John H. Underwood

Experimental Data, Numerical Fit and Fatigue Life Calculations Relating to the Bauschinger Effect in High Strength Armament Steels

Abstract : The uniaxial Bauschinger effect has been evaluated in several high strength steels being considered for armament use. Tests were conducted at plastic strains up to 3.5%. Results of testing show a progressive decrease in Bauschinger effect up to plastic strains of approximately 1% (for all materials investigated), after which there is little further decrease.

Characterization of Steels Using a Revised Kinematic Hardening Model Incorporating Bauschinger Effect

Abstract : A new variant of the nonlinear kinematic hardening model is proposed that accommodates both nonlinear and linear strain hardening during initial tensile loading and reduced elastic modulus during initial load reversal. It also incorporates the Bauschinger effect, as a function of prior tensile plastic strain, during the nonlinear compressive loading phase. The model is shown to fit experimental data from a total of five candidate gun steels. The numerical fits will be employed in subsequent work to predict residual stresses and fatigue lifetimes for autofrettaged tubes manufactured from the candidate steels.

Thermal Damage, Cracking and Rapid Erosion of Cannon Bore Coatings

Abstract : Thermal damage observed at the bore of fired cannons has increased noticeably in the past decade, due to the use of higher combustion gas temperatures for improved cannon performance. Current authors and coworkers recently have described cannon firing damage and proposed new thermomechanical models to gain understanding of its causes, with emphasis on the severe damage that occurs in the steel beneath the chromium plating used to protect the cannon bore. Recent refinements in the models will be used here to characterize some additional damage observations in the area beneath the protective coating of fired cannons. Model results validated by microstructural observations give predictions of near-bore temperature and stress distributions and good agreement with observed depths of hydrogen cracking in the high-strength steel substrate. Interest in damage and failure within a coating is also of concern for cannons, since coating failure leads to extremely rapid erosion of coating and substrate. The slip-zone model of Evans and Hutchinson is adapted here to predict future strength of cannon coatings based on observed crack spacing and microhardness of thermally damaged areas. Results are described for electroplated chromium coatings from fired cannons and for sputtered chromium and tantalum coatings with laser-heating damage to simulate firing. Coating mechanics analysis of fired and laser-heated samples provides an in-situ measurement of coating failure strength, showing that sputtered chromium has more than twice the failure strength of electroplated chromium. An analysis of cyclic shear failure of a coating interface at an open crack shows a six-fold decrease in low-cycle fatigue life compared to the life of a closed crack. Recommendations are given for preventing rapid coating failure and catastrophic erosion of fired cannons, with emphasis on methods to prevent deep, open cracks in coating and substrate.

Influence of Bauschinger Effect on Residual Stress and Fatigue Lifetimes in Autofrettaged Thick-Walled Cylinders

Abstract : This work addresses the influence of Bauschinger effect upon residual stresses and associated fatigue lifetimes for pressurized, autofrettaged thick cylinders. The model employed allows for the variation with radius of Bauschinger Effect Factor (REF) throughout the autofrettaged tube since the percentage plastic strain, which determines BEF, will vary from a maximum value at the bore to zero at the elastic plastic interface. Accounting for BEF variability, it is demonstrated that the residual compressive hoop stress at the inner radius of the tube reaches a maximum value at the percentage overstrain level below which reversed yielding does not occur. Existing experimental residual stress measurements from a variety of sources are shown to support this thesis. This value of overstrain may serve to maximize crack initiation lifetime in autofrettaged thick cylinders. For a tube with significant heat checking and associated initial crack like defects, it is necessary to consider fatigue crack growth rates governed by a crack growth law such as Pariss Law. For a tube of radius ratio 2.0 and at a value of approximately 40 percent overstrain, slightly in excess of that for the onset of reversed yielding, the fatigue lifetime exhibits a maximum value. Fatigue lifetimes achieve a maximum value at overstrain levels in which yielding reaches 1.4 times bore radius and are almost constant thereafter. Furthermore, such extended overstrain leads to a small increase in residual stress at the outside diameter (OD), thus increasing R ratio at that location and reducing fatigue lifetime for crack growth originating at the OD. Existing experimental lifetime measurements are shown to require the inclusion of BEF to properly account for these observed lifetimes.

Finite Element Investigation of Bauschinger Effect in High-Strength A723 Pressure Vessel Steel

The Bauschinger effect has been evaluated in high-strength pressure vessels. A simple initial test suggested that a biaxial Bauschinger effect was present and that it was different from previously published uniaxial Bauschinger results. The difference was believed to be significant, so further investigation was undertaken. Several full-size A723 steel gun sections were heavily overstrained and subjected to slit tests in order to measure opening angles and displacements. These geometries were then modeled with finite element (FE) analysis using both ideal autofrettage stresses and Bauschinger modified stresses, which were based on previously published uniaxial Bauschinger test results. Because techniques available for predicting reverse yielding for overstrained pressure vessels were limited, a simple methodology for predicting the yield surface upon reverse yielding from a series of uniaxial Bauschinger test data was developed and is presented. This methodology, when used in the FE predictions, compares favorably with analytical predictions made previously. Comparisons of slit-opening results measured from pressure vessel sections with FE calculations using uniaxial Bauschinger data are made. The opening displacements comparison between the uniaxial predictions and those measured from the heavily overstrained sections with biaxial stresses are so subtle (<1 mm) that the tests appear to be inconclusive.

Stresses Within Compound Tubes Comprising a Steel Liner and an External Carbon-Fiber Wrapped Laminate

There is increasing interest in techniques that permit weight reduction of thick cylinders, particularly gun barrels. The method examined within this paper involves the external wrapping of a carbon fiber-based laminate jacket around a steel liner. Various design options are examined, e.g., prior autofrettage of a steel liner followed by external tension wrapping and benign wrapping of a stress-free steel liner followed by autofrettage of the assembly. Outcomes include optimum overstrain and machining of the steel liner, identification of a potential crushing failure mode due to radial compressive stresses and techniques for application of tension wrapping which optimize hoop stress within the composite jacket. Finally, possible combinations of fiber orientations are discussed.

Critical Fracture Processes in Army Cannons: A Review

Abstract : Fast fracture in cannons can well be described using elastic-plastic fracture toughness, in combination with comparisons of cannon section size relative to the size required to maintain plane-strain fracture. Fatigue fracture of cannon tubes is modeled from results of full-size fatigue tests that simulate cannon firing. These tests are also the basis of fatigue-intensity- factor modeling of fatigue life, which incorporates material strength, initial crack size, and Bauschinger-modified autofrettage residual stress into life predictions. Environment-assisted fracture in the thermally damaged near-bore region of fired cannons is shown to be controlled by hydrogen. High-strength cannon steels are susceptible to hydrogen; cannon propellant gases provide the hydrogen; and the source of sustained tensile stress is the near-bore thermal damage and compressive yielding. A thermomechanical model predicts tensile residual stress of similar depth to that of observed hydrogen cracks. Coating fracture in the thermal- damage region of fired cannons is characterized and modeled. The Evans/Hutchinson slip-zone concept is extended to calculate in-situ coating fracture strength from observed crack spacing and hardness in the damaged region.

Compressive Thermal Yielding Leading to Hydrogen Cracking in a Fired Cannon

Investigation of environmental cracking of a 1100-MPa yield strength A723 steel cannon tube subjected to prototype firings is described. Metallographic results show cracking of the steel beneath a 0.12-mm protective layer of chromium. Cracks undermine and remove sections of chromium and lead to localized erosion that ruins the cannon. Key features of the firing thermal damage and cracking are: (i ) recrystalization of the chromium to a depth of up to 0.08 mm; (ii ) steel transformation to 0.19 mm below the chrome surface; (iii ) two different periodic arrays of cracks normal to the hoop and axial directions, with mean depths of 0.23 and 0.46 mm, respectively. Time-temperature-depth profiles for the firing cycle were derived via bi-material finite difference analysis of a semi-infinite solid which incorporated cannon combustion gas temperatures and material properties that vary as a function of temperature. The temperature and depth associated with the steel transformation were used to solve iteratively for the convective heat transfer coefficient. This value was further confirmed by the depths of chromium recrystalization and of the crack arrays in the two orientations. A profile of maximum temperature versus depth is used to determine the near-bore applied and residual stress distributions within the tube. The measured volume change of steel transformation is used to determine an upper limit on applied and residual stresses. These stresses are used to determine crack-tip stress intensity factors for the observed crack arrays, and hence provide some explanation for the differential depths of cracking. The near-bore temperature and residual stress distributions are used to help determine the cause of hydrogen cracking and measures to prevent cracking. Compressive yielding due to thermal loading produces near-bore tensile residual stresses, and thereby causes hydrogen cracking. Prevention of cracking is discussed in relationship to hydrogen crack growth rate tests of alternative alloys and coatings.

Thermomechanically Controlled Erosion in Army Cannons: A Review

Metallographic characterization is presented of thermal damage of Cr-coated steel in a fired cannon; Cr and Ta coated steel in a vented-erosion-simulator; and bulk Si 3 N 4 in laser heating. Common features of rapid crack-induced erosion are noted. (i) Cracks form normal to the surface, often permanently open, indicating tensile stress was present at some point during thermal damage. (ii) Softening of Cr and Ta coatings and Si 3 N 4 occurs near the heated surface, verified by metallography and hot hardness. The transformation of steel beneath the coatings is used as an in-situ verification of temperatures that cause thermal damage. (iii) Crack-induced under-cutting of thermal-damage islands is observed for coatings and bulk Si 3 N 4 . A thermomechanical analysis of rapid crack-induced erosion observed in severe cannon firing and firing simulation suggests the following key failure mechanisms common to metals and Si 3 N 4 . (i) High near-bore transient temperatures increase thermal expansion compression and concurrently decrease the elevated temperature strength. For metals, the thermal compression stress greatly exceeds strength, to depths of about 0.5 mm. (ii) Thermal stress exceeding strength produces compressive yielding, which, upon cooling, causes tensile residual stress and cracking. The near-bore residual tension is high enough to cause one-cycle cracking of both Cr and Ta coatings; hydrogen from combustion enters via the cracks and causes cracking in the steel below the coating. For S1 3 N 4 , cracks are encouraged by the low fracture toughness of Si 3 N 4 (iii ) Repeated thermal cycles deepen and widen cracks to form islands that can be undercut, leading to island removal and rapid erosion failure of the cannon. For Cr and Ta coatings, undercutting is by hydrogen cracking in the steel and degradation of the coating interface by combustion products that enter via the cracks. For S1 3 N 4 , undercutting is by direct thermomechanical cracking.

Yield pressure measurements and analysis for autofrettaged cannons

Yield pressure corresponding to a small permanent OD strain was measured in quasi-static laboratory tests of autofrettaged ASTM A723 steel cannon pressure vessels. Yield pressure was found to be a consistent ratio of the yield strength measured from specimens located in close proximity to the area of observed yielding. Yield pressure measurements for dynamic cannon firing with typically a 5-ms pressure pulse duration gave 14% higher yield pressures, attributed to strain rate effects on plastic deformation. Calculated Von Mises yield pressure for the laboratory test conditions, including the Bauschinger-modified ID residual stress and open-end vessel conditions, agreed with measured yield pressure within 3-5%. Calculated yield pressure was found to be insensitive to the value of axial residual stress, since axial stress is the intermediate value in the Van Mises yield criterion. A description of yield pressure normalized by yield strength was given for autofrettaged A723 open-end pressure vessels over a range of wail ratio and degree of autofrettage, including effects of Bauschinger-modified residual stress. This description of yield pressure is proposed as a design procedure for cannons and other pressure vessels.