Characterisation of the effect of lay-up on the delamination growth revealed a complex set of damage characteristics. Tortuous propagation resulted in higher fibre bridge densification in the off-axis laminates, requiring an increased number of experimental tests to validate the characterisation of fibre bridge saturation effects. The main objective of this research is to apply a modified Sørensen model [1] to measure the fibre bridge (FB) stress curve. In addition, the research aims to estimate the full saturation curve. These objectives will contribute to a more efficient and accurate characterisation of fatigue delamination behaviour in composite materials. The procedure was carried out using a model proposed in the literature [2] for unidirectional composites, to estimate positions of the sull-saturated and fibre bridge Paris curves. The proposed model, based on physical considerations of FB stress formation, was applied to a quasi-static curve and a fatigue curve for each lay-up: 0//0, 0//45 and 0//90. Based on the resulting R-curve, the FB stress curve is derived for each case. This approach accurately models the fatigue behaviour and bridging effects in composites by providing the zero bridging curve (Fig. 1). Following a similar procedure, but now adding the full saturation stress integrated along the end opening delamination, provides the additional strain energy in the steady state region (Fig. 1). This approach allows a more efficient and cost-effective characterisation of fatigue delamination behaviour with a reduced number of experiments, and evaluates the feasibility of applying the proposed model to a single quasi-static and fatigue curve. In addition, the proposed method addresses a comprehensive understanding of fatigue behaviour considering the effect of fibre orientation, thereby contributing to a more comprehensive understanding of fatigue behaviour.

Studies have shown that temperature and moisture play a critical role in altering material properties, with both factors contributing to the overall degradation of structural components. This research aims to provide a deeper insight into the complex interplay between environmental factors and fatigue delamination behaviour in composite materials. To this end, the effect of hygrothermal aging and in-service temperature conditions on mode I fatigue delamination was investigated considering the relationship between fracture surface patterns and fatigue propagation behaviour. To investigate the effects of in-service temperature, Mode I delamination tests followed standardised protocols with pristine (unaged) samples tested at -40, 22, and 80ºC. Hygrothermal ageing (HA) was performed in a climatic chamber at 90ºC and 90% relative humidity for 60 days. After reaching an equilibrium moisture content of 1.2%, the samples were tested at in-service temperature levels of -40, 22, and 80ºC. The in-service humidity was set at 50% in all cases to ensure control of the absorbed moisture under the previous HA conditions. The fatigue curves were remarkbly affected by hygrothermal ageing, as a consequence of the change in the material properties. The individual effect of in-service temperature leads to a temperature-dependent variation in the slope of the da/dN versus dU/dN curves. On the other hand, hygrothermal ageing consistently reduced the slope of all curves, compared to pristine samples (Fig. 1a). The results confirm that temperature inversely affects the slope of the fatigue propagation curve, while hygrothermal ageing promotes a tougher fracture behaviour. The fractography investigation reveals that the surface roughness (Fig. 1b) follows the same trend as the dU/dN slope: higher slopes (lower temperature) correspond to less damage (lower tortuosity), while lower slopes (higher temperature) indicate more damage represented by higher surface roughness.

Fibre bridging is an important phenomenon influencing the mode I delamination growth behaviour in composite materials. Accurate modelling of this phenomenon is required in order to be able to account for its effects in damage tolerance evaluation of composite structures. Therefore, this study introduces a novel physical model to isolate and quantify the contribution of fibre bridging to Mode I fatigue delamination. The model distinguishes between monotonic and cyclic components of fibre bridging stress, capturing their individual effects on the strain energy release rate (SERR) in the Paris curve. The monotonic component, based on the Sørensen model, accounts for pre-cracking effects, while the cyclic component is derived by integrating a bridging stress function over the end-opening displacement, with both components modelled by empirical exponential relationships. The model has been validated against established methods such as the Yao model and specific extrapolation techniques, demonstrating improved accuracy in fitting the Paris curve, particularly in accounting for the monotonic influence in the shift of the SERR and the cyclic contribution to the curve slope. Importantly, the model requires only one quasi-static and one fatigue test, reducing the experimental workload. In conclusion, this method provides a more accurate characterisation of fibre bridging effects, making it a robust tool for fatigue delamination analysis.

Delamination fatigue propagation is known to cause a progressive degradation of stiffness and strength in composite laminates. Since delamination tends to follow a preferential plane, fracture resistance is conveniently analysed in terms of dominant loading modes at the crack tip: mode I (opening) and mode II (shearing). To this end, coupon tests can be performed to determine the growth rates under these particular stress states. Paris parameters from such tests are then often used in numerical implementations adopting mesoscale modelling, like in the case of cohesive element traction-separation laws. The majority of coupon tests available in the literature focus on interfaces where the fibres of the upper and lower plies have the same orientation, typically aligned with the direction of delamination growth. However, most practical applications involve multidirectional laminates, where delamination tends to develop at interfaces where the upper and lower plies have mismatching angles. Studying angled interfaces may lead to different results since some fracture phenomena, like fibre bridging and crack migration, are highly dependent on fibre orientations of plies adjacent to the delaminated interface [1].
The present work experimentally explored the various effects of fibre orientation on fatigue delamination growth in the different fracture modes. IM7/8552 carbon fibre epoxy prepreg (Hexcel), a material system commonly adopted in aerospace field, was tested under mode I Double Cantilever Beam (DCB), mode II End-Loaded Split (ELS), and Mixed-Mode Bending (MMB) tests. For all cases a combination of different interfacial fibre orientations were tested and the crack growth rate curves were compared in relation to the observed fracture behaviour.

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.

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.

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.

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.

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.

Damage propagation in fatigue after impact of CFRP is usually monitored by measuring the external area or simply the one-dimensional width of impact delamination using ultrasound inspection. The present work provides experimental evidence demonstrating that this procedure does not account for all the possible damage propagation taking place in CAI fatigue. In the present work, impact and compression after impact fatigue tests were conducted on CFRP laminates. The impact damage, inspected using through-thickness attenuation ultrasound scan, showed the presence of a non-delaminated cone caused by the out of plane compression during impact. In the following fatigue test, first delamination grew internally inside the non-delaminated cone and, only after, outwards, towards the sides of the specimen. In addition to that, the process of stiffness degradation started before any observable damage growth. This provides experimental confirmation to the necessity to reconsider the current definition of fatigue growth in compression fatigue after impact.