Key Engineering Materials Vols. 488-489

Abstract: The paper is focused on finding reasonable proportions for both cube-shaped and cylinder-shaped silicate-based composite specimens subjected to wedge-splitting tests. The analysis is conducted using finite element method code with an implemented cohesive crack model. The aspect of the material’s brittleness, related to the heterogeneity of the material and described by what is termed as the characteristic length of quasi-brittle material, is accented. The results yield some recommendations for the determination of parameters of nonlinear fracture models for cementitious composites by means of wedge splitting tests of laboratory specimens of the two standard shapes.

Abstract: The prediction of the strength of adhesively bonded joints has been investigated using a variety of failure criteria such as maximum stress or strain, and fracture mechanics approaches. Fracture mechanics approaches based on the critical strain energy release rate, for crack propagation are applicable to highly cross-linked structural adhesives and have the advantage of avoiding the explicit consideration of the bi-material singularities inherent in adhesive joints. In the present work, the finite-element simulation of such adhesive joint has been performed and the R-curves of two different rubber-toughened epoxy adhesives were measured using double cantilever beam (DCB) specimens. The FE results are applied to be compared with the experimental results which were reported in the literature.

Abstract: In this paper a theoretical and numerical study of impulsive loads over a granular medium free surface has been developed. We will model a real case, as a high deformable solid impacting a less deformable surface, and consequently subject to micro and macro medium failure. Developing a macroscopic approach we resolve the macro-stress in the homogenized two-dimensional medium and subsequently apply a representative volume element (RVE) analysis modeling to the micro-scale. The problem is developed by energetic approach on an elasto-plastic element using an energy functional containing bulk and surface contributions. Finally, a numerical application is presented.

Abstract: Reliable damage tolerant design of airframe structures relies on the accurate life prediction of fatigue cracks propagating from a detectable size to the critical size, which is challenging due to the complex load sequence effect under spectrum loading. This paper aims at gaining a further understanding of the complex influence of the loading history on fatigue damage through a detailed numerical simulation of the near-tip crack behaviour using the crack closure concept. The spectrum loading is broken down into a number of simple yet representative loading scenarios with overload/underload superimposed onto the baseline constant amplitude fatigue loading. Detailed finite element (FE) simulation of the plasticity-induced crack closure (PICC) has been carried out to catch the transient behaviour of PICC and link it to the fatigue damage. The load interaction effect has been analysed with the aim to identify the possible dominant loading cycle which could simplify the fatigue life prediction process in the industry. It is concluded that more reliable damage tolerant design can be achieved if the load sequence effect on fatigue damage can be taken into account more accurately for a structure under spectrum loadings.

Abstract: In the present study, the mixed-mode fracture toughness of an adhesively bonded composite joint system was examined using a variety of linear elastic fracture mechanics (LEFM) based tests. These tests include the mode I double cantilever beam (DCB), mixed-mode asymmetrical DCB (ADCB) and mode II end load split (ELS) test. The joint system was also evaluated using the wide area lap shear (WALS) test that is often employed by the aerospace industry. While lap shear type tests are relatively simple to perform and post-process compared to their LEFM counterparts, the results can often be misleading and are greatly dependent on the overlap length, thickness of substrate and type of fillet. The experimental tests were also simulated using OpenFOAM, a finite volume based software package. Through this combined experimental-numerical approach, a greater understanding of the influence of the peel ply surface treatment and scrim cloth on the behaviour of the WALS test was achieved.

Abstract: Polycrystalline Cubic Boron Nitride (PCBN) is a super-hard material, which is used in some of the most demanding material removal operations today. PCBN cutting tools are employed in turning of hardened steels as well as machining of other abrasive and aerospace grade alloys. In these applications the tools are subjected to high operating temperatures, abrasive and impact loading. Impact loading can lead to the sudden fracture and hence failure of the tool. Much of the recent developments in this industry have been focused on improving the fracture toughness of the PCBN compact. In this work a hypothesis based on induced thermal stresses at the crack tip has been put forward to explain the observed rate sensitivity of PCBN. The results show that the fracture toughness of PCBN is closely linked to the CBN grain size and that the rate sensitivity can be explained in part by induced thermal stresses at the crack tip at high rates of loading.

Abstract: The variations in shape compared to the initial project affect the performance and the appearance of any mechanical assembly. There are many commercial software that allow to conduct an analysis of tolerances but most of these are based on the assumption that the elements analyzed are rigid. This assumption is not found in many industrial processes. This is the typical case of the automotive industry, which widely uses assemblies of sheet metal elements, which are flexible. The major concern is how much variation (the output) of the assembly will be, as “staked-up” from possible sources of variation (the input). The sources of variation in the assembly process include parts variation, tooling variation, and their interactions. For clarity, only part variation is considered as the input in this paper. The present work aims to perform analyses of tolerances in presence of flexible elements; the analyses involve the finite element method.

Abstract: In the present paper the authors refer about a series of experimental tests, where an aluminium alloy square tube, filled with an aluminium foam, was crushed by a longitudinal load at a speed of 10 m/s. The test apparatus consisted of a sled installed on a very stiff frame moving on appropriate guides, as the specimen was set on a home-made fixture. Two arrangements of square tubes were considered as specimens: a “standard” one and an “optimized” one. Both crushing behaviours and energy absorption capabilities were analyzed experimentally and numerically simulated by means of the explicit FE code LS-DYNA^®; the complete numerical model consisted of the striker, the assemblage of square tubes and the base. A high-speed video recording system was used to capture the images from the physical test. The results from the numerical analyses were compared to those obtained from the experiments: those results showed that the force–deflection response had been overestimated by the numerical model. The authors attempted to justify this inconsistency by considering the influence of the strain rate parameters of the considered Cowper-Symonds analytical model on the results. It was shown that the “optimized” energy absorber exhibited a more desirable force–deflection response than the standard one due to some easy design changes, which involved the insertion of aluminium foam dampers.

Abstract: In this work, results from a study on bolted joints made of unidirectional, quasi isotropic Carbon Fiber Reinforced Polymer (CFRP) composites, subjected to tensile loads, are reported. CFRP composite materials are widely used in the mechanical industry, such as that of aerospace, where requirements of weight reduction and structural high performances are very compelling. Composite materials generally present a high resistance to fatigue and corrosion; however, the presence of joints produces the major problems and a poor design of joints leads to a drastic reduction of the reliability of structures made of these materials. A hybrid bolted joint involving a metal plate, made of aluminum alloy, and a CFRP composite plate has been considered; the plates are held together by a titanium bolt. Experimental results from literature are compared with those obtained through a numerical analysis developed with Abaqus code. Once the CFRP composite has been analyzed and the numerical model validated through numerical-experimental correlations, other possible configurations have been numerically analyzed in order to ensure the highest strength of the examined hybrid joint. Afterwards the effects of bolt-hole clearance on the stiffness and strength of the same joint have been investigated.

Abstract: The influence of glass and polypropylene short fiber addition on fracture response of cement based composites after their exposure to high temperatures up to 900°C and for time intervals up to 2 hours was investigated during this study. These conditions simulate adequately closed those of a common real fire exposure. Measurements of their bending and compressive strength and also of ultrasonic wave propagation before and after their burning were carried out. Three distinct response zones are appeared that arise mainly by the incoming chemo-physical changes of constituents due to the attainment of remarked temperature levels. These changes involve the evaporation of physically bounded water in pores and paste in the region of 100°C, the de-hydration of calcium hydroxide of cement paste in the region of 400°C and the decomposition of calcium carbonate of limestone aggregates in the region of 900°C. The fiber addition at common level volume percentage of about 0.3 %, improve only slightly their fracture features –as first crack formation, energy absorption, even their strength– up to 300 - 400°C after of which this addition appears to have opposite effects. The essential changes seem to concern the improved cohesive features of cement based composites and the desired benefits arise mainly by ensuring better conditions for their durability and the restriction of their fragmentation and brittleness leading to a tougher future response.