Erica Anna Squeo

Shape memory epoxy foams by solid-state foaming

Epoxy foams were produced by means of solid-state foaming and their shape memory properties were evaluated together with other physical properties. Solid-state foaming consists of pressing thermosetting resin powders to produce solid tablets, heating the tablets at high temperature to generate both the formation of pores inside the resin and the resin polymerization. A nanoclay was added to the resin powder before pressing it up to a maximum content of 5 wt%. Unfilled and composite foams were characterized by density measurements and thermal analyses. Subsequently, foam samples underwent up to two thermo-mechanical cycles: each cycle consisted of the storage of a compressed shape and the subsequent thermal recovery. Compression tests were used to measure the effect of the thermo-mechanical cycles on the foams mechanical performances and compressive toughness was extracted from the tests. It was observed that all the foams exhibited good shape memory properties also after cycling: nanoclay filler allows the foams to completely recover the initial shape and to increase the compressive and the specific compressive toughness.

Solid-State Foaming of Epoxy Resin:

A new foaming process has been developed for epoxy resins. Uncured epoxy tablets are fabricated by pressing commercial powders in a steel mold at room temperature and used as foam precursors. The tablets foam when heated in a muffle at high temperature. No blowing agent was added because the foaming mechanism depends on the uncured resin boiling point. The foaming temperature is set to be high enough to rapidly produce the resin boiling but not excessive to avoid the thermal degradation. During boiling, the epoxy resin polymerizes and the bubbles freeze in the final structure. Epoxy foams are obtained by heating the compacted tablets in cylindrical copper molds, having internal diameter equal to the tablet diameter. Several process parameters have been changed in the experiment to understand their correlation with the foaming efficiency. However, the foaming ratio (expressed in terms of the ratio between the final and the initial tablet height) is found to be mainly dependent on the initial tablet density. In optimal conditions, the foaming ratio can rise up to 6. Thermal tests have been performed to evaluate the epoxy powder behavior during the cure, whereas mechanical compression tests were used to evaluate the final performances of the foams.

Solid-state Foaming of Nano–Clay-Filled Thermoset Foams with Shape Memory Properties

A solid-state process was used to produce epoxy foams with different contents of nano-clay (up to 10%wt). The foam properties were studied by means of numerous tests: X-ray analysis, dynamic mechanical analysis, mechanical tests (compression, flexure, indentation, stress relaxation). The straightening effect of the nano-clay filler was investigated and related to the loading conditions and the intrinsic properties of the resin matrix. Small and large foam samples were produced by changing the number of solid precursors during foaming. In the end, shape recovery tests showed that composite foams exhibit remarkably shape memory properties, at least at low filler contents.

Filler–matrix interaction in solid-state foaming of composite foams

Solid-state foaming is a relatively new process for producing closed-cell thermoset foams starting from powders. In this study, the possibility of introducing nanoclay into epoxy foams by solid-state foaming was investigated. The nanoclay was found to affect the foaming efficiency and the foam density; even if large nanoclay aggregates are present in the foams, for a small range of filler content (2.5—3 wt%), the mechanical properties are positively influenced, as shown by specific compressive toughness which attains a maximum value. Electron microscope observations provided important information on the dependence of the cell size on the filler percentage, and the presence of a critical size (about 5 μm) of nanoclay aggregates, below which they are coherent with the matrix.

Density Measurement of Powder Metallurgy Compacts by Means of Small Indentation

In this study, the density of powder metallurgy compacts was measured by means of small indentation. Zinc (Zn) and aluminum (Al) tablets were fabricated by cold compaction of powders: different tablet densities were obtained by changing the packing pressure. Instrumented indentation tests were carried out by means of flat cylindrical indenters having diameters of 1 mm and 2 mm. After a calibration procedure, the slope of the indentation curve or the indentation load can be used for the density evaluation.

Recycling of Waste Epoxy-Polyester Powders for Foam Production

This paper proposes a new foaming technology (solid-state foaming) to produce structural foams from waste thermosetting resins. The proposed technology is easy and does not require specific and expensive equipments. Solid tablets are produced by cold compaction of resin powder, and foam by heating in an oven. Composite foams can be produced by mixing fillers and resin powder before the cold compaction. In the experiment, an epoxy-polyester (EP-PE) resin powder, deriving from the waste of a manufacturer of domestic appliances, was used with montmorillonite (MMT) particles. Resulting foams with a filler content ranging from 0 to 10 wt% were characterized in terms of physical and mechanical properties (by compression tests). Although the effect of the MMT content seems to be negative for the adopted resin, the feasibility of producing composite foams by recycling waste industrial powders is shown. The properties of the unfilled foams are sufficient for many industrial applications.

Numerical Prediction of Residual Stresses in Laser Bending of Stainless Steel Sheet Metals

Stainless steel sheet metals were laser bent by means of a high power diode laser at different values of power and scan velocity. The laser power ranged from 100 to 300 W (with an increment of 50 W); two scan speeds were used, 4 and 8 mm/s, and the number of passes was 2, 4 or 6. In the experimentation, the values of bending angle, microstructure and residual stresses of the laser bended sheet metals were analyzed with regard to the input variables. In particular, residual stresses were evaluated by means of X-ray analysis in terms of first and second order stress. Measurements were performed on the convex surface of the sample in the laser beam action zone. The bending process was numerically simulated by means of a thermo-mechanical finite element model, implemented to predict the sheet metal bending angle as a function of the laser power and scan velocity. The residual stress distribution was extracted from the numerical simulations and its agreement with the experimental observations was discussed. As a general conclusion, the effect of multiple scans is hardly simulated by thermo-mechanical models which do not take into account the material annealing during forming.

Mechanical Characterization of Metal Sheets by Means of Double Indentation

An easy and innovative technique for metal sheet characterization is described. A double indentation is performed on sheets by means of two co-axial small diameter flat indenters made of WC. A very small indentation is left on the sheet, so as to consider this technique a non destructive one, particularly suitable for on-line application. The proposed method was tested on sheets of aluminum alloy (6082 T6) with several thicknesses (nominally 0.6, 0.8, 1 and 1.5 mm). Double indentations were performed changing indenter diameter (1 and 2 mm) and testing rate (from 0.05 to 1 mm/min). In order to make a comparison with indentation tests, flat specimens were cut from the same sheets and standard tensile tests were performed. A very good correlation was found between indentation and tensile test results, showing the effectiveness of the proposed method. A suitable data normalization is necessary to correctly compare indentation and tensile data. The best results were obtained using the smaller diameter indenter. The testing rate seems to be not relevant in the experimented range, suggesting that a fast procedure can be defined on purpose for on-line application.

Diode Laser Assisted Filament Winding of Thermoplastic Matrix Composites

A new consolidation method for the laser-assisted filament winding of thermoplastic prepregs is discussed: for the first time a diode laser is used, as well as long glass fiber reinforced polypropylene prepregs. A consolidation apparatus was built by means of a CNC motion table, a stepper motor and a simple tensioner. Preliminary tests were performed in a hoop winding configuration: only the winding speed was changed, and all the other process parameters (laser power, distance from the laser focus, consolidation force) were kept constant. Small wound rings with an internal diameter of 25 mm were produced and compression tests were carried out to evaluate the composite agglomeration in dependence of the winding speed. At lower winding speeds, a strong interpenetration of adjacent layers was observed.

A numerical-experimental approach for the simulation of tube bending processes

A finite element (FE) numerical-experimental procedure was defined to take into account the real material properties of a tube under bending, as nominal data or data extracted by other specimens could be erroneous. Axial and transverse compression tests were performed on portions of an aluminium alloy tube and a reverse procedure was used to extract the correct values of the material plastic properties. Subsequently, these properties were implemented in an FE model for the simulation of a ram bending process and the numerical results were experimentally validated. The proposed numerical-experimental procedure provides a fast tool to increase accuracy in the numerical simulation of tube bending. As a result, the effect of the clearance between the bended tube and the ram was easily identified and investigated.