discontinuities sized 14.29 mm and 10.72 mm the location of strain localization and the initiation of material fracture was at the edge of the discontinuity while the location for extrusions with a 7.14 mm hole was found to occur at the intersection of the extrusion side walls. Maximum values of the effective strain were found to vary from approximately 60% to 100%. The region of strain localization was consistent with the location where material fracture initiated.

Strain determination of polymeric materials using digital image correlation

Application of digital image correlation (DIC) to polymeric materials has been proven to be a powerful tool for non-contact strain measurement. In this paper the limits of accuracy of this optical strain measurement system under different environmental conditions were investigated, and the technique was applied to the characterization of polypropylene (PP) and PP composites (PP-C) in the pre- and post-yield regimes. As regards accuracy, a fine speckle pattern and a light intensity just below overexposure provided best results. While vibrations related to the operation of the test machine were of minor influence in reducing the strain measurement accuracy, more pronounced effects were found for the operation of the temperature chamber. In characterizing the transverse strain behavior of PP-C, DIC results exhibited smaller values compared to transverse strains determined utilizing a mechanical clip-on extensometer. The latter effect is attributed to viscoelastic creep indentation of the extensometer pins, which mechanically interact with the specimen via the clip-on spring forces of the extensometer, into the surface. For the DIC system, it could be shown that it allows for the proper strain determination both in the pre- and post-yield regimes, and in terms of longitudinal and transverse strains as well as in terms of global average and local strains.

Experimental analysis of the Portevin Le Chatelier effect by means of full field non contact techniques

Strain related phenomena in materials showing the Portevin Le Chatelier (PLC) effect are investigated when subjected to loading conditions both in elastic and in plastic field. Two experimental techniques for strain measurements are compared, thermography and optical strain measurement, both non contact, non destructive and full field. Two materials have been tensile tested with several test speeds. Each specimen has been observed and test evolution has contemporaneously been acquired with both video and thermotracer. Each test has been repeated several times. Aim of this research is to investigate how these experimental techniques may quantitatively describe the above described phenomena. Experimental data are useful to calibrate models describing the mechanical behaviour of materials (i.e. constitutive laws, damage models, etc.) and to focus phenomena which take place during load and corresponding strain phases.

ウィナーフィルタを用いた散乱光パワー推定による分布型光ファイバひずみ計測の空間分解能向上(アクセスシステム及びアクセス用光部品,光無線システム(ROF,FWA等),光映像伝送(CATV含む),オペレーション/保守監視,光計測,光ファイバ,光ファイバケーブル,一般)

ウィナーフィルタを用いた散乱光パワー推定による分布型光ファイバひずみ計測の空間分解能向上(アクセスシステム及びアクセス用光部品,光無線システム(ROF,FWA等),光映像伝送(CATV含む),オペレーション/保守監視,光計測,光ファイバ,光ファイバケーブル,一般)

Electrical ageing of polymers. The role of micro-domains

Polymer materials are disordered systems and their structure is continuously undergoing conformational changes called physical ageing. Furthermore, polymers exhibit a spatially varying morphology and a distribution of their properties and characteristics. In this article, the existence of micro-domains into polymer materials is proved, using an optical strain measurement technique. Then after, the role of these micro-domains is taken into consideration for a phenomenological description of their electrical ageing process. The different parameters affecting the ageing process, such as the local structure evolution and the local field value, are presented in a distributed way. In a first analysis the ageing process is described in a local scale. Afterwards, a statistical description of the totality of processes at play is necessary to present the global effect of the applied stress over the material structure which leads to the failure mechanism.

Effect of microscopic damage events on static and ballistic impact strength of triaxial braid composites

The reliability of impact simulations for aircraft components made with triaxial braided carbon fiber composites is currently limited by inadequate material property data and lack of validated material models for analysis. Methods to characterize the material properties used in the analytical models from a systematically obtained set of test data are also lacking. A macroscopic finite element based analytical model to analyze the impact response of these materials has been developed. The stiffness and strength properties utilized in the material model are obtained from a set of quasi-static in-plane tension, compression and shear coupon level tests. Full-field optical strain measurement techniques are applied in the testing, and the results are used to help in characterizing the model. The unit cell of the braided composite is modeled as a series of shell elements, where each element is modeled as a laminated composite. The braided architecture can thus be approximated within the analytical model. The transient dynamic finite element code LS-DYNA is utilized to conduct the finite element simulations, and an internal LS-DYNA constitutive model is utilized in the analysis. Methods to obtain the stiffness and strength properties required by the constitutive model from the available test data are developed. Simulations of quasi-static coupon tests and impact tests of a represented braided composite are conducted. Overall, the developed method shows promise, but improvements that are needed in test and analysis methods for better predictive capability are examined.

Application of a “concave-side rule” approach for assessing formability of stretch-bent steel sheets

In sheet forming, inhomogeneous through-thickness deformation, (e.g. due to a combined deformation of simultaneous stretching and bending when sheet material is drawn over a defined tool radius), is known to have influence on materials formability (i.e. onset of necking). Thus, it is preferable to take this influence into account when assessing the formability of sheet metal in a forming process. For that reason, in 2003 Tharett and Stoughton introduced the so-called “Concave-Side Rule” (CSR) approach to assess the formability of stretch-bent steel sheets. In this study the predictive quality of the CSR approach is analyzed. Therefore a series of Angular Stretch Bend Tests (ASBT) is performed. H340LAD (micro-alloyed steel) sheet specimens, with a thickness of 1.5 mm are stretch-bent over punches of various radii from 1 mm to 20 mm to produce different severity of bending in the test specimens. Results of optical on-line surface strain measurements of the test specimens are used to calibrate Finite-Element (FE) simulations of the ASBTs. From these FE-simulations, numerical results are used to assess the predictive quality of the CSR approach for H340LAD. The rule is found to be valid for H340LAD sheet material stretch-bent with punch radii R ≥ 10 mm. Whereas predictive quality decreases for more pronounced inhomogeneous through-thickness deformation (i.e. stretch-bending deformation using punch radii R < 10 mm).

A method for the layer compression test considering the anisotropic material behavior

The layer compression test is used in the field of material characterization to determine the onset of yielding and the hardening behavior of a sheet metal within uniaxial compression loading. For testing anisotropic materials the experimental setup has to be adjusted and the evaluation has to be adapted. This could be achieved with the installation of a second optical strain measurement system to get data of both symmetry axes. In the following paper the experimental procedure with two strain measurement systems and the evaluation methodology regarding the two sets of data will be presented and validated with experimental results. The advantage of the layer compression test is discussed in comparison with the results of a standard tensile test.

Characterization of Ductile Core Materials

This article describes an experimental investigation of characterization methods for ductile core materials. Full-field optical strain measurement methods are used to determine the strain distributions in standard testing methods such as block shear and four-point beam testing, particularly for highly ductile cores subjected to large deformations. The results show that the stress and strain fields in both block shear and sandwich beam tests are very different to those assumed by the testing standards. The test methods result in complex post yield states of stress in the core materials, meaning the core shear strength and ultimate shear strain should not be calculated by classical methods in the post yield region.

Optical Strain Measurement Techniques to Assist in Life Monitoring of Power Plant Components

Sensors for monitoring creep strain in high-pressure steam pipes and other power plant components are subjected to very demanding environmental and operational conditions. It is important that the sensors are of a rugged design and that measurement can be made that only relates to creep movements in power plant components. The E.ON UK auto-reference creep management and control (ARCMAC) optical strain gauges have been designed to have this capability. These optical strain gauges are installed across sections of welded steam pipe and other plant components in locations that provide the best monitoring points to reveal the early onset of failure processes. Reported in this paper are recent developments to improve optical creep strain measurement to achieve a 65 microstrain accuracy level with an error of less than 10%. Also reported are trials of combining optical strain gauges with digital image correlation (DIC) to obtain detailed information of the creep strain distribution around the gauges. The DIC data for known defect geometries have been validated with finite element analysis.

Failure analysis of triaxial braided composite

This study focuses on the description of damage and failure behaviour of triaxial braided carbon/epoxy composites under tension. The tensile tests were instrumented with optical surface strain and acoustic emission measurements. Damage was observed using X-ray and microscopy. The damage develops in two stages: (1) intra-yarn cracking: increase of the crack density and crack length, (2) local inter-yarn delamination and conjunction of the intra-yarn cracks. Statistics of crack sizes at both stages were collected and the 3D geometry of cracks was reconstructed. A finite element model of the unit cell of the textile reinforcement is used to predict damage initiation and crack orientation using Puck’s criterion. Progressive damage and stiffness deterioration is modelled using the degradation scheme of Murakami–Ohno and the damage evolution law of Ladeveze, applied to the average stress state of the yarns. Good agreement with experimental damage initiation threshold and non-linear tensile diagrams is found both for loading in fibre and off-axis directions.