G. Giorleo

Non-destructive control of industrial materials by means of lock-in thermography

Lock-in thermography is employed for non-destructive control to evaluate several aspects of industrial interest: inclusions of spurious materials in both carbon-epoxy and glass-epoxy, impact damage and delaminations occurring around holes during drilling in carbon-epoxy, bonding improvements in Certran® after plasma treatments and steel modifications after welding. Phase images are analysed to find quantitative information for industrial characterization.

Comparison between pulsed and modulated thermography in glass–epoxy laminates

Abstract Infrared (IR) thermography is a two-dimensional, non-contact technique which can be usefully employed in non-destructive evaluation of materials. Basically, two different approaches are possible: traditional pulse thermography (PT) and modulated (or lock-in) thermography (MT). The attention of the present work is focused on the peculiar aspects, which characterise the two different techniques. Tests are carried out by considering glass–epoxy specimens and the results obtained, by employing either PT or MT, are compared. The capability of each technique, to detect a defect and give precise information about size, depth and thermal resistance of the defect, is analysed. The advantages and disadvantages of using these techniques are discussed in order to assess the fundamental requirements for the most appropriate choice in quality control processes.

Fatigue lifetime of glass fabric/epoxy composites

Monotonic and fatigue tests were carried out in four-point bending on a glass fabric/epoxy composite, using two different stress ratios. Ultimate failure both in monotonic tests and in fatigue was precipitated by microbuckling phenomena happening at the compression side of the specimens. The experimental results were evaluated adopting a fatigue model statistically implemented, based on the hypothesis of a two-parameter Weibull distribution of the monotonic strength, previously assessed for random glass fibre reinforced plastics failed in tension. The fatigue model was able to account for the effect of the stress ratio on the fatigue life, accurately predicting the classical S–N curve. By the model, the virgin strength for each specimen failed in fatigue was evaluated, and the distributions of the measured and calculated monotonic strength were compared. Some discrepancies between the two distributions, resulting in poor agreement in the tail portions of the curves, were noted. It is shown that the inconsistencies found are probably attributable to the inadequancy of a two-parameter Weibull curve to describe the actual material trend. Better results were obtained by using a three-parameter Weibull distribution.

Experimental Characterization of an Innovative Glare® Fiber Reinforced Metal Laminate in Pin Bearing

An experimental investigation is performed on an innovative Glare® Fiber Reinforced Metal Laminate (FRML), which is produced at the Alenia Aerospazio (Italy), with the aim to characterize its strength and behaviour in the case of mechanical joints. Several specimens are fabricated by varying width and hole-to-edge distance and tested in pin-bearing way without lateral restraints, which is the most critical testing procedure in the simulation of mechanical joints. Specimens, after bearing stress, are analysed in both non-destructive and destructive ways. Non-destructive evaluation is performed by means of lock-in thermography; for a validation of this technique, phase images are compared to photomicrographs. Results prove that a remote infrared imaging system may be a valuable tool to monitor the material behaviour either during the manufacturing processes, or in service.

Pin-bearing strength of glass mat reinforced plastics

An experimental investigation was carried out, in order to find simple analytical methods for the prediction of pin-bearing strength of glass mat reinforced plastics fabricated by hand lay-up. The specimens tested exhibited different failure modes, consisting of net-tension, cleavage, and bearing, depending on the geometry adopted. The parameters affecting the bearing strength were the ratio of the specimen width to the hole diameter D, and the ratio of the edge distance to D. The maximum bearing strength was attained when a pure bearing failure happened. The latter was the only safe failure mode: after its occurrence, the joint was still able to support about 70% of its virgin strength. The experimental data obtained were used to assess a simple procedure for the calculation of the joint load carrying capacity, previously proposed. Since a large scatter was found in the bearing strength of the material, the data were analysed statistically, assuming a probability of failure varying according to a two-parameter Weibull distribution. An excellent agreement was found between the experimental results and the theoretical model. Similar conclusions were drawn applying the previous approach to the residual strength after bearing failure. A limited amount of tests was performed to verify the influence of possible damage induced during hole drilling on the bearing behaviour. It was found that a 20% decrease in bearing strength occurs when delamination is induced by inaccurate drilling. However, the residual bearing strength was substantially unaffected by the drilling damage.

LBW of Similar and Dissimilar Skin-Stringer Joints Part I: Process Optimization and Mechanical Characterization

Laser beam welding of light alloys has always represented a big challenge for both designers and technologists due to the large number of process parameters to take into account and the variable responses of the different materials to be welded. In this paper the results of experimental research on laser beam welds of innovative heat treatable aluminum alloys is reported. The well known T geometry (a stringer welded to a skin) has been considered. Two different skins have been analyzed: AA 2139 and AA 6156, both in form of rolled sheets. Two different stringer have been analyzed: AA 2139 and PA 765, both in form of extruded parts. AA 4047 has been used as filler wire. In the first part of the paper, all the steps leading to the realization of sound welds will be described. The criteria used in order to assess the soundness of a weld was the absence of defects, such as cracks or large pores, verified by means of NDE. In the second part of study, both micro structural analysis and mechanical characterization of welds will be described and discussed. Conclusions will demonstrate the importance of the influence of chemical composition of the parts, above all stringer. The performance of the best welds, however, were very close to those of parent materials.

Infrared thermography in the quality assurance of manufacturing systems

The present work will focus its attention on the aid provided by infrared thermography (IR) in the evolution of manufacturing towards the achievement of highest quality at the lowest price; which is to say: improve manufacturing processes and avoid waste of time. IR can be fruitfully exploited to monitor temperature variations in-process in temperature-dependent manufacturing processes. It is possible to control efficiency of cooling systems in-processes involving material shaping like extrusion and injection moulding. It is also possible to control the temperature rises which can affect the integrity of components and life of tools in material removal processes involving cut, mill, drill. IR is also attractive for non-destructive inspection of end products to assure quality. Experimental tests are carried out to control temperature rises during mill finish, to visualize material inhomogeneities linked to extrusion or injection moulding processes, inclusions of spurious materials during welding or bonding processes and to determine variations of adhesive thickness in metallic joints and composites. Results, presented and discussed here, prove the possibility of using IR as a tool of quality assurance of manufacturing systems.

Using infrared thermography to analyze substrate and adhesive effects in bonded structures

This study was focused on the application of infrared thermography for assessment of technological procedures used to improve adhesion in bonded structures. Infrared thermography is a remote imaging technique which can be fruitfully exploited either for mapping surface temperature in temperature-dependent manufacturing processes, or for nondestructive evaluation of end products. Basically, for nondestructive evaluation two different approaches are possible: traditional pulse thermography (PT) and lock-in thermography (LT) and the use of both techniques is discussed. Several specimens were fabricated to simulate different bonded structures that are used mainly in the aerospace industry. Such bonded structures included: adhesively-bonded aluminum joints with or without surface treatment before bonding, glass-reinforced epoxy composites and carbon-reinforced epoxy composites with or without plasma treatment before bonding. Pulse and lock-in thermographies are both capable of visualizing inhomogeneities in bulk materials, as well as disbonding, delamination, flaws and foreign inclusions in bonded structures. The results show that lock-in thermography is also capable of evaluating the effect of adhesive thickness and the effects induced in bonded structures by substrate surface treatments.

Experimental Evaluation of Properties of Cross-Linked Polyethylene

This article describes several aspects of the production of cross-linked polyethylene. Advantages and disadvantages of chemical, or electron beam irradiation, cross-linking processes are discussed. The material under test is thermoshrinking low density polyethylene, produced at the Megarad s.r.l. (Cassino, Italy) firm. It is mostly used as blanket insulation for low and medium electrical voltage cables and as anticorrosion protection for gas pipes. The performance of end products is verified through destructive tests. In particular, we evaluated the gel fraction, the modulus of elasticity at 150°C and the elongation percent on commercial pipes and on specimens manufactured by varying certain parameters, such as the irradiation dose.

EXPERIMENTAL TECHNIQUES TO CUT AND WELD COPPER BY LASER - A REVIEW

This paper reports on the cutting and welding of copper sheets using a CO2 laser. For the cutting process, the experimental data from tests on 0.2 to 4.0 mm thick copper sheets with a 2 kW CO2 laser are described. The behavior of the critical cutting speed V, cutting widths a, the product V × b and the melted volume versus the thickness (b) for a 2 kW CO2 laser using different focusing lenses has been investigated, The cutting speeds have been found to be significant and micrographic examinations have shown that the laser cutting quality is good. The laser welding of copper sheets is possible by overlapping layers of cupric oxide, CuO, with a small quantity of cuprous oxide, Cu2O grown under laser beam irradiation. This experimental approach, similar to the one used for the cutting process, allows one to increase the copper surface absorption of the laser radiation. The weld tests done in this way have shown a bad quality of the butt joint; in fact, a number of inclusions in the melted zone and growth of ...