1,484 research outputs found
Micromechanical study and simulation of the interlaminar failure of a woven composite laminate
No full textThe laminate load bearing capability is often compromised by the interlaminar failure even though the integrity of the individual lamina remains intact. The out-of-plane components of the stress tensor, defined at the interfaces between plies, are typically responsible for the delamination of multi-layered materials. Failure models for delamination usually make use of both shear and normal interlaminar stresses but their reliable calculation by finite element analysis requires cautions and special techniques. This is particularly true for woven composites in which the fabric architecture may generate unexpected local normal stresses and consequently influence the fracture occurrence. In this paper a numerical analysis was performed using layered solid elements to predict the global response of the laminate. Then sub-modeling techniques are used with several solid elements through the layer thickness in order to obtain accurate interlaminar stresses in the critical regions. As case study, the short-beam configuration under three-point bending condition was selected. Experimental tests show a reduction of the maximum interlaminar shear stress at failure as effect of the span-depth ratio increase for a given laminate thickness. That evidence, assuming a homogenized material behavior, is unpredicted because the span-depth ratio increase has no significant influence on the interlaminar stresses. The work aims to clarify this not well explained phenomenon with a micromechanics approach
Boundary conditions effects on the crack growth mechanism under cycling bending
No full textThe recent increase of train speed and frequency determines a rise of the loads transmitted to the superstructure. Therefore, railway components might experience service loads that have not considered at the design stage. Moreover, wear and backlash modification between components of a mechanical system might be able to modify the internal boundary conditions of the assembly. According to damage tolerance philosophy, an initial flaw is assumed to exist in the fatigue critical location of a structural component, and the analysis of the crack propagation life for such component needs accurate Stress Intensity Factor (SIF) evaluations. In this study, the effects of the boundary conditions on the crack propagation life have been evaluated for a semi-elliptical surface crack having semi-axes a and c and growing from the root of a shoulder fillet notch in a round bar loaded in bending. Two cases have been analyzed: - the shoulder is free from external forces; - the shoulder is in contact with an adjacent generic body. At first, the SIF distribution has been calculated with the Virtual Crack Closure Technique, considering or not the nonlinear effect induced by the contact forces arising from the interaction between the shoulder and the neighboring component. Successively, in both the above cases a two-parameters propagation law has been utilized to predict the evolution of both crack shape and crack depth when a cyclic bending load is applied to the rod. For this purpose, different values of the Stress Concentration Factor at the root of the fillet, and of the initial aspect ratio of the crack front, were considered in the calculations. It is found that the aspect ratio evolves to a unique asymptote, taking or not into account the non-linearity introduced by the contact at the shoulder, and this value depends on the notch severity. The ratio between the dimensionless SIFs obtained with and without the unilateral constraint at the shoulder, βcs / βfs, does not depend on the relative crack depth and crack shape. Also, the effect of the notch severity on the dimensionless SIF appears to be evident only for the portion of the crack front in the vicinity of the free surface
Electron transport through single donors in silicon
No full text-Kavli Institute of Nanoscience DelftApplied Science
On the geometric transferability of the delamination shear limit for CFRP laminate in bending
No full textThe laminate load bearing capability is often compromised when delamination occurs even though the
laminae remain intact. The out-of-plane components of the stress tensor, defined at the interfaces
between plies, are typically responsible for the delamination of multi-layered materials. Commonly used
failure criteria for delamination make use of both shear and normal interlaminar stresses. In this work the
out-of-plane stresses inside a woven laminate were numerically evaluated using a micromechanical
model. Under three point bending the existence of local normal interlaminar stress related to the fabric
architecture was demonstrated. The influence of the due to texture normal interlaminar stress on the
fracture occurrence was discussed. The reduction of the apparent interlaminar shear strength as effect
of the span-to-depth ratio increase was successfully reproduced introducing a dependence of the delamination
shear limit from the stress triaxiality gradient
About the certification of railway rails
Get PDFWhen the compliance with the European Code of some rail steel has to be verified, the need of carrying out the experimental activities in accordance with several testing Standards forces the operator both to solve the problems related to the choice of a suitable testing practice and often to interpret subjectively Standards guidelines. This does not facilitate the comparability and/or the quality of the results produced by several laboratories. With reference to a series of fatigue, fracture toughness and fatigue crack growth tests carried out by the authors on specimens extracted from rails, the main lacks in the current standards, related to both the choice of the control parameters and the testing procedures, are pointed out. Regarding the crack growth testing, several procedures to compute the crack growth rates to be compared with the limits prescribed by the Code are proposed. These procedures have been applied to a data set produced during the aforementioned testing activity, in order to highlight, by comparison of the results obtained by them, the significant differences in the crack growth rate estimates and the magnitude of the errors that can be done due to the lacks in the standard practices currently adopted
A four-parameters model for fatigue crack growth data analysis
Get PDFA four-parameters model for interpolation of fatigue crack growth data is presented. It has been validated by means of both data produced by the Authors and data collected from Literature. The proposed model is an enhanced version of a three-parameters model already discussed in a previous work that has been suitably modified in order to overcome some drawbacks raised when applied to a quite wider experimental data set. Results of validation study have also revealed that the new model, besides interpolating accurately crack growth data, allows to identify the presence of anomalies in the data sets. For this reason, by a suitable filter to be chosen depending on the size and number of anomalies, it can be used to remove them and obtain sigmoidal crack propagation curves smoother than those obtained when the current analysis techniques are used. In the end, possible model parameters correlations are analysed
A three-parameter model for fatigue crack growth data analysis
Get PDFA three-parameters model for the interpolation of fatigue crack propagation data is proposed. It has been validated by a Literature data set obtained by testing 180 M(T) specimens under three different loading levels. In details, it is highlighted that the results of the analysis carried out by means of the proposed model are more smooth and clear than those obtainable using other methods or models. Also, the parameters of the model have been computed and some peculiarities have been picked out
On the mechanical characterization of materials by Arcan-type specimens
No full textThe simple circular notched specimen was originally proposed by Arcan to characterize the elastic properties of fibre-reinforced composites. Unfortunately, its optimized geometry does not allow to measure with reasonable accuracy both the material shear strength and the conditions of failure under a generic biaxial stress state, since the effects of stress concentration on the fillets of the two V-grooves and on the inner circular edges are responsible of premature fractures due to the uniaxial stress states of the notch edges.
In a previous numerical study carried out by a parametric two-dimensional finite element model, some of the Authors of this paper found a new optimal geometry of the Arcan specimen able to minimize the notch effect and achieve a uniform pure shear stress field in the gauge cross-section. In the present paper, starting from such a geometry, a new type of Arcan specimen is proposed, having not uniform thickness. An extensive three-dimensional parametric finite element analysis has been done to define its optimal shape. The numerical results show that the new specimen is able to achieve, with a higher probability, material fracture in the minimum cross-section under a pure shear stress distribution which is more uniform than those acting in the Arcan specimen typologies until now proposed
New (Probabilistic) Derivation of Diaz-Metcalf and Pólya-Szegő Inequalities and Consequences
No full textClassical inequalities of Diaz - Metcalf and Pólya - Szegő are generalized to
probabilistic setting which covers the initial deterministic (both discrete and integral) variants.
From these two inequalities, by the probabilistic derivation method further well -
known inequalities are obtained (that ones by Kantorovich, Rennie and Schweitzer)
The electrons are waves: impossible interview to C.J. Davisson (1881-1958) and G.P. Thomson (1892-1975)
No full textThe author imagines to interview the Nobel Laureates in Physics of the year 1937, who turned upside-down modern physics, demonstrating the wave nature of matter. The answers of C. Davisson and G.P. Thomson are based on
the Nobel Lectures they delivered during the Nobel Prize Award Ceremony
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