An innovative nondestructive technique for the local assessment of residual elastic properties in laminated composites

In this work, an innovative experimental methodology is presented for the assessment of damage severity in composites. The technique aims at determining the local variation of the elastic properties in the damaged region of a composite component. Based on the Impulse Excitation Technique (IET), the vibrational response of the inspected region is isolated by clamping its extremities through vacuum, thus allowing the assessment of local variations. Complementarily, a new analytical approach is derived for the assessment of the residual elastic properties of the damaged area from the measurement of the first resonant frequency. Validation of the proposed methodology is performed on two glass-fibre woven fabric composites, damaged by impact. The material properties of the damaged zone determined through the proposed technique are compared to the results of tensile tests performed on specimens cut from the impacted plates. In particular, the specimens are equipped with optic fibre in order to punctually measure the elastic parameters. Results show that the residual elastic properties assessed with the proposed technique are in very good agreement with those measured through the optic fibre, thus proving the effectiveness of the methodology

A new methodology for thermostructural topology optimization: Analytical definition and validation

In the last few years, the rapid diffusion of components produced through additive manufacturing processes has boosted the research on design methodologies based on topology optimization algorithms. Structural topology optimization is largely employed since it permits to minimize the component weight and maximize its stiffness and, accordingly, optimize its resistance under structural loads. On the other hand, thermal topology optimization has been less investigated, even if in many applications, such as turbine blades, engines, heat exchangers, thermal loads have a crucial impact. Currently, structural and thermal optimizations are mainly considered separately, despite the fact that they are both present and coupled in components in service condition. In the present paper, a novel methodology capable of defining the optimized structure under simultaneous thermomechanical constraints is proposed. The mathematical formulation behind the optimization algorithm is reported. The proposed methodology is finally validated on literature benchmarks and on a real component, confirming that it permits to define the topology, which presents the maximized thermal and mechanical performance

VHCF Response of H13 Steels Produced with Different Manufacturing Processes

Experimental results have recently shown that failures at stress amplitude below the conventional fatigue limit and at Very High Cycle Fatigue (VHCF) are possible. In case of high-strength steels, VHCF failures generally originate from defects/inclusions present within the material and, consequently, the defect population properties (size and quantity) significantly affect the VHCF response of high-strength steels. Different refinement processes are commonly adopted in order to improve steel cleanliness: among the other, the ElectroSlag Remelting (ESR) process allows to obtain very clean high-strength steels, thus possibly inducing a significant enhancement of the VHCF response. The present paper aims at investigating the actual effect of the ESR process on the VHCF behavior of an AISI H13 steel. Fully reversed ultrasonic tension-compression tests are carried out on hourglass specimens manufactured with and without the ESR process. The estimated P-S-N curves highlight the positive effects of the ESR process on the VHCF response of the investigated H13 steel

Dynamic behaviour of polyolefin thermoplastic hot melt adhesive under impact loading conditions

Dynamic behavior of polyolefin thermoplastic hot melt adhesive under impact loading conditions R. Ciardiello1, A. Tridello1, G. Belingardi1, L. Goglio1. 1 Politecnico di Torino, Department of Mechanical and Aerospace Engineering, Torino, 10129, IT. The mechanical behaviour of adhesive joints under impact loadings is an active area of research due to significant industrial interests. Furthermore, the absence of a unique adopted standard for the study of bonded joints under impact loading increases the academic interests for this topic [1]. In this work, the static and the dynamic response of adhesive joints, bonded with a polyolefin hot-melt adhesive (HMA), were investigated by means of Single Lap Joint (SLJ) tests. The adhesive studied in this work is used in automotive application for bonding plastic internal and external plastic components [2], such as plastic bumpers that can be subjected to impacts during its life. The mechanical and thermal properties of this adhesive are presented in [3]. The main aim of this study is to test standard specimens, SLJ, under dynamic impacts with the use of a modified Charpy pendulum in order to compare the differences between static and dynamic behaviour. The substrate used in this activity are made of a polypropylene copolymer with 10% in weight of talc. Figure 1 shows the testing machine with the clamping system of the specimen. These special fixtures were designed by Goglio et al. [4] with the aim to apply a dynamic load on the tested SLJ. The specimen is fixed to the hammer at the front end, as shown in the right part of Figure 1; the back end is connected to a transverse tail, which hits the two stoppers fixed on the pendulum base, shown in the red circle of Figure 1. The fixtures hold the specimen during the fall of the hammer and transmit the load. A tail in aluminium alloy with T cross section was used, in order to guarantee a high stiffness during the impact, without adding excessive inertia to the system. The system is able to perform dynamic tests for SLJ specimens up to 3.75 m/s. Figure 1: Charpy pendulum used for the experimental tests. Mechanical tests show that there is a clear influence of the load rate on force-displacement diagram and on the maximum force for the tested adhesive. Figure 2 illustrates the differences between a representative curve of quasi-static test and dynamic tests with two different velocities. Figure 2: Force vs linear displacement: comparison between quasi-static and dynamic tests. Figure 3 shows the average values of the peak force and absorbed energies. This Figure illustrates that the velocity increase leads to an increase of the maximum force while the adsorbed energy significantly decreases by comparing quasi-static and dynamic tests. Figure 3: Peak loads and absorbed energy of the quasi-static and dynamic tests. Finally, the fracture surfaces of the SLJ specimens were assessed by means of visual inspection. This analysis showed that the joint separation in the quasi-static tests is mostly cohesive, whereas it becomes completely adhesive in dynamic tests. [1] J.J.M. Machado, E.A.S. Marques and L.F.M. da Silva, J. Adhes., (2017). https://doi.org/10.1080/00218464.2017.1282349. [2] G. Belingardi, V. Brunella, B. Martorana and R. Ciardiello, in Adhesives applications and properties, Cap.13, p.341, A. Rudawska Ed. (INTECH, Rijeca, 2016). [3] E. Koricho, E. Verna, G. Belingardi, B. Martorana, and V. Brunella, Int. J. Adhes. Adhes. 68, 169–181 (2016). [4] L. Goglio and M. Rossetto, in Proceedings of ESDA2006 8th Biennial ASME Conference on Engineering Systems Design and Analysis, 637-643 (2006)

Residual properties in damaged laminated composites through nondestructive testing: A review

The development of damage tolerance strategies in the design of composite structures constitutes a major challenge for the widespread application of composite materials. Damage tolerance approaches require a proper combination of material behavior description and nondestructive techniques. In contrast to metals, strength degradation approaches, i.e., the residual strength in pres-ence of cracks, are not straightforwardly enforceable in composites. The nonhomogeneous nature of such materials gives rise to several failure mechanisms and, therefore, the definition of an ulti-mate load carrying capacity is ambiguous. Nondestructive techniques are thus increasingly re-quired, where the damage severity is quantified not only in terms of damage extension, but also in terms of material response of the damaged region. Based on different approaches, many nonde-structive techniques have been proposed in the literature, which are able to provide a quantitative description of the material state. In the present paper, a review of such nondestructive techniques for laminated composites is presented. The main objective is to analyze the damage indexes related to each method and to point out their significance with respect to the residual mechanical perfor-mances, as a result of the working principle of each retained technique. A possible guide for future research on this subject is thus outlined

Assessment of residual elastic properties of a damaged composite plate with combined damage index and finite element methods

In structural component applications the use of composite materials is increasing thanks to their optimal mechanical characteristics. However, the complexity of the damage evolution in composite materials significantly limits their widespread diffusion. Non-destructive tests are thus becoming ever more important. The detecting Damage Index (DId) technique has been recently brought in the realm of the non-destructive characterization tests for components made of composite material. In contrast to other techniques, this methodology allows to quantitatively assess local residual properties. In this paper, the DId technique is adopted in combination with the finite element method. The mechanical response of two composite plates (an 8-layer twill fabric carbon/epoxy) subjected to four-point bending test is firstly used to tune a finite element model of the laminate. Then, an undamaged laminate of the same composite material is progressively damaged through repeated four-point bending tests. Local residual elastic properties are mapped on the plate through the DId technique. A continuous polynomial curve has been considered to account for the variation of the elastic modulus in the finite element model. The resulting force-displacement curve of the numerical analysis is compared to experimental data of damaged plate, resulting in very good agreement. The combination of the experimental activity and the numerical finite element analysis points out the accuracy of the DId methodology in assessing local residual elastic properties of composite materials

VHCF response of heat-treated SLM Ti6Al4V Gaussian specimens with large loaded volume

Abstract Among the materials used for the production of components through Additive Manufacturing (AM) processes, the Selective-Laser-Melting (SLM) Ti6Al4V alloy is widely employed in aerospace applications for its high specific strength and in biomedical applications for its good biocompatibility. Actual structural applications are generally limited to static loading conditions where the large defects originating during the SLM process do not play a significant role for the static failure. On the contrary, the same defects strongly affect the fatigue response of the parts since they act as crack initiation sites that rapidly lead to fatigue failure. In the literature, a lot of research has been carried out to investigate the quasi-static and the High-Cycle Fatigue properties of the SLM Ti6Al4V alloy but there are still few studies on its Very-High-Cycle Fatigue (VHCF) response. In the paper, the VHCF response of Ti6Al4V specimens, which are vertically orientated during the SLM building and then subjected to a conventional heat treatment (2 hours heating in vacuum at 850°C), is experimentally assessed. Ultrasonic VHCF tests are carried out on Gaussian specimens with a large risk-volume (2300 mm3). Fracture surfaces are investigated for revealing the defect originating the fatigue failure. The Stress Intensity Factor Threshold associated to the experimental failures is finally estimated

VHCF response of AISI H13 steel: assessment of size effects through Gaussian specimens

The paper aims at assessing the influence of the Size-Effect (SE) parameter on the VHCF response of the AISI H13 steel. Ultrasonic tests were performed on Gaussian and hourglass specimens characterized by different risk-volumes (volume of material subjected to an almost uniform stress amplitude). Experimental results showed that a significant difference exists between the VHCF strengths of the investigated material obtained by using specimens with different risk-volumes

Comparison between dog-bone and Gaussian specimens for size effect evaluation in gigacycle fatigue

Gigacycle fatigue properties of materials are strongly affected by the specimen risk volume (volume of material subjected to a stress amplitude larger than the 90% of the maximum stress). Gigacycle fatigue tests, performed with ultrasonic fatigue testing machines, are commonly carried out by using hourglass shaped specimens with a small risk volume. The adoption of traditional dog-bone specimens allows for increasing the risk volume, even if the increment is quite limited. In order to obtain larger risk volumes, a new specimen shape is proposed (Gaussian specimen). The dog-bone and the Gaussian specimens are compared through Finite Element Analyses and the numerical results are validated experimentally by means of strain gages measurements. The range of applicability of the two different specimens in terms of available risk volume and stress concentration effects due to the cross section variation is determined