Shreyas' CMH6 Documentation/Notes
Chapter 1 - General Info
Chapter 2 - Guidelines For Property Testing
2.1 Introduction
Sandwich structure has face sheets, core, and a means of connecting the two through an adhesive or a brazing
2.3 Evaluation Of Core Materials
Introduction
Three main types of core in sandwich construction are honeycomb, foam, and balsa wood.
Mechanical properties of interest for core materials include the compressive strength, modulus tensile strength, and shear strength and modulus.
Hygrothermal stability: is a materials ability to retain its physical and mechanical properties when exposed to both heat and moisture.
The Hygrothermal stability of sandwich panels is of interest to the integrity of solar panels.
Aluminum has a higher correction factor based off the cire thickness compared to the non-metallic.
2.4 Evaluation Of CORE-TO-FACE Sheet Bonds
Extremely important in a sandwich structure, as they enable the face sheets and core to work together to make an extremely light and effective structure.
The basic tests this specification requires are the following: climbing drum peel, flatwise tensile, flexural strength, and creep. The tests are conducted at room temperature and at elevated temperature.
Climbing drum peel: A test method used to measure the adhesive bond strength between 2 layers of a material. A drum peels one layer away from the other, and the force required for separation is measured.
Flatwise tensile: A test method that measures the tensile(pulling apart) strength of a material perpendicular to its surface. It determines how well the material resists forces that try to separate its layers
Flexural Strength: A material's ability to resist deformation under a load that causes it to bend. It's a measure of the material's vending strength before it yields or breaks.
Creep: A test that measures the tendency of a solid material to slowly move or permanently deform under the influence of persistent mechanical stresses. This deformation increases over time, even if the load is constant.
2.5 Evaluation of Face Sheet Properties
A sandwich structure is designed so that the face sheets carry the in-plane loading, while the core resists out-of-plane shear loads.
Face sheets develop the running axial loads for a panel in bending, where one is in compression and the other in tension, or for a column both face sheets in compression.
Most important mechanical properties for the face sheets are compressive and tensile strengths and moduli.
Metallic materials such as aluminum alloys are often utilized as face sheets in sandwich structures.
Face sheet behavior can be analyzed in terms of classical laminate theory coupled with Fourier's law of heat flow or Fickian diffusion for moisture equilibrium.
Classical Laminate Theory(CLT): This theory is used to analyze the mechanical behavior of the layered composite face sheets, relating in-plane forces and moments to the resulting strains and curvatures, while considering the properties and orientation of each lamina.
Fourier's Law of Heat Flow: This principle governs the conduction of heat through the material, which helps in understanding and modeling the thermal gradients across the face sheets and the resulting thermal expansion or stresses.
Fickian Diffusion: This law describes the movement of moisture(mass transfer) within the composite material due to concentration gradients, which is essential for predicting moisture absorption, swelling, and potential degradation, especially under various environmental conditions.
2.6 Evaluation Of Sandwich Panels
can be designed to be extremely lightweight and be very stiff and strong
face sheets bonded to a core can be significantly increased over those of a laminate or plate with the same material as the sandwich face sheets for a very small weight increase
The additional bending stiffness and strength depends on the face sheet thickness, the core depth and density, and adhesive bond strength.
Typically a laminate or plate structural panel will need stiffening with bonded or mechanically fastened stiffeners to avoid or reduce buckling, so sandwich panel can achieve the same overall panel
The basic sandwich mechanical properties are the following:
Bending Strength: determined by the thickness and compressive and tensile strength properties of the face sheets and how far apart they are spaced
Bending Stiffness: determined by the thickness and compressive and tensile modulus properties of the face sheets and how far apart they are spaced.
Out of plane shear strength: determined by the thickness and shear strength property of the core
Out of plane shear rigidity: determined by the thickness and shear modulus property of the core
Environmental conditions like temperature, moisture, and fluids can affect the strength and stiffness of a sandwich panel.
Common problems with sandwich structures subjected to temperature and moisture/fluids are those associated with poor core sealing and porous or easily damaged face sheets.
Damage tolerance is more difficult to define for a sandwich structure than for stiffened laminate or plate structure with multiple load paths.
In order to eliminate dangerous stress concentrations, abrupt changes in cross-sectional areas must be avoided whenever practicable by tapering joints, by making small patches round or oval shaped instead of rectangular, and by rounding corners of all large repairs
- ASTM Standard Test Methods for Panel Property
Above are the following ASTM Standard Test Methods that can be used to test sandwich panels.
2.7 Evaluation of Inserts and Fasteners
Sandwich components are often attached to other parts that result in the introduction of concentrated stresses at the attachments.
For very lightly stressed parts, unreinforced fastener holes or subsequently inserted reinforcements will suffice, but in most structural applications local reinforcements must be incorporated during fabrication
The most important mechanical properties for insert and fasteners are generally core shear strength and modulus and face sheet compression(local bearing) strength and modulus.
2.8 Evaluation Of Other Features
Typical edge reinforcements: areas of crushed low-density honeycomb core should be resin stabilized to prevent disintegration under sonic environment.
Chapter 3 - Material Data
3.1 Cores
The core of a sandwich structures serves primarily to separate, support, and stabilize the face sheets such that the desired bending rigidity is attained.
To perform functions at minimum weight, the core material is generally of relatively low density in comparison to the face sheets and adhesive constituents.
Core Types:
Honeycomb
Cross banded
Corrugated
Waffle-Type
Foams
Natural (Balsa wood and other wood)
Honeycomb core is also called open-cell core. Most honeycomb cores can be formed to moderate amounts of single curvature
Honeycomb cores Types A and B are less formable. C, D, E are the more formable options.
Honeycomb core cell size is determined by the diameter of a circle that can be inscribed in a cell.
Thermal conductivity of a honeycomb core depends upon conduction of the material of which the core is made, radiation between face sheets, and convection within the core cell.
Foam cores, plastic cores are foamed, expanded, or processed by other means to reduce the apparent density of the plastic to a practical range for core material.
Desired core properties can be achieved by controlling the expansion process for these cores.
Metallic foamed cores can be produced by mixing molten aluminum-magnesium alloys with suitable foaming agents and cooling the mixture to form a porous solid
Glass foams can also be produced
Even though this core, material is not as strong or stiff as honeycomb core of the same density, it offers advantages:
Core joint elimination, thin and uniform bonding layer between face sheets and core, use of accurately pre molded void free face sheets that can be readily inspected before assembly, good electrical properties, and the flexibility in manufacture.
Wood cores, the selection of natural core materials for sandwich structure is confined to balsa wood with mahogany, spruce, and poplar being used for inserts and edgings
Estimation of core properties
Possible to estimate the density, material, and configuration of core needed to obtain a satisfactory sandwich construction
The elastic moduli and strength of cores of a particular material generally increase as core density increases.
3.2 Face Sheets
They carry the major applied loads
To perform this function the face sheets must be bonded to a core of acceptable quality.
Description of face sheets
Face sheet is in 3 basic categories: adhesively-bonded pre fabricated face sheets, co-cured or co-bonded face sheets with adhesive, and self-adhesive face sheets.
Adhesively-bonded pre-fabricated face sheets: it enables the designer to use the sheet material properties of the face sheet material for structural sizing.
Co-cured or co-bonded face sheets with adhesive: Co-bond refers to an approach where one of the face sheets is pre-cured and the other face sheet is cured simultaneously with the adhesive that bonds the core to that face sheet.
Face sheet properties are highly dependent on processing parameters and core configuration and geometry.
Curing the face sheets materials directly to a honeycomb core may result in waviness or dimpling of the face sheet piles adjacent to the core. Resulting properties of the sandwich should be established by testing representative sections employing production processing techniques and representative tooling approaches.
Self-adhesive face sheets: they are assembled in their uncured state with the core during layup operations, relying exclusively on the face sheet resin material to bond the face sheet to the core. Same thing with the curing as it will cause waviness or dimpling of the face sheet materials.
3.3 Adhesives
Primary surface is to structurally attach the face sheets to the core, reacting shear and peeling forces between the face sheets and the core, and enabling the materials to work together as a system.
Primary considerations for selecting an adhesive for sandwich structures are:
Strength requirements
Service temperature range
Ability to form a suitable fillet at the cell wall - face sheet interface(for a honeycomb core)
Compatibility of cure parameters for face sheet and adhesive (for co-cured or co-bonded face sheet)
Resins for self adhesive face sheets: they provide their own resin to bond the face sheets to the core material.
Adhesive forms/types and uses
Adhesives used in sandwich construction are available in various forms, depending on the application and the fabrication processing for the component.
Chapter 4 - Design and Analysis of Sandwich Structures
4.1 Introduction
layered composite consisting of two or more thin face sheets, bonded to a thicker low-density core, usually a cellular material or corrugated sheet.
The Core material resists shear forces resulting from loads normal to the sandwich plane and provides most of the shear rigidity for the sandwich
The sandwich is analogous to an I-beam, because as much material as possible is placed in the flanges situated farthest from the center of bending or neutral axis. Only enough material is left in the connecting web to make the flanges act in concert and to resist shear and buckling
The core in a sandwich structure takes the place of the web in the I-beam