Acoustic Emission Testing, or AET, is a clever way to listen to the sounds materials make under stress. Think of it like a doctor using a stethoscope to listen to a patient's heartbeat but for materials. When a material starts to deform or crack, even on a microscopic level, it releases tiny bursts of energy in the form of high-frequency sound waves. AET equipment picks up these "acoustic emissions," allowing technicians to detect and monitor potential problems like crack formation or growth before they become critical. It's a technique that has been refined over decades and is a trusted tool in aerospace, automotive, civil engineering, and power generation for assessing the health of vital components.

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AET involves attaching special sensors, called transducers, to the surface of the material or structure being monitored. These sensors are incredibly sensitive and can detect faint, high-frequency acoustic waves traveling through the material.
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When the material is put under stress—perhaps during normal operation, a specific test loading, or even due to temperature changes—any internal changes, like a tiny crack forming or growing, will generate these acoustic emissions. The transducers convert these stress waves into electrical signals. A computer system then amplifies, processes, and analyzes these signals.
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The analysis helps pinpoint where the emissions are coming from, their intensity, and their frequency. This information gives a clear picture of the material's condition. For instance, the strength (amplitude) and pitch (frequency) of the emissions can suggest the type and seriousness of defects, such as cracks, voids, or layers separating in a composite material (delamination). Engineers know exactly where to look for the developing flaw by identifying the location. Unlike other NDT methods, like ultrasonic testing, which sends waves into the material, AET listens to the sounds the material makes as it changes.

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At its core, AET relies on a simple principle: when a material undergoes an irreversible change in its internal structure, it releases energy. Imagine bending a dry twig – just before it snaps, you might hear small cracking sounds. That's a very basic analogy for acoustic emission.
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These internal changes, such as the formation of a new crack, the growth of an existing one, or even localized plastic deformation (where the material permanently changes shape), cause a rapid release of stored elastic energy. This energy travels outwards from the source as elastic waves, also known as stress waves, through the material. These are not typically sounds you can hear with your ear, as their frequencies are usually ultrasonic, often in the 150 to 300 kilohertz (kHz) range.
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These waves are very short-lived, lasting only fractions of a second. By using multiple sensors strategically placed on the material's surface, the system can detect these fleeting signals and, by comparing the arrival times at different sensors, can triangulate the exact location of the defect. A key thing to remember about AET is that it's a passive technique; it listens for defects as they happen or grows during the test rather than actively seeking out static flaws.

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To clarify this, let's consider a practical example: monitoring a railway track. Sensors can be attached directly to the steel rails. As trains pass over the track, they apply significant loads. This stress might cause tiny, invisible cracks to form or existing ones to grow.
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An acoustic emission system continuously listens for the characteristic stress waves generated by these crack events. When such an emission is detected, a signal is sent to a central monitoring system. An alarm can be triggered if the signals indicate a potentially serious flaw. This early warning allows maintenance crews to inspect the specific section of the track and carry out preventive repairs, potentially avoiding derailments or other accidents. This proactive approach is a hallmark of AET's value.

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AET has some distinct features that set it apart from other non-destructive testing techniques:
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• Detects Active Flaws: Acoustic Emission Testing (AET) primarily detects the movement or growth of defects when a material is under stress. Many other NDT methods find flaws based on their physical shape or properties, regardless of whether they are actively changing.
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• Material vs. Geometry Sensitivity: AET is generally more sensitive to the type of material and its behavior under stress and less affected by the complex geometry of the object being tested. Intricate shapes can significantly hamper some other methods.
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• Limited Access Required: Sensors must only be placed on the surface at specific locations, not necessarily all over the part.
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• Whole Structure Testing: AET can often monitor an entire structure or a large component at once from these sensor positions. Other techniques might require scanning local regions sequentially, which can be time-consuming.
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• Noise Challenges: A significant challenge for AET is distinguishing true acoustic emissions from background noise (e.g., mechanical vibrations, hydraulic noise).
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While other NDT methods are excellent at characterizing the size and shape of existing geometric defects, AET excels at telling you if those (or new) defects are active and growing under operational or test stresses. For this reason, AET is often used alongside other NDT techniques to provide a more complete assessment of a component's integrity.

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The idea of listening to materials to understand their condition is not new. The word "crack" itself is onomatopoeic – it sounds like the event it describes.
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Around 6500 BC, pottery makers would listen for audible sounds as their ceramics cooled. These sounds helped them assess the quality and integrity of their work. Even today, you might see someone tap a glass or ceramic item in a shop; they're listening for the clear ring of a sound item versus the dull thud that might indicate an internal crack.
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The term "tin cry" was documented around 3700 BC to describe the characteristic sounds tin makes when deformed. Fast forward to the 1960s, and the field took a significant leap when researchers at Boeing began using high-frequency acoustic signals to detect early signs of failure in aero engines, laying the groundwork for modern AET.

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A typical Acoustic Emission testing setup consists of a few key components:
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• Sensors: These are the system's "ears." Most commonly, they are piezoelectric elements. Piezoelectric materials have the unique property of generating an electrical voltage when they are mechanically stressed (like when an AE wave hits them). These sensitive elements are usually housed in protective casings.
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• Preamplifiers: The initial electrical signals from the sensors are often very weak. Preamplifiers, usually located close to the sensors, boost these signals to a level that can be effectively transmitted and analyzed, improving the signal-to-noise ratio.
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• Signal Processor and Analysis System: This is the brain of the operation. It takes the amplified signals and filters out unwanted noise. It then analyzes the characteristics of the AE signals (like their amplitude, duration, and energy). It can display this data in various charts and graphs for interpretation by a trained technician. This system also typically handles the source location calculations.

Proper sensor installation is crucial for good results. Sensors need to be acoustically coupled to the test object's surface, meaning there needs to be a good path for the sound waves to travel from the material into the sensor. Various couplants are used for this, depending on the application:
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• Glue: Often used for more permanent installations, like on pipes.
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• Magnets: Suitable for attaching sensors to ferromagnetic materials like steel vessels.
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• Grease or viscous fluids: Provide good temporary coupling.
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• Bands: Can be used to strap sensors onto surfaces.
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• Waveguides: These are special rods or wires used to transmit the AE signal from a very hot, cold, or otherwise inaccessible area to a sensor located in a more benign environment. They are used to monitor high-temperature pipework or rotating parts.

In specific environments, like nuclear applications, additional protections such as lead blankets might be used to shield sensors or personnel.

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It's important to understand that Acoustic Emission Waves differ from sound waves traveling through the air we hear. They are elastic waves that radiate through solid materials. Think of them like ripples spreading out from a pebble dropped into a pond but happening within the material itself.
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These waves carry energy and spread out in all directions from their source (e.g., a growing crack). Their frequencies range widely, from the audible spectrum (though these are less common for engineered materials testing) up to around 1200 kHz or even higher. For most industrial NDT applications, the focus is on the ultrasonic range.
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AE signals can generally be categorized into two main types:
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• Burst-type emissions: These are transient, discrete signals that rise quickly and then decay. They are typically associated with sudden, distinct events like the formation or extension of a crack, a fiber breaking in a composite material, or localized plastic deformation.
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• Continuous emissions: These appear as a more sustained, lower-amplitude signal where individual events overlap and are not easily distinguishable. Continuous emissions are often linked to processes like friction (e.g., rubbing between crack faces), material yielding over a larger area, or fluid leakage through a crack.

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As Acoustic Emission Waves travel from their source within a material to the sensors on the surface, their journey isn't always straightforward. The waves are affected by the material they pass through, and understanding these effects is important for accurate interpretation of AE data:
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• Attenuation: This refers to the decrease in the amplitude (strength) of the wave as it travels. Energy is lost due to several mechanisms, including absorption by the material (conversion to heat) and also by the wave spreading out (geometric spreading). Materials with high damping characteristics will attenuate signals more rapidly.
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• Dispersion: In many materials, the speed at which an AE wave travels can depend on its frequency. This phenomenon is called dispersion. Because an AE burst typically comprises a range of frequencies, different frequency components will travel at slightly different speeds. This causes the waveform to spread out or change shape as it propagates, complicating analysis.
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• Diffraction: If an AE wave encounters an obstacle or the edge of the structure, it can bend around it. This is known as diffraction.
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• Scattering: When AE waves encounter discontinuities within the material – such as grain boundaries, inclusions, pores, or existing cracks – they can be deflected or scattered in multiple directions. This can make it harder to pinpoint the source and can also contribute to attenuation.
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By carefully analyzing the characteristics of the received wave patterns and sometimes using information about the material's known acoustic properties, experienced AET practitioners can often infer information about the types of defects present and their locations despite these propagation effects.

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To turn the raw electrical signals from the sensors into meaningful information, AET systems analyze several features of the AE waveform. These parameters help characterize the acoustic emission events:
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• AE Hits (or Events): A "hit" is registered when the signal waveform exceeds a predefined voltage threshold. This helps distinguish genuine AE signals from low-level background noise. Each distinct burst of acoustic emission is usually counted as one hit or event.
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• Peak Amplitude: This is the maximum voltage the signal waveform reaches during a hit. It's a measure of the signal's strength and is often related to the magnitude or intensity of the source event (e.g., a larger crack growth step might produce a higher amplitude signal). It also indicates how easily the signal can be detected.
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• Rise Time: This is the time taken for the signal to go from the point it first crosses the detection threshold to the point it reaches its peak amplitude. Rise time can sometimes provide clues about the nature of the source or the distance the wave has traveled (though dispersion can complicate this).
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• Duration: This is when the AE signal remains above the detection threshold from its first crossing to its last. Signal duration can be useful in distinguishing between different types of sources (e.g., a crack event might be shorter than a friction event).
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• MARSE (Measured Area under the Rectified Signal Envelope): It is often called "AE energy," though not strictly energy. It's a measure calculated from the area under the rectified (absolute value) signal envelope. MARSE is sensitive to both the amplitude and duration of the hit and is often considered a good indicator of the AE event's damage severity or energy release.
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• Counts (or Ringdown Counts): The number of times the signal waveform crosses the detection threshold during a single AE hit. For a burst-type signal, it's essentially the number of oscillations in the "ringdown" part of the signal. Counts are related to the signal's magnitude and duration and can sometimes indicate signal complexity.
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• Count Rate: This is the number of counts detected per unit of time (e.g., counts per second). It can indicate the level of acoustic emission activity.
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By analyzing these parameters, often in combination and looking at their trends over time or under changing load conditions, AET specialists can build a detailed picture of the material's behavior and the development of any damage.

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Acoustic Emission Testing is a versatile technique with a broad spectrum of uses across numerous industries, primarily focused on ensuring safety and reliability:
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• Non-Destructive Testing (NDT) in General: Its fundamental application is to detect and monitor defects, cracks, corrosion, and other forms of damage in a wide array of materials, including metals (steel, aluminum), composites (like those used in aircraft), ceramics, and even concrete.
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• Oil and Gas Industry: AET is critical in monitoring the integrity of pipelines, storage tanks (above-ground and pressure vessels), and offshore structures. Detecting the subtle acoustic emissions from active corrosion, crack growth, or leaks can help identify potential problems like ruptures or leaks before they lead to catastrophic failures, preventing environmental damage and ensuring worker safety.
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• Aerospace: The structural integrity of aircraft components is paramount. AET monitors wings, fuselage sections, landing gear, and engine components during manufacturing (e.g., proof testing) and in-service (structural health monitoring). Detecting changes in acoustic emissions can reveal early signs of fatigue cracking or composite delamination, preventing accidents.
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• Automotive Industry: In automotive manufacturing, AET is applied for quality control and testing critical components such as engines (e.g., detecting valve seating issues), transmissions, bearings, and suspension systems. It can also be used in the development and testing of new materials.
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• Civil Engineering and Construction: AET monitors the health of large concrete structures like bridges, dams, tunnels, and buildings. It can detect the initiation and growth of cracks in concrete, corrosion of reinforcing steel, and other degradation mechanisms, providing valuable data for maintenance planning and ensuring public safety. It also monitors wooden structures and geotechnical applications like slope stability.
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• Power Generation: In power plants (both conventional and nuclear), AET monitors components like boilers, pressure vessels, piping systems, and rotating machinery (e.g., turbines) for signs of damage.
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• Manufacturing Processes: It can monitor processes like welding (detecting crack formation during cooling), cutting, and grinding, as well as material phase transformations.
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• Research and Development: AET is a valuable tool for studying material behavior, fracture mechanics, and damage accumulation processes under various conditions.
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Essentially, anywhere that material degradation poses a risk, AET can offer a powerful way to listen in and catch problems early.

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Acoustic Emission Testing offers several significant benefits, making it a valuable tool for structural integrity assessment and maintenance:
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• Real-time Detection: One of its primary strengths is detecting defect activity, such as crack initiation or growth, as it happens. This provides immediate feedback on the condition of a structure under stress.
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• High Sensitivity to Active Defects: AET is very sensitive to dynamic changes in a material. It can often detect very small increments of crack growth or other damage mechanisms at their earliest stages, providing timely warnings.
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• Precise Defect Location: By using an array of sensors and analyzing the arrival times of the acoustic waves, the emission source (i.e., the location of the active defect) can often be pinpointed with good accuracy, even in large structures. This directs inspectors to the exact area needing attention.
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• Global Monitoring from Fixed Sensors: AET can often monitor an entire structure or a large section simultaneously using a relatively small number of fixed sensors. This eliminates the need for detailed, time-consuming scanning of the entire surface, as some other NDT methods require.
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• Operational Testing: A significant advantage is that AET can often be performed while the equipment or structure is in service and under normal operating loads. This allows for assessment under real-world conditions without requiring costly shutdowns. It can also identify active flaws during proof testing or periodic overloading.
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• No Need to Access Both Sides: Sensors are typically placed on one surface of the object being tested, making it suitable for components where access is limited.
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• Cost-Effective Early Warning: While the initial setup might be costly, AET can be very economical in the long run by detecting damage early and preventing major failures or unplanned downtime.
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These advantages make AET a powerful complement to other NDT methods and a cornerstone of many structural health monitoring programs.

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While AET is a powerful technique, it's important to be aware of its limitations:
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• High Initial Cost: The specialized sensors, data acquisition systems, and analysis software can represent a significant upfront investment.
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• Skilled Operation Required: Interpreting AE data accurately requires well-trained and experienced personnel. Differentiating between genuine defect signals and various forms of noise and then characterizing the source is a complex task.
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• Challenges in Outdoor Environments: AE signals are typically transmitted from sensors via cables. Managing these cables, especially over long distances on structures like bridges or large pipelines in outdoor settings, can be difficult and prone to damage or interference. Wireless sensor technology is helping to mitigate this but may have its own limitations (power, bandwidth).
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• Test Reproducibility: Because AE often relies on the somewhat random nature of crack formation and propagation, tests may not always be perfectly reproducible regarding the exact timing or characteristics of every emission. However, overall trends and activity levels are generally consistent for similar conditions.
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• Signal Strength and Attenuation: AE signals are generally much weaker than those used in active ultrasonic testing. This necessitates highly sensitive sensors and good amplification. Signals can also significantly attenuate (lose strength) as they travel through large or complex structures or materials with high damping, potentially limiting the detection range.
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• Noise Interference: Distinguishing low-level AE signals from ambient noise (e.g., mechanical vibrations, hydraulic sounds, electromagnetic interference, rain) is a major challenge. Sophisticated filtering and signal processing techniques are needed, but noise can sometimes mask true emissions or lead to false alarms.
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• Only Detects Active Flaws: AET generally only detects defects that are actively growing or changing under applied stress. It may not detect static, pre-existing flaws that are not propagating during the test. This is why it's often used in conjunction with other NDT methods that can detect static flaws.
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• Sophisticated Data Processing: Extracting meaningful information from the raw AE data usually requires complex algorithms and software for signal processing, feature extraction, and source localization.
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Understanding these limitations helps decide when and how to apply AET effectively, often as part of a broader inspection and monitoring strategy.

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Integrating Acoustic Emission Testing with a robust Computerized Maintenance Management System (CMMS) like Cryotos can significantly streamline and enhance maintenance operations. While AET provides the raw data on material health, Cryotos CMMS provides the framework to manage this information effectively and turn it into actionable insights.
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Here's how Cryotos CMMS can support your AET program:
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• Scheduling and Tracking AET Inspections: You can schedule recurring AET activities for critical assets directly within Cryotos. The system helps track when tests are due, ensuring they are performed on time as part of your preventive or condition-based maintenance strategy.
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• Work Order Management for AET: When an AET inspection is needed, or if AET findings require follow-up, Cryotos facilitates the creation, assignment, and tracking of work orders. This ensures that AET tasks are properly documented and completed by qualified personnel.
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• Centralized Data Storage and History: AET generates valuable data and reports. Cryotos CMMS allows you to attach these reports directly to the specific asset's record, including waveform data, source location information, and technician analyses. This creates a comprehensive historical log of an asset's condition over time.
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• Linking AET Findings to Corrective Actions: When AET identifies a developing flaw, this information can be logged in Cryotos, triggering further investigation or corrective maintenance work orders. This ensures that issues detected by AET are not overlooked and are addressed promptly.
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• Managing AET Equipment: The sensors, preamplifiers, and analysis units used for AET are assets that require management. Cryotos can help track this equipment, manage calibration schedules, and record any maintenance performed, ensuring your AET gear is always ready and reliable.
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• Trend Analysis and Reporting: By storing AET results consistently within Cryotos, you can leverage its reporting and analytics capabilities. This can help identify trends in defect development, correlate AE data with operational parameters, and assess the overall effectiveness of your AET program in preventing failures.
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Using Cryotos CMMS in conjunction with Acoustic Emission Testing helps bridge the gap between defect detection and effective maintenance execution, ultimately improving asset reliability, safety, and reduced operational costs.

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Acoustic Emission Testing is a sophisticated and highly valuable non-destructive testing method. Listening to the subtle sounds of materials under stress provides a unique window into their internal condition, allowing for the early detection and monitoring of active defects like growing cracks. While it requires specialized equipment and skilled operators, its ability to monitor entire structures in real-time, often during normal operation, makes it an indispensable tool for ensuring the safety and reliability of critical components across a wide range of industries. Its strength lies in identifying problems as they develop, offering a proactive maintenance and failure prevention approach.