Kink Band Formation Research Articles

Synchrotron micro-CT in kink-band formation of UD-CFRP laminates with microdefects

This paper presents the first synchrotron micro-Computed Tomography (micro-CT) study on in-situ pure compressive kink-band failure in Uni-Directional Carbon Fibre-Reinforced Polymer composites (UD-CFRPs) with a notch. The study compares the failure behaviour of baseline samples under standard conditions with defect-rich samples containing micro-defects such as voids and fibre misalignment. Quantitative image-based analysis using the structure tensor technique and Digital Volume Correlation (DVC) reveals changes in fibre orientation and localised strain, respectively, at each increment load. A large 400 μm kink-band inclined at 30° with a half-cosine wave fibre orientation is observed in the baseline samples, while the defect-rich samples form narrower conjugate kink bands (ranging from 77 μm 25 μm) inclined at 45°. Development of kink-band formations/failure of both samples is discussed and compared with literature, providing key implications for the design and safe use of this type of composite layup.

Additive Manufacturing of Flexible Strain Sensors Based on Smart Composites for Structural Health Monitoring with High Accuracy and Fidelity

This research introduces a novel flexible spherical carbon nanoparticle‐based polyurethane conductive ink, which is employed to fabricate strain sensors by a lab‐developed direct ink writing/3D printing system. Rheological tests are performed, and sensors are pasted on glass fiber‐reinforced plastic specimens to study strain gauge behaviors under quasistatic loading. The gauge factor in tensile loading is found to be layer width dependent as decreasing the strain gauge's layer width increases the sensitivity of the strain sensor. A maximum gauge factor of 34 is achieved using a layer width of 0.2 mm, 17 times greater than commercially available metal foil strain gauges. The four‐point bend tests are performed under tension/compression to assess the sensor's strain‐sensing and damage‐monitoring ability. Fractographic analysis is coupled with strain monitoring using the developed sensor, which confirms that the failure progresses from intralaminar failure modes such as ply splitting in tension. At the same time, delamination leads to kink band formation under compression and the eventual failure of load‐bearing fibers. The developed sensor exhibits repeatable performance with low hysteresis and integrated nonlinearity errors for up to 1000 cycles. The developed sensors could be effectively employed for online in situ structural health monitoring of aerospace structures under static and dynamic loading.

Unraveling kinking: A plasticity enhancing failure mode in high strength nano metallic laminates

Kinking is an important and plasticity-enhancing deformation/failure mode in numerous mechanically anisotropic materials including high-strength nano metallic laminates (NMLs). However, our current limited understanding of the mechanics of kinking and its dependence on microstructural attributes is insufficient for thoroughly comprehending and eventually being able to control failure behaviors of materials. In this study, we investigate kinking dependencies on microstructural attributes in NMLs via in situ micropillar compression, multiscale microstructure characterization, dislocation dynamic simulations, and crystal plasticity modeling. By examining several NML systems (Cu/Fe, Ag/Fe, Al-4Mg/Fe), we demonstrate that the development of internal stresses during loading activates local layer-parallel glide triggering kinking in NMLs. Furthermore, this work reveals the effect of key microstructural features including layer thickness, layer waviness, interface barrier strength, and work hardening capacity on kink band formation in NMLs. More broadly, our efforts represent a generically applicable approach for probing large-strain deformation behavior of complex materials via synergetic modeling and experimental efforts.

Mechanical and corrosion properties of biomedical (TiZr)90-xNbxTa5Mo5 medium entropy alloys

High- or medium-entropy alloys with single phase body centered cubic (BCC) structure have attracted significant scientific interests recently, due to their great potentials for biomedical applications. However, BCC high entropy alloys generally do not have good tensile ductility at room temperature. Herein, we utilize the valence electron concentration (VEC) theory to design three (TiZr)90-xNbxTa5Mo5 (x = 5,10,15) biomedical medium entropy alloys (BioMEAs). The BioMEAs showed a single phase BCC structure, a low elastic modulus of 79.2 ± 1.6–82.9 ± 1.2 GPa, a good tensile ductility of 24.4%, and a high specific yield strength of 141.2–137.4 10−3 MPa∙m3/kg. The good tensile ductility of the alloys was found attributed to the cross-slip of mixed dislocations and the formation of kink bands, avoiding the excessive entanglement of dislocations and relieving stress concentration. The increase of Nb content (5–15 at.% Nb) contributed little to the yield strength of the BioMEAs. In addition, the (TiZr)90-xNbxTa5Mo5 alloys showed excellent corrosive resistance in modified Fusayama saliva (MFS) solution, in which passive films composed of a variety of oxides spontaneously developed on the exposed surfaces. Our work has cast new light to develop lightweight and economical BioMEAs with excellent comprehensive properties.

The effect of annealing on kink band formation in Ag/Fe nanolaminates

Kink banding is a common, though not well understood, failure mechanism in anisotropic materials such as nano metallic laminates (NMLs). In this work, we investigate the effect of annealing on kink band (KB) formation in Ag/Fe NMLs prepared by accumulative roll bonding (ARB) using in situ micropillar compression, scanning/transmission electron microscopy (S/TEM), and transmission Kikuchi diffraction (TKD) analyses. Our results show that annealing increases the KB initiation strain, decreases the probability of KB formation, and decreases the load drop magnitude accompanying kink banding in Ag/Fe NMLs. Post-compression analyses reveal that annealing facilitates more uniform deformation of pillars and affects the formation of geometrically necessary grain boundaries (GNBs) near the kink band boundary (KBB). Compared to its as-rolled counterpart, annealed Ag/Fe has wider KBs with blunter KBBs.

Interface-Dominated Plasticity and Kink Bands in Metallic Nanolaminates

The theoretical and computational framework of finite deformation mesoscale field dislocation mechanics (MFDM) is used to understand the salient aspects of kink-band formation in Cu-Nb nano-metallic laminates (NMLs). A conceptually minimal, plane-strain idealization of the three-dimensional geometry, including crystalline orientation, of additively manufactured NML is used to model NMLs. Importantly, the natural jump/interface condition of MFDM imposing continuity of (certain components) of plastic strain rates across interfaces allows theory-driven ‘communication’ of plastic flow across the laminate boundaries in our finite element implementation. Kink bands under layer parallel compression of NMLs in accord with experimental observations arise in our numerical simulations. The possible mechanisms for the formation and orientation of kink bands are discussed, within the scope of our idealized framework. We also report results corresponding to various parametric studies that provide preliminary insights and clear questions for future work on understanding the intricate underlying mechanisms for the formation of kink bands.

Synchronized Formation of Kink Bands in Al/Al<sub>2</sub>Cu Mille-Feuille Structured Alloy

In this study, kink band formation behavior of an Al/Al2Cu eutectic alloy with a mille-feuille structure consisting of a hard Al2Cu phase and a soft Al phase was investigated under compression at room temperature. In-situ surface observations during compression tests revealed multiple micro kink-bands occurring simultaneously within a single lamellar colony. To analyze the formation behavior of micro kink-bands, the spatial distribution of the lamellar structure over a wide area of the specimen was quantitatively evaluated by performing image analysis using the Radon transform. The results showed that the micro kink-bands generated synchronously in one direction and rotated at a uniform angle. Such uniform and fine kink-bands are expected to contribute to the improvement of strength. To unravel the linkage between microstructure, kink band formation, and accompanying kink strengthening, an attempt was made to construct a model that predicts the spacing between micro kink-bands and critical stress for kinking from the features of the lamellar structure. The validity of the model to predict the spacing between kink bands was demonstrated by the experimental results.

Compressive failure and kink-band formation modeling

To increase the use of polymeric structural composites, a major issue is to properly account for intra-laminar failure mechanisms, such as fiber kinking induced under compression. We propose a new continuum damage model that can predict the fiber kinking response at the ply level. The model is based on a previous structure tensor-based model for the response of UD-plies. A novel feature is that the compressive UD-ply response at the macroscale includes the effect of the fiber misalignment shaped as a kink-band that is resolved at the sub-scale. Concepts of computational homogenization are used to include the fiber-shear of the kink-band at the sub-scale. The model calibration is adapted to account for either kink-band formation or shear-splitting depending on the off-axis loading. Finally, the model is validated at the laminate level against experimental data for OHC-tests available in the literature. A good agreement is found for predicted strength values and observed fracture patterns of the laminates. The size effect experienced when different hole sizes are tested is also addressed.

Kink band formation mechanism in α/β Ti-9Cr two-phase alloy

A combination of experimental and numerical studies was carried out to understand the microstructure and formation mechanism of kink bands in α/β two-phase titanium alloys. A deformation kink band was introduced in a Ti-9Cr alloy after uniaxial compression test. The strength changes of the α and β phases inside and outside the kink band were evaluated by nanoindentation tests. A potential hardening effect was revealed in the α phase inside the kink band, while a possible softening effect was located in the β phase at the kink boundaries. Microstructural examination of the kink band was conducted by electron backscatter diffraction measurement. The crystal rotation axis was found to be perpendicular to both the (0001) basal plane of the α phase and one of the (110) planes of the β phase. The crystallographic features were characterized by transmission electron microscopy observation, revealing a prismatic 〈a〉 slip that occurred during the compression to accommodate the rotations around the c-axis. Nanoindentation tests were performed on an undeformed specimen to calibrate the crystal plasticity model. A crystal plasticity finite element model was developed and {110}β-rotation-type kink bands were reproduced in the simulation and confirmed the prismatic slip activity.

Non-local large-strain FFT-based formulation and its application to interface-dominated plasticity of nano-metallic laminates

This paper presents a novel formulation and its robust numerical implementation of strain-gradient (SG) crystal plasticity within a large-strain (LS) elasto-viscoplastic (EVP) fast Fourier transform (FFT)-based micromechanical model. The resulting non-local SG-LS-EVPFFT formulation is used to model and understand the process of kink band formation during layer-parallel compression of nano-metallic laminates (NMLs). NMLs are layered composites with nanoscale thicknesses, thus requiring consideration of the interaction between dislocations and interfaces within the micromechanical model. The length-scale parameter of the SG model is calibrated by simulating a double pile-up and comparing predictions to analytical solution. This required new expressions for the defect energy, resulting in more accurate double pile-up predictions. The calibrated SG-LS-EVPFFT model is then used to simulate layer-parallel compression of copper–niobium NML. Formation of kink bands is predicted, and the model is used to rationalize the microscopic mechanisms enabling the formation process. It is found that accumulation of dislocations at interfaces leads to activation of layer-parallel slip, which in turn leads to kink band formation.

Evaluation of the Failure Mechanism in Polyamide Nanofibre Veil Toughened Hybrid Carbon/Glass Fibre Composites.

The interface of hybrid carbon/E-glass fibres composite is interlayered with Xantu.layr® polyamide 6,6 nanofibre veil to localise cracking to promote a gradual failure. The pseudo-ductile response of these novel stacking sequences examined under quasi-static three-point bending show a change to the failure mechanism. The change in failure mechanism due to the interfacial toughening is examined via SEM micrographs. The incorporation of veil toughening led to a change in the dominant failure mechanism, resulting in fibre yielding by localised kinking and reduced instances of buckling failure. In alternated carbon and glass fibre samples with glass fibre undertaking compression, a pseudo-ductile response with veil interlayering was observed. The localisation of the fibre failure, due to the inclusion of the veil, resulted in kink band formations which were found to be predictable in previous micro buckling models. The localisation of failure by the veil interlayer resulted in a pseudo-ductile response increasing the strain before failure by 24% compared with control samples.

Enhanced longitudinal compressive strength of CFRP composites with interlaminar CNT film prepreg from hot-melt pre-impregnation

High strength, high modulus and the balance of compressive and tensile strength gradually became the main development goals of advanced CFRP composites. However, the low ratio of compressive and tensile strength of CFRP composites have limited the application as structural materials in aerospace field. In this study, CNT film prepreg (CNT-f-p) was prepared by hot-melt pre-impregnation and used to fabricate the interlayer stiffened and reinforced CFRP composites with enhancement of compressive strength, and the effect of CNT-f-p on longitudinal compressive strength and elastic-plastic failure mechanism of composites were firstly investigated from the experimental results and theoretical model. The tensile strength and modulus of CNT-f-p were enhanced due to sufficient and uniform infiltration of CNT network by resin matrix. The interlaminar shear strength and in-plane shear modulus of CFRP/CNT-f-p composites were improved due to effective CNTs bridging and riveted CNTs between resin and fiber in interlaminar region of composites. The compressive strength of CFRP/CNT-f-p composites was enhanced by 13.2%, showing a consistency with predicted results of Jumahat's combined model, which ascribed to restricted kink-band formation and avoided interface delamination during elastic-plastic compression process of composites from improved transverse support and interfacial adhesion. Consequently, this work presented a potential application in fabrication of CFRP composites with high compressive strength.

A micromechanics based model for rate dependent compression loaded unidirectional composites

Strain-rate effects in a unidirectional non-crimp fabric carbon/epoxy composite are addressed. To allow for kink-band formation including strain-rate effects and damage in such composites, the paper advances a recent model focused on compression loading at small off-axis angles. The model is based on computational homogenization with a subscale represented by matrix and fibre constituents at finite deformation. The fibre constituent is assumed to be elastic transversely isotropic and the matrix is viscoelastic–viscoplastic with damage degradation. Novel model improvements of special importance to small off-axis loading relate to the isostress formulation of the homogenized response in transverse shear. In this context, an enhanced homogenized elastic response is proposed based on Halpin–Tsai corrections to account for the nonuniform stress distribution on the microscale. The model captures the strongly rate sensitive kink-band formation due to localized matrix shearing and fibre rotation, confirming the experimentally observed increase in compressive strength for high strain rates.

Kink-band formation in the directionally-solidified Mg/LPSO two-phase alloys

ABSTRACT The variation in the mechanical properties with the volume fraction of the long-period stacking ordered (LPSO) phase in directionally solidified (DS) Mg/LPSO two-phase alloys was examined. Unexpectedly, the yield stress of the DS alloys increases non-monotonically with an increase in the volume fraction of the LPSO phase. The LPSO phase is considered an effective strengthening phase in Mg alloys, when the stress is applied parallel to the growth direction. Nevertheless, the highest strength was obtained in alloys with 61–86 vol.% of the LPSO phase, which was considerably higher than that in the LPSO single-phase alloy. It was clarified that this complicated variation in the yield stress was generated from the change in the formation stress of kink bands, which varied with the thickness of the LPSO-phase grains. Furthermore, the coexistence of Mg in the LPSO phase alloy induced the homogeneous formation of kink bands in the alloys, leading to the enhancement of the ‘kink-band strengthening’. The results demonstrated that microstructural control is significantly important in Mg/LPSO two-phase alloys, in which both phases exhibit strong plastic anisotropy, to realize the maximum mechanical properties.

Impact damage assessment of sandwich structures with CFRP face sheets reinforced by CNTs in thermal environment

The damage behaviour of honeycomb sandwich structures with carbon nanotube (CNT) reinforced composite face sheets under high velocity impact at elevated temperatures is investigated in the present study. In order to endure the out-of-plane loading events such as bird hit, runway debris and hail stone impact, CNTs are introduced into the matrix system with the intention of improving the damage resistance. CNT reinforced carbon-epoxy composites with various weight percentages such as 0.2, 0.4, 0.6 and 0.8 were manufactured as quasi-isotropic laminates. Sandwich structures were prepared and tested under high velocity impact at room temperature and elevated temperature (393 K) using a single stage gas gun. Macro structural observations were performed on the perforated specimens and the damage area, depth, central deflection and bulging height were measured. At room temperature, the impact damage pattern was characterized by matrix cracking, fiber breakage, fiber debris, fiber push away, fiber peel up and push delamination. Plastic deformations of the matrix and kink band formation are the significant damage modes in the thermal environment. Adding a higher proportion of nanoscale reinforcement is not always judicious to get improved impact resistance; rather, the temperature dependency must be considered when designing the CFRP composite for structural applications.

Remarkably high fracture toughness of HfNbTaTiZr refractory high-entropy alloy

• The fracture behavior of the Hfnbtatizr refractory HEA was investigated. • The HEA exhibits a remarkably high fracture toughness of K JIC = 211 MPa•m 1/2 . • Fully-ductile fracture mode with the formation of kink bands was observed. • The excellent damage-tolerance plus high strengths in a wide temperature range endow this HEA a promising structural material. High-entropy alloys (HEAs) with face-centered cubic (FCC) phase such as CoCrFeMnNi or CoCrNi generally exhibit ultra-high fracture toughness but relatively low strength. In contrast, body-centered cubic (BCC) HEAs often display higher strengths, but the few reports on fracture toughness measurement demonstrate low toughness values. Here we show that the BCC HfNbTaTiZr refractory HEA, which possesses a combination of high strength, good tensile ductility and excellent high-temperature properties, also exhibits a remarkably high fracture toughness. By using the "single specimen" compliance method for J -integral measurement according to the ASTM E1820–17 standard, its fracture toughness K JIC was experimentally determined to be 210 MPa m 1/2 , which renders this HEA among the toughest metallic materials. The excellent damage tolerance makes the HEA promising for applications as high-temperature structural materials such as in aerospace field.

Simulation of crystal plasticity in irradiated metals: A case study on Zircaloy-4

A classical crystal plasticity formulation based on dislocation slip was extended to include the mechanisms of dislocation channelling, with associated strain softening which is observed in many alloys post irradiation. The performance of the model was evaluated against experimental data on Zircaloy-4, which included engineering stress-strain response and high-resolution digital image correlation strain mapping. Variants of the model were developed to evaluate the influence on the strain hardening law used, comparing hardening based on a linear relationship for effective plastic strain with that based on the evolution of geometrically necessary dislocations. In addition, governing equations for simulating the interaction between gliding dislocations with various types of irradiation defect were investigated; this included the comparison of isotropic and anisotropic interactions based on the resultant reaction segments for each interaction. It was shown that the engineering stress-strain response measured by experiment could be captured by many of the model variants, but the simulation of characteristic strain heterogeneity was more sensitive to the model used. Direct modelling of the HRDIC experiments indicated that the model successfully predicted the activation of slip systems in many cases and exhibited localised strain distribution as observed in the experiment. In all models localised kink band formation was predicted, which is not observed experimentally, which highlights limitations in modelling of softening materials with a local crystal plasticity approach and a required area of development going forward.

Microstructural factors governing the significant strengthening of Al/Al2Cu mille-feuille structured alloys accompanied by kink-band formation

Development of high-strength Al alloys that can be used at temperatures above 150 °C is strongly desired. To achieve this, Al/Al2Cu mille-feuille structured alloys, in which soft (Al) layer and hard (Al2Cu) layer are alternatively stacked in lamellar form, is recently focused, via inducing kink-band formation as a novel strategy for controlling the mechanical properties. However, the localized formation of large kink bands reduces the yield stress and degrades the ductility of the alloy. There was little information to derive a "general law" for quantitatively predicting an increase in yield stress in mille-feuille structured alloys from a microstructure perspective. In this study, we successfully distinguished the effects of lamellar thickness and colony size on the strength. The results demonstrated that the yield stress governed by kink-band formation is less related to the lamellar thickness in the Al/Al2Cu alloys but it highly depends on the colony size. The decrease in lamellar colony size obtained by increasing the growth rate during the directional solidification of the Al/Al2Cu eutectic alloy can induce the homogeneous formation of tiny kink bands in the alloy, leading to a drastic increment of the yield stress accompanied by ductility. In addition, the formation of tiny kink bands was demonstrated to contribute to tensile deformation. That is, the control of the morphology of kink band changes its role from a fracture mode to a deformation mode. Furthermore, such microstructure control can induce “kink-band strengthening” in the alloys. These experimental results demonstrate that controlling the colony size while maintaining the alignment of lamellae is the key factor for inducing superior mechanical properties of the mille-feuille structured alloy via tiny kink-band formation.

Deformation kinking and highly localized nanocrystallization in metastable β-Ti alloys using cold forging

• Microstructural refinement involving deformation kinking was investigated. • Formation of kink band involves dislocation gliding, pre-kinking and its ripening. • Nanograins ∼15 nm in diameter are produced but highly localized in kink bands. • Localized nanocrystallization results from the intrinsic soft nature of kink bands. • The softening of kink bands originates from the inner degraded dislocation density. Deformation kinking as an uncommon plastic deformation mechanism has been reported in several materials while the relevant microstructure evolution and grain refinement behavior at a large strain remain unclear so far. In this study, the issue was systematically investigated by utilizing cold forging to impose severe plastic deformation (SPD) on Ti-11 V metastable β -Ti alloys. It is found that the formation of kink bands experiences dislocation gliding, pre-kinking and the ripening of pre-kinks in sequences. The kink bands are subsequently thickened through the coalescence of multiple kink bands in a manner of high accommodation. Ordinary dislocation slip is developed as a dominant deformation mechanism when deformation kinking is exhausted. The resulting grain refinement involves transverse breakdown and longitudinal splitting of dislocation walls and cells, which fragment kink bands into small β -blocks. Further refinement of the β -blocks is still governed by dislocation activities, and finally nanograins with a diameter of ∼15 nm are produced at a large strain of 1.2. Alternatively, it is revealed that nanocrystallization is highly localized inside kink bands while the outer microstructure maintains original coarse structures. Such localized refinement characterization is ascribed to the intrinsic soft nature of kink bands, shown as low hardness in nanoindentation testing. The intrinsic softening of kink bands is uncovered to originate from the inner degraded dislocation density evidenced by both experimental measurement and theoretical calculation. These findings enrich fundamental understanding of deformation kinking, and shed some light on exploring the deformation accommodation mechanisms for metal materials at large strains.

Crystal Plasticity Analysis for Kink Band Formation and Kink Strengthening

Crystal Plasticity Analysis for Kink Band Formation and Kink Strengthening