PSI - Issue 5

F. Romano/ Structural Integrity Procedia 00 (2017) 000 – 000

2

722 F. Romano et al. / Procedia Structural Integrity 5 (2017) 721–728 F. Romano/ Structural Integrity Procedia 00 (2017) 000 – 000 a valid design tool to investigate a new design philosophy based on the evaluation of the residual strength of panels with discrete damages. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017. 2 a valid design tool to investigate a new design philosophy based on the evaluation of the residual strength of panels with discrete damages. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017. Keywords: Design; Structural Assessment; Structural Helath Monitoring;

Keywords: Design; Structural Assessment; Structural Helath Monitoring;

1. Introduction

1. Introduction The use of composite materials in the civil aviation is becoming ever more demanding. Their low structural weight and their high specific strength and stiffness, are the key to their success, respect to the traditional metallic materials. On the other hand, their development and application in civil aviation has not been so simple, especially by a certification point of view. Still now, also many improvements are necessary to guarantee more effective operational and maintenance cost reduction, and a higher weight reduction respect to the current one, by fully exploiting their potential high residual strength behaviour. The industrial (traditional) design is based prevalently on the FPF criteria (the structure/laminate is considered as damaged when at least one ply fails), and on the assumption of a pre-existent sizing damage, BVID (EASA, 2010). BVID is defined according to the current first level of maintenance, that consists in the “visual inspection”, i.e. “walking around inspection”. The composite materials are characterised by a high sensitivity to damage and defects. The impact damage is caused by a large variety of sources, ranging from tool drops, runway or ground debris, foot traffic to impact with large objects (e.g. luggage) and hail damage. The strength reduction in the presence of damage must be taken into account in the design process; the approach is dependent on the inspection method used and on its reliability, Kassapoglou ( 2010). A structure with damage below the threshold of detectability, ADL, of the selected inspection method, must be capable of withstanding ultimate load without failure. A structure with damage above the ADL of the selected inspection method, must be capable of withstanding limit load without failure, and must be repaired in order to restore its load carrying capability up to ultimate load. In practice, the most common inspection method currently used is the visual inspection; in this case, the likely impact damage at the threshold of reliable detection, ADL, has been called barely visible impact damage (BVID). The BVID is usually defined as da age that is “barely” visible from a distance of 1.5 meters under a mbient light conditions, and because of its dependence on the inspector and his/her experience level, many attempts to more accurately define BVID have been made by tying the BVID to a specific indentation size. Usually, 0.8-1 mm deep indentation is considered to correspond to BVID, Kassapoglou (2010), Baaran (2009). This level of damage is currently used in the design of the composite aircraft structures. The use, instead, of SHM systems could reduce the conservativism of the current design approach. SHM systems are currently conceived for maintenance purposes of airplane, trying to reduce through-life costs by the adoption of Condition Based Maintenance in place of traditional Time Based Maintenance; but in the last decade, they are also under investigation as support to the design of composite structures to achieve consistent weight reductions. The use of SHM, could dramatically change future design and operation of aircraft. Incorporation of SHM into composite structures has therefore the potential to reduce many of the current uncertainties and could permit increased design allowables leading to lighter and more efficient structures; it can detect the presence of defects and damage in the structure at sizes smaller than those currently assumed for design and certification, BVID. In detail SHM systems can allow to design with higher design allowables thanks to a more reliable detection of the current BVID (0.8-1 mm deep indentation), i proving the static strength (CAI) of the composite structures for a reduced damage size detection. Another aspect, is that the composite structures are traditionally designed supposing that any damage up to the ADL size will not grow under fatigue design loads during all the operative life of the aircraft. The design is mainly based on: The use of composite materials in the civil aviation is becoming ever more demanding. Their low structural weight and their high specific strength and stiffness, are the key to their success, respect to the traditional metallic materials. On the other hand, their development and application in civil aviation has not been so simple, especially by a certification point of view. Still now, also many improvements are necessary to guarantee more effective operational and maintenance cost reduction, and a higher weight reduction respect to the current one, by fully exploiting their potential high residual strength behaviour. The industrial (traditional) design is based prevalently on the FPF criteria (the structure/laminate is considered as damaged when at least one ply fails), and on the assumption of a pre-existent sizing damage, BVID (EASA, 2010). BVID is defined according to the current first level of maintenance, that consists in the “visual inspection”, i.e. “walking around inspection”. The composite materials are characterised by a high sensitivity to damage and defects. The impact damage is caused by a large variety of sources, ranging from tool drops, runway or ground debris, foot traffic to impact with large objects (e.g. luggage) and hail damage. The strength reduction in the presence of damage must be taken into account in the design process; the approach is dependent on the inspection method used and on its reliability, Kassapoglou ( 2010). A structure with damage below the threshold of detectability, ADL, of the selected inspection method, must be capable of withstanding ultimate load without failure. A structure with damage above the ADL of the selected inspection method, must be capable of withstanding limit load without failure, and must be repaired in order to restore its load carrying capability up to ultimate load. In practice, the most common inspection method currently used is the visual inspection; in this case, the likely impact damage at the threshold of reliable detection, ADL, has been called barely visible impact damage (BVID). The BVID is usually defined as damage that is “barely” visible from a distance of 1.5 meters under a mbient light conditions, and because of its dependence on the inspector and his/her experience level, many attempts to more accurately define BVID have been made by tying the BVID to a specific indentation size. Usually, 0.8-1 mm deep indentation is considered to correspond to BVID, Kassapoglou (2010), Baaran (2009). This level of damage is currently used in the design of the composite aircraft structures. The use, instead, of SHM systems could reduce the conservativism of the current design approach. SHM systems are currently conceived for maintenance purposes of airplane, trying to reduce through-life costs by the adoption of Condition Based Maintenance in place of traditional Time Based Maintenance; but in the last decade, they are also under investigation as support to the design of composite structures to achieve consistent weight reductions. The use of SHM, could dramatically change future design and operation of aircraft. Incorporation of SHM into composite structures has therefore the potential to reduce many of the current uncertainties and could permit increased design allowables leading to lighter and more efficient structures; it can detect the presence of defects and damage in the structure at sizes smaller than those currently assumed for design and certification, BVID. In detail SHM systems can allow to design with higher design allowables thanks to a more reliable detection of the current BVID (0.8-1 mm deep indentation), improving the static strength (CAI) of the composite structures for a reduced damage size detection. Another aspect, is that the composite structures are traditionally designed supposing that any damage up to the ADL size will not grow under fatigue design loads during all the operative life of the aircraft. The design is mainly based on: