Experimental and numerical investigations of fracture behavior of magnetostrictive materials

Abstract: The purpose of this work is the characterization of the fracture behaviour of giant magnetostrictive materials subjected to a magnetic field. Both experimental and numerical investigations have been performed, focusing on iron and rare earth alloys, such as the commercially named Terfenol-D. Tests have been carried out on single-edge precracked specimens subjected to three-point bending in the presence of magnetic fields of various intensities and fracture loads have been measured at different loading rates. Recent studies on local stress fields in proximity of crack and notch tips have shown that Strain Energy Density (SED), averaged in a circular control volume which includes a crack tip, could be a robust parameter in the assessment of brittle fracture resistance of several materials. Coupled-field analyses have then been performed on both plane stress and plane strain finite element models and the effect of the magnetic field on fracture resistance of Terfenol-D alloy was predicted in terms of averaged SED. A relationship between the SED's control volume size and the loading rate has also been proposed

the effect of residual stress on fatigue behavior of v notched components a review

Abstract: Fatigue strength of mechanical components in the high cycle regime depends on the intensity of the residual stress field induced by non-homogeneous plastic deformation or the solidification of a local portion of material due to welding operations. In presence of geometric variations modelled as sharp V-notch angle, the residual stress distribution near the notch tip is singular and follows the same solution obtained by Williams in 1952 where the intensity of the asymptotic stress field is quantified by the notch stress intensity factor (NSIF). However, the residual stress varies during fatigue loading until a stationary value is reached. Numerical models were developed for the calculation of the residual NSIFs and their variation under fatigue loading. Taking advantage from these models, new approaches were recently developed which are able to predict the fatigue strength of pre-stressed notched components. A review of such recent advances is described in this work

some recent criteria for brittle fracture prediction under in plane shear loading

Abstract: Different criteria are available in the literature to assess the fracture behaviour of sharp V-notches. A typical and well-known criterion is based on the application of the notch stress intensity factors (NSIFs), which are able to quantify the intensity of the stress fields ahead of the notch tip. This work considers two recent energy-based criteria applied here to sharp V-notches. The first criterion is based on the averaged value of the strain energy density (SED), while the second one called Finite Fracture Mechanics (FFM) criterion is available under two different formulations: that by Leguillon et al. and that by Carpinteri et al. Considering the averaged SED criterion, a new expression for estimating the control radius R c under pure Mode II loading is proposed and compared with the sound expression valid under pure Mode I loading. With reference to pure Mode II loading the critical NSIF at failure can be expressed as a function of the V-notch opening angle. By adopting the three criteria considered here the expressions for the NSIFs are derived and compared. After all, the approaches are employed considering sharp V-notched brittle components under in-plane shear loading, in order to investigate the capability of each approach for the fracture assessment. With this aim a bulk of experimental data taken from the literature is used for the comparison

Fatigue Assessment of Ti–6Al–4V Circular Notched Specimens Produced by Selective Laser Melting

This work is aimed to assess the fatigue strength of Ti–6Al–4V smooth and circular notched samples produced by selective laser melting (SLM). Scanning electron microscopy have been used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Despite the observed fatigue strength reduction induced by circular notch compared to that of sooth specimens, notched samples showed a very low notch sensitivity attributed both to hexagonal crystal lattice characteristics of tempered alpha prime grains and to surface defects induced by the SLM process itself.Досліджується міцність від утомленості плоских зразків (гладкі і з двосторонніми напівкруглими вирізами) зі сплаву Ті–6Al–4V, які виготовлено за технологією вибірної лазерної плавки. Для ідентифікації місця зародження тріщини від утомленості та механізму руйнування фрактограми зруйнованих зразків досліджувались методом сканувальної електронної мікроскопії. Установлено, що міцність від утомленості зразків із концентратором напружень у вигляді двосторонніх напівкруглих вирізів трохи нижча, аніж гладких зразків. Показано, що чутливість даного матеріалу до концентрації напружень дуже низька через специфіку гексагональної кристалічної решітки загартованих первинних альфа-зерен і технологічних поверхневих дефектів, характерних для вибірної лазерної плавки.Исследуется усталостная прочность плоских образцов (гладкие и с двухсторонними полукруглыми вырезами) из сплава Ti–6Al–4V, изготовленных по технологии избирательной лазерной плавки. Для идентификации мест зарождения усталостной трещины и механизма разрушения фрактограммы разрушенных образцов исследовались методом сканирующей электронной микроскопии. Установлено, что усталостная прочность образцов с концентратором напряжений в виде двухсторонних полукруглых вырезов незначительно ниже, чем гладких образцов. Показано, что чувствительность данного материала к концентрации напряжений очень низкая из-за специфики гексагональной кристаллической решетки закаленных первичных альфа-зерен и технологических поверхностных дефектов, характерных для избирательной лазерной плавки

Frontiers of fracture and fatigue: Some recent applications of the local strain energy density

The phenomenon of brittle fracture occurs too often in various branches of engineering being the reason of unexpected termination of anticipated service lives of an engineering objects. This leads to unfortunate catastrophic structural failures resulting in loss of lives and in excessive costs. The theory of fracture mechanics enables the analysis of brittle and fatigue fracture and helps to prevent the occurrence of brittle failure. This field has engaged researchers from various fields of engineering from the early days until today. As its own scientific discipline, it is now less than fifty years old and encourages scientists and engineers to speak the same language when dealing with the design and manufacturing of the classical machinery as well as various intricate devices of nanometer scale, or even smaller, reasoning significant scale effects that arise. Attempting to strike a common ground will connect various physical events/phenomena as a natural result of curiosity arising in course of joint research activities. The interpretation provided by the strain energy density to face different problems and applications is presented in this paper considering some recent outcomes at different scale levels

Notched graphite under multiaxial loading

Cylindrical specimens made of polycrystalline graphite and weakened by circumferential V-notches under mixed mode I/III loading were studied in this research. Different geometries of V-notches varying the notch opening angle and the notch tip radius were tested. Appling various ratios of tensile and torsion loads, the multiaxial static tests have been conducted. Averaged Strain Energy Density (ASED) criterion previously presented by the same authors is employed here for the case of tension and torsion loadings applied in combination. The fracture behavior of the tested joints under multiaxial loading has been successfully predicted using the ASED criterion

Notch stress intensity factors under mixed mode loadings: an overview of recent advanced methods for rapid calculation

Recently some methods for the rapid calculation of notch stress intensity factors (NSIFs) have been developed and three of them are compared in this work. First, the criteria proposed by Lazzarin et al. and Treifi et al. have been reviewed. The former is based on the calculation of the mean value of SED on two different control volume (characterized by two different radius values) centred at the stress singularity point, whereas the latter takes advantage of the strain energy density averaged within two control volumes (semi-circular sector) centred at the notch tip. Then, a new method based on the evaluation of the total and deviatoric SED averaged in a single control volume has been proposed. Finally, plate specimens weakened by different notch geometries have been subjected to the application of the above mentioned methods and the obtained values of the NSIFs have been compared with those derived according to Gross and Mendelson

Length scale parameters to estimate fatigue lifetime of 3D-printed titanium alloy Ti6Al4V containing notches in the as-manufactured condition

The accuracy of the Theory of Critical Distances, Gradient Elasticity and the Averaged Strain Energy Density criterion in estimating fatigue lifetime of notched additively manufactured Ti6Al4V is assessed against numerous experimental results generated under load ratios equal to -1 and 0.1. The 3D-printed Ti-alloy under investigation was tested by keeping the notches in the as-manufactured condition. The common feature of the considered design approaches is that they all make use of a material length scale. The validation exercise based on the generated experimental results demonstrates that the length scale concept can be extended successfully also to the fatigue assessment of notched 3D-printed metallic components

Mode II brittle fracture: recent developments

Fracture behaviour of V-notched specimens is assessed using two energy based criteria namely the averaged strain energy density (SED) and Finite Fracture Mechanics (FFM). Two different formulations of FFM criterion are considered for fracture analysis. A new formulation for calculation of the control radius Rc under pure Mode II loading is presented and used for prediction of fracture behaviour. The critical Notch Stress Intensity Factor (NSIF) at failure under Mode II loading condition can be expressed as a function of notch opening angle. Different formulations of NSIFs are derived using the three criteria and the results are compared in the case of sharp V-notched brittle components under in-plane shear loading, in order to investigate the ability of each method for the fracture assessment. For this purpose, a bulk of experimental data taken from the literature is employed for the comparison among the mentioned criteria

The fatigue behavior of V-notches in presence of residual stresses: recent developments and future outcomes

Residual stresses, arising from welding processes or nonhomogeneous plastic deformations, broadly influence the high cycle fatigue behavior of mechanical components. The presence of V-notches leads to singular residual stresses ahead of the notch tip and the asymptotic stress field can be described by the notch stress intensity factor (NSIF). However, plastic effects induce redistribution of residual stresses during cyclic loading and this variation is accounted in several numerical models developed for the calculation of the residual NSIFs. Due to the development of these models, the fascinating issue of predicting the fatigue strength of pre-stressed notched components has gained widely attention by the researchers and new approaches were recently developed and some of them are here reviewed