William R. Blumenthal

Influence of Temperature and Strain Rate on the Mechanical Behavior of Adiprene L-100

The effect of sample thickness, strain rate, and temperature on the mechanical response of Adiprene-L100 is presented. The compressive stress-train response of Adiprene-L100 was found to depend on both the applies train rate; 0.001 {le} {dot {var_epsilon}} {le} 7,000 s{sup {minus}1}, and the test temperature at high-rate; 77 {le} T {le} 298 K. Due to the slow, dispersive wave propagation in Adiprene-L100, thinner sample thicknesses are needed to assure uniform, uniaxial stress conditions within Hopkinson Bar samples; the optimal sample thickness being dependent on test temperature. Decreasing temperature from 298 to 77 K at 3,000 s{sup {minus}1} was found to increase the maximum flow stress in Adiprene-L100 from 10 to {approximately} 210 MPa.High-strain-rate (2000 s{sup -1}) compression measurements utilizing a specially-designed Split-Hopkinson-Pressure Bar have been obtained as a function of temperature from -55 to +50{degree}C for the plastic-bonded explosive PBX 9501. The PBX 9501 high-strain-rate data was found to exhibit similarities to other energetic, propellant, and polymer-composite materials as a function of strain rate and temperature. The high-rate response of the energetic was found to exhibit increased ultimate compressive fracture strength and elastic loading modulus with decreasing temperature. PBX 9501 exhibited nearly invariant fracture strains of {approximately}1.5 percent as a function of temperature at high-strain rate. The maximum compressive strength of PBX 9501 was measured to increase from {approximately}55 MPa at 50{degree}C to 150 MPa at -55{degree}C. Scanning electron microscopic observations of the fracture mode of PBX 9501 deformed at high-strain revealed transgranular cleavage fracture of the HMX crystals.

Determination of Non-Symmetric 3-D Fiber-Orientation Distribution and Average Fiber Length in Short-Fiber Composites

A mathematical procedure is proposed for recovering from image analysis the three-dimensional (3-D) non-symmetric density distribution of fiber orientation and the average fiber length in short-fiber composites. The determination of fiber-orientation distribution and average fiber length is essential for assessing the mechanical and physical properties of a short-fiber composite. The average fiber length can be obtained from one micrograph, while the determination of 3-D fiber orientation requires micrographs from two orthogonal planar faces of a composite sample. In addition, a simple procedure is proposed to obtain the single-angle fiber-orientation distribution from one micrograph. This distribution is often needed for predicting the mechanical and physical properties in the direction normal to the plane of the photographed sample cross section. In processing the fiber-orientation density data obtained from a micrograph, a cumulative fiber-orientation density curve is used to derive the fiber-orientation function, which is simpler and more accurate than the histogram that has been commonly used. Previously reported procedures all assume symmetries in fiber-orientation distribution, and are therefore highly idealized. Without assuming any symmetry, the present procedure improves upon previous works.

Response of aluminium-infiltrated boron carbide cermets to shock wave loading

Shock-recovery and shock-spallation experiments were performed on two compositions of aluminium-infiltrated B4C cermets as a function of shock pressure. Sixty-five per cent volume B4C-Al cermets were recovered largely intact after shock loading up to pressures of ca. 12 GPa which permitted a critical study of the microstructural changes produced by the shock. Significantly, shock loading to between 12 and 13 GPa produced a combination of dislocation debris, stacking faults and deformation twins in a small fraction of the B4C grains. Fragmentation of shock-loaded 80% B4C-Al samples prevented meaningful microstructural investigation. Spall-strength testing also provided indirect evidence for the Hugoniot elastic limits (HEL) of these composites. Spall-strength calculations based on an elastic equation of state for 65% B4C-Al indicated that the elastic regime extended up to shock pressures of ca. 10 GPa, or approximately 65% of the HEL of polycrystalline B4C. A complete loss of spall strength was then observed at the transition to a plastic equation of state at a pressure of 12 GPa which coincided with observations of plasticity within the B4C-substructure. This study demonstrated that composites containing a highly ductile phase combined with a high compressive strength ceramic phase could support high dynamic tensile stresses by resisting the propagation of catastrophic cracks through the brittle ceramic substructure.

A composite reinforced with bone-shaped short fibers

Using a model composite system, the authors have demonstrated the concept that bone-shaped short-fiber composites can yield both high strength and toughness, thus avoiding reliance on the interfacial properties as the limiting factor for improving the strength and toughness of short-fiber composites. The higher yield strength and Young`s modulus of bone-shaped short fiber composites demonstrate that bone-shaped short fibers more effectively reinforce the composite matrix, most likely due to more effective cracking bridging and load transfer. The results suggest that an optimized bone morphology, coupled with a weak interface, has the potential to significantly improve both the strength and toughness of short fiber composites. Further study is underway to optimize the fiber morphology to obtain the best combination of strength and toughness.

Characterization of Nicalon fibres with varying diameters: Part I Strength and fracture studies

Experimental studies have been conducted to examine the strength and fracture behaviour of monofilament Nicalon SiC fibres with diameters ranging from 8 to 22 μm. The effects of varying fibre diameter, flaw location and flaw population on the mechanical response of individual fibres were investigated by recourse to extensive fractographic analysis performed on fibres fractured under tensile loading. Results indicate that variations in fibre diameter influence the apparent fibre fracture toughness (K1c), with higher K1c values observed for decreasing fibre diameters. Observations also suggest that the location of the critical flaw may play a role in the fracture of Nicalon fibres. Tensile strength values are shown to increase as the normalized distance of the critical flaw from the fibre centre increases, while critical flaw population appears to be strongly dependent on location. The ratio of K1c to geometry factor (Y) is observed to remain constant with varying flaw location. In addition to surface flaws, three distinct internal flaw populations are seen to cause fracture in Nicalon fibres. Based on these experimental findings, a statistical characterization of the strength of Nicalon fibres with varying diameters is presented in Part II of this paper.

Influence of temperature on the high-strain-rate mechanical behavior of PBX 9501

High-strain-rate (2000 s−1) compression measurements utilizing a specially-designed Split-Hopkinson-Pressure Bar have been obtained as a function of temperature from −55 to +50 °C for the plastic-bonded explosive PBX 9501. The PBX 9501 high-strain-rate data was found to exhibit similarities to other energetic, propellant, and polymer-composite materials as a function of strain rate and temperature. The high-rate response of the energetic was found to exhibit increased ultimate compressive fracture strength and elastic loading modulus with decreasing temperature. PBX 9501 exhibited nearly invariant fracture strains of ∼1.5 percent as a function of temperature at high-strain rate. The maximum compressive strength of PBX 9501 was measured to increase from ∼55 MPa at 50 °C to 150 MPa at −55 °C. Scanning electron microscopic observations of the fracture mode of PBX 9501 deformed at high-strain revealed predominantly transgranular cleavage fracture of the HMX crystals.

Influence of Temperature and Strain Rate on the Compressive Behavior of PMMA and Polycarbonate Polymers

Compression stress‐strain measurements have been made on commercial polymethylmethacrylate (PMMA) and polycarbonate (PC) polymers as a function of temperature (−197C to 220C) and strain rate. A split‐Hopkinson‐pressure bar (SHPB) was used to achieve strain rates of about 2500 s−1 and a servo‐hydraulic tester was used for lower strain rate testing (0.001 to 5 s−1). The mechanical response of these transparent polymers is quite different. The strength of PC is weakly dependent on strain rate, only moderately dependent on temperature, and remains ductile to −197C. In contrast, the strength of PMMA is linearly dependent on temperature and strongly dependent on strain rate. Significantly, PMMA develops cracking and fails in compression with little ductility (7–8% total strain) at either low strain rates and very low temperatures (−197C) or at high strain rates and temperatures very near ambient.

Influence of temperature and strain rate on the mechanical behavior of PBX 9502 and Kel-F 800™

Compression measurements were conducted on plastic-bonded explosive PBX 9502 and its binder, Kel-F 800™, as a function of temperature from −55 °C to +55 °C using an improved split Hopkinson pressure bar at high strain rates (≈1400 s−1) and at low strain rates (≈0.001 to 0.1 s−1) at ambient temperatures. PBX 9502 exhibits lower dynamic compressive strength, but is much less sensitive to strain rate and temperature, than PBX 9501. In contrast, the mechanical response of the Kel-F 800™ binder is stronger than pure (or plasticized) Estane™, but is again less strain rate and temperature dependent. The effects of longitudinal and transverse loading orientations (due to preferred orientation of TATB) and virgin versus recycled TATB on the properties of PBX 9502 are presented.

The tensile strength of short fibre-reinforced composites

Tensile strength is one of the most important mechanical properties of structural short fibre composites, and its prediction is essential for composite design. This paper develops a strength theory for three-dimensionally oriented short fibre-reinforced composites. The contribution of direct fibre strengthening to the composite strength is derived using a maximum-load composite failure criterion. Other strengthening mechanisms, such as residual thermal stress, matrix work hardening and short fibre dispersion hardening are also incorporated into the calculation of composite strength. In the derivation of direct fibre strengthening, the strain and stress of short fibres with different inclination angles were first derived, and the direct fibre strengthening was calculated from the maximum total load these short fibres can carry in the composite loading direction.

Characterization of Nicalon fibres with varying diameters Part II Modified Weibull distribution

Diameters vary significantly in a tow of commercial NicalonTM fibres, which is one of the most attractive ceramic reinforcements for structural composites. It was found that the strength distribution of Nicalon fibres could not be adequately characterized using either single- or bi-modal Weibull distribution. A recently proposed modified Weibull distribution can account for the effect of varying diameter in the characterization of fibre strength. To verify the validity of the modified Weibull distribution, comprehensive mechanical testing and fractographic studies have been conducted on Nicalon SiC fibres with diameters varying from 8 to 22 μm. The experimental results have been reported in Part I. Part II of this paper further modifies the derivation of the modified Weibull distribution to yield a relationship which is similar in form, but soundly based on experimental findings. Factors considered in the modified Weibull distribution include the dependence of fracture toughness and flaw density on fibre diameter, both of which may vary with fibre diameter, as reported in Part I. Comparison with experimental data shows that the current modified Weibull distribution works very well, while both single-modal and bi-modal Weibull distributions are inadequate for describing Nicalon fibres with varying diameters.