From minute cracks and checks to imperceptible voids undetectable through the use of X-ray, ultrasonic detection equipment is instrumental in the successful discovery of defects in all kinds of materials. Extensive use of composite products is standard in the aerospace industry. For this reason, continuous testing and assessment is a key strategy to detect flaws and ensure companies remain compliant. That’s where ultrasonic testing comes in—UT makes it possible to maintain the highest quality and safety levels. But how exactly does ultrasonic work and why is it so important for the aerospace industry? Let’s discuss.
What Is Ultrasonic Inspection?
Ultrasonic testing (UT) is a non-destructive testing (NDT) technique that transmits ultrasonic waves via material or object to characterize or detect flaws. Because defects are normally “weaker” than the surrounding material, they vibrate differently when subjected to the same pressure wave from an acoustic pulse. Typically, UT inspections utilize short pulse waves, i.e., 0.1 to 15MHz to locate flaws in materials. That said, it’s not unusual to use higher frequencies up to 50MHz.
Ultrasonic inspection is used in a variety of industries, including aerospace, steel and aluminum construction, metallurgy, manufacturing, automotive, and other transportation sectors.
How Does Ultrasonic Inspection Work?
The material undergoing UT inspection must first use a couplant, typically water or gel. This couplant is critical as it facilitates the transmission of sound waves between the transducer and object undergoing UT inspection.
Four primary UT inspection methods exist.
1. Pulse-echo
Pulse-echo testing uses one transducer that serves both as a transmitter and receiver. The transducer is the means by which electrical energy is converted into acoustic energy and back again.
In this process, a sound (ultrasonic waves) is transmitted in one direction into the material. Echoes of this sound are reflected and received by a single transducer to provide information on the test object. A diagnostic device displays and analyzes the returning waves using an amplitude—showing the magnitude and distance of the reflection.
2. Through-transmission
This UT testing method, also known as attenuation, involves the transmission and reception of ultrasound over two transducers. Again, the sound is transmitted in one direction through the object and received at the other end of the object.
The first transducer called the pulser transducer, emits the pulse while the receiver transducer receives the signals on the other side of the component after traveling through a couplant. An interruption in the sound path points to an existing imperfection—expect a low sound that will allow you to identify the defects.
The advantage of this technique is high-speed non-destructive inspection. However, this method requires access to both sides of the test object and does not provide depth/amplitude information on individual interfaces.
3. Resonance
Resonance is the preferred method for testing of bond-lines in bonded structures. This technique uses standing waves, and the phenomenon occurs when piezoelectric crystals are excited at their fundamental resonating frequency.
4. Phased Array
Phased Array Ultrasonic Testing (PAUT) is a more comprehensive ultrasonic testing method that uses ultrasonic phased arrays with multiple ultrasonic elements and electronic time delays to emit and receive ultrasounds. The elements undergo pulsation through computer-calculated time frames (hence the name phased). This process generates a focused beam that will help in flaw detection.
Phased Array ultrasonic testing optimizes the detection of discontinuities in aerospace composites. In addition, it allows for very fast component coverage which can be highly cost-effective.
Main Types of UT Inspection
Besides the methods, ultrasonic testing can fall under two types.
- Contact. A UT inspection technique that’s ideal due to its portability and accessibility. Those carrying out this test will use it when only one material side is reachable. It is also the go-to method for bulky items.
- Immersion. A factory-based testing approach suitable for curved components and ultrasonic technique development. The immersion process uses water and not gel as a couplant. Plus, it uses the pulse-echo method to identify flaws. This UT type can work for various wall thicknesses, that’s why it applies to other industries besides aerospace.
Role of UT Inspection of Aerospace Composites
Composites are a synergistic combination of materials, a product resulting from combining multiple objects with significantly different chemical and physical attributes to address each material’s weakness. Keep in mind that this fusion does not eliminate each object’s identity. Instead, once the fibers for each element come into contact with another, you end up with a durable, low-weight, efficient and robust product.
Ultrasonic inspection is the most widely used non-destructive inspection method for the examination of aerospace composites. UT inspection is critical as it enables a high level of accuracy when trying to identify and measure flaws, voids, and other defects. This process sends sound waves ranging between 500KHz and 20MHz through a receiver circuit and then evaluates the signal.
It is this analysis of information that allows technicians to measure the density and internal structure of components. If there are no flaws, the sound waves will transmit seamlessly. On the same note, these waves will bounce off the material if there’s a defect or void. Technicians will then develop a 3D visualization of the object to calculate the distances between the identified flaws.
As an expert in aerospace manufacturing, Avior uses each of the above UT inspection methods to ensure the quality and compliance of its composite products. By using these various methods, we are able to identify potential flaws in our composite products.
Examples of flaws found in composites are:
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Advantages and Limitations of UT Inspection
Advantages
Ultrasonic testing on aerospace composites works because of the following reasons.
- Extensive penetration allows flaw detection in thick materials
- Highly sensitive, increasing its accuracy
- High versatility and variety of techniques
- These techniques work even when you can only access one side of the material
- Generally more accurate than other non-destructive testing techniques
- UT inspection offers instant findings allowing companies to take immediate next steps
- Non-hazardous
- High portability
- Offers comprehensive details about defects, including their nature and size
- Automated
Limitations
UT inspections present a few challenges.
- Need a trained operator
- Only certified personnel can inspect and evaluate the findings
- Liquid couplant is required
- Surface conditions affect test performance
- Component shape/configuration can affect test results
- Can produce false-positive results
- UT inspection for irregularly shaped materials is complicated
- Discontinuity detection is affected by orientation
- Test object must be able to properly conduct sound. Due to unfavorable geometry or undesirable interior structure, some objects may be unsuitable for ultrasonic testing
- Dead Zone: discontinuities just beneath the surface may not be detectable
- Near Field: the interface zone in front of the transducer can degrade detection and evaluation
- You must eliminate paint before commencing the inspection process
Avior UT Inspection Services
Due to its non-destructive nature, ultrasonic inspection lists among the most preferred evaluation methods. Moreover, it has a wide range of applications offering highly accurate flaw detection on ferrous and thick materials and objects accessible from one side.
Avior uses several ultrasonic inspection methods to test the quality of its aerospace composite products. We are Nadcap accredited for non-destructive testing (NDT) and are qualified to Boeing, Airbus and Spirit Aerosystems specifications for ultrasonic NDT on composite parts in our Aerospace testing lab. We’ve recently entered into an agreement with a major aerospace OEM to provide UT inspection services using our new 10-axis UT inspection equipment. Contact us for more information about our composite products and UT inspection services.
Further Readings You May Be Interested In
- Aerospace Non-Destructive Testing: 6 Nadcap Approved Methods
- Overview of Fluorescent Penetrant Inspection in Aerospace
