Adhesive bonding structures are widely used in a variety of engineering fields. Their overall strength is dependent on the cohesive properties involving local interface fracture. In the present ...research, the influence of the adhesive thickness on the cohesive properties and the overall strength of metallic adhesive bonding structures are investigated, with the cohesive zone model employed to equivalently simulate the adhesive layers with various thicknesses. A theoretical approach has been developed to determine the cohesive parameters for the present model when the adhesive thickness is varied. And then some numerical examples are given to explore the adhesive thickness-dependence overall strength of the adhesive joints, followed by some comparisons with the existing experimental results. Furthermore, the variations of both the cohesive parameters and the overall strength with the various thicknesses are influenced by some intrinsic characteristics of adhesives, which are investigated finally. The results show that both the cohesive parameters and the overall strength of metallic adhesive bonding structures are much dependent on the adhesive thickness, and the variations of overall strength resulting from the various thicknesses have discrepancy due to the toughness and strain hardening capacity of adhesives.
The indentation test is a localized testing technique; therefore, the role of the material size-effect and local non-uniformity is of much importance. The influence of the heterogeneity in ...size-independent materials has been studied previously. The present work detailedly investigated the influence of the material size-effect and heterogeneity (inclusions near the indenter tip) on the indentation hardness using a size-dependent strain gradient plastic theory. And it was found that when considering the material size-effect, shallow hard inclusions in the heterogeneous materials more significantly enhance the material indentation hardness compared with the size-independent materials which are based on the conventional plastic theory. This hardening effect is believed to be related to the elevation of the load and local constraints of large deformation. The effect of material inhomogeneity mainly comes from the non-uniformity of the structure rather than the inclusion modulus itself especially when the size-effect is involved, and the transition range of the inclusion modulus’ influence is pretty narrow. The effect of non-uniformity becomes negligible after the initial inclusion depth is larger than its diameter. The horizontal offset of the indenter from the inclusion is also of much sensitivity to the influence of the heterogeneous indentation. This paper focuses on the scaling relationships in micro- and nanoindentation, the influence of non-uniformity in microscopic materials is studied and supplemented as well.
The structure of a hard film mounted on a soft viscoelastic substrate exists widely in nature and in industrial production, in which the hard film plays an important role. In this study, in order to ...accurately describe the indentation behaviors of hard film/soft viscoelastic substrate systems and to separate the mechanical properties of the hard film from the overall indentation response, an analytical relationship between the indentation loads and displacements was derived and presented herein based on plate theory, Hankel transformation, the elastic-viscoelastic correspondence principle, and Boltzmann superposition principle. In addition, finite element simulations and experiments were performed to investigate the effectiveness and applicability of the theoretical model. The results showed that the analytical solution agreed extremely well with the experiments and simulations. By analyzing the analytical solution, it was found that, due to the presence of the hard film, the viscous behaviors of the hard film/soft viscoelastic substrate system depended not only on the viscous properties of the soft substrate but also on the elastic properties of the soft substrate. Surprisingly, the relaxation time of the hard film/soft viscoelastic substrate system became longer than that of the soft substrate, but the ratio of the decay modulus to the equilibrium modulus was lower than that of the soft substrate. The above findings remind us that it is impossible to simply judge whether the hard film inhibits or promotes the soft substrate viscous behaviors. Our research can help readers gain insights into the indentation response characteristics of hard film/soft viscoelastic substrate systems, and it provide a theoretical basis for the measurement of the mechanical properties of a hard film mounted on a soft viscoelastic substrate.
The structure of a hard film mounted on a soft viscoelastic substrate exists widely in nature and in industrial production, in which the hard film plays an important role.
Nanomechanical properties of polymer samples were calculated using an adhesive contact model appropriate for AFM indentation problems. A series of Polydimethylsiloxane (PDMS) samples were indented by ...the sharp indenter in the air by using an AFM, and dozens of the force-displacement curves of each sample were obtained. An adhesive contact model suitable for sharp indentation with adhesion was established based on the same assumptions of the JKR model which is only suitable for spherical indentation at small penetration depth. Differences between sharp indentation problems with and without adhesion were discussed, and the limitations of the traditional adhesion model were given. The elastic modulus was obtained by fitting experimental force-displacement curves with theoretical ones, and results were compared to those macroscopic values in literature. The adhesion energy between the indenter and the sample surface was accurately calculated using the adhesion model based on the calculated elastic modulus. The influence of the indenter tip angle on the calculation results of the elastic modulus was also discussed theoretically. In this study, the mechanical properties of polymer samples were calculated at the nanoscale considering the adhesion effect.
•Break down of H-P effect.•GB absorption of deformation is the main mechanism.•The area of plastic zone depends on the GB.
The hardness measured by the nanosize indenter under atomic indentation is ...examined for the cases of nanocrystalline nickel by means of molecular dynamics (MD) simulations. The grain size effect observed is different from the one by uniform deformation or deep indentation. The results show that hardness can only show inverse Hall–Petch (H-P) effect, no H-P effect is observed with the grain size up to 40nm. Grain boundary (GB) absorption of the localized strain is the main deformation mechanism when the indenter size and the depth both come to nano size. The area of plastic zone generated beneath the tip is strongly dependent on the GB density, sample with small grain size results in larger plastic area, which leads to the softer response of hardness.
The self-assembly of surface-order structures based on the surface wrinkling of stiff film-compliant substrate structures (SFCS) is potentially useful in the fabrication of functional devices, the ...manufacture of superhydrophobic or self-cleaning surfaces, and so on. Due to the influence of the intrinsic characteristic length (
g
), the surface wrinkling behavior of SFCS at the micro scale is different from that at the macro scale. In this work, based on the strain gradient theory, a trans-scale surface wrinkling model for SFCS is established. First, the effectiveness of this model is verified by previous experiments. Then, based on the model and dimensional analysis, the effect of
g
on the surface wrinkling behavior is investigated, and the scaling relationship of surface wrinkling of SFCS at different scales is analyzed. The results show that the influence of
g
cannot be neglected when the film thickness decreases to the one comparable to
g
. At the micro scale,
g
will lead to the increase of the critical wrinkling wavelength and load. In addition, the scaling relationship of surface wrinkling at the micro scale will not follow the traditional one. Our study explains the underlying mechanism of the dissimilarity of surface wrinkling behaviors of SFCS at different scales and lays a theoretical foundation for the precise control of surface-order structures.
Adhesive joints have a wide range of applications in the civil engineering, automotive and aircraft industries. In the present research, we use the finite element method to systematically study the ...overall strength and interface failure mechanism of single lap joints, which are subjected to tensile loading, focusing on the effects of various system parameters including fracture energy of the adhesive layer, overlap length and adhesive layer thickness on the load-bearing capability of the joints. The results show that the overlap length and the adhesive fracture energy have combined influences on the load-bearing capability. On the other hand, a preliminary damage analysis of the adhesive layer is carried out, considering the situations when the loads arrive to the peak values. Furthermore, the interface behavior is investigated, including the interface stress analysis and interface slip. The rotation of the joint during loading and its influence factors are studied as well. Obtained results suggest that the interface stress distributions are related to the slip and the rotation angle.
Background
Inappropriate macrophages phenotype transition contributes to the development of ulcerative colitis, and the poly (ethylene glycol)-block-poly (
d, l
-lactic acid) (PEG-PLA) nanoparticles ...delivery system can be utilized to improve the cryptotanshinone (CTS)-based therapy.
Methods
We used a single emulsification method to prepare CTS-encapsulated nanoparticles (NP
CTS
). The therapeutic efficacy of NP
CTS
was evaluated in dextran sulfate sodium (DSS)-induced colitis mice. Then the proportion of total macrophages and M2-like macrophages were assayed with flow cytometry, and the relative content of pro-inflammatory cytokines in the colon was detected with Western blot. Bone-marrow-derived macrophages (BMDMs) were induced into M1-like macrophages, which were further incubated with NP
CTS
to repolarize into M2 subtype
.
Results
Cryptotanshinone could induce the transition of M1 subtype to M2 subtype as indicated by up-regulated expression of arginase 1 (ARG1), interleukin (IL)-10, and CD206.
In vivo
, orally administrated NP
CTS
accumulated in the colon-infiltrated macrophages in colitis mice. It further revealed that NP
CTS
significantly alleviated colitis symptoms as indicated by increased body weight and colon length, decreased tumor necrosis factor (TNF)-α, IL-1β, and IL-6 content in the colon, and diminished total macrophage proportion (CD45
+
CD11b
+
F4/80
+
) and up-regulated M2 proportion (CD45
+
CD11b
+
F4/80
+
CD206
hi
).
Conclusion
Oral administration of NP
CTS
could ameliorate ulcerative colitis with the conversion of M1-like macrophages to M2-like macrophages.
Indentation hardness is found to be related to indentation depth when indentation test is applied on homogeneous materials under small indentation depth, which shows strong size effect in the ...indentation. While in contrast, indentation hardness has a very limited relationship with indentation depth when it is large, showing distinct scaling relationships between hardness and material properties. Previous studies on scaling relationships under deep indentation condition of elastic-perfectly plastic homogeneous materials have been carried out systematically by finite element analysis. In this paper, a heterogeneous material, particle-reinforced matrix composite is detailed studied to investigate its scaling relationships under deep indentation with different particle positions and material properties by finite element analysis.
Display omitted
•Special grain boundary can be eliminated by twinning and dislocation activities.•All twinning bands are formed by atomic slip along ordinary twinning direction.•All dislocations are ...nucleated by atomic slip along anti-twinning direction.•Grain boundary elimination enhances resistance to intergranular fracture.•Conditions for grain boundary elimination are proposed to improve grain boundary design.
Grain boundary structures with high resistance to intergranular fracture are the target of grain boundary design. In this work, grain boundary elimination in front of crack tip is observed in two special bcc iron bicrystals through molecular dynamics simulations under mode I loading. Grain boundary elimination depends on crack advance direction and enhances resistance to intergranular fracture. Direction-dependent elimination leads to directional anisotropy of intergranular crack propagation. By analytical analysis and molecular dynamics simulation, grain boundary elimination is found to be attributed to the activities of twinning and dislocation. All twinning bands are formed by atomic slip along ordinary twinning direction, but all dislocations are nucleated by atomic slip along anti-twinning direction. Mechanisms of twinning formation and dislocation nucleation are revealed by calculating energy barriers of atomic slip and shear stress field. According to the mechanism of grain boundary elimination, conditions for grain boundary elimination are proposed to find all special grain boundaries. Results show that twist grain boundaries cannot be eliminated under mode I loading, while tilt grain boundaries with axes of 〈110〉 can. This work provides a good reference for grain boundary design.
PDF
