How Impact Resistance Impacts Carrier Profits
Upping the impact resistance of thermoplastic sheet can boost the durability of thermoformed parts, or enable a 200-seat aircraft to shed 8000 lbs. (3629 kg)
(FIG. 1) In Izod Impact Strength Testing, the operator releases a pendulum that breaks the notched sample. The amount of energy absorbed by the sample determines the distance the pendulum swings past it, as indicated by the pointer on the half-round graduated scale shown.
(FIG. 2) A technician adjusts the height from which an impactor is released during Gardner Impact Testing.
(FIG. 3) Mean failure height of one 6565-rated sheet product (set of six taller specimens) is 70 in. (17.78 cm) versus 24 in. (60.96 cm) for the second grade (set of six shorter specimens), a performance advantage of 192 percent according to Gardner Impact Strength test results.
(FIG. 4) Aircraft tray table thermoformed of proprietary high-impact thermoplastic alloy from Boltaron
(FIG. 5) The Model 3050 FeatherWeightTM premium economy seat from TIMCO Aerosystems weighs as much as 25 percent less than similar seating. Shrouds, seat dividers, end caps, tray tables and other plastic components are formed by Multifab Inc. of Boltaron proprietary sheet.
Because aircraft manufacturers and aircraft interior suppliers are limited to a relatively small number of thermoplastic sheet products that comply with strict FAA regulations, significant differences in impact resistance between compliant products are often overlooked but at great long-term expense to the carrier. The reason is that sheet products exhibiting higher impact resistance yield one of two advantages: 1) thermoformed parts with greater durability that look new longer and decrease replacement costs, or 2) thermoformed parts of equivalent durability achieved using thinner, lighter sheet that cuts fuel costs throughout the useful life of the aircraft's interior.
The baseline: sheet products that meet aircraft fire, smoke and heat release standards
High-performance thermoplastic alloy sheet products that qualify for aircraft interior applications must be certified as meeting stringent FAA (Federal Aviation Administration) standards for flammability, heat release and smoke generation including: FAR 25.853 A1 F, Part IV-6565; FAR 25.853 (d); FAR 25.853 A1(i); FAR 25.853 A1.
While it is fundamental that the sheet purchased carry the above certifications, it is also critical to realize that thermoplastic formulations developed to meet these standards typically incur reductions in impact resistance of 50 to 75 percent versus comparable non-aircraft-rated grades.
Since the properties of available fire-rated sheet products can vary widely, the specifier should identify materials that offer the highest impact resistance and other properties desirable for any given fire rating, product category and/or price point.
Impact testing basics every sheet specifier should know
Two widely accepted methods for measuring the impact resistance of thermoplastic sheet products are Izod Impact Strength Testing (FIG. 1) and Gardner Impact Testing (FIG. 2).
With Izod Impact Strength Testing (ASTM D-256), a pendulum held at a specific height is released, breaking the notched specimen. The amount of energy absorbed determines the distance the pendulum swings past the sample, as indicated by a pointer on a graduated scale. The more energy the test specimen absorbs, the lower the needle will indicate.
The most common test specimen measures 2.5 x 0.5 x 0.125 in. (63.5 x 12.7 x 3.2 mm), and is cut with a distinct notch on one of its sides. The specimen is secured to the testing unit below the notch, causing it to break at the notch, with results generally expressed in ft-lb/in or J/m.
The distance the pendulum travels after breaking two different heavy-gauge specimens (and specimens having high impact resistance) can vary significantly, yielding meaningful test results. However, because the weight of the pendulum relative to the strength of two different thin-gauge specimens (and specimens having low impact resistance) is far greater, differences in the distance the pendulum travels beyond the specimens is far less, decreasing the accuracy of this method for evaluating sheet products typically utilized for thermoforming of aircraft interior parts.
Widely utilized for accurately measuring differences in impact resistance between sheet products of thinner gauge and/or lower impact resistance, is Gardner Impact Testing (ASTM-D4272) also known as Falling Dart Impact Testing, in which an "impactor" or "dart" (of variable weight) is released from various heights onto a specimen. After multiple impacts, the height from which half of the specimens failed is expressed as "mean failure height" in inches/centimeters or as "mean failure energy" in in./lb or kg/cm.
It is important to note that impact resistance for most aircraft rated sheet products is expressed in terms of Izod Impact Strength, which can understate the performance difference between two different sheet products meeting the FAA "6565" standard.
For example, the published Izod Impact Strength of one 6565-rated sheet product is 5 ft-lbs/in (265 J/m), versus 3 ft-lbs/in (159 J/m) for another 6565-rated sheet product, a performance advantage of 67 percent.
However, Gardner Impact Testing of the same two sheet grades (FIG. 3) in 60 mil thickness determined mean failure height of the first grade to be 70 in. (177.80 cm) versus 24 in. (60.96 cm) for the second grade, a performance advantage of 192 percent, illustrating the importance of this test method to specifiers of aircraft rated sheet products.
Higher impact resistance can improve the durability of thermoformed components (FIG. 4), and/or allow components to be thermoformed using thinner, lighter sheet products having equivalent impact resistance.
For example, a new Model 3050 FeatherWeightTM seat (FIG. 5) from TIMCO Aerosystems, which is thermoformed by Multifab Inc. using the higher-performing sheet grade described in the aforementioned test, can reportedly reduce overall weight of a 200 seat aircraft by more than 8000 lb. (3629 kg), yielding significant long term fuel savings.
Producing sheet products that comply with strict FAA requirements for fire, smoke and heat release incurs sacrifices in impact resistance and other physical properties, dramatically affecting the durability of components and/or fuel efficiency of the aircraft. It is therefore incumbent on the specifier of thermoformed interior components to identify significant differences in available sheet products by evaluating reliable test data obtained not only through Izod Impact Strength Testing, but also Gardner Impact Testing.