Prof. Wolfgang Knauss
Professor of Aeronautics and Applied Mechanics
Wolfgang Knauss received his B.S., M.S. and Ph.D. from the California Institute of Technology. He has received the following awards: Woodrow Wilson Foundation Fellowship, the National Aeronautics and Space Administration Fellowship, Tau Beta Pi Engineering Honorary Society, Elected Fellow, Institute for the Advancement of Engineering, 1971; National Academy of Sciences Lecturer to the U.S.S.R., 1977; Elected Fellow, National Academy of Mechanics, 1980; Senior U.S. Scientist Award by the Alexander von Humboldt Stiftung, 1986/87; Fellow, Society for Experimental Mechhanics, 1987; Murray Medal, Society of Experimental Mechanics, 1995; Fellow, American Society of Mechanical Engineers, 1996; Corresponding Member of the International (Russian) Academy of Engineering, 1996.
Application of Scanning Tunneling Microscopy to Problems of Interfacial
Strength Design in Composite Structures
The mechanical strength of interfaces is the basis of composite material
strength. Because the region where the material properties of the two solids
making up the interface is very small (micron and submicron scale),
mechanics related measurements are difficult to perform since optics can no
longer serve for observation purposes. Accordingly, electron (tunneling)
microscopy is being developed to perform observations at the submicron
range. These developments are of interest in the evolution of high strength
composite materials for aircraft/rocket designs as well as for
microelectronic devices subjected to a variety of environmental influences
in the manufacturing process.
Constitutive Behavior of Matrix Materials for High Temperature Composites
High-speed flight is the most important driver for aerospace engineering in
the next decades. High-speed is invariably linked to the exposure of
structures to high temperatures, and thus the rush is on for raising the
temperature capabilities of structural composites. Polymer based composites
are targeted for use in the 600/700=B0F range, with an important limitation
set by creep and related viscoelastic failure behavior. Both structural
creep as well as time-dependent fracture are governed by the high
temperature viscoelastic behavior of the matrix material.
In order to make the development and use of such high temperature composites
efficient requires the analytical characterization of the polymers in order
to compute micromechanical characteristics and failure processes. This
knowledge is particularly important for understanding the fracture behavior
of these matrix materials and the composites into which they are
incorporated. This (NASA) program is intended to develop such constitutive
description for the next generation of polymer based aerospace materials.
Fracture Behavior of Non-Linearly Viscoelastic Solids Related to Adhesive
Bonding in Solid Propellant Rockets (Shuttle Booster)
One of the areas of mechanics, which is currently attracting much interest,
is that of interface separation between joined solids. For time-independent
behavior, the motivation for this interest comes from a need to understand
the fundamentals of internal cohesion in composites which exhibit a large
amount of interfacial contact between its separate phases as well as from
the failure mechanics of microelectronics, the increasing complexity of
which call for a proportionately increasing need to understand their failure
mechanics. The construction of solid propellant rocket motors depends
similarly to a large degree on one's ability to bond the propellant charge
to the rocket casing.
Fatigue of Thermoplastic Matrix Materials for Composites
Although thermoplastic matrix materials are hailed as being very tough in
composite applications, we possess very little fundamental knowledge about
their behavior under fatigue loading. To gain insight into the fatigue
failure process, the development of microscopic energy absorption processes
are studied at the tip of propagating cracks through the development of
"crazes". Observations of this crack tip through a microscope are recorded
and processed by computer in real time to measure crack growth with a
resolution of 1 micron; simultaneously changes in the craze structure at the
crack tip as observed by optical interferometry are monitored to assess the
degradation of the craze material to examine how the material at the crack
tip breaks down under repeated loading and with slow crack growth.
Time-Dependent Buckling of Structures Made of Fiber-Composites
The introduction of thermoplastic, tough matrix materials into composite
design brings with it an increased sensitivity to time-dependent or delayed
failure. This phenomenon is heightened by the sensitivity of these materials
to accelerated creep under even moderate temperature increases
(100-150=B0C). This study is concerned with the gradual occurrence of
buckling in composite structure because of the viscoelasticity of its matrix
component. The delayed buckling may occur either in a gross structural mode
or at the fiber level (compression crimping). Non-uniform temperature
distributions through the skin of a high speed aircraft will be particularly
detrimental because it not only accelerates the creep process in the hot
part of the skin but also contributes to the out-of-plane deformation which
strongly lowers the in-plane load needed to cause structural instability.
Failure of and Crack Propagation in (Particulate) Composites Incorporating
Microdamage in High Deformation Gradients
There are many materials which fail through crack propagation, but in which
the earlier stages of failure are identified by the evolution of many
microfractures distributed spatially in high strain regions which ultimately
become the failure regions. Failure is then the result of the coalescence of
these microflaws into a macroscopic fracture. A basic problem is to
characterize the behavior of the disintegrating and increasingly
discontinuous material, riddled with microcracks, in terms of continuum
concepts. It is the purpose of this study to deal with this
discrete/continuous characterization on both the analytical and experimental
basis.
Adhesion and Interfacial Fracture Mechanics
One of the most dominating issues for the strength of future
high-strength/low weight materials is the characterization and performance
of the interface between the two or more phases making up the composite. The
toughness of the composite is most strongly influenced by the interface
strength, though the highest value for the latter does not necessarily
produce the best composite.
Similarly, the adhesive bonding of aerospace structures requires a markedly
improved understanding of the interfacial fracture process before designs
are to benefit from that very promising weight-saving technology. Issues to
be examined relate to methods of characterizing interfacial strength, the
development of fracture criteria to aid the structural designer. These
developments are to emphasize time dependent processes (viscoelasticity and
fatigue) in order to impact long term durability (tens of years) based on
short term (laboratory) evaluations. The program anticipates drawing on the
results from the Scanning Tunneling Microscopy to address questions of
interfacial strength in composites at the submicron level.
Geometry-Induced Failure of Composite Structures for Future Aircraft
Besides inventing new and strong composite materials, their use in future
aerospace designs requires new concepts of design that are different from
those associated with metallic structures. For many structural problems the
spanning of the size scale between the material microstructure and the
macroscopic dimensions of a full scale structure requires a failure
characterization at the macroscopic level but with a full understanding of
the micromechanics of the failure process involved. Thus a new way of
characterizing the failure behavior of these types of materials needs to be
devised which, while recognizing the phenomena at the microscale, cast the
failure behavior into more macroscopic concepts. This problem is known in
the industry as the "Problem of Scaling". It is particularly important in
the class of geometries that involve sharp dimensional changes within
structural components, e.g., stiffeners on skins, panel reinforcements,
junctions of struts, etc.
Recent Publications
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