چكيده به لاتين
The application of adhesively bonded joints (ABJs) in automobile, aeronautical and other industries is increasingly growing. Many researchers use fracture mechanics and damage mechanics for predicting mechanical behavior and strength of ABJs. Critical strain energy release rate is an important input parameter in both fracture mechanics and damage mechanics approaches. Many researchers have investigated pure mode I fracture energy measurement which has led to standard test method for adhesive joints. However, for pure mode II and mixed-mode I/II loading conditions, there are no international standards.
In the first part of this research, some issues of the end-notched flexure (ENF) test method for mode II fracture energy measurement are pointed out. In this part, the effect of notch and pre-crack lengths on the fracture energy obtained from ENF is studied and, it turns out that, this parameter has significant impact on the obtained results. A modification based on finite element (FE) analysis is proposed to overcome this drawback. In this field the conventional data reduction approaches such as direct beam theory (DBT) and compliance-based beam method (CBBM) are studied and a new approach based on compliance calibration method (CCM) is proposed for brittle adhesives. Experiments and numerical analyses showed the advantage of proposed technique. In the second part, a specimen based on semi-circular bend (SCB) is proposed for measuring mode I, mode II and, mixed-mode I/II fracture energies. Advantages of this method are: low cost and easy manufacturing, no need for special jig and fixtures, no need for special sensors and actuators, wide range mixed-mode ratio coverage from pure mode I to pure mode II, applicable for both ductile and brittle adhesives, lower risk of plastic deformation in adherends.
The SCB specimen is first tested with a brittle adhesive. A wide range of FE analyses with linear elastic formulation is conducted for this specimen and shape factors are obtained for many geometrical parameters. The digital image correlation (DIC) technique is used to validate FE results. Finally, the fracture envelope of adhesive joint is obtained. Double cantilever beam (DCB), single-leg bending (SLB) and ENF tests are conducted and very good agreement is shown to exist between their results and the SCB results.
Finally, the SCB specimen is used to obtain the fracture envelope of a ductile adhesive. There are two basic differences between ductile and brittle adhesives: (i) constant phase angle during crack propagation should be proved. (ii) Data reduction scheme. A wide range of FE simulations are used to find proper geometrical parameters that show the least variation in mixed-mode phase angle during crack propagation. Cohesive zone modelling (CZM) technique is used for data reduction by inverse method and the results are validated with DCB test.
