Determination of mechanical properties of the bones of the skull

This paper presents the initial results of a research project concerning the mechanism of head injury. In order to begin to define the mechanism, it is necessary to determine mechanical properties of the various skull bones, organize them into constitutive equations, and develop a structural model of the skull. The material presented is concerned primarily with the development of experimental procedures and the results which have been obtained. The specimen-testing program has been split into four parts: (1) The procural of 3/4-in. and 11/2-in.-diam plugs from human skulls at autopsy and the precise determination of specimen location and orientation; (2) the fabrication and strain gaging of small test specimens for basic tension, compression, tension-compression, and shear tests; (3) the conducting of tests; and (4) the correlation of experimental findings with microscopic structure by standard and nonstandard techniques of histology.     

Effects of ultrahigh vacuum on the response characteristics of strain gages

Strain gages are extensively used in spacesimulation research; yet, little if any information has been reported about the environmental effects on the gage installations themselves. Since ultrahigh vacuum affects the physical and chemical properties of materials, one may expect that the response characteristics of strain-gage installations will also be affected. A study was initiated to determine the behavior of a number of strain-gage installations subjected to ultrahigh-vacuum environments. This paper presents the results of the first phase of the program. The gages and adhesives were selected to provide optimum chance for failure in order to establish a time parameter for following tests and, more importantly, to verify the extent and nature of failure possible under the environmental test conditions. Data were obtained on a number of strain-gage-per-formance characteristics. The performances of the gage installations varied widely being dependent in part upon gage and adhesive composition, whether a gage was used as an active or inactive device, and the level of strain to which a gage was subjected. Detailed pre- and post-test examinations showed that there was little permanent damage to any of the installations.     

Dynamic photoelastic and dynamic finite-element analyses of dynamic-tear-test specimens

The transient response of the dynamic-tear-test specimen of a brittle material, Homalite-100, was investigated by dynamic photoelasticity and dynamic finite-element method. The dynamic stress-intensity factors obtained from dynamic photoelasticity and dynamic finite-element analyses were in reasonable agreement with each other. The dynamic finite-element analysis also showed that the dynamic-fracture-initiation toughness could be determined from the dynamicstrain response of a strain gage located near the crack tip in conjunction with a simple static analysis. Dynamic-fracture-toughness vs. crack-velocity relation was also obtained.     

Response of circular clamped plates to square-wave stress pulses

An account is given of an experimental investigation of the reeponse of clamped circular mild-steel plates of various thicknesses subjected to rectangular stress pulses over a small circular region. The stress pulses were transmitted to the plates through a 1/2-in.-diam shock bar and the strain-time responses of the plates were measured. The stress-wave interactions between the bar and the plates were measured for a number of thicknesses and the effect of the applied stress on the extent of the plastic deformation was determined. It was found that the elastic response was accurately predicted by the theory of Sneddon and the plastic response behaved according to a simple modification of this theory. The interaction between the stress pulse and plates of various thickness was theoretically predicted and found to be in excellent agreement with experimental measurements. The final plate deflections were theoretically predicted using a rigid viscoplastic theory and was in substantial agreement with the data. From this theory, the data were analyzed to determine the visco-plastic constant or relaxation time of the material. It is proposed that this testing arrangement is a suitable and convenient method for determining dynamic yield properties under biaxial-loading conditions.     

Elastic waves in truncated cones

An experimental investigation of elastic waves produced by the axial collision of strikers with truncated 2024 aluminum cones with apex angles of 0.48, 5.38, 20, and 30 deg was performed. Wave propagation was initiated at the small end of all four cones and at the large end of the 0.48-deg and 5.38-deg cones. The striker consisted of a 1/2-in.-diam steel ball or a soft phenol-impregnated fiber cylinder. In most cases, impact was caused by firing the striker from an air gun at approximately 1300 ips; in an additional series of tests, a steel ball was dropped on the cone. The metamorphosis of the pulse at the surface of the target was recorded using both foil and semiconductor resistance strain gages. Data were obtained for periods ranging from 200 to 500 μsec; this permitted the observation of several reflections from the ends of the specimen. In several instances, cylindrical aluminum rods were glued to the cone to form a composite target; this permitted observation of the initial pulse incident on the conical section both from surface strain gage and sandwiched crystal records. Studies were also conduced to ascertain the stress distribution across the base of the 20-deg cone. Initial pulse records were employed to predict the surface response in the target using the one-dimensional equation of elastic wave propagation in a cone of infinite length. Reasonable agreement between the data and the results of calculations based on the analysis was obtained.     

Static and dynamic behavior of concrete and granite in tension with damage

A series of dynamic and static tensile-splitting experiments were performed on concrete and granite specimens to investigate the effect of induced damage on their tensile strength. These experiments were performed as part of a larger effort investigating the penetration process into the two materials. The strain rate each specimen was subjected to remained constant for these experiments, while the level of induced damage was increased. Damage was induced into the specimens through repeated drop-weight impacts and quantified using a statistical technique. The dynamic splitting experiments were performed using a split Hopkinson pressure bar (SHPB), while the static splitting experiments were conducted per the ASTM standard procedures D3967 and C496. As part of the investigation, photoelastic dynamic tensile-splitting experiments were also performed to establish the validity of using static relations for the determination of dynamic tensile strength. The experiments showed that the static splitting strength was highly dependent on the orientation of the induced damage with regard to the applied loading; however the dynamic tensile strength decreased with increasing damage with no apparent dependency on the random damage orientation. Photoelastic experiments have shown that the mechanism of failure changes for the dynamically tested damaged specimens, reducing their dependence on damage orientation.     

A ten-inch extensometer measuring small low-frequency strains

A strain gage is described for the measurement of small low-frequency dynamic strains in concrete structures. It is in the form of a demountable extensometer, and its sensitivity is attained by a combination of mechanical amplification, electronic amplification, the use of semiconductor gages and the adoption of suitable filters. The gage has been successfully used for measuring strain in the range 0.1–5.0 microstrain at about 3 Hz on a 10-in, gage length.     

Application of carbon-powder-polymer composite as a continuous crack-length sensor

This paper explores the application of carbon-powder-impregnated polymeric composites for measuring crack extensions. The strain sensitivity of the gage material is shown to be very small. In the first series of tests, the gage material is characterized by measuring the change in electrical resistance due to machined slits for various gage lengths. The measured response is compared with the response predicted from a very simple electrical model. On the basis of good correlation and repeatability, the usefulness of such gages to measure crack extensions is assessed by a second series of tests. Further work to improve the gage response by optimizing the shape of the gage and making the gage and the adhesive layer thinner is proposed. The presented concept, with improvements, can result in a reliable, inexpensive crack gage requiring inexpensive instrumentation. Paper was presented at the 1990 SEM Spring Conference on Experimental Mechanics held in Albuquerque, NM on June 3–6.     

Development of high-temperature microsample testing

Microsample tensile testing has been established as a means of evaluating the room temperature mechanical properties of specimens with gage sections that are tens to hundreds of microns thick and several hundred microns wide. The desire to characterize the mechanical response of materials at elevated temperatures has motivated the development of high-temperature microsample testing that is reported here. The design of specially insulated grips allows the microsamples to be resistively heated using approximately 2 V DC and currents ranging between 2 to 6 A. An optical pyrometer with nominal spot size of 290 μm and 12 μm diameter type K thermocouples was employed to measure and verify the temperature of the microsamples. The ability of the pyrometer to accurately measure temperature on microsamples of different thicknesses and with slightly different emissivities was established over a temperature range from 400°C to 1100°C. The temperature gradient along the length and thickness of the microsample was measured, and the temperature difference measured in the gage section used for strain measurements was found to be less than 6.5°C. Examples of elevated temperature tensile and creep tests are presented.     

Strain-history effect on isotropic and anisotropic plastic behavior

An experimental investigation was performed to evaluate the effect of strain history on an initially isotropic material. A hot-rolled 2.5-in.-diam bar of SAE 1045 steel provided all the test specimens. Axial and circumferential compression data indicated that the steel was isotropic. Additional tension and torsion data indicated that the steel was an isotropic-hardening von Mises material; this was also confirmed by proportionate loading of thin-walled cylinders such that the ratio of axial to circumferential stresses was either 0, 1/2, 1, 2 or ∞. Two additional sets of cylinders were preloaded either in simple axial tension or as closed-ended cylinders to an effective plastic strain of 0.006 before they were proportionately loaded. The preloading had a pronounced effect on yield surfaces for reloading if the effective plastic strain on reloading was only slightly greater than that for the preloading. The effect of preloading on the yield surfaces was small when the effective plastic strain was three to four times that for the preloading. Hill's anisotropic theory was used to predict stress-strain relations for several of the reloaded cylinders. Good agreement was obtained between theory and experiment.     

M-shaped Specimen for the High-strain Rate Tensile Testing Using a Split Hopkinson Pressure Bar Apparatus

An experimental technique is proposed to determine the tensile stress–strain curve of metals at high strain rates. An M-shaped specimen is designed which transforms a compressive loading at its boundaries into tensile loading of its gage section. The specimen can be used in a conventional split Hopkinson pressure bar apparatus, thereby circumventing experimental problems associated with the gripping of tensile specimens under dynamic loading. The M-specimen geometry provides plane strain conditions within its gage section. This feature retards necking and allows for very short gage sections. This new technique is validated both experimentally and numerically for true equivalent plastic strain rates of up to 4,250/s.     

Mechanical characterisation of a viscoplastic material sensitive to hydrostatic pressure

The present paper deals with the characterisation of the static mechanical behaviour of an energetic material all along its lifespan. The material behaviour is viscoplastic, damageable and sensitive to hydrostatic pressure. For such materials, existing models have generally been developed in the framework of transient dynamic behaviour. These models are not suitable for a static study. Therefore a specific experimental protocol and an associated model are developed. Characterisation is derived from both uniaxial compressive, tensile tests and triaxial tests. Plastic behaviour is described by means of a parabolic yield criterion and a new hardening law. Non-associated plastic flow rule and isotropic damage complete the model. The performance of the model is illustrated through the simulation of various loading paths.     

Propagation of a longitudinal pulse in wire ropes under axial loads

Measurements of the velocity of propagation of longitudinal pulses in wire ropes as a function of applied tension are reported. Twelve 3/8-in.-diam cables are investigated which differ in configuration, i.e., number of wires per strand and number of strands, and in the material from which the wires and the core are fabricated. The velocity of longitudinal waves is found to increase with increasing tension, approaching the velocity in a solid steel bar as the applied load is increased toward the failure load of the wire rope. The material from which the wires are fabricated and the number of strands, rather than the number of wires in a strand or its core material, appear to significantly affect the velocity of longitudinal pulses.     

Design of Inserts for Split-Hopkinson Pressure Bar Testing of Low Strain-to-Failure Materials

In the present study a new insert design is presented and validated to enable reliable dynamic mechanical characterization of low strain-to-failure materials using the Split-Hopkinson Pressure Bar (SHPB) apparatus. Finite element-based simulations are conducted to better understand the effects of stress concentrations on the dynamic behavior of LM-1, a Zr-based bulk metallic glass (BMG), using the conventional SHPB setup with cylindrical inserts, and two modified setups—one utilizing conical inserts and the other utilizing a “dogbone” shaped specimen. Based on the results of these computational experiments the ends of the dogbone specimen are replaced with high-strength maraging steel inserts. This new insert-specimen configuration is expected to prevent specimen failure outside the specimen gage section. Simulations are then performed to validate the new insert design. Moreover, high strain-rate uniaxial compression tests are conducted on LM-1 using the modified SHPB with the new inserts. An ultra-high-speed camera is employed to investigate the changes in failure behavior of the specimens. Additional experiments are conducted with strain gages directly attached to the gage section of the specimens to determine accurately their dynamic stress–strain behavior.     

Doubly Reinforcing Cementitious Beams with Instrumented Hollow Carbon Fiber Tendons

Surface mounted strain gages are used to characterize the behavior of polymer-enhanced cementitious beams designed to withstand reverse loadings. These unique composite structures are doubly reinforced with hollow carbon fiber (graphite) tendons equipped with strain gages and the study includes section design, materials considerations, structural testing, and finite element analysis. The primary purpose of strain gage integration is to insure that the stress in the materials remains within the elastic range so that damage does not occur. A finite element model is developed to characterize the structural response in the elastic range and a hybrid approach is suggested in which displacement, strain, and stress can be obtained with a single strain gage. The ability to characterize structural performance beyond the elastic range is also demonstrated by analyzing data obtained from displacement-controlled tests.     

Strain measurements in tubes during rapid transient heating

A measuring apparatus using reluctance-type displacement transducers was successfully used for measuring radial thermal strains in 1-in.-diam × 8-in.-long thin-walled tubes of molybdenum, tantalum and 304 stainless steel. Wall-temperature rates of approximately 300° F/sec were accomplished by rapid heating with a plasma jet and strains at temperatures up to 2500° F were recorded. Excellent agreement between experimental results and a theoretical solution based on temperature profiles was found for temperatures to 2000° F.     

A machine for static and dynamic triaxial testing

A machine has been developed for studying the static and dynamic triaxial constitutive behavior of large specimens of geologic and construction materials. Test specimens can also contain a cylindrical tunnel cavity to permit study of tunnel-reinforcement structures and rock-structure interaction. The specimens are 0.3 m in diameter and 0.3 to 0.45 m high; the model tunnels can be up to 50 mm in diameter. Static and dynamic triaxial loads can be applied with maximum pressures of 200 MPa in static tests and 100 MPa in dynamic tests. Dynamic loading can also be superimposed on a static preload as large as 20 MPa. To facilitate study of tunnel reinforcement, the tunnel is maintained at ambient pressure, with access at both ends for instrumentation and photography. Example results show the influence on tunnel deformation of loading rate as well as the presence of joints and their orientation. For a given allowable tunnel closure, substantially greater pressures can be sustained under dynamic loading than under static loading, and substantially greater pressures can be sustained by an intact specimen than by a jointed specimen.     

Modal Analysis of a Servo-Hydraulic High Speed Machine and its Application to Dynamic Tensile Testing at an Intermediate Strain Rate

This paper describes the experimental procedure to identify the predominant frequencies of the high speed testing machine by conducting modal analysis. The effects due to the predominant frequencies of the system and loading rate on the magnitude of system ringing and the flow stress were analyzed by using a single degree-of-freedom (SDOF) spring-mass-damper model. The system was then used to study the dynamic tensile behavior of two engineering materials, i.e., polyethylene (PE) fabric-cement composite and Alkaline Resistant (AR) glass fabrics at an intermediate strain rate. The stress oscillations in the response of these materials due to system ringing were addressed. The failure behavior of each material was studied by examining high speed digital camera images of specimens during the test. The validity of the dynamic tensile tests was investigated by examining the condition of dynamic stress equilibrium—a criterion used in split Hopkinson pressure bar (SHPB) tests. The results show that the quantitative criterion for a valid SHPB test is also applicable to dynamic tensile tests of these materials at the intermediate strain rate.     

A Study on the Evaluation of the Fracture Process Zone in CCNBD Rock Samples

The safety of many civil and mining concrete and rock structures including pre-existing crack networks is fundamentally affected by the mechanical behaviour of the material under static and cyclic loading. In cyclic loading case, cracks can grow at a lower load level compared to the monotonic case. This phenomenon is called fatigue due to subcritical crack propagation and depends on the behaviour of the fracture process zone (FPZ). This study presents the results of laboratory diametrical compression tests performed on Brisbane tuff disc specimens to investigate their mode-I (tensile) fracture toughness response to static and cyclic loading and relevant FPZ. The FPZ and fracture toughness response to cyclic loading was found to be different from that under static loading in terms of the ultimate load and the damage mechanisms in front of the chevron crack. A maximum reduction of the static fracture toughness (K _IC ) of 42 % was obtained for the highest amplitude increasing cyclic loading test. Detailed scanning electron microscope (SEM) examinations were performed on the surfaces of the tips of the chevron notch cracks, revealing that both loading methods cause FPZ development in the CCNBD specimens. When compared with monotonic FPZ development, the main difference with the cyclically loaded specimens was that intergranular cracks were formed due to particle breakage under cyclic loading, while smooth and bright cracks along cleavage planes were formed under static loading. Further, the SEM images showed that fatigue damage in Brisbane tuff is strongly influenced by the failure of the matrix because of both intergranular and transgranular subcritical fracturing.