Non-destructive testing of structural composites and adhesively bonded composite joints: pulsed thermography

Structural adhesive bonding is an enabling technology for the implementation of composite assemblies in automotive applications. Therefore, the quality and reliability of the composite bond must be assured. An advanced thermal non-destructive test (NDT) method, pulsed thermography, was evaluated for its capability to assess joint quality in an adhesively bonded composite pickup truck box. Pulsed thermography, used under in-plant, pre-production conditions as would exist during process start-up and optimization trials, was shown effective in determining both the quality of the structural adhesive bonds and the quality of the composite itself. With one exception, NDT showed that bonding was performed correctly, i.e. the bond was continuous and properly placed. The exception was a ‘starved’ bond-line that we believe exists due to poor fixturing at that location. Pulsed thermography illustrated the effects of environmental and mechanical exposure on the bonded joints. Finally, the NDT method was able to show mechanical damage to the composite itself, identifying impact damage not visible to the unaided eye.     

Design of adhesively bonded joints between fibre-reinforced composites and metal

An analogy is made between engineering joints and tendons to bone attachment. The latter have similarities with composites to metal joints. It was observed that the morphology of the attachment in the form of a ridge allows for increased strength due to friction between a tendon and a ridge. This paper proposes the same geometry to be applied to the engineering joint mentioned. Static tensile tests on carbon fiber-epoxi-aluminium specimens indicate that this approach may improve the strength of the joint.     

Laser-based non-destructive testing techniques for the ultrasonic characterization of subsurface flaws

Laser generation and detection of ultrasound has been used for remote characterization of flat-bottomed holes, acting as subsurface flaws, drilled into an aluminium test sample. A pulsed Nd:YAG laser system was used to generate the ultrasonic source whilst a HeNe laser interferometer detected the subsequent surface displacements. The technique allows both the width and depth of flawa to be assessed. Moreover, an analysis of the data shows that the method is particularly suited to detection of flaws ≤ 4 mm below the surface of the sample, a region which is difficult to inspect by conventional contact ultrasonic techniques.     

Nonlinear analysis of adhesively bonded tubular single-lap joints for composites in torsion

The nonlinear analysis of tubular single-lap adhesive joints with composite adherends was performed by incorporating the nonlinear behavior of the adhesive into the analysis. For this purpose, the stress and strain in the laminated composite tube were first calculated under a general loading scheme, and then the iterative solution of the joints was derived by including the nonlinear properties of the adhesive. The stress distributions in the adhesive were investigated for different types of composite adherends and compared with the results of the linear analysis. The effect of the bonding length on torque transmission capability was also taken into consideration. The results indicate that the nonlinear analysis relieves the stress concentration at the edge of the joint, resulting in a more accurate prediction of joint strength.     

Applications of non-destructive testing techniques for post-process control of additively manufactured parts

This paper presents an overview on the principle of operation for post-process inspection non-destructive testing (NDT) techniques. The techniques include visual inspection, liquid penetrant testing, magnetic particle testing, eddy current testing, ultrasonic testing, and radiography. The applications of these NDT techniques in additive manufacturing (AM) and their suitability for defects detection of additively manufactured parts are reviewed. The sensitivity, and the advantages and disadvantages of each technique are evaluated. The types of defect, and the detectability of these defects by NDT techniques are assessed. The applicability of each NDT technique for different categories of AM process is discussed. The categories of AM are, namely, material extrusion, powder bed fusion, vat photopolymerisation, material jetting, binder jetting, sheet lamination, and directed energy deposition.     

A review of non-destructive techniques used for mechanical damage assessment in polymer composites

Polymer composite materials are being increasingly used in primary load-bearing structures in several advanced industrial fields such as aerospace vessels, railway wagons and mega-scaled wind turbines where detection of subcritical damage initiation can significantly reduce safety issues and maintenance costs. It is therefore crucial to inspect these composite structures in order to assess their structural health and to ensure their integrity. Non-destructive testing techniques (NDT) are used for this purpose, making it possible to monitor mechanical damage of composite materials under in situ or ex situ service conditions. This paper reviews the capabilities of the most common NDT techniques used to inspect the integrity of composite materials. Each technique has a detection potential and cannot allow a full diagnosis of the mechanical damage state of the material. Thus, depending on the occurring damage mechanism and the conditions of use, one technique will be preferred over another, or several techniques should be combined to improve the diagnosis of the damage state of the structures.     

Physical and numerical models for the shear load-carrying capacity testing of steel-concrete bonded joints

Owing to the very good adhesion between epoxy resins and steel or concrete it is quite possible to achieve coupling of these materials by means of bonding alone, without using any additional coupling means such as anchors or dowels. The shear-bearing capacity and properties of such joints, from the point of view of their practical application, are the subject of the investigations reported. For the tests done in this work, physical models were designed by which it was possible to determine the stress distribution and approximate values of the shear-bearing capacity as a function of properties of the coupled materials. The concept of the physical models used was tested by numerical models.     

An innovative testing technique for assessing the VHCF response of adhesively bonded joints

In the last few years, the use of adhesive joints for structural applications has rapidly increased and adhesives are more often subject to fatigue loads during their in‐service life. In presence of a rapidly varying load, such as a high‐frequency vibration, adhesively bonded joints may undergo fatigue lives in the Very High Cycle Fatigue (VHCF) region that are significantly larger than those investigated in usual high‐cycle fatigue tests. The present paper proposes an innovative testing technique for performing accelerated fully reversed tension‐compression VHCF tests on adhesive butt‐joints. The procedure for the design of the adherends is described and then experimentally validated. Ultrasonic VHCF tests are finally carried out on a cyanoacrylate butt‐joint up to 10⁹ cycles: experimental results show that the proposed testing equipment permits an effective assessment of the VHCF response of the adhesive in a limited testing time.     

The role of interlayers in diffusion bonded joints in metal-matrix composites

Solid state bonded joints in a particulate metal-matrix composite containing 17 vol% SiC in Al-Li 8090 alloy had planar bond interfaces with particle-particle (P-P) interfaces and particle-matrix (P-M) interfaces aligned in and parallel to the bond interface. The insertion of a matrix interlayer into the MMC-MMC bond interface changed the type of particle interface and interface area fractions in the two new bond planes created. In these planes P-P interfaces became P-M interfaces and the P-M area fraction either increased or fell to zero depending on the particle symmetry with respect to the bond plane. The implications for the mechanical properties of diffusion bonded joints in MMCs are discussed.     

The applicability of plate wave techniques for the inspection of adhesive and diffusion bonded joints

The role of plate waves in the inspection of adhesive and diffusion bonded joints is examined. This involves a review of the modal techniques which have been proposed for the measurement of the adhesion and cohesion properties of adhesive joints and the presentation of some of our own studies on the detection and characterization of an unwanted layer of brittle alpha case in diffusion bonded titanium. It is concluded that Lamb waves, which occupy the whole joint, are viable in principle but are limited in both applications by their strong sensitivity to the material properties and the thicknesses of the adherends and their relative insensitivity to those of the bondline layer. On the other hand, embedded modes, which propagate along an embedded layer, are largely insensitive to the adherends, the dispersion curves showing a major improvement in sensitivity to the properties of the layer and to the boundary conditions between the layer and the adherends. The drawback is that their exploitation is limited in practice because it is difficult to excite and detect them. True modes offer good potential but require access to the ends of the joints. In attempting to excite leaky modes, minima of the reflection coefficient, commonly used to measure Lamb wave dispersion curves in immersion coupled plates, do not correspond to the dispersion curves because the acoustic impedance of the adherends is too large. Therefore, although measurement of the minima offers good potential for inspection, this is a response technique and cannot be associated directly with the plate modes. In neither of the examples studied could an interface wave exist at a single interface between the bondline layer and an adherend. However, in general such modes could be rather attractive for inspection, provided that their wavelengths are much smaller than the layer thickness, because they are sensitive to the interface region but not to the thicknesses of the layers, and they are relatively simple to measure.     

Designing for damage tolerant bonded joints

This paper discusses the damage tolerant design methodology for adhesively bonded joints and bonded repairs. Attention is focused on the effects of rate dependence, creep, and strain/load holds on the load carrying performance of structural adhesives.     

Delamination damage analyses of adhesively bonded lap shear joints in laminated FRP composites

Three-dimensional non-linear finite ele- ment analyses have been carried out to evaluate the out-of-plane stresses in the adhesive layer existing between the lap and the strap adherends of the Lap Shear Joint (LSJ) in laminated FRP composites for varied delamination lengths. The delaminations are presumed to be pre-embedded in the thin resin rich layer existing between the first and second plies of the strap adherend. Sublaminate technique has been used to model the LSJ with the delamination. Contact finite element analyses have been performed in order to avoid interpenetration of delaminated surfaces. The effects of varied delamination lengths on the peel and interlaminar shear stresses and the individual modes of Energy Release Rate (ERR) in the delamination zones are highlighted in this paper. It is seen that three-dimensional effects exist near the free edges of the overlap end of the joint. The delamination propagation significantly affects the stress distributions in the adhesive layer existing between the lap and the strap adherends of the LSJ. The variations of interlaminar stresses and ERRs on both the delamination fronts are found to be significantly different and thus, indicate that the propagation of delamination does not occur at same rate at the two delamination fronts. This may throw some light to the evaluation of structural integrity of the LSJ in the presence of pre-embedded delaminations.     

无损检测在木材应用中的现状及发展趋势

无损检测技术在木材检测中的地位日益重要,本文主要从应力波检测、超声波检测、X射线检测和机械应力检测四个方面介绍了无损检测在木材中的应用,阐述了其基本原理和特点,概述了发展历史和研究现状,并展望了未来研究方向及工作重点。     

A simple methodology for the design of metallic lap joints bonded with epoxy/ceramic composites

In the present study it is proposed a shape factor that allows correlating the static strength of two single lap joints with different geometries. The formula is valid for epoxy/ceramic composites used as adhesive in special applications in oil and gas industry and highly resistant metallic adherends. Therefore, only a few tests performed in particular joint taken as reference are necessary for assessing the structural integrity of any similar joint with arbitrary geometry. Results from tensile tests performed in joints with different bonding areas are compared with model prediction showing a good agreement.     

Additive manufacturing process monitoring and control by non-destructive testing techniques: challenges and in-process monitoring

This paper examines on the obstacles specific to the implementation of non-destructive testing (NDT) techniques in additive manufacturing (AM). The general challenges impeding the adoption of AM for volume production of parts, and the use of NDT techniques to ameliorate some of these challenges are studied. These challenges include the lack of understanding of AM materials, and insufficient standards for the mechanical testing and NDT of additively manufactured parts. An overview on the principle of operation for in-process inspection NDT methods is presented. The techniques include thermography, and acoustic emission testing. The applications of these NDT techniques in AM and their suitability for defects detection of additively manufactured parts are reviewed. The sensitivity, and the advantages and disadvantages of each technique are evaluated. The types of defects, and the detectability of these defects by NDT techniques are assessed.     

Influence of scrap rubber addition to Portland I concrete composites: Destructive and non-destructive testing

The aim of the present work was the recycling of rubber from automobile tyre treads, as a partial substitute of fine aggregates in concrete. Composites obtained were characterized by destructive and non-destructive testing, in order to find a liable application.

According to results obtained, it was found that when weight proportion increased and particle size of scrap rubber decreased (0.59 and 0.29 mm), flow and density of composite in the fresh state decreased, as well as compressive strength and splitting tensile strength in the dry state. Previous treatment of rubber with NaOH and silane (A-174) did not produce significant changes on compressive strength and splitting tensile strength of composites, when compared to the untreated rubber–concrete composite. On the other hand, from the behavior showed by the ultrasonic pulse velocity with time, one can infer that the addition of rubber decreased this variable, being the effect more notorious when rubber content increased. This was due to the greater volume that rubber occupies, as well as water absorption. It can be concluded that the ultrasonic pulse velocity was relatively independent of particle size and coupling agent employed.     

Elastic analysis of hybrid bonded joints and bonded composite repairs

The authors extend the closed-form bonded joint linear elastic analysis method of Delale et al. [Delale F, Erdogan F, Aydinoglu MN. Stresses in adhesively bonded joints: a closed-form solution. J Compos Mater 1981;15:249–71] and Bigwood and Crocrombie [Bigwood DA, Crocombe AD. Elastic analysis and engineering design formulae for bonded joints. Int J Adhes Adhes 1989;9(4):229–42] to include the composite deformation mechanisms and the thermal residual strains that arise in hybrid metal-composite joints such as those presented by bonded composite repairs applied to metallic aircraft structures. The analytical predictions for the adhesive stresses and the compliance are compared to the results of a linear elastic finite element model that has itself been validated by comparison with experimental results. The results are applied to the problem of coupled linear extension and bending of a bonded composite repair applied to a cracked aluminum substrate. The resulting stress intensity factor and crack-opening displacement in the repaired plate are compared to the results of a three-dimensional finite element analysis, and also exhibit excellent results. Throughout the text, observations are made regarding the practical application of the results to failure prediction in hybrid joints, whereby the authors demonstrate the need for consistency in the analytical methods used to determine the fatigue and failure of composites from the coupon level to the analysis of the final structural details.     

Intelligent sensing for non-destructive testing using eddy currents

When a metallic surface is tested by the eddy-current technique, defects produce events on the electrical signal coming from the sensor whose particular signature is a function of different factors such as the shape, size and position of any defects. Thanks to the introduction of modern digital signal processing methods, the quality of detection of events in the signal may be appreciably improved. The reliability of the decision mechanism, entrusted with reducing non-detection and limiting false alarms, is then increased and allows consideration of automatic testing. The work presented here was done jointly by the Institut de Recherches de la Sidérurgie Française (IRSID) of the Usinor-Sacilor group and the aboratoire d' Automatique et de Recherche Appliquée (LARA) of the Centre de Recherche en Automatique de Nancy (CRAN).