Machining Damage Research Articles

Increased effects of machining damage in beryllium observed at high strain rates

Tensile tests at both low and high strain rates, and also impact shear tests, were performed on a weldable grade powder-source beryllium. Impact energies increased by a factor of 2 to 3 from the as-machined level after etching or annealing. Similar increases in the ductility from machining damage removal were observed from the tensile data at the higher strain rate (10 s/sup -1/) while an insignificant increase in elongation was measured at the lower strain rate (10/sup -4/ s/sup -1/). High strain-rate tests appear to be more sensitive and reliable for evaluating machining practice and damage removal methods for beryllium components subjected to sudden loads. 2 tables.

Effect of annealing and etching on machining damage in structural beryllium

Machining damage can seriously degrade the mechanical properties of beryllium. In this study substantial machining damage was introduced in two different structural grades of beryllium. Two methods of relieving the machining damage were studied: annealing at various temperatures and etching away various amounts of surface material. For the depth of damage introduced in the study (approximately 100 Σm (0.004 in.)), removing the machining damage in the first 25 to 50 Σm (0.001 to 0.002 in.) by either etching or annealing restored the mechanical properties of the beryllium. Microhardness was useful for ascertaining the extent of machining damage.

Fractographic evidence of machining damage in beryllium

Fractographic evidence of machining damage in beryllium

Probability based fracture mechanics for impact penetration damage

This paper presents a new method for assessing the residual strength of structural components which have sustained impact penetration damage. The method is based upon the application of the Weibull probability density function to account for the significant scatter in residual strengths which result from the random nature of impact damage detail (cracks, holes, and spall surfaces). To measure the extent of an impact damage pattern, a new parameter is presented which is based upon the solution for an inclined crack in a uniaxial tension field and an experimentally obtained fracture criterion for mixed mode crack configurations. Test results are presented for residual strengths of machined damage specimens to ascertain the statistical nature which arises from extreme damage detail, smooth holes and sharp cracks. Finally, probability of failure curves are given for both machined (7075-T6) and impact damage (7075-T6 and 2024-T81) in aluminum uniaxial tension panels.

Surface topography and integrity effects on sliding wear

The effect of surface roughness and integrity on the sliding wear of metals was investigated experimentally. The results are consistent with the delamination theory of wear. The initial wear rate was influenced by the surface roughness and the applied load but the steady state wear rate was independent of the initial roughness. Under low applied loads delamination of smooth surfaces commences soon after sliding is initiated, whereas the delamination of rough surfaces is delayed until the original asperities are worn. Consequently, under low loads the initial wear rate of a smooth surface is higher than that of a rougher surface. The opposite is found under high loads since original asperities are removed immediately. It is also shown that machining damage to the surface or the subsurface (in the form of deformation, voids and cracks) accelerates the initial wear rate of the machined surface.

Fracture‐Strength Anisotropy of Sapphire

At 22°C, the bend strength of as‐ground sapphire bars with different orientations of tensile surface and tensile axis varies from 4×104 to 12×104 psi as a result of the anisotropies of machining damage and fracture energy. Generally, the strength‐controlling surface flaws caused by grinding are of constant size; failure from such flaws is affected by grinding‐induced slip and twinning. Surface striations, which occur when there is negligible subsurface slip, can lead to stress concentrations, whereas twins can lead to increases in flaw size; each of these effects reduces strength. In addition, the large anisotropy in fracture strength, which is not predicted by the elastic anisotropy, is controlled by the anisotropic fracture energy. Approximation of the attractive forces across the fracture plane based on elastic moduli and interplanar distance can be used to understand the fracture energy anisotropy.

Design and performance characteristics of an experimental crop conditioning system for difficult climates

Study of established grass mowing and conditioning machines has enabled functional short-comings to be identified and objective crop treatment criteria to be formulated. Subsequent development has led to an experimental crop conditioning system which takes account of the special requirements imposed by difficult climatic conditions during the conservation season. It aims at confining physical treatment to the plant surface and applying it with varying severity along the length of plants and relative to crop yield, so that inherent differences in drying rates are reduced. To carry the objectives into effect, experimental conditioning rotors with open V-form beaters have been combined with reciprocating and drum-type rotary mowers. The beaters are forward-acting in the vertical plane and allow limited crop movement laterally and abrasive slip longitudinally. By guiding the crop cut-ends-first into the beaters, most work is done on the basal parts. The treated crop retains much of its structural strength and, therefore, can be formed into loose swaths resistant to settling. Relative to the control treatment of mowing followed by tedding or turning, the experimental mower-conditioners have given average drying rate improvements to the baling stage of up to about 60%; this was identical with crimping but lower than for flail mowing. Average dry matter recovery at baling was 10–15% above the control; this was also similar to crimping but higher than for flail mowing. Good resistance of experimentally treated crop to leaching by rain can be associated with a low level of deep tissue damage. Consideration is being given to applying the experimental beater rotor to the prevention of machine damage by foreign objects in forage harvesters.

Grinding induced machining damage in beryllium

An investigation has been made of the machining damage induced in beryllium by surface grinding. A series of damage free specimens was prepared from a bar extruded from a block of hot-pressed beryllium powder. Each specimen was then ground on one large surface using a different set of grinding conditions, as dictated by the requirements of a quarter-replicate factorial experiment which involved eight grinding parameters each taken at two levels. The damage induced by the surface grinding was evaluated by testing each specimen in a four point bending mode. The tests were conducted so that the maximum tensile strain due to bending occurred at the experimentally ground surfaces. A statistical analysis of the results enabled the grinding conditions giving rise to the least and the greatest amount of machining damage to be predicted. In a second series of experiments, these predictions were found to correlate with the metallographically observable damage and the mechanical test data obtained from a further series of bend test specimens.

Grinding induced machining damage in beryllium

Grinding induced machining damage in beryllium

Sub-surface damage and edge sharpness in finish machining

Machined surfaces are usually characterized by the presence of sub-surface plastic deformation. In the present study, orthogonal cutting tools having exaggeratedly large edge radii (model tools) were used to machine aluminium, zinc and brass workpieces at various cutting speeds (32 in./min and 50–300 ft/min). The experimental results are in general agreement with the approximate theoretical prediction of von Turkovich 1, which states that the depth of the deformed layer is equal to F 3K , F being the resultant cutting force and K the dynamic yield shear stress.

Lightning protection of turbine generators

At present, it is the general practice to apply lightning protective equipment at the terminals of large unit-connected generators. There is a hesitation on the part of industry to omit protective equipment, even though analysis of the problem indicates that no protection is needed. This is primarily because the cost of the added insurance or protective devices is trivial when compared with the investment in a large machine, the expense of damage to the machine, and the expense of the loss of a large unit of generating capacity.