To help reduce aircraft weight, investigations have been carried out on materials suitable for replacing metal food tray table arms.
Mohammaed Moniruzzaman, product development engineer, SABIC’s Innovative Plastics, talks us through the process
The airline industry is continually striving for weight reductions to improve fuel economy. As an example, a company such as American Airlines, which operates a fleet of around 600 planes, could save up to 11,000 gallons of fuel annually by removing just one pound from each of its aircraft. However, lowering fuel consumption by just one gallon also translates into a reduction in CO2 emissions of 19.4 pounds.
To meet demands, the Innovative Plastics strategic business unit of SABIC has developed several high-performance engineering thermoplastic compounds that, along with smart (optimised) plastic design, have the potential to replace metals. These high-modulus, high-strength materials are part of LNP’s Thermocomp product line.
So, a study has been carried out to validate the replacement of aluminium with LNP Thermocomp polyetherimide (PEI)-carbon fibre compounds in a food tray table arm.
The thermoplastic composite tray arm, which was designed by Vaupell Holdings and intended for 1:1 replacement of aluminium, was side-gated and the bottom of the arm was open-ended and contained a rib. The company used different thermoplastic composite materials with varying tensile modulus and strength properties to fabricate the tray arms, then tested the load-withstanding capability of these using a custom-built testing system. The company found that almost all the parts failed in a narrow range of load (60 to 80 lbf) irrespective of the strength of the materials, indicating that the material properties were not efficiently transferring to the tray arms.
To overcome this, Vaupell approached SABIC’s Innovative Plastics to develop a tray arm capable of withstanding a peak load of 120lbf or higher.
Two compounds were developed from SABIC’s Innovative Plastics Ultem PEI resin and aerospace-grade carbon fibres, and these contained 30 wt% and 40 wt% of fibre reinforcement (Thermocomp EC006PXQ and EC008PXQ, respectively). The compounds also contained proprietary flow modifiers that not only improved the flow but also boosted the mechanical properties; EC006PXQ and EC008PXQ have excellent FST (Flame-Smoke-Toxicity) properties, and are FAR 25.853 compliant.
In addition 30 wt% and 40 wt% carbon fibre-filled PPS compounds – Thermocomp OC006XXQ and OC008XXQ, respectively – were used in the study to compare their performance in the tray arms with the EC006PXQ and EC008PXQ materials.
EC006PXQ and EC008PXQ materials are approximately 50% lighter than aluminium. Although the tensile modulus of aluminium is higher than EC006PXQ and EC008PXQ materials, the stiffness-to-weight ratio of the EC008PXQ material is comparable to aluminium and the strength-to-weight ratio of EC006PXQ and EC008PXQ materials are better than die-cast aluminium (7075-O) and comparable to machined aluminium (7075-T6).
To test the materials, the tray arm was attached to a modified MTS system from Instron, with a bottom and a top fixture and pins; with the top fixture also attached to the load cell of the system. The tray arm was supported by an 8mm side support, similar to the Vaupell test set-up.
Finite Element Analysis
Abacus finite element analysis (FEA) software was used for simulating the loads, boundary conditions and resulting stresses and deflections on the design geometry of the tray arm; with non-linear geometric effects and non-linear material properties considered. Thermocomp EC008PXQ compound was modelled as an isotropic elastic material using the data from the stress-strain curve of this material. For boundary conditions (how the part is supported and constrained), the tray table arm rested on the pin at the bottom and a vertical load of 120 lbf was applied on the top end.
FEA showed the locations of stress in the part, especially around pin-loaded holes, ribs or areas in contact with support structures. In this study, structural FEA was also used in combination with mould flow analysis (MFA) to understand the fibre orientation and weld line locations in the tray arm and predict their impact on the mechanical performance of the tray arms.
The stress distribution information obtained from the FEA was utilised for the design improvements. The goal was to decrease stress in critical areas and better accommodate the as-moulded material properties in the part. Once we were confident in the analysis results, these design changes were translated to tooling, and then the parts were moulded for testing and validation.
The bottom part of the tray arm was open-ended with a single rib, and tests showed the failure location. So, SABIC’s Innovative Plastics suggested that a rib be added to the location where the failure was observed. Before modifying the tray arm tool, FEA was performed on this open-end, dual-rib tray arm to predict the stress levels at different locations of the arm. FEA indicated that with the 8mm side support the maximum stress concentration was close to the new rib and the side support location, and the maximum stress value (273 Mpa) was very close to the tensile strength of the PEI compounds. In order to reduce the stress on the rib location, further FEA was performed with increased side supports. As the side support width was increased from 8mm to 20mm, the maximum stress was still close to the rib and the side support location, but the maximum stress was reduced from 273 Mpa to 261 Mpa.
The tray arm tool was then modified to add the second rib. The location of the gate was kept unchanged for the dual-rib design. Tray arms were moulded with the PEI and PPS materials (EC006PXQ, EC008PXQ, OC006XXQ and OC008XXQ) and the right tray arms were tested with the modified MTS system. These were tested with 8mm and 20mm stainless steel side supports, but all the samples failed along the new rib, which was in line with the FEA prediction.
As predicted by the FEA, the open-end, dual-rib tray arms performed better and were able to withstand a higher load with the 20mm side support. The PEI-based compounds survived higher loads than the PPS-based compounds, but in both cases the peak load/break load was not close to the target peak load of 120lbf. Although the FEA indicated that with the 20mm pin support and 120lbs of applied load, the maximum stress would be below the break stress (tensile strength) of the PEI materials, the tray arms actually failed earlier than FEA suggested. There could be several reasons behind this premature failure:
• In FEA the composite was modelled as isotropic (uniform) for simplicity of analysis purposes, whereas the test materials were actually anisotropic (strength and stiffness vary in flow vs. cross-flow fibre orientation).
• The tensile strength of the materials was measured in the flow direction; most of the fillers (carbon fibres) were aligned in the tensile bars in this flow direction. In the actual part, not all the fibres were aligned in the same direction.
• The gate location could affect the filler size distribution and loading of the fillers at different locations of the tray arm. With the side gate, the carbon fibres hit the side wall of the tray arms perpendicularly, which could significantly reduce the carbon fibre length and ultimately affect the strength of the materials.
The gate location will also affect the weldline location in the tray arm. A weldline around a hole will typically only be as strong as the base resin since composite fibres turn sideways where flow fronts intersect and do not bridge the weldline in order to reinforce it.
Given the fact that the tray arm’s load-withstanding capability would be lower than the tensile strength of the constituting material due to the orientation of the fillers in different directions at the bottom of the tray arm, a redesign of the part was necessary. FEA iterations were performed to determine a design that would minimise the stress at the critical rib/core intersection of the tray arm.
Several configurations were considered: closed-end, dual-rib design, closed-end, multiple-rib (spider rib) core-out design and entirely filled core-out design. Vaupell assisted with design input for the spider-rib configuration. With the spider rib and entirely filled design, the maximum stress was localised at the side support location, not at the rib/core intersection. Moving the maximum stress to the side support location was advantageous as there was an additional support at that location and the fillers were more aligned in one direction in that region compared to the circular bottom part of the tray arm.
The cored-out, fully filled design exhibited lowest stress near the core among all the designs evaluated by FEA in this study, but there was concern about sink in the moulded parts and a slight increase in part weight; therefore, the closed-end, spider-rib design was considered for translation to tooling.
Before translating the closed-end, spider rib tray arm, MFA was performed to predict the influence of gate location (side gate vs. end gate) with this design. In an end gate situation, MFA indicated a weld line close to the rib that was located opposite to the gate. But, with the side gate, the weld line moved to a location where the stress level was lower.
As the weld line is a weak point in the structure, it would be advantageous to move it to a location where the stress level is lower. However, having the gate at the end also has its advantages: the fillers will experience a curved path at the entrance and therefore they are less likely to crush into smaller lengths. Also, with the end gate, the filler loading will be better controlled at the critical area. Therefore, both the end gate and side gate designs were evaluated in the actual tray arms.
Both the right and the left tray arms were tested with 8mm and 20mm side support. The performance of the tray arms improved significantly with the open-end, dual-rib design moved to the close-end, spider-rib design. For example, for the EC008PXQ material, the peak load for the right tray arm with open-end, dual-rib design and 8mm side support was 82lbf, whereas for the closed-end, spider-rib right tray arm, the peak load was 149lbf , an 80% improvement approximately.
For the left tray arm, the peak load was lower for both the EC006PXQ and EC008PXQ materials, although this may have been due to the difference in test set-up between the right and left tray arm.
The outstanding improvement in the load-withstanding capability of the tray arm with the end-gated, spider-rib design, needed further refinement. During the testing, a tiny deflection was observed in the stress strain curves that appeared much earlier than the peak load. The deflection was barely noticeable in the stress strain curve, but was accompanied by a soft cracking sound during the testing. After this tiny deflection point, the tray arm survived for a long time and the peak load was much higher than this deflection point.
To investigate the origin of the problem, the test was stopped manually when the sound was heard and the test part was minutely examined. A tiny crack was observed at the rib/core intersection just opposite the gate location. Mould flow analysis indicated that the weld line would be located close to this rib/core intersection with the end gate design. Therefore, the weld line, the weak point of the structure, was generating an initial crack. Although the tray arm survived increasing amount of load for a long period after the initial crack, the concern was that this initial crack would affect the fatigue properties of the tray arm, i.e., after repeated use. To achieve robust performance from the tray arms, this initial crack needed to be prevented or delayed up to a certain load – in this case, a minimum of 120lbf.
So, the tray arm tool was modified to change the gate location from the end to the side. The MFA suggested that the weldline location should be in a low stress zone with the side gate moulding, so theoretically, this configuration should delay or prevent the initial crack.
Actual test data supported this prediction. The closed-end, spider-rib, side-gated tray arms (both right and left) made with the EC006PXQ material did not show any initial crack (with 8mm and 20mm side support) and survived up to a maximum load of 213lbf (right arm, 20mm support). The right and left tray arms made with EC008PXQ material also did not show any initial crack while tested with 20mm side support and survived up to a maximum load of 192lbf. With 8mm side support, the right tray arms did not show any initial crack. Some of the left tray arms (made with EC008PXQ compound) did show an initial crack while tested with 8mm side support, but the crack occurred at very high load so it was not of much concern.
Besides the peak load/break load of the tray arm, two other factors need to be considered. Firstly, a stiffer arm will be able to withstand higher load before it deflects to a certain distance. Also, the tray arms should not be too ductile; high elongation will cause a large deflection of the arm that will shake the food tray.
Depending on the performance requirements of the tray arm, EC006PXQ or EC008PXQ could be the material of choice. Tray arms made with EC008PXQ material will provide better stiffness and lower deflection with great load-bearing capacity, while tray arms made with EC006PXQ material will provide even greater load-bearing capacity with some compromise in stiffness and deflection.
The thermoplastic composite tray arms made with the EC006PXQ and EC008PXQ compounds will not only help deliver excellent performance, but also provide significant weight savings and cost benefits.
Results indicated that the tray arms made with the EC006PXQ and EC008EXQ materials result in weight savings of 46% and 44%, respectively, as compared to the machined aluminium tray arms. In a 200-seat passenger plane, this will potentially save around 45lbs and will translate into estimated savings of 0.5 million gallons of fuel savings and a reduction of 9.9 million lbs in CO2 emissions yearly for an airline the size of American Airlines.
SABIC’s Innovative Plastics