The publication presents a cruciform specimen for the determination of cyclic crack growth data under biaxial loading. The design of the specimen with slotted loading arms allows good decoupling between the two loading directions. For different initial crack geometries, the solutions for the stress intensity factors K I and K II as well as the crack-parallel T-stress are calculated by linear elastic finite element analysis (FEA) with the program ABAQUS. For two specimens with the same geometry made of aluminium alloy 6061 T651, the crack growth behaviour is measured at different Tstresses at a stress ratio of R=0.7 and overloads. It is shown that the crack retardation after an overload with crack-parallel tensile stress is less than without it. The reason for this behaviour is considered to be the reduced plasticity at the crack tip due to the higher triaxiality of the stress state.
In this study, the cyclic crack growth behavior of biaxially loaded cruciform specimens is investigated and compared with an uniaxially loaded cruciform specimen together with the Paris law to detect differences between uniaxial and multiaxial experiments and to understand the occuring mechanisms. The tests are performed on a planar‐biaxial test system and a stress ratio of R = 0.1. Biaxial loading takes place both in‐phase as well as out‐of‐phase, after an initial technical crack is generated, whereby the phase shift loading leads to a constantly changing biaxial force ratio . The crack path curved after changing from in‐phase to out‐of‐phase loading and subsequently the cracks grow straight again. Both cracks of each specimen have an almost identical crack path indicating a symmetrical stress distribution on the specimen. The calculation of the stress intensity factor range for the curved crack path is done by FE‐calculations.
The absence of sufficient knowledge of the heterogeneous damage behaviour of textile reinforced composites, especially under combined in-plane and out-of-plane loadings, requires the development of multi-scale experimental and numerical methods. In the scope of this paper, three different types of plain weave fabrics with increasing areal weight were considered to characterise the influence of ondulation and nesting effects on the damage behaviour. Therefore an advanced new biaxial testing method has been elaborated to experimentally determine the fracture resistance at the combined biaxial loads. Methods in image processing of the acquired in-situ CT data and micrographs have been utilised to obtain profound knowledge of the textile geometry and the distribution of the fibre volume content of each type. Combining the derived data of the idealised geometry with a numerical multi-scale approach was sufficient to determine the fracture resistances of predefined uniaxial and biaxial load paths. Thereby, CUNTZEmathsizesmall’s three-dimensional failure mode concept was incorporated to predict damage and failure. The embedded element method was used to obtain a structured mesh of the complex textile geometries. The usage of statistical and visualisation methods contributed to a profound comprehension of the ondulation and nesting effects.
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