Kink Bands Research Articles

Geometric structures of micropolar continuum with elastic and plastic deformations based on generalized Finsler space

Elasto-plastic deformations of micropolar continuum are discussed by a non-Riemannian geometry. The non-locality of micropolar continuum is described in a second-order vector bundle of displacements and microrotations. With a decomposition of total elasto-plastic field, geometric quantities are divided into the elastic and plastic components independently. Especially, when an intrinsic parallelism of displacements and microrotations holds, integrability conditions of the elasto-plastic field are represented by a torsion tensor or the curvature of nonlinear connection. Then, Burgers and Frank vectors and an energy release rate around crack tips are related to the torsion tensor or the curvature of nonlinear connection. Moreover, the non-locality of microrotation is discussed based on a kink band as a disclination. It is found a generalized expression of Burgers vector which can describe the kink interface including the disclination.

Damage mechanism characterisation of plain weave ceramic matrix composites under in-plane shear using in-situ X-ray micro-CT and deep-learning-based image segmentation

This paper investigates the microstructure and damage evolution of plain weave ceramic matrix composites under in-plane shear using the in-situ X-ray computed tomography approach. The actual microstructure and damage evolution of material under several successive loading levels have been revealed precisely. A deep-learning-based image segmentation method has been employed to accurately identify material damages, including matrix cracks, delamination, fibre pull-outs and interface debonding. Besides, three-dimensional visual characterisation and quantitative analysis of damage have been carried out to gain a better understanding on the failure mechanisms. For longitudinal tows under shear, the cracks propagate along the loading direction and cause the splitting and sliding failure of tow; For transverse tows under shear, they undergo coupling effects of shear and tension, which results in kink band damage with fibre bridging mechanism. Finally, the connection of longitudinal tow splitting and transverse tow kinking leads to the catastrophic fracture of CMCs under in-plane shear.

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.

Deformation behavior of LPSO phases with regulated morphology and distribution and their role on dynamic recrystallization in hot-rolled Mg–Gd–Y–Zn–Zr alloy

Three relatively novel initial microstructures of Mg–Gd–Y–Zn–Zr alloy were regulated by heat treatment, the deformation behavior of long period stacking ordered (LPSO) phases with different morphology and distribution and their role on recrystallization during thermal deformation were systematically investigated. The interdendritic block-shaped LPSO phases exhibit higher deformation resistance, and shearing, bending, kinking, tearing, dissolution, together with the breaking of plate-shaped lamellae produced by dissolution, are the main deformation mechanisms for them to coordinate deformation. As for the lamellar LPSO phases, deformation behaviors such as shearing, bending, kinking and breaking mainly occur. The interdendritic block-shaped LPSO promote recrystallization through the particle-stimulated nucleation (PSN) mechanism, and the PSN effect is enhanced with the increasing phase fraction, also, the broken block-shaped LPSO have better PSN effect. Recrystallized grains tend to nucleate at the kink of lamellar LPSO and form recrystallized grains bands along the kink bands; the fragmented fine lamellae have better PSN effect, promoting numerous recrystallization nucleation in the severely deformation zone, meanwhile, the joint pinning effect of the fragmented lamellae and precipitates β-phase results in the smaller recrystallized grains size. However, the intragranular continuous large-sized lamellar LPSO strongly pin the dislocation movement and hinder migration of recrystallized grain boundaries along the basal plane and c-axis of lamellar LPSO, making it difficult to achieve extensive lattice rotation, which is not conducive to the recrystallization nucleation. The most remarkable work in this paper is that we clarified the mechanism of different LPSO phases, especially lamellar LPSO phase, on recrystallization behavior, and these findings will provide microstructure regulation guidance for thermal-deformed Mg–Gd–Y–Zn–Zr alloys containing LPSO.

Microstructure and mechanical properties of the Mg98.5Zn0.5RE1 (RE=Gd and/or Y) alloys processed by equal channel angular pressing

In this work, Mg98.5Zn0.5RE1 (RE = Gd and/or Y) and Mg97Zn1Y2 (at%) alloys were subjected to homogenization and equal channel angular pressing (ECAP) to systematically compare the microstructure evolution and mechanical properties. The results indicate that, instead of the lamellar LPSO phases in the as-cast Mg-Zn-Y series alloys, the β-Mg5(Gd, Zn) phases appear in the as-cast Mg98.5Zn0.5Gd1 alloy which tend to be dissolved during homogenization. The furnace cooling after homogenization promotes the precipitation of numerous lamellar LPSO phases in all the studied alloys which inhibit the DRX behavior but facilitate the operation of kinking deformation during ECAP for the Mg98.5Zn0.5RE1 alloys, especially the Mg98.5Zn0.5Gd1 one. After the 5p-ECAP process at 350 °C, the larger density of kink bands, more LAGBs (low-angle grain boundaries) and lower SFB (Schmid factor for basal slip) combine to endow the Mg98.5Zn0.5Gd1 alloy with the higher strength-ductility synergy, exhibiting the tensile yield strength (TYS) of 214 MPa and elongation of 9.6%. The Mg97Zn1Y2 alloy with the larger volume fraction of interdendritic LPSO phases and dynamically recrystallized (DRXed) regions exhibits the superior ultimate tensile strength (UTS) of 277 MPa and moderate elongation of 7.2%.

Kink band orientation of 3D printed continuous carbon fiber composites under compressive loading

For continuous carbon fiber-reinforced plastic (cCFRP), in the case of loading in the longitudinal direction, occurrence of kink band failure causes compression strength to be lower than the tensile strength. Furthermore, 3D-printed cCFRP may exhibit intrinsic compressive behavior depending on the printing orientation. In this study, two types of rectangular specimens were fabricated using the Markforged Mark Two > 3D printer. The first was the in-plane specimen, whose width direction was parallel to the print bed and thickness direction was along the layup direction. The second was the out-of-plane specimen, whose width direction was the layup direction and thickness direction was parallel to the print bed. Compression tests were performed, and the experimental results were compared. As a result, the compressive strengths of both types were similar, and the kink bands always grew in the lay-up direction. For the out-of-plane specimens, nonlinear stress–strain curves were observed. This may be due to the delamination crack growth caused by the low strength in the lay-up direction and the existence of initial defects. This study experimentally investigated the effect of two different printing orientations of 3D printed cCFRP to its strength and failure mode, and this work shows the importance of considering structural deformation due to compression loading in 3D printed cCFRP.

Effect of long-period stacking ordered phase on dynamic recrystallization in Mg-Y-Zn-Zr alloy processed by backward extrusion

During the backward extrusion process, the impact of the long-period stacking ordered (LPSO) phase on dynamic recrystallization (DRX) in the Mg-10Y-2Zn-0.5Zr alloy was studied. The results show that DRX at the α-Mg/18R-LPSO interface belongs to the discontinuous dynamic recrystallization (DDRX) mechanism, and the DRX at the α-Mg/14H-LPSO interface belongs to the continuous dynamic recrystallization (CDRX) mechanism. Under the action of strain force, the 18R-LPSO interface migrates locally to form a “convex” structure. It was easy to produce high-density dislocations around the structure, which promoted DDRX at the α-Mg/18R-LPSO interface. The lamellar 14H-LPSO would undergo severe kink and form a large number of kink bands, providing favorable nucleation sites for CDRX grains. With the increase of the extrusion strain force, the lamellar 14H-LPSO kink angle gradually increases, which promotes the nucleation of CDRX grains at the kink band boundary. In addition, the production of 14H-LPSO kink activates prismatic <a>slip, pyramidal <a>slip and pyramidal <c+a>slip in the α-Mg matrix.

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.

Quasi in-situ investigation on kinking behavior of Fe-Cr-Al alloy during thermal compression

As a rare plastic deformation mechanism, kinking has attracted substantial attention because of its additional softening effect on the matrix and its heterogeneous refinement on the original grains. However, the details of kinking deformation and the influence of crystal orientation on itself have not been sufficiently elucidated yet, thereby resulting in inadequate microstructure optimization associated with kinking. In this work, the kinking formation process in Fe-Cr-Al alloy during thermal compression deformation has been investigated by using quasi in-situ electron backscatter diffraction (EBSD). By employing the average Schmid factor (Average-SF) model, the influence of crystal orientation on kinking behavior is predicted accurately. The results suggest that the softening effect of kinking on the matrix can be attributed to the fact that the nucleation of pre kink bands (pre-KBs, misorientation <15°) leads to a pronounced reduction in geometrically necessary dislocations (GNDs) density within them. Besides, the ripening behavior of pre-KBs is closely associated with the variation of GNDs density and the overall slip activation capability of the matrix, and is essentially dependent on crystal orientation. The harder the crystal orientation, the smaller the Average-SF, the higher the increment rate of GNDs density, the larger the ripening rate pre-KBs. Under equivalent strain, orientations close to 〈111〉// compression direction (CD) have the highest probability of forming mature KBs with high angle grain boundaries (HAGBs, >15°), whereas KBs in those close to 〈001〉//CD almost always remain as pre-KBs with low angle grain boundaries (LAGBs).

High-strain-rate deformation: Stress-induced phase transformation and nanostructures in a titanium alloy

Microstructural evolution in adiabatic shear bands of a Ti-10V-2Fe-3Al (wt.%) alloy subjected to high strain-rate dynamic loading was investigated in detail via high-resolution transmission electron microscopy (HRTEM), focusing on the underlying plastic deformation mechanisms. The alloy after solution treatment consisted of a coarse body-centered cubic β single-phase microstructure, which exhibited double-yield and stress-collapse characteristics. The former was a result of stress-induced martensite transformation accompanied by the formation of nanotwins in the martensite, and the latter was caused by thermal softening stemming from dynamic recovery. Substantial grain refinement with three orders of magnitude grain size reduction occurred in the center of adiabatic shear bands due to severe plastic deformation along with dynamic recovery. Multiple deformation mechanisms in the adiabatic shear bands were identified, including the formation of face-centered orthorhombic α″ martensite, {111}α″ type I nanotwins, and hexagonal close-packed α phase, where distinctive nano-twinned structures and phase transformation from α″ to α with an orientation relationship of {1¯11}α″//{1¯101}α were observed. The transition area of the adiabatic shear bands contained mainly high-density dislocation regions and plate-like structures with relatively fewer nanoscale grains, where the transition from nanokinks to nanotwins along with phase transformation was also revealed, corroborating that kink bands served as nanotwin nucleation sites. The mechanisms of the microstructural evolution in relation to the mechanical response during the high strain-rate dynamic loading were discussed.

Kwinking as the plastic forming mechanism of B19[formula omitted] NiTi martensite

Irreversible plastic forming of B19′ martensite of the NiTi shape memory alloy is discussed within the framework of continuum mechanics. It is suggested that the main mechanism arises from coupling between martensite reorientation and coordinated [100](001)M dislocation slip. A heuristic model is proposed, showing that the (201̄)M deformation-twin bands, commonly observed in experiments, can be interpreted as a combination of dislocation-mediated kink bands, appearing due to strong plastic anisotropy, and reversible twinning of martensite. We introduce a term ’kwinking’ for this combination of reversible twinning and irreversible plastic kinking The model is subsequently formulated using the tools of nonlinear elasticity theory of martensite and crystal plasticity, introducing ’kwink interfaces’ as planar, kinematically compatible interfaces between two differently plastically slipped variants of martensite. It is shown that the (201̄)M kwink bands may be understood as resulting from energy minimization, and that their nucleation and growth and their pairing with (100)M twins into specific patterns enables low-energy plastic forming of NiTi martensite. We conclude that kwinking makes plastic deformation of B19′ martensite in polycrystalline NiTi possible despite only one slip system being available.

The elasto-plastic nano- and microscale compressive behaviour of rehydrated mineralised collagen fibres.

The hierarchical design of bio-based nanostructured materials such as bone enables them to combine unique structure-mechanical properties. As one of its main components, water plays an important role in bone's material multiscale mechanical interplay. However, its influence has not been quantified at the length-scale of a mineralised collagen fibre. Here, we couple in situ micropillar compression, and simultaneous synchrotron small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) with a statistical constitutive model. Since the synchrotron data contain statistical information on the nanostructure, we establish a direct connection between experiment and model to identify the rehydrated elasto-plastic micro- and nanomechanical fibre behaviour. Rehydration led to a decrease of 65%-75% in fibre yield stress and compressive strength, and 70% in stiffness with a 3x higher effect on stresses than strains. While in agreement with bone extracellular matrix, the decrease is 1.5-3x higher compared to micro-indentation and macro-compression. Hydration influences mineral more than fibril strain with the highest difference to the macroscale when comparing mineral and tissue levels. The effect of hydration seems to be strongly mediated by ultrastructural interfaces while results provide insights towards mechanical consequences of reported water-mediated structuring of bone apatite. The missing reinforcing capacity of surrounding tissue for an excised fibril array is more pronounced in wet than dry conditions, mainly related to fibril swelling. Differences leading to higher compressive strength between mineralised tissues seem not to depend on rehydration while the lack of kink bands supports the role of water as an elastic embedding influencing energy-absorption mechanisms. STATEMENT OF SIGNIFICANCE: Characterising structure-property-function relationships in hierarchical biological materials helps us to elucidate mechanisms that enable their unique properties. Experimental and computational methods can advance our understanding of their complex behaviour with the potential to inform bio-inspired material development. In this study, we close a gap for bone's fundamental mechanical building block at micro- and nanometre length scales. We establish a direct connection between experiments and simulations by coupling in situ synchrotron tests with a statistical model and quantify the behaviour of rehydrated single mineralised collagen fibres. Results suggest a high influence of hydration on structural interfaces, and the role of water as an elastic embedding by outlining important differences between wet and dry elasto-plastic properties of mineral nanocrystals, fibrils and fibres.

Quantification of damage evolution in cross-ply polymer composites under longitudinal compression by fast computed tomography and semi-automated segmentation

Compressive failure in a notched cross-ply IM7/8552 carbon fiber reinforced polymer (CFRP) laminate is studied experimentally by time-lapse in-situ synchrotron radiation computed tomography (CT). The focus is on tracking and quantifying the fiber damage evolution in the 0° plies under compression for the first time via a specimen design that exhibits quasi-stable damage accumulation. A novel segmentation algorithm has been developed and applied to extract morphological features of interest including the kink band width, inclination angle, and the extent of propagation from the notch. In the four innermost 0∘ plies, shear-driven fiber fracture initiates from the notch tip and propagates as a single fracture plane inclined about 45° to the compression axis. This shear-driven fiber fracture transitions to a kink band at a distance of about 300μm from the notch, which propagates at a shallower angle. During the later stages of the test, the kink bands propagate in the reverse direction to areas that previously initiated by shear-driven fiber fracture, which may explain why post-mortem observations seldom reveal the fiber shear fracture mechanism. The fiber damage initiation and evolution appears distinctly different from the classical parallel plane kink bands formed by plastic microbuckling under uniaxial compression in that the dominant mechanism appears to be shear-driven fiber fracture.

Strong-anisotropy-induced instability and discussion on kink band under complex loads

Compressive instability, such as the occurrence of kink bands, is common in composite materials and other highly anisotropic layered materials, limiting their use. Most previous research has focused on the microstructure and heterogeneity in studying the compressive instability of composites. In this paper, to highlight and quantify the influence of anisotropy, we investigate the deformation instability of effective homogeneous solids with strong anisotropy. Firstly, we re-write the local instability criterion from an energy perspective and verify its applicability. Even when the heterogeneity of the composites is eliminated, the derived criterion can reasonably predict the kink band instability and can be reduced to the well-known Rosen’s estimate. The strong stiffness anisotropy plays a dominant role in kink band instability. For unidirectional fiber-reinforced composites, increasing the modulus of the hard phase contributes little to an increase in the compressive strength because, as the anisotropy increases, the critical strain decreases and becomes approximately inversely proportional to the anisotropy degree. The kink band instability is also investigated under complex loads. We find that lateral tension increases the critical kink band load, while lateral compression or shear decreases the critical kink band load. To estimate the instability, we develop a series of instability phase diagrams that can be used as a guideline for predicting the compressive instability of composite materials. We have found that reducing the anisotropy of the material is the main way to increase the critical load of kink band instability, which is important for material design and engineering applications.

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.

Strengthening of αMg and long-period stacking ordered phases in a Mg-Zn-Y alloy by hot-extrusion with low extrusion ratio

An as-cast sample and two hot-extruded samples with different extrusion ratios (R) of Mg97Zn1Y2 alloy containing the HCP α matrix (αMg) and the long-period stacking ordered phase (LPSO) of about 25-vol%, were used in tensile deformation in situ neutron diffraction experiments, to elucidate the effects of uniquely different microstructural evolutions in αMg and LPSO with varying the R value to the mechanical properties. αMg behaved as the soft phase and LPSO as the hard phase, and hot-extrusion improved the strength of both. At the R value of 5.0, dynamic recrystallization partly occurred in αMg creating a bimodal microstructure, consisting of a deformed αMg component and a recrystallized αMg component. The yield strength of αMg was significantly improved due to the optimal effects of dislocation density accumulation, texture development in the deformed αMg component and grain refinement in the recrystallized αMg component. With increasing the R value to 12.5, grain growth as well as dynamic recrystallization were promoted in αMg, causing a decrease in the yield strength of αMg, but the elongation was improved due to the presence of weakly textured grains. In contrast, the strength of LPSO increased monotonously with increasing the R value due to the developments of kink bands and texture. The density of kink bands saturated at R=5.0, but the texture of LPSO became stronger monotonously with increasing the R value. At R=12.5, the strength improvement of LPSO compensated for the strength decrease of αMg, and maintained the high ultimate tensile strength of the alloy.

Microstructural Changes Caused by the Creep Test in ZK60 Alloy Reinforced by SiCp at Intermediate Temperature after KOBO Extrusion and Aging.

In this study, we investigated the creep properties of ZK60 alloy and a ZK60/SiCp composite at 200 °C and 250 °C in the 10-80 MPa stress range after the KOBO extrusion and precipitation hardening process. The true stress exponent was obtained in the range of 1.6-2.3 for both the unreinforced alloy and the composite. The apparent activation energy of the unreinforced alloy was found to be in the range of 80.91-88.09 kJ/mol, and that of the composite was found to be in the range of 47.15-81.60 kJ/mol, and this indicated the grain boundary sliding (GBS) mechanism. An investigation of crept microstructures using an optical microscope and scanning electron microscope (SEM) showed that at 200 °C, the predominant strengthening mechanisms at low stresses were the formation of twin, double twin, and shear bands, and that by increasing the stress, kink bands were activated. At 250 °C, it was found that a slip band was created in the microstructure, and this effectively delayed GBS. The failure surfaces and adjacent regions were examined using SEM, and it was discovered that the primary cause of failure was cavity nucleation around precipitations and reinforcement particles.

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.

Compressive strength prediction of carbon fiber-reinforced pultruded profiles including realistic volumetric fiber orientations

A compressive strength prediction method considering 3D fiber orientation distributions is developed. The method quantifies volumetric fiber orientations using the structure tensor image analysis method on X-ray computed micro-tomography data. The estimated fiber orientations are subsequently mapped to a finite element mesh. The finite element model is solved using a three-dimensional non-linear constitutive model implemented as a user-subroutine in AbaqusTM. A model capable of describing the elastic–plastic behavior of fibrous composites and simulating strain localization in the form of kink bands. The compressive strength of a carbon-epoxy pultruded profile is predicted based on three volumetric subsets of the profile and compared with experimental obtained compression test measurements. The strength predictions of the three volumetric subsets are in the range of 16%–42% higher than the experimental results. It is argued that the experimental results do not represent the full potential of the material compressive strength due to stress concentrations at the load introduction and that the model predictions can describe a potential for the actual compressive strength of the carbon-epoxy pultruded profile.

Numerical Investigation of Kink Strengthening Mechanism due to Kink Band in Long-Period Stacking Ordered Magnesium Alloy

A numerical investigation of the kink strengthening mechanism in long-period stacking ordered magnesium alloy is presented. A higher-order gradient crystal plasticity model is introduced, and the reproducing kernel particle method is adopted for the numerical procedure. The specimen, including a kink band with several kink angles, which is defined as the angle between the inside and outside of the kink band, is considered. A simple shear analysis is conducted to evaluate the kink strengthening due to the kink band. The numerical results suggest two main origins of kink strengthening, namely, orientation and defect strengthening. The former is caused by the spatial distribution of the slip direction due to kink, and the latter is the strengthening induced by crystal defects around the kink boundary. The amplitude of kink strengthening depends on the kink angle, and an optimal kink angle, which maximizes the kink strengthening, might exist. The kink strengthening exhibits a Hall–Petch-like behavior, i.e., the correlation between the inverse of the square root of the kink band width and flow stress is almost linear.