Optical Strain Measurement Research Articles

Influence of Different Yield Loci on Failure Prediction with Damage Models

Advanced high strength steels are widely used in the automotive industry to simultaneously improve crash performance and reduce the car body weight. A drawback of these multiphase steels is their sensitivity to damage effects and thus the reduction of ductility. For that reason the Forming Limit Curve is only partially suitable for this class of steels. An improvement in failure prediction can be obtained by using damage mechanics. The objective of this paper is to comparatively review the phenomenological damage model GISSMO and the Enhanced Lemaitre Damage Model. GISSMO is combined with three different yield loci, namely von Mises, Hill48 and Barlat2000 to investigate the influence of the choice of the plasticity description on damage modelling. The Enhanced Lemaitre Model is used with Hill48. An inverse parameter identification strategy for a DP1000 based on stress-strain curves and optical strain measurements of shear, uniaxial, notch and (equi-)biaxial tension tests is applied to calibrate the models. A strong dependency of fracture strains on the choice of yield locus can be observed. The identified models are validated on a cross-die cup showing ductile fracture with slight necking.

Prototyping of automotive components with variable width and depth

Roll forming enables the manufacturing of longitudinal components from materials that combine high strength with limited formability and is increasingly used in the automotive industry for the manufacture of structural and crash components. An extension of conventional roll forming is the Flexible Roll Forming (FRF) process where the rolls are no longer fixed in space but are free to move which enables the forming of components with variable cross section over the length of the part. Even though FRF components have high weight saving potential the technology has found only limited application in the automotive industry.A new flexible forming facility has recently been developed that enables proof of concept studies and the production of FRF prototypes before a full FRF line is built; this may lead to a wider uptake of the FRF technology in the automotive industry. In this process, the pre-cut blank is placed between two clamps and the whole set up moves back and forth; a forming roll that is mounted on a servo-controlled platform with six degrees of freedom forms the pre-cut blank to the desired shape.In this study an initial forming concept for the flexible roll forming of an automotive component with variable height is developed using COPRA® FEA RF. This is followed by performing experimental prototyping studies on the new concept forming facility. Using the optical strain measurement system Autogrid Compact, material deformation, part shape and wrinkling severity are analysed for some forming passes and compared with the numerical results.The results show that the numerical model gives a good representation of material behaviour and that with increasing forming severity wrinkling issues need to be overcome in the process.

A macroscopic approach to simulate the forming behaviour of stitched unidirectional non-crimp fabrics (UD-NCF)

Thin composite shell structures manufactured from stitched unidirectional non-crimp fabrics (UD-NCF) in a liquid composite moulding process provide high lightweight design capabilities. For complex geometries, the forming process of the textile pre-products is challenging and requires intensive investigations to avoid defects like macroscopic wrinkling or fibre gapping. In contrast to biaxial engineering fabrics, UD-NCF has been investigated only sparsely in terms of its forming behaviour, both experimentally and numerically. To close this gap, a macroscopic forming simulation model for UD-NCF is proposed in this work, including a new non-orthogonal and linear strain measure for large strains and the corresponding nominal stress measure. To parameterize the constitutive laws and to investigate the multiaxial deformation behaviour of UD-NCF, supplementary off-axis-tension tests with optical strain measurement are performed in this work. The new forming simulation model is validated via forming simulation of a hemisphere test and of a preforming process of an automotive component.

Bending Limit Curves in Sheet Metal Bending Evaluation

Bending and hemming process are used in automotive industries for assembling the car body panel.The main failure mechanism under bending loads is the intercrystalline fracture. This is due to the fact that the Forming Limit Curve (FLC) describes first occurrence of membrane instability and no material failure in consequence of an intercrystalline fracture at bending.The FLC fails to predict the formability in hemming processes since difference in failure mechanism. A new failure criterion, the so-called Bending Limit Curve (BLC) has been developed. In this work, the left hand side BLCs are experimentally determined for Advanced High Strength Steel grade DP1000, Stainless Steel grade SUS430 and Deep Drawing Steel grade SPCC having a thickness of 1.0 mm. The influence of various bending radii and level of pre-strain on the bending strains are investigated and discussed by using the Three Point Bending Test. Bendability of investigated materials are evaluated by using optical strain measurement system GOM-Aramis to determine maximal achievable bending strain on the specimens. The developed left hand side BLCs were found to be higher level than conventional FLCs. The bigger bending radius established lower bending limit strain. The higher bending strain was obtained from the higher pre strain level.

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.

Localisation of plastic deformation in friction stir and electron beam copper welds

Optical strain measurement through digital image correlation during tensile testing was utilised to study the effect of strain rate and welding defects on localisation of plastic deformation in friction stir welded and electron beam welded phosphorous-alloyed oxygen-free copper. Welds cut from full-size nuclear waste copper canisters and additionally made defective welds were studied. Optical strain measurement represents a new way of studying deformation of materials and obtaining local material data. The data was used to construct strain maps and estimate local true stress–strain curves. Using Gini coefficients to quantify strain localisation, it appears that the lower strain rate resulted in slightly faster localisation of plastic deformation with lower stress level.

On the Indentation Resistance of a PC Layer on PVC Foam Substrate

The indentation response of a polycarbonate face sheet bonded to a PVC foam substrate is investigated experimentally and numerically. The deformation mode involves foam crushing, and membrane stretching of the PC layer at large indenter displacements; this is quantified using optical strain measurement techniques. The bottom corners of the foam substrate lifts off its underlying support when the foam layer is sufficiently thin. Peak load is dictated by tensile failure of the foam on the bottom face. Finite element simulations suggest that a deep foam core prevents this lift‐off and results in a greater load carrying and energy absorption capacity.

Fracture toughness of injection molded, man-made cellulose fiber reinforced polypropylene

This investigation focuses on the fracture toughness of injection molded compact tension (CT) specimen of man-made cellulose fibers reinforced composites with PP as their matrix and different varying fiber contents. The influence of the fiber orientation and the addition of a coupling agent on the fracture toughness was determined using an optical strain measurement and a micro computer tomography. It was verified that a reinforcement with man-made cellulose fibers leads to significantly higher values of fracture toughness and J-Integral in comparison to glass fiber reinforcement. Furthermore, it was demonstrated that the majority of fibers in the CT specimen show an orientation perpendicular to the flow direction of the injection molding process. Thus, a notch direction parallel to the flow direction leads to significantly higher values. This is a result of less local strains around the crack path, as well as of a higher amount of fiber pull-outs in the fractured surface. The coupling agent MAPP creates stronger fiber-matrix adhesion, which results in increasing values of the fracture toughness but a decreasing of the J-Integral values due to less fiber pull-outs.

The fatigue life of notched magnesium sheet metals with emphasis on the effect of bands of twinned grains

Notched AM50 sheet metal specimens with four different stress concentration factors are tested under quasi-static and cyclic uniaxial loading, and the results are analyzed and compared with those of unnotched specimens. Special attention is given to the formation of bands of twinned grains and the inhomogeneous local strain field in the vicinity of notches, which is measured by an in situ optical strain measurement technique. It is shown that the notch sensitivity is reduced, if bands of twinned grains occur. In the range between 500 and 1000 load cycles to fracture, the endurable load is only slightly reduced by the presence of a notch for completely reversed fatigue tests. In contrast, specimens are notch sensitive under loading conditions, where no twin formation occurs.

Material Characterization and Validation Studies for Modeling Ductile Damage during Deep Drawing

Modern high strength steels can exhibit ductile damage evolution till fracture which drops ductility for certain loading conditions. This complex formability characteristic as well as shear induced fracture cannot be described by standard methods like forming limit curves. As a remedy, the continuum damage models (CDM) can be applied. In this study, a variant of Lemaitre CDM was investigated to model ductile fracture of an advanced high strength steel, DP1000 in deep drawing processes. This model variant scales the effect of maximum shear stress on damage evolution to predict especially the shear cracks under low stress triaxialities. A decelerated damage evolution due to compressive stress components is taken into account. The material characterization for the damage model requires several tests such as tensile tests with notch, shear test and biaxial cupping tests to capture some characteristic stress states that can occur during deep drawing and further sheet forming processes. An inverse parameter identification methodology using the finite element modeling of whole samples is applied. From the experimental data, force displacement curves and optical strain measurements were used. Test parts were simulated for the validation of the model integrated in the commercial software LS-Dyna. The identified model parameters were finally validated on a cross die cup of high strength steel. The model predictions show good agreement with experimental observations.

Experimental Investigation and Numerical Modeling of the Bond Shear Strength of Multi-layered 6000 Series Aluminum Alloys

The use of high-strength aluminum alloys is one key factor for the realization of lightweight design in industrial applications. Besides alloying, it is possible to produce high-strength sheet materials using the Accumulative Roll Bonding (ARB) process. Based on a repetitive stacking and roll bonding process, multi-layered sheets with a nanocrystalline grain structure are produced. The bond formation is created by a cold welding process due to the high pressure in the rolling gap and cracks in the oxide film of the aluminum sheet material caused by the thickness reduction. The experimental investigation of the bond strength is crucial for the assessment of the formability of the multi-layered sheet material. High bonding quality is necessary in order to prevent failure mechanisms like delamination in subsequent forming operations. Furthermore, a numerical model, which contains discrete layers over the sheet thickness as well as their bonding mechanisms will be implemented in finite element analysis. This enables a process layout for forming operations by the prediction of delamination effects. Within this investigation, an experimental test method is presented to characterize the bond shear strength of multi-layered sheet material. The use of an optical strain measurement system enables the identification of the material behavior over the sheet thickness during the shear test. For this investigation a 16-layered aluminum alloy AA6014 is used. However, the bond shear strength of the last bonded respectively middle layer of the sheet material is investigated. The 16-layered material has 15 bonds over the thickness but the one in the middle has the lowest bond strength as it was only roll bonded once compared to the others, which were rolled at least twice. The bonding mechanisms are modelled using a tiebreak contact formulation in the finite element software LS-DYNA. Concluding, the numerical model is validated by comparison with the experimental results.

Strain measurement and determining coefficient of plastic anisotropy using digital image correlation (DIC)

The deformation and strain measurements have always been one of the main issues in structural engineering. Measuring displacements accurately without damaging structures are an important aim in experimental analysis. For an example, the existing structures need to be evaluated whether they are still fit for purposes or not. To assess complex structures, we need an acceptable and enough amount of data Current strain measurement technologies are not capable of evaluating compounded structures. This paper investigates on an alternative method that uses digital images to provide a full displacement and strain field called Digital Image Correlation (DIC); This technique compares the digital images taken from digital cameras, before and after deformation. We also calculated the coefficient of plastic anisotropy. An aluminium 1050 specimen was loaded in tension according to ASTM E8 testing method while the Sony camcorder captured the video of the specimen being loaded. Once the video was captured, the images were snapshot from video strip using a built-in function of the camera. After that, the images were processed using the Matlab software. The results determined by the extensometer and the optical strain measurement method were compared to each other. This comparison shows that if the test’s setup could be configured in a good way, the error will be less than 2%.

Comparison between different Fiber Optical Strain Measurement Systems Based on the Example of Reinforcing Bars

Fiber optical measurement systems come to the fore after having been intensively investigated during the last two decades. One example is Fiber-Bragg-Gratings (FBG). This technology allows a sufficient strain measurement at predefined points. Another new technique deals with Rayleigh backscatter to realize a distributed strain measurement over the total length of an optical fiber. Such measurement is recognized to provide a high potential in civil engineering, especially in reinforced concrete.This paper deals with some preliminary comparisons between Rayleigh backscatter and FBG based on tension test with reinforcing bars. Therefor optical fibers were placed in notches, milled into the reinforcing bars. Analysis of the test data showed a strong correlation between the optical measurement systems and theoretical results up to the yield point of the steel. Furthermore, the advantages of distributed strain measurement were uncovered. In a last step, the paper offers a prospect concerning future applications for the Rayleigh backscatter system.

Tree biomechanics.

Abstract This review of tree biomechanics covers recent research publications that offer new insights into how trees respond to mechanical loads and resist failure, especially in winds. Biomechanical studies using dynamic rather than static structural methods are showing that the form of the tree influences dynamic response in winds and differences in tree morphology can produce extreme behaviours such as very little or nearly critical dissipation of stem oscillations. Biomechanical studies on trees that measure the forces, stresses and strains are reviewed including a new method of optical strain measurement where small dots (speckle) sprayed on tree surfaces show the strain distribution of trunks and branches under load. The role of stress and strain in tree growth is currently an important research question and the review includes papers that are indicating that strain may be the factor, more than stress, in triggering adaptive growth response. The importance of the form of trees influencing their wind loading is leading to investigations of the importance of branches, especially in winds where they act like coupled oscillators attached to an oscillating trunk to modify sway behaviour. Branch unions are being studied and new concepts on their attachment are also reviewed. Collectively, these developments are leading to a better understanding of tree biomechanics and the complex dynamic response of trees in wind.

Role of the interface between distributed fibre optic strain sensor and soil in ground deformation measurement.

Recently the distributed fibre optic strain sensing (DFOSS) technique has been applied to monitor deformations of various earth structures. However, the reliability of soil deformation measurements remains unclear. Here we present an integrated DFOSS- and photogrammetry-based test study on the deformation behaviour of a soil foundation model to highlight the role of strain sensing fibre–soil interface in DFOSS-based geotechnical monitoring. Then we investigate how the fibre–soil interfacial behaviour is influenced by environmental changes, and how the strain distribution along the fibre evolves during progressive interface failure. We observe that the fibre–soil interfacial bond is tightened and the measurement range of the fibre is extended under high densities or low water contents of soil. The plastic zone gradually occupies the whole fibre length when the soil deformation accumulates. Consequently, we derive a theoretical model to simulate the fibre–soil interfacial behaviour throughout the progressive failure process, which accords well with the experimental results. On this basis, we further propose that the reliability of measured strain can be determined by estimating the stress state of the fibre–soil interface. These findings may have important implications for interpreting and evaluating fibre optic strain measurements, and implementing reliable DFOSS-based geotechnical instrumentation.

Edge crack sensitivity of lightweight materials under different load conditions

This study addresses the analysis of edge crack sensitivity of DP800 steel and AA5182 aluminum alloy in dependency of punching and machining operation as well as load case of subsequent forming. The inserting of a round hole by punching with defined punch-to- die-clearance, milling and drilling is compared. Subsequent forming is performed by standardized hole expansion test and by Nakajima-tests with three different specimen geometries. Local strain distribution at the surface for Nakajima-tests is measured by optical strain measurement technique and investigated in order to evaluate local deformation before failure. Additionally, resulting hole expansion ratio λ is determined. Significant higher X as well as local strain values εmax are achieved by machined holes. This is directly coupled to higher local formability and stretchability for both materials. Furthermore, the load condition has a strong impact on the edge crack sensitivity of the material. Prior failure is observed with changing stress conditions using different specimen geometries also influencing the reachable maximum failure strain. Higher edge crack sensitivity is observed for DP800, which is in good accordance to the material properties in terms of ductility and strength. These data in dependency of the process parameter can be used for the design of automotive components.

Model updating techniques for damage detection in concrete beam using optical fiber strain measurement device

The article aims at detecting and quantifying early structural damages using deterministic and probabilistic model updating techniques. To achieve this purpose, local information in a form of optical strain measurement is employed. The strategy consists in updating physical parameters associated to damages, such as Young’s modulus, in order to minimize the gap between the numerical strain obtained from finite element solves and the strain sensor outputs. Generally, the damage estimation is an ill-posed inverse problem, and hence requires regularization. Herein, three model updating techniques are considered involving different type of regularization: classical Tikhonov regularization, constitutive relation error based updating method and Bayesian approach.An illustration of these three approaches is proposed for localizing and quantifying an early damage in a real 8m post-tensioned concrete beam. Numerical results show that all the methods properly localize the damaged area and give similar estimation of the damage level.

Investigations on fracture curves in strain and stress space for advanced high strength steel forming

Conventional forming limit curves (FLCs) are inappropriate for describing formability for advanced high strength (AHS) steel sheets, since such steel grades experience fracture without localized necking occurrence. The aim of this work was to develop a fracture curve (FC) for the AHS steel grade DP980. The FC was determined by means of the Nakajima stretch forming test and tensile tests of various sample geometries, by which shear fracture governed. An optical strain measurement system was used to capture strain histories of deformed samples up to failure. From these results, fracture strains were gathered and plotted in a strain space. Subsequently, the strain based curve was transformed to space between stress triaxiality and plastic strain. Hereby, effects of anisotropic yield function, namely, the Hill’48 model on obtained stress fracture loci were investigated. In order to verify applicability of the determined limit curves, a Mini-tunnel part was pressed and simulated. It was found that the stress based FC do predict failure of the DP980 steel sheet more accurately than the strain based F C.

Determination of FLD for Ti-6Al-4V Titanium Alloy Sheet

Ti-6Al-4V is the most widely applied titanium alloy in technology and medicine due its good mechanical properties combined with low density and good corrosion resistance. However, poor technological and tribological properties make it very difficult to process, including the problems with sheet-metal forming. The best way to evaluate sheet drawability is to use Forming Limit Diagram (FLD), which represents a line at which failure occurs. FLD allows for determination of critical forming areas.The FLDs can be determined both theoretically and experimentally. Recently, special optical strain measurement systems have been used to determine FLDs.In this study, material deformation was measured with the Aramis system that allows for real-time observation of displacements of the stochastic points applied to the surface using a colour spray. The FLD was determined for Ti-6Al-4V titanium alloy sheet with thickness of 0.8 mm. In order to obtain a complete FLD, a set of 6 samples with different geometries underwent plastic deformation in stretch forming i.e. in the Erichsen cupping test until the appearance of fracture.The real-time results obtained from the ARAMIS software for multiple measurement positions from the test specimen surface were compared with numerical simulations of the cupping tests. The numerical simulations were performed using the PamStamp 2G v2012 software dedicated for analysis of sheet-metal forming processes. PamStamp 2G is based on the Finite Element method (FEM). The major and minor strains were analysed. The effect of friction conditions on strain distribution was also taken into consideration

Challenges During Microstructural Analysis and Mechanical Testing of Small-Scale Pseudoelastic NiTi Structures

Most investigations on NiTi-based shape memory alloys involve large-scale bulk material; knowledge about the martensitic transformation in small-scale NiTi structures is still limited. In this paper, we study the microstructures of thin NiTi layers and their mechanical properties, and we discuss typical challenges that arise when experiments are performed on small samples. A physical vapor deposition (PVD) process was used to deposit thin NiTi wires with a cross section of 15 × 15 μm2 and dogbone-shaped samples 5 × 500 μm2. Microstructural properties were characterized by X-ray diffraction, electron backscatter diffraction, and scanning electron microscopy. Moreover, tensile tests were performed using optical strain measurements in order to observe martensite band formation during cyclic loading. The surfaces of the crystalline wires reflect the columnar growth of NiTi during deposition. The wires exhibit pseudoelastic material behavior during tensile testing. Fracture typically occurs along the columns because the column growth direction is perpendicular to the straining direction. Electropolishing removes these local stress raisers and hence increases fracture strains. Our results demonstrate that the pseudoelastic properties of the PVD-processed materials agree well with those of conventional NiTi, and that they provide new opportunities to study the fundamentals of martensitic transformation in small-scale model systems.