High strain rate compressive response of porous metal matrix composites

Vehicle weight reduction is an important priority for civilian and military automobiles. Increasing prices of fuel and awareness for environmental concerns for the emissions make it necessary to reduce the vehicle weight and achieve higher fuel efficiency. In military vehicles weight reduction is also required to accommodate heavy armor, increase mission length, and increase payload. While new lightweight materials and technologies are required to meet the goals of weight reduction, the new focus on safety and security takes a front seat in developing and selecting materials for these applications. Energy absorption in automotive parts during crash is an important design criterion. Most existing knowledge about material properties and energy absorption capabilities is based on quasi-static compression. The present work is aimed at extending the present science and technology by (a) exploring new lightweight materials that are relevant to such applications and (b) understanding strain rate sensitivity in mechanical properties, energy absorption capability, and failure mechanisms of such new materials under dynamic compression. The material characteristics are studied in the compressive strain rate range of 10 -4--3x103 s-1. A method for intermediate strain rate range compression, usually 10--1000 s-1, is designed and validated for several material types. The study includes three metal matrix composite systems filled with hollow particles. Such materials are called syntactic foams. Aluminum alloys and magnesium alloys are selected as two promising matrix material systems in syntactic foams. In addition, steel based syntactic foams are also studied because of widespread use of steel and possibilities of weight reduction through incorporation of hollow particles. The test instrumentation and methods developed in this work are used to also study polyvinyl chloride based closed cell polymer foams. These compliant materials are very challenging to study and applicability of developed procedures on these materials significantly enhances confidence in the developed test protocols. The results show that compressive strength, energy absorption, and failure mode can be strain rate sensitive. It is not necessary that all these properties show strain rate sensitivity at the same time. The failure mode is observed to change remarkably in magnesium alloys and composites at high strain rates. Energy absorption mechanisms change in aluminum alloys at high strain rates and mechanical properties of steel syntactic foams change with strain rate. These observations need to be incorporated in the design process when applications of such materials are developed.