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This third part of the review on defects as root cause of fatigue failure addresses cavities (pores, micro-shrinkages, unmelted regions), defective microstructures and microcracks as material defects and defects due to local damage during manufacturing, service and maintenance such as dents, scratches and localized corrosion. In addition, damage due to contact fatigue and the effect of surface roughness are discussed in the context of fatigue failure. Also addressed is the competition between different kinds of defects in controlling the initiation and early growth of fatigue cracks.
During their life cycle, composite structures used in aircraft structures can be subjected to high- and low-velocity impact loading. High velocity impact damage is usually easy to detect as it creates visible external damage. Low-velocity impacts are more complex to assess because, although significant damage can be generated internally, there can be little indication of external damage on the impacted surface, leading to the term BVID. Impact damage, especially barely visible impact damage, is therefore of primary concern for design and maintenance of modern aircraft composite structures. As a result there is a concerted research effort to improve the damage resistance and tolerance of these materials.
One of the great strengths of X-ray computed tomography over conventional inspection methods (ultrasound, thermography, radiography) is that it can image damage in 3D. or inter-ply interfaces. An X-ray computed tomography (CT) data processing methodology is developed to extract the through-thickness distribution of damage in curved or deformed composite panels. The method is applied to [(0°/90°)2]s carbon fibre reinforced polymer (CFRP) panels subjected to low velocity impact damage (5 J up to 20 J) providing 3D ply-by-ply damage visualisation and analysis. Our distance transform approach allows slices to be taken that approximately follow the composite curvature allowing the impact damage to be separated, visualised and quantified in 3D on a ply-by-ply basis. In this way the interply delaminations have been mapped, showing characteristic peanut shaped delaminations with the major axis oriented with the fibres in the ply below the interface. This registry to the profile of the panel constitutes a significant improvement in our ability to characterise impact damage in composite laminates and extract relevant measurements from X-ray CT datasets.
The modeling and prediction of the behavior of reinforced concrete under impact load is still an engineering challenge. The scientific community has put a lot of effort into the development of this knowledge, especially after the unfortunate events of 09/11 in Manhattan. The main concern is with nuclear power plants and how to design structures that can withstand such extreme situations. An experimental investigation has been carried out to collect fundamental data and to develop a deeper understanding of the effect of impact damage on the load capacity of concrete plates. The paper presents the process on the definition of procedures and the first results of an experimental investigation on the damage and residual load capacity of reinforced concrete plates after impact load. Two types of reinforced concrete plates measuring 1.5 x 1.5 x 0.3 m were subjected to the impact of a flat-nose hard projectile. The two types were casted with the same reinforcement and 80 or 40 MPa concrete. After the impact, the plates go through planar tomography, visual inspection and an ultimate load capacity test. The results showed that the planar tomography can be used for the assessment of internal damage on concrete plates, as long as the number of scans in each direction is more than four. The visual inspection gave a good. The formation of cone cracking after the impact load showed the highest influence on the remaining load capacity of the concrete plates. More plates will be tested to confirm the indications at different damage conditions.
The modeling and prediction of the behavior of reinforced concrete under impact load is still an engineering challenge. The scientific community has put a lot of effort into the development of this knowledge, especially after the unfortunate events of 09/11 in Manhattan. The main concern is with nuclear power plants and how to design structures that can withstand such extreme situations. An experimental investigation has been carried out to collect fundamental data and to develop a deeper understanding of the effect of impact damage on the load capacity of concrete plates. The paper presents the process on the definition of procedures and the first results of an experimental investigation on the damage and residual load capacity of reinforced concrete plates after impact load. Two types of reinforced concrete plates measuring 1.5 x 1.5 x 0.3 m were subjected to the impact of a flat-nose hard projectile. The two types were casted with the same reinforcement and 80 or 40 MPa concrete. After the impact, the plates go through planar tomography, visual inspection and an ultimate load capacity test. The results showed that the planar tomography can be used for the assessment of internal damage on concrete plates, as long as the number of scans in each direction is more than four. The visual inspection gave a good. The formation of cone cracking after the impact load showed the highest influence on the remaining load capacity of the concrete plates. More plates will be tested to confirm the indications at different damage conditions.