Impact events are common throughout an aircraft's lifespan, and the use of carbon fiber reinforced polymers (CFRP) in aircraft structures complicates their management. Unlike metals, CFRP dissipates energy through mechanisms such as delamination and fiber fractures, creating complex damage patterns that are difficult to detect and may propagate over time due to fatigue, threatening the structural integrity of the aircraft.

While compression after impact (CAI) fatigue has been widely studied, gaps remain in fully understanding this phenomenon. This study provides a comprehensive analysis of CAI fatigue. Ultrasound and digital image correlation (DIC) techniques revealed delamination growth within the impact cone, which later extended beyond the initial damage area, challenging prior assumptions about plateau phases of no-delamination growth. Further analysis of acoustic emission using a convolutional neural network allowed to classify the damage modes and study their progression under different load conditions. Distinct load levels were found to trigger different damage modes, leading to different phases of propagation.
Additionally, a surrogate impact damage fatigue test and a numerical modeling approach are presented to study some delamination growth patterns observed in CAI fatigue under simplified conditions.
@en

Fibre reinforced polymer composites have found increasing use in aircraft structures. This means that fleet managers need damage assessment tools for such materials, in order to decide on appropriate sustainment strategies. Developing such tools is hindered by the difficulty of generalising from lab tests to predict the behaviour of full-scale structures. A more focussed research effort is needed to address the knowledge gaps that prevent such generalisations. This paper highlights the limitations of current non-destructive inspection techniques, and how this can lead to misinterpretation of experimental results. Furthermore, it discusses differences between coupons and full-scale structures caused by 1D vs 2D delamination growth and the effects of lay-up. New test methods to deal with these issues are discussed and some preliminary results are presented. Finally, the paper discusses the prediction of residual strength and the difficulties of determining a suitable end-point for damage propagation analyses. While the residual strength in the presence of a given damage can be accurately modelled for small components, if sufficiently computational resources are available, further development is needed to develop tools that can be implemented for sustainment of operational aircraft.@en

Although several studies have been performed, the compression after impact (CAI) failure of CFRP is still not entirely understood. It is still unclear what sequence of events determines the onset of failure in CAI tests and how the different damage modes are involved in this process. To experimentally investigate this matter, the present work relies on acoustic emission (AE) monitoring and advanced acoustic signal analysis. A series of preliminary tests was conducted to correlate damage modes with recorded acoustic waveforms. Four types of waveforms were separated and associated to different damage modes. Following the preliminary tests, AE was monitored in actual CAI tests. A damage accumulation study was conducted combining three indicators, namely wavelet packet components, sentry function and energy b-value. The results evidence different phases in the damage accumulation process that were not shown in previous literature. In all specimens, the onset of the unstable damage accumulation appeared to be triggered by an intermediate frequency acoustic event associated to a combination of matrix cracking and fiber-matrix debonding, occurring at 80% of failure displacement.

@en

In previous literature, a plateau phase in fatigue growth of impact delamination projected area in CFRP was found. Explaining this plateau phase still represents a knowledge gap. In the present work, echo-pulse and through thickness transmission ultrasonic scan inspections were combined with acoustic emission monitoring to explain this plateau phase. Before the onset of growth outside of projected area, growth of delamination in the central impact cone and growth of smaller delamination was observed. Low-frequency acoustic emissions localized in the impact damage were recorded even before any type of growth was measured. The provided evidence suggests that an actual plateau phase may not exist if all the damage mechanisms are properly considered.

@en

Impacts on carbon fiber reinforced composites (CFRP) can produce a complex internal damage comprising multiple delaminations, which is hard to detect from visual inspection. This situation is known as barely visible impact damage (BVID). Considering that every airplane faces several impacts during its operational life, and that the majority of exposed surfaces in new generation aircraft is made of CFRP, there is a high chance that some aircraft will be flying with unnoticed impact damage. For this reason, BVID damage tolerance must be taken into account in design. The FAA and EASA dictate a no-growth design philosophy for BVID. Although multiple delaminations are present, BVID fatigue growth is usually assessed by measuring only the projected delaminated area with ultrasound inspections. This is done to simplify the damage description and because of the limitations in ultrasound inspection methodologies. In the present work, we show two cases of delamination propagation that are neglected following this procedure. Our experimental monitoring of delamination propagation with different ultrasound techniques shows a) growth inside the impact cone and b) faster growth of shorter delamination. The conclusion is that the projected area description is insufficient, since a no-growth in the projected area does not necessarily correspond to a no-growth in the actual damage.@en