Effect Of Pre-existing Defects Research Articles

Experimental study on the effect of waviness defects on composite material impact dynamics

The effect of pre-existing defects in composite materials on impact tolerance is poorly understood. In this study, carbon fibre reinforced polymers with waviness defects were used to explore how pre-existing defects affect impact events. An algorithm was introduced to provide insights into damage mechanics during impact experiments using damage-time maps. These maps provide information on damage: location, severity and initiation time. The morphology of damage measured using damage-time maps were compared with measurements from pulse-echo ultrasound and found to have a high level of agreement. This study found severe waviness bands acted as a barrier preventing delamination growth, resulting in a unique damage shape without causing higher damage area. By using the introduced algorithm, the amount of time required to understand complex impact events is substantially reduced.

Synchrotron-based multiscale study on phase transformation in a cold-rolled NiTi shape memory alloy: Effects of preexisting defects

Deformation and phase transformation of a cold-rolled, textured, shape memory alloy sheet Ni50.6Ti49.4 are investigated via real-time, in situ synchrotron X-ray diffraction and digital imaging correlation. The specimens are subjected to tensile loading along two orthogonal directions. Bulk stress–strain curves (macroscale), strain fields (mesoscale) and X-ray diffraction patterns (microscale) are obtained simultaneously. Martensite nucleation is highly correlated with the preexisting defects. Different from the widely observed Lüders bands within defect-free NiTi, uniform martensite phase transformation occurs in the cold-rolled alloy as a result of the relatively widely distributed preexisting defects, giving rise to progressive growth of martensite, relatively uniform strain fields, the sigmoidal stress–strain curves and increased strain hardening rate. Different favorably oriented martensite variants are induced due to the initial texture, and the martensite phase transformation is more active for loading along the less favorable orientation, giving rise to the anisotropy in the phase transformation.

Micro- and macrocracking behaviors in granite and molded gypsum containing a single flaw

The effect of preexisting defects on fracture behaviors is significant for the stability analysis of rock engineering. To study the effect of preexisting flaws on the micro- and macrocracking behaviors of rocks, photographic monitoring together with strain field analysis, passive acoustic emission (AE) and active ultrasonic testing were carried out on gypsum and granite samples containing a single flaw. Based on coupled analysis of the acousto-optic-mechanical characteristics, it is suggested to identify micro- and macrocrack initiation by AE monitoring rather than stress–strain behaviors, photographic observation and P-wave measurement. The micro- and macrocrack initiation stresses generally increase with the flaw angle. The microcrack initiation stress ranging from 0.22σc to 0.62σc (peak strength) coincides with the onset of AE events, and the macrocrack initiation stress ranging from 0.60σc to 0.85σc coincides well with a dramatic increase in AE energy for flawed granite. The crack types are classified into nine different categories. Macrocrack coalescence is observed before reaching σc, and violent rock failure occurs seconds after σc is achieved for flawed granites, which differs from that process for gypsum samples. The cracking process of flawed granites experiences three typical stages and is highly dependent on the flaw angle. A stress drop accompanied by a significant increase in the AE energy rate appears several times. With an increasing flaw angle, the count of stress drop and the cumulative AE energy decrease, while the maximum value of the AE energy rate and the average hit energy increase gradually. The energy and stress release reduces the violent failure degree of rock. The AE energy correlated with the cracking magnitude changes more violently than the AE event before peak failure. P-wave attenuation increases sharply after macrocrack initiation. A sharp reduction in velocity is always accompanied by a significant increase in the AE rate. Compared with the global cracking damage of rock characterized by AE characteristics, the fracture identified by P-wave velocity is localized.

Fracture behavior of solid electrolyte LATP material based on micro-pillar splitting method

The NASICON type solid electrolyte LATP is a promising candidate for all-solid-state Li-ion batteries considering energy density and safety aspects. To ensure the performance and reliability of batteries, crack initiation and propagation within the electrolyte need to be suppressed, which requires knowledge of the fracture characteristics. In the current work, micro-pillar splitting was applied to determine the fracture toughness of LATP material for different grain orientations. The results are compared with data obtained using a conventional Vickers indentation fracture (VIF) approach. The fracture toughness obtained via micro-pillar splitting test is 0.89 ± 0.13 MPa∙m1/2, which is comparable to the VIF result, and grain orientation has no significant effect on the intrinsic fracture toughness. Being a brittle ceramic material, the effect of pre-existing defects on the toughness needs to be considered.

Effect of pre-existing defects in the parent fcc phase on atomistic mechanisms during the martensitic transformation in pure Fe: A molecular dynamics study

Molecular dynamics (MD) simulations are used to study the effect of different defect configurations and their arrangements in the parent fcc phase on atomistic mechanisms during the martensitic transformation mechanisms in pure Fe. The defect configurations considered are stacking faults (SF) and twin boundaries (TB) in single crystal fcc. Three arrangements of these defect structures are considered: parallel TB, intersecting SF, and intersecting SF and TB. Each of these defect configurations affect the transformation mechanisms and transformation temperatures. Parallel TB are the lowest-barrier sites for the atomic shear and thus accelerate the transformation process. The fcc phase with parallel TB follows the Nishiyama-Wasserman (NW) martensitic transformation mechanism. On the other hand, intersecting SF impede the atomic shear and thus retard the transformation. The atomistic transformation mechanism in this case first follows the hcp to bcc Burgers path and then the fcc to bcc Olson-Cohen model. The intersecting SF and TB result in a combined effect of both the previous cases. The simulation results show the occurrence of different atomistic mechanisms during martensitic transformation depending on the type of defects present in the parent fcc phase.

Effects of Pre-Existing Defects on Nanoimprint Process Investigated by Molecular Dynamics Simulation

In current study molecular dynamics (MD) simulations are performed to reveal effects of pre-existing defects on nanoimprint of single crystal aluminum ( Al ) thin films. Simulation results suggest that critical force required for plastic deformation initiation decreases with the increase of vacancy volume fraction, but increases with the interstitial volume fraction. It is found that adsorption phenomenon is affected by pre-existing defects, as adsorption phenomenon weakens with the increasing interstitial volume fraction. Pre-existing defects have significant influence on the deformation mechanisms and imprint forces during the nanoimprint processes. Simulation results also show that surface deformation is strongly affected by pre-existing defects.

Effect of Pre-Existing Defects on Fracture of Electromagnetically Accelerated Projectiles: Comparison of Simulations and Experiments

Abstract Induction coil gun uses electromagnetic forces to accelerate metallic tubular projectiles for impact studies. It consists of a helical coil and a cylindrical projectile placed coaxially with the coil. Electromagnetic flux linkage between the coil and the projectile sets up axial and radial forces on the projectile. The axial force accelerates the projectile out of the coil, while the radial force leads to pinching . In certain experiments, projectiles are non-uniformly pinched and undergo plastic buckling. This is manifested in longitudinal cracks that degrade system efficiency. These cracks cannot be handled in a R-Z axisymmetric model. Hence we have developed a composite model consisting of MHD R-Z (axisymmetric) and 2D-Hydrodynamic X-Y (plane geometry) models to study deformation and longitudinal cracking. Our study predicts that an initial perturbation in projectile thickness along the θ direction can grow and lead to localization of plastic strains causing longitudinal cracking.

The effect of pre-existing defects on the strength and deformation behavior of α-Fe nanopillars

The effects of two types of pre-existing defects, dislocations and clusters, on the strength and deformation behavior of body-centered cubic Fe nanopillars with a diameter of ∼150nm were investigated using in situ nanocompression in a transmission electron microscope. The plastic deformation of nanopillars containing high initial dislocation densities was observed to be relatively continuous, proceeding via a series of small- and intermediate-scale strain bursts that were associated with the movement/escape of dislocations and the formation of slip bands. Mechanical annealing was observed in nanopillars with high dislocation densities. When the dislocation density was reduced by in situ heating, the nanopillars were much stronger and the plastic deformation behavior transformed to a more abrupt and explosive mode. The introduction of a dispersion of solute atom clusters into nanopillars caused further strengthening as a higher stress level is required for dislocations to pass the clusters. The strengthening effect of cluster dispersion in nanopillars is comparable to that observed in the bulk steel. These phenomena are universal for Fe nanopillars with different crystallographic orientations.

A molecular dynamics study of self-diffusion in the cores of screw and edge dislocations in aluminum

Self-diffusion along screw and edge dislocations in Al is studied by molecular dynamics simulations. Three types of simulations are performed: with pre-existing vacancies in the dislocation core, with pre-existing interstitials, and without any pre-existing defects (intrinsic diffusion). We find that diffusion along the screw dislocation is dominated by the intrinsic mechanism, whereas diffusion along the edge dislocation is dominated by the vacancy mechanism. Diffusion along the screw dislocation is found to be significantly faster than diffusion along the edge dislocation, and both diffusivities are in reasonable agreement with experimental data. The intrinsic diffusion mechanism can be associated with the formation of dynamic Frenkel pairs, possibly activated by thermal jogs and/or kinks. The simulations show that at high temperatures the dislocation core becomes an effective source/sink of point defects and the effect of pre-existing defects on the core diffusivity diminishes.

Effect of Pre-Existing Defects on Reliability Assessment of High-K Gate Dielectrics

Response of the high-k gate dielectrics to low voltage stresses was studied by probing high-k transistors with various voltage/time measurements at different temperatures. The observed dependence of the transistor threshold voltage on stress time was attributed to electron trapping at pre-existing defects in the high-k dielectric rather than stress-induced trap generation. The dominance of the contribution from the reversible electron trapping on the pre-existing defects in the low voltage stress response raises the question on the applicability of the conventional reliability assessment methodology to the high-k dielectrics.