Strength and Failure Characteristics of SMC-R Composites under Biaxial Loads

Sheet molding compound composites containing randomly oriented short fibers (SMC-R) are among the most commonly used composites in the automotive and many non-aerospace applications. They are also finding a few niche applications in the aerospace industry. Many studies have reported on static and fatigue properties of SMC-R composites under uniaxial loading conditions. However, there are many applications in which they may be subjected to biaxial loads and their biaxial properties have not been reported in the literature. This study considers the strength and failure characteristics of SMC-R under biaxial loading conditions that were generated using various combinations of normal and shear stresses ranging from uniaxial tension to shear. Glass fiber and Carbon fiber SMC-R were the materials used for quasi-static tests and carbon fiber SMC-R was used for fatigue tests. In addition to determining the strength properties, this study also includes damage development process and failure prediction under biaxial loading. Finite Element Analysis was used to understand and verify the stress distribution in the specimen. It was observed that the presence of shear stress decreases the tensile stress at failure for both glass and carbon fiber SMC-R. Depending on the stress biaxiality ratio, macroscopic damage development in both materials was initiated at 95 to 99% of the peak load. For both materials, a knee load was observed above which the material behaved non-linearly. Finite element analysis confirmed the damage development location in quasi-static tests. The biaxial failure load prediction seems to follow Hill’s anisotropic yield criterion. The carbon fiber SMC-R exhibited a high degree of scatter in both quasi-static strength and fatigue life. Weibull analysis was performed to determine its strength characteristics of this material.Master of Science in EngineeringAutomotive Systems Engineering, College of Engineering and Computer ScienceCollege of Engineering and Computer ScienceUniversity of Michigan-Dearbornhttps://deepblue.lib.umich.edu/bitstream/2027.42/138099/1/Strength and Failure Characteristics of SMC-R Composites under Biaxial Loads.pdfDescription of Strength and Failure Characteristics of SMC-R Composites under Biaxial Loads.pdf : Thesi

Analysis of Tubular Adhesive Joints in Aluminum and Composite Structures under Crush and Tensile Loads

As the automotive industry moves toward developing lightweight crashworthy structures, it is expected that a multi-material solution involving steels, aluminum alloys and high-performance composites will become increasingly common in future vehicles. Joining a variety of materials with different physical, mechanical, and thermal characteristics is one of the major challenges for such multi-material designs. Adhesive joining is emerging as one of the key joining methods in multi-material structures, since in general, adhesives are compatible with most materials under consideration for lightweight vehicles. There are many body, chassis and powertrain components in vehicles that are designed with tubular sections. A few examples of these components are the front rails, underbody frames or sub-frames, instrument panel crossbeams, drive shafts and spaceframe structures. Increasing use of hydroforming and closed-section extrusions will lead to even more use of tubular sections, especially in crush-resistant components, such as front rails and roof rails. Tubular joints are also used in buses and other heavy vehicle constructions. Unlike the seam adhesive joints between thin sheets or panels, there has not been much research and design studies on tubular adhesive joints in which a tube is fitted in another tube of the same material or different materials. In a crash condition, tubular structures are designed to crush in a controlled manner. In addition to the crush mode, crush energy absorption and peak crush load are the two most critical parameters to consider for improved crashworthiness. If the tubular structure is made of adhesive joints, it is important that the joint failure does not occur before controlled crushing of the joined tubes. The crush characteristics are affected by joint geometry and material properties. Hence, the key objective of this research is to develop a crush resistant tubular adhesive joint in aluminum-aluminum, composite-composite, and composite-aluminum structures using finite element analysis. A Design of Experiments approach is used to understand the interactions between different joint parameters and their effects. Since such tubular structures are likely to be subjected to different forms of loading, the dissertation aims to present optimal tubular adhesive lap joint design choices for maximum energy absorption under crush load and joint failure strength under tensile load using finite element analysis.Ph.D.College of Engineering & Computer ScienceUniversity of Michigan-Dearbornhttp://deepblue.lib.umich.edu/bitstream/2027.42/170776/1/Monish Ramakrishnan Final Dissertation.pdfDescription of Monish Ramakrishnan Final Dissertation.pdf : Dissertatio

Static and fatigue behavior of a carbon-fiber SMC-R composite under combined tensile and shear stresses

Randomly oriented short fiber reinforced sheet molding compound composites (SMC-R) are among the most commonly used composites for automotive and many non-automotive structural applications. Many studies have reported the static and fatigue properties of SMC-R composites under uniaxial stress conditions. However, there are many applications in which they may be subjected to biaxial stress conditions and their biaxial properties have not been reported in literature. The current study considers the static and fatigue characteristics of a randomly oriented carbon fiber SMC-R under both uniaxial and biaxial loading conditions with various combinations of tensile and shear stresses. A butterfly shaped Arcan specimen was used to conduct the biaxial stress tests. Since both static strength and fatigue life data exhibited significant variability, statistical analysis was conducted using Weibull distribution to analyse the variations in these properties of the material. In biaxial conditions in which a shear stress is imposed along with a tensile stress, the tensile strength of the material decreases with increasing shear stress. A failure envelope drawn based on von Mises equation shows reasonable agreement with the experimental results. As a percentage of the static strength, the fatigue strengths at 1 million cycles in tension and combined equal tension and shear are significantly higher than the fatigue strength in pure shear.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/172327/1/pc26545.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/172327/2/pc26545_am.pd