Roger J. Morgan

The effect of moisture on the physical and mechanical integrity of epoxies

The effect of specific combinations of moisture, heat, and stress on the physical structure, failure modes, and tensile mechanical properties of diaminodiphenyl sulphone (DDS)-cured tetraglycidyl 4,4′diaminodiphenyl methane (TGDDM) epoxies [TGDDM-DDS (27 wt% DDS)] are reported. Sorbed moisture plasticizes TGDDM-DDS epoxies and deteriorates their mechanical properties in the range 23 to 150° C. Studies of the initiation cavity and mirror regions of the fracture topographies of these epoxies indicate that sorbed moisture enhances the craze initiation and propagation processes. The effect of tensile stress-level, applied for 1 h on dry epoxies, on the subsequent moisture sorption characteristics of the epoxies was also investigated. Such studies indicate that the initial stages of failure that involve both dilatational craze propagation and subsequent crack propagation enhance the accessibility of moisture to sorption sites within the epoxy to a greater extent than in the latter stages of failure which involve crack propagation alone. The amount of moisture sorbed by TGDDM-DDS epoxies is enhanced by ∼ 1.6 wt% after exposure to a 150° C thermal spike, as a result of moisture-induced free volume increases in the epoxies that involve rotational—isomeric population changes.

The microscopic failure processes and their relation to the structure of amine-cured bisphenol-A-diglycidyl ether epoxies

Electron and optical microscopy are used to study the relation between the structure and the microscopic flow and failure processes of diethylene triamine-cured bisphenol-A-diglycidyl ether epoxies. By straining films directly in the electron microscope, these epoxies are found to consist of 6 to 9 nm diameter particles which remain intact when flow occurs. It is suggested that these particles are intramolecularly crosslinked molecular domains which can interconnect to form larger network morphological entities. Epoxy films, either strained directly in the electron microscope or strained on a metal substrate, deform and fail by a crazing process. The flow processes that occur during deformation are dependent on the network morphology in which regions of either high or low crosslink density are the continuous phase. The fracture topographies of the epoxies are interpreted in terms of a crazing process. The coarse fracture topography initiation regions result from void growth and coalescence through the centre of a simultaneously growing poorly developed craze which consists of coarse fibrils. The surrounding smooth slow-crack growth mirror-like region results from crack propagation either through the centre or along the craze—matrix boundary interface of a thick, well developed craze consisting of fine fibrils.

The structure, modes of deformation and failure, and mechanical properties of diaminodiphenyl sulphone-cured tetraglycidyl 4,4'diaminodiphenyl methane epoxy*

The tensile mechanical properties of diaminodiphenyl sulphone (DDS) — cured tetraglycidyl 4,4′diaminodiphenyl methane (TGDDM) epoxies [TGDDM-DDS (12 to 35 wt% DDS)] are reported as a function of temperature and strain rate. TGDDM-DDS (20 to 35 wt% DDS) epoxies, which exhibit broadTgs near 250° C, are not highly cross-linked glasses because diffusional and steric restrictions limit their cross-link density. TGDDM-DDS (10 to 20wt% DDS) epoxies are more brittle with lowerTgs as a result of lower molecular weights and/or lower cross-link densities. Electron diffraction and X-ray emission spectroscopy studies indicate that TGDDM-DDS (>25wt% DDS) epoxies contain crystalline regions of unreacted DDS which can be eliminated from these epoxies during cure resulting in microvoids. TGDDM-DDS (12 to 35wt% DDS) epoxies predominantly deform and fail in tension by crazing, as indicated by fracture topography studies. These glasses also deform by shear banding as indicated by right-angle steps in the fracture topography initiation region and mixed modes of deformation that involve both crazing and shear banding. No evidence was found for heterogeneous cross-link density distributions in TGDDM-DDS (15 to 35wt% DDS) epoxies on straining films in the electron microscope.

Characterization of Microcrack Development in BMI-Carbon Fiber Composite under Stress and Thermal Cycling

The objective of this research was to determine the effect of thermal cycling on the development of microcracks in bismaleimide (BMI)-carbon fiber composites (5250-4 RTM/IM7 4-harness satin weave fabric). By clamping composite specimens on the radial sides of half cylinders having two different radii (78.74 and 37.96 mm), two different strain conditions with respect to the neutral axis (0.406 to 0.406% and -0.843 to 0.843%) were applied to the composites. Three different thermal cycling experiments: (1) -196 to 250°C, (2) 23°C to (i) 150°C, (ii) 200°C, (iii) 250°C, and (3) -196 to 23°C were performed as a function of stress, number of thermal cycles, heating or cooling rate, and humidity conditions. An in situ monitoring microscope was used to observe the microcrack development during the experiment. The results suggest that there is a higher probability of microcracking with increasing number of thermocycles, higher prestrain, and humidity. The principle findings are that the full cycles from 196 up to 250°C cause the most significant microcrack development. Observations indicate that the high-temperature portion of the cycle under load causes fiber–matrix interface failure. Subsequent exposure to higher stresses in the cryogenic temperature region results in composite matrix microcracking due to the additional stresses associated with the fiber–matrix thermal expansion mismatch.

An initial and progressive failure analysis for cryogenic composite fuel tank design

Thermal residual stresses, internal pressure stresses, and acceleration stresses during launch were evaluated and quantified for cryogenic composite fuel tank design. Both failure initiation and progression of graphite/epoxy laminate system (IM7/977-2) [0/90/90/0/0/90]s and graphite/BMI laminate system (IM7/5250-4) [0/90/90/0/0/90]s were investigated using the non-isothermal classical laminate and plate theory (CLPT) and the maximum stress failure criterion. The thermal residual stresses in the transverse direction are the dominant stresses on each ply in the launch stage. After initial ply cracking, through-the-thickness temperature change of a laminate related to fuel leakage as well as a laminate stiffness matrix change was applied to the progressive failure analysis. The fuel leakage-based progressive analysis shows a higher number of initial ply cracking does not necessarily mean a higher chance of matrix cracking in all plies. The graphite/BMI laminate has such an advantage as transverse thermo-mechanical resistance over the graphite/epoxy laminate at an initial exposure to —253°C and 1500 kPa. In terms of complete laminate matrix cracking, however, the graphite/ epoxy laminate is more resistant to transferring stresses to other plies than the graphite/BMI laminate.

Modes of deformation and failure of polycarbonate

Abstract Electron and optical microscopy studies of the modes of deformation and failure of polycarbonate are reported. The high toughness of glassy polycarbonate is controlled by the ease of shear-band deformation and the surface craze characteristics. Such crazes form in tension prior to macroscopic necking and cold-drawing and serve as sites for ultimate fracture. The surface craze characteristics and the role they play in the fracture processes are reported as a function of strain-rate (10−2−10+2 min−1) from scanning electron microscopy studies of the fracture topographies and edges of polycarbonate specimens fractured in tension at room temperature. The mechanism by which surface crazing in polycarbonate is enhanced by handling is also reported. The surface regions that come into contact with islands of finger-grease are plasticized, and fabrication stresses within these regions relax near Tg at a faster rate than in the unplasticized surroundings. Microcracks which are produced at the boundary between the plasticized and unplasticized regions serve as sites for craze initiation and growth. The craze processes in thin polycarbonate films strained directly in the electron microscope are also reported. Undeformable ∼10 nm sized nodular regions were observed during the craze flow processes in these thin films.

The failure processes, morphology and mechanical properties of a co-polyimide glass based on benzophenone tetracarboxylic acid dianhydride

The phyiscal structure, failure processes and mechanical properties of solution-soluble copolyimide films based on benzophenone tetracarboxylic acid dianhydride are reported as a function of sample preparation. The failure processes and mechanical response are modified by the presence of residual solvent and microvoids, which are produced by the elimination of solvent clusters from the glass. The polyimide is amorphous, with the exception of a few isolated clusters of poorly formed spherulites and networks of 50 to 500 nm wide lamellae. The deformation modes observed when thin films were strained directly in the electron microscope were crazing, shear-band deformation and an edge-yielding phenomena. Edge-yielding, which has characteristics of both crazing and shear-banding, occurred in ∼1 μm wide bands which were 20 to 30° to the tensile stress direction. Shear-band deformation occurred in fine ∼-100 nm wide bands, which exhibited a sharp boundary between themselves and their surroundings. TEM indicated that the shear strain was uniform within these bands. Microvoids, 1.5 to 15 nm diameter, were found to initiate shear bands some of which were ∼ 1 nm wide. These bands increased in width by tearing at the microvoid initiation sites.

Characterization of the Electron Beam Curing of Cationic Polymerization of Diglycidylether of Bisphenol A Epoxy Resin

The characterization of electron beam (E-beam) curing of diglycidyl-ether of bisphenol A-diaryliodonium hexafluoroantimonate epoxy resin-initiator system is reported as a function of (i) diaryliodonium hexafluoroantimonate catalyst (initiator) concentrations of 0.1-10 parts per hundred (phr) and (ii) total electron beam doses of 5-150 kilogray (kGy). The in situ E-beam temperature of the resin is monitored as a function of dose-time characteristics. The degree of cure is monitored after radiation exposure by Fourier transform infrared spectrometry (FTIR) and the glass transition temperatures (Tg) by differential scanning calorimetry (DSC). The degree of cure and cure rate increased with total dose exposure and initiator concentration. The maximum cure rate occurred at 5 kGy exposure and, thereafter, decreased as reactive species concentration decreased. The maximum in situ E-beam temperature of 76°C was recorded for the resin containing 10 phr of initiator, with a maximum degree of cure of 94% and a glass transition temperature of 86 C, indicating that the cure reactions under E-beam are glassy state diffusion controlled. The resin glass transition temperatures are considerably lower than the thermally cured glass transition temperatures of 170 C because of H2O termination reactions at the lower E-beam cure temperatures that result in a poor cross-linked network. In addition, the diaryliodonium hexafluoroantimonate catalytic activity for epoxide cationic polymerization is retarded by H2O. E-beam exposure causes the diaryliodonium hexafluoroantimonate to dissociate into active catalytic species, such as HSbF6, well below 100°C compared to catalytic thermal induced dissociation near 200°C. The E-beam cure reaction rate is modeled as a function of degree of cure and dose exposure by a standard autocatalytic kinetic model.

Transverse Cracking of M40J/PMR-II-50 Composites under Thermal—Mechanical Loading Part II — Experiment and Analytical Investigation

In this study, the effects of thermal cycling combined with mechanical loading on the microcracking of M40J/PMR-II-50 are investigated. Characterization of the failure mechanisms are conducted based on the critical parameters which cause composite microcracking, as presented in Part I. Based on the test results in Part I, the tests with intermediate in-plane lamina strain (0.175—0.350%) and an increased number of thermal cycles are added. Elevated temperature thermal cycling (23—250°C) is also added to the original test plan to investigate the thermal cycling temperature amplitude effect on microcracking of the composites. Observations indicate that the elevated temperature exposure under mechanical loads causes an easy fiber/matrix debonding. Subsequent exposure to cryogenic temperatures results in fiber/matrix debonding due to the high thermal stresses associated with fiber/ matrix thermal expansion mismatch. Crack propagation under cryogenic exposures is shown to be dominant with an increasing number of thermal cycles, especially when combined with high temperature exposure associated with high amplitude of cyclic thermal stresses.

Transverse Cracking of M40J/PMR-II-50 Composites under Thermal—Mechanical Loading: Part I — Characterization of Main and Interaction Effects using Statistical Design of Experiments:

In this study, a novel conduction heating-based thermal cycling apparatus combined with large deflection bending is developed and utilized to identify the critical controlling parameters for microcracking of [90/0]1s, M40J/ PMR-II-50 high modulus carbon fiber/polyimide composite laminate under synergistic environmental conditions. The synergistic test involves four controlling parameters namely, average in-plane mechanical strains (0 and 0.488%), thermal cycling temperature amplitudes (—196—23 °C and —196—250°C), number of thermal cycles (1 and 8), and heating rate (1 and 4°C/min). The 2k factorial design is used for the four factors to provide their quantitative primary and interaction effects on crack density with a minimum number of experiments. The experimental results indicate that the number of thermal cycles is the primary controlling factor (41%), while the thermal cycling temperature amplitude (25%) or the in-plane strain (22%) is the secondary factor. The number of thermal cycles also exhibits a significant interaction effect on the development of microcracks when it was combined with either the temperature amplitude of thermal cycling (7%) or mechanical in-plane strain (5%).