The structure of the DS-Panel refers to a double-sinusoidal sheet sandwich panel (Fig. 1, left) and the material is PEEK reinforced with graphene and long fibers. It possesses outstanding strength–stiffness, durability, crashworthy- ness and damping capacity. New concept processing equipment, namely a mixer/reactor, a filament liquid impregnation device and a toothed hot-rolling machine were developed. Product competitiveness is enhanced by the extra-durability, resulting from the graphene reinforcement. The product covers a wide application area, including automotive, aerospace, marine and railroad. Its behavior is characterized by the capabilities.

a. OUTSTANDING PERFORMANCE

Higher performance and functional capacity of the panels used in transport is required in order to reduce weight, production/maintenance cost and offer more functions.
The DS-sheet/ panel exploits the top thermoplastics of the PAEK family (like PEEK from Victrex plc, VESTAKEEP from Evonik GmbH, PEKK Kepstan from Arkema, KetaSpire PEEK from Solvay), which is widely validated in many critical applications, the last 35 years. The prototype equipment for mixing is reducing the reaction time from minutes to seconds, without graphene damage (attrition). Further, sufficient filament impregnation was attained by intimately mixing matrix-filaments in liquid form and then hot rolling of laminates ‘by a pair of co-rotating toothed rollers’, a method inspired from the theory of 3-D gearing (Fig. 1, right).

dspanel

Fig. 1: The sinusoidal in X, Y periodic function and the sheet or core of the panel with orthogonal fibers.

b. EXTRA-DURABLE SHEET OR PANEL UNDER FATIGUE LOADING

The main problem in fiber-reinforced polymer panels is their limited durability. Fatigue is responsible for the majority of failures for panels used in transport where they are continuously subjected to vibration and other fluctuating loads. PEEK sustains more than 10^(6 )fatigue cycles at the high stress level of 75 PMa. CNTs or graphene absorb strain energy by the creation of a multitude of fine nano-scale cracks and by participating in the fracture process as nano-scale bridges. Graphene, under certain architecture of the reinforced matrix can absorb, many times more fracture energy by forcing the fatigue crack to continuously deflect/bifurcate. The deflection is related with graphene size, orientation and volume fraction.

c. THE CRASHWORTHINESS CAPACITY

Concerns regarding the performance of polymer composite structures subjected to impact have been a factor limiting their use. The proposed panel faces the problem at the material and at the structure level. The high impact strength of PEEK, its top specific energy absorption capacity when reinforced with carbon fibers (180 KJ/Kg), the high fracture energy due to graphene extensive crack tip deflection, all these create an unparallel energy absorption efficiency. In addition, the interlaminar Mode I fracture toughness G_IC parallel to the carbon fibers ranges from 1,56 to 2,4 KJ/m^2 , the highest resistance to crack growth between the fibers for all composites. The basic element is a circular, smooth, closed conical shell geometry, without stress concentrations, maximizing energy absorption. Then, the fiber reinforced cell/ core can accommodate an extensive structural elasto-plastic deformation (up to 30%) upon increasing loading on the facing sheets, which is combined with the work hardening behavior of the matrix.

d. DAMPING AND SOUND ABSORPTION

The problem of composite panels is that they exhibit inferior acoustic performance levels and high structure-borne vibration transmissibility due to their stiffness and lightweight. This has a negative effect on structural integrity and passenger comfort. The required vibrational and acoustic energy dissipation is directly proportional to the material damping and the stored strain energy. Damping should be attained without sacrificing stiffness. The PEEK material (loss factor η= 0,003) and its reinforcement (graphene, PEEK/graphene composite η=0,02) absorb inherently considerable energy. The multi-reinforced (nanolevel plus fibers) cell/ core walls are inclined, causing upon loading excessive shear deformation, which maximizes damping.

e. THE LOW COST COMPARED TO PERFORMANCE

There is a justified concern about the cost of advanced products of nanotechnology. Although PEEK is an expensive thermoplastic (about 75 Euro/Kg), its cost contribution in producing the panel is < 7%. The costs of other similar thermoplastics of the PAEK family reduce this contribution to < 5%. The functionalized graphene oxide cost is < 1,2 %. However, the PEEK processing temperature (> 380^0 C) and melt viscosity (> 240 Pa s) generate several processing difficulties considerably lengthen production times. The new concept processing devices reduce the production time, more than four times . Particularly, mixing (usually by stirring) consumes most of the processing time. The market price of the panel should be lower than the competing panels, although their polymer cost is much lower.

Table 1: Mechanical properties of the multi-reinforced core of the panel

Density (gr/cm3)

0.03

Specific shear strength in-plane (MPa / (gr/cm3))

22

Specific flatwise comp. strength (MPa/(gr/cm3))

31

Shear modulus in-plane (MPa)

37

Modulus in flatwise compression (MPa)

508

Loss factor, nx

0.09