Competing failure mechanisms in thin films: Application to layer transfer

We investigate the origin of transverse cracks often observed in thin films obtained by the layer transfer technique. During this process, two crystals bonded to each other containing a weak plane produced by ion implantation are heated to let a thin layer of one of the material on the other. The level of stress imposed on the film during the heating phase due to the mismatch of thermal expansion coefficients of the substrate and the film is shown to be the dominent factor in determining the quality of the transferred layer. In particular, it is shown that if the film is submitted to a tensile stress, the microcracks produced by ion implantation are not stable and deviate from the plane of implantation making the layer transfer process impossible. However, if the compressive stress exceeds a threshold value, after layer transfer, the film can buckle and delaminate, leading to transverse cracks induced by bending. As a result, we show that the imposed stress σ_m —- or equivalently the heating temperature -— must be within the range −σ_c<σ_m<0 to produce an intact thin film where σ_c depends on the interfacial fracture energy and the size of defects at the interface between film and substrate

An energy-based model of longitudinal splitting in unidirectional fiber-reinforced composites

Unidirectional fiber-reinforced composites are often observed to fail in a longitudinal splitting mode in the fiber direction under far-field compressive loading with weak lateral confinement. An energy-based model is developed based on the principle of minimum potential energy and the evaluation of effective properties to obtain an analytical approximation to the critical stress for longitudinal splitting. The analytic estimate for the compressive strength is used to illustrate its dependence on material properties, surface energy, fiber volume fraction, fiber diameter, and lateral confining pressure. The predictions of the model show good agreement with available experimental data

Mechanics of large electrostriction in ferroelectrics

The complex arrangement of domains observed in ferroelectric crystals is a consequence of multiple energy minima of the crystal free energy density. Since the total energy is a sum of the free energy, and electrical and mechanical work, switching between the different energetically equivalent domain states can be achieved by both electrical and mechanical means. For many ferroelectric materials, this results in an electrostrictive phenomenon resulting from domain switching. In the current study, the electrostrictive behavior of single crystal ferroelectric perovskites has been investigated experimentally. Experiments have been performed in which a crystal of barium titanate is exposed to a constant compressive stress and an oscillating electric field and global deformation is measured. The combined electromechanical loading results in a cycle of stress and electric field induced 90-degree domain switching. The domain switching cycle results in a measurable strain response theoretically limited by the crystallographic unit cell dimensions. Induced strains of more than 0.8% have been measured in barium titanate. Larger strains of up to 5% are predicted for other materials of the same class

Study on drug utilization pattern in conservative management of patients with pancreatitis in a tertiary care teaching hospital

Background: Drug utilization plays a key role in helping the healthcare system to understand, interpret and improve the prescribing, administration and use of medications. The objective of the study was to assess the drug utilization pattern of drugs prescribed in conservative management of pancreatitis patients in a tertiary care teaching hospital.Methods: This prospective observational study was conducted in department of surgery, Rajah Muthiah Medical College and Hospital, Annamalai University. The study period was six months from November 2019 to April 2020. A total of 115 patients were selected based on inclusion and exclusion criteria.Results: The study population containing of 90% (n=104) males and 10% (n=11) females were included in the study. Most of the patients between age group of 25-44 years (48%). The most common etiology was alcoholic 68% (n=78). 7% of population with comorbidities (5% hypertension, 2% diabetes mellitus). In analgesics, tramadol (28%) and in antibiotics cefotaxime 21% was most prescribed. Lactated ringer solution (36%) was prescribed most as intravenous hydration. 7% (n=8) and 3% (n=4) of patients receiving enteral nutrition and parenteral nutrition. The average number of drugs per prescription was 8.5. Out of 1205 drugs, 61% of drugs prescribed in generic form, 39% were prescribed in brand name. The study analyzed that 69% of drugs were prescribed from essential drugs list (EDL) 2019 and 84% drugs prescribed from national list of essential medicines (NLEM) 2015. Number of prescriptions with injection were 79.07%. The overall antibiotic encounter rate 15%. Prescribed daily dose/defined daily dose (PDD/DDD) ratio of folic acid was 5.Conclusions: Drug utilization study can help in evaluating the quality of care given to the pancreatitis patients and promote rational use of medicines

Stress fluctuation, crack renucleation and toughening in layered materials

It has been established that contrast in the elastic properties can lead to enhancement of fracture toughness in heterogeneous materials. Focussing on layered materials as a model system, we show that this enhancement is a result of two distinct phenomena – first, fluctuations in stress leading to regions where the stress intensity at the crack is considerably smaller than that of the macroscopically applied value; and second, the lack of stress intensity when a crack is at a compliant to stiff interface thereby requiring renucleation. Using theoretical, computational and experimental methods, we study two geometries – a layered material and a layered material with a narrow channel – to separate the two phenomena. The stress fluctuation is present in both, but renucleation is present only in the layered medium. We provide quantitative estimates for the enhanced toughness

Differential Sandwich Theorems For Certain Analytic Functions.

We give some applications of first order differential super ordinations to obtain sufficient conditions for normalized analytic functions f(z) to satisf

On Planar Harmonic Mappings And Minimal Surfaces

The project seeks to study the geometric properties of planar harmonic mappings (and related mappings), and aims to apply the findings in the investigation of minimal surfaces

Plastic Deformation in Laser-Induced Shock Compression of Monocrystalline Copper

Copper monocrystals were subjected to shock compression at pressures of 10–60 GPa by a short (3 ns initial) duration laser pulse. Transmission electron microscopy revealed features consistent with previous observations of shock-compressed copper, albeit at pulse durations in the µs regime. The results suggest that the defect structure is generated at the shock front. A mechanism for dislocation generation is presented, providing a realistic prediction of dislocation density as a function of pressure. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle-Grady relationship

Toughening and asymmetry in peeling of heterogeneous adhesives

The effective adhesive properties of heterogeneous thin films are characterized through a combined experimental and theoretical investigation. By bridging scales, we show how variations of elastic or adhesive properties at the microscale can significantly affect the effective peeling behavior of the adhesive at the macroscale. Our study reveals three elementary mechanisms in heterogeneous systems involving front propagation: (i) patterning the elastic bending stiffness of the film produces fluctuations of the driving force resulting in dramatically enhanced resistance to peeling; (ii) optimized arrangements of pinning sites with large adhesion energy are shown to control the effective system resistance, allowing the design of highly anisotropic and asymmetric adhesives; (iii) heterogeneities of both types result in front motion instabilities producing sudden energy releases that increase the overall adhesion energy. These findings open potentially new avenues for the design of thin films with improved adhesion properties, and motivate new investigation of other phenomena involving front propagation.Comment: Physical Review Letters (2012)