Direct Partition of Mixed-Mode Fractures Using Digital Image Correlation

Composite materials are very important to the aerospace industry because they maximise weight reduction in aircraft as well as providing several other advantages, for example, reduced fuel consumption. Although composite materials already provide great benefits to the aerospace industry, their full potential is not currently being realised due to their susceptibility to delamination. Delamination is a type of failure mode suffered by laminated materials in which constituent layers debond and separate from each other. It results in a significant loss of structural stiffness and is often accompanied by catastrophic structural failure.

To combat delamination, composite parts are often over-designed increasing the cost, weight and volume of a structure. This is in great part because the resistance to fracture, the 'fracture toughness', of a laminate, is not easily predicted while the consequences of delamination are severe. Fractures deform in different modes: in mode I opening, mode II shearing and mode III tearing. Since the fracture toughness of each pure fracture mode is typically different, the overall fracture toughness of a mixed-mode fracture depends on the relative proportions of each fracture mode, and this is called the 'fracture mode partition'. Determining the fracture mode partition is therefore a very important task. It has however turned out to be a complex and highly controversial problem.

Many researchers have worked on the problem over the years by deriving analytical theories and carrying out numerical simulations; however, reliably and accurately validating these results with experiments has proved problematic. The conventional mixed-mode fracture experiment applies loads to a cracked specimen until the crack grows and then uses the measured critical load to calculate the total fracture toughness. The fracture mode partition cannot be directly determined nor the in-depth mechanics of delamination understood. Instead comparisons can only be made between the measured fracture toughness and the material's failure locus to approximate the partition. This indirect measurement has made it very difficult to validate any of the partition theories proposed in the literature and has no doubt contributed to the confusion and controversy surrounding the topic.

Digital image correlation (DIC) is a technology that is becoming increasingly available. It is an optical method that can provide full-field non-contact accurate measurements of deformation. It has great potential to circumvent the shortcomings of the existing mixed-mode fracture experiments because it can accurately reveal the mechanics near to the crack tip. This research project will respond to this need for a test method to directly measure fracture mode partitions. It aims to develop a methodology to determine the fracture mode partition of a crack by examining its near-crack tip strain field using DIC. This will allow the various partition theories in the literature, including the principal investigator's (PI's) own ones, to be either validated or invalidated, new partition theories to be developed and tested, and the conditions of applicability of a particular partition theory to be determined. There are however several challenges to overcome, in particular related to the application of DIC to this problem and the scales of observation required.

The proposed research has great potential to result in new test standards for the direct measurement of fracture mode partitions, considerably enhancing the knowledge and skills of the structural mechanics research community, and also providing industrial engineers with a method to accurately characterise the mixed-mode failure behaviour of the laminated materials they use. The in-depth physics of the fracture mechanics of advanced composite materials will be revealed, which will contribute towards their full potential being harnessed without over-design against the danger of delamination.

Composite materials are very important to the aerospace industry because they maximise weight reduction in aircraft as well as providing several other advantages, for example, reduced fuel consumption. Although composite materials already provide great benefits to the aerospace industry, their full potential is not currently being realised due to their susceptibility to delamination during fabrication, assembly, and service. To combat the problem of delamination, composite parts are often over-designed and this increases the cost, weight and volume of a structure. This is in great part because the fracture toughness of a laminate is not easily predicted while the consequences of delamination are severe. The fracture toughness of a laminate depends on the mode partition of a fracture, that is, the relative proportions of each fracture mode, and is not an intrinsic material property. Determining the fracture mode partition has turned out to be a complex and highly controversial problem but nevertheless, one that is very important to solve.

This research project will respond to this need. It aims to develop a methodology to determine the fracture mode partition of a crack by examining its near-crack tip strain field using digital image correlation (DIC). This will allow the various partition theories in the literature, including the principal investigator's (PI's) own ones, to be either validated or invalidated, new partition theories to be developed and tested, and the circumstances under which a particular partition theory is and isn't appropriate to be determined.

The practical output of the proposed research has great potential to result in new test standards for the direct measurement of fracture mode partitions. This would considerably enhance the knowledge and skills of the research community of structural mechanics. It would also provide industrial engineers with a method to accurately characterise the mixed-mode failure behaviour of the laminated materials they use. Further knowledge output created from the project will explicitly reveal the in-depth physics of fracture mechanics of advanced composite materials, which would contribute towards their full potential being harnessed without over-design against the danger of delamination. Making full use of the unique properties of composites will result in new products, improvements to existing products and the opportunity to respond to demand for composites in new sectors and/or applications.

A number of companies are currently investing heavily in composite research, for example Airbus and Rolls Royce. Composites research has been identified as a national priority area by both the Department for Business Innovation and Skills and the Technology Strategy Board (TSB). All of these interested parties recognise that new composite technologies will need to be developed to meet future needs. The results from the proposed research will clearly be of great interest to them.

To reach the academic and industrial audiences, the results will be: (1) Presented in a plenary lecture at the 18th International Conference of Composite Structures (ICCS18). (2) Published in at least one article in a prestigious journal. (3) Presented in a seminar at the Faculty of Engineering at the University of Porto (FEUP). (4) Circulated in reports to Technical Committee 4 of the European Structural Integrity Society (ESIS TC4). (5) Disseminated at international events and workshops, including the twice-yearly meeting of ESIS TC4 but also, for example, the events and meetings organised by the National Composites Network (NCN). (6) Potentially included in the 2 chapters of an AIAA book on the subject of mixed-mode partitioning, which the PI is currently preparing to contribute.

To further reach an industrial audience, the PI intends to communicate with companies that work with composite materials to organise visits, to deliver presentations, and to distribute reports.