High Resolution Digital Image Correlation Research Articles

Understanding strain localisation behaviour in a near-α Ti-alloy during initial loading below the yield stress

Near-α Ti-alloys such as TIMETAL® 834 are known for their superior mechanical properties at high temperature, and as such are found in applications where high strength and improved fatigue performance at elevated temperatures (>450°C) are required. However, these alloys can be susceptible to cold-dwell fatigue; a failure mechanism that is not well understood. The present work investigates the strain localisation behaviour during cold creep and the implications it has in terms of dwell susceptibility for two different bi-modal microstructures. Slip traces and strain distributions have been analysed for different material conditions by employing High-Resolution Digital Image Correlation (HRDIC) in combination with orientation mapping. Using this approach, it was possible to distinguish deformation patterns in primary α grains and transformed-β colonies, loaded incrementally to stress levels of 70%, 80% and 90% of the yield stress. Different prior β-grain morphologies didn’t affect the average strains when stresses are low; but strain distributions have been affected by the β-grain morphology. Material with coarse transformation product accumulated larger amounts of plastic strain compared to material with fine transformation product, at the same relative stress levels. At low stress levels, slip bands have been detected both in primary α, as well in the transformed-β phase, cutting through the lamellae, for the material condition with a coarse transformation product; on the other hand, for the material conditions with a fine transformation product, slip bands are localised only in primary α grains at low stress levels. It was also found for both conditions that at low stress levels slip bands are found in grains that are well oriented for basal slip. Based on these observations it is discussed if b-ligaments are significant obstacles to dislocation movement. Finally, the requirement of crystal-plasticity modelling to take into account differences in crystallographic orientations and the elastic and plastic anisotropy of HCP-titanium will be discussed and considered.

Direct measurements of slip irreversibility in a nickel-based superalloy using high resolution digital image correlation

Fatigue crack nucleation in crystalline materials typically develops due to highly localized cyclic slip. During a fatigue cycle, reverse slip differs locally from slip in the forward direction particularly in precipitate-containing materials such as superalloys. In this paper we report the first direct measurements of irreversibility at the scale of individual slip bands by high-resolution digital image correlation (DIC) in a polycrystalline nickel-based superalloy. Quantitative measurements of the slip irreversibility are challenging for regions of material that have a size that captures the microstructure and its variability. High spatial resolution at the nanometer scale during experimental measurements is needed to observe slip localization during deformation. Moreover, large fields are also needed to obtain the material response over statistically representative populations of microstructural configurations. Recently, high resolution scanning electron microscope (SEM) digital image correlation (DIC) has been extended for quantitative analysis of discontinuities induced by slip events using the Heaviside-DIC method. This novel method provides quantitative measurements of slip localization at the specimen surface. In this paper, the Heaviside-DIC method is used to measure slip irreversibility and plastic strain accumulation in a nickel-based superalloy. The method detects bands with high levels of irreversibility early in cycling that ultimately form fatigue cracks upon further cycling. The local microstructural configurations that induce large amounts of plasticity and slip irreversibility are correlated to crack nucleation locations.

Initial plasticity stages in Mg alloys containing Long-Period Stacking Ordered phases using High Resolution Digital Image Correlation (HRDIC) and in-situ synchrotron radiation

The development of strain incompatibilities between phases in an extruded MgY2Zn1(at.%) alloy during the initial stages of plastic deformation was studied using Synchrotron X-ray Diffraction experiment during in-situ tensile test and High-resolution Digital Image Correlation (HRDIC). The alloy microstructure is characterised by long LPSO fibers, crystalographically aligned non-DRXed grains with the basal plane parallel to the extrusion direction and randomly oriented fine DRXed grains. Localized transgranular shear bands form in DRXed grain areas first, just after yield. The total deformation is also accommodated by non-basal slip in the reinforcing microstructural components: non-DRXed and LPSO phases. The local deformation transmitted in these two phases is the main responsible of the appreciate ductility observed in Mg–Y–Zn alloys containing high volume fraction of LPSO phases.

Digital image correlation for microstructural analysis of deformation pattern in additively manufactured 316L thin walls

In additive manufacturing, the process parameters have a direct impact on the microstructure of the material and consequently on the mechanical properties of the manufactured parts. The purpose of this paper is to explore this relation by characterizing the local microstructural response via in situ tensile test under a scanning electron microscope (SEM) combined with high resolution digital image correlation (HR-DIC) and Electron Backscatter Diffraction (EBSD) maps. The specimens under scrutiny were extracted from bidirectionally-printed single-track thickness 316L stainless steel walls built by directed energy deposition. The morphologic and crystallographic textures of the grains were characterized by statistical analysis and associated with the particular heat flow pattern of the process. Grains were sorted according to their size into large columnar grains located within the printed layer and small equiaxed grains located at the interfaces between successive layers. In situ tensile experiments were performed with a loading direction either perpendicular or along the printing direction and exhibit different mechanisms of deformation. A statistical analysis of the average deformation per grain indicates that for a tensile loading along the building direction, small grains deform less than the large ones. In addition, HR-DIC combined with EBSD maps showed strain localization situated at the interface between layers in the absence of small grains either individual or in clusters. For tensile loads along the printing direction, the strain localization was present in several particular large grains. These observations permit to justify the differences in yield and ultimate strength noticed during macroscopic tensile tests for both configurations. Moreover, they indicate that an optimization of the process parameters could trigger the control of microstructure and consequently the macroscopic mechanical behavior.

Strain visualization of growing short fatigue cracks in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint: Intergranular cracking and crack closure

Physically short fatigue crack growth behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint was investigated in-situ in SEM to reveal effects of crack closure and tortuous crack growth path on crack-tip straining behavior with the help of high resolution digital image correlation technique. The crack growth path and associated fatigue damage at microstructural scale was characterized by electron backscatter diffraction and electron channeling contrast imaging techniques. Results showed that the short crack growth was influenced by both local microstructure and global strength gradient. The short crack could deflect intergranularly when meeting with one large grain due to misfit deformation and strain localization. The crack closure, which was accompanied during loading and unloading in terms of crack surface contact, shielded the real crack-tip and manifested itself well in both compressive strain at crack wake and strain development at crack-tip. It is indicated that strain localization at lathy boundaries and formation of sub-grains was responsible for the fatigue of short cracks.

Enhancement of the strength-ductility relationship for carbon nanotube/Al–Cu–Mg nanocomposites by material parameter optimisation

Abstract Heterogeneous carbon nanotube (CNT)/Al–Cu–Mg composites, consisting of CNT-free coarse grain (CG) bands and CNT-rich ultrafine grain (UFG) zones, were fabricated to significantly enhance their strength-ductility. Their mechanical behavior as well as extra-strengthening and elongation increase mechanisms were investigated in detail, with the help of high-resolution digital image correlation (DIC) and extended finite element method (XFEM). A narrow CG band and medium CG content were found to be beneficial in increasing the strength-ductility. Under optimized conditions, a heterogeneous 3 vol% CNT/Al composite exhibited more than 100% elongation increase with nearly no ultimate tensile strength loss as compared to the uniform composite. Geometrically necessary dislocations were induced between the CG and UFG zones, leading to extra-strengthening beyond the rule-of-mixtures. Local strain and micro-crack propagation analyses based on DIC and XFEM indicated that strain localization was greatly suppressed and micro-cracks were effectively blunted because of the existence of CG bands, leading to considerably enhanced elongation. Finally, a model was proposed to assist the selection of the heterogeneous structure parameters for the strength-ductility design of the nanocomposite. The calculated safety zones for CG parameter selection were in well agreement with the experimental results.

In situ characterization of a high work hardening Ti-6Al-4V prepared by electron beam melting

A multi-phase Ti–6Al–4V prepared by electron beam melting and thermal post treatments has been shown to exhibit increased strength and ductility over standard wrought or hot isostatic pressed Ti–6Al–4V. The mechanical improvements are due to a prolonged, continuous work hardening effect not commonly observed in Ti alloys. In situ x-ray diffraction and high resolution digital image correlation are used to examine the strain partitioning between the phases during tensile loading with post-mortem electron microscopy to characterize the deformation behavior in each phase. Specimens heat treated between 850 and 980 °C were tested and the effect of annealing temperature on the micromechanical response is discussed. It is shown that the work hardening is the result of composite load-sharing behavior between three mechanically distinct microstructures: large α lamellae and a martensitic region of fine acicular α' and a third phase not previously reported in this alloy.

Following Microstructures during Deformation: In situ X-ray/Neutron Diffraction and HRDIC

The mechanical behavior of three engineering materials is studied employing in situ deformation methods. The study covers metastable austenitic steels with different stacking fault energies during multiaxial loading, a Ti-6Al-4V alloy processed by electron beam melting during uniaxial deformation and a commercial nanocrystalline NiTi alloy during multiaxial deformation. The experimental results obtained by in situ X-ray or neutron diffraction elucidate the load transfer and phase transformation mechanisms, information that is averaged over a relatively large volume containing a statistically representative number of grains. Complementary in situ high resolution digital image correlation allows details to be revealed regarding the localized strain accommodation and slip activity with a sub-grain spatial resolution. It is demonstrated that the synergy of the different length-scale investigations provides a better understanding of the complex relationship between microstructure and deformation behavior in these materials.

Comparing local deformation measurements to predictions from crystal plasticity during reverse loading of an aerospace alloy

Cyclic loading is of great importance in aerospace applications but the origins of the change in flow stress with load path are poorly understood and difficult to predict. Many crystal plasticity models are capable of modelling macroscopic flow behaviour during non-monotonic load paths, such as the Bauschinger effect often seen when reverse loading. Although these models represent the microstructure explicitly, it remains unclear whether they capture reversibility at the local microstructural scale, or simply fit the macroscopic hardening response better by using more fitting parameters. Here we present experimental surface deformation data acquired using high resolution digital image correlation (HR-DIC) during a reverse load cycle. With resolution greater than 200nm, these experiments reveal the discrete nature of deformation in the form of localised crystallographic slip bands. These results are then compared statistically, over many hundreds of grains, to crystal plasticity simulations of reverse loading. Simulation results are taken from both finite element method (CPFEM) and fast Fourier transform (CPFFT) encompassing the two popular numerical techniques for full-field crystal plasticity models.

New In Situ Imaging-Based Methodology to Identify the Material Constitutive Model Coefficients in Metal Cutting Process

A great challenge of metal cutting modeling is the ability of the material constitutive model to describe the mechanical behavior of the work material under the deformation conditions that characterizes this process. In particular, metal cutting generates a large range of state of stresses, as well as strains and strain rates higher than those generated by conventional mechanical tests, including the Split-Hopkinson pressure bar tests. A new hybrid analytical–experimental methodology to identify the material constitutive model coefficients is proposed. This methodology is based on an in situ high-resolution imaging and digital image correlation (DIC) technique, coupled with an analytical model of orthogonal cutting. This methodology is particularly suitable for the identification of the constitutive model coefficients at strains and strain rates higher than those found in mechanical tests. Orthogonal cutting tests of nickel aluminum bronze alloy are performed to obtain the strains and strain rates fields in the cutting zone, using DIC technique. Shear forces derived from stress integrations are matched to the measured ones. Then, the constitutive model coefficients can be determined, which is performed by solving a sequential optimization problem. Verifications are made by comparing the strain, strain rate, and temperature fields of cutting zone from experiments against those obtained by finite element simulations using the identified material constitutive model coefficients as input.

Identification of active slip mode in a hexagonal material by correlative scanning electron microscopy

Metals with a hexagonal close packed structure can deform by several different slip modes with different Critical Resolved Shear Stresses, which provides a great deal of complexity when considering mechanical performance of Mg, Ti and Zr alloys. Hence, an accurate but also statistically meaningful analysis of active slip systems and their contribution to plasticity is of great importance for the understanding of deformation mechanism. In the present study, a correlative scanning electron microscopy-based method of slip trace analysis has been utilised to provide statistical, accurate information of slip behaviour in a weakly textured Ti–6Al–4V alloy with a plastic strain of ∼2%. This is achieved through grain orientation mapping by Electron Backscatter Diffraction and strain mapping by High Resolution Digital Image Correlation. The initial identification of slip mode was performed by comparing the slip trace captured in the high-resolution effective shear strain map with all theoretical slip planes with an angle acceptance criterion of ±5°. Ambiguity in slip mode identification was further resolved using the Relative Displacement Ratio method, which enables the determination of the Burgers vector directly from the displacement data. The correctness of the identified slip modes has been confirmed by detailed dislocation analysis using Bright Field Scanning Transmission Electron Microscopy on thin foils extracted from specific grains employing Focused Ion Beam. This detailed investigation demonstrates the robustness of the slip trace analysis based on grain orientation and high-resolution strain mapping.

On the crack initiation and heterogeneous deformation of Ti-6Al-4V during high cycle fatigue at high R ratios

In engineering practice, mean stress corrections are employed to assess the fatigue performance of a material or structure; albeit this is problematic for Ti-6Al-4V, which experiences anomalous behavior at high R ratios. To address this problem, high cycle fatigue analyses were performed on two Ti-6Al-4V specimens with equiaxed alpha microstructures at a high R ratio. In one specimen, two micro-textured regions (MTRs) having their c-axes near parallel and perpendicular to the loading direction were identified. High-resolution digital image correlation (HR-DIC) was performed in the MTRs to study grain-level strain localization. In the other specimen, DIC was performed on a larger area and crack initiation was observed in a random-textured region. To accompany the experiments, crystal plasticity finite element simulations were performed to investigate the mechanistic aspects of crack initiation, and the relative activity of different families of slip systems as a function of R ratio. A critical soft-hard-soft grain combination was associated with crack initiation indicating possible dwell effect at high R ratios, which could be attributed to the high-applied mean stress and high creep sensitivity of Ti-6Al-4V at room temperature. Further, simulations indicated more heterogeneous deformation, specifically the activation of multiple families of slip systems with fewer grains being plasticized, at higher R ratios. Such behavior is exacerbated within MTRs, especially the MTR composed of grains with their c-axes near parallel to the loading direction. These features of micro-plasticity make the high R ratio regime more vulnerable to fatigue damage accumulation and justify the anomalous mean stress behavior experienced by Ti-6Al-4V at high R ratios.

Material response, localization, and failure of an aluminum alloy under combined shear and tension: Part I experiments

This two-part series of papers presents complementary experiments and analyses of ductile failure of an aluminum alloy in the challenging regime of moderate to low stress triaxiality. Part I uses a custom tension-torsion experimental setup to establish the response and the onset of failure under proportional and non-proportional stress histories that form the basis for the numerical simulations in Part II. The test section of the adopted tubular specimen allows large localized strains to develop unimpeded, which were captured using high-resolution digital image correlation (DIC). The measured failure strains exhibit a monotonic decrease as triaxiality increases, similar to the results of Scales et al. (2016). A difference is that the present failure strains are calculated by integrating the work-compatible equivalent plastic strain increments over the loading history, which necessitates the use of a constitutive model. It is shown that for triaxialities greater than 0.2 the integrated failure strains are very close to the values measured directly from the last DIC image. Furthermore, the choice of constitutive model does not significantly affect the measured strains. For the four corner-path experiments, the state of stress and equivalent strain at failure were found to be close to the values of the corresponding radial paths.Microscopic evaluations indicated that the material exhibited limited void growth until just prior to rupture. Part II of this series investigates the extent to which properly-calibrated plasticity models can reproduce the high strains in the localized zones.

Aggregate-driven reconfigurations of carbon nanotubes in thin networks under strain: in-situ characterization

This work focuses on the in-situ characterization of multi-walled carbon nanotube (CNT) motions in thin random networks under strain. Many fine-grain models have been devised to account for CNT motions in carbon nanotube networks (CNN). However, the validation of these models relies on mesoscopic or macroscopic data with very little experimental validation of the physical mechanisms actually arising at the CNT scale. In the present paper, we use in-situ scanning electron microscopy imaging and high-resolution digital image correlation to uncover prominent mechanisms of CNT motions in CNNs under strain. Results show that thin and sparse CNNs feature stronger strain heterogeneities than thicker and denser ones. It is attributed to the complex motions of individual CNTs connected to aggregates within thin and sparse CNNs. While the aggregates exhibit a collective homogeneous deformation, individual CNTs connecting them are observed to fold, unwind or buckle, seemingly to accommodate the motion of these aggregates. In addition, looser aggregates feature internal reconfigurations via cell closing, similar to foam materials. Overall, this suggests that models describing thin and sparse CNN deformation should integrate multiphase behaviour (with various densities of aggregates in addition to individual CNTs), heterogeneity across surface, as well as imperfect substrate adhesion.

In-plane and out-of-plane deformation at the sub-grain scale in polycrystalline materials assessed by confocal microscopy

High-resolution digital image correlation (HR-DIC) techniques have become essential in material mechanics to assess strain measurements at the scale of the elementary mechanisms responsible of the deformation in polycrystalline materials. The purpose of this study is to demonstrate the use of laser scanning confocal microscopy (LSCM) coupled with DIC techniques to deepen knowledge on the deformation process of a polycrystalline nickel-based superalloy at room temperature. The LSCM technique is capable of detecting both in-plane and out-of-plane strain localization within slip bands at the sub-grain level. The LSCM observations are consistent with previous in-situ scanning electron microscopy (SEM) studies: The onset of crystal plasticity occurs primarily near Σ3 twin boundaries with macroscopic loading in the elastic domain (macroscopic stress as low as 80% of the 0.2% offset yield strength (Y.S.0.2%)). This intense irreversible strain localization occurs with either a high Schmid factor (μ > 0.43) or a significant elastic modulus difference between the pair of twins (ΔΕ > 100 GPa). In the plastic deformation domain, transgranular slip activity following slip systems with the highest Schmid factor is mostly responsible for the deformation at the grain level, thus leading to strain percolation. The simultaneous in-plane and out-of-plane deformation assessment via the HR-LSCM-DIC technique was found to be essential for the identification of active slip systems. Finally, the HR-LSCM-DIC technique enabled the quantification of the glide amplitude involved in the three-dimensional shearing process at the grain level that solely in-plane measurements cannot provide.

Characterisation of irradiation enhanced strain localisation in a zirconium alloy

High resolution digital image correlation (HRDIC) has been used to quantify the effect of proton irradiation on strain localisation in Zircaloy-4. Confinement of slip to dislocation channels in irradiated material lead to intense, planar slip bands with high effective shear strain values within channels. More diffuse, homogenous slip was observed in non-irradiated material, with the highest strains measured near grain boundaries. By comparing experimental slip trace angles from HRDIC with theoretical slip trace angles determined using grain orientations from electron backscatter diffraction (EBSD), the active slip plane was determined. Understanding the localised deformation of irradiated materials relative to their microstructure is essential for valid predictions of the structural integrity, and therefore design life, of components in nuclear applications.

Digital image correlation strain measurement of thick adherend shear test specimen joined with an epoxy film adhesive

Structural bonding and bonded repairs of composite materials become more and more important. Understanding the strain within the bondline leads to suitable bonding design. For new design approaches the strain distribution within the bondline has to be analyzed. Thus, often finite element analysis (FE) are used. However, a huge challenge is the availability of reliable material properties for the adhesives and their validation. Previous work has shown that it is possible to measure the small displacements resulting within thin epoxy film adhesives using high resolution digital image correlation (DIC). In this work a 2D DIC setup with a high resolution consumer camera is used to visualize the strain distribution within the bondline over the length of the joint as well as over the adhesive thickness. Therefore, single lap joints with thick aluminum adherends according to ASTM D 5656 are manufactured and tested. Local 2D DIC strain measurements are performed and analyzed. Two different camera setups are used and compared. The evaluation provides reliable material data and enables a look insight the bondline. The results of the full field strain data measured with DIC are compared with numerical simulations. Thus, material models as well as chosen parameters for the adhesive are validated. Compared to extensometers, giving only point-wise information for fixed measuring points, the DIC allows a virtual point-wise inspection along the complete bondline. Furthermore, it allows measuring close to the bondline to reduce the influence of adherend deformation.

Direct Measurements of Slip Irreversibility in a Nickel Base Superalloy Using High Resolution Digital Image Correlation

Fatigue crack nucleation in crystalline materials typically develops due to highly localized cyclic slip. During a fatigue cycle, reverse slip differs locally from slip in the forward direction particularly in precipitate-containing materials such as superalloys. In this paper we report the first direct measurements of irreversibility at the scale of individual slip planes in a polycrystalline nickel base superalloy. Quantitative measurements of the slip irreversibility is challenging to be performed for regions of material that have a size that captures the microstructure and its variability. High spatial resolution at the nanometer scale during experimental measurements is needed to observe slip localization during deformation. Moreover, large fields are also needed to obtain the material response over statistically representative populations of microstructural configurations. Recently, high resolution scanning electron microscope (SEM) digital image correlation (DIC) has been extended for quantitative analysis of discontinuities induced by slip events using the Heaviside-DIC method. This novel method provides quantitative measurements of slip localization at the specimen surface. In this paper, the Heaviside-DIC method is used to measure slip irreversibility and plastic accumulation in a nickel base superalloy. The local microstructural configurations that induces large amounts of plasticity and slip irreversibility are captured and correlated to crack nucleation locations.

Strain Accommodation in a Superelastic NiTi Alloy: A High Resolution Digital Image Correlation and Transmission Electron Microscopy Study

Strain Accommodation in a Superelastic NiTi Alloy: A High Resolution Digital Image Correlation and Transmission Electron Microscopy Study

Surface toughening – a concept to decrease stress peaks in bonded joints

ABSTRACTBy using fiber-reinforced plastics as the material for lightweight structures, adhesive bonding becomes an increasingly focused solution for joining these components. In comparison to bolted joints, adhesive bonding has a lot of known benefits, e.g. sealing, no weakening of adherends and a homogeneous load transfer. However, the stress distributions in single-lap joints are not homogeneous all over. Both the shear and peel stress distribution have a peak at the overlap ends of the joint and a lower loaded part in the middle, as shown by Volkersen and Goland and Reissner. In order to reduce these stress concentrations, there are known concepts for the modification of the joint. In this paper, adherend chamfering, different adhesive spew fillet geometries, and the mixed adhesive joint are investigated and compared to a novel local adherend surface toughening concept by using a thermoplastic PVDF layer. A high-resolution digital image correlation is used to visualize the deformation in the bondline. While the state-of-the-art used concepts do not show a high increase of joint strength for single lap joints, the joint strength for the surface toughening specimens can be increased by up to 84%.