SPF-DB Core Inflation Pressure Calculator

Aerospace Structures: Design multi-sheet titanium SPF/DB sandwich panels, internal gas inflation pressure, truss angle, and web thinning.

Sandwich Panel Architecture

Panel Construction Type:

Total Core Height H (mm): Internal separation between skins

Cell Pitch P_pitch (mm): Center-to-center bond pitch

Core Sheet Thickness t₀ (mm):

Skin Sheet Thickness (mm):

Ti-64 Flow Stress σ (MPa): At 900°C, ε̇ = 5×10⁻⁴ s⁻¹

Target Corner Fillet Radius (mm): Tight fillet requires high pressure

Inflation Parameters & Weight Savings

Peak Inflation Pressure

-- bar

-- MPa

Final Web Thickness

-- mm

Weight Savings vs Solid

-- %

Truss Core Angle

-- °

Areal Weight

-- kg/m²

Equivalent Bending EI

-- × solid

Initial DB Dwell Pressure (Step 1): 2.5 - 3.5 MPa (900°C for 60 min)

Core Inflation Cycle (Step 2): Dynamic ramp over 45 - 90 min

Final Sizing DB Pressure (Step 3): -- bar

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Engineering SPF/DB Hollow Sandwich Structures

Superplastic forming / diffusion bonding (SPF/DB) enables the fabrication of ultra-stiff, lightweight titanium sandwich panels widely deployed in gas turbine wide-chord fan blades, nacelle acoustic liners, and supersonic military aircraft access doors.

1. Process Phases

Diffusion Bonding Phase (DB): Cleaned titanium sheets with pattern-printed yttria stop-off are loaded into a sealed ceramic tool at $900^\circ\text{C}$. External press pressure ($2.5\text{--}3.5\text{ MPa}$) welds unmasked contact lands.
Core Blow Forming Phase (SPF): Argon gas is injected into internal micro-tubes at controlled strain rates ($10^{-4}\text{ s}^{-1}$), peeling and inflating the core webs until contact with outer skin panels is established.
Final Filleting & Sizing Phase: Gas pressure is boosted to final peak pressure ($20\text{--}40\text{ bar}$) to drive secondary diffusion bonding between inflated core flanges and outer skins while consolidating corner fillets.

2. Web Thinning & Cell Geometry

The core sheets stretch during inflation. The final web thickness $t_{\text{web}}$ is governed by initial core sheet thickness $t_0$, cell pitch, and core height $H$:

t_web = t₀ · (Pitch / 2) / √[ H² + (Pitch / 2)² ]

Frequently Asked Questions

What is the SPF/DB manufacturing process for hollow sandwich structures?

Superplastic Forming combined with Diffusion Bonding (SPF/DB) is a multi-sheet manufacturing route where titanium sheets are first screen-printed with a ceramic stop-off pattern (e.g. yttria $\text{Y}_2\text{O}_3$) to prevent bonding at designated areas. The pack is sealed, diffusion bonded under external pressure at $900^\circ\text{C}$ where bare titanium meets, and then internally inflated with argon gas, expanding the unbonded core sheets into a lightweight hollow truss structure.

What is the difference between 3-sheet and 4-sheet SPF/DB configurations?

In a 3-sheet panel, a single core sheet is printed with staggered stop-off strips on opposite faces and inflated between two outer face skins, forming a corrugated triangulated truss. In a 4-sheet panel, two internal core sheets are bonded together along web seams and expanded into two outer skins, allowing vertical webs, rectangular cells, or pyramidal architectures with superior torsional stiffness.

Why is corner fillet radius critical in SPF/DB pressure scheduling?

As the expanding core touches the outer skins, the remaining corner radius $R$ requires a gas pressure $p = \frac{\sigma \cdot t}{R}$ to sharpen. Forcing a tight $1.5\text{--}2.0\text{ mm}$ fillet into die corners requires peak gas pressures of 20 to 35 bar (2.0–3.5 MPa). Inadequate final pressure leaves large triangular void roots that severely degrade joint peel and shear fatigue resistance.