Non-contact ultrasonic inspection by Gas-Coupled Laser Acoustic Detection (GCLAD).

Gas-Coupled Laser Acoustic Detection (GCLAD) is an ultrasonic, non-contact detection technique that has been recently proven to be applicable to the inspection of mechanical components. GCLAD response raises as the intersection length between the probe laser beam and the acoustic wavefront propagating in the air increases; such feature differentiates the GCLAD device from other optical detection instruments, making it a line detection system rather than a point detector. During the inspection of structures mainly extending in two dimensions, the capability to evidence presence of defects in whichever point over a line would enable moving the emitter and the detector along a single direction: this translates in the possibility to decrease the overall required time for interrogation of components compared to point detectors, as well as generating simpler automated monitoring layouts. Based on this assumption, the present study highlights the possibility of employing the GCLAD device as a line inspection tool. To this end, preliminary concepts are provided allowing maximization of the GCLAD response for the non-destructive testing of components which predominantly extend in two dimensions. Afterwards, the GCLAD device is employed in pulse-echo mode for the detection of artificial defects machined on a 12 mm-thick steel plate: the GCLAD probe laser beam is inclined to be perpendicular to the propagation direction of the airborne ultrasound, generated by surface acoustic waves (SAWs) in the solid which are first reflected by the defect flanks and subsequently refracted in the air. Numerical results are provided highlighting the SAW reflection patterns, originated by 3 mm deep surface and subsurface defects, that the GCLAD should interpret. The subsequent experimental campaign highlights that the GCLAD device can identify echoes associated with surface and subsurface defects, located in eight different positions on the plate. B-scan of the component ultimately demonstrates the GCLAD performance in accomplishing the inspection task.

Gas-Coupled Laser Acoustic Detection technique for NDT of mechanical components.

The use of non-contact ultrasonic detection techniques is essential to develop an apparatus capable of inspecting components in service, operating in adverse environmental conditions or whose surfaces are challenging to access. Among these techniques, Gas-Coupled Laser Acoustic Detection (GCLAD) is based on measuring the deviation that a laser beam sustains when intersecting an ultrasonic wavefront. The method has been recently developed, being hence unestablished in the non-destructive testing (NDT) field. The objective of the present work is to expand knowledge on the physical operating principles of the GCLAD system; this is obtained by investigating, both theoretically and experimentally, the influence of specific working parameters on the detected signals with the aim of maximizing the ultrasonic amplitude. These analyses are essential for proposing effective non-destructive investigation methodologies benefitting from the employment of the GCLAD technique. Based on the information obtained and from an NDT standpoint, test configurations are proposed with a different relative arrangement between the GCLAD system and the structure; these configurations provide peculiar results also based on the type of wave under investigation (surface or bulk waves). Finally, to highlight the capabilities of the GCLAD system in inspections of mechanical pieces, signals detected experimentally on a component (i.e., a railway axle) are presented in the presence and absence of a surface defect.

Injury risk assessment based on pre-crash variables: The role of closing velocity and impact eccentricity.

Thorough evaluations on injury risk (IR) are fundamental for guiding interventions toward the enhancement of both the road infrastructure and the active/passive safety of vehicles. Well-established estimates are currently based on IR functions modeled on post-crash variables, such as velocity change sustained by the vehicle (ΔV); thence, these analyses do not directly suggest how pre-crash conditions can be modified to allow for IR reduction. Nevertheless, ΔV can be disaggregated into two contributions which enable its apriori calculation, based only on the information available at the impact instant: the Crash Momentum Index (CMI), representing impact eccentricity at collision, and the closing velocity at collision (Vr). By extensively employing the CMI indicator, this work assesses the overall influence of impact eccentricity and closing velocity on the risk for occupants to sustain a serious injury. As CMI synthesizes indications regarding ΔV, its use can be disjointed from the ΔV itself for the derivation of high-quality IR models. This feature distinguishes CMI from the other eccentricity indicators available at the state-of-the-art, allowing for the contribution of eccentricity on IR to be completely isolated. Because of this element of originality, special attention is given to the CMI variable throughout the present work. Based on data extracted from the NASS/CDS database, the influence of the CMI and Vr variables on IR is specifically highlighted and analyzed from several perspectives. The feature ranking algorithm ReliefF, whose use is unprecedented in the accident analysis field, is first employed to assess importance of such impact-related variables in determining the injury outcome: if compared to vehicle-related and occupant-related variables (as category and age, respectively), the higher influence of CMI and Vr is initially highlighted. Secondly, the relevance of CMI and Vr is confirmed by fitting different predictive models: the fitted models which include the CMI predictor perform better than models which neglect the CMI, in terms of classical evaluation metrics. As a whole, considering the high predictive power of the proposed CMI-based models, this work provides valuable tools for the apriori assessment of IR.

Interference-based amplification for CW laser-induced photoacoustic signals.

Thanks to their low cost, compactness and suitability of use in industrial environments, CW lasers represent a viable alternative to more traditional pulsed lasers for non-contact inspection of structures, both under static and working conditions. However, the lower energy density typically reached by CW lasers requires adequate attention to obtain ultrasonic signals with sufficient amplitude. CW lasers can be modulated by TTL sequences with different duty cycles to constructively use the excitation of a double ultrasonic wave, occurring when the laser is both turned on and off. An appropriate choice in terms of number and spacing of the TTL pulses allows for the optimization of the ultrasonic response from the excitation standpoint: an increase in the amplitude of the ultrasound is enabled, and its frequency band modified. Such a solution allows to better adapt the ultrasonic wave to the receiver band, maximizing the global efficiency. In this work, the influence of the employed modulation sequence on the ultrasonic signal is analysed; by the TTL optimization proposed, application of the technique to the exemplary case of a rail represents a first implementation of CW lasers to the inspection of an industrial component.

Vehicle stiffness assessment for energy loss evaluation in vehicle impacts.

The energy loss in a vehicle-to-vehicle impact is an important compatibility indicator between the real event and the reconstruction made by the expert. For its estimate, stiffness characteristics for the vehicle are typically referred to, evaluated by the residual deformation obtained in a crash test. In frontal vehicle into rigid barrier crash tests, residual deformation on the vehicle front is generally measured as the bumper displacement in respect to its undeformed shape; nevertheless, in real crashes, the bumper is subject to restitution which differs from the rest of the vehicle structure, or is often detached. For these reasons, the present work proposes a methodology for vehicle stiffness calculation based on the dynamic crush experienced by the vehicle in a crash test; this datum is independent from bumper restitution. To apply the methodology, a video analysis approach is proposed for the derivation of dynamic crush: dynamic crush can be obtained without the processing of accelerometer signals, which are not always publicly available from consumer programs. The proposed video analysis approach represents a valid alternative to already existing procedures, because of its superior accuracy. By application of this approach to dynamic crush calculation in some crash tests, the drawbacks deriving from the possible use of residual deformation of the bumper are shown; consequently, it is highlighted how residual deformation measures should be cautiously employed in the accident reconstruction process.

Vehicle accident reconstruction by a reduced order impact model.

Road accident reconstruction by simulation represents an important step to determine what happened, as well as responsibilities of subjects involved in the event. To allow the reconstruction, a large variety of well-established simulative approaches are available on the market, e.g., impulse-momentum models, finite element method and multi-body systems: the choice on the appropriate methodology mainly depends on the reconstruction expert's needs in terms of calculation time and accuracy in the description of the event's overall dynamics. Most of the available techniques do not simultaneously provide detailed information about kinematics and deformation due to the impact, or considerable calculation resources are required to accomplish the task. The present work thus introduces a special-purpose, reduced order model devoted to accident reconstruction: discretization in a 2D domain of the sole vehicle contour allows to limit the calculation time. The ruling equations on which the 2D road accident reconstruction is based are given in detail, to demonstrate the approach. Referring to five vehicle-to-vehicle impact cases, the compatibility between the actual event dynamics and the results of simulations allowed to emphasize the method potential in the application field.

Coherence assessment of accident database kinematic data.

The analysis and research of accidents aimed at improving the safety of vehicles and infrastructures are typically based on the retrospective investigation of data that are collected in in-depth accident databases. In particular, kinematic data related to accidents (impact velocity, velocity change of the vehicles, etc.) make possible the identification of correlations between impact severity and injury risk (IR), as well as assessing the effectiveness of vehicle protection systems. The necessary condition to conduct suitable and significant analyses is to utilise data which are correct and representative of national statistics, i.e., congruent with physical laws governing the accident phenomena. Whereas representativeness can generally be retrospectively verified, the checks on kinematic data coherence during codification are rarely performed. The present work describes a procedure to verify the internal coherence of kinematic data collected in in-depth accident databases. The introduced checks allow the identification of parameters, which are not internally coherent because the accident reconstruction model employed is inappropriate or improperly used. These checks pertain to physical laws on which road accident reconstruction is based, i.e., momentum conservation, compatibility of velocity triangles, and energy conservation. Moreover, they can be modified and expanded to consider other parameters, making the methodology virtually applicable to any database. In the case of vehicle-to-vehicle collisions, the application of the procedure to detect incongruent data inside two real databases demonstrates how their number is often not negligible. Furthermore, consequences can be substantial for both direct and secondary analysis, i.e., determining IR curves (for example, logistic regression on input data) and identifying IR associated to an accident. Accordingly, the application of checks is particularly recommended during both analysis and collection phases to confirm the congruence of collected data; consequently, the quality of investigation is enhanced.

Analysis of mechanics of side impact test defined in UN/ECE Regulation 129.

OBJECTIVE: This article discusses differences between a side impact procedure described in United Nations/Economic Commission for Europe (UN/ECE) Regulation 129 and scenarios observed in real-world cases. METHODS: Numerical simulations of side impact tests utilizing different boundary conditions are used to compare the severity of the Regulation 129 test and the other tests with different kinematics of child restraint systems (CRSs). In the simulations, the authors use a validated finite element (FE) model of real-world CRSs together with a fully deformable numerical model of the Q3 anthropomorphic test device (ATD) by Humanetics Innovative Solution, Inc. RESULTS: The comparison of 5 selected cases is based on the head injury criterion (HIC) index. Numerical investigations reveal that the presence of oblique velocity components or the way in which the CRS is mounted to the test bench seat fixture is among the significant factors influencing ATD kinematics. The results of analyses show that the side impact test procedure is very sensitive to these parameters. A side impact setup defined in Regulation 129 may minimize the effects of the impact. CONCLUSIONS: It is demonstrated that an artificial anchorage in the Regulation 129 test does not account for a rotation of the CRS, which should appear in the case of a realistic anchorage. Therefore, the adopted procedure generates the smallest HIC value, which is at the level of the far-side impact scenario where there are no obstacles. It is also shown that the presence of nonlateral acceleration components challenges the quality of a CRS and its headrest much more than a pure lateral setup.

Three-dimensional finite element analysis of different implant configurations for a mandibular fixed prosthesis.

PURPOSE: The distribution of stresses in bone, implants, and prosthesis were analyzed via three-dimensional finite element modeling in different implant configurations for a fixed implant-supported prosthesis in an edentulous mandible. MATERIALS AND METHODS: A finite element model was created with data obtained from computed tomographic scans of a human mandible. Anisotropic characteristics for cortical and cancellous bone were incorporated into the model. Six different configurations of intraforaminal implants were tested, with the number of implants varying from three to five and the distal implants inserted either parallel to the other implants or tilted distally by 17 or 34 degrees. A prosthetic structure connecting the implants was designed, with 20-mm posterior cantilevers for the parallel implant configurations, and a load of 200 N was applied to the distal portion of the cantilevers. Stresses were measured at the level of the implant, the prosthetic structure, and the bone. Bone-level stresses were analyzed at the implant-bone interface, at the external cortical bone surface, distal to the terminal implant, and in the cancellous bone along the implant body. RESULTS: A three-parallel-implant configuration resulted in higher stress in the implant and bone than configurations with four or five parallel implants. Configurations with the distal implants tilted resulted in a more favorable stress distribution at all levels. CONCLUSION: In parallel-implant configurations for fixed implant-supported mandibular prostheses, four and five implants resulted in similar stress distribution in the bone, framework, and implants. A distribution of four implants with the distal implants tilted 34 degrees (ie, the "All-on-Four" configuration) resulted in a favorable reduction of stresses in the bone, framework, and implants.

Resistance to wear of four matrices with ball attachments for implant overdentures: a fatigue study.

PURPOSE: The study evaluated in vitro the retention force and the wear resistance over simulated function of four matrix components of ball attachments for implant-retained overdentures. MATERIALS AND METHODS: Four types of matrices for ball attachments were evaluated in a fatigue study simulating 5500 cycles of insertion and removal. The matrices used were (1) a Teflon matrix supported by a metal housing, (2) a titanium matrix, (3) a gold alloy matrix, (4) an O-ring matrix using the red color ring for medium retention. Dimensional changes of the ball attachments were investigated with a profilometer. RESULTS: The Teflon matrices showed an increase of 27% in retention at 5500 cycles while the gold alloy matrices showed an increase of 50% in retention in the first 500 cycles and remained relatively stable up to 5500 cycles. On the other hand, titanium matrices and O-ring matrices exhibited progressive loss of retention ending with 68% and 75% of retention loss, respectively, at 5500 cycles. Dimensional analysis by profilometer revealed significant wear on the ball attachment only for titanium matrixes. CONCLUSIONS: Gold alloy and Teflon matrices showed the highest retention values without retention loss after 3 years of simulated function. Titanium and O-ring matrices presented a continuous loss of retention with the highest wear on the ball attachments when combined with the titanium matrix.

Simplified method for evaluating energy loss in vehicle collisions.

A method is proposed for the evaluation of energy loss in road vehicle collisions. The energy loss evaluation is an essential task to reconstruct the dynamics of a road accident. The proposed method combines the simplicity of visual evaluation, typical of the method based on EES (equivalent energy speed), with flexibility, in order to evaluate the energy loss on any kind of vehicle deformation profile, of the methods based on measuring residual crush. The method is based on linearizing the damage profile, so that it is possible to predetermine the analytical expression of the kinetic energy loss in relation to only two parameters that characterise the shape of the damage. The stiffness of the vehicle is determined by estimating the geometric parameters of the damage starting from a photograph of generic damage, with documented EES, on a vehicle of the same model as the one under investigation. The proposed method was validated performing crash tests and using data from crash tests found in the literature. The method estimate with sufficient accuracy the kinetic energy loss in deformation on vehicles. The method, thanks to its simplicity and versatility, can constitute a valid alternative to the classic procedures for evaluating energy loss commonly utilised.