Debonding and interlaminar delamination are typical failure modes when it comes to fiber-metal laminates and composites respectively. Accurately characterizing interface properties plays an important role in modeling and improving mechanical attributes of bonded and laminate structures. The Double Cantilever Beam (DCB) test is widely used to determine Mode I interlaminar facture toughness of composite materials and structures. This traditional approach is based on optical tracking of scales hand-marked on the test piece edge. This makes the method prone to uncertainties as it is dependent on human skills. Accurate crack length identification in the determination of the Mode I strain energy release rate is the main challenge, in particular in thin laminates.
In a recent paper published in Engineering Fracture Mechanics Johannes Reiner, Juan Pablo Torres and Martin Veidt at the University of Queensland and the Defence Materials Technology Centre in Australia present a novel Top Surface Analysis (TSA) method that overcomes human errors by a top surface analysis of the DCB test pieces through Digital Image Correlation (DIC). This automated measuring and data analysis approach determines simultaneously crack length and crack tip opening displacement. The method minimizes human errors because it is based on clear-cut post-processing of generated surface data by non-contact DIC. DCB tests were performed with a 10kN load cell at room temperature. The authors set the cross-head displacement rate at 3mm/min and recorded load displacement, which was synchronized with the DIC software Aramis.
The researchers painted a fine speckle pattern on the surface of the test pieces over a selected area of interest. This created a textured pattern on the specimen surface. They set the facet size and distance at 19×19 pixels2 and 16 pixels respectively. Each facet had an overlap distance of 3 pixels with its immediate neighbors. The facet size as well as the quality of the speckle pattern determine the spatial resolution, and consequently the accuracy of the measurements.
The methodology presented in this paper compares the outcomes of the Top Surface Analysis method with those of the manual crack tracking approach that implements optical side-view observation of markings on printed scales.
For carbon fiber reinforced polymer laminates, the authors observed that delamination initiated consistently at around 100N. A typical saw-toothed response in woven composite laminates was observed after the initiation of delamination. This is due to the fiber microstructure that inhibited delamination growth which results in a stick-slip load decrease up to ultimate failure. Seven surface lines on the DCB specimens were analyzed. TSA gave consistent resistance estimation in all the seven surface lines. Conventional side analysis estimated slightly lower Mode I at crack initiation.
For glass fiber reinforced polymer laminates, the authors observed that the delamination initiated at a maximum load of 11-13N. A saw-toothed response was noted during crack propagation. A high resolution side view using DIC gave fractured speckle patterns that led to incomplete pattern recognition. In contrast, TSA guaranteed continuous digital image correlation measurement of the displacement pattern on the top surface. Above all, it was possible to determine the strain energy release rate along the width of the DCB specimen, which was previously only possible in computational studies.
The Top Surface Analysis appears to be a promising candidate for simplifying Double Cantilever Beam analyses while increasing precision and repeatability. This crack length inspection approach and data analysis is simple to set up and to analyze and can be automated.
Johannes Reiner1, 2, Juan Pablo Torres1,3 , Martin Veidt1,2,3 . A novel Top Surface Analysis method for Mode I interface characterization using Digital Image Correlation. Engineering Fracture Mechanics, volume 173 (2017), pages 107–117.Show Affiliations
1 School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
2 Queensland Centre for Advanced Materials Processing and Manufacturing (AMPAM), The University of Queensland, Brisbane, QLD 4072, Australia
3 Defence Materials Technology Centre, Hawthorn, Victoria 3122, Australia
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