Engineering topics

Bakis, Charles E.

Publications and source records attributed to Bakis, Charles E..

7 records

DOE OSTI2020

Comparison of woven and stitched out-of-autoclave E-glass/epoxy composites subjected to quasi-static and cyclic tensile loads

Tensile quasi-static and fatigue behaviors of out-of-autoclave, quasi-isotropic, E-glass/epoxy composites with woven and stitched fiber architectures are compared. Some specimens were cyclically loaded to failure, while others were tested for residual modulus and ultimate strength. Both laminates survived 10 7 cycles at a maximum stress of 94.8 MPa but failed prior to 10 7 cycles at a maximum stress greater than or equal to 124.1 MPa. For the stress-lifetime diagram and residual strength, the woven laminate had superior properties below 10 4 cycles and the stitched laminate had superior properties above 10 5 cycles. While intralaminar crack density was higher in the stitched laminate, the woven laminate demonstrated a higher percentage of delaminated area.

Materials Science↗

DOE OSTI2019

Sandwich panels with battery cores

Systems and methods are disclosed for enhancing the construction and application of batteries by providing a battery system comprising at least one electrochemical panel with at least one electrochemical cell disposed therein, which is configured to enable multi-purpose functionality and applicability of the battery system. Embodiments provide for electrodes and/or current collectors of each electrochemical cell that are transversely orientated with respect to each face of an electrochemical panel. Some embodiments provide for a separator pouch disposed about at least one electrode and/or current collector to electrically insulate the electrode and/or current collector from a structure component of the device. Each electrochemical panel is configured to enable electrical communication with another electrochemical panel and/or an ancillary electric circuit. Each electrochemical panel is configured enable utilization thereof as a structural component of an ancillary structure.

Rahn, Christopher D.↗

DOE OSTI2019

Transverse Young's modulus of carbon/glass hybrid fiber composites

Hybrid fiber composites offer designers a means of tailoring the stress–strain behavior of lightweight materials used in high-performance structures. While the longitudinal stress–strain behavior of unidirectional hybrid fiber composites has been thoroughly evaluated experimentally and analytically, relatively little information is available on the transverse behavior. The objective of the current investigation is to present data on the transverse modulus of elasticity of unidirectional composites with five different ratios of carbon and glass fiber and to compare the data with predictive and fitted models. The transverse modulus increases monotonically with the proportion of glass fiber in the composite. Finite element analysis was used to evaluate different ways to model voids in the matrix and allowed the unknown transverse properties of the carbon fibers to be backed out using experimental data from the all-carbon composite. The finite element results show that the transverse modulus can be accurately modeled if voids are modeled explicitly in the matrix region and if modulus is calculated based on stress applied along the minimum interfiber distance path between adjacent fibers arranged in a rectangular array. The transverse modulus was under-predicted by the iso-stress model and was well predicted by a modified iso-stress model and a modified Halpin–Tsai model.

Materials Science↗

DOE OSTI2019

Reactive Molecular Dynamics Simulations of the Atomic Oxygen Impact on Epoxies with Different Chemistries

Atomic oxygen (AO) is one of the most abundant species present in the lower earth orbit and is responsible for the aggressive degradation of polymers used in spacecraft structures. In this investigation, we use ReaxFF reactive force field molecular dynamics simulations to evaluate the disintegration of several different thermosetting epoxy polymers subjected to hypervelocity AO impact. Our simulations indicate that epoxy with aromatic curative displays higher resistance to the AO impact because of its stable benzene functionality. Decreased cross-linking density and increased simulation temperature both lead to faster disintegration of the polymer. Our simulation results indicate that ReaxFF force field simulations can be a useful tool to evaluate the response of various thermosetting epoxies to AO impact and identify promising candidate materials for spacecraft applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1992

Experimental investigation of cyclic thermomechanical deformation in torsion

An investigation of thermomechanical testing and deformation behavior of tubular specimens under torsional loading is described. Experimental issues concerning test accuracy and control specific to thermomechanical loadings under a torsional regime are discussed. A series of shear strain-controlled tests involving the nickel-base superalloy Hastelloy X were performed with various temperature excursions and compared to similar thermomechanical uniaxial tests. The concept and use of second invariants of the deviatoric stress and strain tensors as a means of comparing uniaxial and torsional specimens is also briefly presented and discussed in light of previous thermomechanical tests conducted under uniaxial conditions.

Ellis, John R.↗

NASA NTRS1989

A test method to measure the response of composite materials under reversed cyclic loads

A test method to measure the response of composite materials under reversed cyclic loads is described. The method approximates the long-term response of a component by permitting the composite specimen to respond to the imposed loads and fail in an unconstrained mode rather than in a constrained mode. The method has been successfully used for the reversed cyclic loading of unnotched and notched graphite/epoxy and graphite/PEEK laminates of various stacking sequences. Included in the paper are monotonic tensile and compressive stiffness and strength data at several points in the fatigue lifetime, and damage development information obtained via X-ray radiography for quasi-isotropic T300/5208 and AS4/PEEK laminates with unloaded circular holes.

Bakis, Charles E.↗

NASA NTRS1989

Effects of matrix toughness on fatigue response of graphite fiber composite laminates

The effect of the matrix toughness on the fatigue response of graphite-fiber-composite laminates was investigated by comparing the fatigue-response results, obtained using the test method of Bakis et al. (1989), for two graphite-epoxy specimens subjected to constant-amplitude fully-reversed loading, namely, (0/ +45/90/ -45)s4 AS4/PEEK samples with drilled center holes and T300-5208 samples. It was found that, although similar matrix damage modes were observed in the two materials, several significant differences occurred in the S-N data and in stiffness data taken over the lives of the two materials, especially at high cyclic stresses.

Simonds, Robert A.↗