1. Introduction
Lately, aerospace industry has shown great interest in the usage of thermoplastic matrix composites for primary and secondary applications due to their recyclability and welding capability. Spirit AeroSystems’ “ASTRA” [
1], Netherlands Aerospace Centre “STUNNING” [
2,
3,
4,
5] and German Aerospace Center (DLR) “LuFoV-3 TB-Rumpf” [
6] fuselage panels demonstrators have been developed with a common purpose, the integrated manufacturing of skin, stiffeners, and frames that are leading in a tremendous reduction of manufacturing time and cost via the process of consolidation.
Thermoplastics are highly desirable for advanced structural applications not only due to the ongoing efforts in reforming manufacturing processes but also because of their exceptional properties, which have been well-known for decades. Despite their advantageous properties, thermoplastics have been overshadowed by thermosets in certain sectors, primarily due to historical challenges in manufacturing that have been recently addressed with the introduction of new processing technologies, leading to cost reductions.
One significant advancement is the development and exploration of Automated Fiber Placement (AFP), a method that incorporates manufacturing parameters from additive manufacturing [
7,
8]. AFP has emerged as a transformative technology, replacing costly and limited autoclave-based procedures. This technological shift has contributed to the increasing prominence of thermoplastics in structural applications.
Research on bearing failure in thermoplastic composites has been a focal point, covering various aspects such as drilling procedures, fastener materials, plate materials, manufacturing processes, and the geometry of joints and holes. This body of research aims to understand and optimize the performance of thermoplastic composites under bearing forces. Specific studies have delved into the intricacies of drilling procedures [
9,
10,
11], the influence of fastener materials [
12], characteristics of plate materials [
13,
14], manufacturing techniques [
15], and the geometric aspects of joints and holes [
16].
By addressing these factors, researchers seek to enhance the overall understanding of how thermoplastic composites respond to bearing forces. This knowledge is crucial for advancing the application of thermoplastics in structural components, opening new possibilities and considerations for the design and manufacturing of high-tech structures. As these advancements continue, thermoplastics are poised to play an increasingly significant role in shaping the future of structural materials and applications.
The drilling procedure in composite materials often leads to significant damage around holes, with prevalent issues like delaminations and fiber breakage. A study by A. Dickson et al. [
9] demonstrated a transformative approach using additive manufacturing for thermoplastic composites, eliminating the need for drilling. In their research, this method substantially increased bearing strength, showing improvements of up to 29% for a single lap joint and an impressive 63% for a double lap joint.
Another innovative strategy, as highlighted by Y. Chenxi et al. [
12] involves the use of compatible thermoplastic CF-PEEK (Carbon Fiber-Reinforced Polyether Ether Ketone) pins against titanium. This technique aims to reduce mass and eliminate the need for sealants, crucial for preventing galvanic corrosion that could lead to catastrophic failures, especially in environments with high electric conductivity that may initiate fires. The study demonstrated that CF-PEEK pins, applied through hot-press and cooling processes, exhibit higher specific strength compared to titanium. Moreover, increasing fiber volume fraction (Vf) and decreasing cooling rates were identified as factors enhancing the strength of these joints.
The ongoing competition between thermoplastic and thermoset composites, particularly in the aerospace industry, has prompted investigations into their respective merits. In addressing this question, B. Vieille and L. Taleb [
14] conducted a comparative study between thermoplastic composites, specifically those with PPS (Polyphenylene Sulfide) and PEEK matrices, and common thermosets with epoxy matrices under bearing schemes with considerations for environmental conditions. The results indicated that under room temperature and dry (RTD) conditions, thermoplastic composites exhibit higher bearing strength. However, in severe hydrothermal conditions, especially after the glass transition temperature, this advantage is diminished.
Furthermore, B. Vieille et al. [
15] delved into the impact of the manufacturing procedure on bearing strength capability. Comparisons between stamping and consolidation revealed that the stamping method enhances bearing strength for a CF-PPS (Carbon Fiber-Reinforced Polyphenylene Sulfide) thermoplastic composite. Collectively, these studies contribute to an evolving understanding of how various factors, ranging from manufacturing techniques to material choices, influence the bearing performance of thermoplastic composites in aerospace applications.
Many studies suggest that crack propagation initiation and fracture toughness are not influenced by the interface angle of the plies in contact, while others believe that cross-plies lead to enhanced resistance to delamination [
17]. Specifically, M.M. Shokrieh et al. [
18] investigated the interface fiber angle for DCB specimens and concluded that maximum bridging stress is fiber-angle-dependent, whereas the crack tip opening displacement is independent. In fact, with a raise in interface fiber angle there is a corresponding raise in fracture toughness of the interface.
Another important fact that should be taken into consideration is the inelastic and viscoelastic behavior of thermoplastic composites. Many pillar studies have proven that thermoplastic composites exhibit non-linear stress–strain curves due to the plasticity of the matrix, especially as fiber angle increases (off-axis tension). Sun and his colleagues have done much research towards the development of a one-parameter plasticity damage model for thermoplastic composites, both experimentally and numerically [
19,
20,
21] and with temperature considerations (viscoelastic behavior) [
22].
Regarding modelling techniques for bearing strength evaluation, P. Camanho and F. Mathews [
23] have created a 3D model for strength prediction of mechanically fastened joints in composite materials, taking into consideration progressive damage at elastic properties of the material and a 3D failure criterion. This can be used for all three accepted types of failure in fastened joints, bearing, tension and shear-out. Similarly, the same authors [
24] have used 3D numerical models to quantify stress fields at the interface between layers together with delamination criterion. It is concluded that out of plane tightening pressure affects the joint efficiency and that clamping pressure between the washer and the laminate leads to higher strength. Moreover, P. Camanho and M. Lambert [
25] have developed a numerical methodology for determining damage, final failure and failure mode of composite fastened joints, applied only for double shear and quasi-isotropic laminates.
Last but not least, another aspect of this work is the implementation of Double Cantilever Beam and End Notch Flexure test results for Cohesive Zone Modelling capabilities and delamination quantification (interlaminar damage). A few works towards fracture characterization of thermoplastic materials have been surveyed. R. Giusti and G. Lucchetta [
26] have recently tested a woven thermoplastic composite under Mode I and II schemes and validated numerical models using various cohesive Zone Models, such as bilinear and trilinear traction separation laws. Similarly, P. Ghabezi and M. Farahani [
27,
28], have examined the effect of the usage of nano-particles on bridging laws and cohesive zone modelling with extensive reference to various parameter considerations for cohesive laws. The aim of this study is the investigation of thermoplastic matrix composites used in mechanically fastened joints for aerospace applications. Numerical modelling is found to be inadequate due to contact issues, instability factors and cohesion definition. Simple techniques are suggested to tackle these difficulties. Moreover, the results of this survey are supporting the design of thermoplastic composite multi-stiffened panels via the Experimental Building Block approach [
29].
In this study, double-shear-bolted joint configuration tensile tests of a CF-LMPAEK are taking place to exploit the enhanced bearing strength capabilities of thermoplastic composite materials, together with the survey of validity of the present standards for aerospace applications. In addition, a fast and efficient numerical methodology is strategically selected using cohesive zone modelling, inelastic considerations, and interface type discrepancies. Modelling is assisted by DCB and ENF tests outcomes conducted previously by the authors [
29]. CF-LMPAEK, which is used in several applications by aerospace industries [
30,
31], presents consistent types of failure (local damages, total fracture via bearing profile) which can be perfectly predicted by the simplified approach presented here. Bolted joint configuration tests are part of an extensive experimental campaign, aiming to fully characterize the capabilities of thermoplastic composites in aerospace industries.