Issue 35

O. Demir et alii, Frattura ed Integrità Strutturale, 35 (2016) 330-339; DOI: 10.3221/IGF-ESIS.35.38 331 I NTRODUCTION lthough many fracture mechanics problems seen in practice can adequately be analyzed by taking into account only mode-I conditions, there are still many problems that are subjected to mixed mode loading, for which mode- I analysis approaches are not sufficient. The mode mixity of the problem can be due to orientation of an initial defect existing in the structure caused by imperfections or processes such as manufacturing operations. Another source of mixed mode loading on the crack is due to the nature of loads that exist on the structure. For such situations, analyses of the cracked structure under mixed mode loads and related criteria for fracture conditions are needed. The most basic type of mixed mode fracture is mode-I/II, in which both mode-I (opening) and mode-II (shearing) loads act on the crack tip. There are various studies and criteria that exist in the literature for mixed mode-I/II fracture. Maximum tangential stress (MTS) criterion [1], minimum strain energy density (SED) criterion [2], maximum energy release rate (MERR) criterion [3, 4], maximum tangential strain criterion (MTSN) [5] are some of the most common criteria used to understand the fracture behavior of materials under mixed mode-I/II loading conditions. Koo and Choy [6], Pook [7] and Tanaka [8] also proposed different criteria for in-plane mixed mode problems. Numerous experimental and numerical analyses were performed and various types of specimens [9-11] were introduced by researchers and used in experimental analyses. Richard also performed many detailed and comprehensive studies using new and modified CTS specimen and proposed new criteria that includes empirical formulations involving KI, KII, equivalent stress intensity factor (SIF) and crack deflection angle separately [12-14]. In this study, results from finite element analyses of the test system composed of compact tension shear specimen, loading apparatus and pins are presented. Using the results from analyses of the assembly for different load mixity angles for the specimens, corresponding fracture analyses are also performed on the specimen submodel and mixed mode stress intensity factors computed. Results from experimental studies on a new type of specimen, called T-specimen, which has smaller dimensions and requires less material, are also presented. Fracture experiments of the finite element models of CTS and T specimen are also conducted to check the validity of some of the existing criteria for mixed mode-I/II fracture conditions and to develop a further refined mode-I/II fracture criterion (Part 2). The outline of the paper is as follows: In the next section, details of the finite element models including fracture submodels are given. This is followed by description of the test procedure and the corresponding experimental results. Finally, preliminary fracture results from a new type of mixed mode specimen are presented. F INITE ELEMENT MODELING OF MODE - I / II FRACTURE n this section, details and results of the finite element models of CTS specimen are presented. First, finite element models, boundary conditions and loads are described. In the second subsection, results of the finite element models in terms of stress intensity factors are presented. Description of Finite Element Models In Fig.1, overall and exploded views of the mixed mode-I/II test assembly solid model for 0° loading angle and finite element model and dimensions of CTS (compact tension shear) specimen are shown. As can be seen in Fig.1 (a), mixed mode loading clevises are designed to allow the loading axis to pass through the specimen center under different loading angles (0°, 15°, 30°, 45°, 60°, 75°, 90°). CTS specimen proposed by Richard [13] is used in fracture analyses and experiments (Fig. 1 (b)). (a) (b) Figure 1 : Overall and exploded views of the mixed mode-I/II test assembly solid model and finite element model and dimensions of CTS specimen. A I