Tube to Foam Interface (Tim)

Outline Discuss what’s known about the tube to foam interface – Describe problem – Issues – Theoretical Calculations – Anecdotal Experience 30/05/20122Tube to Foam Interface

Problem Heat Flow – The heat generated within the active components on a stave is removed through evaporating CO2 in small- bore tubes. – The heat is conducted from the facesheets to the tube via thermally-conducting carbon foam and two interfaces made using thermally-conducting adhesive. Geometry – Tubes (S/steel so far) 1/8” (3.175mm) OD x (0.50, 0.25, 0.22mm wall) – For the future 2.2mm OD x 0.14mm wall (ABCN130) – Thermally-conducting Foam Strip-staves: Two 10mm wide ‘bars’ Pixels:Full width ‘slabs’ Materials – Tubes 316L / 304L stainless steel (current staves & stavelets) CP2 titanium (only used in UK ‘nearly glue-less’ stavelet – Thermally-conducting foam Pocofoam Allcomp foam – Tube-to-foam adhesives CGL (compliant) Hysol EA9396 (30% BN by wt) (rigid) 30/05/20123Tube to Foam Interface

Issues / Concerns Thermal Performance – Is the thermal impedance of the tube-foam interface good enough to mitigate against thermal runaway Stave Mechanical Stability – Are the temperature-induced stave deformations (and associated stresses) small enough (stable enough) to ensure good tracking performance. Longevity – Will the thermal impedance of the joint deteriorate over time? – Could stresses induce creep 30/05/20124Tube to Foam Interface

Comparison Compliant Adhesive Could the adhesive ‘slump’? i.e. separate out under – Gravity – Capillary flow Could the adhesive ‘migrate’ away from the interface? – Closed Cell foams – Open Cell foams Could the adhesive become less compliant? – Irradiation induced ‘curing’ may produce a ‘rigid’ joint over time. Rigid Adhesive Could thermally-induced stresses lead to:- – Large dimensional changes – High stresses which might promote cracking & ultimate failure of thermal path Could the need to accommodate dimensional changes complicate stave mounting? – Fixations in Z – Mounting brackets What are the effects of long- term creep? 30/05/20125Tube to Foam Interface

Open and Closed-cell Foams Allcomp Foam – Open structure from low density (0.05g/cc) open cell foam – ppi (0.25mm) Pocofoam – Closed cell structure with voids typically 0.5mm diameter – Voids volume equivalent to 0.22mm thick glue layer 30/05/20126Tube to Foam Interface

Thermal Properties Second largest impact (after fluid htc) Doubling the thermal impedance of the foam glue reduces the coolant temperature headroom by 2⁰C (≈ 10%) 30/05/20127Tube to Foam Interface

Stave CTE Assuming Rigid Foam & Rigid Glue Simple 1D model (Classical Laminate Theory) – 2 Face sheets (all 0/90/0) K13D2U / RS-3[80gsm/29%RC] K13C2U / EX-1515[100gsm/40%RC] K13C2U / EX-1515[45gsm/40%RC] – 2 Cooling tubes S/Steel:3.185mm OD x 0.22mm wall (x2) Effective thickness 0.037mm Titanium:2.2mm OD x 0.14mm wall (x2) Effective thickness 0.017mm – 2 Bus Tapes 0.025mm Kapton cover-layer 0.025mm / 0.05mm Aluminium screen 0.100mm Kapton 30/05/20128Tube to Foam Interface

Stave CTE Assuming Rigid Foam & Rigid Glue Bare Facesheets + S/S Tube Facesheets + Ti Tube Facesheets + Tape (50) Facesheets + Tape (25) Facesheets + Tape(50) + S/S Tube Facesheets + Tape(50) + Ti Tube Facesheets + Tape(25) + S/S Tube Facesheets + Tape(25) + Ti Tube 30/05/20129Tube to Foam Interface

Historical Data Jones (2009) – Crude measurement of relative CTE of CLG & Rigid epoxy – Measurement of thermal performance vs Thermal cycling (15 cycles) Sutcliffe (2010) – Stavelet FEA LBNL (2010) – Thermal Cycling of 12cm rigid-glued prototypes 30/05/201210Tube to Foam Interface

Jones2009 Clip-type extensometer 30cm prototypes – CGL – ER2074 (rigid epoxy) Zero thickness glue line 0.1mm glue line Cool down to -40C and allow to rise back to room temperature 30/05/201211Tube to Foam Interface % strainCTE (ppm) ER2074(0.0) ER2074(0.0) ER2074(0.1) CGL

Jones cm single-tube prototypes – Equivalent thermal performance for CGL and rigid epoxy – Thermal cycling shows no deterioration of average temperature above cooling tube at -40⁰C 30/05/201212Tube to Foam Interface

Sutcliffe (2010) Standard UK build – 0/90/0 K13D2U/RS3 (80gsm,29%RC) – 1/8” s/steel tubes CTEs – No Al screen 0.02mm contraction CTE = 1e-6 – 0.05mm Al screen 0.042mm contraction CTE = 2e-6 30/05/201213Tube to Foam Interface

Critical Stresses Foam stress (likely) to cause failure – but structures survive! Two explanations – Glue bridging between facesheet and tube – Simple FEA assumes linear material properties but materials testing reveals otherwise UK Stave core design employs end close- outs to protect foam 30/05/201214Tube to Foam Interface

Mechanical Materials Measurements Pocofoam has different properties in orthogonal directions and a failure stress of typically 0.8MPa Allcomp (K9) – 130ppi has uniform characteristics and failure stress >2.5MPa Failure Tensile loading of foam Unloading of foam Strain Stress (MPa) 30/05/201215Tube to Foam Interface

LBNL (2010) Construction – Length 12cm – Hysol 9396/BN(30% by weight) to bond tube & facings to foam – K7 foam – One SS tube(2.8mm OD) – One Ti tube(2.2 mm OD) Thermal cycle and irradiation(time sequence) – 900 cycles (20C -35C) then – 1 cycle to -70C then – 1 cycle to about -175C with LN2 then – Irradiation to 50 MRad, then to 150 Mrad total No change in Thermal performance – No difference in SS and Ti tubes (with given ID/OD). Not a surprise (from FEA). – No significant change in thermal performance for any sample after any thermal cycle sequence, including LN2 – Effect of irradiation up to 1 GRad is <10% increase in  T. – Thermal performance with K9 foam is significantly better than with K7 foam, by about 25% 1630/05/2012Tube to Foam Interface

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Stave 1.3m x 0.12cm 0/90/0 250 um thick K13D2U facings, gsm pre-preg CGL around s/steel pipe, Allcomp foam LBNL/BNL (2012) Stave 1.2m x 0.12m Co-cured facings, low density 45 gsm cf pre-preg. 0/90/0 370 um thick K13C2U + bus facings CGL around steel pipe, Allcomp foam 30/05/201219Tube to Foam Interface

Stave stiffness independent of temperature (simple support 120 cm apart) Comparison of Stave Stiffness, Room Temp and Chilled Stave is about 10% stiffer when chilled with -30 deg-C coolant. Bus cable glue layers responsible??? (simple support 120 cm apart) 30/05/201220Tube to Foam Interface

Co-cure stave contraction 30/05/201221Tube to Foam Interface

1.3 m stave and 1.2 m co-cure stave contraction Stave Contractions – 1.2m co-cure stave contracts 0.2mm CLT predicts 0.118mm assuming completely free tube NB Stave is held together with 2 x 15g of Hysol – equivalent to 0.082mm thickness spread over stave area – 1.3 m stave expands ~ um CLT predicts 0.040mm assuming completely free tube Similar glue mass / thickness Tube Length Changes – Pipe moves into co-cure stave ~ 100 um, into 1.3 m stave ~ 160 um – Free stainless steel pipe should contract ~ 1mm when cooled ~ 50 deg-C – Question is: does pipe really contract 1 mm? If so, see analysis of 1.3 m stave on next slide 30/05/201222Tube to Foam Interface

1.3 m stave contraction analysis But we expected pipe to be mostly fixed at U-bend 30/05/201223Tube to Foam Interface

Summary & Conclusions Stresses in staves come from – Bus tapes (primarily the aluminium screen) – Core assembly adhesive – Cooling tubes (if assembled with rigid epoxy) Evolving stave design reduces potential stresses – Smaller bore tubes (ABCN130) – Titanium (Progress in joining technology) – Bus-tape screen (0.05mm -> 0.025mm -> ‘0’ ?) CGL – Experience since 2008 Many staves built showing good thermo-mechanical performance – Concerns about migration into foam structure addressed by lining channel with rigid epoxy – Reliance on ‘sliding’ properties over long service life in high radiation environment – Some evidence that tube is not completely ‘free’ Rigid Adhesive (Hysol9396/30%BN) – Experience since 2009 – Thermal cycling (tens to many hundreds) No failures for ‘nominal’ cycling (Room temp to -40C) – FEA shows stresses in all components (in particular the foam) have large safety margins for ‘nominal’ excursions and indicate that structures will survive large (160C) excursions. 30/05/201224Tube to Foam Interface