Tube to Foam Interface

(Tim)

Tube to Foam Interface 2

Outline• Discuss what’s known about the tube to foam

interface– Describe problem– Issues– Theoretical Calculations– Anecdotal Experience

30/05/2012

Tube to Foam Interface 3

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)

30/05/2012

Tube to Foam Interface 4

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

30/05/2012

Tube to Foam Interface 5

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?

30/05/2012

Tube to Foam Interface 6

Open and Closed-cell Foams• Allcomp Foam

– Open structure from low density (0.05g/cc) open cell foam

– 100-130ppi (0.25mm)• Pocofoam

– Closed cell structure with voids typically 0.5mm diameter

– Voids volume equivalent to 0.22mm thick glue layer

30/05/2012

Tube to Foam Interface 7

Thermal Properties

• Second largest impact (after fluid htc)• Doubling the thermal impedance of the foam glue reduces

the coolant temperature headroom by 2⁰C (≈ 10%)

Fluid htc (=>4000)

Foam Glue (BN/Hysol)

CFRP Kx

Cable Bus Ky

CFRP Ky

Foam Kxyz

Baseline

Sensor Glue =>IRS2125, 0.44W/mK

ALL Ks halved

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Coolant Temperature Headroom wrt -30C [C]

4 8 126 1021Approximate Power Headroom Factor wrt 0.65mW/mm2(0C)

30/05/2012

Tube to Foam Interface 8

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

30/05/2012

Tube to Foam Interface 9

Stave CTE Assuming Rigid Foam & Rigid GlueK1

3D2U

[80g

sm/2

9%RC

]0/9

0/0

K13C

2U[1

00gs

m/4

0%RC

]0/9

0/0

K13C

2U[4

5gsm

/40%

RC]0

/90/

0

K13D

2U[8

0gsm

/29%

RC]0

/90/

0/SS

/0/9

0/0

K13C

2U[1

00gs

m/4

0%RC

]0/9

0/0/

SS/0

/90/

1

K13C

2U[4

5gsm

/40%

RC]0

/90/

0/SS

/0/9

0/2

K13D

2U[8

0gsm

/29%

RC]0

/90/

0/Ti

/0/9

0/0

K13C

2U[1

00gs

m/4

0%RC

]0/9

0/0/

Ti/0

/90/

0

K13C

2U[4

5gsm

/40%

RC]0

/90/

0/Ti

/0/9

0/0

K13D

2U[8

0gsm

/29%

RC][t

ape/

0/90

/0]s

K13C

2U[1

00gs

m/4

0%RC

][tap

e/0/

90/0

]s

K13C

2U[4

5gsm

/40%

RC][t

ape/

0/90

/0]s

K13D

2U[8

0gsm

/29%

RC][]

s with

0.0

25m

m sc

reen

K13C

2U[1

00gs

m/4

0%RC

][]s w

ith 0

.025

mm

scre

en

K13C

2U[4

5gsm

/40%

RC][]

s(0.

025)

K13D

2U[8

0gsm

/29%

RC][]

s(0.

05)+

SS

K13C

2U[1

00gs

m/4

0%RC

][s](0

.05)

+SS

K13C

2U[4

5gsm

/40%

RC][]

s(0.

05)+

SS

K13D

2U[8

0gsm

/29%

RC][]

s(0.

05)+

Ti

K13C

2U[1

00gs

m/4

0%RC

][]s(

0.05

)+Ti

K13C

2U[4

5gsm

/40%

RC][]

s(0.

05)+

Ti

K13D

2U[8

0gsm

/29%

RC][]

s(0.

025)

+SS

K13C

2U[1

00gs

m/4

0%RC

][]s(

0.02

5)+S

S

K13C

2U[4

5gsm

/40%

RC][]

s(0.

025)

+SS

K13D

2U[8

0gsm

/29%

RC][]

s(0.

025)

+Ti

K13C

2U[1

00gs

m/4

0%RC

][]s(

0.02

5)+T

i

K13C

2U[4

5gsm

/40%

RC][]

s(0.

025)

+Ti

-1.5E-06-1.0E-06-5.0E-070.0E+005.0E-071.0E-061.5E-062.0E-062.5E-063.0E-06

Stav

e 1D

Mod

el C

TE

BareFacesheets

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 Tube30/05/2012

Tube to Foam Interface 10

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

30/05/2012

Tube to Foam Interface 11

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

0 60 120 180 240 300 360 420 480 540 600-0.01

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04ER2074 (0.0mm)ER2074 (0.0mm)ER2074 (0.1mm)CGL

Time (s)

Ther

mal

Str

ain(

%)

30/05/2012

0 60 120 180 240 300 360 420 480 540 600-0.01

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

-50

-40

-30

-20

-10

0

10

20

30

40

ER2074 '0' Stave Temperature

Time (s)

Ther

mal

Str

ain

(%)

Tem

pera

ture

(deg

C)

% strain CTE (ppm)ER2074(0.0) 0.0233 4.57ER2074(0.0) 0.0242 4.75ER2074(0.1) 0.0187 3.67CGL 0.0091 1.78

Tube to Foam Interface 12

Jones2009• 30cm single-tube prototypes

– Equivalent thermal performance for CGL and rigid epoxy– Thermal cycling shows no deterioration of average

temperature above cooling tube at -40⁰C

0 1 2 3 4 5 6-50

-40

-30

-20

-10

0

10

20

30

Time (h)

Tem

pera

ture

(deg

C)

30/05/2012

Tube to Foam Interface 13

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

30/05/2012

Tube to Foam Interface 14

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

30/05/2012

Tube to Foam Interface 15

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

0

0.5

1

1.5

2

2.5

3

3.5Failure

1 2

1

2

Tensile loading of foam

Unloading of foam

Strain

Stre

ss

(MPa

)

30/05/2012

Tube to Foam Interface 16

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%30/05/2012

Tube to Foam Interface 1730/05/2012

Tube to Foam Interface 1830/05/2012

Tube to Foam Interface 19

area[cm^2] 1560

Weight [g]Pipe 106.3Foam 45.6CGL 15Facings 104.8CF Tubes 38.7Closeouts 4.2Hysol 30.7Honeycomb 24.7

Total 370

fitings 16.6

new total 386.6

Stave 1.3m x 0.12cm

0/90/0 250 um thick K13D2U facings, 80-100 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

Weight [grams]

facing 1 84.7facing 3 84.6two cf tubes 18.3Stainless Steel Pipe+Fittings 76.1closeouts 3.4CGL on foam and pipe 15.3foam 50.4honeycomb 25.3hysol 30% BN facing #1 15.6hysol 30% BN facing #3 16.8

Total Stave Weight 390.4

Co-cure Stave Component Weights

30/05/2012

Tube to Foam Interface 20

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)

30/05/2012

Tube to Foam Interface 21

Co-cure stave contraction

30/05/2012

Tube to Foam Interface 22

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 ~ 20-35 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

30/05/2012

Tube to Foam Interface 23

1.3 m stave contraction analysis

But we expected pipe to be mostly fixed at U-bend

30/05/2012

Tube to Foam Interface 24

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.30/05/2012