Measurement of the Width Change by Video-Extensometer for the Determination of the Plastic Strain Ratio (r-value) Johannes Aegerter, Stefan Keller, Torsten Schröter 2015-10-15 (1) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Plastic stain ratio r The plastic strain ratio describes the ratio of true (plastic) width strain vs. true (plastic) thickness strain: r = b / a (1) Before straining F After straining Ll 0 o b 0 o olo bo a o Ll b a True plastic length strain l = ln True plastic width strain b = ln L Lo b bo Source: Quelle: S. Keller, Hydro Aluminium Rolled Products GmbH F True plastic thickness strain a a = ln ao (2) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Plastic strain ratio r F r = b / a (1) Stronger deformation in width direction L o l b o a o r = 1 Isotropic material (same deformation in width and thickness ( b = a ) Original crosssectional area S o Stronger deformation in thickness direction F Source: S. Keller, Hydro Aluminium Rolled Products GmbH Increase of straining (3) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Plastic strain ratio r Problem: Measurement of thickness reduction Solution: Law of constant volume: or V = const. L o * b o * a o = L * b * a l + b + a = 0 (Equation only valid for rectangular test pieces up to the begin of local necking) r b a ln ln b b o a a o b ln bo b L ln o b L o (4) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Determination of the r-value according to ISO 10113 Manual method: 1. Marking the original gauge length L o 2. Measurement of the original width b o of the test piece 3. Applying a force to achieve the specified strain level 4. Releasing the force 5. Measurement of the gauge length L and the gauge width b 6. Calculation of the r-value according the equation below r b ln bo b L ln o b L o (5) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Determination of the r-value according to ISO 10113 Automatic method: Automatic and continuous measuring of width change and Length change during tensile testing Calculation of the r-value at different strain levels according the equation shown before Figure: Zwick GmbH (6) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Determination of the r-value according to ISO 10113 Automatic method: 1. Continuously automatically measuring of width change and length change in the tensile test 2. Calculation of the r-value at different strain levels according the equation shown before 3. Specification of extensometers for measuring length and width change: class 1 of ISO 9513 or better Advantages of the automatic method: 1. Tensile test properties and r-value in one test 2. Less time required, because measurement of test piece after straining is not necessary 3. Use of robotic systems possible 4. Plastic strain ratio for different degrees of straining in one test available (7) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
System for length and width measurement In this case: System with 4 levels for measurement of the width change Possible problems of the described r-value evaluation: Length and width change is measured over different volumes Test piece slips between the pins for change of width measurement Width change extensometer must be hold mechanically in the middle of the gauge length of the (length) extensometer Figure: Hydro Aluminium Rolled Products GmbH, RDB (8) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Material with inhomogeneous yielding AA5182-H111: Stress vs. strain respectively width vs. strain 300 20,5 250 20,0 Spannung Stress R [MPa] 200 150 100 R p0,2 : 130,3 MPa R m : 276,7 MPa A g : 22,1 % Tactile measurement of width change using with 4 measurement levels 19,5 19,0 18,5 Breite Width b [mm] b [mm] 50 A 80mm : 24,0 % 18,0 0 0 5 10 15 20 25 30 Strain D h e [%] [%] 17,5 (9) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Tensile test machine with tactile (length) extensometer and video-extensometer for width change measurement Video-extensometer for change of width measurement Mirror Figure: Hydro Aluminium Rolled Products GmbH, RDB Tactile Extensometer for measurement of the length strain (lower sensor arms) (10) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Material with inhomogeneous yielding AA5182-H111: Width vs. strain 19,30 19,25 20,2 20,0 19,8 Width Breite b in [mm] mm 19,20 19,15 19,10 19,6 19,05 Width Breite b [mm] in 19,4 19,2 19,0 18,8 18,6 18,4 Ebene 1 Ebene 2 Ebene 3 Ebene 4 Ebene 5 Ebene 6 Ebene 7 Ebene 8 Ebene 9 Ebene 10 19,00 10 11 12 13 14 15 Dehnung in % Strain e [%] 18,2 0 5 10 15 20 25 Strain Dehnung e [%] e in % (11) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Material with inhomogeneous yielding AA5182-H111: Stress vs. strain resp. width vs. strain 300 20,5 250 20,0 Spannung Stress R [MPa] 200 150 100 50 R p0,2 : 130,7 MPa R m : 278,6 MPa A g : 21,9 % A 80mm : 24,2 % Optical measurement of width change using 10 measurement levels Averaged curve from 10 single curves 19,5 19,0 18,5 18,0 Breite Width b [mm] b [mm] 0 0 5 10 15 20 25 30 Strain Dehnung e [%] e [%] 17,5 (12) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Measurement of the change of width by videoextensometer Advantages: 1. Larger number of measurement levels 2. Measuring levels over the complete gauge length equally distributed 3. Measuring levels follows the test piece during the test, test piece does not slip between the measurement levels 4. No mechanical ensue of the width extensometer necessary 5. Storage of even single measurement level is possible Points which have to be considered: 1. Scaling of the width measurement is necessary 2. Optical distortion especially in the edge region of the field of view is possible (13) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Width scaling by a cylindrical reference bar Ø 10 h7 Possible problems: 1. Optical distortion leads to imprecise width values for the single width levels Test piece appears conical 2. Different gripping systems for cylindrical reference bar and for the flat test pieces to be tested 3. Diameter of the reference bar may deviate substantially from the width of the test piece to be tested (14) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Scaling of the width by using a 20 mm wide parallel strip Approaches for solving the problems: 1. Optical distortion: Mathematic correction by using a polynomial fit of 4 th degree 2. Adaption of the reference on the geometry and dimension on the material to be tested: Reference from flat strip same gripping system for scaling and testing 20 mm flat strip for 20 mm width of the test piece (15) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Width measurement on a flat test piece Lösungsansätze: Very good conformity of the width values in the different measurement levels by Mathematical correction of the optical distortion Scaling by using a 20 mm strip (16) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Possible online-validation of the system in the running testing activity 1. Normally, original width b o of every test piece is determined before the test by tactile measurement 2. After applying the preload, fixing the edges of the extensometer and detecting of the width levels the width data of the 10 levels were checked regarding the following criteria: Comparison of the average of the 10 levels with tactile measured original width b o : tactile width measurement correct? Adjustment of video-extensometer / lighting conditions o.k.? Comparison of level 1 and level 10: Test piece preparation regarding parallelism o.k.? Comparison of minimum and maximum width: Test piece preparation regarding tolerances on shape o.k.? 3. For every criterion a separate tolerance can be given 4. If all criteria are fulfilled the test will be continued, otherwise the test will be stopped and the operator will get an information (17) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Tactile and optical width change measurement Average value and scatter range from 5 parallel tests 1,40 1,30 1,20 1,10 r-value r-wert 1,00 0,90 0,80 1,230 1,219 1,230 1,239 r2-20/ag-1% r8-12 0,70 Material: AA1050-O 1050A-O / Z250 / A 80mm / 90 Testing direction: 90 to the rolling direction 0,60 0,50 Tactile Taktile change Breitenänderungsmessung of width measurement Optische Optical change Breitenänderungsmessung of width measurement (18) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Tactile and optical width change measurement Average value and scatter range from 5 parallel tests 0,70 0,65 0,60 r-value r-wert 0,55 0,680 0,679 0,673 0,676 r2-20/ag-1% r8-12 0,50 0,45 Material: 5182-O AA5182-O / Z250RED / A 80mm / 90 Testing direction: 90 to the rolling direction 0,40 Tactile Taktile change Breitenänderungsmessung of width measurement Optische Optical change Breitenänderungsmessung of width measurement (19) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Summary The automatic width change measurement for the r-value determination is state of the art and usually integrated in standard tensile testing The number of measurement levels for tactile width change measurement is often limited to a small single-digit quantity, only the average value is normally stored Tactile width change measurements are often very sensitive to mechanical faults The optical width change measurement by video-extensometer enable a greater number of measurement levels and their values could be stored separately Alternative concepts for scaling the video-extensometer are available (e. g. for the correction of the optical distortion) The inspection of the following points can be integrated in the test procedure: test piece preparation, values of the original width measurement and adjustment of the video-extensometer The optical width change measurement results in more stable r-values with small scatter ranges (20) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany
Hydro Aluminium Rolled Products GmbH Research & Development Bonn Johannes Aegerter Georg-von-Boeselager-Str. 21 53117 Bonn T: +49 (0)228-552-2386 F: +49 (0)228-552-2017 E: johannes.aegerter@hydro.com www.hydro.com (21) 2015-10-15 Johannes Aegerter testxpo 24 th International Forum for Materials Testing Ulm, Germany