Tuesday, June 4, 2019

Specifications of Damage Assessment Technique

Specifications of impose on _or_ oppress Assessment TechniqueSpecifications of defame Assessment TechniqueDamage Tolerance (DT), is the ability of a material to function and resist fracture after a permanent change/ blot has taken place for a given period of time. The Damage Tolerance is an essential attribute of a structural component, whose ruin could result in catastrophic loss of life or property.Damage Tolerance AnalysisThe DT addresses two points concerning an initially defected/ aggrieved structure.First, it determines fracture load for a specified defect size. Meaning that the load where material failure occurs, when a specified defect size, exists is determined.Second, it predicts the needed length of time for a sub-critical defect to grow to the size that causes fracture at given load. In this sequel, it is fabricated that the defect can extend and propagate in a sub-critical manner1.The determination of fracture/failure load for a defected component will be the pris tine target of the DT depth psychology that will be carried out for the projects needs.To apply the DT method, the fatigue target separate out levels need to be calculated. The last can be derived from the last-ditch abide forable speech pattern levels. The ultimate allowable stress levels can be basically determined by the loading conditions applied to the specified component. In every(prenominal) structural case, the specific target stress value for any given primary structural component is adjusted for its specific geometry and material properties. Therefore, the prediction of the required length of time for a sub-critical defect to grow to the size that causes fracture at given load will be attempted to be determined. Of grade to achieve predicting the required length of time for a sub-critical defect to grow to the size that causes fracture at given load, the respective material info should be available.Damage Tolerance Analysis ProcedureIt is hit the sackn that a stru cture usually fails by one or combination of failures. These failures can be elastic /inelastic deformations, buckling, fatigue or accidental impact, etc.In the flow chart of Figure 4.1 a possible procedure that might be choose to carry out the numerical DT analysis of a structural component is depicted. This flow chart, as it can be seen, covers the case of static and fatigue load. contempt of this, distributively case can be assessed and examined individually if all the required inputs be available.1 Damage Tolerance Analysis of Aero Structural Components, TATAFigure 4.1 Damage Tolerance Analysis ProcedureIt is intended to develop a fully parametric numerical regulate of the structural (metallic and/or composite) component for the DT modelling victimization the aid of suitable Finite Element softw atomic number 18. The work plan is to write a scripting code in the FE softwares parametric design language to allow defect geometric characteristics (e.g. defect location, damage size etc.) and external data (e.g. geometric characteristics, material properties, loading and boundary conditions of the component) to be automatically inserted to the numerical model of the structural component.The aim is to develop a code with the following featureseasy data insertion by uploading the geometry of the component,apply loading and boundary conditions to the DT modular unit, anddata import for different types of components with minimum modifications.Moreover, it will be attempted to achieve a sufficient mesh density for the Finite Element model (refined FE model) that will provide adequate accuracy by minimizing the discrepancies of the results in a prescribed range. At the same time, it is desired that the model will not consumes immoderate computational time and effort for its solution.It will be attempted to incorporate an adaptive mesh routine for increasing the mesh density at the defect regions according to certain condition/criteria, such as the minimum ener gy condition. This technique will be employed in case that Byzantine geometry is well-nigh to be analysed.Eventually, a post-processing macro-routine will also be programmed to process the results from the analysis solution. A possible way for verifying whether the structural component can break with defect or repair is required is by calculating the Margin of Safety (MS) for the examined component. Using the maximum stress (von-Mises) obtained from the numerical analysis, static margin of condom (MS) obtained for limit and ultimate loads can be calculated respectively. By employing the suitable failure criteria a possible reduction of the components structural strength will be defined.Required inputs for Damage Tolerance AnalysisIn this section the inputs that will be required for carrying out the numerical structural analysis are listed in the following paragraphs. A brief explanation about each required input is also presented in order the reader to comprehend the compulsion of each characteristic.Components geometric characteristicsThe geometric characteristics of the component can be separated in three distinguished categories, which both of them have to be defined with accuracy. These categories arecomponents geometry (the geometric characteristic of the reference-no defect- component),type of defect, andgeometry of the defect.And they are explained in more detail below.Components Geometry Initially, the geometric characteristics of the component are required for carrying out a numerical analysis that will assess the DT of the investigated component. In more detail, the geometric characteristics of the investigated component-in case of a flat plate- are its width, length, its thickness etc.For the case where a more complex geometry needs to be investigated, CAD drawings will be required. It is preferred to provide EXIS with the drawings in electronic format in a neutral file (e.g. .igs or .stp format) to allow the smooth insertion of these file to th e FE software. Additional inputs will be required, depending the case, for de-featuring the CAD model and eventually generate an equivalent simplified FE model. It is expected the last to have less elements/nodes and because less computational demands.TypeofdefectThe type of flaw detected by the Non Destructive procedure (e.g. crack, delaminations, etc.) will be an advantage to be known and given as input before developing the FE model for the DT analysis, Figure 4.2. It will be helpful to know the defect type DT Analysis of the structural component will examine, so the appropriate modelling procedure and discretization method in the defects region will be employ.Delaminations(single-andmulti-level)Freeedgedamages(delaminations,notches,loosefibers,etc.)Figure 4.2 Type of defect to be analysedDefectGeometryIt is essential when performing a numerical structural analysis of a defected component to know the geometry of the defect. Basically in this case the geometry of the defect is p art of the geometry of the analysed and investigated component.Therefore, the non-destructive method (PA or IRT) will initially detect the damage and generate an image. The image processing procedure will allow to calculate the size of the defect and its location. The geometric characteristics that are expected to extract are the area, width (w), height (h) and the coordinates of the defected area into the component. An estimation of the through thickness damage will be also required for the FE model. The last is expected to be provided by the software that will perform the non-destructive testing.It might be prerequisite to model the defect using an equivalent area of a circle or rectangular, in order to avoid modelling complex defect shapes, such as of Figure 4.3. In case that this approach is utilized, a validation procedure will be performed.Figure 4.3 Geometrical characteristics of the defectMaterial automatonlike propertiesTo carry out a numerical DT analysis of a component, it is necessary to insert the appropriate material models into the FE model to simulate the exact demeanour of the component and its strength. In case the metallic components are examined, the material properties that need to be available are the followingModulus of Elasticity (E), andPoisson Ratio ().Regarding composite components, massive CFRP is the main category that is believed that is about to be investigated, since a wide range of currently flying aircrafts, such as the Airbus 3XX family is using them.Therefore, in case that monolithic composite material the material properties that define the material model will be inserted to the FE model. The following material properties can adequately define the demeanor of the material model.Material properties of the ply Eij, Gij and vij, where Eij Gij and vij is the Young, Shear modulus and Poissons ratio in the i,j direction respectively. The subscripts i, j can take values from 1 to 3 and they are used for defining the material coordinate system, Figure 4.4.Ply thickness.Stacking sequence and number of plies, Figure 4.4.Axial intensity level of laminate in 1, 2 and 3 direction.Shear Strength of laminate in 12, 23 and 31 direction.If the material inputs are not available from the designer of the component, then they will be taken from the open literature.Figure 4.4 good example stacking sequence of a CFRP composite and material coordinate systemCharacteristic example of a set of three dimensional failure criteria is the maximum stress failure criteria. They are presented below (ij are the calculated layer-stress components in the (ij) direction and the denominators are the ultimate strengths in the corresponding direction).MaximumStressFailureCriteriaThe nominator of each ratio is the maximum stress value that is developed in the structure.The denominator is the strength of the material in each direction.Whenever the ratio is less than one then no failure occurs, otherwise material failure exists.It desir ed not having any failure when ultimate load is applied to the component.Restraints and Boundary conditionsThe stress state of a composite structural member is strongly related with the way this member is attached to the rest of the aircraft structure. Knowing that in the area of the defect/damage stress concentration exists, the correct definition of the boundary conditions is essential. To this end, the boundary conditions of the structural component must be provided in order to apply the respective degrees of freedom/ constraints to the FE model.Loading conditionsFor carrying out the DT analysis, the loading conditions exerted to the composite component need to be defined. It is essential for the numerical analysis the applied load to be known hence it is a required input. In more detail the limit load that is applied to the structural component needs to be known before ladder the analysis.Additional inputsAdditional inputs might be required to perform the DT analysis of a speci fic component with a specific type of defect. In that case additional inputs might be needed.

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