Certification of composite air vehicles is accomplished through extensive test campaigns to demonstrate that the structure meets all strength, durability, damage tolerance, and environmental resistance requirements. The “building block” certification approach entails evaluating large quantities of coupon-level tests to establish basic material properties based on which testing is scaled up. Progressing in each scaling level (elements, subcomponents, and full-scale components), fewer articles are tested which also serve as design verification and validate configurations for analytical predictions. While the processes for defining the building block test campaign are well defined and specify the use of various adjustment factors (environmental, statistical, and load enhancement factors), they are based on the coupon-level. These adjustment factors thus can be overly conservative, ignoring the benefits of scaling effects observed for most failure modes. For example, the failure strength of a component containing a flaw is typically underpredicted when assessed using knockdown factors derived from the coupon-level which disregard scaling and local load redistribution effects. Similarly, environmental impacts determined using coupons can also be overly conservative compared to effects at scale. In this work, a standard seven-point bend element test configuration which is representative of a common aerospace structural component is proposed for inclusion in future building block certification efforts. This configuration enables a more efficient means of assessing the effects of manufacturing and in-service nonconformances. The configuration consisting of a single stiffening element attached to a skin or cover, is large enough to capture scaling effects of many common failure modes and defects, while also being small enough to fit into environmental chambers during tests, enabling environmental compensation factors determination at the element level. With the test configuration exerting out-of-plane loads mimicking actual complex structural loads, damage at the skin/stiffener interface undergo mixed-mode crack propagation. In this initial assessment, a secondary bonded skin/stringer construction was investigated to determine impact of surface preparation techniques. Mechanical abrasion was compared to an atmospheric plasma treatment (APT) process at room temperature and elevated temperature, saturated environmental conditions. The results of these tests support the adoption of this test process as a standard for use in building block testing to reduce the degree of conservatism in future composite vehicle design.

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Seven-Point Bend Test Method to Evaluate Skin-Stringer Post-buckling Separation: Surface Preparations and Moisture Conditioning Effects

  • Waruna Seneviratne,
  • Mohamed Shafie,
  • Chris Boshers,
  • Harishanker Nadason,
  • John Tomblin

摘要

Certification of composite air vehicles is accomplished through extensive test campaigns to demonstrate that the structure meets all strength, durability, damage tolerance, and environmental resistance requirements. The “building block” certification approach entails evaluating large quantities of coupon-level tests to establish basic material properties based on which testing is scaled up. Progressing in each scaling level (elements, subcomponents, and full-scale components), fewer articles are tested which also serve as design verification and validate configurations for analytical predictions. While the processes for defining the building block test campaign are well defined and specify the use of various adjustment factors (environmental, statistical, and load enhancement factors), they are based on the coupon-level. These adjustment factors thus can be overly conservative, ignoring the benefits of scaling effects observed for most failure modes. For example, the failure strength of a component containing a flaw is typically underpredicted when assessed using knockdown factors derived from the coupon-level which disregard scaling and local load redistribution effects. Similarly, environmental impacts determined using coupons can also be overly conservative compared to effects at scale. In this work, a standard seven-point bend element test configuration which is representative of a common aerospace structural component is proposed for inclusion in future building block certification efforts. This configuration enables a more efficient means of assessing the effects of manufacturing and in-service nonconformances. The configuration consisting of a single stiffening element attached to a skin or cover, is large enough to capture scaling effects of many common failure modes and defects, while also being small enough to fit into environmental chambers during tests, enabling environmental compensation factors determination at the element level. With the test configuration exerting out-of-plane loads mimicking actual complex structural loads, damage at the skin/stiffener interface undergo mixed-mode crack propagation. In this initial assessment, a secondary bonded skin/stringer construction was investigated to determine impact of surface preparation techniques. Mechanical abrasion was compared to an atmospheric plasma treatment (APT) process at room temperature and elevated temperature, saturated environmental conditions. The results of these tests support the adoption of this test process as a standard for use in building block testing to reduce the degree of conservatism in future composite vehicle design.