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ASTM D8637/D8637M-25
Standard Test Method for Mode I Dominant Face Sheet-to-Core Fracture Toughness of Sandwich Constructions
Standard Test Method for Mode I Dominant Face Sheet-to-Core Fracture Toughness of Sandwich Constructions
D8637_D8637M-25
ASTM|D8637_D8637M-25|en-US
Standard Test Method for Mode I Dominant Face Sheet-to-Core Fracture Toughness of Sandwich Constructions
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D8637/D8637M Standard Test Method for Mode I Dominant Face Sheet-to-Core Fracture Toughness of Sandwich Constructions>
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Significance and Use
5.1 In a sandwich construction, face sheet-to-core bond integrity is necessary to maintain face sheet stability and permit force transfer between the face sheet and core. Knowledge of a sandwich construction’s resistance to interface region fracture is useful for product development and material selection. Furthermore, a measurement of the Mode I fracture toughness is useful for establishing design allowables used in damage tolerance analyses of composite sandwich structures with the same sandwich construction.
5.2 This test method serves the following purposes:
5.2.1 To compare quantitatively the relative values of GIc for sandwich constructions with different constituents.
5.2.2 To compare the relative values of GIc for sandwich constructions consisting of the same component materials (face sheet, resin materials and core) but with differing dimensions such as core height and face sheet thickness.
5.2.3 To compare qualitatively the relative through-the-thickness location of face sheet-to-core separation (which could be a face sheet-to-core disbond or a fracture of the core in the interface region) for sandwich constructions of either the same component materials but with differing dimensions or for sandwich constructions with differing component materials.
5.2.4 To develop face sheet-to-core separation failure criteria for sandwich composite damage tolerance and durability analyses.
5.2.5 To qualitatively evaluate the bondline failure mode as part of bonding process qualification.
5.3 This test method may be used to determine one or both of the following two Mode I fracture toughness values:
5.3.1 Crack Initiation Fracture Toughness—A single value of crack initiation fracture toughness shall be reported. This GIc(init) value is based on the force value shown in Fig. 2(a) and described in 5.3.1.1. The crack length at crack initiation, a0, shall be used to calculate GIc(init) as described in 13.2.
5.3.1.1 Crack Initiation/Maximum Force (Init/Max)—The force at crack initiation (corresponding to the initiation value for GIc) is that value at the intersection of the force-displacement curve with a line drawn from the origin and offset by an increase in compliance from the original linear region of the force-displacement curve (Fig. 2(a)). This compliance increase is determined using the method described in 13.2.1. If the intersection of this line, corresponding to an increased compliance, with the force-displacement curve occurs after the point of maximum force, the maximum force should be used to calculate GIc(init).
5.3.2 Crack Propagation Fracture Toughness—In the SCB test, the crack will grow in either a stable or an unstable manner depending on the sandwich construction being tested. The propagation fracture toughness values, GIc(prop), are calculated using the area method (described in 13.3) when a prescribed crack growth increment and a complete force-displacement response have been recorded (see Fig. 2(b)). Propagation fracture toughness values can be influenced by resistance curve (R-curve) effects, such as fracture process zone development. Changes in propagation fracture toughness with crack growth can also be indicative of a change in the through-the-thickness location of the crack relative to the face sheet-to-core interface. Therefore, the through-the-thickness location of the face sheet-to-core separation must be monitored and reported for the increment of growth used in the calculation of the propagation fracture toughness, GIc(prop).
Scope
1.1 This test method describes the determination of the Mode I dominant fracture toughness, GIc, associated with face sheet-to-core separation of sandwich constructions using the single cantilever beam (SCB) specimen. This separation can consist of a disbond of the face sheet-to-core interface or a fracture of the core near the face sheet-to-core interface region.
1.2 This test method is limited to use with sandwich constructions with face sheets consisting of continuous-fiber or discontinuous-fiber (tape, fabric, or both) reinforced composites in which the laminate is balanced with respect to the test direction. 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. This test method may prove useful for other types and classes of sandwich constructions; however, certain interferences have been noted (see 6.8).
1.3 The measured fracture toughness of the face sheet-to-core interface region is a Mode I dominant structural property that is a function of the dimensions and constituent materials of the sandwich construction and the method of manufacturing.
1.4 Certain face sheet/core combinations may cause fracture loading conditions that deviate from a Mode I dominant condition, resulting in a fracture mode not addressed by this standard. Therefore, only combinations of face sheet and core materials that yield Mode I dominant fracture loading conditions in the SCB specimen are permissible. Further details are provided in 6.7. Mode I dominant fracture in the SCB specimen is defined as having a Mode-I component of energy release rate (ERR) equal to at least 90 % of the total ERR.
1.5 The values stated in SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other, and values from the two systems shall not be combined.
1.5.1 Within the text the inch-pound units are shown in brackets.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.