Defect Characteristics in Ultrasonic Testing
This study utilizes the Ondar-3600 ultrasonic flaw detector to investigate the ultrasonic waveforms of typical defect surfaces in steel and aluminum. Through qualitative analysis of surface-reflected defect waveforms and dynamic ultrasonic characteristics, the research characterizes the pulse-type ultrasonic waveforms of typical surface-reflected defects such as bubbles and shrinkage cavities. The results demonstrate that while the ultrasonic waveforms of surface-reflected defects (e.g., bubbles and shrinkage cavities) exhibit striking similarity, subtle variations in the detected waveforms arise from differences in acoustic field parameters (e.g., acoustic impedance) at the defect interfaces.
1. Introduction
With the rapid advancement of modern technology, industries such as aerospace, aviation, and high-speed rail have established stringent requirements for material quality and performance. Critical components in these systems demand particularly rigorous material structures, where mechanical properties serve as a key criterion for material selection. However, limitations in casting technology often result in various defects during the production of steel and aluminum materials. These defects, such as shrinkage cavities caused by thermal expansion and contraction, or porosity induced by carbon dioxide, can severely compromise the mechanical properties of these materials. Such defects not only lead to substantial economic losses but may also cause serious accidents. Therefore, conducting non-destructive testing (NDT) for defects in steel and aluminum is crucial. Analyzing the fundamental characteristics of ultrasonic defect waveforms in various types of defects provides critical guidance for identifying the specific types of defects present in these materials.
Steel and aluminum are widely used in modern society, making their defect detection a common practice. Non-destructive testing (NDT) technologies for these materials have advanced rapidly, with multiple detection methods available. Ultrasonic testing stands out as one of the five standard NDT techniques in industry and is the most widely used. Ultrasonic waves exhibit minimal energy loss during propagation through steel and aluminum, enabling deep detection. However, when encountering interfaces with abrupt acoustic impedance changes, acoustic phenomena like reflection and refraction occur. Notably, ultrasonic waves cannot penetrate gas-solid interfaces.
This study employs ultrasonic flaw detection technology to examine material notches. The research focuses on simulating typical shrinkage and porosity defects in materials by fabricating geometrically regular curved reflective notches (circular-shaped) from steel and aluminum using specialized equipment. These notches are then measured using Type-A ultrasonic flaw detection technology.
2. Principles and Methods
Ultrasonic Testing Principle: In uniform media, ultrasonic waves exhibit excellent directional propagation with minimal energy attenuation. However, when propagating through media, they encounter interfaces with abrupt acoustic impedance changes (e.g., defects like porosity or shrinkage cavities). These discontinuities cause acoustic phenomena including reflection, scattering, and attenuation. By analyzing variations in reflected or transmitted waves, material defect information can be obtained.
The ultrasonic reflection characteristics of material defects are related to factors such as defect orientation and geometric shape. Different types of defects exhibit distinct waveforms due to variations in their interface geometries. Therefore, the defect characteristics of materials can be analyzed by examining the features of ultrasonic reflection waveforms, as illustrated in Figure 2, which demonstrates the reflection patterns of several different types of defects.
Experimental Method: This study employs the ultrasonic dynamic waveform method, which involves monitoring the amplitude variations of defect waves as the probe moves across the material's surface. During probe movement, the most significant change in defect wave characteristics occurs at the peak value. Typically, the ratio of pulse signal heights displayed on the ultrasonic flaw detector's screen (i.e., the peak ratio of defect waves detected by ultrasonic testing) equals the ratio of ultrasonic sound pressure.
Ultrasonic flaw detector is a kind of instrument which uses ultrasonic to detect the internal defect of material. This paper mainly uses the ultrasonic flaw detector of Ondar 3600 model to study the simulated notch of steel and aluminum. The ultrasonic probe selected in the experiment is a single straight probe, its nominal frequency is set to 2.5, nominal K value is set to 0.0, and the current sound path is set to 125.0.
Calibration procedures: For steel specimens, the calibration process utilizes standard steel components in the laboratory. The ultrasonic detection waveform is adjusted by modifying the instrument's sound velocity setting, with the experimental configuration employing a 3000 m/s sound velocity and a gain of 9.0. For aluminum specimens, the calibration employs complete aluminum specimens, with the instrument sound velocity set at 2960 m/s and a gain of 40.0. As the primary focus of this thesis is the analysis of defect waveform characteristics, these calibration experiments enhance the accuracy of the detection results.
3. Preparation of Experimental Samples
This study primarily utilizes steel and aluminum samples. To simulate shrinkage cavities and porosity defects, the research focuses on their characteristic geometrical features: continuous surfaces with spherical interfaces, which are classified as curved reflection-type defects. Accordingly, mechanical processing was conducted on both materials to create regular curved reflection-type notches (i.e., hole-shaped notches) for experimental purposes. The processing conditions for these simulated defects in steel and aluminum are detailed in Table 1.
4. Experimental Results and Analysis
In this paper, the ultrasonic flaw detector of Ondar 3600 model is used to measure the simulated hole notch of steel and aluminum materials.
4.1 Crack Waveform Analysis
4.1.1 Analysis of the notch waveforms of steel samples
The defect waveforms obtained by ultrasonic testing of the simulated orifice notch at different positions on the upper surface of the steel sample are shown in Figure 4.
Analysis of the experimental steel sample waveforms reveals that the aforementioned patterns represent defect waves generated by ultrasonic testing of a semi-circular reflector. Given the thin steel sample, the baseline wave in (a) exhibits significant amplitude and broad waveform before the defect wave appears, with rapid attenuation. When the defect wave emerges, the baseline wave amplitude diminishes. After the probe moves a certain distance, the detected defect wave becomes sharply defined, while the baseline wave gradually fades away.
4.1.2 Waveform Analysis of Aluminum Sample Notch
The amplitude of the bottom echo is lower in the direction of the notch extension under the same gain, and the amplitude of the defect wave increases gradually with the increase of the detection surface.
Comparing the notch waveforms of two samples made of different materials reveals a clear pattern: Under identical gain conditions, both the defect wave and bottom wave amplitudes in steel are greater than those in aluminum, with the steel waveform exhibiting sharper edges. This occurs because the larger notch size in steel increases the contact area between the ultrasonic wave and the interface. When the ultrasonic beam emitted by the detector undergoes scattering and attenuation near the medium's reflective surface due to abrupt acoustic impedance changes (i.e., wave reflection), the more reflected waves return to the probe, resulting in stronger electrical pulse signals. Consequently, the displayed defect wave peaks appear larger. Figure 5 shows the defect waveform obtained from ultrasonic testing of a hole notch on the opposite side of the aluminum sample's surface.
Field tests have identified several typical surface reflection defect waveforms: Pores generally exhibit nearly spherical geometries. Ultrasonic flaw detection reveals monotonous and sharply defined reflection waveforms, with significant variations during probe movement. Most cases show clearly demarcated wave boundaries. Porosities, however, display irregular shapes. Their ultrasonic waveforms closely resemble pores but feature clustered reflections, often with overlapping waveforms and alternating amplitude peaks. Small amplitude peaks frequently appear near the main defect wave peaks. Analysis of these detected waveforms demonstrates that simulated defect waveforms closely match actual porosity and pore characteristics. Therefore, the simulated notch patterns of steel and aluminum materials can be utilized to analyze such flaw detection waveforms.
4.2 Dynamic Waveform of Ultrasonic Testing
4.2.1 Dynamic Waveform of Steel Sample Notch
The curved notch on the steel surface was inspected using the probe. During the probe's left-to-right movement, the relative peak value of the defect wave was recorded every 2.0mm displacement. Using the experimental data, a dynamic waveform diagram of the steel sample's notch was plotted with probe displacement on the x-axis and the relative peak value of the defect wave on the y-axis, as shown in Figure 6. Analysis of the ultrasonic dynamic waveform reveals that the defect wave peak value exhibits significant changes corresponding to the probe's position, with a consistent trend. Initially, as the probe approaches the notch, the peak value gradually increases but remains relatively stable. When the probe reaches near the notch center, the peak value undergoes a sharp increase, reaching its maximum at the interface measurement point. Subsequently, as the probe continues to move, the peak value progressively decreases until the defect wave completely disappears.
4.2.2 Dynamic waveform diagram of aluminum sample hole notch:
The notch on the upper surface of the aluminum sample was inspected. As the probe moved from left to right, the relative peak value of the defect wave was recorded every 1.0mm. Similarly, a dynamic waveform diagram of the curved notch in the aluminum sample was plotted, as shown in Figure 7. Analysis of the ultrasonic dynamic waveform of the aluminum sample revealed that the trend of the notch's dynamic waveform curve closely resembled that of the steel sample. However, a comparison of the dynamic waveforms of the two materials clearly showed that, due to the larger notch size in the steel sample, the maximum peak value of the defect wave measured in the steel sample was approximately twice as large as that in the aluminum sample. Additionally, during the detection process, the probe moved twice as far to inspect the notch in the steel sample compared to the aluminum sample.
The dynamic waveform analysis of ultrasonic testing of steel and aluminum shows that the more the contact surface in the direction of the acoustic beam, the more the ultrasonic energy will be reflected back to the probe and received by the instrument.
5. Conclusion
(1) The ultrasonic defect waveforms detected by the pore type notch are very close to the actual flaw waveforms of the material, such as porosity and shrinkage defects. The defect waveforms are relatively simple, and the waveforms will change significantly when the probe moves.
(2) The variation law of dynamic waveform curve of ultrasonic is related to the geometry and size of the interface. Therefore, the ultrasonic dynamic waveform detection method can be used to qualitatively analyze the defects in the material in practice, especially for the defects such as shrinkage cavity and porosity.