ISO 6892.pdf

M etallic Ma te ia ials ls fo forr T Te e ns nsiil Testi g | IS

6892-1:200

Pa t 1: Met o d o f Te Test at Ambient Te peratu e D e s cription o the Testi ng Requir ments

strai n rate can be, unambiguou ly, based on the strain

The ew ISO 6892-1:2009 (Met llic materials – Tensile

mea sured by the xtensometer and the control of the strai rate can be achie ed using the ignal from the exte someter.

testi g – Part 1: Method of Test at Room Temperature) is a signi icant event f r anyone perf orming tensil tests on metallic materials. The new standard replaces both the previous version o ISO 6892 and the widely-used EN 100 2-1:2001 standard. ISO 892-1:2009 incorporates i

any detailed

impr vements over the older ve sion; howeve , the most signi icant change are in the area of test con rol, where the new standard introduces a new test contr l criteria “10. Testing Rate Based on Strain Rate Control (Method A)”. The aim of the new test con rol is to reduce the varia ion on the m chanical pro erties by red cing the variability in the te sting conditio s. The essential feature o the new test contr l is the requi ement to maintain the strai rate applied to the test pi ce within ±2 % of the specified rate. This contrasts with the test control requirements of the EN10002-1:2001 and th older versi n ISO 6892, which specifi d a combination of stress and train rate control (stress rate in the elastic region

igure 1 – Test sp eeds at different calculation points for ISO 6892-1 ( ethod A and Me thod B).

follo ed by strain ate) and allo ed considerable variation in th rates e.g. a :10 variation in the strain rates when dete mining Lower Yield (ReL) and only an upper limit on the strai rate when d termining Pr of Strength ( p) (Figure 1). Sinc the mechanical properties of many met llic materials, including commonly used steels, d pend on strai rate, reduci g the variability in the strai rate will increase the preci ion of the measured prope ties (Figure 2). In addition to reducing the allowed variation i the strain rate or all tests, the new standard also clarifi s the test conditions for the wo types of yield behavior isplayed by metallic materials. For metals that demonstra e a smooth transition from the elastic to plastic region (Fi ure 3), the strai distribution in the gauge section of the aterial is uniform through the offset yield (Rp) and up to the

Fiigure 2 – Tests s eeds at 6MPa a d 60MPa to ISO 6892-1:2009 ( ethod B).

maxi um tensile stress (Rm). In this case, the definition of

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Figure 3 – Without and with yield po ints graphs.

Man metals, how ver, display a “yield point”. In this case, the s ress increas s in proportion to the strain (the elastic

new standard intr duces the re uirement that the rate chan ge should be gradual in or er to avoid in troducing a

region) and then a the upper yi ld point (ReH) there is a sudden decrease in the stress. urthermore, hen a metal

stee p change in t e stress–strain curve, which could poss ibly be misint erpreted as a material characteristic.

yield in this way t e strain distriibution along he parallel

Figu e 4 shows ho w an abrupt strain rate increase could resullt in a false R result.

length is no longer uniform and instead it is localized in narrow regions kn wn as Luders bands. Thes Luders bands often form utside the ex ensometer g uge length and nder these c nditions the train measured by an exte someter can actually decr ase despite t e fact that the s rain over the entire parallel section of the specimen is increasing. In this egion it is im ossible to co trol the strai rate using the signal from the extensometer. In the lates version of ISO 6892-1, it i made clear hat the strain rate fter a yield is to be based n the extension rate applied divided by the parallel length of the specimen. ISO 892-1:2009 utlines two distinctly different methods of strain rate control, eLe (strain rate) and eLc (estimated strai rate over th parallel leng h). eLe is bas d on direct feed ack from the extensometer or closed-loop strain control, whereas e c is based on a calculated ate and is in closed-loop crosshead position ontrol.

Fiigure 4 – Fast sp eed change show ing false Rm.



stan dard. Using a intelligent algorithm, the Instron®

Inst ron Soluti n Instr n testing ma hines are able to meet the demanding requirements of ISO 6892-1:20 9, both Method A, based on strain rate control, and Meth d B, based o stress rate.

mac hine swaps to position control, as detailed in the stan dard, allowin it to maintai the standar defined esti ated strain r ate through the discontinuous yielding regi n. At the end of this yielding region with he onset of strai n hardening the machine t en moves to a final rate

M at erials Tes ing Machiines

that it maintains u ntil the concl sion of the test.

Our electromecha ical or static- ydraulic machines can be equi ped with a range of clip-on or high-resol tion auto atic extensometers for strain rate contr l. With many gripping solutions vailable, Instron® has a suitable gripping mechanism for almost ll material ty es.

M ethod A M at erials wit

No Yield Point

Figure 5 shows a t pical curve o a specimen hat exhibits no yi ld point or continuously yi lding behavior. Construction lines show points

here typical calculations

Fiigure 5 – BH3 sc een shot of conti nuously yielding

aterial.

for ISO 6892-1 ha e been deter ined, including Rp0.2 and Rm. Construction li es or markers are availabl for almost all calculations in lueHill® 2 an BlueHill 3 for a quick and easy visual indication of the correct result bei g calculated. ISO 892-1 details test speeds hat must be dhered to withi a tolerance of ±20% while certain material properties are calculated. Th re are four s eed ranges in total, with reco mendations as to which s ould be used at each poin of th test. Figure focuses on he yield region of the test curv . The red lines show the st ain rate bein maintained well

ithin the ±2 % allowable limits.

M at erials wit Yield Poi n t Fiigure 6 – BH3 sc een shot of yield region with strai rate and

The iagram (Figure 7) shows a ypical curve f a specimen that xhibits yield oint behavio or discontinuously yielding behavior. Construction lines show points where typical calculations for IS 6892-1 hav been deter ined, inclu ing ReH and eL.

± 20% error band.



M ethod B This tress-controlled method h s far wider allowable speed ranges than Method A. From the start of the test up until 50% of the specified yield strength of the material under test the ma hine can be run at any con enient speed. From this 50% point the ystem shoul be run at a stress rate between 2MPa/s an 60MPa/s, d pending on the

odulus of elasticity of the

aterial. Thro gh yield and

as calculations su h as ReH, ReL, Rp and Rt are performed the r te of separation of the cro shead must e kept cons ant within se strain rate li its. Followin these yield proof strength based calculations the te t speed can be increased to a train rate not greater than .008/s. Unle s otherwise agreed, the choice of method (A or B) and

Fiigure 7 – BH3 sc een shot of disco ntinuously yieldi g material.

the t st rates within them are at the discretio of the prod cer running the tests provided they com ly with the requirements of the relevant se tion of ISO 6 92-1:2009.

Inter ational Orga ization for Standardization, Metallic materials -- Tensile testing -- Part 1: Method f test at roo temperature, temperature, ISO 892-1:2009, International Organization or Standardization, Genev .

ISO 6892.pdf

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