Non-contact Digital Image Correlation Research Articles

Experimental Investigation of Load-Bearing Capacity in EN AW-2024-T3 Aluminum Alloy Sheets Strengthened by SPIF-Fabricated Stiffening Rib.

The aluminum strength-to-weight ratio has become a highly significant factor in industrial applications. Placing stiffening ribs along the surface can significantly improve the panel's resistance to bending and compression in aluminum alloys. This study used single-point incremental forming (SPIF) to fabricate stiffening ribs for 1 mm and 3 mm thick aluminum alloy EN AW-2024-T3 sheets. A universal compression machine was used to investigate sheet deformation. The resulting deformation was examined using non-contact digital image correlation (DIC) based on several high-resolution cameras. The results showed that deformation progressively escalated from the edges toward the center, and the highest buckling values were confined within the non-strengthened area. Specimens with a larger thickness (3 mm) showed better effectiveness against buckling and bending for each applied load: 8 kN or 10 kN. Additionally, the displacement from the sheet surface decreased by 60% for sheets 3 mm thick and by half for sheets 1 mm thick, which indicated that thicker sheets could resist deformation better.

Additive Manufacturing and Characterization of Sustainable Wood Fiber-Reinforced Green Composites

Enhancing mechanical properties of environmentally friendly and renewable polymers by the introduction of natural fibers not only paves the way for developing sustainable composites but also enables new opportunities in advanced additive manufacturing (AM). In this paper, wood fibers, as a versatile renewable resource of cellulose, are integrated within bio-based polylactic acid (PLA) polymer for the development and 3D printing of sustainable and recycle green composites using fused deposition modeling (FDM) technology. The 3D-printed composites are comprehensively characterized to understand critical materials properties, including density, porosity, microstructures, tensile modulus, and ultimate strength. Non-contact digital image correlation (DIC) technology is employed to understand local stress and strain concentration during mechanical testing. The validated FDB-based AM process is employed to print honeycombs, woven bowls, and frame bins to demonstrate the manufacturing capability. The performance of 3D-printed honeycombs is tested under compressive loads with DIC to fully evaluate the mechanical performance and failure mechanism of ultra-light honeycomb structures. The research outcomes can be used to guide the design and optimization of AM-processed composite structures in a broad range of engineering applications.

Experimental Investigation and Mechanical Model of Tensile Behaviors of Membrane with Seam

The contradiction between the spatial demand of a large-scale membrane structure and the existing membrane processing method encourages the development of membrane splicing technology. However, there has been very limited research on the tensile mechanical properties of the seamed membrane. A detailed experimental investigation of the tensile behavior of polyimide membranes with various seam situations is presented in this paper, including pure membranes and seamed membranes with wide or narrow bond regions. The tensile phenomena, elastic behaviors, fracture performances, and wrinkle configurations of the membrane specimens with different length-to-width aspect ratios are analyzed based on the uniaxial tensile tests and noncontact digital image correlation measuring technology. Furthermore, the analytical models are developed to predict the bond stress distribution and the required minimal seam width. The test results indicated that a seam with a width of 12.5 mm could not provide sufficient bonding force, whereas a seam with a minimum width of 17.6 mm could establish effective seamed membranes. Additionally, the elastic modulus of the seamed region increased 53.5% more than that of the pure membrane using the seam details. The out-of-plane deformation and the number of wrinkles of the seamed membrane specimens are all much greater than those of the corresponding pure membrane specimens for all length-to-width aspect ratios. Moreover, the count of wrinkles can explain the phenomena that the fracture strengths of membranes in group 2 are higher than those in group 1, and there is a slight increase in the fracture strengths of membranes in group 1 with the rise of specimen widths. This principle in this work can be regarded as the basis of the design and application of seams in larger-scale membrane structures.

Investigation of deformation behaviour of uniaxially loaded sand grains using a novel high-resolution imaging apparatus and ensemble machine learning models

ABSTRACT In geotechnical engineering, the time-dependent behaviour or ageing behaviour is vital for applications such as earthwork compaction and liquefaction potential assessment. This study introduces a novel test apparatus to understand micromechanical factors and deformations at grain contacts. Using a non-contact Digital Image Correlation (DIC) technique, deformations were measured with a 10 μϵ spatial resolution. This enabled quantification of grain creep and contact maturing deformations, surpassing previous experimental methods. To model this complex behaviour, Machine Learning (ML) models, including an artificial neural network (ANN) and long-short term memory neural network (LSTM), were used, achieving a 1-2% error rate with experimental results. The integration of ML offers a promising tool for predicting long-term grain strains, enhancing the assessment of structures' serviceability with the studied materials.

The yielding behavior and plastic deformation of oxygen-free copper under biaxial quasi-static and dynamic loadings

This study investigated the yield behavior of oxygen-free copper under complex stress states and different loading rates. An optimized cruciform specimen and a novel electromagnetic biaxial split Hopkinson bar (EBSHB) were used to perform the dynamic biaxial tension tests at three different load ratios. The non-contact digital image correlation technique was adopted for strain measurement. Quasi-static and dynamic yield surfaces were obtained. Classical yield criteria, including the Tresca and von Mises yield criteria, were evaluated based on the experimental results. The von Mises criterion is found to provide better prediction of the yield surface under both quasi-static and dynamic loadings. The Johnson-Cook model from uniaxial test results provides a moderate prediction for experimental curves, while that from biaxial test results shows better prediction. Dynamic biaxial tests are proven necessary to acquire the constitutive models that can accurately describe the mechanical properties of copper under complex stress states.

Microstructure, mechanical properties and fracture toughness of ECAPed magnesium matrix composite reinforced with hydroxyapatite ceramic particulates for bioabsorbable implants

Employing ceramic particles like hydroxyapatite to fabricate magnesium-based bio-composite has received incredible attention in orthopedic applications due to their bioactivity and biocompatibility. Besides, applying severe plastic deformation methods like equal channel angular pressing (ECAP), in addition to eliminating defects induced by casting, causes the metallurgical and mechanical behavior improvement of Mg/HA bio-composites. In this paper, the combined effects of HA particles and the ECAP process with Bc route on the microstructure, mechanical properties, and fracture toughness behavior of Mg/HA bio-composite fabricated through the electromagnetic and mechanical stir casting method were evaluated. Furthermore, a non-contact digital image correlation (DIC) method was adopted to accurately assess the elastic and plastic parameters and crack propagation during the fracture toughness test. Microstructural observation demonstrated that the addition of HA particles and, subsequently, applying four passes of the ECAP process on the bio-composites not only decreased the grain size by a considerable amount of 92%, but also resulted in a homogenous microstructure. Moreover, according to the results of the mechanical tests, after four-pass ECAP, although the compressive yield stress of Mg/HA bio-composite did not significantly change, the highest tensile yield stress, ultimate tensile, and compressive strength were accomplished. Among all elastic and plastic parameters, the elastic modulus and strength coefficient have continuously risen with adding HA and increasing the number of ECAP passes. However, the strain hardening exponent has decreased noticeably with the addition of HA, and the ECAP process compensated for some of this reduction. In addition, reinforcing magnesium with HA particles and improving the dislocation density caused by increasing the number of ECAP passes has led to increased material resistance against crack growth, wasted energy, and, consequently, fracture toughness. In this case, the four-pass ECAPed bio-composite showed the highest fracture toughness, approximately 28 MPa m0.5, 70% more than the pure Mg.

Microstructure and mechanical properties of gas metal arc welded CoCrFeMnNi joints using a 410 stainless steel filler metal

The use of filler materials during fusion-based welding processes is widely used to regulate and modify the composition of the welded joints aiming at producing a desired microstructure and/or achieving an improvement in its mechanical performance. Welding of high entropy alloys is still a new topic and the impact of different filler materials on the microstructure and mechanical properties is yet unknown. In this work, gas metal arc welding of the CoCrFeMnNi high entropy alloy using 410 stainless steel as a filler wire was performed. The microstructural evolution of the welded joints was evaluated by optical microscopy, scanning electron microscopy aided by electron backscattered diffraction, high energy synchrotron X-ray diffraction and thermodynamic calculations. Meanwhile, the mechanical behavior of the welded joint, as well as the local mechanical response were investigated with microhardness mapping measurements and with non-contact digital image correlation during tensile loading to failure. The weld thermal cycle promoted solid state reactions in the heat affected zone (recovery, recrystallization and grain growth), which impacted the microhardness across the joint. The role of the 410 stainless steel filler material in the solidification path experienced by the fusion zone was evaluated using Scheil-Gulliver calculations, and a good agreement with the experimentally observed phases was observed. Despite the addition of the 410 stainless steel filler was not conducive to an increase in the fusion zone hardness, the associated bead reinforcement promoted an improvement in both the yield and tensile strengths of the joint compared to a similar weld obtained without filler material (355 vs 284 MPa and 641 vs 519 MPa, respectively). This allows to infer that the addition of filler materials for welding high entropy alloys is a viable method for the widespread use of these novel materials. In this work, by coupling microstructure and mechanical property characterization, a correlation between the processing conditions, microstructure and mechanical properties was obtained providing a wider basis for promoting the application of gas metal arc welding of high entropy alloys for industrial applications.

Microstructure and mechanical properties of gas metal arc welded CoCrFeMnNi joints using a 308 stainless steel filler metal

In this paper, gas metal arc welding of a CoCrFeMnNi high entropy alloy was performed using 308 stainless steel filler wire. Electron backscatter diffraction and synchrotron X-ray diffraction were used to determine the microstructural evolution, while microhardness mapping and non-contact digital image correlation were employed to assess the local mechanical response across the welded joints. Further, thermodynamic calculations were implemented to support the understanding of the microstructure evolution. Through a systematic analysis of the microstructure evolution and mechanical properties, it is established a correlation between welding process, microstructure and mechanical properties. Besides, this work lays the foundations for the use of low-cost arc-based welding technologies for successful joining and application of welded joints based on high entropy alloys.

Deformation Characteristics of Sandy Soil around a Plate Anchor under Lateral Loading

The lateral loading of a plate anchor is a complicated process that involves complex anchor-soil interactions. The deformation characteristics of the soil around an anchor have an important effect on its lateral bearing capacity. In this paper, the noncontact digital image correlation (DIC) technique is used to study the distribution and variation of the soil deformation field under a laterally loaded anchor in sandy soil. The results show that the sand density and embedment ratio significantly affect the distribution and influence range of the active and passive zones around the anchor. The active zone behind the rod gradually decreases with increasing sand density until ultimately disappearing, and the passive zone increases. The maximum influence height occurs in the passive zone behind the rod in dense sand, and the influence range of the passive zone in front of the rod expands with an increasing embedment ratio. Shear bands form during the lateral loading process, which are accompanied by dilatancy in the shear process. The motion path of the rotation center in loose and medium sand is initially rigid translational and then becomes rigid rotational, while the opposite trend occurs in compact dense sand. The results provide important guidance for the development of predictive models for anchor lateral loading and design.

Investigation on Temperature-Dependent Multiaxial Ratchetting of Polycarbonate by a Novel Experimental Method

A novel method to heat the multiaxial hollow thin-wall specimen was proposed, where its gauge length section can be heated by the liquid (i.e., water) filled inside the specimen instead of the closed furnace. This method realizes the direct measurement of multiaxial strain on the surface of specimen at different temperatures by the noncontact digital image correlation. By utilizing the proposed method, the multiaxial stress-control cyclic tests were performed to investigate the multiaxial ratchetting of polycarbonate (PC) at different temperatures. It is found that the multiaxial ratchetting of PC depends greatly on the test temperatures and the multiaxial ratchetting strain increases with increasing the test temperature. The temperature-dependent multiaxial ratchetting is also influenced by valley stresses. The unrecoverable part of deformation in the multiaxial ratchetting strain increases with increasing temperature.

Investigations on material properties and residual stresses in cold-formed high strength steel irregular hexagonal hollow sections

This paper presents an experimental investigation on the material properties variation and residual stress distribution within the cold-formed high strength steel (HSS) irregular hexagonal hollow sections (IHexHS) with two different fabrication methods. The test specimens were manufactured through press-braking and gas metal arc welding (GMAW). Tensile coupons tests were conducted on specimens fabricated from the critical locations within cold-formed HSS irregular hexagonal hollow sections, namely the flat portions, corner portions of either half or quarter sections. New material models to predict the material properties for the tensile coupons with both rounded responses and yield plateau followed by significant strain hardening were proposed. In conjunction with conventional tensile coupon testing, non-contact digital image correlation (DIC) measurement through which strain fields along the gauge length before and after the occurrence of diffuse necking was carried out to obtain the accurate strain field after necking. Moreover, the residual stresses measurements for HSS IHexHS were also performed, membrane and bending residual stresses distributions on the investigated sections were measured in longitudinal directions with 59 strips cut by wire-cutting method and more than 708 strain readings obtained. Results of the residual stress distributions and magnitudes are presented and discussed. Based on the measurement results, predictive models for residual stress distribution were developed and can be subsequently applied for predicting structural behaviour of the cold-formed HSS IHexHS.

Deformation Characteristics of Bucket−Soil Interaction in Sand during Jacked Penetration

Grouped buckets are an important type of foundation for offshore structures. Because of interactions between buckets, jacked penetration above a group of two buckets is a complicated process that involves complex bucket-soil interactions. In this study, non-contact digital image correlation was applied to study the deformation of sandy soil during jacked penetration. Bucket-group efficiency is negatively correlated with sand density and bucket diameter, and the incremental filling ratio is negatively correlated with sand density and positively correlated with bucket diameter. The graph of the incremental filling ratio versus penetration depth curves is a polyline for a bucket diameter of 5 cm and a hyperbola for 7.5 cm. Buckets in groups experience superposition of their deformation fields, which leads to a partial loss of shear strength. Sand density, bucket diameter, and bucket spacing markedly influence the deformation field of a bucket group, consistent with the force-displacement curves. The bucket-group effect retards the formation of a soil plug.

Combined approach for fatigue crack characterisation in metals

In this work a comprehensive characterisation of the fatigue crack growth of an Al 2024 alloy is presented. A powerful Christopher-James-Patterson (CJP) model is used for multi-parameter characterisation of the crack state at a few different locations through the fatigue crack growth. The CJP model of crack tip displacements and stress fields was proposed in order to better capture the influences on the applied elastic stress field of the plastic enclave that is generated around a growing fatigue crack. The model does this through a set of elastic stresses applied at a notional elastic-plastic boundary, and it has been shown to accurately model plastic zone shape and size, whilst its ability to predict the effective range of stress intensity factor during a fatigue cycle has been verified. Thus, the model can predict the driving force component of the crack and additional components that account for different shielding mechanisms. CJP model is combined with full-field non-contact Digital Image Correlation (DIC) so that the model can be fed at any moment with real experimental information. The results are then correlated with Paris law data for Al 2024 alloy through Scanning Electron Microscopy (SEM) observations of the fracture surface. In this work the effect of crack length and load level are thoroughly studied and validated with fatigue crack growth rate measurements.

Accurate Measurement of Strain in Noncontact Surface Deformation Using Subset-Based Digital Image Correlation.

The measurement of strain using some contact techniques has some drawbacks like less accuracy and it takes larger computation time for finding each location of subpixels. Thus, a faster noncontact Digital Image Correlation (DIC) mechanism is utilized along with the traditional techniques to measure the strain. The Newton-Raphson (NR) technique is considered to be an accepted mechanism for accurate tracking of different intensity relocation. Generally, the issue regarding the DIC mechanism is its computational cost. In this paper, an interpolation technique is utilized to accomplish a high precision rate and faster image correlation; thereby it reduces the computation time required for finding the matched pixel and viably handles the rehashing relationship process. Hence, the proposed mechanism provides better efficiency along with a reduced number of iterations required for finding the identity. The number of iterations can be reduced using the Sum of Square of Subset Intensity Gradients (SSSIG) method. The evaluation of the projected scheme is tested with different images through various parameters. Finally, the outcome indicates that the projected mechanism takes only a few milliseconds to match the best matching location, whereas the prevailing techniques require 16 seconds for the same operation with the same step size. This demonstrates the effectiveness of the proposed scheme.

Experimental analysis of control technology and deformation failure mechanism of inclined coal seam roadway using non-contact DIC technique

The deformation and failure forms of inclined coal seam roadway under the joint action of dip angle and various geological conditions are complex, and there is a lack of targeted support measures, which brings great problems to the stability control of roadway surrounding rock. In order to safely and economically mine inclined coal seams, taking the engineering geology of Shitanjing No. 2 mining area as the background, and the physical similarity model of right-angle trapezoidal roadway in inclined coal seam, in which the non-contact digital image correlation (DIC) technology and the stress sensor is employed to provide full-field displacement and stress measurements. The deformation control technology of the roadway surrounding rock was proposed, verified by numerical simulation and applied to engineering practice. The research results show that the stress and deformation failure of surrounding rock in low sidewall of roadway are greater than those in high sidewall, showing asymmetric characteristics, and the maximum stress concentration coefficients of roadway sidewall, roof and floor are 4.1, 3.4 and 2.8, respectively. A concept of roadway "cyclic failure" mechanism is proposed that is, the cyclic interaction of the two sidewalls, the sharp angles and roof aggravated the failure of roadway, resulting in the overall instability of roadway. The roadway sidewall is serious rib spalling, the roof is asymmetric "Beret" type caving arch failure, and the floor is slightly bulging. On this basis, the principle of roadway deformation control is revealed and asymmetric support design is adopted, and the deformation of roadway is controlled, which support scheme is effective.

Experimental study on a new method to forecasting goaf pressure based key strata deformation detected using optic fiber sensors

The spatial distribution of rock mass stress in goaf seriously affects the stable operation of underground reservoir. Measurement of stress distribution in the goaf of coal mines is difficult due to the complex internal environment and other restrictive conditions of goaf. This paper presents a new method of monitoring the deformation of key stratum in overburden by optical fiber sensing, and then characterizing the pressure of goaf. The internal mechanism of key stratum breakage and the variation of goaf stress is investigated by physical model with the geometric dimension of 3 (length) × 1.3 (height) × 0.4 (width) m), in which the Brillouin Optical Time Domain Analysis (BOTDA) fiber-optic sensing technology and Fiber Bragg Grating (FBG) technology are used to monitor the stability of key stratum and the non-contact Digital Image Correlation (DIC) technique is employed to provide real-time deformation measurements. Combining the fracture mechanics model of key strata with the parabolic function model of subsidence trajectory, the stress evolution model of coal seam mining space is established for the first time, the goaf pressure change and the corresponding relation between the key layer breakage was quantitatively characterized. The research results show that: The fracture location of the key strata is consistent with the peak stress location of the goaf. The breaking limit strain threshold value of the key strata based on optical fiber sensing detection is 4000 με. Through the inversion of optical fiber monitoring results, the fracture degree and range of overburden rock in goaf are obtained, thus reflecting the position and change of the peak pressure in goaf. The consistency between key layer deformation and goaf pressure variation is verified.

Assessment of load distribution mechanism in geocell reinforced foundation beds using Digital Imaging Correlation Techniques

The pressure distribution below geocell reinforced foundation beds depends on several factors like height of geocell, weld distance (pocket size), infill material, etc. This paper describes a method to determine the load dispersion mechanism through a geocell layer using a non-contact Digital Image Correlation technique. Series of plate load tests were performed in a test tank having plan dimensions of 0.6 m × 0.6 m and height of 0.65 m. The loading was applied through a strip footing of width 120 mm. Two miniature geocells having heights of 60 mm and 75 mm were used for reinforcing the base layer of the foundation section. The studies were performed with two types of geocells having smooth and rough surfaces. The relative density (RD) of the base and subgrade layers were maintained as 80% and 40%, respectively during the tests. The movement of soil particles was recorded by capturing images at different stages of the test. The images were processed using a Digital Image Correlation (DIC) software to obtain displacement contours and vectors. The results show that the geocells distribute the load over a wide area of the foundation soil. The dispersion angle was found to depend on the height of geocell and the roughness of the geocell surface. The load dispersion angles obtained from Digital Image Correlation analyses of unreinforced and reinforced sections have been compared with similar published results.

Determination of Load-Carrying Capacity of C-Profile Glued Ti-Al Column under Temperature Environment.

The paper deals with an examination of the behaviour of glued Ti-Al column under compression at elevated temperature. The tests of compressed columns with initial load were performed at different temperatures to obtain their characteristics and the load-carrying capacity. The deformations of columns during tests were registered by employing non-contact Digital Image Correlation Aramis® System. The numerical computations based on finite element method by using two different discrete models were carried out to validate the empirical results. To solve the problems, true stress-logarithmic strain curves of one-directional tensile tests dependent on temperature both for considered metals and glue were implemented to software. Numerical estimations based on Green–Lagrange equations for large deflections and strains were conducted. The paper reveals the influence of temperature on the behaviour of compressed C-profile Ti-Al columns. It was verified how the load-carrying capacity of glued bi-metal column decreases with an increase in the temperature increment. The achieved maximum loads at temperature 200 °C dropped by 2.5 times related to maximum loads at ambient temperature.

The role of brass texture on the deformation response of 7075-T651 aluminum alloy under equi-biaxial tension

Cruciform specimen is utilized to study the influence of equi-biaxial stress state on the plastic strain response of AA7075-T651 alloy until fracture. The failure ductility as a function of stress state is evaluated using non-contact digital image correlation technique. The distinct biaxial deformation behavior in contrast to uniaxial response is attributed to strong crystallographic texture developed during prior hot-rolling reduction. The weakening of initial brass texture upon equi-biaxial deformation is also evident from the orientation distribution analysis. Increased activity of grain boundary assisted cracking due to increased stress triaxiality due to biaxial loading is evident, when compared to uniaxial stress state. • Cruciform specimen is used to capture the plastic strain response of 7075-T651 aluminum alloy until fracture. • Severe reduction in ductility with concomitant improvement in yield strength observed under biaxial condition. • Increased occurrence of grain-boundary assisted cracking corroborates decreased ductility under equi-biaxial stress state. • Brass texture component {0 1 1} < 5 3 ‾ 3> contributed to the observed biaxial strengthening effect.

Composite timber-steel encased columns subjected to concentric loading

Novel composite timber-steel encased columns were proposed to fully exploit advantages and address short-comings of timber and steel; two construction materials with different mechanical properties and complementary benefits. The encased steel bars were adhesively bonded inside the timber casing to achieve near full composite action. The load carrying capacity, failure mechanisms, stiffness, and ductility of 48 hybrid members under concentric axial compression were experimentally evaluated and compared with bare timber columns, where the benefits of hybridising timber with steel bars are highlighted. The composite columns were designed with intermediate slenderness and varying steel core area, steel grade (300PLUS, 5.8, N500 and 8.8), timber species (Radiata Pine and Douglas fir) and timber grade (MGP10 and F5). When compared to bare timber columns, the proposed composite columns demonstrated increased axial capacity, stiffness, and ductility. Analytical expressions based on principles of structural mechanics and existing design provisions are developed to estimate the stiffness and strength of the composite columns. Non-contact Digital Image Correlation (DIC) was employed to measure strain and predict crack initiation. The practical benefits of composite timber-steel encased columns are also discussed, notably the potential to express the materiality of wood and offer a market for low-grade timber with smaller cross sections.