Amer Hameed

A Comparison of Methods for Predicting Residual Stresses in Strain-Hardening, Autofrettaged Thick Cylinders, Including the Bauschinger Effect

High-pressure vessels, such as gun barrels, are autofrettaged in order to increase their operating pressure and fatigue life. Autofrettage causes plastic expansion of the inner section of the cylinder, setting up residual compressive stresses at the bore after relaxation. Subsequent application of pressure has to overcome these compressive stresses before tensile stresses can be developed, thereby increasing its fatigue lifetime and safe working pressure. This paper presents the results from a series of finite element models that have been developed to predict the magnitude of these stresses for a range of end conditions: plane stress and several plane-strain states (open and closed ended, plus true plane strain). The material model is currently bilinear and allows consideration of strain hardening and the Bauschinger effect. Results are compared to an alternative numerical model and a recent analytical model (developed by Huang), and show close agreement. This demonstrates that general purpose finite element analysis software may be used to simulate high-pressure vessels, justifying further refining of the models.

The high strain-rate behaviour of selected tissue analogues

The high strain-rate response of four readily available tissue simulants has been investigated via plate-impact experiments. Comparison of the shock response of gelatin, ballistic soap (both sub-dermal tissue simulants), lard (adipose layers) and Sylgard(®) (a potential brain simulant) allowed interrogation of the applicability of such monolithic tissue surrogates in the ballistic regime. The gelatin and lard exhibited classic linear Hugoniot equations-of-state in the US-uP plane; while for the ballistic soap and Sylgard(®) a polymer-like non-linear response was observed. In the P/σX-v/v0 plane there was evidence of separation of the simulant materials into distinct groups, suggesting that a single tissue simulant is inadequate to ensure a high-fidelity description of the high strain-rate response of complex mammalian tissue. Gelatin appeared to behave broadly hydrodynamically, while soap, lard and Sylgard(®) were observed to strengthen in a material-dependent manner under specific loading conditions at elevated shock loading pressures/stresses. This strengthening behaviour was tentatively attributed to a further polymeric-like response in the form of a re-arrangement of the molecular chains under loading (a steric effect). In addition, investigation of lateral stress data from the literature showed evidence of operation of a material-independent strengthening mechanism when these materials were stressed above 2.5-3.0GPa, tentatively linked to the generically polymeric-like underlying microstructure of the simulants under consideration.

Hydrocode Simulation with Modified Johnson-Cook Model and Experimental Analysis of Explosively Formed Projectiles

The formation of mild steel (MS) and copper (Cu) explosively formed projectiles (EFPs) was simulated in AUTODYN using both the Johnson-Cook (JC) and modified Johnson-Cook (JCM) constitutive models. The JC model was modified by increasing the hardening constant by 10%. The previously established semi-empirical equations for diameter, length, velocity, and depth of penetration were used to verify the design of the EFP. The length-to-diameter (L/D) ratio of the warhead used in the simulation varied between 1 < L/D < 2. To avoid projectile distortion or breakup for large standoff applications, the design of the EFP warhead was modified to obtain a lower L/D ratio. Simulations from the JC model underestimated the EFP diameter, resulting in an unrealistically elongated projectile. This shortcoming was resolved by employing the JCM model, giving good agreement with the experimental results. The projectile velocity and hole characteristics in 10-mm-thick aluminum target plates were studied for both models. The semi-empirical equations and the JC model overestimated the projectile velocity, whereas the JCM model underestimated the velocity slightly when compared to the experimental results. The depths of penetration calculated by the semi-empirical equations in the aluminum (Al) target plate were 55 and 52 mm for Cu and MS EFPs, respectively.

A study of the residual stress distribution in an autofrettaged, thick-walled cylinder with cross-bore

It may be necessary to provide a radial opening such as gas evacuator holes, or an opening to operate the unlocking of the bolt mechanism by means of exhaust gases, in a gun barrel, which is a thick walled cylinder. A three dimensional finite element analysis has been performed to evaluate the effect of introducing a radial cross-bore in an autofrettaged thick-walled cylinder. From the analysis of the cross-bored autofrettaged cylinder, it was observed that there is a severe localized change in the residual stress profile in the vicinity of the cross-bore. The residual circumferential stress increases in compression at the bore. Similarly it increases in tension at the outer diameter, thus making the outer diameter more vulnerable to fatigue failure or crack initiation under stresses arising as a result of firing. Analyses were also performed by varying the cross-bore diameter and it was observed that, by increasing the diameter of the radial hole, the residual circumferential stress at the bore reduces, while it increases at the outer diameter, with an increase in the cross bore diameter The re-pressurization pressure of an autofrettaged cylinder with radial cross-bore was found to be approximately 65 percent less than the actual autofrettage pressure in a particular case discussed in this paper. A comparison is also made with the residual stress field which would result if the cross-bore was machined before autofrettage.

A Novel Scheme for Computing Gun Barrel Temperature History and Its Experimental Validation

An accurate modeling of gun barrel temperature variation over time is important to assess wear and the number of shot fires needed to reach cook-off. Using lumped parameter methods, an internal ballistics code was developed to compute heat transfer to the gun barrel for given ammunition parameters. Subsequently the finite element method was employed to model gun barrel temperature history (temperature variation over time). Simulations were performed for a burst of nine shots and the results were found to match satisfactorily to the corresponding experimental measurements. Wear or erosion of the barrel during a gun fire is very sensitive toward the maximum bore surface temperature. The proposed scheme can accurately simulate gun barrel temperature history; hence improved wear calculations can be made with it. An important and unique advantage of the developed scheme is that it easily couples internal ballistics simulations with the finite element methods.

Automotive damper model for use in multi-body dynamic simulations

Abstract The subject of this study is the Warrior armoured personnel carrier rotary suspension damper. This is a simple passive device where energy is dissipated by the resistance to oil flowing through an orifice. In principle the mechanism of this damper is simple, but the requirement for a compact unit that can withstand high dynamic loads means that the design is complex in detail. Because of this complexity it is possible for air to remain in the oil chambers if care is not taken when priming the device. Measurements show that the nonlinear response of the damper is primarily due to turbulent oil flow through the orifice and the compression of entrapped air, but that friction is negligible. The most appropriate model for this system in multi-body dynamic simulations is to represent the damper as two discrete components: a non-linear dashpot (representing oil flow through the orifice) and a non-linear spring (representing the compression of entrapped air). From measurements, a splined curve is determined that describes the non-linear torque-velocity relationship due to orifice flow characteristics. By applying Henrys law and assuming isothermal compression, a function is derived that describes the compression of air in oil. This modelling method closely reproduces the measured response of the damper.

Transient Thermal Analyses of Midwall Cooling and External Cooling Methods for a Gun Barrel

Liquid cooling methods are often used for thermal management of a large caliber gun barrel. In this work, transient thermal analyses of midwall-cooled and externally cooled gun barrels were performed. At first, a novel simulation scheme was developed for the computation of the gun barrel temperature history (temperature variation over time), and its experimental validation was performed. In the computational scheme an internal ballistics code, GUNTEMP8.EXE , was developed to simulate the total heat transfer per cycle for the given ammunition parameters. Subsequently, a finite element (FE) model of the barrel was developed in ANSYS 11.0. Heat transfer to the barrel was approximated by an exponentially decaying heat flux. The FE model was solved to compute for barrel temperature history. Simulations were performed for a burst of 9 cycles, and the results were found to agree with the experimental measurements. Subsequently, the simulation scheme was extended to analyze a burst of 40 cycles at 10 shots per minute (spm). Three cases were investigated as follows: (1) a naturally cooled gun barrel, (2) a gun barrel with midwall cooling channels, and (3) an externally cooled gun barrel. Natural cooling was found insufficient to prevent cook-off, whereas midwall and external cooling methods were found to eliminate any possibility of it. In the context of a self-propelled howitzer, a midwall-cooled gun barrel connected to an engine cooling system was also analyzed.

Numerical Analysis of the Effect of Machining on the Depth of Yield, Maximum Firing Pressure and Residual Stress Profile in an Autofrettaged Gun Tube

A multi-linear kinematic, two dimensional finite element model incorporating Bauschinger effect, developed using ANSYS commercial software is used to determine the effect of machining both at the bore and at the outside diameter, on the depth of yield, maximum firing pressure and final residual stress field present in an autofrettaged gun tube. The model, which is in good agreement with experimental findings, clearly shows that the reduction in maximum compressive circumferential stress is more sensitive to internal machining than to external machining; the depth of yield remains stable and there is no movement of the elastic-plastic interface, relative to its location before material removal. If the internal machining removes material in which reverse yield has occurred, the maximum firing pressure is not affected. The finite element analysis supported by experimental evidence thus leads to an optimization technique for gun tube design.

A sealed capsule system for biological and liquid shock-recovery experiments.

This paper presents an experimental method designed to one-dimensionally shock load and subsequently recover liquid samples. Resultant loading profiles have been interrogated via hydrocode simulation as the nature of the target did not allow for direct application of the diagnostics typically employed in shock physics (e.g., manganin stress gauges or Heterodyne velocimeter (Het-V)). The target setup has been experimentally tested using aluminium flyer plates accelerated by a 50-mm bore single-stage gas-gun reaching projectile impact velocities of up to ~500 ms(-1) (corresponding to peak pressures of up to ca. 4 GPa being experienced by fluid samples). Recovered capsules survived well showing only minor signs of damage. Modelled gauge traces have been validated through the use of a (slightly modified) experiment in which a Het-V facing the rear of the inner capsule was employed. In these tests, good correlation between simulated and experimental traces was observed.

Investigation of Driving Force Variation During Swage Autofrettage, Using Finite Element Analysis

Swaging is one method of autofrettage, a means of prestressing high-pressure vessels to increase their fatigue lives and load bearing capacity. Swaging achieves the required deformation through physical interference between an oversized mandrel and the bore diameter of the tube, as it is pushed along and through the bore of the tube. A finite element (FE) model of the swaging process, developed previously by the author in ANSYS , was configured for comparison with an earlier model; this allowed the accuracy of further properties of the ANSYS model to be investigated. Driving force was the main property of interest, specifically how it varied with mandrel slopes and parallel midsection, to allow direct comparison with the earlier model. The variation of driving force with respect to coefficient of friction was investigated; driving force increased in near proportion, but a subtle trend indicated a further study of stress component be made. This was followed by a two-pass swage process. Close agreement was found with empirical data and the discrepancies observed between the two models are explained by the relatively coarse mesh used by the earlier model. This further verifies the sensitivity of the model described here.