JEC COMPOSITES MAGAZINE - Issue #56 - April 2010 - 89

KNOW-HOW
Simulation

Mastering simulation methods and tools for composite design
There is an urgent need to better understand and predict the strength and life of composite materials and structures. JEC and Stanford University teamed up to provide a micromechanics-based simulation method and tools that meet this demand.
PRANAV DHOJ SHAH, PHD CONSULTANT & JOSÉ DANIEL D. MELO, PHD VISITING PROFESSOR DEPARTMENT OF AERONAUTICS AND ASTRONAUTICS STANFORD UNIVERSITY

he use of ultra light-weight and fatigue- and corrosionresistant composite materials and structures would result in enormous benefits, energy savings and durable infrastructures. Fuel economy and environmental pollution are directly related to the weight of transportation systems. Fatigue and corrosion resistance means low or no maintenance. For infrastructures such as bridges and buildings, lighter weight implies simpler and quicker construction. This article assesses the use of science-based prediction of composite strength and life for designing and manufacturing structural components. The methodology aims at enhancing confidence in simulation by making static and fatigue tests of structures predictable. The use of composites has been compromised for years by the use of quasi-isotropic laminates, also known as the “black aluminium syndrome”. Far too many trained-in-metal engineers try to impose their experience and concept in isotropy, allowable strain and fracture mechanics to a highly anisotropic, laminated material system; clearly this approach is archaic. An empirically-based building block approach using a unidirectional ply as the starting material has dominated composite design since its inception fifty years ago. This empiricism needs to be replaced by model-based simulation and the ply-based hierarchy should be substituted by a constituent-based (fibre, matrix and interface) methodology. With this approach, measurements will be simpler in concept and easier to implement while standards will be generic. Thus, simulationaided measurements will deliver more information about the failure process of composites with fewer but more discriminating data. In this constituent-based methodology, the hygrothermal response of a matrix in a ply is assumed to remain the same as in its bulk form. Its behaviour as a function of time and temperature is best characterized by a shift factor function and is assumed to remain the

same for deformation as well as properties like strength and life. From this, multi-scale models can be developed that are scientifically consistent and practical for design. Many current practices of using notched specimens to generate design allowable, proverbial use of knock-down factors and point design for fatigue and damage tolerance are ready to be challenged and replaced. A set of science-based standards can make a quantum jump in the use of composites that will exploit their full potential and provide many unmet societal needs. Composites can be more competitive and more reliable with this new approach than with what has been achieved using current empirically-based methodologies. With this in mind, JEC and Stanford University collaborated to offer a series of design forums and composite design workshops to empower engineers with these advanced concepts and tools in composite design. Some of the concepts and materials covered are illustrated in the following sections.

Static failure
Figure 1 shows the ultimate strength of a smooth laminate (red) and of open hole and slot notched laminates (blue and green respectively). The ultimate strength of a smooth laminate is the upper bound, i.e. intact laminate with no damage. The open circular

Fig. 1: Biaxial strength of smooth and notched composite panels

No56 April 2010 / jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #56 - April 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #56 - April 2010