Study of damage of t-joint components by using different non-destructive techniques

The present research is focused on the use of different non-destructive techniques for detecting damage in CFRP composite structures obtained by an innovative technological process: Automated Fiber Placement. The component was a T-joint stringer adhesively bonded to a skin panel. The aim of the present work is to show the capability of these techniques to provide complementary information for detecting the damage in composites. Automated Fibre Placement consists in an automatic deposing of prepeg or dry plies on a specific mould. The innovation lies in the possibility to reduce the time of the manufacturing process of large and complex structures by using a robotic arm that contemporary deposes fibre tows and pre-polymerizes them. The resulting products present higher quality in terms of surface finish, internal flaws absent and higher mechanical properties. The T-joint component tested in the present research was addressed to both static and cyclic tests. After the damage was induced in the material it was performed a qualitative and quantitative study of the damage by using different nondestructive techniques: Thermoelastic stress analysis (TSA), Ultrasound tests (UT) and displacement/strain measurements provided by strain gages. Processing and post-processing procedures were developed to analyze the data from each tests and finally the comparison of the results allowed a complete characterization and an overview of damage in the component by observing specifically where and when it occurred and how many regions of the component were interested. Finally, dimension, shape and depth where assessed

Sizing the length of surface breaking cracks using vibrothermography

Ultrasound excited thermography is used to determine the length of vertical surface breaking cracks. Two methods are proposed based on the analysis of the maximum temperature reached along the line containing the crack and the time at which the maximum temperature occurs. It is shown that, for short bursts, the full width at half maximum of the maximum temperature curve provides the crack length, and that the time at which the maximum temperature occurs increases beyond the crack tip. The range of application of the methods is analysed and the validity is checked taking data on samples containing artificial calibrated cracks.This work has been supported by Ministerio de Economía y Competitividad (DPI2016-77719-R, AEI/FEDER, UE), by Gobierno Vasco (PIBA2018/15) and by Universidad del País Vasco UPV/EHU (GIU16/33)

Characterizing Subsurface Rectangular Tilted Heat Sources Using Inductive Thermography

In this study, we characterize the lateral dimension, depth, and inclination of buried tilted rectangular heat sources from time domain temperature data measured at the surface. The heat sources are representative for planar defects that emit heat in thermographic tests with internal burst excitation. We present a semi-analytical expression for the evolution of the surface temperature distribution. The emitted flux, dimensions and inclination of the heat source are determined by fitting the model to two perpendicular surface temperature profiles and the temperature history at one point of the surface. We show that the sensitivity of the data to the geometrical parameters of the heat source decreases as the angle it makes with the surface increases. The study also shows that the optimum duration of the excitation corresponds to a thermal diffusion length covering the distance from the surface to the deepest end of the heat source. The accuracy and precision of the results for different noise levels and inclinations have been tested by fitting the model to synthetic data with added noise. Fittings of experimental induction thermography data on 3D printed photo-polymeric resin samples containing calibrated Cu slabs confirm that it is possible to characterize tilted rectangular heat sources from surface temperature data.This research was funded by Ministerio de Ciencia e Innovación, grant number PID2019-104347RB-100, AEI/FEDER, UE, by Gobierno Vasco, grant number PIBA 2018-15, and by Universidad del País Vasco UPV/EHU, grant number GIU19/058

A multianalysis thermography-based approach for fatigue and damage investigations of ASTM A182 F6NM steel at two stress ratios

Infrared thermography allows an alternative energy-based approach for studying the fatigue behaviour of materials to better understand damage phenomena. In particular, the methodology of infrared thermography can explain the complex dissipative mechanisms promoted by the input parameters, such as the loading ratio, can rapidly provide information about the fatigue strength, and has low cost. In this work, analysis of the thermographic sequences of ASTM A 182 grade F6NM steel obtained during fatigue testing provided four thermal indexes that were used to investigate the thermoelastic and plastic behaviour of material. Fatigue tests at two opportunely chosen loading ratios (R = −0.1, R = 0.5) were performed to investigate the relation between the material behaviour and each index at a specific loading ratio. Finally, estimation of the fatigue strength by means of suitable analysis procedures allowed for an investigation of the damage behaviour of materials under specific loading conditions

A Comparison among Different Ways to Investigate Composite Materials with Lock-In Thermography: The Multi-Frequency Approach

The main goal of non-destructive testing is the detection of defects early enough to avoid catastrophic failure with particular interest for the inspection of aerospace structures; under this aspect, all methods for fast and reliable inspection deserve special attention. In this sense, active thermography for non-destructive testing enables contactless, fast, remote, and not expensive control of materials and structures. Furthermore, different works have confirmed the potentials of lock-in thermography as a flexible technique for its peculiarity to be performed by means of a low-cost set-up. In this work, a new approach called the multi-frequency via software approach (MFS), based on the superimposition via software of two square waves with two different main excitation frequencies, has been used to inspect a sample in carbon fiber reinforced polymers (CFRP) material with imposed defects of different materials, sizes and depths, by means of lock-in thermography. The advantages and disadvantages of the multi-frequency approach have been highlighted by comparing quantitatively the MFS with the traditional excitation methods (sine and square waves)

Evaluation of damage in composites by using thermoelastic stress analysis: A promising technique to assess the stiffness degradation

The stiffness degradation represents one of the most interesting damage phenomena used for describing the fatigue behaviour of composites. A critical aspect of modelling the damage is represented by the simulation of the whole behaviour of the composite and by the assessment of the actual stiffness for the models validation. In this work, the stiffness degradation of quasi-isotropic carbon fibre reinforced polymer (CFRP) obtained by automated fibre placement has been assessed by means of thermoelastic stress analysis. The amplitude of temperature signal at the mechanical frequency (thermoelastic signal) was considered as an indicator of material degradation and compared with the data provided by an extensometer. The correlation between thermoelastic and mechanical data allowed to build a new experimental model for evaluating and predicting material stiffness degradation by just using thermoelastic data. The proposed approach seems to be very promising for stiffness degradation assessment of real and complex mechanical components subjected to actual loading conditions

Assessment of the quality of adhesive bond in t-joints coupons by using thermoelastic stress analysis

Adhesively bonded joints represent an interesting alternative to mechanical joints due to the advantages over conventional mechanical fasteners: continuity of the structure, high strength-to-weight ratio, design flexibility. The aim of this work is to assess and predict the quality of aeronautical adhesive bonded CFRP T-joints made by the automated fibre placement process by means of the Thermoelastic Stress analysis (TSA) technique used as non-destructive technique. The results provided by TSA technique, in terms of debonded area, were compared to the well-established lock-in thermography technique showing the capability of TSA to evaluate the quality of T-joints. The approach allows to perform a cost-efficient characterisation process by means of non-destructive evaluations

Mechanical Behaviour of Stainless Steels under Dynamic Loading: An Investigation with Thermal Methods

Stainless steels are the most exploited materials due to their high mechanical strength and versatility in producing different alloys. Although there is great interest in these materials, mechanical characterisation, in particular fatigue characterisation, requires the application of several standardised procedures involving expensive and time-consuming experimental campaigns. As a matter of fact, the use of Standard Test Methods does not rely on a physical approach, since they are based on a statistical evaluation of the fatigue limit with a fixed probabilistic confidence. In this regard, Infra-Red thermography, the well-known, non-destructive technique, allows for the development of an approach based on evaluation of dissipative sources. In this work, an approach based on a simple analysis of a single thermographic sequence has been presented, which is capable of providing two indices of the damage processes occurring in material: the phase shift of thermoelastic signal φ and the amplitude of thermal signal at twice the loading frequency, S2. These thermal indices can provide synergetic information about the mechanical (fatigue and fracture) behaviour of austenitic AISI 316L and martensitic X4 Cr Ni Mo 16-5-1; since they are related to different thermal effects that produce damage phenomena. In particular, the use of φ and S2 allows for estimation of the fatigue limit of stainless steels at loading ratio R = 0.5 in agreement with the applied Standard methods. Within Fracture Mechanics tests, both indices demonstrate the capacity to localize the plastic zone and determine the position of the crack tip. Finally, it will be shown that the value of the thermoelastic phase signal can be correlated with the mechanical behaviour of the specific material (austenitic or martensitic)

Thermoelastic stress analysis as a method for the quantitative non-destructive evaluation of bonded CFRP T-joints

Adhesive bonding is a material joining process in which an adhesive, placed between the surfaces, solidifies to produce a strong bond. In this regard, adhesively bonded joints represent an interesting alternative to mechanical joints and provide many advantages over conventional mechanical fasteners: continuity of the structure, high strength-to-weight ratio, design flexibility, and easiness of fabrication. In the present research, the Thermoelastic Stress Analysis (TSA) technique has been used as a non-destructive tool for evaluating the mechanical behaviour of aeronautical adhesive bonded CFRP T-joints made by automated fibre placement process. Moreover, the thermoelastic signal was used for determining the debonded areas after the pull-off tests. The results in terms of the measured debonded area were compared to the well-established lock-in thermography technique. The capability of the Thermoelastic Stress Analysis to perform an in-depth study of the quality of T-joints has been demonstrated

Evaluation of Effectiveness of Heat Treatments in Boron Steel by Laser Thermography

The applicability of active thermography as a non-destructive method to distinguish heat treated from not-treated boron steel has been investigated. While the usual hardness semi-destructive tests influence the inspected surface, laser thermography is capable of verifying the effectiveness of heat treatment in boron steel in a non-destructive way without any surface modification. The procedure has been verified on two plates of boron steels with different structures (100% ferritic–pearlitic and 100% martensitic)