Beryllium window experiment at HiRadMat

1Chris Densham, 1Tristan Davenne, 1Andrew Atherton, 1Otto Caretta, 1Peter Loveridge, 2Patrick Hurh, 2Brian

Hartsell, 2Kavin Ammigan, 3Steve Roberts, 3Viacheslav Kuksenko, 1Michael Fitton, 1Joseph O’Dell, 2Robert Zwaska

1 STFC Rutherford Appleton Laboratory, UK2 Fermilab, US3 Oxford University (Materials for Fission and Fusion Power), UK

Objectives of experimentIdentify design limits for beam windows for the next generation of proton accelerator driven facilities by:

• Exploring the onset of failure modes (flow behaviour, crack initiation, or fracture, and other degradation) of various beryllium grades/forms under controlled conditions at simultaneous high localized strain rates and temperature rises.

• Identifying and quantifying any potential thermal stress wave limits for beryllium windows under intense pulsed beam conditions and how they may differ between grades/forms

• Comparing measurements to non-linear failure simulations for

validation/modification of material models through the use of state-of-the art material analysis techniques

• Investigating the potential effects of resonance, with constructive superposition of stress waves, in windows of particular thicknesses/geometries.

Model Inputs

Fluka and MARS Energy Deposition calcsMax energy density = 0.2 GeV/cc/primary Temperature jump = 1.7K/bunch or 493K/spill

HiRadMat Proton Beam ParametersBeam kinetic energy - 440GeVBeam Sigma – 0.3 - 0.5mmBunch spacing - 25nsNumber of protons/bunch = 1.7e11Number of bunches – 288Spill duration - 7.2μs

Stress simulations (Static and inertial)LS-Dyna, Autodyn and ANSYS Beryllium window – temperature dependent strength propertiesBilinear and elastic-viscoplastic hardening models Window dimensions:

Radius range = 5-25 mmThickness range = 0.15-1 mm(0.15mm chosen such that bunch spacing=2*t/cL)

Model inputs

Beryllium Material Data

[ITER MATERIAL PROPERTIES HANDBOOK 1997] [Mechanical Properties of Structural Grades of Beryllium at High Strain Rates, US Air Force Materials Laboratory, 1975]

Stress Strain curves for Beryllium S-65B

Yield Strength of Beryllium S-65B

Combined ITER and US Air Force data used to implement Bilinear Kinematic Hardening material model in ANSYS Classic

Literature data on mechanical properties of beryllium at high strain rates

Approximated using elastic viscoplastic model in LS-Dyna simulations

Beryllium Material Data

Tangential Modulus = 4.62 GPa

Bi-linear kinematic model used in ANSYS

Beam Induced Stress

Bi-linear Model

Static structural analysis of thermal stresses induced by beam pulse

Temperature dependent material properties Window properties:

25mm radius, 1mm thickness Temperature jump of 360°C

Bi-linear

Von Mises Stress [MPa] 268

Total Strain 1%

Axial Dispacement [μm] 5.71

Beam induced stress & strain

Edge strain simulation results

Be slugs: R = 20 mm, L = 30 mm Beam centered at r/R = 0.9 Beam sigma: 0.3 mm Elastic viscoplastic material model

(LS-DYNA) Temperature and strain rate

dependent [1]

LS-DYNA model showing beam location and temperature after 288 bunches.

Dynamic simulations

Dynamic strain responseE: elastic material modelEVP: elastic viscoplastic model

288 bunches

36 bunches

ΔT = 120 °C Max. εtot,equiv = 0.08 %

ΔT = 980 °C Max. εtot,equiv = 2.0 %

Plastic strain: generation of permanent surface displacement

y-displacements, σ = 0.3 mm

• y-displacements (bump height) range from 2 – 10 µm (σ = 0.3 mm, t = 0.25 – 3 mm) – well within resolution of modern profilometers

• Damage model being developed to better predict onset of fracture and fracture morphology after cool-down (fracture of centre spot expected)

Results are for 0.25 mm window, elastic viscoplastic material model

At maximum intensity:(288 bunches/pulse)

Surface deformation versus beam sigma / intensity

Beam and Applied Pressure

A pressure is applied to one side of window as is the case in an actual beam window

Investigate whether addition of beam pulse could produce significant stress peak

Window is constrained at periphery edge. Investigated the influence of altering the window radius and thickness and

magnitude of pressure load.

Stress response of window under beam and pressure load of 4 bar

Realistic load case: beam pulse + applied pressure

Interim conclusion

Applying a pressure to the window in conjunction with beam loading does not appear to induce a higher stress peak in the window (good result for actual beam windows!)

Nevertheless, it may still be a valid method of detecting window failure e.g. by using an on-line leak detector

Realistic load case: beam pulse + applied pressure

Outline conceptual design of experiment

Multiple samples exploiting long interaction length in beryllium.Samples include:• Different commercial grades of

Be• Thick & thin windows• Unstressed and pre-stressed

Online instrumentation

Strain measurements: strain gages positioned on surface of beryllium slugs to measure axial strain circumferential strain

Laser Doppler Vibrometer to compare surface vibrations with simulations and provide independent check on rms beam spot size

Optical pyrometer to measure peak temperature rise (another check on beam size)

HRMT14 experiment: Equipped Inermet specimen for strain measurements [2]

Off-line materials analysis

• Profilometer/AFM to analyse window surface profile and measure out-of-plane plastic deformations.

• Advanced microscopy systems for micro-structural and crystallography evaluation (SEM, EBSD, EDS) and potential crack/failure analysis.

Proposed experimental methodology

1. Polish samples before irradiation and characterise using AFM, SEM, EBSD, EDS, nanoindentation and, possibly, micromechanical methods

2. Carry out experiments:– Scan beam across samples with increasing number of

bunches per spill– Carry out multiple shots on single locations to investigate

whether beam effects saturate or accumulate

3. Repeat measurements in step 1 to identify effects of pulse beam interation

Material analysis techniques

• Used before and after in-beam experiment to quantify effects of pulsed beam interaction with material

Atomic Force Microscopy

Used to measure surface bump dimensions

Electron backscatter diffraction (EBSD)

• Electron backscatter diffraction is a technique for the scanning electron microscope which allows crystal orientations in a polycrystalline material to be measured.

• Maps of crystal orientation can be collected using EBSD. They remove any ambiguity regarding the recognition of grains and grain boundaries in the sample.

• We intend to use EBSD to see how the material flows during plastic deformation and, if a crack develops, how the flow results in fracture

Nanoindentation

Used to measure changes in hardness across sample after irradiation

Focussed Ion Beam (FIB) Methods

Zeiss Nvision dual beam FIB-SEM

Sample

10

The DualBeam Advantage:The DualBeam Advantage:

TheCoincidencePoint

FIB technique advantageous for:• Site specific regions• Small volumes – reduction in hazards e.g. activity,

toxicity, etc.

Micromechanical Testing

Steve RobertsOxford University Materials

2mm

23

Why microcantilevers?

• Need for a sample design that can be machined in surface of bulk samples.

• Bend testing allows fracture as well as elastic and plastic properties to be investigated.

• Suitable for measuring individual microstructural features.

• Testing of samples only available in small volumes.

• Geometry that can be manufactured quickly and reproducibly.

1um

3um 2um

3um

4um

0.00E+00

1.00E+09

2.00E+09

3.00E+09

4.00E+09

5.00E+09

6.00E+09

7.00E+09

0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000

Yiel

d St

ress

(Pa)

Beam Depth (μm)

Simple Yield Stress

Neutron Irradiated

Ion Irradiated

Un-Irradiated

Power (Neutron Irradiated)

Power (Ion Irradiated)

Power (Un-Irradiated)

Neutron-irradiated

Unirradiated

Ion-irradiated

Micromechanical testing Fe-6%Cr – yield stress

6.0

4.0

2.0

0.00.0 2.0 4.0 6.0 8.0

Yield Stress (GPa)

Beam depth (mm)

0.1mm

Energy-dispersive X-ray Spectroscopy (EDS)

• Used to measure migration of impurities e.g. to grain boundaries

Summary of measurements

1) Plastic deformation out-of-plane profile.2) Vibration (strain gauges) response (onset of yielding, fracture timing (in cool-down cycle?))3) Crack/fracture detection through microscopy4) Fracture surface morphology through microscopy (inter-granular?)5) Grain orientation and residual strain through microscopy (EBSD)6) Visual (High Speed or High Resolution Camera) to capture any unforeseen events

Interpretations of measurements

1. Do measurements match the macro-scale simulations and/or material/damage models? (Validation, Benchmarking)

2. Are results consistent across the various Be grades and conditions tested? Can materials characterisation explain any differences noted?

3. Do results indicate that certain grades/conditions/orientations exhibit better resistance to thermal stress waves?

4. Does resonance between bunches have a measureable effect?5. Can one primary failure mode be identified for all material

grades/conditions or does the failure mode differ depending upon material/grade/condition?

6. Was anything observed that was not expected?

Extra Material

Influence of Cp

Temp dependent CpConstant Cp

Temp

Dependent Cp Constant Cp

Temp [°C] 362.8 461

Von Mises Stress [MPa] 268 273

Plastic Strain 0.009 0.014

Influence of Cp

30

Stress at window centre following a single bunch. Note ‘small’ magnitude of stress waves and significant reduction in stress wave magnitude within several bunches

Axial stress at window centre during first six bunchesAxial wave magnitude increases for first three bunchesno significant constructive interference of axial waves observed

Inertial stresses from single pulse

31

Inertial Stress – complete pulse

Stress resulting from entire pulse (288 bunches)Plastic deformation starts at about 2μsPeak stress is 260MPaInertial stress waves don’t appear to significantly add to stress

Axial strain rate < 25000 s-1

Radial strain < 900 s-1

Strain rate reduces once plastic deformation occurs

Inertial stress from complete spill

32

Inertial Stress – complete pulse

Plastic work occurs on beam axis

Axial Deformation of 0.6microns with 0.15mm thick window

Strain growth rate changing at yield point

Inertial stress from complete spill