PROF. GEORGE O RADING

A comparison of the influence of different runner designs on the mechanical properties of aluminium castings has shown that filtered rectangular runners (FRRs) yield aluminium castings with higher and more reliable mechanical properties than the conventional unfiltered rectangular runners. Unfiltered vortex flow runners have also been shown to improve the reliability of the modulus of rupture of cast aluminium alloys over unfiltered rectangular runners. In the present paper, experimental results of a comparative study on the effect of the unfiltered vortex flow and FRR designs on the tensile strength of permanent mould aluminium castings are reported. The results show that an FRR yields castings with higher and more reliable tensile strengths than the unfiltered vortex flow runner. Castings poured into a mould with an FRR had strengths between 269 and 291 MPa and a Weibull modulus of 50·2 while castings poured into the vortex flow runner had strengths between 255 and 280 MPa and a Weibull modulus of 40·3.

The neutron diffraction technique was used to determine the residual stress field in welded compact tension specimens of the aluminium-lithium alloy M 2095. The deep penetrating characteristic of neutrons was exploited to evaluate the through-thickness variation in residual stress. Moreover, insight into the redistribution of these stresses was gained by extending a fatigue crack through the residual stress field and re-examining the stress distribution. The specimen without a crack was found to have a high compressive stress (of the order of -135 MPa) ahead of the notch. This rose to a maximum tensile stress of about 50 MPa, 22 mm from the notch, followed by a drop to negative values further ahead of the notch. It was observed that the magnitude of the stresses changed on moving into the thickness of the specimen. However, the form of the graph showing stress versus distance ahead of the notch remained unchanged. When fatigue cracks of different lengths were introduced, the magnitude of the stress close to the tip first increased with crack length, before decreasing and then rising again. Nevertheless, the form of the graph remained unchanged and the stress at the crack tip remained compressive. The paper concludes that any study of the response of a component to mechanical loading involving a residual stress field must take these factors (i.e. through-thickness stress variation and stress redistribution) into consideration.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

A 3D FEM computer program was developed to establish the stress distributions and SCFs in thick walled cylinders with radiused entry flush and non-protruding cross bores under internal pressure. The displacement formulation and eight noded brick isoparametric elements were used. The Frontal solution technique was used due to the limited computing facilities. The variation of SCF with entry radius to main bore radius ratio was established for varying cylinder outer radius to main bore radius ratio and cross bore to main bore radius ratio. For low values of cylinder outer radius to main bore radius ratio (≤2.25), the lower values of cross bore to main bore radius ratio resulted in lower SCFs. For high values of cylinder outer radius to main bore radius ratio (>2.25),  the lower values of cross bore to main bore radius ratio resulted in higher SCFs. The cylinder with cylinder outer radius to main bore radius ratio of 2.25 was found to be a transition geometry. For very small values of cross bore to main bore radius ratio, the SCFs converged to a value of 2.2 for entry radius to main bore radius ratio of 0.2. The cylinder outer radius to main bore radius ratio of 1.75 to 3 cylinders had a constant SCF value of 2.3 at cross bore to main bore radius ratio of 0.05 and entry radius to main bore radius ratio of 0.2. A new categorization of cylinders earlier proposed in the study of plain cross-bored cylinders is further validated.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

A 3D FEM computer program was developed to establish the stress distributions and SCFs in thick walled cylinders with radiused entry flush and non-protruding cross bores under internal pressure. The displacement formulation and eight noded brick isoparametric elements were used. The Frontal solution technique was used due to the limited computing facilities. The variation of SCF with entry radius to main bore radius ratio was established for varying cylinder outer radius to main bore radius ratio and cross bore to main bore radius ratio. For low values of cylinder outer radius to main bore radius ratio (≤2.25), the lower values of cross bore to main bore radius ratio resulted in lower SCFs. For high values of cylinder outer radius to main bore radius ratio (>2.25),  the lower values of cross bore to main bore radius ratio resulted in higher SCFs. The cylinder with cylinder outer radius to main bore radius ratio of 2.25 was found to be a transition geometry. For very small values of cross bore to main bore radius ratio, the SCFs converged to a value of 2.2 for entry radius to main bore radius ratio of 0.2. The cylinder outer radius to main bore radius ratio of 1.75 to 3 cylinders had a constant SCF value of 2.3 at cross bore to main bore radius ratio of 0.05 and entry radius to main bore radius ratio of 0.2. A new categorization of cylinders earlier proposed in the study of plain cross-bored cylinders is further validated.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

The inaccessibility of commercial software has necessitated the development of low-cost, general-purpose finite element method (FEM) computer programs for structural analysis. Using the FEM program, the elastic, elastic-plastic, residual and service stresses and displacements in a closed ended, thick-walled cylinder under internal pressure were established. The displacement formulation was implemented and eight-noded brick isoparametric elements chosen. The frontal solution technique and the incremental theory of plasticity were used, as only limited computing facilities were available. The results were found to be in very good agreement with the through-thickness analytical values. The benefits of autofrettage were demonstrated and an optimum overstrain of 16% established for a cylinder with a thickness ratio of 2. The material economy achieved through autofrettage and the limitations imposed are illustrated. The FEM program could therefore be reliably used for other complex geometries and load conditions.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

A 3D FEM computer program was developed to establish the stress distributions and SCFs in thick walled cylinders with radiused entry flush and non-protruding cross bores under internal pressure. The displacement formulation and eight noded brick isoparametric elements were used. The Frontal solution technique was used due to the limited computing facilities. The variation of SCF with entry radius to main bore radius ratio was established for varying cylinder outer radius to main bore radius ratio and cross bore to main bore radius ratio. For low values of cylinder outer radius to main bore radius ratio (≤2.25), the lower values of cross bore to main bore radius ratio resulted in lower SCFs. For high values of cylinder outer radius to main bore radius ratio (>2.25),  the lower values of cross bore to main bore radius ratio resulted in higher SCFs. The cylinder with cylinder outer radius to main bore radius ratio of 2.25 was found to be a transition geometry. For very small values of cross bore to main bore radius ratio, the SCFs converged to a value of 2.2 for entry radius to main bore radius ratio of 0.2. The cylinder outer radius to main bore radius ratio of 1.75 to 3 cylinders had a constant SCF value of 2.3 at cross bore to main bore radius ratio of 0.05 and entry radius to main bore radius ratio of 0.2. A new categorization of cylinders earlier proposed in the study of plain cross-bored cylinders is further validated.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

A model based on reaction kinetics and elemental diffusion is pro­posed to account for the presence of double inflection in the hardness profiles of the heat-affected zone (HAZ) in weld­ments of AI-Li-X alloys tested without postweld heat treatment (PWHT). Such profiles are particularly evident when 1) the base metal is in the peak-aged (T8 or T6) temper condition prior to welding; 2) the welding process is a high-heat input process, i.e., gas tungsten arc (GTA), gas metal arc (GMA) or plasma arc (PA) welding; and 3) a filler alloy deficient in lithium (i.e., AA 2319) is used. In the first part of this paper, the theoretical mechanisms are presented. It is proposed that the double inflection appears due to complete or partial reversion of the semi-coherent, plate-like precipitates (i.e., ө', T₁ or S'); coarsening of the platelike precipitates at constant volume fraction; precipitation of d' as a result of natural aging; and diffusion of lithium from the HAZ into the weld pool due to the concentration gradient between the weld pool and the base metal. In the second part (to be published in next month's Welding Journal), experimental validation of the model is provided using weld-ments of the AI-Li-Cu Alloy 2095.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

In a previous paper (Ref. 1), details were presented of a theoretical model describing the evolution of the hardness profiles in the heat-affected zones (HAZ) of AI-Li-X weldments. The intent of the model was to qualitatively predict the general shape of such a profile, which indicates points of double inflection. In the present paper, experimental results are presented to validate the model. Panels of AI-Li-Cu Alloy 2095 in the peak aged (T8) condition were welded by the gas tungsten arc (GTA) process using AA 2319 filler metal. Conventional transmission electron microscopy (TEM) studies were conducted on specimens taken from specific points across the HAZ to estimate the relative ratios of T₁, (AI₂CuLi) and _d' (AI₃Li) precipitates, as well as incoherent grain boundary phases. Electron probe microanalysis (EPMA) was used to determine the variation of concentrations of elements across the HAZ, while the hardness profile was determined using Vickers micro-hardness measurements. The hardness profile and the associated pattern of phases present agree well with the information predicted qualitatively by the previously described model.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

The fatigue crack growth (FCG) characteristics of a cast Al-Cu alloy A206 in the T7 temper condition were studied in laboratory air at a frequency of 30 Hz and an R-ratio of 0.2. Two different (as cast) thicknesses and two different casting conditions (casting with and without a chill) were investigated with the aim of characterizing the effect of porosity on the FCG behavior. The level of porosity in each sample was determined by the point-counting method and varied from 2.4 to 4.0% before hot isostatic pressing (HIP).<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

The study covered both near-threshold and mid-range regimes of FCG. The results indicate that for this limited range of porosity, there exists no systematic relationship between casting conditions (i.e., chilled or non-chilled) or the as-cast thickness and the average level of porosity. However, in the near-threshold regime, the FCG resistance decreased as the level of porosity increased, while in the mid-range of FCG, the FCG rate was fairly independent of the amount of porosity.

Application of HIP led to a reduction in the porosity level from about 4% to less than 0.4%, but this was accompanied by improvement in the FCG resistance only, close to the threshold. The results are rationalized in terms of the effect of porosity on the micro-mechanisms of fatigue crack growth and the initiation of secondary cracks caused by the size, distribution and morphology, of the pores.

The fatigue crack growth (FCG) characteristics of a cast Al-Cu alloy A206 in the T7 temper condition were studied in laboratory air at a frequency of 30 Hz and an R-ratio of 0.2. Two different (as cast) thicknesses and two different casting conditions (casting with and without a chill) were investigated with the aim of characterizing the effect of porosity on the FCG behavior. The level of porosity in each sample was determined by the point-counting method and varied from 2.4 to 4.0% before hot isostatic pressing (HIP).<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

The study covered both near-threshold and mid-range regimes of FCG. The results indicate that for this limited range of porosity, there exists no systematic relationship between casting conditions (i.e., chilled or non-chilled) or the as-cast thickness and the average level of porosity. However, in the near-threshold regime, the FCG resistance decreased as the level of porosity increased, while in the mid-range of FCG, the FCG rate was fairly independent of the amount of porosity.

Application of HIP led to a reduction in the porosity level from about 4% to less than 0.4%, but this was accompanied by improvement in the FCG resistance only, close to the threshold. The results are rationalized in terms of the effect of porosity on the micro-mechanisms of fatigue crack growth and the initiation of secondary cracks caused by the size, distribution and morphology, of the pores.

The fatigue crack growth (FCG) characteristics of a cast Al-Cu alloy A206 in the T7 temper condition were studied in laboratory air at a frequency of 30 Hz and an R-ratio of 0.2. Two different (as cast) thicknesses and two different casting conditions (casting with and without a chill) were investigated with the aim of characterizing the effect of porosity on the FCG behavior. The level of porosity in each sample was determined by the point-counting method and varied from 2.4 to 4.0% before hot isostatic pressing (HIP).<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

The study covered both near-threshold and mid-range regimes of FCG. The results indicate that for this limited range of porosity, there exists no systematic relationship between casting conditions (i.e., chilled or non-chilled) or the as-cast thickness and the average level of porosity. However, in the near-threshold regime, the FCG resistance decreased as the level of porosity increased, while in the mid-range of FCG, the FCG rate was fairly independent of the amount of porosity.

Application of HIP led to a reduction in the porosity level from about 4% to less than 0.4%, but this was accompanied by improvement in the FCG resistance only, close to the threshold. The results are rationalized in terms of the effect of porosity on the micro-mechanisms of fatigue crack growth and the initiation of secondary cracks caused by the size, distribution and morphology, of the pores.

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Fracture mechanics principles have been used to study the effects of welding on the rate of fatigue crack growth (FCG) in a low-carbon structural steel. The steel concerned is used widely in the fabrication of the structural frame-work of passenger buses. Tests were carried out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and midrange regimes of crack growth were studied. In the midrange regime, the FCG rate was highest in the HAZ and lowest in the WM. Near the threshold, the FCG rate was highest in the BM and lowest in the WM. The results are explained in terms of microstructural changes due to welding, welding residual stresses, and fracture mechanisms. Recommendations to reduce the incidence of fatigue cracking have been made.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />