Due to the very large number of tests offered, descriptions of those most commonly requested have been gathered into the following groups:. Core Shear Properties of Sandwich Constructions by ASTM C Historical Version s — view previous versions asttm standard Translated Version s: Sandwich constructions are increasingly being used as high-strength, light-weight components, and therefore, standard test methods are being developed and used to determine critical properties and qstm assessments. Permissible core material forms include those with continuous bonding surfaces such as balsa wood and foams as well as those with discontinuous bonding surfaces such as honeycomb. Referenced Documents purchase separately The documents listed below are referenced within the subject standard but are not provided as part of the standard. From 3-point loading results the following calculations can be made: Wider load pads with rubber pads may assist in distributing the loads.
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A number in parentheses indicates the year of last reapproval. A superscript epsilon e indicates an editorial change since the last revision or reapproval. This standard has been approved for use by agencies of the Department of Defense. Scope 1. Permissible core material forms include those with continuous bonding surfaces such as balsa wood and foams as well as those with discontinuous bonding surfaces such as honeycomb.
Within the text the inch-pound units are shown in brackets. The values stated in each system are not exact equivalents; therefore, each system must be used independently of the other. Combining values from the two systems may result in nonconformance with the standard. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Referenced Documents 2.
Terminology 3. Terminology C denes terms relating to structural sandwich constructions. Terminology D denes terms relating to plastics. Terminology E 6 denes terms relating to mechanical testing. Terminology E and Practice E dene terms relating to statistics.
In the event of a conict between terms, Terminology D shall have precedence over the other terminologies. Current edition approved Sept.
Published October Originally approved in Last previous edition approved in as C — No further reproductions authorized. Summary of Test Method 4.
Force versus deection measurements are recorded. Failure of the sandwich facing preceding failure of the core or core-to-facing bond is not an acceptable failure mode. Use Test Method D to determine facing strength. Signicance and Use 5. Tests to evaluate core shear strength may also be used to evaluate core-to-facing bonds. NOTE 1—Core shear strength and shear modulus are best determined in accordance with Test Method C provided bare core material is available.
NOTE 2—Concentrated loads on beams with thin facings and low density cores can produce results that are difficult to interpret, especially close to the failure point. Wider load pads with rubber pads may assist in distributing the loads. Interferences 6. A specic material factor that affects sandwich cores is variability in core density.
Important aspects of sandwich core specimen preparation that contribute to data scatter include the existence of joints, voids or other core discontinuities, out-of-plane curvature, and surface roughness. Specimens tested in various environments can exhibit signicant differences in both strength behavior and failure mode.
Critical environments must be assessed independently for each specic combination of core material, facing material, and core-tofacing interfacial adhesive if used that is tested. In other cases, facing failure can cause local core crushing. When there is both facing and core failure in the vicinity of one of the loading points it can be difficult to determine the failure sequence in a post-mortem inspection of the specimen as the failed specimens look very similar for both sequences.
For some core materials, the shear strength is a function of the direction that the core is oriented relative to the length of the specimen. Apparatus 7. The instrument s shall have an accuracy of mm [ Non-standard 3- and 4-point loading congurations have been retained within this standard a for historical continuity with previous versions of Test Method C , b because some sandwich panel designs require the use of non-standard loading congurations to achieve core or bond failure modes, and c load-deection data from non-standard congurations may be used with Practice D to obtain sandwich beam exural and shear stiffnesses.
The bars shall have sufficient stiffness to avoid signicant deection of the bars under load; any obvious bowing of the bars or any gaps occurring between the bars and the test specimen during loading shall be considered signicant deection.
The recommended conguration has a 25 mm [1. The tips of the V-shaped loading bars shall have a minimum radius of 3 mm [0. Loading bars consisting of 25 mm [1. Also, the load and support span lengths tend to increase as the specimen deects when cylindrical loading bars without V-grooved loading pads are used for example, rolling supports. The velocity of the movable head shall be capable of being regulated in accordance with NOTE 4—The use of crosshead or actuator displacement for the beam mid-span deection produces inaccurate results, particularly for 4-point loading congurations; the direct measurement of the deection of the mid-span of the beam must be made by a suitable instrument.
Report the method of sampling. In cases where the standard specimen conguration will not produce a desired failure, a non-standard specimen shall be designed to produce a core or bond failure mode. The depth of the specimen shall be equal to the thickness of the sandwich construction. The specimen length shall be equal to the support span length plus 50 mm [2 in.
Limitations on the maximum specimen width are intended to allow for the use of simplied sandwich beam calculations; plate exure effects must be considered for specimens that are wider than the restrictions specied above. However, if the facings are too thick, the transverse shear force will be carried to a considerable extent by the facings, thus leading to a high apparent core shear strength as computed by the equations given in this standard.
Chamber conditions shall be monitored either on an automated continuous basis or on a manual basis at regular intervals. This chamber shall be capable of maintaining the gage section of the test specimen at the required test environment during the mechanical test. Sampling and Test Specimen 8. For the standard test conguration, facings consisting of a laminated composite material shall be balanced and symmetric about the sandwich beam mid-plane.
The calculations assume constant and equal upper and lower facing stiffness properties. This assumption may not be applicable for certain facing materials such as aramid ber composites which have signicantly different tensile and compressive moduli or which exhibit signicant non-linear stress-strain behavior.
The test requestor is responsible for specifying the facing thicknesses to be used for the calculations in this test method.
For metallic or precured composite facings which are secondarily bonded to the core, the facing thickness should be measured prior to bonding. In these cases the test requestor may specify that either or both measured and nominal thicknesses be used in the calculations.
For co-cured composite facings, the thicknesses are generally calculated using nominal per ply thickness values. Take precautions when cutting specimens from large panels to avoid notches, undercuts, rough or uneven surfaces, or delaminations due to inappropriate machining methods.
Obtain nal dimensions by water-lubricated precision sawing, milling, or grinding. The use of diamond coated machining tools has been found to be extremely effective for many material systems. Edges should be at and parallel within the specied tolerances. Record and report the specimen cutting preparation method. Calibration 9. Conditioning Procedure NOTE 6—Determine specic material property, accuracy, and data reporting requirements prior to test for proper selection of instrumentation and data recording equipment.
Estimate the specimen strength to aid in transducer selection, calibration of equipment, and determination of equipment settings. Store the specimens in the conditioned environment until test time, if the test environment is different than the conditioning environment.
Measure the specimen length and width with an accuracy of mm [ Measure the specimen thickness with an accuracy of mm [ Record the dimensions to three signicant gures in units of millimeters [inches]. If the ultimate strength of the material cannot be reasonably estimated, initial trials should be conducted using standard speeds until the ultimate strength of the material and the compliance of the system are known, and speed of testing can be adjusted.
However, cases such as elevated temperature testing of a moist specimen place unrealistic requirements on the capabilities of common testing machine environmental chambers. In such cases, the mechanical test environment may need to be modied, for example, by testing at elevated temperature with no uid exposure control, but with a specied limit on time to failure from withdrawal from the conditioning chamber.
Record any modications to the test environment. Remove any remaining preload, zero the strain gages and balance the LVDT. Load the specimen until failure or until a deection equal to the specimen thickness is reached. NOTE 7—Some core materials do not exhibit a well-dened fracture failure with sudden loss of load-carrying capacity, rather failures are characterized by a protracted yield of the core in shear, resulting in large core-shear deformation while continuing to carry load.
Tests of such materials should be stopped within the limits of linear beam theory. If any initial failures are noted, record the force, displacement, and mode of damage at such points. Potential initial non-catastrophic failures that should be reported include: facesheet delamination, core-to-facesheet disbond, partial core fracture, and local core crushing.
Record the mode, area and location of each initial failure. Use the failure identication codes shown in Table 1. Record the method used to determine the initial failure visual, acoustic emission, etc. Record the maximum force, the failure force, the head displacement and the LVDT deection at, or as near as possible to, the moment of ultimate failure. Shear failures of the sandwich core or failures of the core-to-facing bond are the only acceptable failure modes.
Failure of one or both of the facings preceding failure of the core or core-to-facing bond is not an acceptable failure mode. Validation Retests shall be performed for any specimen on which values are not calculated. Calculation An example of a transition region is shown in Test Method D Determine the slope of the forcedisplacement curve above and below the transition point using chord values over linear regions of the curve.
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