Local Strain Method Research Articles

Local strain engineering in Janus MoSSe nanoribbons induces tunable electronic structures and remarkable magnetic moments

Local strain, as a small degree and single direction strain method, can effectively regulate the structures and electronic properties of armchair Janus MoSSe nanoribbon, so that the system can be transformed from the original 0.467 eV indirect band gap into 0.259 eV (3-zig), 0.117 eV (3-arm), 0.080 eV (6-arm) and 0.139 eV (9-zig) direct band-gap semiconductor according to the different strain degrees and directions. Compared with traditional MoS2 and MoSe2 nanoribbons, Janus MoSSe nanoribbon shows relatively stable band structure under local strain. The structure and electronic properties of Janus MoSSe nanoribbon are anisotropic when the local strain is along different directions. Due to the broken mirror symmetry of the Janus system and the appearance of in-plane local polarization, the spin polarization effect of Janus nanoribbon under local strain is more remarkable. When the local strain degree C = 0.167 is along the zigzag direction and the local strain C ⩾ 0.056 is along the armchair direction, the Janus nanoribbon exhibits half-metallic properties and surprisingly induces a magnetic moment. For the local strain along the armchair direction, the total magnetic moment of the system can be up to 2.05 μB when C = 0.111. A local strain method is applied to the nanoribbon system, which can effectively regulate the geometric configuration and electronic structure without external doping, and introduce magnetism, providing the possibility for expanding nanoribbons as potential nanoelectronic and spintronic materials.

Numerical implementation of a multiaxial cyclic plasticity model for the Local Strain method in low cycle fatigue

Very often computations on structural elements or machine components subjected to variable loading require the use of an advanced finite element model. This paper reports the numerical implementation of a model for multiaxial cyclic elasto-plastic behaviour, developed to extend the tools of the Local Strain method to fatigue under multiaxial conditions. A basic computer code for axial–torsional loads was developed with the commercial software MATLAB and a more sophisticated one, for general multiaxial loads, was developed as a UMAT subroutine in ABAQUS. Stress integration was introduced in the two usual forms: implicitly and explicitly. A comparison of the results obtained with the implicit and explicit formulations revealed that, under certain loading conditions, the outcome of the process depends very much on the particular integration scheme used.

Numerical implementation of a multiaxial cyclic plasticity model for the Local Strain Method in low cycle fatigue

Very often computations on structural elements or machine components subjected to variable loading require using an advanced finite element model. This paper reports the numerical implementation of a model for multiaxial cyclic elasticplastic behaviour developed to extend the tools of the local deformation method under fatigue to multiaxial conditions. A basic computer code for axialtorsional loads was developed with the commercial software Matlab and a more sophisticated code based on the finite element model for general multiaxial loads was developed as a UMAT subroutine in Abaqus. Stress integration was introduced in the two usual forms: implicitly and explicitly. A comparison of the results obtained with the implicit and explicit formulations revealed that, under certain loading conditions, the outcome of the process depends on the particular integration scheme used.

The use of the specific heat loss to analyse the low- and high-cycle fatigue behaviour of plain and notched specimens made of a stainless steel

Development of experimental equipments able to provide the whole temperature field have given impulse to energy related fatigue studies, since temperature is an indicator of the heat loss. In the present paper the specific heat dissipation is assumed to be a fatigue damage parameter. A simple equation and the associated experimental technique previously published by the authors to estimate the specific energy loss is first recalled. Then it is shown that the fatigue behaviour of a stainless steel material can be summarised by a log–log plot of the specific heat loss versus number of cycles characterised by a constant slope from few hundreds to two million cycles. Further it has been found that plain and notched fatigue data collapse into the same energy-based scatter band. Finally a comparison of the proposed approach with the classical local strain method is performed.

Relations between the S-N, ε-N and da/dN-ΔK Curves of Materials

The S-N,  -N and da/dN- K curves are the basic data of fatigue property for materials, and they are used in the stress-life method, the local strain method and the LEFM (Linear elastic fracture mechanics) crack propagation method to predict fatigue life of structures, respectively. In the present paper, the relations between these fatigue curves and their probability of predicting each other were discussed through several fatigue-life models. The discussed subjects include (1) predicting the S-N curves from the da/dN- K curves of materials, (2) predicting the S-N curves from the  -N curves, and (3) predicting the da/dN- K curves from the  -N curves. It is shown that there are certain relations between the S-N,  -N and da/dN- K curves of materials and it is predictable from one curve to another.

A local strain method for the evaluation of welded joints fatigue resistance: the case of thin main‐plates thickness

ABSTRACTCurrent procedures for evaluating fatigue strength of welded structures may not be consistent with the real fatigue behaviour of welded joints. A local strain method for the prediction of the WELded joints FAtigue REsistance (WELFARE), by local strain measurements at the weld toe, was recently proposed on the basis of fatigue tests on more than 10 series of welded joints (T, cruciform, angular and butt joints) in structural steel, with 10–25 mm main‐plate thickness. This paper reports fatigue test results obtained from 30 cruciform and butt welded joints (3–5 mm thick) under two load ratios (0.1 and −1) in order to extend the applicability of the method to thin welded joints.

Corrosive fatigue of structures with dimple holes under spectrum loading

ABSTRACT Experimental investigation and life prediction are made for structures with straight holes and dimple holes in both laboratory ambient conditions and 3.5% NaCl solution under spectrum loading. With the aid of acoustic emission, label loading and fractography technique, the crack‐growth data are obtained in both environments. Stress‐intensity factors for the corner crack emanated from the dimple hole are evaluated by three‐dimensional (3D) finite element (FE) method. Life prediction of the tested structures is made by using the modified FASTRAN‐II code on the basis of corresponding fatigue crack‐growth rate curves. It is shown that once the initial defect size a0 is determined by one set of test data, it can be used together with the 3D crack‐growth method in order to provide accurate life prediction in alternative structure and loading condition. By comparison, life prediction by the traditional local strain method is also made which is shown to be less reliable than the 3D crack‐growth method.

Influence of strain estimation methods on life predictions using the local strain approach

The influence of strain estimating techniques on life predictions using the local strain approach has been studied. Strain prediction methods included Neuber's rule in its original form and a generalized form, the equivalent strain energy density method, and the finite element analysis using ABAQUS code. Life predictions were made using the local strain method for two aluminium coupons (7050-T7451), one with a Low K t of 1.4 and the other with a High K t of 3.25, under two different load spectra. The predictions were compared to the results of five fatigue tests per coupon type and spectrum. The comparisons showed that the strain estimation technique has little influence on the prediction accuracy compared to the experimental data scatter. Also, a significant difference in relative prediction accuracy between the High and Low K t coupons was found. It was attributed to different proportions of time spent in fatigue initiation and propagation for the two coupons.