Applications of Carbon Fibre Reinforced Polymers (CFRP) at temperatures over 150–200 °C are becoming common in aerospace and automotive applications. Exposure of CFRP to these temperatures can lead to permanent changes in their mechanical properties. In this work, we investigated the effect of thermal ageing in air on the strength of carbon fabric/epoxy composites. To this end, accelerated artificial ageing at different temperatures was performed on carbon fabric/epoxy specimens. The flexural and interlaminar shear strengths of the aged specimens were assessed by three-point bending and short beam shear tests, respectively, and compared to those of unaged samples. For ageing at temperatures below the glass transition temperature of the resin, Tg, a moderate reduction of strength was found, with a maximum decrease of 25% for 2160 h at 75% Tg. On the other hand, a rapid strength decrease was observed for ageing temperatures above Tg. This was attributed to degradation of the epoxy matrix and of the fibre/epoxy interface. In particular, a 30% strength decrease was found for less than 6 h at 145% Tg. Therefore, it was concluded that even a short exposure to operating temperatures above Tg could substantially impair the load-carrying capability of CFRP components.
Resin transfer molding (RTM) technologies are widely used in automotive, marine, and aerospace applications. The need to evaluate the impact of design and production critical choices, also in terms of final costs, leads to the wider use of numerical simulation in the preliminary phase of component development. The main issue for accurate RTM analysis is the reliable characterization of the involved materials. The aim of this paper is to present a validated methodology for material characterization to be implemented and introduce data elaboration in the ESI PAM-RTM software. Experimental campaigns for reinforcement permeabilities and resin viscosity measurement are presented and discussed. Finally, the obtained data are implemented in the software and then compared to experimental results in order to validate the described methodology.
Composite materials usage in several industrial fields is now widespread, and this leads to the necessity of overcoming issues that are still currently open. In the aeronautic industry, this is especially true for Barely Visible Impact Damage (BVID) and humidity uptake issues. BVID is the most insidious kind of impact damage, being rather common and not easily detectable. These, along with the ageing that a composite structure could face during its operative life, could be a cause of fatal failures. In this paper, the influence of water absorption on impacted specimens compressive residual strength was studied. Specimens were impacted using a modified Charpy pendulum. Two different locations were chosen for comparison: Near-Edge (NE) and Central (CI). Accelerated hygrothermal ageing was conducted on impacted and reference nonimpacted coupons, placing them in a water-filled jar at 70 °C. Compressive tests were performed in accordance with the Combined Loading Compression (CLC) test method. A Dynamic Mechanical Analysis (DMA) was performed as well. The results showed the influence of hygrothermal ageing, as expected. Nevertheless, the influence of impact location on compressive residual strength is not clearly noticeable in aged specimens, leading to the conclusion that hygrothermal ageing may have a greater effect on composite compressive strength than the analysed BVI damage.
In this work has been estimated the compressive strength of a unidirectional lamina of a carbon/epoxy composite material, using the cross-ply and angle-ply laminates. Over the years various methods have been developed to deduce compressive properties of composite materials reinforced with long fibres. Each of these methods is characterized by a specific way of applying load to the specimen. The method chosen to perform the compression tests is the Wyoming Combined Loading Compression (CLC) Test Method, described in ASTM D 6641 / D 6641M-09. This method presents many advantages, especially: the load application on the specimen (end load combined with shear load), the reproducibility of measurements and the experimental equipment quite simplified. Six different laminates were tested in compressive tests. They were realized by the same unidirectional prepreg, but with different stacking sequences: two cross-ply [0/90]ns, two angle-ply [0/90/±45]ns and two unidirectional laminates [0]ns and [90]ns. The estimate of the compressive strength of the unidirectional laminates at 0°, was done by an indirect analytical method, developed from the classical lamination theory, and which uses a multiplicative parameter known as Back-out Factor (BF). The BF is determined by using the experimental values obtained from compression tests.Finally, extrapolated data were compared with prepreg manufacturer datasheet.
SOMMARIO.In questo lavoro è stata stimata la resistenza a compressione di una lamina unidirezionale di un composito a fibre di carbonio e matrice epossidica, utilizzando dei laminati cross-ply ed angle-ply. Negli anni sono stati sviluppati vari metodi per la caratterizzazione a compressione di materiali compositi rinforzati a fibre lunghe, ognuno dei quali contraddistinto da una specifica modalità di applicazione del carico al provino. Il metodo scelto per eseguire le prove di compressione è il Wyoming Combined Loading Compression (CLC) Test Method, descritto nella normativa ASTM D 6641/D 6641M-09. Tale metodo presenta molti vantaggi, tra i quali: la modalità di applicazione del carico sul provino (combinazione di carico di taglio e di estremità), la riproducibilità delle misure e la facilità di utilizzo del telaio Wyoming. Sono stati testati a compressione sei laminati, realizzati a partire da uno stesso prepreg unidirezionale: due crossply [0/90]ns ognuno con una diversa sequenza di laminazione, due angle-ply [0/90/±45]ns ognuno con una diversa sequenza di laminazione e due laminati con fibre solo a 0° od a 90°. Il calcolo della resistenza a compressione della lamina unidirezionale a 0°, è stato fatto utilizzando un metodo analitico indiretto, sviluppato a partire dalla teoria classica della laminazione e che fa uso di un parametro moltiplicativo noto come Back-out Factor (BF). Il BF viene determinato utilizzando i valori sperimentali ricavati dalle prove di compressione. Infine i valori di resistenza così ottenuti sono stati comparati con quelli presenti nella scheda tecnica fornita dal produttore del prepreg.
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