Abstract This study investigates the applicability of Long Pulse Thermography (LPT) for detecting artificial defects in carbon-fiber-reinforced polymer (CFRP) composites. The work aimed to determine which types of defects can be detected in a CFRP panel using active infrared thermography and to assess the influence of defect type, size, and depth on their detectability. The test specimen was a CFRP laminate manufactured from unidirectional prepreg using the autoclave process. The panel had a variable thickness, and artificial defects were introduced at different locations to simulate typical manufacturing- and service-related damage. The investigated defects included: prepreg foil, simulating interlayer insertion caused by errors during prepreg lay-up; flat-bottom holes, simulating delamination caused, for example, by low-energy impacts; paper inserts, simulating interlayer insertion caused by the inclusion of a foreign body during manufacturing; flat-bottom holes filled with epoxy resin, simulating resin-rich areas; and graphite foil, simulating interlayer insertion defects. Thermographic measurements were performed using long-pulse thermal excitation, and the thermal response was analyzed during the cooling phase. The results showed that defect detectability depended strongly on defect type and increased with increasing defect diameter. Flat-bottom holes and resin-rich defects produced the clearest thermal indications and the highest detection repeatability, whereas prepreg foil and paper inserts yielded less distinct signals, particularly for smaller diameters. Under the applied test conditions, graphite foil inserts were not detected. The results confirm that LPT is a promising non-destructive testing method for the rapid inspection of CFRP structures; however, its effectiveness is limited for defects generating low thermal contrast.
Abstract Damage tolerance analysis provides the analytical basis for structural fatigue assessment and inspection planning in transport category aircraft and is required for certification under contemporary airworthiness regulations. In current practice, damage tolerance analysis commonly relies on generalized assumptions for initial flaw size and prescriptive inspection intervals without explicitly incorporating differences in nondestructive inspection detection capability. A baseline damage tolerance analysis was conducted on a simplified structural configuration selected to illustrate the underlying concepts without unnecessary analytical complexity, using established industry and regulatory assumptions. The analysis was repeated with the initial crack length defined by inspection capability. The results demonstrate that the assumed initial flaw size governs inspection initiation and is directly related to the maximum probable undetected flaw required by certification regulations, resulting in substantial shifts in inspection timing. A cumulative probability of detection formulation is introduced to evaluate inspection strategies in terms of total detection reliability, enabling alternative combinations of inspection method and frequency to achieve equivalent detection performance. This approach provides flexibility in inspection method selection and interval definition and may enable the use of lower-cost or more readily deployable inspection techniques while maintaining the required level of structural reliability. The proposed framework establishes a direct linkage between crack growth prediction and inspection capability, improving analytical consistency while preserving regulatory intent.
Abstract Vibration-induced fatigue cracks in turbine blades can lead to engine failure and the forced withdrawal of an aircraft or other turbine-powered machine from service. One means of preventing such cracks is to reduce blade vibration through increased damping. This paper presents a concept for enhancing the vibration damping of a turbine blade by implementing damping mechanisms within the blade structure itself. A prototype blade was manufactured using additive technology, which enabled the creation of internal pockets containing non-melted metallic powder. Lattice bars and pins of various geometries, and consequently different natural frequencies, were built into the pockets as elements fixed to the solid structure, with the function of absorbing vibration energy and transferring it into the surrounding powder volume, where the energy is dissipated. The blade was subjected to experimental testing on an electrodynamic shaker to determine its natural frequencies, mode shapes, and modal damping ratio. A significant increase in the damping ratio was observed relative to a reference Solid blade manufactured using the same technology. While the results demonstrate the effectiveness of the proposed solution, they also reveal pronounced vibration nonlinearity. Accordingly, subsequent work will focus on optimizing the blade damping structures and developing a correlated computational model.
Abstract Drilling-induced residual stresses can significantly impact the fatigue performance of aluminum alloy components. This paper presents an experimental and numerical study on the impact of rotational speed and feed rate on residual stresses experienced while drilling Al2024-T351 in a dry cutting environment. Drilling experiments were carried out using an 8 mm HSS drill at rotational speeds from 375 to 1250 rpm and feed rates between 0.12 and 0.30 mm/rev. The residual hoop stresses near the hole entrance were measured by surface-mounted strain gauges. For residual stress simulation, a coupled thermo-mechanical finite element model was developed using Abaqus/Explicit. Results showed that increasing rotational speed decreases tensile residual stress, and increasing feed rate greatly increases it. The numerical model reproduces the experimental trends but slightly overpredicts the magnitude of the stress. The findings provide practical guidance for selecting drilling parameters to minimize residual stress in aerospace aluminum alloy component.
Carbon-fiber-reinforced (CFR) composites in aircraft structures are subjected to complex, multiaxial loading conditions that may induce fatigue damage prior to final failure. To ensure structural safety, reliable failure criteria must be established for both undamaged and fatigue-affected materials. This study presents experimental investigations of tubular CFR composite specimens subjected to combined axial force and internal pressure, generating complex stress states in the thin-walled gage section. The specimens were loaded to failure along various stress paths, enabling construction of a failure surface in principal stress space. Three distinct failure modes were observed: resin matrix puncture, longitudinal cracking, and circumferential cracking with specimen separation. A probabilistic approach was introduced to account for the large scatter in experimental data, replacing deterministic failure stresses with stress values corresponding to specified survival probabilities. The results indicate that the maximum principal stress criterion, formulated in three-dimensional principal stress space with axes aligned to fiber directions, provides a suitable framework for the investigated composite. Incorporating probabilistic assessment improves reliability in predicting composite failure under complex loading.
