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This paper focuses on investigating the forming behaviour of sandwich materials composed of steel sheets and glass fibre-reinforced polyamide 6 (GF-PA6), i.e., thermoplastic-based fibre metal laminates (FML). Stretching and forming limit curve (FLC) determination of FML with different cover/core layer thickness ratios at various forming temperatures, i.e., at room temperature (RT), 200 and 235 °C, are the main approaches for characterizing their formability. In addition, the formability of mono-materials and non-reinforced sandwich materials is investigated as a reference. For a successful test and reliable results, several technical issues are considered, such as the suitable lubrication configuration and digital image correlation at elevated forming temperatures. The results revealed that the formability of non-reinforced sandwich materials with different core layer thicknesses exhibited compared formability to their monolithic steel sheet and no remarkable improvement in their formability with increasing the temperature up to 200 °C. Conversely, the formability of FML shows significant improvement (approx. 300%) with increasing temperature with a forming depth of about 33 mm at 235 °C compared to only 12 mm at RT.

This study was carried out during two successive seasons 2016 and 2017 on six years old ‘Wonderful’ pomegranate trees (Punica granatum L.). Trees were grown on sandy loam soil conditions in a private orchard located at Burg El-Arab, Alexandria governorate, Egypt. The trees were spaced 4x5 m apart and irrigated by drip irrigation system and received similar cultural practices adapted in the orchard. The effect of some growth regulators, nutrient elements and kaolin on cracking and fruit quality of pomegranate ‘Wonderful’ cultivar was studied. The experiment was designed as randomized complete block design with five replicates. Each block contained 11 treatments (control, naphthalene acetic acid (NAA) at 25 and 50 mg/l, gibberellic acid (GA3) at 100 and 150 mg/l, boric acid at 0.1% and 0.3%, calcium chloride at 1% and 2% and kaolin at 4% and 6%). Results revealed that, boric acid at 0.3% gave the highest mean values of fruit set (%), number of fruits/tree, yield (kg/tree) and anthocyanin; while, naphthalene acetic acid at 50 mg/l recorded the maximum mean values of fruit weight, fruit diameter, volume and grain weight, and gave the minimum mean values of fruit drop and fruit cracking (%). On other hand, gibberellic acid (GA3) at 150 mg/l gave the maximum yield (kg/tree), in the second season and the minimum fruit cracking (%), whereas, calcium chloride at 2% gave the best results of TSS (%), acidity (%), and vitamin C, total and reducing sugars were higher than the control for all treatments. Finally, kaolin at 6% gave the lowest percentage of sunburn and the highest percentage of juice, as compared with the control and other treatments during both seasons.

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In this article, we continue the investigations on the forming behavior of the steel/polymer/steel (SPS) sandwich composites as introduced in Part 1 [1]. In this paper, the main focus lies on bending conditions, validated by analytical and numerical methods. A wide variety of SPS layer configurations and thicknesses were tested under three-point bending conditions considering different bending angles (60, 90 and 150°) and different punch radii (1.5, 3, 6 and 12 mm). The results are validated in terms of the bending forces, springback degree, strain field distribution, and thickness reduction, where a good matching between the numerical and analytical results with the experimental ones was reached.

Metal-polymer-metal sandwiches can find promising applications in the automotive field thanks to their lightweight and formability . The paper focuses on the effect of low velocity impacts on the residual mechanical behavior. Experimental stepwise tests are run on undamaged and impacted specimens with different combinations of thickness and grade for the outer steel skins and the inner polymeric core. Surface temperature evolution is thermally monitored during the tests with the aim to characterize the induced damage and to identify a parameter able to quantify the residual strength of the panel. Several approaches have been considered. The analysis of the thermal amplitude trend with the lock-in thermography evidences a variation in the thermal behavior of the specimens, defining a corresponding damage stress σD. We found a 20% σD difference between undamaged and damaged specimens. Moreover, impacted specimens experience a temperature and stress concentration at the impact area dependent on the indentation . Based on these results, we evidence the possibility to relate impact indentation with the damage stress estimated by thermography and with the stress concentration factor induced by the impact. Therefore, thermography is a useful and valid tool for post-impact damage detection, monitoring and quantification of these multi-layer sandwich materials.

Recently, the applicability of thermoplastic fibre-metal-laminates (FMLs) in the automotive and aerospace fields has gained more attention due to their lightweight potential and improved flexural stiffness. Therefore, this study deals with investigating the structural properties of FMLs top-hat crashboxes under quasi-static and highly-dynamic bending conditions. Accordingly, the energy absorption characteristics and failure modes including the arisen microscopic defects are identified. For improved structural integrity, the hat structures were produced using a one-step thermoforming process, in which the forming and bonding of the FMLs layers take place simultaneously. Different metallic skin sheets (aluminium and steel) combined with glass fibre reinforced polyamide cores with different fibre orientations and thicknesses were considered. Also, finite element analysis using LS-Dyna™ was carried out on the Al-based FMLs to calculate the energy absorption behaviour and analyse the failure modes. To characterize the monomaterials and FMLs, tensile tests at different temperatures and strain rates were performed. The results state that with the thermoforming process, arbitrary FMLs material combinations and thicknesses can be processed. Varied thicknesses in the different hat structure regions are expected due to the flow of the polyamide matrix. Moreover, increasing the core thickness and utilizing the 0°/90° fibre orientation can improve the bending stiffness. Several defects such as delamination, plastic deformation and fibre cracking arose during the bending test. Finally, the simulation results revealed good agreement with the experimental ones. Due to the model simplifications, some failure modes could not be replicated in the simulation. This requires further studies.

In part 1, a fundamental study on mechanical properties and deep drawing of a steel/polymer/steel sandwich (SMs) combination was accomplished investigating one type of reinforced SMs (R-SMs). In this part, the effect of reinforcements (REs) with different geometries, sizes, materials and locations on stretching and deep drawability is introduced. Additionally, the influence of reinforcing SMs with different mechanical properties and skin/core thicknesses is considered. Therefore, a deep drawable steel grade with thicknesses 0.49 mm and 0.24 mm was utilized. For the core polyolefin (PP-PE) foils with thicknesses of 0.3 mm and 0.6 mm were used. Substantial reduction of the deep drawability was found if the RE is located in the forming region (close to the punch), however, only locally if located in the flange area. Inserting the RE in the flange has no remarkable effect on the drawing force but on the forming potential represented by the cup height at failure. The stretching results show that increasing the RE size reduces the forming potential. The RE creates a sharp interface with the neighbour core layer accelerating failure. A negligible effect of the RE geometry was found, however solid REs show reduced forming limits compared to the mesh-like ones.

Recently, increasing development efforts of components made of fibre metal laminates (FMLs), with thermoplastics due to their numerous advantages are remarkable. One-step adhesion and forming approach is an innovative solution to process such laminates reducing the cycle time and delivering failure-free components simultaneously. Hat shaped sections were the chosen geometry in this paper. This process is described considering the temperature, pressure and time progress in addition to the temperature distribution inside the FMLs in different locations. In order to describe the structural behaviour of the hat profiles, top-hat crashboxes were produced and tested via quasi-static three-point bending tests. The investigations were carried out by experimental and finite element simulation. Several parameters are discussed in detail, the top layer (steel and aluminium), the core layer thickness (0.5 and 1.0 mm) and glass fibre orientations (0°/90° and ±45°). The results revealed that by doubling the core thickness, the forming forces and accordingly the absorbed deformation energy were doubled at reduced weight increase. Comparing the experimental and numerical results for the Al-based FMLs, a good agreement regarding the energy absorption deformation mode and the force–displacement progress was reached.

Since years, metal/polymer/metal sandwich sheets (MPM) are in the focus of research, not only because of their lightweight potential, but also for their arbitrary customization in aviation as well as automotive applications fields. Because of the limited forming potential of the MPM containing thin metallic skin sheets in combination with epoxy resins, improving their shaping ability is a challenge. In this paper, studies on forming of MPM sheets by electromagnetic forming (EMF), a die less, high strain rate, free-forming process were carried out. EMF bending tests at different bending radii and complex forming process (comparable to stretch forming) were attempted at different energy discharges. The results revealed that forming the MPM sheets is possible with EMF with improved results in terms of the strain values and forming depth compared to the conventional stretch forming process with fixed tools. In the EMF bending tests, several damage types could be observed due to the inappropriate selected energy level and coil/sample size ratio. These parameters have to be optimized in a follow-up study to define a proper process window.

Steel/polymer/steel sandwich structures have a high potential in weight reduction at high stiffness values. The aim of this study is to investigate the adhesion properties of galvanized steel/Polyamide 6 sandwich composites, which are produced by hot pressing following a parametric design of experiments plan at varied temperatures, times and pressures. One of the most interesting phenomenon hindering a good adhesion strength was the formation of sub-millimeter sized bubbles during the hot pressing. Possible reasons and methods to avoid these bubbles were investigated using scanning electron microscopy, X-ray photoelectron spectroscopy , thermogravimetric analysis , ultraviolet photoelectron spectroscopy, metastable induced electron spectroscopy and X-ray diffraction. The proposed solution is to apply a heat treatment on the galvanized sheet prior to pressing, where varied treating regimes were applied. To improve the adhesion quality , mechanical/physical treatments were utilized as well. Furthermore, a compatible adhesive agent was applied in combination with the different surface treatments, where the lap shear strength reached at least 23 MPa, high enough for a delamination-free shaping afterwards.

Steel/polymer/steel sandwich materials (SMs) with varying core thickness were produced using the roll bonding technology. At constant skin thickness, the effect of the core thickness change on the mechanical properties, particularly their specific stiffness and strength, was investigated. Additionally, the SMs were locally reinforced with steel inlays to improve their behaviour against dimension changes or the degradation of the core in case of local loading or joining. The effect of the reinforcement (RE) on the strain distribution under deep drawing conditions was studied. Strains were measured using photogrammetry. To take into account the influence of anisotropy in this sense, an essential anisotropy evaluation was carried out for the SMs to assess their forming behaviour. The results revealed an adequate agreement between the measured mechanical properties of SMs and the estimated values following the rule of mixtures. Up to a core volume fraction of approx. 0.37, the specific stiffness and strength values of the SMs are only slightly reduced compared to the monolithic steel material. The deformation analysis shows a significant influence of the inserted REs on the strain distribution and failure and consequently the position of cracking, occurring at the bottom/edge region in front of the RE.