Asymmetry In Compression Research Articles

A cohesive-frictional force field (CFFF) for colloidal calcium-silicate-hydrates

Calcium-silicate-hydrate (C-S-H) gel is a cohesive-frictional material that exhibits strength asymmetry in compression and tension and normal-stress dependency of the maximum shear strength. Experiments suggest the basic structural component of C-S-H is a colloidal particle with an internal layered structure. These colloids form heterogeneous assemblies with a complex pore network at the mesoscale. We propose a cohesive-frictional force field (CFFF) to describe the interactions in colloidal C-S-H materials that incorporates the strength anisotropy fundamental to the C-S-H molecular structure that has been omitted from recent mesoscale models. We parameterize the CFFF from reactive force field simulations of an internal interface that controls mechanical performance, describing the behavior of thousands of atoms through a single effective pair interaction. We apply the CFFF to study the mesoscale elastic and Mohr–Coulomb strength properties of C-S-H with varying polydispersity and packing density. Our results show that the consideration of cohesive-frictional interactions lead to an increase in stiffness, shear strength, and normal-stress dependency, while also changing the nature of local deformation processes. The CFFF and our coarse-graining approach provide an essential connection between nanoscale molecular interactions and macroscale continuum behavior for hydrated cementitious materials.

Constitutive response of calcium‐silicate‐hydrate layers under combined loading

AbstractCalcium‐silicate‐hydrate (C‐S‐H) is the main hydration product for ordinary Portland cement (OPC) materials that exhibits a layered structure containing interfaces that controls the system response to shear deformation at the nanometer scale. In this work, we used molecular statics simulations to study the mechanical behavior of an atomistic model of C‐S‐H under combined loading conditions that are typical of structural applications of these materials. Combined loading is implemented by first compressing or stretching the atomistic structure to impose an external hydrostatic pressure, and then loading the system through both heterogeneous and homogeneous shear deformation. By utilizing two different shear methodologies, we were able to isolate the interface behavior from the bulk response. Our results show several qualitative similarities with that of macroscale cementitious materials including pressure sensitivity of the maximum shear strength and strength asymmetry in compression and tension. This indicates that the well‐known cohesive‐frictional behavior of cementitious materials is fundamental to interfaces between C‐S‐H grains at the nanoscale. Comparing differences in our results with nanoindentation experiments motivate future investigations of the effect of C‐S‐H particle size and morphology on strength scaling properties at the mesoscale. These mesoscale model interactions should include the normal‐stress or pressure dependency that we observe.

Comprehensive Modeling of Shape Memory Alloy Material Response Using a Multimechanism-Based Inelastic Model

Despite significant work over many years, using either one of the micromechanics based or phenomenological approaches, there are still prospects of much improvement to be made in constitutive modeling of shape memory alloys (SMAs). This is especially true if we sufficiently target the general scope in the modeling, i.e., including such important attributes as (1) multiaxiality; (2) possible asymmetry in tension and compression; (3) accounting for pseudoelasticity and pseudoplasticity; (4) rate dependence; and (5) effects of shape memory training under cyclic loading. The desire for comprehensive modeling of SMA materials provides the primary motivation for the inelastic model described here. Its theoretical formulation employs two main ingredients: (1) multiplicity of hardening mechanisms and (2) the partition of the energy storage/dissipation that is so vital in capturing the extremes of pseudoelasticity and pseudoplasticity in SMAs. For the purpose of validation and assessment part of the model capabili...

Strengthening Mg–Al–Zn alloy by repetitive oblique shear strain with caliber roll

Grain refinement of an Mg–3Al–1Zn–0.2Mn (wt.%) alloy was conducted by severe plastic working with caliber rolling. The alloy’s yield strength was increased to more than 400 MPa, which is twice that of an initially extruded alloy, and its yield asymmetry in compression against tension was reduced by the weakened basal texture caused by the shear deformation.

Reverse effect of tensile force on sidewall curl for materials with tensile/compressive strength difference

Sidewall curl occurring by the removal of tool surfaces after forming is one adverse phenomenon that should be effectively reduced in sheet metal forming operations. Among several process parameters controlling sidewall curl, a constraint tensile force is widely used along with attainable formability by introducing blank holder and drawbead. The classic but common knowledge is that sidewall curl is suppressed for conventional sheet metals as the constraint tensile force increases. Interestingly, however, for magnesium alloy sheets that have unusual asymmetry in tension and compression it has been recently reported that springback increases as the tensile force increases within a specific range of tension. The major deformation in the sidewall usually consists of bending and unbending under tensile force. Therefore, this unique stress-strain response of sheet materials with strength-differential, including magnesium alloys, should be considered for an accurate estimation of sidewall curl. In the present study, a semi-analytical bending/unbending theory incorporating characteristic constitutive behavior of magnesium alloys was developed to evaluate the moment-curvature relationship for various levels of constraint tensile forces. The present analysis proved that the reverse effect of constraint tensile force on sidewall curl was caused by the lower resistance to plastic yielding in compression with proper combination of applied tensile force.

Response of NiTi shape memory alloy at high strain rate: A systematic investigation of temperature effects on tension–compression asymmetry

A systematic experimental investigation into the effects of temperature on the stress–strain response of NiTi (55.6 wt.% Ni) shape memory alloy at high strain rate was conducted and compared with the behavior at quasi-static strain rate. Dynamic compression and tension tests, to total strains of 16–24%, were performed at high strain rate (∼1200/s) and under quasi-static conditions at temperatures from −196 to 400 °C. Since the superelastic range for the NiTi alloy used in this work is A f (2 °C) < T < M d (150 °C), it was possible to study the high strain rate deformation of both the austenite phase (where stress-induced martensite transformation occurs) at temperatures >0 °C and the (thermally induced) martensite phase at lower temperatures. The results indicate differences in the stress–strain response of thermally induced martensite and stress-induced martensite, in terms of plateau stress characteristics and critical stress (as determined by 0.2% strain offset). The observations illustrate a complex interplay of test temperature, stress state (compression and tension) and martensite type (thermally induced vs. stress induced) that lead to the asymmetry in compression vs. tension response of the shape memory alloy in both quasi-static and dynamic loading conditions. This asymmetry was captured in the variation of critical stress with temperature that exhibited a three-stage character, with the critical stress being higher in compression than in tension. These findings have significant implications for the understanding and exploitation of the underlining functional characteristics of shape memory alloys, especially at high strain rates.

Mechanical behavior of thin Cu films studied by a four-point bending technique

ABSTRACTFour-point bending experiments in combination with thermal cycling of thin films on substrates were performed in a dedicated apparatus. Strains up to ±0.8% could be imposed into Cu films of 0.2, 0.5, and 1.0 μm thickness on Si substrates by bending the substrates at various temperatures in high vacuum. After relief of the bending, the residual stress was measured by the wafer-curvature method. At temperatures below 250°C, the yield behavior is asymmetric in tension and compression. The amount of plastic strain introduced by external bending increases with film thickness, but the absolute values of the introduced plastic strains are very low throughout. At higher temperatures, there is no clear thickness dependence and no asymmetry in tension and compression. The results are discussed in connection with the formation of misfit dislocations during plastic deformation of thin films.