Experiment Izod and Charpy

8/13/2019 Experiment Izod and Charpy

1/7

Experiment: Mechanical Testing- Impact & Hardness TestingObjectiveThe main objective of this experiment is to introduce two important basic mechanical propert tests!hardness"using #oc$well testing% and impact "using harp testing%' hauvenet(s riterion will be applied toidentifoutliers and probabilit paper will be used to ascertain whether or not the repeated measure hardnessdata isnormall distributed'AbstractThe mechanical properties of materials are ascertained b performing carefull designed laboratorexperiments that replicate) as nearl as practical) the service conditions' *actors to be considered includethe nature of the applied load"s%) its( duration and the applicable environmental conditions' It is possiblefor the load to be tensile) compressive) or shear) and its magnitude ma be constant with time) or it mafluctuate continuousl ' +pplication time ma be for onl a fraction of a second) or it ma extend over aperiod of man ears' ,ervice temperature can also be an important factor'Mechanical properties are of concern to a variet of parties "e'g') producers and material consumers)research organi ations and government agencies% that have differing interests' onse.uentl ) it isimperative that there be some consistenc in the manner in which tests are conducted) and in the

interpretation of their results' This consistenc is accomplished b using standardi ed testing techni.ues'Establishment and publication of these standards are often coordinated b professional societies' In the/nited ,tates the most active organi ation is the +merican ,ociet for Testing and Materials "+,TM%! its

+nnual 0oo$ of +,TM ,tandards comprises numerous volumes that are updated earl ' + large number of the +,TM ,tandards are relate directl to mechanical testing techni.ues including specifications for measuring the hardness and impact properties of materials'Background Impact TestingToughness is a measure of the amount of energ a material can absorb before fracturing' It becomes of engineering importance when the abilit of a material to withstand an impact load without fracturing isconsidered' Impact test conditions were chosen to represent those most severe relative to the potentialfor fracture) namel ) "1% deformation at a relativel low temperature) "2% a high strain rate "i'e') rate of deformation%) and "3% a triaxial stress state "which ma be introduced b the presence of a notch%'Two standardi ed tests) the harp and I od) are commonl used to measure Impact Energ "sometimes

referred to as 4otch Toughness%' The harp 5-notch " 54% techni.ue is most commonl used in the/nited ,tates' *or both harp and I od) a 5-notch is machined into a bar specimen with a s.uarecrosssection'

+ standard 5-notch test apparatus is schematicall illustrated in *igure 1' The load is applied asan impact blow from a weighted pendulum hammer that is released from a coc$ed position at a fixedheight 6h6' The specimen is positioned at the base as shown' /pon release) a $nife edge mounted onthe pendulum stri$es and fractures the specimen at the notch) which acts as a point of stressconcentration for the high velocit impact blow' The pendulum continues its swing) rising to a maximumheight 6h(6) which is lower than 6h6' 0ased on the difference between h and h() the energ absorption of the specimen is computed' The primar difference between the harp and I od techni.ues lies in themanner of specimen support as is indicated in *igure 7' 5ariables including specimen si e and shape) aswell as notch configuration and depth can influence the test results'#olfe and 0arsom "189 % developed a correlation between 54 and *racture Toughness "; Ic% that isgivenb :


2/7

in a set of repeated measures data'


3/7

determining the ade.uac of the 4ormall ?istributed assumption'Example 1: The following length data were collected for the measurement of the length of a single steelrod'

+ppl hauvenetDs riterion to determine if suspect data points can be eliminated) plot the data onprobabilit paper) and comment on the normalit of the distribution'K#eading X i , cm1 7'L72 7'L33 7'37 7'LLK 7'L1L 7'LK

7'K89 7'L88 7'1 7'L9T e mean o, t e data set is given b+-#amp$e standard deviation can be computed as-The following table presents the data in #an$ rder) the Mean ?eviations "di% and the ratio of the mean

deviations to the standard deviation:ank Order /i cm1 di di (s C auvenet3s Criterion

1 7'3 '2K2 2'LK 7 #eject2 7'K8 ' 32 '33LK +ccept3 7'L1 ' 12 '12L2 +ccept7 7'L3 ' 9 ' 971 +cceptK 7'L7 ' 19 '1983 +cceptL 7'LK ' 29 '287K +ccept

7'LL ' 39 '388 +ccept9 7'L9 ' K9 'L1 +ccept8 7'L8 ' L9 ' 1K2 +ccept1 7' ' 9 '92 7 +ccept6? 4 T E IMI4+TE THE ?+T+ I4T *# M THE ?+T+ ,HEET6BBB

,ince d max / S for the ten readings is 1'8L "see Table 1%) the first data point can be eliminated from thedataanal sis) but to determine whether or not the data is 4ormall "Caussian% ?istributed) the data shouldnow betabulated in ran$ order with appropriate cumulative percentages and the rejected data points not included:"N,ince there are now onl 8 data points) the J 0elow Gi will be in increments of 1=8 G 1 or 11'11J%L

ank Order /i cm1 % Be$o4 /i1 7'K8 11'112 7'L1 22'223 7'L3 33'337 7'L7 77'77K 7'LK KK'KKL 7'LL LL'LL

7'L9 '9 7'L8 99'998 7' 88'88The data are now plotted on robabilit aper as shown below to evaluate the 4ormalit of the data'The mean value of the revised data set) without the hauvenet rejected data points included) shouldnow be computed' *or this problem)G #E5 O 7'LK cm)if the data is 4ormall ?istributed then an approximatel linear fit should pass through K J ver near 7'LK cm "see *igure K%' ne would conclude that the data appears to be linearl distributed based on


4/7

the probabilit plot shown in *igure K'#od ength) G i "cm%*igure 2 > Example ?ata lotted on robabilit aper'

Background " 5ardness TestingHardness is a measure of a material(s resistance to locali ed plastic deformation "e'g') a small dent or scratch%' Hardness testing involves a small indenter being forced into the surface of the material beingtestedunder controlled conditions of load and rate of application' The depth or si e of the resulting indentation ismeasured) which in turn is related to a hardness number! the softer the material) the larger and deepertheindentation) and the lower the hardness index number' Measured hardnesses are onl relative "ratherthanabsolute% thus care must be ta$en when comparing values determined b different techni.ues'Macro-Hardness "#oc$well and 0rinell% testing is the most commonl applied materials test in industrdue toseveral factors:1) Macro-Hardness Testing is simple to perform and does not re.uire highl s$illed operators!2) Through the use of different loads and indenters) hardness testing can be used for determining thehardness and approximate strength of most metals and allo s including soft bearing materials andhighstrengthsteels!3) Hardness readings can be ta$en in a few seconds with minimal preparation! and4) *or #oc$well hardness testing) no optical measurements are re.uired! all readings are direct'There are two common methods applied to measure material hardness macroscopicall : #oc$well and0rinell'#oc$well hardness differs from 0rinell hardness testing in that the indentation si e is measured in 0rinelltesting while #oc$well hardness is determined b the depth of the indentation made b a constant appliedload' *or thin test samples or samples for which the relativel large 0rinell or #oc$well indentations mustbeavoided) the ,uperficial #oc$well hardness test is often emplo ed' ,uperficial #oc$well hardness testingemplo s lower loads to the indenter in order to minimi e penetration'The ock4e$$ 5ardness Test consists of measuring the additional depth to which an indenter is forcedb a heav "major% load be ond the depth of a previousl applied light "minor% load as illustrated in*igures L and ' +pplication of the minor load eliminates bac$lash in the load train and causes theindenter to brea$ through slight surface roughness and to crush particles of foreign matter) thuscontributing to much greater accurac in the test'The minor load is applied first) and a reference or 6set6 position is established on the measuring device or the #oc$well hardness tester' Then the major load is applied at a prescribed) controlled rate' ?iamond- one 0rale Indenter used in #oc$well Hardness Testing "2G%' @Metals Hdb$) 1882A9*igure 7 > Indentation in a wor$piece made b application of "a% the minor load) and "b% the major load) on a diamond 0raleindenter in #oc$well hardness testing' @Metals Hdb$ - ?es$ Edition) 1882A'Hardness value based on the difference in depths of indentation produced b minor and major load'?iamond indenters are used mainl for testing materials such as hardened steels and cementedcarbides' ,teel-ball indenters) available with diameters of 1=1L) 1=9) 1=7 and 1=2 in') are generallused for testing materials such as soft steel) copper allo s) aluminum allo s and bearings metals'?esignations and t pical applications of various indentors are listed in Tables 2 and 3'Table 2 > #oc$well Hardness ,cale ?esignations for ombinations of T pe of Indenter and Major oad'@Metals Handboo$ - ?es$ Edition) 1882A8Table 3 > T pical +pplications of #egular #oc$well Hardness ,cales' @Metals Handbood - ?es$ Edition) 1882AThe minor load is alwa s 1 $g in regular #oc$well hardness testing' 4o #oc$well hardness value isexpressed b a number alone' + letter has been assigned to each combination of load and indenter) as


5/7

indicated in Tables 1 and 2' Each number is suffixed first b the letter H "for hardness%) then the letter #"for #oc$well%) and finall the letter that indicates the scale used' *or example) a value of L in the #oc$well

scale is expressed as L H# ) etc' ne #oc$well number represents an indentation of ' 2 mm" ' 9in'%'Therefore a reading of L H# indicates an indentation from minor to major load of "1 - L % G ' 2mm O

' 9 mm or ' 32 in' + reading of 9 H#0 indicates an indentation of "13 - 9 % G ' 2 mm O '1mm'The metal immediatel surrounding the indentation from a #oc$well hardness test is cold wor$ed thusmultiplereadings cannot be ta$en at the same point on a material(s surface' If multiple tests are conducted on asingle part the indentations should each be a minimum of 3 indentation diameters apart' The depth of material affected during testing is on the order of ten times the depth of the indentations'Therefore) unless the thic$ness of the metal being tested is at least ten times the depth of the indentation)an accurate #oc$well hardness test cannot be expected' In addition to the limitation of indentation depthfor a wor$piece of given thic$ness and hardness) there is a limiting factor on the minimum material width' Iftheindentation is placed too close to the edge of a wor$piece) the edge will deform outward and the #oc$wellhardness number will be decreased accordingl ' Experience has shown that the distance from the centerof the indentation to the edge of the wor$piece must be at least 3 times the diameter of the indentation toensure an accurate test'

+ fundamental re.uirement of the #oc$well hardness test is that the surface of the wor$piece beingtestedbe approximatel normal to the indenter and that the wor$piece must not move or slip in the slightestdegreeas the major load is applied' +s one point of hardness represents a depth of onl ' 9 inches) amovement of onl ' 1 inches could cause an error of over 1 #oc$well numbers' The support must beof

sufficient rigidit to prevent its permanent deformation in use' Indenters are not calibrated below values of2or above values of 8K) thus if readings are outside of this range then a different indenter must beselected'Minimum thic$ness re.uirements) conversions between various #oc$well scales and round wor$corrections1are tabulated and are available in the Materials Testing aborator 'The Brine$$ 5ardness Test consists of appl ing a constant load) usuall K to 3 $ilograms) on ahardened steel ball-t pe indenter) 1 mm in diameter) to the flat surface of a wor$piece' The K -$ilogramload is usuall used for testing nonferrous metals such as copper and aluminum allo s) whereas the3 -$ilogram load is most often used for testing harder metals such as steels and cast irons' The load is heldfor a specified time "1 to 1K seconds for iron or steel and about 3 seconds for softer metals%) after whichthediameter of the recovered indentation is measured in millimeters' This time period is re.uired to ensurethatplastic flow of the wor$ metal has stopped'0rinell hardness is evaluated b ta$ing the mean diameter of the indentation "two readings at right anglestoeach other% and calculating the 0rinell hardness number "H0% b dividing the applied load b the surfacearea of the indentation according to the expression:


6/7

HB = L/( D/2)[D - (D 2 - d 2)1/2]where is the load in $ilograms) ? is the diameter of the ball in millimeters) and d is the diameter of theindentation in millimeters' Highl hardened steel "or other ver hard metals% cannot be tested b ahardened steel ball b the 0rinell method because the ball would plasticall deform and flatten duringpenetration' Tungsten carbide balls are recommended for 0rinell testing materials of hardness from 777H0 up to L2 H0 "indentation of 2'7K mm in diameter%' However) higher 0rinell values will result when

using carbide balls instead of steel balls because of the difference in elastic properties' The degree of accurac that can be attained b the 0rinell hardness test is greatl influenced b the surfacesmoothness and therefore the test surface should be filed) ground) machined or polished with emerpaper such that the indentation diameter is clearl enough defined to permit its( accurate measurement'Ceneral precautions of 0rinell hardness testing include the following:1. Indentations should not be made on a curved surface having a radius of less than 1 inch'2. The load should be applied in such a wa that the direction of loading and the test surfaceare perpendicular to each other within 2 o '3. The thic$ness of the wor$piece being tested should be such that no bulge or mar$ showingthe effect of the load appears on the side of the wor$piece opposite the indentation' In anevent) the thic$ness of the specimen shall be at least 1 times the depth of the indentationThe 0rinell hardness test has three principal limitations as follows:1. ,i e and shape of the wor$piece must be capable of accommodating the relativel largeindentations'2. 0ecause of the relativel large indentations) the wor$piece ma not be usable after testing'3. The limit of hardness range -- about 11 H0 with the K -$g load to L2 with the 3 -$g load-- is generall considered the practical range'#uper,icia$ ock4e$$ 5ardness Testing emplo s a minor load of 3 $ilograms) and a major load of 1K)3 or 7K $ilograms' Pust as in regular #oc$well testing) the indenter ma be either a diamond brale or steel balldepending mainl on the ductilit of the material being tested' ,uperficial #oc$well hardness values arealwa s expressed with a number suffixed b a number and a letter that show the load=indentercombination'*or example) if a load of 3 $g is used with a diamond indenter and a reading of 9 is obtained) the resultisreported as 9 H#3 4 "where H indicates hardness) # indicates #oc$well) 3 indicates a major load of3 $g)and 4 indicates the use of a diamond brale indenter%'

+ll ,uperficial #oc$well hardness tests are started from the 6,et6 position' ne #oc$well superficialhardnessnumber represents an indentation of ' 1 mm or ' 7 inches' Therefore) a reading of 9 H#3 4indicates an indentation from minor to major load of "1 - 9 % G ' 1 mm O ' 2 mm or ' 9 inches'11

orrelation between Hardness and Tensile ,trength > 0oth tensile strength and hardness are indicatorsof ametal(s resistance to plastic deformation' onse.uentl ) the are roughl proportional' +s a rule of thumbfor steels "steels onl B%) the H0 and Tensile ,trength are related according toT# 6*a1 7 8.9: / 5B or T# psi1 7 :00 5B.

+ comparison=conversion between various hardness scales is provided as *igure K'*igure K > omparison of ,everal Hardness ,cales' @ allister) 1881A12

;ab e


7/7

sample is curved then adjust our data accordingl based on the #ound

Experiment Izod and Charpy

Documents

Transcript of Experiment Izod and Charpy