WOLFGANG G. KNAUSS

Professor of Aeronautics and Applied Mechanics

e-mail: wgk@aero.caltech.edu

B.S., California Institute of Technology, 1958.
M.S., California Institute of Technology, 1959.
Ph.D., California Institute of Technology, 1963.

Field of Study

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.

For a short list of publications by Professor Knauss and his group, Selected Publications.

Figure

High resolution moire interferometry of crack parallel displacements. Displacements between adjacent fringes = 0.83 microns. The width of the picture represents 1 cm.

Evolution of voids at the tip of a crack in a polymer in the process of (discontinous) crack propagation. The width of the picture represents about 6 mm.