Tata Steel Slide D. P. Norman, S. Danks, M. Lansbergen Strain Rate Sensitivity in Engineering Applications 20th May 2015 Engineering Integrity Society Product Integrity Testing: An Integral Part of the Development Process Tata Steel, Swinden Technology Centre, Rotherham. Strain Rate Sensitivity in Engineering Applications Tata Steel Overview Topics Covered What is high strain rate sensitivity (SRS) Applications where SRS is required Case studies - Crush Box Finite Element (FE) Model - Vehicle FE Model - 56kph Full Frontal Crash into Rigid Wall Strain rate testing method Strain rate testing standards Crush box testing Application to FE Material Data Supply to Customers Slide 2
Strain Rate Sensitivity in Engineering Applications Tata Steel What is Strain Rate Sensitivity (SRS)? SRS is a change in material properties with strain rate Slide 3 Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 4 Applications where SRS is Required Impact, Forming, Fracture Impact Testing - Automotive vehicle impacts - Nuclear transport flask shock absorber - Orion Space Capsule - LCD-TV - Cooker - Barbie Vehicle Side Impact Forming and Fracture - SRS helps to prevent strain localisation, giving a better strain distribution and
helps give more realistic fracture predictions Automotive Forming Analysis Hole Expansion Fracture Correlation Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 5 Crush Box (60x60x500mm drop weight 136kg) When SRS is not included crush force is ~20% less crush displacement is ~20% greater Without SRS With SRS Force-Displacement Comparisons Strain Rate Sensitivity in Engineering Applications Strain Rate Distribution Strain rate (1/s)
High strain rates are concentrated in a region close to the impact, most strain rates are below 500/s Tata Steel Slide 6 Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 7 Vehicle Crash ULSAB 56kph Full Width Frontal Crash into Rigid Wall Upper Load Path (1) Middle Load Path (2) Analysis is run with and without SRS and the results compared Lower Load Path (3)
Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 8 Strain Rates Distribution 56kph Impact Into Rigid Wall 3 elements with strain rates >200/s Strain Rate (1/s) For the subset of elements that have strain rates > 1/s: 87% have strain rates less than 30/s 94% have strain rates less than 50/s 99.97% have strain rates less than 200/s 3 elements in the model have strain rates above 200/s they are located in front section of the main longitudinal (see arrow) Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 9 ULSAB Frontal Crash Deformation D
With SRS, D=946mm No SRS, D=843mm Crush distance is 103mm greater when SRS is not included Upper Load Path (1) Middle Load Path (2) Lower Load Path (3) Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 10 Main Longitudinal Viewed from Underneath With SRS Final length 1213mm Max Axial Force 111.2kN Without SRS Middle Load Path (2) Final length 1159mm Max Axial Force 94.7kN
Without SRS, the force is 18% less and crush distance is 74mm greater. The part without SRS also buckles in the middle Strain Rate Sensitivity in Engineering Applications Tata Steel ULSAB Frontal Crash Study Importance of SRS in Vehicle Crash Performance When SRS is modelled: - structure is stronger - deformation is lower - structure can be better optimised to reduce weight When SRS is not modelled: - structure is softer - deformations are larger - buckling can occur as an unwanted consequence - structure design will tend to be too conservative Slide 11 Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 12 Influence of Pre-Strain from Manufacturing Lower Load Path Lower Load Path
Lower Longitudinal Plastic Strain due to Forming Including the forming strains and thickness changes together with SRS can have a further influence on crash performance Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 13 Strain Rate Testing Procedure 2 Servo-Hydraulic Machines: 1. 0.001 to 1/s (slow rate) 20kN Capacity 2. 10 to 200/s (medium/ high rate) 25kN Capacity 1 2 Strain Rate Sensitivity in Engineering Applications Tata Steel
Slide 14 Test Sample - The test coupon is mounted on alignment pins in the holes - The test coupon is then clamped to the static and moving ends Note: the elongated side of the specimen is clamped to the static end (this reduces the vibration and enables the dynamic loads to be captured more faithfully) - A slack adaptor is used to allow the machine to accelerate to the test speed before loading the test coupon Strain Rate Sensitivity in Engineering Applications Tata Steel Load Measurement Load: Load cell is used for slower strain rates (up to 1/s) Strain gauge is used for higher strain rate tests (10/s-200/s) For higher strain rates, the load is calculated from the strain gauge strain measurement P=AEe (P-Load, A-Area, E-Elastic Modulus, e-strain) Validated by calibrating P vs e on the slow rate machine Slide 15
Strain Rate Sensitivity in Engineering Applications Tata Steel Extension Measurement Extensometer measurement Up to 1/s Extension-Time DIC measurement From 10/s to 200/s Extension-Time Slide 16 Up to 1/s: - An extensometer with an initial gauge length of 12.5mm is used From 10/s to 200/s: - Speckled paint is applied to the sample surface - The test is filmed with a
high speed camera (40,000 fps) - A contour plot and graph of extension vs time is obtained - Digital Image Correlation (DIC) tracks the motion of 2 points on a line (equivalent to an extensometer, gauge length 12.5mm) Strain Rate Sensitivity in Engineering Applications Test Data Output Tata Steel Slide 17 Strain Rate Sensitivity in Engineering Applications Tata Steel Tata Steel Benchmarking 1. Tata Steel vs Aachen Data Benchmarking shows a good match between Tata Steel and Aachen test data 2 coils of the same batch of material have been
tested at Tata Steel. The scatter is similar to the Tata Steel vs Aachen data shown in Figure 1. Tata Steel 2% Strain Tata Steel 6% Strain 2. Tata Steel (2 Coils from Same Batch) Slide 18 Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 19 Tata Steel SRS Data Benchmarking Activity Tata Steel has spent a significant amount of time developing an optimised test method for SRS testing at Tata Steel Test geometry, test measurements and data processing methods have all been carefully developed to give the optimum quality of measured SRS data From time to time, Tata Steel also validates SRS test data by sending the same batch of material to be tested at other European test houses, to compare the results This provides further validation of the Tata Steel SRS test data, and gives further confidence in the data we provide
Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 20 Strain Rate Testing Standards BSI: BS EN ISO 26203-2:2011 Metallic materials. Tensile testing at high strain rates. Servo-hydraulic and other test systems http://shop.bsigroup.com/ProductDetail/?pid=000000000030182902 ISO: ISO 26203-2:2011 Metallic materials -- Tensile testing at high strain rates -- Part 2: Servo-hydraulic and other test systems www.iso.org/iso/catalogue_detail.htm?csnumber=46275 Steel Industry Groups: VDEh: Verein Deutscher Eisenhttenleute Stahl-Eisen-Prfblatt (SEP) 1230 Ermittlung mechanischer, Eigenschaften anBlechwerkstoffen bei hohen Dehnraten im Hochgeschwindigkeitszugversuch-1. Ausgabe, Entwurf 04.2006 ESIS: European Structural Integrity Society P7-00 Procedure for Dynamic Tensile Tests, ISSN 1616-2129, August 2000 WorldAutoSteel: International Iron and Steel Institute, Recommendations for Dynamic Tensile Testing of Sheet Steels - August 2005 http://c315221.r21.cf1.rackcdn.com/HighStrainRate_Recommended_Procedure.pdf Strain Rate Sensitivity in Engineering Applications Tata Steel
Slide 21 Strain Rate Testing Standards Developments in Strain Measurement To measure the gauge length extension directly, a non-contact, inertia free system is required (i.e. laser measurement or Digital Image Correlation (DIC)). Before 2007, it was considered acceptable to derive the gauge length extension from the machine displacements - this required a test coupon design that minimised the coupon strain outside of the gauge length (i.e. a very sudden transition from gauge width to full sample width) and a correction function to convert machine displacement to gauge length extension. Shorter, narrower gauge lengths could be used, which enabled higher strain rates to be tested. The ISO standard now requires the strain to be measured directly this gives obvious benefits, but has led to longer and wider gauge lengths, which tends to lower the maximum strain rates that can be achieved using servo-hydraulic equipment Not all global test standards have made direct strain measurement a requirement, but in Europe this practice is now established. Tata Steel use DIC methods for measuring the strain in the gauge length directly DIC for high speed tests is also limited by the maximum speed and resolution of the camera currently we use a frame rate of 40000 frames per second (fps) to measure strain rates of up to 200/s for a gauge length of 12.5mm Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 22 Crash: Axial Crash test Test specifications
60 Standard geometry Closed Top Hat (CTH) Ro 107.5 60 Ro Trigger t Length = 500 [mm] R0 = 4 [mm] Triggers are applied on side wall and closing plate Weld pitch = 30 [mm] (another pitch is possible) Adhesive and spot weld-bonded are possible Crash test - 3 test speeds: 15 20 Cross section CTH Quasi-static and Intermediate Dynamic
CTH specimen Quasi-static and Intermediate Dynamic Servo-hydraulic machine (0.1 mm/s 100mm/s) Quasi-static compression: 0.33 mm/s Intermediate compression: 100 mm/s Load measurement Load cell Displacement measurement Cross-head displacement of machine Drop Weight Tower Height = 0 10 [m] Impact velocity = 0 14 [m/s] (= 50 [kph]) Impact mass = 50 250 [kg] Load measurement Load cell Displacement measurement 3 point laser measurement Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 23 Crash: Axial Crash test - Analysis For each CTH is analysed: Force-displacement signal Closed Top Hat Damage Matrix (visual damage inspection)
Force-displacement signal analysis Mean Force Peak Force 250 Fpeak Force Displacement (50 km/h) Mean Force - F_mean (50 km/h) Force [kN] 200 150 100 50 0 0 50 100 150 Displacement [mm]
200 250 Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 24 Crash: Axial Crash test - Analysis Closed Top Hat Damage Matrix (visual damage inspection) Legend Spot-weld failure mode Material damage stage CTH Damage Matrix Fold LF 0.5 O 1 O 1.5 P 2 O 2.5
X 3 O 3.5 P 4 O 4.5 O LS LC Location F RC S S N S S S C S RS RF O
X P O O O P O O Location B C Fold 0.5 Fold 1.5 Fold 1 Fold 2 Fold 2.5 Fold 3 Fold 3.5 Fold 4 Fold 4.5
RF RC LC RC F N S C ? O P X LF LS LC F B RC RS RF Undeformed
Deformed Necking MATERIAL DAMAGE STAGE Surface fracturing Cracking Rolling Direction (RD) Damage 0 to RD DAMAGE DIRECTION Damage 45 to RD Damage 90 to RD Spot weld intact Plug failure SPOT WELD FAILURE MODE Interfacial failure Left Flange CTH Left Side wall CTH Left Corner CTH Front CTH Backing Plate CTH Right Corner CTH Right Side wall CTH Right Flange CTH Material Damage Stage Spot-weld failure Mode Necking Surface fracture
Crack Interfacial Plug Strain Rate Sensitivity in Engineering Applications Tata Steel Crash: Example of analysis result HX340LAD+Z - 1.5 mm Comparison mean and peak force for: HCT780X+Z - 1,5 mm 70 60 Mean Force [kN] Axial Crash results [ 140 kg / 14000 mm/s ] HCT600X+Z - 1,5 mm 80 50 40
30 20 10 0 Quasi-Static Intermediate Dynamic 0.33 mm/s 100mm/s 14000 mm/s HX340LAD+Z - 1.5 mm HCT600X+Z - 1,5 mm HCT780X+Z - 1,5 mm Quasi-Static Intermediate Dynamic 0.33 mm/s 100mm/s
14000 mm/s 250 200 Peak Force [kN] HX340LAD+Z 1,5 [mm] HCT600X+Z 1,5 [mm] HCT780X+Z 1,5 [mm] Slide 25 150 100 50 0 Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 26 Application to FE Data Processing Elastic part of the curve is cut off
Engineering strain is converted to plastic engineering strain (elastic strain component (s/E) is subtracted from total strain to give plastic strain) Data beyond maximum load is removed (true stress strain conversion is not valid beyond the point of maximum load) Engineering plastic stress-strain curve is converted to true stress-true strain: s=s(1+e), e=ln(1+e) eplastic=etotaleelastic Key to symbols E = Elastic Modulus s = engineering stress e = engineering strain s= true stress e = true strain Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 27 Application to FE - Bergstrom-van-Liempt (BvL) Data Fit BvL model is a 3D surface - Static part is based on dislocation theory (a function of strain) - Dynamic part is based on thermal activation of dislocation movement (a function of strain
rate and temperature) s0 = static yield stress sm = stress increase parameter for strain hardening = strain hardening parameter for large strain behaviour = strain hardening parameter for low strain behaviour e0 = pre-deformation parameter n = exponent for the strain hardening behaviour s*0 G0 m' k T e 0 = = = = = = limit dynamic flow stress maximum activation enthalpy (eV)
power for the strain rate behaviour -5 Boltzmann constant = 8.617 . 10 eV/K absolute temperature (K) limit strain rate for thermal activated movement Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 28 Strain Rate Law Comparisons - Different Strain Rate laws significantly influence the shape of the stress-strain-strain rate curves - BvL is based on the fundamental physics of dislocation movements under static and dynamic conditions - Bergstrom-van-Liempt (BvL) is recommended by Tata Steel Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 29 Strain Rate Model Behaviour (BvL, Cowper Symonds, Johnson Cook) Validating SRS material models can be difficult - the large range
of strain rates observed has a tendency to smooth out the SRS response in the part Observed strain rates CS - Cowper Symonds, JC - Johnson Cook, BvL - Bergstrom van Liempt CS JC BvL Crush Displacements No SR Crush Force vs Displacement Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 30 Material Data Supply to Customers Customers
Question Answer Question Answer SALES / MARKETING Question AURORA-TEAM Manja Boon Michael Abspoel Marc Scholting Marcel Lansbergen AURORA-DATABASE Strain Rate Sensitivity in Engineering Applications Tata Steel Slide 31 Strain Rate Sensitivity (SRS) in Engineering Applications Summary Including SRS and forming strains - Increases the strength of a structure
- Reduces deformation - Leads to better optimisation of structure design SRS testing requires - Loading at constant speed - A non contact gauge length extension measurement method - A strain gauge to measure the load - A fast data capture system FE modelling requires - A process to convert test data to smooth plastic stress-strain data FE model validation - FE model validation is difficult to achieve in practical examples
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