Materials Science and Engineering Lecture Notes PDF

EGYPTIAN CHINESE UNIVERSITY FACULTY OF ENGINEERING STRUCTURES AND PROPERTIES OF MATERIALS MCE132 Dr. Shady Abdelnasser, PhD Department of Energy and Renewable Energy Engineering 1 STRESS-...

EGYPTIAN CHINESE UNIVERSITY FACULTY OF ENGINEERING STRUCTURES AND PROPERTIES OF MATERIALS MCE132 Dr. Shady Abdelnasser, PhD Department of Energy and Renewable Energy Engineering 1 STRESS-STRAIN CURVE & MEASURING DEVICES 2 Different types of loads 3  Different types of loads Tension: Pulling or stretching the workpiece Compression: Pushing or crushing the workpiece Bending: Folding the workpiece  Introduction Stress-Strain Curve ❑ The action or process of deforming or distorting is referred to as deformation. When you apply a force to a sample, it will either compress, stretch or bend in reaction to the force. ❑ The amount of force exerted to a unit area is referred to as Stress. (σ = F/A) ❑ The amount of stretching or compressing that occurs as a result of a stress reaction is referred to as Strain. ( = Lfinal – Linitial)/Linitial = ( = ΔL/L0) ❑ Every material reacts to stress differently Elastic and Plastic Deformation Elastic deformation or Elasticity is the deformation that alleviate when the external forces that caused the change and the stress connected with it are removed. Plastic deformation or Plasticity is a persistent/permanent deformation or change in the shape of the sample caused by the effect of the force. The primary distinction between elastic and plastic deformation is that elastic deformation is reversible, but plastic deformation is irreversible. Difference Between Elastic and Plastic Deformation  Stress-Strain Curve Elastic deformation only occurs =σε within the elastic limit. In this Where, σ is the stress region, a material can restore its E is the Young's modulus original shape once the stress is ε is the strain. removed. the ratio of stress and strain will be constant obeying Hooke’s law. Hooke’s law states that the stress is directly proportional to strain. The constant ratio of stress to strain in this region is defined as Young’s modulus (SI unit: Pascal, Pa) also known as stiffness. Stiffness is a material's ability to return to its original form after being subjected to a load. Laboratory Testing Devices Measuring Devices ❑The main target of all the devices is to determine the load- deformation relationship in order to calculate the material mechanical properties Laboratory Testing Devices ❑ Testing Machines ❑ Dial Gauges ❑ LVDT ❑ Strain Gauges ❑ Load Cells ❑ Data Acquisition System 10 Laboratory Testing Devices Testing Machines ❑The Device that used to apply the load on the test specimen and measure this load with the required accuracy. ❑There are different types of testing machines (Tension – Compression, Bending…etc) Bending Tension Compression 11 Laboratory Testing Devices Dial Gauges ❑ Mechanical device ❑ Dial gauge or dial indicator is a mechanical device used to measure a small linear displacements or dimensional variation in the workpiece ❑ The dial gauge is attached at two points between which the relative movement is measured. It can be attached to frames or holders. ❑ The dial gauge can measure up to a reading of 0.01 or 0.001 mm 12  Laboratory Testing Devices ❑ The working principle of the dial gauge is dependent on the movement of the spindle ❑ The upward pressure on the spindle/plunger is transferred to the system of gears and then is indicated on the face of the dial by the pointer ❑ Practically, A dial gauge is a measuring device that mechanically magnifies the amount of linear movement of a plunger using gears, converts it into the amount of rotation of a pointer, and reads the amount of displacement on a scale plate ❑ The movement of the pointer represent the amount of spindle 13 movement Dial Gauge Components MAIN COMPONENTS OF DIAL GAUGE External Circular dial body Round graduated dial Pointer Gear train Plunger/Spindle Laboratory Testing Devices Extensometer (measuring the extension of a material under load) ❑ Mechanical device ❑ An extensometer is a device that is used to measure changes in the length of a sample. It is useful for stress-strain measurements and tensile tests. Its name comes from "extension-meter". ❑ A device used to measure the axial deformations of the axially loaded elements. 15 Laboratory Testing Devices LVDT ❑ Electrical device ❑ Acronym for Linear Variable Differential Transducer ❑ Generally, transducers convert one type of energy (linear motion) of a sample to which it is coupled, into something you can measure on the other end, an electrical signal. ❑ LVDT is used to measure the small movements or deformations of the test specimens. ❑ LVDTs vary widely in sensitivity and range. The sensitivity of commercial LVDTs range from 0.003 to 0.25 V/mm. 16  Laboratory Testing Devices ❑ The basic principle is based on the mutual induction as the current flowing in one coil that induces a voltage in an adjacent coil ❑ Metallic part moving across a magnetic field. The output voltage related to the value of the movement (i.e. deformation) LVDT circuit 17  Laboratory Testing Devices LVDT Concept ❑ The device has a core shell structure. The shell contains one primary and two secondary electric coils. An electric voltage is input to the LVDT and an output voltage is obtained. When the core is moved, an output voltage is obtained. The relationship between the core position and output voltage represent the characteristic response (curve) of LVDT, where the deformation can be determined 18 Laboratory Testing Devices LVDT Response The response (Characteristic curve) is plotted between linear displacement to output voltage. 19 Laboratory Testing Devices Strain Gauge ❑ Electrical device ❑ Strain gauges are used to measure small deformations (strain) of a specimen within a certain gauge length. ❑ Gauge length varies from 5 mm to 15 mm. 20 Laboratory Testing Devices Strain Gauge ❑ Electrical strain gauge consists of foil or wire bonded to thin base of plastic or paper. An electric current is passed through the element (foil or wire). ❑ As the element is strained, its electrical resistance changes proportionally. ❑ The strain gauge is bonded via an adhesive to the surface on which the strain measurement is desired. ❑ As the surface deforms, the strain gauge deforms, the resistance changes. ❑ Since the amount of resistance is very small, ohmmeter cannot be used. Wheatstone bridge (resistance bridge) circuit can be used to detect small changes in resistance. 21 Laboratory Testing Devices Strain Gauge Concept ❑ Deformation of test specimen (because of the applied load) will be transferred to the gauge (due to bond). Increasing the length of the gauge will increase the electric resistance of the gauge. ❑ The increase of the electric resistance of the gauge is linearly proportional to the deformation occurred in the test specimen because of the applied load as given in the following equation :  = L R=ρ L/A L0 22 Laboratory Testing Devices Gauge Factor The sensor gauge factor (GF) of a strain gauge is a characteristic coefficient that relates the change of the resistivity of gauge element relative to its strain ε. In other words, GF is the ratio of the fractional change in resistance to the strain. = (R/R0 /L /L0) 23  Laboratory Testing Devices Difference between Strain Gauge & Dial Gauge Mechanical Dial gauge Electrical Strain Gauge Advantages Easy to use Small device Cheap Easy to use Can be used several times and Very high accuracy and precision for a long period Can read measurements from both Don’t need supplementary static and dynamic loads devices Can read strain in both compression and tension Used in research Disadvantages Large in size Need supplementary devices (read- Limited magnification for out units) deformation Expensive Measures deformation from Only one use for one position static load only Can be affected by temperature and Can not measure deformation humidity and must be isolated from dynamic load Can be affected by the internal friction between its parts 14 Laboratory Testing Devices Load cell ❑Force measuring device – measures the amount of the applied load. ❑A load cell converts a force (applied load) into digital values that the user can read and record. ❑They do this with strain gauges attached to the body of the load cell. 25 Laboratory Testing Devices 26 Laboratory Testing Devices Load cell ❑Strain gauge load cell widely used in industrial applications. When a load/force/stress is applied to the sensor, it changes its resistance. This change in resistance leads to a change in output voltage when a input voltage is applied. When under load, the body of the load cell deforms slightly. It can be used to calibrate machines. 27  Laboratory Testing Devices Load cell ❖ In this device, strain gauges are attached (bonded) to a member within the load cell, which is subjected to either axial loading or bending. ❖ An electric voltage is input to the load cell and the output voltage is obtained. Load cell output is measured using a digital meter. ❖ You can measure the output of a load cell from the digital value 28 Laboratory Testing Devices Data Acquisition system ❑ Data acquisition is the process of extracting, transforming, and transporting data from the source systems to the data processing system to be displayed, analyzed, and stored. ❑ A data acquisition system is a collection of software and hardware that allows one to measure or control the physical characteristics of something in the real world. 29  Signal Conditioning Analog and digital signals are the types of signals carrying information. The major difference between both signals is that the analog signals have continuous electrical signals, while digital signals have non-continuous electrical signals. The difference between analog and digital signal can be observed with the examples of different types of waves. Calibration What is calibration? ❑ Make sure that the readings shown by the testing machine represent the correct value of the load affecting the specimen. Target ❑ calculating percentage of error How testing machines are calibrated? ❑ By putting the load cell in the place of specimen in the testing machine When should you calibrate your machine? ❑ In the industry after manufacturing and before selling the machine ❑ First time after buying (new machine) ❑ When it is moved from one place to another ❑ Periodically each month 18 Calibration Calibration can be calculated by: ❑ % Error = {(Machine Reading – True load ) / (True load)} x 100 19 Definitions Accuracy ❑ To which extent a device can give a correct estimate and reading of the measured value. The accuracy value is corelated to the percentage of error. Sensitivity ❑ The least value, the device can read. Capacity ❑ The highest value, the device can read. Range ❑ The difference between the highest and the lowest values the device can read. Precision ❑ To which extent can a device read the same value when exposed to the same force. For example: A reading of 2.9 tons when applied actual load 3 tons, and the reading is repeated for the same value. 20 What does materials science and engineering study? Synthesis & Processing Composition Structure Materials Science and Engineering Materials science and Engineering Studies the composition and structure of materials across length scales from macrostructure to microstructure to control materials properties through synthesis and processing. Studying: Composition: the chemical makeup of a material. The combination of different elements, compounds, or substances that make up a material. Structure: The arrangement of materials' internal components. The description of the arrangement of atoms. Synthesis refers to how materials are made from naturally occurring or man-made chemicals. Processing means how materials are shaped into useful components to cause changes in the properties of different materials. The series of steps or operations used in the manufacture of raw-materials into finished goods. Materials science and Engineering Engineering project requires : Money - Men - Material - Method (4 M,s) Basic requirements for Material Selection : o Economy o Performance o Safety o Mechanical/non-mechanical properties Economy depends on : o Availability of raw materials o Cost: Raw material & Construction cost (manufacturing – transportation) o Maintenance 2 Materials science and Engineering Main Objective of a Materials Engineer Develop or select materials or devices that have the best cost performance ratio ( C P R ) for a particular application. (how much value we get for each unit of cost we spend). ❖ It is necessary for an engineer to be familiar with the properties of engineering materials. ❖ Only when you fully understand the materials’ properties, Right selection of materials can be made for a construction activity 37