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Essay: Designing, selecting an appropriate material based on mechanical properties

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  • Published: 22 September 2015*
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The word Damage was first used to predict the creep rupture of metals. The concept of continuum factor and effective stress was first introduced by Kachanov in 1958 in his study of fracture mechanics later than this study was taken to develop damage factor by Robotnov [1]. Damage mechanics is a topic of applied mechanics which heavily relies upon continuum mechanics which is a study of failure of materials. Damage mechanics is concerned with the demonstration of damaged material suitable for engineering predictions, such as degradation of material properties such as elastic moduli and proportional limits of the material as the material propagates towards ultimate failure. Therefore in Damage Mechanics the damage variables were introduced to emulate material degradation on a macro mechanics scale generating CDM (continuum damage mechanics).
Further investigation was done in the area of fracture mechanics to discover the stability conditions of macro-cracks such as large straining conditions and disruptions in fracture mechanics produced micro-voids and micro-cracks. Thus in fracture mechanics it was concluded that without the microscopic study of the material it’s too complicated to work on practical engineering analysis such as analysis on damage state (crack) and its effect on the structure, so models were made based on micro-mechanics in order to investigate the damage phenomena which signifies the causes of crack nucleation, defects in materials (damage initiation), and propagation with in the material and this phenomena of damage mechanics describes the actual damage and how it is computed. Furthermore it was observed using microscopy (micro mechanics) the main cause of damage in material deformation was ductile fracture and intergranular fracture. [1] or [2]
In the recent era of industry due to progression in safety factors, technology and the needs of new materials the work in damage mechanics theory has expanded its base and made it easy for us to examine the structure or materials resistance to crack initiation during the design phase for the material to be approved. For the stability of a material it is important to discover the maximum value of load and the number of cyclic load a material can sustain as the cracks initiate hence for this it’s vital to know about the structural integrity and the time period for which the material can endure the load. As a result, damage mechanics theory performs number of tests using different state variables to characterise the effects of damage on the toughness and the lifetime of material, which is damaged due to different mechanical loads applied. With these tests results and reports, adjustments can be done on the design feature of the material in-order to reduce the risks of failure or defects. This theory allows estimating the materials failure under various loading conditions through stress-strain graphs and also shows the survival of the material under these loads. It also comprises of studying the materials behaviour under different kind of loads either tensile or compressive stress e.g. the bending of the Tacoma Bridge which was poorly constructed causing it to oscillate and thus collapsed.
This theory is functional during many different engineering applications such as projects on bridges, crane with a pile driver, and aircraft design (wing plates). The main purpose of this theory is to study the changes in shape, elasticity, stiffness, micro-hardness and certify if the structure in its damaged situation is suitable for the projected use in-order to predict how long it could withstand the force in its damaged state before it fails or bends. So this project is based on designing, selecting an appropriate material based on mechanical properties, and hence using finite element analysis (FEA) to approximate the damage effects on the materials behaviour. Bending strength tests proposes the ways of determining the modulus of elasticity in bending the strength of flat metallic materials in the form of metal strip or sheet. The method used for bending is cantilever beam test which measures the angle of deflection and bending moment of materials metallic strip subjected to continuous loading in bending. In-order to find out the models stiffness, strength, stress and strains, in the state of bending the following software’s were used:
SolidWorks is a 3D Mechanical CAD software which is based on solid Modelling and operates using parametric feature based method to create models, parts and assemblies by assembling parts or components in-order to make a perfect model or structure. It also contains finite element analysis (FEA) design simulation tool or program which is used to solve multi-physics simulations such as linear and non-linear. It contains variety of elements and materials in the Materials library in-order to simulate a specific type of model or structure. The Model is built in SolidWorks using a 2D or 3D sketch based on the requirements. The Model is then simulated based on the type of study required linear, static or non-linear followed by many other selections available, meshing the model is essential for predicting the results accurately. The software helps in analysing the weak areas of the material where damage can occur. Modifications can be done on the design feature of the structure before the model is prepared to be tested and presented in-order to reduce the risks for defects and failure which saves the time and cost production.
AutoCAD is a program application made for 2D, 3D and Technical CAD drawings. This software is widely used by industries, such as engineering, architecture for constructing sustainable buildings and inventing hybrid automobiles. This Software is only used for designing 2D and 3D CAD drawings for the model so that these drawing could be either imported in solid works or Comsol for the FEA simulation to be carried out in-order to see the bending of the structure including stress and strains occurring on the model.
Comsol Multi-Physics
Comsol Multi-physics is based on Finite Element analysis and it provides solutions for multi-physics modelling. It is similar to solid works simulation but has better number of selections available based on the type of study whereas solid works is limited to some studies. In-order to simulate the required type of model or structure this software also contains a variety of materials in the materials library. For running the simulation the file has to be imported either from Solid works or AutoCAD or can be created in Comsol. This software contains variety of options for meshing such as fine, extra fine or extremely fine which gives more accurate results. The Graphs are plotted based on the given values of load and parameters.
Instron Tensile Testing Machine
These machines are based on estimating the mechanical properties of materials by performing tests on tension, compression, fatigue and hardness. This machine is used to test the compressive strength of the materials. The sample was tested on this machine in-order to find the ultimate tensile stress, yield stress, stress & strains caused on the thin central section. The results achieved were to estimate the damage done on the model

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