Step on an airplane and you expect every last rivet or weld to have exceeded the most strenuous quality testing. It isn’t just the materials themselves, but the assembly. Newer materials such as carbon fiber and resin bonding agents are used in novel or lighter design, but doors may well still operate on familiar hydraulics and pins. We have all watched the many screws or rivets on those flexing wings, as they seem to fall wide open for landing. Translate this into the extreme environments of military and space, where the newest technologies are utilized, and it is clear that force testing is absolutely central to safety.
In just 110 years, human flight has moved from two ingenious bicycle makers twisting a wooden airframe, to plans for a manned mission to Mars. Fatalities in space flight may be 0.9%, among astronauts who recognized the risks, but for the rest of us, we hope that no air disaster has involved mechanical failure. We travel with confidence because of the rigorous testing of forces on every component, not just by trusting predictive design. Testing is to extremes, and materials testing machines will apply the forces in precise ways, in any direction.
Strength and performance testing falls into three areas: materials testing, joints and adhesives, and component testing. It involves the most accurate digital instruments, laboratory testing frames, and portable jigs to ensure highly repeatable and controlled methods. This ensures that the same methods used for design or for newly manufactured parts can be applied in ongoing maintenance checks.
For testing materials, samples are produced to standardized shape profiles. Easily handled in a testing laboratory, these can be made or die-stamped, in different orientations from donor source material, which may itself be anisotropic. Typical tests include compressive and tensile strength, deformation elasticity and recovery, hardness, and resistance to puncture, penetration and tear. Vibration and repeated stress is part of the life of aeronautic structures, and specialized testing machines are available for multi-axis fatigue testing, using cruciform or star-shaped samples. These tests all assess the properties of the materials themselves, and are well suited to controlled laboratory testing, where specific environmental variables can also be introduced. Extreme forces may be required, like sample containment, or a test to be performed over extended temperature ranges. Data for multiple stresses may need to be captured, or data from high-speed photography, acoustic or thermal sensors.
Gripping a sample is likely to be fairly standardized, because of their regular and symmetrical shapes, so fixtures for holding a sample will usually be readily available. Properties and behaviors of a material can then be extended into the design process and become predictive of performance by extrapolation. The bottom line of materials testing is that sample preparation and application methods are precisely codified in international standards, and repeatable in any testing laboratory, anywhere.
Clearly, materials testing with precise and accredited equipment is essential for ongoing quality assurance in supplied materials. This is especially true for new composites, and production methods such as laminated additive manufacturing (3D printing in differing multiple-layer materials). Without available standardized data, or mass-production uniformity in manufacture, performance post-manufacture must be tested.
Where new materials are used to replace traditional ones, typically for their weight/strength ratio, design can change substantially. Jointing methods will probably be very different, which in turn means that historical experience of points of failure and weakness is being replaced. Helicopters are now 90% composite by weight, and commercial aircraft 50%.
Joints and adhesives
For testing fixings, fasteners, seams, welds and bonds, samples can be made according to specification, to test the intrinsic design of a joint or the strength of an adhesive. Conditions on the laboratory bench can be carefully controlled and monitored, and subsequently, samples can be microscopically examined for the way a bond, for example, has disintegrated in a destructive test, or a crack has propagated.
International test standards prescribe the dimensions and preparation of samples, including curing conditions of resins (for example), and the speed of application of tensile or compressive forces. The test may focus on the jointing method rather than the materials involved, and the testing methods themselves are likely to be computerized for fully repeatable, controlled operations. In this way, prescribed methods can be reliably used for developing new, optimized jointing solutions. As with materials testing, ultimate strength—or test to destruction under tension, compression, torque, or a combination—can be applied, and compared against standardized results.
Very similar tests are required on high-performance coatings designed for protection of external surfaces in extreme conditions. These tests usually involve pull-off from within a scored-out surface area, using an adhered dolly, requiring preparation and curing time. In the laboratory, conditions can be highly controlled, but a portable instrument and test jig may need to be taken to the final structure. In either case, the result is not just a pull-off force measurement. Examination of the manner of disbondment is important: the coating may peel or lift away (adhesive failure), disintegrate (cohesive failure), or pull away the surface of the substrate.
Component testing involves handling completed assemblies. Again, bench testing offers the most controlled conditions, especially where repeated use to failure is important, but may only be suited to smaller parts. In the case of clips, locks, levers and handles, correct application in situ requires the test instrument to be taken to the location, perhaps with a special lightweight jig for positioning to ensure stable, fully repeatable testing by hand.
Component parts need not be part of airframe structural integrity to be safety critical. From electrical connectors to push-button controls and switches, and from springs and fasteners to hand controls, their mechanical specifications can be tested. What does it take to actuate a touch-screen or membrane type control, and again after a thousand uses? A programmable universal tester coupled to the switch circuit can compare electrical and mechanical performance once or over many cycles, applying very precise pressure.
How secure are surface-mount components on a PCB under shear forces, or flexing? With suitable test fixtures you can have the answers, including precise test data, and repeatability in your method. This may reveal not just the point of failure, or falling out of tolerance, but early signs of wear or disintegration.
Wire terminal connectors may be welded, soldered or crimped for a secure and strong cable joint. The connector may be push-fit, or a bolted loop or fork, and straight or angled. One joint may be for easy insertion and withdrawal, the other must never fail. In the assembly of complex wiring harnesses, overall integrity also matters. Measuring a pull-off force is straightforward, but holding the terminal without deformation requires care.
New manufacturing methods
In aerospace, weight is especially important for fuel economy and emissions. Aluminum replaces copper in cables, structural composites replace aluminum in airframes, whilst smaller 3D printed parts may be of titanium and carbon fiber, not just ABS plastic or nylon.
3D parts can be automatically manufactured using complex multilayered materials, and have embedded components such as captive metal fasteners and mounts. They have quite different properties from traditionally cast and CNC-machined parts, which may not even have been manufacturable as a single piece. The new components can be stronger and more durable than their metal counterparts, to be used in airframes, interior structures, even engine parts.
Testing of a complex part that has been 3D printed in, say, nylon and carbon fiber, in multiple layers of different shapes and, for example, with embedded nuts, requires testing the finished part as a whole. Three-point bending of the piece and tensile strength of captive nuts are both essential, because they reflect a novel material design, not just a novel design in a well-understood material. The same is true of vacuum-assisted resin transfer molding, for example, as used for fuselage panels, radomes and sonar domes. Injection molding and 3D printing of parts that used to be machined or cast, creates parts with new internal stress characteristics.
Seals may be co-molded for improved integrity, making separated testing more difficult. Further, metal linings and inserts may be combined with plastics for temperature shielding or wear resistance, and differential thermal expansion becomes a factor in testing the finished assembly. Given aerospace environments, testing new materials and new manufacturing methods may require the same force testing but in an environmental chamber at reduced or elevated temperatures.
So you feel safer now, as you step off an aircraft? Don’t forget that the fabric of your seat, the seatbelt, the air conditioning knob, the trolley and even the bottle of water in flight were all tested in a very similar way. (And it is also why your suitcase has survived the rigors of the conveyor.)