Gian Piero Malfense Fierro

Residual fatigue life estimation using a nonlinear ultrasound modulation method

Predicting the residual fatigue life of a material is not a simple task and requires the development and association of many variables that as standalone tasks can be difficult to determine. This work develops a modulated nonlinear elastic wave spectroscopy method for the evaluation of a metallic components residual fatigue life. An aluminium specimen (AA6082-T6) was tested at predetermined fatigue stages throughout its fatigue life using a dual-frequency ultrasound method. A modulated nonlinear parameter was derived, which described the relationship between the generation of modulated (sideband) responses of a dual frequency signal and the linear response. The sideband generation from the dual frequency (two signal output system) was shown to increase as the residual fatigue life decreased, and as a standalone measurement method it can be used to show an increase in a materials damage. A baseline-free method was developed by linking a theoretical model, obtained by combining the Paris law and the Nazarov–Sutin crack equation, to experimental nonlinear modulation measurements. The results showed good correlation between the derived theoretical model and the modulated nonlinear parameter, allowing for baseline-free material residual fatigue life estimation. Advantages and disadvantages of these methods are discussed, as well as presenting further methods that would lead to increased accuracy of residual fatigue life detection.

Nonlinear imaging (NIM) of flaws in a complex composite stiffened panel using a constructive nonlinear array (CNA) technique

&NA; Recently, there has been high interest in the capabilities of nonlinear ultrasound techniques for damage/defect detection as these techniques have been shown to be quite accurate in imaging some particular type of damage. This paper presents a Constructive Nonlinear Array (CNA) method, for the detection and imaging of material defects/damage in a complex composite stiffened panel. CNA requires the construction of an ultrasound array in a similar manner to standard phased arrays systems, which require multiple transmitting and receiving elements. The method constructively phase‐match multiple captured signals at a particular position given multiple transmit positions, similar to the total focusing method (TFM) method. Unlike most of the ultrasonic linear techniques, a longer excitation signal was used to achieve a steady‐state excitation at each capturing position, so that compressive and tensile stress at defect/crack locations increases the likelihood of the generation of nonlinear elastic waves. Moreover, the technique allows the reduction of instrumentation nonlinear wave generation by relying on signal attenuation to naturally filter these errors. Experimental tests were carried out on a stiffened panel with manufacturing defects. Standard industrial linear ultrasonic test were carried out for comparison. The proposed new method allows to image damages/defects in a reliable and reproducible manner and overcomes some of the main limitations of nonlinear ultrasound techniques. In particular, the effectiveness and robustness of CNA and the advantages over linear ultrasonic were clearly demonstrated allowing a better resolution and imaging of complex and realistic flaws.

Nonlinear thermosonics and laser vibrometry for barely visible impact damage of a composite stiffener panel

Two methods have been evaluated in order to locate barely visible impact damage (BVID) in a composite stiffener panel. A nonlinear thermosonics technique and a nonlinear laser vibrometer technique were evaluated. Damaged regions were excited using a piezo shaker in both methods. Evaluation of the damaged regions was done by first determining the second and third order nonlinear harmonic response of the damaged regions. This was then used to determine the excitation frequency. By evaluating the presence of nonlinear responses in the output signal it is possible to excite the damaged structure at frequencies that give high heat generation and high displacements at the damaged regions. The results showed that both methods can be used to locate damaged regions, although it was shown that the stiffener impedes the propagation of the exciting wave and that these tests should be carried out in-between stiffeners in order to maximise the excitation and heating of damaged regions. Furthermore, both methods allowed for excitation of damaged regions over a large area.

A combined linear and nonlinear ultrasound time-domain approach for impact damage detection in composite structures using a constructive nonlinear array technique

HighlightsAn excitation method is proposed to detect local defect resonance.Image segmentation method is proposed to highlight damaged regions.A combined linear and nonlinear imaging method is presented. ABSTRACT Discovery and evaluation concerns of barely visible impact damage in composite materials is a well‐known issue in industries using these materials. This work proposes a frequency sweep method where damage assessment is conducted with respect to the time domain. Firstly, a combined linear and nonlinear ultrasound imaging technique is proposed, which focuses on the excitation of damage/defect regions using a frequency sweep methodology from multiple transducer locations. Secondly, the method deconstructs time domain signals, which allows for the visualisation of linear and nonlinear ultrasound components independently. While, a filtering and frequency band separation method was used to exploit defect responses over different frequency ranges and provide time domain visualisation at the damage region. Finally, image segmentation was employed to automate the damage sizing procedure, while a binary imaging method was used to remove false positive damage regions produced by material vibration mode excitation (fundamental frequency responses) by using the nonlinear responses as a baseline‐free tool. The results showed that the combined linear and nonlinear results provided more accurate results than a purely linear or nonlinear approach, furthermore the results were shown to be equivalent to those of a standard phased array system. The ability of the method to visualise nonlinear outputs in time can improve the understanding of nonlinear ultrasound mechanisms while provides a clear argument that a complete approach, incorporating both linear and nonlinear methods should be regarded as the future of NDT/E systems.

Nonlinear elastic imaging of barely visible impact damage in composite structures using a constructive nonlinear array sweep technique

ABSTRACT Linear and nonlinear ultrasound imaging methods highlight different damage features: the linear method detects large stiffness changes, while the nonlinear technique identifies small impedance mismatches, such as microcracks or closed delaminations. Typically, nonlinear ultrasound techniques detect damage/defects in materials by measuring higher order harmonics. These harmonics can be difficult to measure due to low magnitude and signal to noise ratios (SNR): hence large excitation amplitudes are needed, which can further complicate the reliability of these methods as equipment nonlinearities can be generated. To overcome these issues, exciting at specific frequencies, known as local defect resonances (LDR), produce a much larger displacements at the damaged regions. However, estimation of LDR is time‐consuming, complex and not an easily automated process. A coupled baseline‐free linear and nonlinear ultrasonic imaging approach is proposed, using a Constructive Nonlinear Array Sweep excitation and an image subtraction method for identifying damage in layered materials. The signal sweep method uses a narrow band frequency excitation to increase the probability of detection of a LDR frequency. The novel imaging approach was employed using laser vibrometry measurements in various complex composite structures to assess barely visible impact damage, critical for the aircraft industry. The results showed better estimation of impact damage when compared to classical linear or nonlinear ultrasonic methods leading to improved reliability of aircraft inspections.

Structural health monitoring of the loosening in a multi-bolt structure using linear and modulated nonlinear ultrasound acoustic moments approach

Applying highly accurate clamp loads in bolted joints during assembly and inspections is essential for estimation of the integrity of a joint and reduction of disastrous failures. Non-destructive post-assembly and in-service inspections of joint integrity are vital and significantly reduce maintenance and associated repair costs. Therefore, a bolt control technology able to provide precise direct measurement of bolt loosening state during assembly and in-service is needed. This work proposes an in situ structural health monitoring approach based on the evaluation of linear and nonlinear modulated acoustic moments for the assessment of the loosened state of bolts in a multi-bolted structure. Linear and nonlinear ultrasound methods’ detection accuracy and robustness can be highly dependent on correct frequency selection. The structural health monitoring method suggested uses material resonance and a frequency sweep methodology coupled with a cross-correlation method which identifies significant frequency pairs or higher harmonics used to determine bolt loosening. The proposed approach was tested and successfully validated on three different bolted structures showing that loosening of the structure can be identified accurately with a limited number of transducers. The solution provides a qualitative solution, which identifies degradation in the torque of a bolted structure; furthermore, the developed structural health monitoring method has the potential to become an automatic tool for monitoring the loosened state of bolts in critical complex structural components.

Nonlinear imaging (NIM) of barely visible impact damage (BVID) in composite panels using a semi and full air-coupled linear and nonlinear ultrasound technique

Two non-contact methods were evaluated to address the reliability and reproducibility concerns affecting industry adoption of nonlinear ultrasound techniques for non-destructive testing and evaluation (NDT/E) purposes. A semi and a fully air-coupled linear and nonlinear ultrasound method was evaluated by testing for barely visible impact damage (BVID) in composite materials. Air coupled systems provide various advantages over contact driven systems; such as: ease of inspection, no contact and lubrication issues and a great potential for non-uniform geometry evaluation. The semi air-coupled setup used a suction attached piezoelectric transducer to excite the sample and an array of low-cost microphones to capture the signal over the inspection area, while the second method focused on a purely air-coupled setup, using an air-coupled transducer to excite the structure and capture the signal. One of the issues facing nonlinear and any air-coupled systems is transferring enough energy to stimulate wave propagation and in the case of nonlinear ultrasound; damage regions. Results for both methods provided nonlinear imaging (NIM) of damage regions using a sweep excitation methodology, with the semi aircoupled system providing clearer results.

Damage detection in composites using nonlinear ultrasonically modulated thermography

This paper proposes a novel nonlinear ultrasonically stimulated thermography technique for a quick and reliable assessment of material damage in carbon fibre reinforced plastic (CFRP) composite materials. The proposed nondestructive evaluation (NDE) method requires narrow sweep ultrasonic excitation using contact piezoelectric transducers in order to identify dual excitation frequencies associated with the damage resonance. High-amplitude signals and higher harmonic generation are necessary conditions for an accurate identification of these two input frequencies. Dual periodic excitation using high- and low-frequency input signals was then performed in order to generate frictional heating at the crack location that was measured by an infrared (IR) camera. To validate this concept, an impact damaged CFRP composite panel was tested and the experimental results were compared with traditional flash thermography. A laser vibrometer was used to investigate the response of the material with dual frequency excitation. The proposed nonlinear ultrasonically modulated thermography successfully detected barely visible impact damage in CFRP composites. Hence, it can be considered as an alternative to traditional flash thermography and thermosonics by allowing repeatable detection of damage in composites.

A constructive nonlinear array (CNA) method for barely visible impact detection in composite materials

Currently there are numerous phased array techniques such as Full Matrix Capture (FMC) and Total Focusing Method (TFM) that provide good damage assessment for composite materials. Although, linear methods struggle to evaluate and assess low levels of damage, while nonlinear methods have shown great promise in early damage detection. A sweep and subtraction evaluation method coupled with a constructive nonlinear array method (CNA) is proposed in order to assess damage specific nonlinearities, address issues with frequency selection when using nonlinear ultrasound imaging techniques and reduce equipment generated nonlinearities. These methods were evaluated using multiple excitation locations on an impacted composite panel with a complex damage (barely visible impact damage). According to various recent works, damage excitation can be accentuated by exciting at local defect resonance (LDR) frequencies; although these frequencies are not always easily determinable. The sweep methodology uses broadband excitation to determine both local defect and material resonances, by assessing local defect generated nonlinearities using a laser vibrometer it is possible to assess which frequencies excite the complex geometry of the crack. The dual effect of accurately determining local defect resonances, the use of an image subtraction method and the reduction of equipment based nonlinearities using CNA result in greater repeatability and clearer nonlinear imaging (NIM).