This technical publication explains the innovative ‘invariant-based’ approach as a much more effective way to design and test composite materials than the traditional building block method. This comprehensive textbook was written by two eminent research academics from the Department of Aeronautics and Astronautics at Stamford University in the USA: Prof. Stephen W. Tsai is Emeritus Professor, Structures and Composites Laboratory (SACL) and Dr. Jose Daniel D. Melo, a Visiting Scholar in Aeronautics & Astronautics at Stamford.
Tsai and Melo’s new concept of using only a trace based invariant approach to stiffness analysis, and the omni-envelope based approach to strength analysis of CFRP laminated composites, is a very efficient and accurate way to determine stiffness and strength properties of composite laminates, so shortening the design and validation cycle time of new products using composite materials. The authors demonstrate that using trace-based technology now makes the design and testing of composites no different or more difficult than for metals.
Instead of hundreds of coupons as the building blocks of a strength pyramid, Tsai and Melo’s new trace based approach explains the need to test only one block. This single block is the uniaxial tensile and compressive stiffness and strength of ; the unidirectional  specimen (the master coupon) provides baseline data for the strength, stiffness and failure analysis of material laminates with similar configurations. One panel of  can instantly map out the predicted stiffness and strength of an ‘as-built’ laminate. According to the authors, testing unidirectional  specimens should provide enough information to predict the failure envelope of a laminate. Then, laminates can be tested “as-built” and data compared to the predicted properties using a master stress-strain template, which greatly reduces the number of tests required. This approach also enables as-built laminate data to indicate the quality of the processing and the effects of any defects.
The authors use “logic and ingenuity” to rate as-built laminates in their lamination efficiency in a master stress-strain template. Templates are easy to build (in Excel) and to compare directly different laminates and different materials. Using this innovative methodology, improvements in materials and processes can be demonstrated from test data obtained in minutes instead of having to wait for specimen laminate test data that can take days if not weeks to produce. With fewer specimens to be tested, emphasis can be placed on the quality of laminates as reflected by the conversion of stiffness and strength from processing and its associated defects. Environmental effects such as hot-wet conditions can also be accounted for through their respective trace values.
This new approach to composite materials design and testing offers significant benefits for testing laboratories, reducing the number and complexity of tests, as well as for design engineers using simulation analysis techniques, speeding up the NPD process and creating new opportunities to test and develop new composite materials and processes.
Publication date: 2015
• Stress-strain relations
• Ply stiffness
• In-plane stiffness
• Flexural stiffness of symmetric laminates
• Stiffness of general laminates
• Failure criteria
• Strength of laminates
• The invariant-based approach to stiffness
• The invariant-based approach to strength
• New opportunities in design & testing and mechanical testing of composites
About the authors:
Stephen W. Tsai, is Emeritus Professor, Structures and Composites Laboratory (SACL), Department of Aeronautics and Astronautics at Stamford University, USA, where he has been since 1990. He has a BS degree in Mechanical Engineering (1952) and D.Eng (1961) postgraduate degree, both from Yale University. Before moving to Stamford University, he worked at the US Air Force Materials Laboratory from 1968 until 1990. He is a Member of US National Academy of Engineering.
Daniel Melo has been Vice President of the Federal University of Rio Grande do Norte – UFRN, BRAZIL since May, 2015. From July 2011 to May 2015 he served as Dean of the School of Engineering and Architecture, at UFRN. Prof. Jose Daniel Melo received his Ph.D. in Mechanical Engineering from Colorado State University (2002), M.S. in Mechanical Engineering from the University of Maine (1993), B.S. degrees in Mechanical Engineering (1990) and in Civil Engineering (1997) from the Federal University of Rio Grande do Norte. He has been a Visiting Scholar at the Department of Aeronautics and Astronautics of Stanford University since 2008.
Prof. Melo has worked in the field of Composite Materials for over 20 years. His main research interests and contributions are in mechanical characterization and failure of polymer composites.