Standard Testing Methods for Natural Fiber based Hybrid Sandwich Composites

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Standard Testing Methods for Natural Fiber based Hybrid Sandwich Composites

Sandesh Kiran.S1

1 Research Scholar Department of Mechanical Engineering,

University Visvesvaraya College of Engineering, Bangalore, India

  1. M Rajaprakasp

    2 Professor,

    Department of Mechanical Engineering, University Visvesvaraya College of Engineering, Bangalore, India

    Abstract: This paper provides brief summary of standard test methods conforming to ASTM Standards applicable to hybrid sandwich composite panels. Technical parameters of applicable test standards are detailed to help the design engineers in the selection and utilization of sandwich test methods. Apart from ASTM specified test methods for evaluating physical and mechanical properties of cores and sandwich composites, some the other important testing requirements like flammability and environmental compliance is also described.

    Keywords: Natural fibers, Sandwich Panels, Composites, Test Methods, ASTM Standards, Environmental compliance, Flammability

    INTRODUCTION

    Light weight, low cost and ecofriendly characteristics of natural fibers based composites have become the need of the hour due to the rising prices of petroleum products and their environmental risks. Natural fibers based hybrid sandwich composites (NFHC) are also being used extensively in the interior parts of automobiles, generic commercial and construction industry. For product designer or manufacturer of hybrid sandwich composite panels, testing is a critical step in the process of establishing the acceptability of a product before it reaches the market. Test results are the conformation declaration to the customer about trustworthiness and product performance.

    NFHCs main constituents are hydrophilic natural fibers and hydrophobic polymer(s), problems such as cracking, bending, and strength reduction may occur in case of long-term use due different characteristics of core, face sheet and adhesives/resin system. Problem may arise in the sandwich composite panels due to the environment where it need to function e.g. moisture, UV resistance and thermal changes. However, due to the lack of standardized testing methods to evaluate the performance and durability of NFHC, it is difficult to give the orientation for the product development and to protect the consumers

    interest. Consequently International Standards are being established in order to encourage technology development in the NFHC production field and to protect consumers from NFHC products natural challenges by establishing stiff regulatory requirements. The five major issues with natural fiber based composite materials commercial and industrial applications are listed below.

    • misapplication of the product,

    • poor design,

    • inadequate control of materials,

    • poorly controlled manufacturing techniques,

    • Misapplication of materials.

      Adequate testing is required as means for preventing such failures. Testing of natural fiber based hybrid sandwich composites is especially important due to the fact that properties and performance of core face sheet and adhesive/resin system varies with the base materials, manufacturing processing, and design parameters. Standardized and/ or special tests are necessary to aid in materials selection, process development, design, and quality control (including verification of product performance). Purposes of Testing for Natural fiber based hybrid sandwich composites are

    • Assess quality of raw materials.

    • Evaluate and optimize materials.

    • Evaluate and optimize manufacturing process variables.

    • Determine effects of equipment and tool design.

    • Establish engineering design information.

    • Measure quality and reproducibility of end item.

    As expected, safety is the driving force behind many requirements. Protecting end-user from fire, smoke, toxic fumes, etc., may seem obvious but these concerns had no firm guidelines (and resulting punitive action) until after a series of tragic accidents in the 1980s and 1990s.

    Fig .1 Honeycomb based hybrid sandwich panels

    There are many tests that can be used to obtain the mechanical properties and structural integrity of honeycomb cores and sandwich panels. This paper discusses the various tests pertaining to basic honeycomb core and sandwich panel tests per STM Standards Test Methods and the Military Standard 401B, Sandwich Constructions and Core Materials: General Test Methods.

    ASTM STANDARDS SPECIFIC TO SANDWICH PANELS

    ASTM stands for American Society for Testing and Materials. ASTM D30 Committee on Composite Materials has developed 20 internationally recognized standards for the assessment of physical and mechanical properties of sandwich composites. The D30 subcommittee responsible for sandwich standards (D30.09) has revised the majority of its documents, and has developed six new test methods for sandwich composites. ASTM D30 .09 subcommittee oversees development of standard test methods, practices, guides, and terminology in the area of sandwich construction which includes honeycomb, foam, and balsa

    cores with nonmetallic or metallic facings. ASTM has 20 composite materials specific standards published as shown in Table 2; these can be divided into two groups: core material standards and sandwich structure standards.

    Standards are continually revised to meet end-users needs and to adoption of technological advancements. Eight standards (C 271, C 273, C 297, C 364, C 365, C 366, C 393 and F 1645) undergone substantial revision to accommodate conversion into dual unit (SI and inch- pound) standards, revision of scope to define applicable core material forms, addition of sections on terminology, interferences, sampling, calibration, validation and reporting. New revision of the standards outline the sampling rates and minimum number of data points for digital data collection and recording [6]. Table 1 illustrates the list of properties specific to sandwich panels. Table 2 tabulates the list of tests specific to cores and linking ASTM specs. (Specifications) with MIL(Military) standards. Table 3 tabulates the list of tests specific to sandwich panels.

    Table 1. Properties of Interest for Sandwich Panels [1]

    Sl.No

    MECHANICAL PROPERTIES

    Sl.No.

    THERMAL PROPERTIES

    1

    Tensile properties

    1

    Thermal expansion

    Tensile strength

    2

    Thermal shrinkage

    Modulus of elasticity

    3

    Thermal conductivity

    Elastic limit

    4

    Specific heat

    Yield strength

    5

    Heat distortion temperature (deflection Temperature under load)

    2

    Compressive properties

    6

    Flammability (/lame resistance)

    Strength

    7

    Ignition properties

    Modulus

    8

    Maximum safe operating temperature

    3

    Flexural properties

    9

    Flow temperature

    Strength

    10

    Brittleness temperature

    Modulus

    ELECTRICAL PROPERTIE

    4

    Shear properties

    5

    Impact strength

    1

    Electrical resistance (insulation resistancevolume and surface

    6

    Properties at high rates of loading

    2

    Dielectric strength and dielectric breakdown voltage

    (dynamic properties)

    3

    Dielectric constant and power factor (loss factor)

    7

    Shear properties

    4

    Arc resistance

    Strength

    MISCELLANEOUS PHYSICAL PROPERTIES

    Modulus of rigidity

    8

    Bearing strength

    1

    Specific gravity (density)

    9

    Surface hardness

    2

    Porosity

    Indentation hardness

    3

    Machinability

    Scratch resistance

    4

    Punching quality

    Abrasion resistance

    Mar resistance

    CHEMICAL AND PERMANENCE PROPERTIES

    10

    Creep properties (creep-rupture and stress-relaxation)

    1

    Resistance to chemical reagents

    11

    Fatigue (cyclic properties)

    Acids

    12

    Poisson's ratio

    Bases

    13

    Notch sensitivity

    Solvents

    14

    Shalterproofness

    Fuels

    15

    Shockproofness

    Bacteria and fungi

    16

    Tear resistance

    Salt spray

    OPTICAL PROPERTIES

    2

    Water absorption

    3

    Water vapor permeability (diffusion)- gas transmission rate

    1

    Spectral transmission (haze)

    4

    Accelerated service (temperature and humidity)

    2

    Index of refraction (refractive index)

    5

    Sunlight and weather exposure (aging)

    3

    light diffusion

    6

    Effects of radiation

    4

    Crazing resistance

    7

    Impact sensitivity (LOX)

    5

    Internal stress; transparent plastics

    8

    Toxicity

    6

    Optical uniformity and distortion

    9

    Volatile loss (outgassing)

    7

    Surface stability, optical

    10

    Stress-crazing

    Table 2. Specifications applicable for Cores [2]

    Table 3. Specifications applicable for Sandwich Panels [2]

    FIRING ORDER OF TESTING REQUIREMENTS

    Most of the time, design engineers tend to do mechanical testing of sandwich panel specimens but environmental compliance test like flammability test, smoke & toxicity test, water absorption test are extremely important. If test specimen pass all mechanical, thermal and physical testing and fails in environmental compliance test, than it is not worthwhile to peruse because of the fact that newly developed material may be harmful to human use and for the environment. Thus it is always advisable to start testing from the compliance stand point of view. Tabulation covered in Table 5 provides details of Firing order of testing requirements

    Table 5. Firing Order and Priority of Tests for Hybrid Composite Panels

    TEST SPECIMEN CONDITIONING REQUIREMENTS

    Test samples needs to be conditioned before starting the tests. Temperature and humidity to which specimen is expected to be tested can affect the properties of the sandwich panels. ASTM D618-61 provides the outline for specimen conditioning requirements before actual testing.

    Table 4. Specimen conditioning requirement for Sandwich Panels [1]

    This paper intends to provide summary of minimum and basic set of tests essential for validation of natural fiber based hybrid sandwich panels. Design engineers and manufacturers of the natural fiber based sandwich panels needs to select applicable tests based on form, fit, functional requirements and customer driven requirements. Different types of honeycomb based sandwich panels are shown in Fig.2.

    Fig .2 Types of Honeycomb based hybrid sandwich panelsType

    1. Aluminium Core Glass fiber faceesheet based Hybrid Panel

    2. Paper based circular cell type veneer facesheet based Panel

    3. Polyproplyne based circular cell type PVC facesheet based panel

    4. Polycorbonate based circular cell type wood venner based hybrid panel

      FLAMMABILITY

      Flammability Tests both on laminates and sandwich panels must conform to fire safety standards in all parts of applications occupied by humans. Flammability tests can be used for assessing the risks and impact of a fire. Tests will measure the heat and smoke release rates by materials and products when exposed to a set level of radiant heat. The results reveal how the material will perform under laboratory conditions in response to heat and flame. The tests also measure heat release from injection, duration before ignition and progressive flame involvement on the surface of a material. The test procedure for Flammability test of sandwich panels are usually according to ASTM D 3014 standards. The test is carried out by mounting a specimen vertically or horizontally on a vertical stand and ignites it with a Bunsen burner for 10 sec. The flame height, time of burning and weight percent retained by the specimen is determined. The test procedure for Flammability test of composite laminates in vertical and horizontal direction was according to UL 94 V and UL 94 HB respectively [3].

      VERTICAL BURN TESTING

      In vertical direction test, a small 3/4th inch high blue flame is applied to the bottom of the specimen for 10 sec, withdrawn, then reapplied for an additional 10 sec, the duration of flaming and glowing is noted as soon as the specimen is extinguished. A layer of cotton is placed beneath specimen to determine whether dripping material will ignite it during the test period.

      HORIZONTAL BURN TESTING

      In horizontal direction test, burner ignited to produce 1 inch high blue flame. A depth of 1/4th inch flame applied to specimen for 30 sec without changing the position of the burner and is removed from the burner. If the specimen burns to the 1 inch mark before 30sec the flame is withdrawn. If the specimen continues to burn after removal of the flame, the time for the flame front to travel from the mark 1 inch from the free end to the mark 4.0 inch from the free-end is determined and rate of burning is calculated.

      Fig.3 Flammability Test set up

      1. Vertical Burn Test (b) Horizontal Burn Test

45° FLAME TEST

45° Flame Test is typically use in aerospace application where sandwich panel are mounted on a fixture at an angle of 45° and placed under Bunsen burner flames. Test samples will be place under flame for a specified time frame (12 second or 60 second). After burning and cooling, the test samples burn pattern will be analyzed for pass/fail criteria.

Fig.4 Flammability Test set up for 45o burn test

BURN-THROUGH RESISTANCE TEST

The Burn-Through Resistance test specifically applies to materials used to measure how well a material will prevent fire from spreading (either in orout) in the usage area in the event of an in-flight fire. This test measures the amount of smoke produced by a material when it burns. During a fire, dense and noxious or impossible to see and breathe, so designing a material that emits less smoke is critical.

The Burn-Through Resistance test is very critical for aerospace products and FAA (federal aviation authority) has laid out clearly defined requirements. If a 1,700° F flame does not burn a hole through the product within five minutes and the temperature above the product does not exceed 400° F, the product passes. As stated above, protecting passengers from the danger of an in-flight fire, and the resulting smoke and toxic fumes, is at the core of material requirements by the FAA. In addition to liners,

floor panels and interior panels, the Smoke Density Test is also conducted on honeycomb materials when used in the interior of an aircraft.

REQUIREMENT BASED SPECIFIC TESTS

The Requirement based tests are tests aligned with specific industry like aerospace, medical and marine applications where regulatory governing body lays down stringent regulation for the use of sandwich composites panels. Most of the requirement based tests may not be applicable for commercial and generic application because of the nature of the requirements. The family of requirement based tests consists of wear and tear test, Impact (Puncture) Test, Roller Cart Test, Material Construction Tests and Edge Bearing Test. Test criteria for Requirement based tests are tabulated and illustrated in Table 6.

Table 6. Requirement based Tests

CORE TESTING

While core used in sandwich panels does resist shear loadings, its primary purpose is to keep the facesheets separated and thus maintain a high section modulus (a high "moment of inertia" or "second moment of the area"). The core material is typically of relatively low density (e.g., honeycomb or foam), which results in high specific mechanical properties of the panel under favorable

loadings (in particular, high flexural strength and stiffness properties relative to the overall panel density). Sandwich panels are particularly efficient in carrying bending loads, although they have other important load application uses as well. For example, they provide increased buckling and crippling resistance to shear panels. Additionally, they provide buckling resistance to compression members. Fig.4 illustrates different tests used specifically on cores.

Fig 4 Sandwich Core specific testing

  1. Tensile Strength test for Core (b) Flatwise Compressive Strength Test

    (c) 3-Point Bending (d) Test 4-Point Bending Test (e) Shear Fatigue Test

    DENSITY OF SANDWICH CORE MATERIALS

    ASTM D271 is the standard method for determining the density of sandwich core materials. For sandwich core materials, its properties may be proportional to density. Specimens are cut from fabricated sandwich panels and conditioned as per standard test conditions and then weighed. Specimen dimension are maintained to be at 304.8mm x 304mm (12 inch x 12 inch) with thickness and needs to have five Specimens for averaging. Unit of Density is Kg/m3.

    Density = 1,000,000 * Wt. / (l * w * t) Wt. = weight of sandwich panels

    L = length

    w = width

    t = thickness

    WATER ABSORPTION OF CORE MATERIALS

    Test method as per ASTM C272 is applied for structural core materials such as honeycomb or foam for the determination of water absorption characteristics when immersed or in high relative humidity environments. ASTM C272 outlines three methods: Method A which recommends for 24 hour immersion; Test Method B which recommends for Elevated Temperature Humidity; and Test Method C for Maximum Percent Weight Gain. At least five samples with dimensions 76.2mm x 76.2mm (3.0 inch x

      1. inch) with applicable thickness are cut from fabricated sandwich panels. The % increase in weight is given by

        Increase in Weight, % = (W D) / D * 100 W = wet weight

        D = dry weight

        CORE SHEAR PROPERTIES

        ASTM C273 test method is used to determine core shear properties of a sandwich construction material using loading plates bonded to the outer surfaces or skins (face sheets). The test is applicable for either continuous (foams) or discontinues bonding surfaces (honeycomb). The bonded loading plates are either pulled in tension or pushed in compression on a Universal Test Machine in order to shear the material within 3 to 6 minutes. Specimen size is 50.8mm x 190.5mm(2inch x 7.5inch) with applicable thickness of 25.4mm (1inch). Data from ASTM C273 will help in calculation of Ultimate core shear strength, 2% offset shear stress, Engineering shear strain, Core shear modulus and identification of Failure type.

        TENSILE STRENGTH OF HONEYCOMB CORE MATERIALS

        The honeycomb bond strength is a fundamental property that can be used to determine if honeycomb cores

        can be manipulated during cutting, machining and forming without breaking the nodes. Tensile strength of the honeycomb core materials are tested as per ASTM C363. The binding strength of the traction node is the tensile stress that causes the failure of the honeycomb due to the breaking of the link between the nodes. It is usually a failure of the peeling type. This test method provides a standard method for obtaining tensile knot joint strength data for quality control, acceptance specification testing and research and development.

        FLATWISE COMPRESSIVE STRENGTH OF SANDWICH CORES

        Test method as per ASTM C365 is used to determine compressive strength of the core. Core materials which can be tested as per ASTM C365 are continuous (foams) and discontinue bonding surfaces (honeycomb) stabilized or unstabilized. The conditioned specimens (if required) are loaded in to the Universal Test Machine with ram head speed maintained at 0.50 mm/min (0.020 in/min). A batch of five specimens will be tested at different thickness levels.

        • Continuous bonding surfaces minimum facing area shall be 625 mm2 (1.0 incp)

        • Discontinuous cellular bonding surfaces 60 cell minimum per test specimen

    Flatwise Compression Testing provides results for the calculation of Ultimate Strength and analysis of failure type/mode.

    THICKNESS MEASUREMENT OF SANDWICH CORES

    This specification covers the procedures for measuring the thickness of flat sandwich cores as per ASTM C366. There are two types of methods for measuring the thickness of sandwich cores. One is Test Method A which involves Roller-Type Thickness Tester and the second method is Test Method B which involves Disk-Type Thickness Tester.

    CORE FLEXURAL SHEAR PROPERTIES

    The test method ASTM C393 determines flexural strength, core shear strength of the core material used in hybrid sandwich panel construction. Core materials applicable include continuous (foams) and discontinue bonding surfaces (honeycomb). There are two types of tests used in the industry to evaluate the flexural strength of the core material. They are Short Beam and Long Beam Test. Both this tests provide flexural strength and shear allowable stress values for core material. In Short Beam Tests, effect of shear in flexure behavior plays important role and in Long Beam Test, the effect of shear in flexure design is neglected [4].

    The conditioned specimens are loaded into a 3-point or 4-point loading fixture on the Universal Test Machine and ram speed maintained at 6 mm/min (0.25 in/min). The specimen size is 75 mm x 200 mm (3.0 in x 8.0 in) at

    various levels thickness. From 3-point or 4 point loading tests core materials Core Shear Ultimate Stress, Core Shear Yield Stress and Facing Stress can be calculated. FS, the ultimate flexural strength is calculated by

    FS = P max / ((d+c) b)

    Where (P max) is the maximum load before failure, (d) is sandwich thickness and (c) is thickness of the core and

  2. is the sandwich width.

    Fig. 4. Three point bending test Fig. 5. Four point beding test

    SHEAR FATIGUE – SANDWICH CORE MATERIAL

    This test determines the effect of repeated shear loads on sandwich core materials as per ASTM C394. These test methods provide a standard method of obtaining the sandwich core shear fatigue properties. Usually the most critical stress to which a sandwich panel core is subjected is shear stress and the effect of repeated shear stresses on the core material can be very important. ASTM C394 uses the same test fixture as the present ASTM C273 standard. However, the core material is to be tested without face sheets. That is, the core is bonded directly to the bonding plates.

    DIMENSIONAL STABILITY OF SANDWICH CORE MATERIALS

    Sandwich panel cores may change planar dimensions when heated. This phenomenon can be associated with the effects of heating upon the core material itself, as well as changes in core moisture content resulting from the heating cycle. It is prudent to know if this may be problematic with regard to the intended final part dimensions.

    ASTM-D6772 provides a standard method of characterizing the dimensional stability of sandwich core materials for design properties, material specifications, research and development applications, and quality assurance. Factors that influence dimensional stability of sandwich core materials and shall therefore be reported include the following: core material, methods of material fabrication, core geometry, core thickness, core thickness uniformity, cell wall thickness, specimen geometry, specimen preparation, heating and cooling environments (including temperatures and humidity levels), and specimen conditioning (both prior to and after heating).

    POISSONS RATIO

    Test method D 6790 is used to determine the Poissons ratio of honeycomb core materials. A flat square sample of core is bent around a cylinder of known radius (610 mm is recommended), making sure that the specimen remains in contact with the cylinder along its centerline. Using a straightedge that spans the specimen, depth and chord measurements are taken, and then used to calculate the anticlastic curvature radius of the core. The Poissons ratio

    is then calculated by dividing the cylinder radius by the anticlastic curvature radius.

    STATIC ENERGY ABSORPTION PROPERTIES OF SANDWICH CORE MATERIAL

    The test method used determines the static energy absorption properties (compressive crush stress and crush stroke) of honeycomb sandwich core materials is ASTM.D7336. These properties are usually determined for design purposes in a direction normal to the plane of facings as the honeycomb core material would be placed in a structural sandwich construction. Permissible core materials for this test are honeycomb and foam. The procedures contained within this standard are intended to assess the crush stress and crush stroke properties of the sandwich honeycomb core material under static compressive loading. The dynamic crush stress of the honeycomb core material may vary from that measured under static loading, depending upon factors such as honeycomb core material thickness, core material density, impact velocity, etc.[6].

    Factors that influence the compressive crush stress and crush stroke are honeycomb core material, methods of material fabrication, core material geometry (nominal cell size), core material density, specimen geometry, specimen preparation, specimen conditioning, environment of testing, specimen alignment, pre-crush procedure, pre- crush depth, loading procedure, and speed of testing.

    FLUID MIGRATION TEST

    Test method F 1645/F 1645M is used to determine the rate of water migration within honeycomb core materials. A single cell in a core sample is filled with water, and is subjected to a constant hydrostatic pressure by maintaining a 915mm (36inch) tall water column height. The amount of water transferred into the honeycomb core (primarily due to diffusion through the cell walls) within a 24-hour period is determined. The requirements for maintaining the head of water throughout the test is within ±0.5% to keep the water pressure consistent in the primary core cell.

    SANDWICH PANEL SPECS.

    The testing of sandwich panels has necessarily existed for as long as these panels have been used as structural

    materials in high-performance applications. However, the refinement and standardization of these various test methods is much more recent. The most important question to be answered is, "What properties or characteristics of the sandwich panel need to be evaluated?" The answer is based on how the panel might fail. The panel can fail in any one of several different ways, depending upon the geometric and fabrication characteristics of the panel and loading

    pattern on the panel. Some the examples are, a factsheet can fail in tension, compression, shear or local buckling. Additionally, the core can fail in shear or by crushing. A face sheet can separate from the core due to excessive shear or normal tensile stress in the adhesive bond. Test methods have been developed to isolate and simulate each of these specific failure models [5].

    Fig 6 Sandwich Composite Panel specific testing

    1. Tensile Strength test for sandwich panel (b) 3-Point Bending Test

(c) Test 4-Point Bending Test (d) Drum Peel Test (e) Drop Impact Test

(f) Sound Absorption Test

STANDARD TERMINOLOGY OF STRUCTURAL SANDWICH CONSTRUCTIONS

This terminology as per ASTM C274-07 covers terms necessary for a basic uniform understanding and usage of the language specific to structural sandwich constructions. The simplest structural sandwich is a three layered construction formed by bonding a thin layer (facing) to each side of a thick layer (core).

TENSILE STRENGTH OF SANDWICH CONSTRUCTIONS

The Tensile Strength Test of sandwich construction as per ASTM C297 provides information on core-to-face bonding stability, load transfer and flatwise tensile strength of sandwich core material. ASTM C297 is applicable for continuous bonded surfaces such as foams or discontinuous bonded surfaces such as honeycomb materials. The data is often used to specify a material, to design parts to withstand application force and as a quality control check for materials. Since the physical properties of many materials can vary depending on temperature, it is sometimes appropriate to test materials at temperatures that simulate the intended end use environment.

Test specimens are cut from sandwich panels, and the faces of the test specimens are bonded to aluminum blocks.

The adhesive must then be allowed the appropriate conditioning before testing. One of the aluminum blocks is mounted in a fixture on the stationary side of a universal test machine, while the opposite aluminum block is mounted in a fixture on the moving side of the universal test machine. The assembly is then pulled apart at the suggested standard head displacement speed of 0.50 mm/min (0.020 in/min) to failure, or at a speed set to produce failure within 3 to 6 minutes. Failure between an aluminum block and the sandwich structure is not an acceptable failure mode. For discontinuous cellular bonded surfaces, specimen test geometry is based on cell size. For continuous bonded surfaces, the minimum facing size is 625 mm2 (1.0 in2).

EDGEWISE COMPRESSIVE STRENGTH OF SHORT SANDWICH CONSTRUCTION

ASTM C364 / C364M is the standard method of obtaining sandwich edgewise compressive strengths. The results from the test helps in review of panel design properties, material specifications, research and development applications and quality assurance. The list of items, that can influence edgewise compressive strength to be reported are materials, fabrication method, face sheet lay-up orientation, core orientation, results of any

nondestructive inspections, specimen preparation, specimen dimensions and associated measurement accuracy, speed of testing, failure mode, and failure location. This test method covers the compressive properties of structural sandwich constructionin a direction parallel to the sandwich facing plane.

FLEXURE CREEP OF SANDWICH CONSTRUCTIONS

Test method per ASTM C 480/C480M covers the determination of the creep characteristics and creep rate of flat sandwich constructions loaded in flexure, at any desired temperature. Permissible core material forms include those with continuous bonding surfaces (such as foams) as well as those with discontinuous bonding surfaces (such as honeycomb).

LABORATORY AGING OF SANDWICH CONSTRUCTIONS

Test methods ASTM C481 covers the determination of the resistance of sandwich panels to severe exposure conditions as measured by the change in selected properties of the material after exposure. The exposure cycle to which the specimen is subjected is an arbitrary test having no correlation with natural weathering conditions.

SANDWICH PLATE FLEXURE

The ASTM D6416 defines the flexural testing of sandwich composite panels subjected to a distributed load. The properties obtained from this testing include the panel face-sheet strain, bending stiffness, shear stiffness, panel strength and panel failure modes. Test method standardized in D 6416/D 6416M determines the two-dimensional flexural stiffness and strength properties of sandwich composite plates subjected to a distributed out-of-plane load. The procedure for testing a square panel, is to place the panel on simply-supported four sides that is uniformly loaded over a portion of its surface using a water-filled bladder. The bladder contact area has a readily definable geometric shape. Surface pressure is increased by moving the test frame, which compresses the bladder against the panel surface. Panel deflection, surface strains and bladder pressure are measured continuously throughout the test[6].

SANDWICH FLEXURAL STIFFNESS

The test method as per ASTM D7249 provides information on sandwich flexural stiffness, core shear strength, shear modulus, facing compressive strength and facing tensile strength. Core materials applicable include continuous (foams) and discontinuous bonding surfaces (honeycomb). Bonded strain gages are applied to the samples and conditioned. The specimens are installed into the 4-point bend test fixture of the Universal Test Machine. The fixture and specimen are aligned specimen perpendicular to the loading bars with the bars perpendicular to the plane of the specimen facing. Test samples are tested with compressive force until failure. Specimen size is 75 mm x 600mm (3.0 inch x 24.0 inch. Five specimens will be tested at thickness. From 4-point bend test results in Force-Displacement Behavior and

Facing Ultimate Stress, Ultimate Facing Stress, Effective Compressive and Tensile Chord Modulus, Effective Sandwich and Flexural Stiffness can be calculated. Results also revels the Failure type/mode.

SANDWICH BEAM FLEXURAL AND SHEAR STIFFNESS

Test method ASTM D7250 provides Standard Practice for Determining Sandwich Beam Flexural and Shear Stiffness. This test covers the determination of the flexural and transverse stiffness properties of flat sandwich constructions subjected to bending in such a way that the applied moments produce curvature of the sandwich cladding planes. Permitted core material forms include those with continuous bonding surfaces (such as foams) as well as those with discontinuous bonding surfaces (such as honeycomb). This test is applicable for the sandwich beams that exhibit a linear force-strain response.

CLIMBING DRUM PEEL TEST

Test method ASTM D1781 is used to evaluate adhesion between flexible facing of a sandwich structure and its core. The peel resistance may indicate degree of cure for relatively brittle adhesives. Sample dimension varies with type of material. For sandwich panels, 76 mm x 305 mm (3in x 12 in) including ~25 mm (1 inch) overhang of one facing at each end. Test samples will be tested with a cross head speed of 25mm/min (I inch) to a total length of 6 inch (152 mm). After the specimen facing was peeled, the cross head motion of the test machine was reversed and the crosshead returned to its starting position. Test was again repeated on the same specimen to find the average load required to rewind the peel skin back (Fo).The average load (Fp) required to peel the skin from the core is the average of five peaks and five troughs of the load deflection curve obtained during testing . The average peel torque (T) can be obtained from the equation mentioned below.

T = [(ro-ri)(Fp-Fo)]/W

where, the radius of the flange plus one half the thickness of the loading straps (ro) is 63.5mm and ri represents the radius of the drum plus one half the thickness of the adherend being peeled which is 51.206mm[7].

ACOUSTIC PROPERTY MEASUREMENT

Sound absorption property of sandwich panels usually performed using the impedance tube method. The sound absorption coefficient (SAC) of sandwich composites was measured through a transfer function technique with the impedance tube facilities. The SAC is defined as the ratio of the absorbed sound energy and the incident sound energy. The measurements were according to ISO10534-2 standard (International organization for standards) at medium to high frequencies from 250 to 10,000 Hz at 25

°C and 60% relative humidity, and at least three specimens were tested for each group. Samples were backed up by a rigid wall which reflected all the incoming sound energy. Two microphones were mounted at the wall of the tube to measure the sound pressure. Therefore, if the incident sound energy was known and the transmission sound energy could be measured with the aid of the transfer function, the sound energy absorbed by the materials could be obtained [8].

DROP WEIGHT IMPACT TEST

The test method according to ASTM 7136 outlines the Drop weight impact test. A known weight is dropped in a vertical direction and the impact energy is calculated from the dropped height. Since the falling weight either stopped on the test specimen, or destroyed the specimen completely and passed through it, the only result that could be obtained is of a pass or fail the test.

Instrumented impact test devices are made with electronic sensing instrumentations that can record the load on the test specimen continuously as a function of time and specimen deformation prior to fracture. The instruments can record strain or stress against time for the entire period of an impact test. The data from the acquisition system provides complete representation of impact test history rather than a value from a single calculated number. Another advantage is that the test times can be reduced because of its automated data acquisition system. Generally instrumented drop weight test equipment are considered to be the best general impact tests devices presently available.

CONCLUSIONS

For sandwich composite based product designer or a manufacturer, testing is a critical step in the process of establishing acceptability of a product before it reaches the market. Test results conforms the customer that they are s getting a product that meets or exceeds there requirement and satisfaction to design engineer. Over the past decade or more, natural fiber based sandwich panels are gaining lot of attention due to various advantages compared to synthetic fiber based sandwich panels. Multiple tabulations were created starting from tabulation of the applicable properties for the sandwich panels, tabulation for core testing, tabulation of sandwich panel testing, tabulation for normalizing and tabulation of tests based on firing order or priority of requirement. Testing of sandwich panel is as important as designing of the panels with customer requirements and industrial regulations. Firing order of testing based on priority of requirements will help the design engineer to narrow down the applicable set of tests. Testing requirement for generic sandwich panels tabulated and segregated by mechanical, thermal, electrical, optical and miscellaneous tests. All the tests documented in this paper may not be applicable for natural fiber based sadwich panels but designer has to choose the best set of tests based on form, fit and functional requirements of the sandwich panel.

REFERENCE

  1. George Lubin, Handbook of composites, 1995

  2. Tom Bitzer, Honeycomb Technology materials, design, manufacturing, application and testing, 1997

  3. https://www.fire.tc.faa.gov/

  4. Standard Test Method for Core Shear properties of Sandwich Constructions by Beam Flexure. ASTM designation C393/C 393M-06

  5. https://www.compositesworld.com/columns/sandwich-panel- test-methods.

  6. A. J. Sawicki, Development of Standard Test Methods for Sandwich Composites Under ASTM Committee D30

  7. Standard test method for climbing drum peel for adhesives. ASTM designation D1781, Annual book of ASTM Standards, Volume 15.06;

  8. Zhang, Jichun & Shen, Yiou & Jiang, Bing & Li, Yan. (2018). Sound Absorption Characterization of Natural Materials and Sandwich Structure Composites. Aerospace. 5. 75. 10.3390/aerospace5030075.

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