Variation in fatigue properties of cast A359-SiC composites under total strain controlled conditions: Effects of porosity and inclusions

The effects of casting defects, such as inclusions and porosity on fatigue properties of permanent mold cast bars of A359-20 vol% SiC particle composites cast and heat treated under identical conditions, were studied under total strain control conditions. The fatigue fracture surfaces and the near fracture regions were observed using optical stereo-microscope and scanning electron microscope (SEM) to identify the effects of microstructure and casting defects on fatigue properties of pairs of specimens tested under the same total strain that showed large scatter in fatigue life. It was found that the largest porosity size on the fracture surfaces of the fatigue specimens measured was around 700 μm, whereas the average dendrite arm spacing was 60–100 μm and the size of SiC particle clusters in the near fracture region was around 100 μm. Fracture surfaces of the composite bars showed that fatigue cracks frequently initiated from porosity present near the surface of the samples. Test specimens containing larger porosity size were observed to have lower fatigue life. However, some specimens with larger amounts of inclusions had a lower fatigue life than those with larger porosity size within the range observed in this study. The size and amount of inclusions, and the size and shape of the porosity near the surface seems to have the greatest influence in decreasing fatigue life. Fatigue strength, and fatigue ductility constants for permanent mold cast A359-20 vol% SiC composite were determined from the strain controlled fatigue data. They were as follows: fatigue strength exponent (b) is −0.1145, fatigue strength coefficient $({\sigma }_{\text{f}}^{\prime })$ is 558 MPa, fatigue ductility exponent (c) is −0.6066 and fatigue ductility coefficient (${\epsilon }_{\text{f}}^{\prime })$ is 0.0108.     

Fracture toughness determination of adhesive and co-cured joints in natural fibre composites

Adhesive bonding has become more efficient in the last few decades due to the adhesives developments, granting higher strength and ductility. On the other hand, natural fibre composites have recently gained interest due to the low cost and density. It is therefore essential to predict the fracture behavior of joints between these materials, to assess the feasibility of joining or repairing with adhesives. In this work, the tensile fracture toughness ($G_{\text{n}}^{\text{c}}$) of adhesive joints between natural fibre composites is studied, by bonding with a ductile adhesive and co-curing. Conventional methods to obtain $G_{\text{n}}^{\text{c}}$ are used for the co-cured specimens, while for the adhesive within the bonded joint, the J-integral is considered. For the J-integral calculation, an optical measurement method is developed for the evaluation of the crack tip opening and adherends rotation at the crack tip during the test, supported by a Matlab sub-routine for the automated extraction of these quantities. As output of this work, an optical method that allows an easier and quicker extraction of the parameters to obtain $G_{\text{n}}^{\text{c}}$ than the available methods is proposed (by the J-integral technique), and the fracture behaviour in tension of bonded and co-cured joints in jute-reinforced natural fibre composites is also provided for the subsequent strength prediction. Additionally, for the adhesively-bonded joints, the tensile cohesive law of the adhesive is derived by the direct method.     

Chloride penetration into concrete structures in the marine atmosphere zone – Relationship between deposition of chlorides on the wet candle and chlorides accumulated into concrete

The relationship between chlorides from marine aerosol and chlorides accumulated into concrete is discussed in this paper. The experimental programme comprised an environmental characterisation, with climatic and chloride deposition data, and a study of chloride penetration into concrete based on natural exposure of specimens in a marine atmosphere zone. Results show that salt concentration in marine aerosol strongly decreases in the first meters from the sea. Chlorides present in the atmosphere can be studied using the wet candle method and correlated with chlorides accumulated into concrete. This relationship can be represented by the equation $C_{\text{tot}}=C_0+k_{\text{d}}·\sqrt{D_{\text{ac}}}$, where k_d is a coefficient which depends on concrete and environmental characteristics, C_tot is the average total amount of chlorides accumulated into concrete, C₀ is the chloride content in concrete before exposure and D_ac is the accumulated dry deposition of chlorides.