Optical Strain Measurement System Research Articles

Dual-actuator mixed-mode bending tests on structural adhesive joints

In this article, a novel mixed-mode bending (MMB) test setup is presented and used to investigate the fracture behavior of the structural adhesive SikaPower®-498. The novelty lies in the implementation of two actuators that can be controlled independently of each other. This opens up investigation possibilities that go beyond those of the MMB standard configuration. For example, a varying mode-mixity can be prescribed during testing, allowing identification of an entire fracture envelope from a single experiment. Additionally, an optical fiber strain measurement (OFS) system is used to determine the curvatures of the substrates, allowing for a detection of the actual crack tip position and an analysis of the loading conditions behind the crack tip. The presented work focuses on the experimental calibration of the compliance change during crack growth, on tests performed at constant (external) mode-mixity using a mode partitioning approach based on the J-integral, as well as on tests carried out under varying mode-mixity. Additional aspects of the test setup such as stable crack growth regime or differences between the results of different evaluation methods are discussed. Finally, the obtained fracture envelopes are compared with references from literature, including for the first time a comparison between mixed-modes I+II and I+III.

Buckling Analysis of Thin-Walled Composite Structures with Rectangular Cross-Sections under Compressive Load.

The purpose of this research was the analysis of the stability of compressed thin-walled composite columns with closed rectangular cross-sections, subjected to axial load. The test specimens (made of carbon-epoxy composite) were characterized by different lay-ups of the composite material. Experimental tests were carried out using a universal testing machine and other interdisciplinary testing techniques, such as an optical strain measurement system. Simultaneously with the experimental studies, numerical simulations were carried out using the finite element method. In the case of FEA simulations, original numerical models were derived. In the case of both experimental research and FEM simulations, an in-depth investigation of buckling states was carried out. The measurable effect of the research was to determine both the influence of the cross-sectional shape and the lay-up of the composite layers on the stability of the structure. The novelty of the present paper is the use of interdisciplinary research techniques in order to determine the critical state of compressed thin-walled composite structures with closed sections. An additional novelty is the object of study itself-that is, thin-walled composite columns with closed sections.

Mechanical properties evaluation of metacarpophalangeal joint prosthesis with new titanium-nickel memory alloy: a cadaver study

ObjectiveNi-Ti memory alloys are unusual materials for hard-tissue replacement because of their unique superelasticity, good biocompatibility, high strength, low specific gravity, low magnetism, wear resistance, corrosion resistance and fatigue resistance. The current study aims to evaluate its mechanical properties and provide biomechanical basis for the clinical application of the prosthesis.MethodsTen adult metacarpophalangeal joint specimens were randomly divided into a prosthesis group (n = 5, underwent metacarpophalangeal joint prosthesis) and a control group (n = 5, underwent sham operation). Firstly, the axial compression strength was tested with BOSE material testing machine to evaluate its biomechanical strength. Secondly, these specimens were tested for strain changes using BOSE material testing machine and GOM non-contact optical strain measurement system to evaluate the stress changes. Thirdly, fatigue test was performed between groups. Lastly, the mechanical wear of the metacarpophalangeal joint prosthesis was tested with ETK5510 material testing machine to study its mechanical properties.ResultsAxial compression stiffness in the prosthesis group was greater than that in the control group in terms of 30 ° and 60 ° flexion positions (P < 0.05). There was no statistically significant difference between two groups with regards to axial compression stiffness and stress change test (P > 0.05). In the fatigue wear test, the mean mass loss in the prosthesis group’s prosthesis was 17.2 mg and 17.619 mm3, respectively. The mean volume wear rate was 0.12%. There was no statistically significant difference in the maximum pull-out force of the metacarpal, phalangeal, and polymer polyethylene pads between the prosthesis group and the control group specimens.ConclusionsNi-Ti memory alloy metacarpophalangeal joint prosthesis conforms to the biomechanical characteristics of metacarpophalangeal joints without implants, and the fatigue strength can fully meet the needs of metacarpophalangeal joint activities after joint replacement.

Differential temperature compensation method for edge filtering strain sensors

The edge filtering system is a common optical fiber strain measurement system. However, temperature changes in the measurement will interfere with the results. A differential temperature compensation method for edge filtering strain sensors is proposed. An additional temperature compensation FBG is added to the traditional edge filtering interrogation system. In the experiment, we analyze the influence of temperature on the sensor and filter. Temperature cross-sensitivity can be eliminated by differencing two FBGs, and the influence of temperature on the filter can also be eliminated. Experiments show that the system's temperature compensation ability is ideal, the influence of temperature on the strain test is reduced by about 80 %, and the sensitivity of the system is 0.003 mV/με. This method can reduce the influence of temperature on the edge filter measuring system.

Damage growth and failure detection in hybrid fiber composites using experimental in-situ optical strain measurements and smoothing element analysis

In previous study the failure initiation and development in hybrid fiber laminates was successfully monitored and determined. In current investigation a novel damage monitoring approach is proposed for hybrid laminates by combining different optical strain measurement techniques namely digital image correlation (DIC), fiber Bragg grating sensors (FBG) and infrared thermography (IRT) with smoothing element analysis (SEA). This viable experimental procedure eliminates the effects of global/local nature of optical strain measurement systems on heterogeneous damage accumulation and is a two-step approach. First, all optical sensing systems together with conventional strain gauges are utilized concurrently to indicate the differences in the measured strains and monitor damage accumulation under tensile loading. This demonstrates how failure events disturb the measurement capabilities of optical systems, which can cause a miscalculation of hybrid effect in hybrid-fiber laminates. The second step involves the utilization of SEA algorithm for discretely measured DIC displacements to predict a realistic continuous displacement/strain map and rigorously mitigate the inherent noise of the full field optical system. Remarkably, for large deformation states in hybrid composites, the combination of SEA/DIC enables early prediction of susceptible damage zones at stress levels 30% below material strength.

Forming Limit Diagrams of Low-Carbon Steels Obtained Using Digital Image Correlation Technique and Enhanced Formability Predictions Incorporating Microstructural Developments

In the present work, drawing quality (DQ) and interstitial-free (IF) steel sheets were subjected to limiting dome height tests for determining strain-based forming limit diagrams (FLDs) experimentally and effect of dynamic (variable) material properties, such as work hardening ‘n’ and plastic anisotropy ( $$\bar{r}$$ ), on FLDs has been studied and simulated by PAMSTAMP™ finite element software. Dot prints on the steel specimens and an efficient optical strain measurement system GOM™ which works on the digital image correlation technique principle were used to measure limiting strains accurately instead of conventional circle grid analysis technique and strain measurement by traveling microscope. The dynamic (variable) material properties (n) and ( $$\bar{r}$$ ) were estimated by studying the microstructural developments in terms of changes in grain average misorientation and crystallographic texture, respectively, at different strains and strain paths, during deformation. In our proposed work, a novel technique of incorporating dynamic (variable) material properties (n and $$\bar{r}$$ ) in FE simulations was carried out during FLDs predictions. Though marginal but improved predictions in FLDs were observed in both IF and DQ steels. In addition to strain-based FLDs, stress-based FLDs were also determined for both grades. Interestingly, in both cases it was noticed that IF steel had higher formability than DQ steel.

Experimental and Numerical Investigation of Single Point Incremental Forming of Aluminium Alloy Foils

Single Point Incremental Forming (SPIF) is a flexible process to manufacture sheet metal parts that is well adapted and profitable for prototypes or small batch production. Compared to traditional sheet forming technologies this relatively slow process can be used in different applications in automotive and aircraft industries, in architecture engineering and in medical aids manufacturing. In this paper indirectly obtained axial forming force on SPIF of variable wall angle geometry were studied under different process parameters. The estimation of the forces on AlMn1Mg1 sheets with 0.22 mm initial thickness is performed by continuous monitoring of servo motor currents. The deformation states of the formed parts were analysed using the ARGUS optical strain measurement system of GOM, while the roughness measurements were carried out by a System of Mitutoyo. Some initial Finite Element Analysis simulations and a crack monitoring method together with an interaction plot of forming speed, incremental depth, tool diameter and lubrication were also reported.

Statistics of Fracture in 3-Point Bending

CrachFEM have a built-in failure probability model. The failure probability can be derived from the scatter of one material batch, next to the deterministic prediction of the CrachFEM failure risk. In this paper a 3-point bending experiment is used to mimic the in-house forming operation of Faurecia and to derive the failure probability. The failure probability of three materials have been determined with a 3-point bending experiment in combination with an optical strain measurement system. A post-processing procedure has been setup for this purpose. Although the three materials are somewhat comparable, the failure probability is different and will reveal whether a material is critical or suitable for the production process.

A Review of Cruciform Biaxial Tensile Testing of Sheet Metals

Biaxial testing technology enables the comprehensive study of sheet-forming performance and has great potential developmental and commercial value. This work is a review of cruciform biaxial tensile testing technology, as used in actual production, and provides evidence for sheet metal forming processes to describe problems that remain to be solved and to suggest new ideas for future study. The testing apparatus including cruciform biaxial tensile testing machines, heating devices, and optical strain measurement systems are discussed. Some typical descriptions of material tests to achieve stress and strain curves, the yield loci, and limit strains relating to both ambient and high temperatures are also presented. Moreover, finite element modeling as applied to specimen design, material deformation prediction, and material model determination is investigated. Furthermore, the sample design methods and a few examples are summarized in spite of the lack of geometry standard. Future prospects are mainly focused on the establishment of specimen geometry standard, reverse modeling technique for high temperature tests, and the actual applications of biaxial tensile tests.

A Simplified Analytical Solution for the Necking Semi-empirical Stresses based on Aramis System

To make comparison between classic necking stress solution Bridgman formula and ChenChi formula, the difference of necking surface profile assumptions in these two formulas were discussed theoretically. Then, the uniaxial tension experiment on 16 groups of carbon structural steels Q235 and Q345 were carried out at room temperature. Associate with the necking displacement measured by Aramis optical dynamic 3D strain measurement system, the parameter equation of necking surface profile was determined. The comparison results indicate that ChenChi formula is more accurate than Bridgman formula in calculating the necking stress distribution. Furthermore, to overcome the difficulty of measuring the radius curvature R in these formulas, an empirical formula of the geometric dimensions in the necking section was applied and the parameter was corrected based on the necking strain measured by Aramis system. As a result, the obtained equation in this study can provide an improved calculation method for the analysis of three-dimensional stress in necking and a reference for future research on the necking and ductile fracture of structural steels.

Damage evolution and deformation behaviour of dry and saturated sandstones: Insights gleaned from optical measurements

Microscopic properties have a marked influence on the macroscopic mechanical response of sandstones. Improved understanding of the mechanical behaviour of these rocks caused by their microscopic properties is imperative for the successful design of rock structures. Here we experimentally investigate the damage evolution and deformation behaviour of three types of sandstones from Australia with contrasting petrographic properties under dry and saturated conditions with the aid of an optical strain and deformation measurement system (ARAMIS camera). More accurate and representative strain measurements are obtained using ARAMIS camera images compared to conventional strain measurement techniques. The results revealed that the quartz content of sandstone alone is not indicative of its strength, as the sandstone with the highest quartz content showed the lowest strength and this was attributed to the superior influence of other microscopic properties, such as grain size and porosity. However, the quartz content displayed a significant influence on the water-weakening behaviour of the sandstones, where the sandstone with the greatest quartz content was least affected by saturation and the sandstones with considerable clay mineral content showed greater weakening due to water. It was also found that some clay minerals are susceptible to swelling in the presence of water, and this can increase the initial crack volume and affect deformation characteristics. ARAMIS camera images clearly illustrated a more brittle behaviour by the sandstone with the greatest quartz content and notable early deformation by the sandstones with greater clay mineral content. Moreover, it was found that the overall deformation of sandstones was influenced by the swelling capacity of the constituent clay minerals, in addition to their composition by percentage.

Experimental and numerical studies on indentation and perforation characteristics of honeycomb sandwich panels

Aluminum sandwich panels with honeycomb core have been widely used as energy absorption structure in lightweight design. This study aimed to characterize the indentation and perforation behaviors of sandwich structures with different geometric configurations. The specimens with four characteristic geometric variables, namely, facesheet thickness, core height, honeycomb core thickness and side length of hexagon cell were tested experimentally. Photographs of cross-sectional view near the loading area and failure modes in the tests were investigated in detail. For the first time, digital image correlation (DIC) technique through an ARAMIS™ real-time optical strain measurement system was adopted for capturing the deformation process of lower skin by acquiring the displacement-time data. Three typical damage modes were identified from the force-displacement curves with different geometric parameters and configurations. It was found that the thickness of facesheet has the most significant effects on both force-displacement curves and energy absorption capacity. Changes in the core parameters have relatively small influences in total energy absorption but sizeable effects on the force-displacement curve and failure modes. A finite element model for predicting damage evolution was also developed and validated through the force-displacement relation and deformation process on the bottom skin. The damage mechanisms of the sandwich panel subject to quasi-static indentation and perforation were analyzed through the numerical models. The present study contributed on understanding how the geometric parameters affect the characteristics of indentation and perforation, thereby providing useful guidelines for its potential applications in impact engineering.

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.

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.

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.

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.

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

Optical Investigation of Component Made by FDM Technology

3D printed plastic components are nowadays frequently used parts in all areas of industrial sphere. These components are often made by FDM technology. The main advantage of this technology is quick manufacturing process, price and also possibility of producing complex parts. This paper is aimed to the component made by FDM technology from the view of their strength. Specially designed chair was loaded by different force and the maximum load before destruction was measured. Chair was under inspection of two different optical strain measurement systems working on different principles. System PONTOS working on the principle of digital photogrammetry and system ARAMIS working on the principle of digital image correlation were used. These systems were used in order to investigate and identify weak places of this chair.

Numerical Investigation of Dry and Lubricated Sheet Metal Forming Processes

The current global development towards efficient and sustainable usage of resources as well as a stronger environmental awareness motivates lubrication abandonment in metal forming. Dry forming processes accomplish besides a green production technology also a shortage in production steps and time. However, the change of the tribological conditions influences the material flow during the forming operations and has therefore to be taken into account for the design of complex sheet metal forming operations. The aim of this study is a comparison of dry and lubricated processes by numerical as well as experimental investigations. To ensure reliable results a test setup is necessary which provides a discrete control of the process parameters. Furthermore, an analysis of the local material flow by an optical strain measurement system during the whole test procedure should be possible. These requirements are well fulfilled by the so called Nakajima test, which is typically used for the characterisation of the formability of sheet metals. The influence of varying friction coefficients on the material behaviour is discussed based on the numerical model built up in the Finite Element Software LS-Dyna. The numerical results show a good conformity with the experimental outcomes by identifying the strain localisation. Based on the gained knowledge of the investigations an increase of process understanding for dry forming operations will be derived.