Full-field Optical Strain Measurement Research Articles

Assessment of Fatigue Resistance of Aluminide Layers on MAR 247 Nickel Super Alloy with Full-Field Optical Strain Measurements

This work presents the results of fatigue tests of MAR 247 alloy flat specimens with aluminides layers of 20 or 40 µm thickness obtained in CVD process. Fatigue test was conducted at amplitude equal to half of maximum load and ranging between 300 and 650 MPa (stress asymmetry ratio R = 0, frequency f = 20 Hz). Additionally, 4 of the tests, characterized by the highest amplitude, were accompanied with non-contact strain field measurements by means of electronic speckle pattern interferometry and digital image correlation. Results of these measurements allowed to localize the areas of deformation concentration identified as the damage points of the surface layer or advanced crack presence in core material. Identification and observation of the development of deformation in localization areas allowed to assess fatigue-related phenomena in both layer and substrate materials.

DEFORMATION CHARACTERISTICS OF CHOPPED FIBRE COMPOSITES SUBJECTED TO QUASI-STATIC TENSILE LOADING

This work presents deformation behaviour of cost effective chopped fiber composites. Use of chopped fibre is advantageous for manufacturing however complex shape parts production technology could be challenging. Batches of samples with different fibres composition were subjected to uni-axial tensile loading to obtain overall materials properties and inspection of proper manufacturing based on local deformation inhomogeneities. Both crossbeam displacement and optical strain measurement were used for elastic characteristics evaluation. Deformation response was derived from full-field optical strain measurements based on digital image correlation method. Relatively large variation of mechanical properties testing of samples was found.

The Strain Rate Dependent Material Behavior of S-GFRP Extracted from GLARE

S-Glass fiber reinforced epoxy (S-GFRP) extracted from GLARE has been experimentally characterized at three distinct strain-rates (5 × 10−4 s−1, 10 s−1, and c.a. 2000 s−1) and in four loading directions (0°, 90°, and 45° to the fiber direction and in the through-thickness direction). A novel specimen clamping mechanism was developed and full-field optical strain measurement was applied. With the aid of these techniques, a significant increase in failure-strength and apparent elastic modulus in all loading directions, particularly in fiber direction, was observed with an increased strain-rate; strain to failure increased in the fiber direction and decreased in all other loading directions.

Comparison of Numerical and Experimental Strain Measurements of a Composite Panel Using Image Decomposition

Image decomposition is used to address the problem of accurately and concisely describing the strain in an inhomogeneous composite panel that is bolted to a vehicle structure. In-service, the composite panel is subject to structural loads from the vehicle which can cause unintended damage to the panel. Finite element simulations have been performed with the plan to establish their fidelity using full-field optical strain measurements obtained using digital image correlation. A methodology is presented based on using orthogonal shape descriptors to decompose the data-rich maps of strain into information-preserved data sets of reduced dimensionality that facilitate a quantitative comparison of the computational and experimental results. The decomposition is achieved employing the Fourier transform followed by fitting Tchebichef moments to the maps of the magnitude of the Fourier transform. The results show that this approach is fast and reliably describes the strain fields using less than fifty moments as compared to the thousands of data points in each strain map.

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.

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.

Assessing the Feasibility of Monitoring the Condition of Historic Tapestries Using Engineering Techniques

The findings of a year-long programme carried out by a multidisciplinary engineering/conservation team are described. A mass-produced textile material that can be used to represent tapestries is identified and mechanical tests are detailed which demonstrate it behaves in a similar way to tapestry. The feasibility of using optical fibre sensors, full-field optical strain measurement techniques and thermography for monitoring tapestry degradation is assessed. The results of preliminary findings are presented and a rationale is developed for in-situ quantitative strain monitoring of tapestries.

Interface structure and strain development during compression tests of Al2O3/Nb/Al2O3 sandwiches

The interface structure of an Al2O3/Nb/Al2O3 sandwich produced by solid-state diffusion bonding was investigated in detail by various transmission electron microscopy (TEM) methods. The joint possessed at one interface a $$ {\hbox{(110)}}_{{{{\rm Nb}}}} {{ \,||\, (0001)}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} $$ , $$ {\hbox{[1{{$\bar{1}$}}0]}}_{{{{\rm Nb}}}} {{ \,||\, [2{{\bar{1}}}{{\bar{1}}}0]}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} $$ , and on the other interface a $$ {\hbox{(1{{${1}$}}0)}}_{{{{\rm Nb}}}} {{ \,||\, (0001)}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} $$ and $$ {\hbox{[1{{$\bar{1}$}}0]}}_{{{{\rm Nb}}}} {{ || [0{{\bar{1}}}{{\bar{1}}}0]}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} $$ orientation relationship. At both interfaces, misfit dislocations formed to compensate the lattice mismatch as found by high-resolution transmission electron microscopy (HRTEM). Electron energy-loss near edge structure (ELNES) studies revealed that the interface is terminating with an Al layer resulting in Al–Nb bonds. Identical sandwiches were investigated on the meso- and macroscopic scale by performing compression tests and simultaneously monitoring the strain development at (001)Nb and $$ (1{{\bar{{{1}}}}}0)_{{\rm Nb}} $$ crystal faces. The full-field optical strain measurements (FFOM) revealed that the strain is localized at the interfaces when observed at the (001)Nb face while it is along the maximum shear directions of 36–54° inclined to the interface when observed at the $$ (1{{\bar{{{1}}}}}0) $$ face. The strain localization along a specific maximum shear direction results in the cleavage of Al2O3, always initiating from the interface possessing the $$ {\hbox{(110)}}_{{{{\rm Nb}}}} {{ \,||\, (0001)}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} $$ and $$ {\hbox{[1{{$\bar{1}$}}0]}}_{{{{\rm Nb}}}} {{ \,||\, [0{{\bar{1}}}{{\bar{1}}}0]}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} $$ orientation relationship.

On the Calibration of Optical Full-Field Strain Measurement Systems

There are no standard reference materials suitable for the calibration of full-field optical strain measurement systems. This is hindering the uptake of the technology by industrial end-users since optical metrology instrumentation and procedures cannot be easily integrated into quality assurance systems. The EU-funded SPOTS project is developing a physical reference material (PRM) and measurement protocol that should provide the basis of a calibration standard for establishing the traceability of strain values obtained with optical devices. This paper describes a PRM based on a parametric design of monolithic four-point bend test that can reliably generate a known strain field over a range of specimen sizes. Measurements acquired from strain gauges and LVDTs compared well with data obtained from ESPI, digital image correlation, photoelasticity and thermoelasticity studies, demonstrating excellent repeatability and inter-laboratory reproducibility.

Malrotation in total knee arthroplasty: Effect on tibial cortex strain captured by laser-based strain acquisition

Malrotation of the tibial and femoral components has been recognized to be a clinical complication affecting the performance and durability of total knee arthroplasty. This study used a novel strain acquisition technique to determine the effect of tibio-femoral component malrotation on tibial torque and strain distribution of the proximal tibial cortex with a cemented fixed-bearing posterior-stabilized knee. Using electronic speckle pattern interferometry, strain on the proximal tibia of human cadaveric knees was obtained in response to 1500N axial loading for neutrally aligned tibial and femoral components, and for 10 degrees internal and external malrotation between the tibial and femoral components. Local strain gage measurements were combined with full-field optical strain measurements to quantify effects on tibial cortex strain and strain distributions caused by the 10 degrees malrotations. In addition, tibial torque was measured for incremental degrees of tibio-femoral malrotation. Tibio-femoral malrotations as small as 2 degrees caused tibial torque in excess of 4 Nm. At 10 degrees malrotation, tibial torque significantly increased to over 8 Nm (P<0.001) as compared to neutrally aligned components. Local strain gage results significantly increased from 500 muepsilon to 632 muepsilon compressive strain in response to 10 degrees external malrotation, and to 1000 muepsilon compressive strain in response to 10 degrees internal malrotation. Full-field optical strain reports yielded the highest strain of 2153 muepsilon for 10 degrees internal malrotation 30 mm below the joint line. Laser-based strain measurement technology provides novel capabilities to capture cortex strain fields. The sensitivity of cortex strain and torsion to small amounts of tibio-femoral malrotation may explain factors contributing to aseptic implant loosening of the tibial component.

Compressive deformation of niobium sandwich-bonded to alumina

Abstract The deformation behavior of polycrystalline niobium with a thickness of 0.7–2.8 mm sandwich-bonded to alumina was studaed by using surface strain mapping during compressive deformation. A full-field optical strain measurement technique was employed in determining the 3-dimensional displacement and strain fields for one side surface of the bonded Nb layer, including the interfaces. The measured stress–strain curves consist of three well-defined stages of elastic deformation, yielding, and plastic deformation with hardening until fracture of alumina. Strain and displacement maps for each stage illustrate that the deformation of bonded Nb is highly influenced by the constraints from the interfaces. The yielding of Nb starts from the immediate vicinity of interfaces and extends to the whole metal layer. After yielding, strain localization maintains at the interfaces untid interface debonding or fracture of alumina occurs. The strain localization at interfaces is discussed with respect to the microstr...

Dynamic testing of large AISI-316L steel specimens behaviour using LDTF

The paper presents the results of a series of dynamic experiments on AISI-316L steel specimens having a cross section of 2000 mm 2 and a gauge length of 350 mm subject to dynamic traction up to rupture. Results for dynamic tests at a nominal strain rate up to 50 s −1 are compared to quasi-static ones, showing an important increase in both yield stress and flow stress. The experimental techniques are described, with special emphasis on full-field optical strain measurements based on Moiré interferometry. A 1-D numerical analysis of the experiments is presented.

Full-field optical strain measurement having postrecording sensitivity and direction selectivity

An optical method of strain measurement is described which possesses numerous advantages over conventional and holographic interferometry. The method consists of imaging the coherently illuminated object by a lens, and double-exposing a photographic plate in a misfocused image plane before and after deformation of the object. The processed photographic plate or ‘specklegram’ is subsequently Fourier-filtered to provide fringe patterns representing derivatives of surface displacements with respect to any desired direction and with variable and controllable sensitivity.