Materials Lab: Introduction (PDF)

Materials Lab: an Introduction Safety and Mechanical properties  Office and Office hours  Ahmed Mohamed Gabr  Ismail Medhat  [email protected]  [email protected]  S1.414  S1.211  Office hours: TBD  Office hours:...

Materials Lab: an Introduction Safety and Mechanical properties  Office and Office hours  Ahmed Mohamed Gabr  Ismail Medhat  [email protected]  [email protected]  S1.414  S1.211  Office hours: TBD  Office hours: TBD  Grade distribution Graded Items % of Final Grade Classwork 10% Quizzes 20% (Best 1 out 2) Midterm exam 20% (Theoretical) Project 30% Report 10% Presentation 10% 100% Materials Lab, what is it? Materials Lab, what is it? Safety in lab  Keep the lab area clean at all times. Place trash in appropriate receptacles. Return equipment immediately to where it was taken from.  Never use lab equipment that you are not approved or trained by your supervisor to operate.  If an instrument or piece of equipment fails during use, or isn't operating properly, report the issue to the lab engineer right away. Never try to repair an equipment problem on your own.  Never leave an ongoing experiment unattended.  Never smell or taste chemicals.  Report all injuries, accidents, and broken equipment or glass right away, even if the incident seems small or unimportant.  Don't Eat or Drink in the Laboratory Mechanical Properties An overview Why study Mechanical properties? Why study Mechanical properties?  Understand the capabilities of the materials at hand  Notice the behavior and reaction under certain loads  Predict the failure rate and determine a lifetime Objectives  Understand the reason for studying Mechanical properties and testing  Get a feel for some terms like Stress and Strain  Differentiate between types of stress like the tensile and compressive  Be able to do some analysis on the Stress-Strain Curve Types of Mechanical Properties  Stiffness Young’s Modulus or Elastic Modulus  Toughness and Resilience Measure of how metal absorbs energy  Ductility The ability to deform plastically without fracture  Strength The maximum Load material can handle before fracture  Hardness Resistance to abrasion and indentation. Force vs ∆ L  Normalizing the Force into Stress by dividing over the Area  Normalizing the Delta L into Strain by dividing over the L Stress (σ)  Stress is a measurement of how the material reacts to external influences.  It's simply the external pressures divided by the material's cross sectional area. Types of Stresses Strain (ε)  Strain is defined as the amount of distortion experienced by the body as a result of applied force divided by the body's initial dimensions. Types of Strain  Lateral Strain  Shear strain Volumetric strain   Longitudinal strain  Normal Strain  Tension  Compression Stress (σ)  Different types of stresses depending on:  Direction of the load  The cross sectional area affected  Normal  Parallel Direction of the Load  Axial/Vertical  Shear or Transverse Tensile Test  To understand the behavior of materials under axial tensile (pulling away) loading, the tensile test is performed  Materials investigated in it :  Metals  Plastics  Some natural materials  Ceramics (but rarely) Stress-Strain Curve  Stress-Strain curves are plotted to visualize the mechanical properties of the materials.  Key Properties determined:  Elastic Modulus  Yield point  Ultimate Tensile Strength  Ductility Elastic and Plastic Region  There are 2 zones in most Stress-Strain Curves:  Elastic Region; no permanent deformation after load is released. The linear part of the curve (The grey region)  Plastic Region; permanent deformation after the load is released. The non- linear part of the curve (The white region) Elasticity (E)  The ratio of direct stress to longitudinal strain under elastic limit is known as young modulus of elasticity (denoted by E)  It has the same unit as the stress (N/m2 or Pascal (Pa)) the most common unit being the GPa or the MPa  It is a material related property and may be changed with some heat treatments or mechanical action.  The higher its value the more rigid the material and vice versa. Yield point  It marks the end of the elastic region and the start of the plastic (non-linear region) Yield point  If the graph is not linear from the start, to determine the yield point we take an offset yield point at 0.2% strain for Metals and 5% strain for plastics Strength vs. Stress  Ultimate Strength is the MAXIMUM stress the specimen can hold before fracturing  Thus Strength is different than Stress  One is the maximum  Other is the reading at different load Yield vs. Ultimate tensile Strength  Failure at the Yield is an  At the Ultimate the material excessive deformation fractures completely, thus  It will damage the catastrophic failure surrounding structure or stop  If it was elastic, and was in working correctly but it will application, thus it would not break thus the failure is have deformed plastically contained ruining the structure surrounding it Ex. Determine the tensile strength of this alloy  The maximum stress on the complete stress strain plot  The tensile strength is approximately 370 MPa Ductility  Ductility is the percent elongation until fracture point, the larger the more ductile and malleable the material is.  Calculated using the change of the length divided by the original length  The strain is an indicator of the material’s ductility Ductility  Ductility is observed in the specimens here loaded under tension until failure  The most ductile will have a pronounced necking effect and a decreasing diameter till fracture point (c)  The most brittle will show a sharp fracture surface perpendicular to the direction of the loading (a)  With intermediate ductility a measure of necking and sharp surface will appear (b) Stress-Strain Curve: Different Materials Different Materials have different Stress- Strain curves  Ceramics show highest elastic modulus YET brittle  Plastics show most ductility yet least strength  Our bones seem to have no separation between the elastic and plastic region  Most metals increase linearly in the Elastic region, then non-linearly in the Plastic region Stress: Engineering vs. True  As materials are put under stress (for example Tension), the diameter decreases as the length increases, called Necking  When calculating the Stress applied, the variable Force should be divided by the changing cross-sectional area; this is called True Stress  For Practical Applications that is difficult to do and the variable Force is divided by the initial cross-sectional area; called Engineering Stress Compression Test  Understanding the behavior of materials under compressive loading  Some applications mainly have a compressive load, which makes this test perfect for them:  Concrete  Bars and Beams in buildings  Some metals Compression Test  Understanding the elastic compressive fracture of Brittle materials  Determining the Ductile fracture limits of materials Ex.  A load of 10,000 N is applied to a cylindrical specimen of a steel alloy that has a cross-sectional diameter of 12 mm.  Will the specimen experience elastic and/or plastic deformation? Why?  If the original specimen length is 500 mm, how much will it increase in length when a stress of 150 MPa is applied? Is it a permanent elongation? Ex.8 solution  Area= 113.09mm2 Stress=88.419MPa < 350 therefore Elastic  Elastic modulus can be calculated 200MPa/0.001= 200GPa  At stress= 150MPa, strain equals 150/200,000= 7.5x10-4  Strain= delta L/ original Length, therefore delta L= 0.375mm  That is the increase in length and it is NOT permanent because the stress is in the Elastic region References  https://weldguru.com/mechanical-properties-of-metals/  https://www.bu.edu/moss/mechanics-of-materials-stress/