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Mechanical Testing of Materials

The document provides an overview of mechanical testing techniques for materials, including destructive tests like tensile, hardness, and impact testing, as well as non-destructive tests. It emphasizes the importance of materials testing in assessing mechanical properties and quality control, particularly post-processing like heat treatment and welding. Various testing equipment and standards are discussed, along with explanations of phenomena such as creep and fatigue in materials under stress.

Engineering◦
Related topics:
Mechanical Engineering•Material Testing•
MATERIAL TESTING DESTRUCTIVE/ MECHANICAL TESTS JJ TECHNICAL SOLUTIONS (www.mechieprojects.com)
CONTENTS • INTRODUCTION • TENSILE TESTING • HARDNESS TESTING • IMPACT TESTING • CREEP AND FATIGUE TESTING
Why materials testing is required ? • To test mechanical properties like UTS, YS, % elongation etc. which is used as a design input. • Make informed choices in using raw materials. • To check properties of material after heat treatment. • To check material property after welding/ brazing etc. • Ensure batch and production quality • As part of maintenance and to prevent failure in usage Factor of Safety is the ratio comparing the actual stress on a material and the safe useable stress.
Two types of material testing!!! • Destructive Testing/Mechanical Tests – • The material may be physically tested to destruction or indentation. • To measure the strength, hardness, toughness, etc. • Example: Tensile testing, Impact testing, Hardness Testing etc. • Non-Destructive Tests (NDT) • Samples or finished articles are tested before being used and as routine maintenance checks. • Example: Radiography, Dye Penetration tests etc.
Tensile Testing • Is done on a Universal Testing Machine (UTM) • Uses a load cell and an extensiometer to apply measured force to an test specimen and %age elongation respectively . • The load v/s elongation is then plotted and studied • Variables such as strain, stress, elasticity, tensile strength, ductility and shear strength are measured and computed. • Test specimens can be round or flat. • Standard for Tensile testing: ASTM A370 (Standard: ASTM A370; ASTM B557; ASTM E8; Shore: ISO6892-1; EN2002-1)
 STRENGTH  DUCTILITY  ELASTICITY  STIFFNESS  MALLEABILTY  MODULUS of TOUGHNESS  MODULUS of RESILIENCE Tensile Test Equipment – UTM
A machine which applies a tensile force (a force applied in opposite directions) to the specimen, and then measures that force and also the elongation. This machine usually uses a hydraulic cylinder to create the force. The applied force is determined by system pressure, which can be accurately measured.
HYDRAULIC UNIVERSAL TESTING MACHINE • A universal testing machine (UTM), also known as a universal tester, materials testing machine or materials test frame, is used to test the tensile stress and compressive strength of materials . It is named after the fact that it can perform many standard tensile and compression tests on materials, components, and structures.
 Round Specimens: Forgings, Plates (t>20) etc  Flat specimens: Sheets (t<20), rubber, ceramics etc.  Standard specimen sizes available in ASTM A370/ IS 1608 standard  Major Test variables: Specimen size Strain rate- Different for metals, rubbers, ceramics etc
Specimen Nomenclature  Tensile testing at room temperature  Tensile testing at elevated temperature  Tensile testing  Fasteners  Forgings  Plates/ Sheets  Tubes/ bars  Polymers/ rubber etc
Tensile Test Results  Cup and cone fracture signifies a ductile material  A shear fracture indicates a brittle material Brittle Ductile Very Ductile (Ductile)
Glass (Brittle) Ductile M.S. -Ductile Aluminium Rubber High Carbon Steel
Youngs Modulus (E)
Proof Stress for Load – Extension graph Proof Stress for Stress – Strain graph Proof Stress  Sometimes the stress strain graph doesn’t has a distinctive Yield Point  For such material yield point is defined using load corresponding to 0.1% or 0.2% proof stress  The stress that causes a % increase in gauge length.  It can be found by drawing a line parallel to the straight part of the graph.  A value can be taken from the vertical axis.
HARDNESS TESTING (Standard: Brinell:ASTM E10; Rockwell: ASTM E18; Vickers: ASTME384; Shore: ASTM E2240) Hardness is the ability to withstand indentation or scratches Hardness measurement is also used to check response to heat treatment of a particular materials Types of Hardness Measurement  Scratch Hardness: Uses Mohs Scale  Indentation Hardness  Brinell/Rockwell/ Vickers: For Metals  Shore: Rubber, Polymers  Rebound Hardness: Uses Scleroscope
Hardness Testing Machine • The indenter is pressed into the metal • Softer materials leave a deeper indentation
Brinell Hardness Test (BHN) • Uses spherical shaped indentor. • Cannot be used for thin materials. Used for large components • Ball may deform on very hard materials. Used on softer material • Surface area of indentation is measured. Spherical indenter is shot with desired load force at the target The diameter of the indentation caused is recorded The diameter of the indentation tells about the Brinell Hardness Number (BHN)
Rockwell Hardness Tests (HRC) • Gives direct reading. • Rockwell B (ball) used for soft materials. • Rockwell C (cone) uses diamond cone for hard materials. • Flexible, quick and easy to use. • Used on forgings, castings etc. BHN = HRC × 10 (approximately)
Vickers Hardness Test (HV) • Measures Small Samples • Uses square shaped pyramid indenter. • Accurate results. • Measures length of diagonal on indentation. • Usually used on very hard materials and for surface hardness measurement
Impact Tests • Impact test determines material toughness. • Measure an objects ability to resist high-rate loading • The impact strength of a material in the presence of a flaw or notch and fast loading conditions is determined. • The amount of energy absorbed by a material before fracture, is determined. • Impact test: • Charpy Impact Test • Izod Impact Test (Standard: ASTM A370/ ASTM E23/EN 100045-1/ ISO 148)
Impact Tests Types of Notch Specimens Impact Test –Equipment Izod v/s Charpy Tests • Charpy Test • Izod Test • Types of Notch specimens • V notch • U notch • Keyhole notch
Izod Impact Test • Strikes from a height at =167 Joules. • Test specimen is held vertically. • Notch faces striker. • Strikes form higher position with 300 Joules. • Test specimen is held horizontally. • Notch faces away from striker. Charpy Impact Test Izod v/s Charpy
Creep  The elongation of a material due to constant high load at constant higher temperature is called creep.  Problems with creep increase when the materials are subject to high temperature or the materials themselves have low melting points such as lead. Creep can cause materials to fail at a stress well below there tensile strength.  During Creep testing, the tensile specimen is subjected to sufficiently high load and temperature, to produce a time-dependent inelastic strain  The strain in the specimen varies with time .  For an appropriate constant stress and elevated temperature, a strain-time plot ( creep curve is plotted (Standard: ASTM 1983/ BIS 1987/ ISO 1987) Creep Curve Creep Test Methodology
Fatigue  Fatigue is due to the repeated loading and unloading.  When a material is subjected to a force acting in different directions at different times it can cause cracking. In time this causes the material to fail at a load that is much less than its tensile strength, this is fatigue failure. Vibration for example is a serious cause of fatigue failure.  Fatigue can be prevented with good design practice. 1. A smooth surface finish reduces the chance of surface cracking. 2. Sharp corners should be avoided. 3. Corrosion should be avoided as this can cause fatigue cracks. (Standard: ASTM D7791/ ASTMF218/ ISO/TC 164)
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This is purely an academic work and has no financial or other interest. The authors are acknowledge the various reference sources and are grateful to all the people and firms who’s sources have been used The results achieved in this should be independently verified.