Cohesive Zone Modeling of the Interface Fracture in Full-Thermoplastic Hybrid Composites for Lightweight Application

With the increasing demand for lightweight and high-performance materials in the automotive and aerospace industries, full-thermoplastic hybrid composites have emerged as a pivotal solution, offering enhanced mechanical properties and design flexibility. This work aims to numerically model the fracture strength in full-thermoplastic hybrid composites made by forming and overmolding organosheets. The mode I fracture was investigated by modeling the behavior of T-joint specimens under a tensile test following the cohesive zone modeling (CZM) approach. The sample was designed to replicate the connection between the laminate and the overmolded part. Double cantilever beam (DCB) specimens were manufactured with organosheets and tested to mode I opening to determine the interlaminar fracture toughness. The fracture toughness out of the mode I test with DCB specimens was used to define the CZM parameters that describe the traction-separation law. Later, due to the particular geometry of the T-join specimens that under tensile load work close to pure mode I, the cohesive parameters were determined by inverse analysis, i.e., calibrating the theoretical models to match experimental results. The fracture resistance T-joint specimens appeared dependent on the fiber-bridging phenomenon during the delamination. In particular, the presence of fiber-bridging visible from the experimental results has been replicated by virtual analyses, and it is observed that it leads to a higher energy value before the interface's complete breakage. Moreover, a correspondence between the mode I fracture toughness of the DCB specimen and T-joint specimens was observed.

Study on the Overmolding Process of Carbon-Fiber-Reinforced Poly (Aryl Ether Ketone) (PAEK)/Poly (Ether Ether Ketone) (PEEK) Thermoplastic Composites

This paper used poly (aryl ether ketone) (PAEK) resin with a low melting temperature to prepare laminate via the compression-molding process for continuous-carbon-fiber-reinforced composites (CCF-PAEK). Then, poly (ether ether ketone) (PEEK), or a short-carbon-fiber-reinforced poly (ether ether ketone) (SCF-PEEK) with a high melting temperature, was injected to prepare the overmolding composites. The shear strength of short beams was used to characterize the interface bonding strength of composites. The results showed that the interface properties of the composite were affected by the interface temperature, which was adjusted by mold temperature. PAEK and PEEK formed a better interfacial bonding at higher interface temperatures. The shear strength of the SCF-PEEK/CCF-PAEK short beam was 77 MPa when the mold temperature was 220 °C and 85 MPa when the mold temperature was raised to 260 °C. The melting temperature did not significantly affect the shear strength of SCF-PEEK/CCF-PAEK short beams. For the melting temperature increasing from 380 °C to 420 °C, the shear strength of the SCF-PEEK/CCF-PAEK short beam ranged from 83 MPa to 87 MPa. The microstructure and failure morphology of the composite was observed using an optical microscope. A molecular dynamics model was established to simulate the adhesion of PAEK and PEEK at different mold temperatures. The interfacial bonding energy and diffusion coefficient agreed with the experimental results.

Investigation of the adhesive strength in a combined compaction and back-injection process to produce back-injected self-reinforced composites (SRCs)

This publication investigates the adhesion between an injection molded component and a self-reinforced composite (SRC) produced in a combined compaction and back-injection process to produce back-injected self-reinforced composites. To study the influence of the process, the parameters barrel temperature, time of injection, and tool temperature were varied. In addition, samples were taken at different positions along the flow path. In light of the orthotropic material behavior of SRCs, investigations were conducted to see whether different loading cases lead to different mechanical behavior. Shear-off and pull-off tests revealed a different strength as a function of the loading type. In the shear-off tests, a mean strength of 11.37 MPa was recorded over the entire test series, while the measured mean strength in the pull-off tests is considerably lower, 4.04 MPa. The type of failure is determined with the aid of SEM images, and the influence of the microstructure of the thermoplastic fibre materials on the adhesion is set out. It is shown that, as of a sufficiently high level of adhesion, failure occurs within the fibres.

Influences on the mechanical properties of SRCs in a combined compacting and back injecting process

In this research paper, the effects of the combined compacting and back-injection process to produce back-injected self-reinforced composites on the mechanical properties of the self-reinforced composites (SRCs) are investigated. For this purpose, the parameters barrel temperature, time of injection and holding pressure were varied for the back injection. Tensile and bending tests were carried out on the SRCs. The results show that the mechanical properties depend to a large extent on the process parameters. The measured tensile strength varies between approx. 186 and 86 MPa, the stiffness between approx. 3500 and 2000 MPa. The flexural strength is measured between approx. 75 and 5 MPa, the flexural modulus between approx. 5480 and 650 MPa. Flexural tests are more suitable for evaluation of the consolidation, as tensile tests cannot evaluate the adhesion of the fabric layers to each other in the SRCs. Microscopic examinations show that consolidation by the back-injected melt can lead to smaller cross-sections in the SRCs compared to an area that was not back-injected. At high barrel temperatures, melting of individual fabric layers can occur, which explains, among other things, the drop in the mechanical properties of the SRCs.

Effect of the temperature gradient on the interfacial strength of polyethylene/polyamide 6 during the sequential injection molding

Effect of the temperature gradient on the interfacial strength of polyethylene (PE)/polyamide 6 (PA6) during the sequential injection molding (SIM) was investigated in this article. PE grafting with maleic anhydride (PE- g-MAH) was added into PE matrix to enhance the interfacial strength through the formation of the copolymer at PE/PA6 interface. The results showed that the reaction at PE + PE- g-MAH/PA6 interface was incomplete due to the steep temperature gradient built from the melt (the second part) to the solid (the first part) during SIM. However, the interfacial reaction proceeded further during annealing through the diffusion of the reactive groups into PE + PE- g-MAH/PA6 interface so that much more copolymers were formed at PE + PE- g-MAH/PA6 interface, resulting in obvious improvement of the interfacial strength after annealing. It was also found that the relationship between interfacial strength and PE- g-MAH content was a linear relation through data fitting under gradient cooling during SIM. However, the relationship became the square relation after annealing at 220°C for 10 min. Through the rheological analysis and morphological observation, it was concluded that the interfacial temperature was a very important factor to control the interfacial strength during SIM.

Interfacial crystalline structures in injection over-molded polypropylene and bond strength

This paper describes interfacial crystalline structures found in injection overmolded polypropylene components and the relationship of these structures to bond strength between the components. The combined effects of the development of hierarchical gradient structures and the particular thermomechanical environment near the interface on the interfacial crystalline structures were investigated in detail by PLM, SEM, DSC, WAXD, and infrared dichroism spectroscopy. The experimental results showed that during molding there was competitive formation of interfacial crystalline structures consisted of "shish-kebab" layer (SKL) and a transcrystalline layers (TCL). Variation in shear stress (controlled by injection pressure and injection speed) plays an important role in the formation of the SKL. The formation of TCL is influenced by the thermal environment, namely melt temperature and mold temperature. Increasing within certain limits, interfacial temperature and the thermal gradient near the interface promotes β-iPP growth. The relationship between interfacial crystalline structures and interfacial bond strength was established by lap shear measurement. The interfacial bond strength is improved by enhancing the formation of TCL, but reduced if SKL predominates.