Tech BriefsTensile Testing Copper Berylliumby Technical Service DepartmentBrush Wellman Inc.

Copper beryllium’s mechanical properties are mostfrequently measured by the simple uniaxial tensile test.The test provides information that can be used incomponent design. Tensile test information is also usedin materials acceptance and in the process control ofoperations such as stamping, bending, rolling,machining, drawing and slitting. The test itself isrelatively simple, but interpretation and use of the testdata for copper beryllium alloys require a thoroughunderstanding of both the test procedure and the alloy'sbehavior during testing.

The Tensile Test

Depending upon the sample configuration and the testequipment available, a wide range of test fixtures, grips,and stress and strain measuring devices can be used.Brush Wellman uses the test procedure described inASTM E-8 (Standard Test Methods of Tension TestingMetallic Materials) for measurement and certification oftensile properties on all copper beryllium shipments.This test procedure allows wide latitude in selection oftest conditions, equipment, and sample configurations.

Copper beryllium’s properties are moderately strain ratesensitive for test speeds in the strain rate range of0.005-0.2 min-1. The test is conducted at a constantspeed or strain rate, although the speed may bestep-increased after the sample reaches the yield stress inorder to minimize testing time for high elongationalloys.

For testing of copper beryllium strip, reduced section(“dog bone”) test pieces are most often used, but straightsided strip samples are also acceptable. The test sampleaxis is oriented along the longitudinal or the rollingdirection of the strip. Any sample preparation techniquethat produces a smooth, stress-free edge can be used.The edges should be lightly polished with emery cloth toremove nicks or burrs that can cause premature andinaccurate test results. Brush Wellman’s testing of strip

products uses a 0.5 inch (12.5 mm) wide test samplewith a 2 inch (50 mm) gauge length.

For heavy gauge or bulk copper beryllium products,tensile test bars are machined to either a 0.35 inch (9mm) diameter by 1.40 inch (36 mm) long gauge sectionor 0.5 inch diameter by 2 inch gauge section. As withstrip samples, ASTM E-8 requires a gauge length that isat least four times the gauge diameter. Tensile barmachining is done to a 63 microinch (1.5 µm) maximumrms surface finish, with at least 0.003 inch (0.08 mm)depth cut on the final machining pass to minimizesurface stress in the bar.

The output from the tensile test is the engineeringstress-strain curve shown schematically in Figure 1.Stress, on the vertical axis, is the test load divided by thecross sectional area of the sample. Stress is expressed inunits of pounds/in2 (psi or lbf/in2), thousands ofpounds/in2 (ksi) or, in metric units of Newton’s/mm2

(N/mm2) or megapascals (MPa). A megapascal isdefined as 1 N/mm2. Less frequently, metric stress isgiven in kilograms/mm2 (kg/mm2 or kgf/mm2). Table 1lists conversions for stress units.

psi x 1000 = ksiksi x 6.895 = N/mm2

ksi x 6.895 = MPaksi x 0.703 = kg/mm2

kg/mm2 x 9.807 = MPaMPa x 1 = N/mm2

MPa x 0.102 = kg/mm2

MPa x 0.145 = ksiN/mm2 x 0.145 = ksikg/mm2 x 1.422 = ksiTable 1. Conversion Factors for Stress Units

Strain, on the horizontal axis of the stress-strain curve, isthe elongation of the test sample divided by the length ofthe gauge section. Since the units of strain are inch/inchor mm/mm, strain is expressed either as a dimensionlessdecimal or as a percentage. Sometimes the gauge lengthis provided with the strain units such as “% in 2 inches”.

Analysis of the stress-strain curve allows the de-termination of yield strengths, tensile strength, elastic(Young’s) modulus, uniform strain, and total strain.Reduction in area measurements, when required forround test samples, are made from the test specimensafter completion of the test.

The Stress-Strain Curve

The initial portion of the stress-strain curve (Figure 1) islinear and the numerical value of the slope (stressdivided by strain) of the linear portion is the ElasticModulus of the material. The elastic modulus, alsocalled Young’s Modulus, measures the resistance of thematerial to small deformations and is a measure of thestiffness of the material. The greater the elasticmodulus, the smaller the strain resulting from theapplication of a given stress. The traditionaldimensional units of elastic modulus for metals aremillions of psi (msi), thousands of ksi or, in metric units,gigapascals (GPa).

STRAIN

STRESS

Total Strain Uniform Strain

Tensile Strength

Yield Strength

Elastic Limit

Elastic Strain Region

Plastic Strain Region

Figure 1. Stress-Strain Curve

An alternative stiffness measurement, the secantmodulus, is sometimes used in design as an indication ofapparent stiffness when the test sample is strainedbeyond the elastic limit, as shown in Figure 2. Calcu-lation of the secant modulus requires an accuratestress-strain curve and a specific strain value. Thesecant modulus is not a pure materials property in that adesign element (the strain value) influences the value ofthe secant modulus. The secant modulus will always beless than the elastic modulus.

The initial linear portion of the stress-strain curve iselastic. When the load is removed, the sample willexhibit no permanent set. Deformation beyond theelastic region and into the plastic strain region will

always result in some permanent set or strain when theload is removed and the elastic deformation is recovered.

STRESS

STRAIN

Slope = Elastic Modulus

Slope = Secant Modulus at e1 Strain

e1

Figure 2. Elastic Modulus and Secant Modulus

The Yield Strength of the material is defined as the stressrequired to generate a given permanent set in the testsample. To be unambiguous, the yield strength shouldbe specified with its numerical strain or permanent setvalue, such as 0.01%, 0.2%, or 0.5%. The 0.2% yieldstrength (also called 0.2% offset yield) is the mostfrequently measured yield strength, and when thenumerical strain value is omitted, it is assumed to be0.2%. Prior to computerized tensile test equipment, theyield strength was determined graphically from the stressstrain curve by constructing a line parallel to the elasticdeformation line, and offset to the right of the origin bythe appropriate strain value. The intersection of theconstruction line with the stress-strain curve is the yieldstrength. The yield strength measurements areinfluenced by the accuracy of this construction whetherit is performed by a computer or done graphically.

The transition point between the elastic and plasticregions (the 0% yield strength) is termed the ElasticLimit. For many nonferrous alloys, the transitionbetween elastic and plastic behavior is very gradual.The elastic limit, or any low offset yield strength meas-urement, is difficult to precisely determine withouthighly sensitive equipment. Computer generated elasticmodulus and elastic limit measurements are provided bynewer test equipment, but their accuracy can besignificantly influenced by inaccurately determinedslopes in the elastic region. Inaccurate slopemeasurements can be caused by failure to compensatefor flexure of the test machine or by the early onset ofplastic deformation in the test sample. The precisionelastic limit test measures the yield strength at 0.0001%offset (1 microstrain or 1 microinch strain per inch ofgauge).

Because of the measurement difficulties, elastic limit andvery low offset (below 0.2%) yield strengths are notroutinely reported for copper beryllium. Information isavailable from Brush Wellman’s Customer TechnicalService Department.

As the stress-strain curve moves further into the plasticregion, the stress required to accommodate theelongation of the test sample continues to increase untilit reaches a maximum. The maximum is called theTensile Strength or ultimate tensile strength (UTS). Upto this point in the tensile test, the test sample haselongated (and reduced its cross section) uniformlyalong the gauge length. At the maximum strength ortensile strength, the tensile test piece becomes dimen-sionally unstable and additional deformation is non-uniform and very localized. The test piece begins toneck down at the point where it will eventually fracture.This is reflected in the stress-strain curve by the decreasein stress for strain values above the ultimate tensilestrength. Because of the necking of the test piece, thepoints of maximum stress and maximum strain are notcoincident.

Strain or Elongation measurements from the tensile testprovide an indication of the alloy’s ductility orformability. Total strain is the value most frequentlyreported, and as shown in Figure 1. It is the strainrecorded at the conclusion of the test when the testsample fails. Total strain includes the elastic strain, theuniform strain and the nonuniform strain during thenecking of the test sample after the material reached theultimate tensile stress. In component design work, theuniform strain value is of greater importance than thetotal strain since it measures only the “usable” deforma-tion, to the point of the ultimate tensile stress, themaximum allowable design stress. On the other hand, inmaterials characterization for metal processingoperations such as machining, stamping, or slitting,where the metal deformation leads to fracture, totalstrain is more meaningful than uniform strain.

Interpretation of Test Data

For each shipment of its copper beryllium product,Brush Wellman will certify the following standardtensile properties: ultimate tensile strength, 0.2% offsetyield strength, and elongation. While thesemeasurements will, for most applications, characterizethe alloy’s performance, component performance insome applications may be affected by the alloys nearelastic properties (low offset yield strength) for whichthe standard tensile test is insensitive. When alloy

samples, showing identical tensile test certifications,perform differently in service, more sensitive tensiletesting or additional materials characterization may berequired to identify the problem.

Tensile test data will accurately reflect performancewhen the use conditions closely approximate the actualtest conditions. This rarely occurs since most useenvironments impose stress conditions which are morecomplex than the uniaxial tensile test. While tensile testdata may provide indications of performance underconditions of compression, bending, or plane strain,additional information is required to accurately predictperformance under non-tensile conditions.

Hardness testing provides an indication of materialstrength, but since it does not measure the alloy’sstrength, it cannot be used as a substitute for tensile testdata. The hardness test evaluates a relatively smallvolume of metal, and can be influenced by amicrostructure which is not homogeneous. The tensiletest, because of the size of the test piece, is less sensitiveto variations in structure.

For heavily cold or hot worked products, tensileproperties are not isotropic. The relationship of theoff-axis tensile properties to the properties reported inthe longitudinal direction depends upon the property(modulus, strength, elongation), the alloy, the micros-tructure, and the degree of deformation in the material.In cold worked strip, the transverse elastic modulus isslightly higher and elongation slightly lower thanmeasured in the longitudinal direction.

Tensile test ranges for all tempers of copper berylliumalloy products are provided in Brush Wellman’spublication, “Guide to Copper Beryllium Alloys”.

Unless otherwise specified, tensile properties aremeasured at room temperature. Near room temperature,-100-300°F (-70-150°C), copper beryllium’s tensileproperties are temperature insensitive. Tensile data atelevated and cryogenic temperatures are available fromBrush Wellman.

If you require further information or technicalassistance, please contact Brush Wellman’s CustomerTechnical Service Department at (800) 375-4205.

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