Impact Tests and Temper Brittleness

• Impact tests indicate a material's toughness, especially its capacity to resist mechanical shock
• Brittleness isn't always revealed during a tensile test

Temper Brittleness

• Nickel-chromium constructional steels can suffer from "temper brittleness" due to faulty heat treatment
• Tensile tests might show similar tensile strengths and elongations for both correctly and incorrectly heat-treated materials
• Impact tests would reveal the difference, as the unsatisfactory material is more brittle than the correctly treated one

Izod and Charpy Impact Tests

• A heavy pendulum on ball-bearings strikes a test-piece after release from a fixed height
• The tests differ in the test-piece form and the pendulum's swing height
• Striking energy is partly absorbed in breaking the test-piece

Energy Measurement

• A drag pointer leaves marks at the pendulum's highest point of swing
• The amount of mechanical energy used in fracturing the test-piece is indicated

Testing Machine details

• The Avery-Denison universal impact-testing machine can be used for Charpy or Izod impact tests
• For the Izod test, the release position pendulum is from the lower position, the striking energy is 170 J
• For the Charpy test, the release position is from the upper position, the striking energy is 300 J
• Each test has its own scale graduation set

Izod Impact Test

• This test uses a standard notched test-piece clamped firmly in a vice
• The striking energy is about 163 J
• The notch initiates fracture, and its dimensional accuracy must be standard
• A standard gauge ensures the dimensional accuracy of the notch

Fractured Cross-Section Examination

• Most ductile materials = 'fibrous' nature, appears dull and 'silky'
• Plastic flow of the crystalline structure has occurred
• Very brittle materials = bright, sparkling, and 'crystalline'
• Crystals aren't plastically deformed
• Fracture follows the crystal boundaries
• Many metals = combination of ductile and 'crystalline' areas

Measurement of Notch-Ductility

• For steels it possible to estimate % of crystalline area of the fractured surface of the test-piece
• It can be used as a measure of the notch-ductility

Izod Impact Test on Metal Specimen

• Aim of the test is to determine the impact strength through the Izod test
• An impact testing machine and a metal specimen are used

Impact Test Procedure

• Fit the proper striker to the hammer's bottom with the clamping piece for impact test
• Fit the latching take for impact test firmly to the bearing housing at the columns' side
• Determine the frictional loss by free fall test, raise the hammer by hand
• Latch in, release it by operating lever
• Confirm that friction loss doesn't exceed 0.5% of the initial potential energy; add if needed
• Fix the Izod test specimen in the support using clamping screw and key; notch faces pendulum striker
• Release the pendulum; operate its brake after one swing
• Read the reading pointer on dial and note the indicate value. Lastly, remove broken specimen
• Notch impact strength depends on specimen and notch shape, therefore, the values may not be compared

Charpy Impact Testing vs Izod Impact Testing

• Materials tested: Metals vs. Plastics & Metals
• Types of notches: U-notch and V-notch vs. V-notch only
• Specimen position: Horizontally, notch away from the pendulum vs. Vertically, notch toward the pendulum
• Striking point: Middle of the sample vs. Upper Tip of the sample
• Common dimensions: 55 x 10 x 10 mm vs. 64 x 12.7 x 3.2 mm (plastic) or 127 x 11.43 mm round bar (metal)

Precautions

• Measure specimen dimensions carefully
• Locate the specimen so the hammer strikes the middle
• Note down readings carefully

Charpy Impact Test

• Originates from continental Europe
• The test-piece is supported at each end (unlike the Izod test)
• The Izod test uses test-piece held cantilever (supported on only one end)
• Pendulum load varies so that impact energy is either 150 J or 300 J

Impact Properties and Defects

• Defects such as cavities, slag particles, and grain-boundary segregates have a limited effect on tensile strength and hardness
• They impair impact properties
• Defects act as stress raisers, especially under shock loading

Impact Test Results

• A much greater 'scatter' of results occurs on a single cast material
• A casting is likely to be less homogenous in its structure that is a wrought testing material
• Directionality fibre can have effect on impact toughness than on tensile strength or ductility in wrought materials

Charpy Impact Test on Metal Specimen

• The aim is to perform the Charpy impact test on materials
• The Vernier calipers and steel rule for measurements
• Measures a material's resistance to fracture under sudden load
• Mild steel is used and the test pieces are square

Impact test theory

• Able to absorb energy during plastic deformation, known as toughness of a material
• The test specimen used is a Square Cross-section, the specimen may have 1-3 notches
• An impact test signifies ability of material to absorb energy during plastic deformation
• Test Specimen : Square Cross section notch(es)

Testing Machine Specifications

• Top face of grips and face holding the specimen vertical, angle is 90°
• Tip of hammer angle is 75°°10
• Angle between normal to the specimen and underside face of the hammer at striking point is 100°C10
• Hammer speed at impact = 3.99 m/sec
• Striking energy is roughly 168 J

Energy Relations

• Longitudinal axes must lie in the swing plane of the hammer's center of gravity plus notch is positioned at the top of the grips

Energy Relations Calculation

• Impact strength is the energy absorbed in breaking the specimen, the equal difference between energy in the pendulum before and after impact

Losses and Corrections

• Corrections may needed for air bag loss, friction in machine bearing, and energy used in broken piece movement
• Can be determined follows, angle B,, indicator at zero and without the pendulum swinging.

Reporting Test Results (ASTM E 23)

• Type, model, capacity type, size of Machine used
• Type and size of specimen used
• Maximum linear velocity
• Energy loss due to friction. Plus energy of blows/specimen absorbed
• Temperature of specimen
• Appearance of surface following fracture
• Amount of specimens that failed to break

Fatigue Testing

• Fatigue testing is about a material under fluctuating or action of 'live load'
• Fatigue Failure: the press invariably hints darkly at 'metal fatigue'
• Metal Fatigue : effects of fluctuating and alternating force to a member in engineering, it will be subjected to a live load

S/N Curves

• Wohler Machine: Alternates stress of a test-piece, and reduce reasonable time for the fatigue test
• Infinite Reversals approx. 20 million cycles
• The S/N Curve: stress S is plotted against cycles endured

Fatigue/Endurance Limit

• Curve becomes horizontal at a stress, which will be endured for infinite reversals
• Non-ferrous Materials do not have a defined limit
• A limited life is the only design option for eventually regardless of the stress. The material fails to fatigue

Fatigue Failure Characterisitcs

• Failure may begin with a keyway, a fillet, a microstructural defect, or even a tool mark
• Due to unforeseen high frequencies
• Operating stress fluctuates or alternates at its fatigue limit

Fatigue Testing

• Determines fatigue behavior using fatigue testing machines

Fatigue Terminology and Theory

• Subjecting a material below its yield point can result to failure from crack formation (Fatigue)
• Fatigue is affected by component size, relative magnitude of static and fluctuating loads, and the # of load reversals
• During rotation, the bending stress at the upper fibres varies : Max Compressive to Max Tensile
• During rotation, bending stress rotates lower fibres from max tensile to : Max compressive
• Standard mirror polished specimen is rotated fatigue testing machine, while loaded ending causes specimen to be subject to : Completly reversed stress cycle

Fatigue Limit

• Stress is infinite below dotted, the material never fails
• The stress (the dotted line ) is defined as the max value of completely reversed bending stress)
• Stress vs time fluctuate with Omin and Omax( stress and completely reversed with O V

Material Fatigue, S-N , Bending

• S vs. N curve: (Wohler curve) plots materials fatigue and endurance
• The graph shows Stress amplitude , difference between max and min stress divided and the # load reversals
• Log Scale numbers graph and the results
• If the cycling is done indefinitely , it is done below

Fatigue Testing Machine Basics

• Sample is subject to Bend force
• This force bending is when or where sample and how sample is supported

Test Procedures

• Place the circular, specified dimensions Circular sample
• Place the set up and apply Force via bending , hang sample weight
• Start Test, sample will the alter surface
• Let The Machine do Its work and calculate cycles
• Take Results
• When the test has started, the sample has rotated at desired rate
• Sample will Interchange, experience alternate Tension
• Stop at ,Controller will cycle Precautions
• The specimen must be into the machine
• Accurately Take the readings
• Carefully loads

Metallographic Examination

• Metal, surface for Micro structure w/ etching to make an analysis
• Metallography allows alloy identifier, predictive capabilities
• The materials must have a structural quality in order to find preparation

Metallography Aims

• Find something failed
• Improve the designs /select metals

Preparatory process

• In short, Planar surfaces, and Polish
• Chemical
• Other etchin

Metallography procedure

• Use for characterization/failure mode
• Weld, corrosion

Preparation and examination

• Grinding Step: remove any and prepare site
• Polished Cloth types : woven, hard
• To polish you will need:
• Diamond
• Diamond
• Spray

Etching

• The layer of chemical etching
• Leading surface, grain size
• Always do In small increments to avoid overdoing it

Examination

• Must be able to
• Do macro and micro
• Follow procedures

Welded Sections process

• Follow metal graphic procedure
• Immerse 2 min saturated to remove lossely adhering product
• For materials analysis metallography helps determine correct processing and material failure reasons

Grinding

• Purpose is to remove damage from cutting, planarize the speciment
• It removes material approaching the area of interest
• Abrasive is silicon carbide
• Application procedures involve:
• Soft Non ferrous: initial grinding with 320SiC, followed with 400,600,1200
• Soft Ferrous: 240 grit initial, with 320, 400, 600
• Hardware: remove 120-180 grit
• Ceramics are hard, the semi fixed minimizes the forms
• Composites: initial is meteral. Secondary is focusing on the ceramic.

Polishing

• Is the most important, completely eliminate precious damage
• Is used to remove the fine grinding
• Factors include cloth, abrasive quality’s, pressure polishing’s
• • 3 CLOTHS
• Woven cloth, ‘Hard polished
• Non Woven
• Flocked cloth- Super polished/shorter polishing
• Diamond: cut ability, due to its hardness

Diamonds

• Poly -lots small cuts, results in high removable and shallow
• Mono: few cuts and blocklike

Etching process

• Removes the surface
• The etchant attacks

Microscopic Examination

• Cut to size , resin in mold
• Polished / Etched in chemical

Theory

• Structural exams provide by list
• Grain area sizes

The process analysis

• Sulphur: amounts have to minimum and even
• Steel grade
• Detect in steel
• Show silver

Inspection

• The inspection must be the standard
• If not it would reject to meet code
• Detection and removing faulty materials
• To not lower by letting it reach customers

Purpose of inspection

• Detect good part from bad and why
• If process is is approaching correct tolerance range
• The measure of process

Methods

• Two types 100% inspect or sample inspection
• Inspect means to look at strategic locations
• If its not destruction you sample

Sampling

• Destruction test
• Widly using more now with machines and tools
• Automatic reduces variables

Back draws

• Its not its value, to why part
• Its still up to human to pass

Tolerances

• That should
• ASME - That specific can have tolerance
• Ensure that
• Cost reduction
• To be acceptable

Important Cons

• To see everything fits
• Tolerance and accumulate , and may make shaft to large
• You test every dimension
• The device must
• May want add
• Its not only good just test
• Dimensions are and are
• To also test your finishes, such as electro plating

Inspector responsibilities

• Perform visual
• Assist in receipting
• Verifence to standards
• Prepare documents plus maintanance . ISO
• Communication and be the trainer for others
• Log , collect
• Methods , procedures

Impact tests can assess a material's toughness and resistance to mechanical shock, which tensile tests may not reveal. Temper brittleness in nickel-chromium steels, caused by heat treatment, reduces toughness without affecting tensile properties. Izod and Charpy tests use a pendulum to measure energy absorbed during fracture.