A.M. Venter

Comparative measurements on autofrettaged cylinders with large Bauschinger reverse yielding zones

A comparative study to map the residual strain/stress states through the walls of autofrettaged thick-walled steel cylinders has been conducted with neutron diffraction, Sachs boring and the compliance methods. Test samples with different wall thickness ratios were prepared to have significant amounts of reverse yielding due to the Bauschinger effect. All three methods indicate that the autofrettage action primarily influenced the hoop stresses, which varied rapidly close to the bores from compressive to tensile within the first half of the wall thickness. Quantitatively, results from the neutron diffraction and Sachs boring techniques compare favourably across large regions of the cylinder walls, while the compliance results showed different features. The existence of reverse yielding close to the cylinder bores has been sensed to different magnitudes and distances from the bores. Plastically yielded material regions identified from the neutron diffraction results correlated well with theoretical modelling.

High-energy synchrotron X-ray analysis of residual plastic strains induced in shot-peened steel plates:

Shot peening has long been recognized as a method for treating metal surfaces in order to improve their resistance to crack initiation and propagation. In view of shot-peening control, it would be useful if methods for direct measurement or estimation of residual stresses and depths of deformed layers were available. The focus of the present paper is on the presentation of high-energy synchrotron X-ray diffraction that allows better understanding of shot-peening residual strain profiles. Furthermore, it attempts to show that considerable elucidation of shot-peening effects can be attained if plastic strain profiles (eigenstrains) are considered, rather than residual stresses. Indeed, bending analysis shows that residual stresses depend on the sample geometry, and on the thickness in particular. In contrast, the experimental analysis presented in this paper suggests that the plastic strain profile induced in a particular material, on the other hand, is a function of the peening intensity alone and does not depend on the specimen thickness.

Quantum critical behavior of the (Cr86Ru14)1−xVx alloy system

Electrical resistivity, Hall effect, specific heat, and neutron diffraction measurements are used to investigate magnetic and quantum critical behavior in a commensurate spin-density-wave (CSDW) antiferromagnetic (Cr86Ru14)1−xVx alloy system. The x−T magnetic phase diagram obtained from the measurements depicts a critical point, occuring at T=0K for a critical V concentration xc=0.104. This point is classified as a CSDW-type quantum critical point using recently suggested methods for classification of quantum critical points from Hall measurements.

Neutron Diffraction Investigation of Residual Stresses Induced in Niobium-Steel Bilayer Pipe Manufactured by Explosive Welding

Recently, reliable and hermetic joining of stainless steel to niobium pipes has been achieved with the explosive bonding technique. Joining of these two materials are essential to ensure production of a bimetallic transition element of pipe-type for its further use as a part of charged beam acceleration systems of the new generation. A non-destructive neutron diffraction investigation of the tri-axial strains along a radial cross-sectional line through the joint section has been performed. Residual stress results indicate inherently different natures in the residual stress values within the respective pipe sections. In the external stainless steel pipe the residual stresses are tensile, showing a sudden increase to 600 MPa as the interface is approached, whilst being compressive in the internal niobium pipe, not exceeding 650 MPa. A characteristic abrupt stress discontinuity exits at the interface region.

Influence of Quenchant Hydrodynamics and Boiling Phase Incipient Temperature Shifts on Residual Stress Formation

Quench heat treatments are used in metallurgical applications to alter material mechanical properties such as hardness and strength. Although these conventional heat treatments have been used for many decades, specific influences of material properties and heat transfer conditions during quenching are not very well understood. In particular, predictions based on steady-state boiling heat transfer coefficients disagree with observations, leading to the use of average uniform heat transfer coefficient applied over entire component surfaces as a rule of thumb, with adjustments made for particular processes and components. This paper investigates the effects of multiphase boiling heat transfer and transitional nucleate boiling regimes on the final residual stress states within components. The results of this study show that correct representation of heat transfer conditions provides significant improvements over the current quench modeling techniques, ultimately allowing production of engineering components with superior mechanical properties, reduced distortion, and well-controlled beneficial residual stress states.

Neutron Diffraction Measurement and Finite Element Modelling of Residual Strains Due to Bath and Spray Quenching of AISI 316L Stainless Steel Cylinders

This paper describes the thermo-elasto-plastic analysis of bath and spray quenched AISI 316L cylinders. A suitably detailed continuum deformation analysis approach presented here is implemented within the framework of commercial Finite Element (FE) package ABAQUS. The results of the numerical analysis are compared with the residual elastic strains measured experimentally using neutron diffraction. The good agreement between measured and modelled residual elastic strains provides a basis for careful analysis of the residual elastic strain development resulting from two different quench methods. The conclusions drawn from the analysis provides a better understanding of quench processsing, so that the effects of different heat removal efficiencies of such processing technique can be taken advantage of to generate favourable residual elastic stress and deformation and microstructural distributions with quench processed components.

Neutron Strain Investigations of Laser Bent Samples

Bending of metal plates with high-energy laser beams presents a flexible materials forming technique where bending results from the establishment of a steep temperature gradient through the material thickness. This inevitably leads to non-uniform thermal expansion/contraction and subsequently residual stresses. Non destructive residual strain mapping with neutron diffraction through the 8mm thickness of a series WA 300 grade structural steel plate samples, focused on the region straddling the centerline of the heating bead location, shows the presence of large residual stress fields. Directly below the laser track the longitudinal strains are tensile and dominant, normal strains compressive and transverse strains slightly tensile. The magnitudes of the strains decrease outside the width of the laser beam footprint. The first laser pass induces throughthickness strains close to yield, whereafter their magnitudes decrease with increased number of laser beam passes. A comprehensive mapping of the longitudinal stresses as function of the number of laser passes is given.

Multi-scan laser forming: Synchrotron strain scanning and microstructure evolution

Residual elastic strain and microstructural transformations resulting from high thermal gradients introduced by industrial-strength laser forming of mild steel are reported in this article. An 8 mm thick steel plate was bent to a total bending angle of 1.2° by running a laser three successive times across the length of the plate. Thin through-thickness slices of the plate were prepared for synchrotron energy-dispersive X-ray diffraction experiments from which lattice strains were calculated by measuring peak shifts in the diffraction patterns. The diffraction patterns were analysed by means of Rietveld refinement using the general structure analysis system allowing the spatially resolved mapping of relevant strain components needed for full-field eigenstrain determination in the plate. Comparison of the measured residual elastic strain fields and metallographic analyses of the steel plate suggests that a correlation exists between the residual elastic strains and metallurgical processes. The present study serves the purpose of developing a better understanding of the laser forming process and its implications on structural integrity and in-service reliability of components. It also provides a foundation for further in-depth studies into the interaction between lasers and structural materials

Benchmarking studies of the MPISI Material Science Diffractometer at SAFARI-1

The measurement performance of the new material science instrument MPISI at the SAFARI-1 research reactor in South Africa has been comprehensively benchmarked for strain scanning applications. In addition to the traditional VAMAS Ring and Plug specimen, the benchmarking was extended to a project that required sub-millimeter spatial resolution. For the latter the MPISI performance has been compared with experimental investigations on the same sample set at the KOWARI instrument at ANSTO. Overall, good data quality and agreement have been observed between the two instruments. Introduction A new material science neutron diffractometer named MPISI (Materials Probe for Internal Strain Investigations; Zulu name for spotted hyena) has recently been established at the SAFARI-1 research reactor in South Africa. As part of the commissioning program the well characterized VAMAS Ring and Plug specimen [1] was investigated to confirm the general instrumental performance in terms of attainable strains and positional resolution. Various projects have since been performed on MPISI with residual stress investigations in standard geometries using fully embedded and large gauge volumes. Resolving steep stress gradients close to surfaces however require higher precision setups and measurement strategies to achieve the desired positional accuracies. This entails positioning the apertures as close as possible to the sample, in conjunction with very accurate directional alignment to the instrument center of rotation (CoR) to ensure precise definition of the measurement gauge volume. In addition the sample reference point should be determined to within 10% of the minimum dimension of the gauge volume since poorer accuracy may lead to significant spurious strains in data points treated for partially submerged gauge volumes. By performing comparative studies on the same samples the correspondence between the data quality, stress resolving ability and overall instrument performance can be determined. Experiment design To assess our high-spatial resolution measurement strategy, the depth dependences of the in-plane stresses were determined in thin laser-shock peened (LSP) aluminium plates. LSP is a new laser surface treatment technique that can introduce compressive residual stresses extending beyond 1 mm in depth into metallic components to enhance fatigue life and stress corrosion performance [2]. Investigations were done on the MPISI instrument using the smallest practically achievable gauge volume that was selected as a trade-off between the strain positional resolution and measurement times. The investigations were subsequently extended to the KOWARI neutron strain scanner [3] at ANSTO which is a well-established high performing instrument that, with its higher neutron flux, enabled achieving even smaller through-depth spatial resolution. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 413-418 doi: http://dx.doi.org/10.21741/9781945291173-70 414 Sample description. Four AA6056-T4 aluminium plate samples treated to different laser-shock peen parameters have been investigated on both the MPISI and KOWARI neutron strain scanners. The plate sizes were 200 mm (along the rolling direction) x 50 mm x 3.2 mm where the top surface of each sample was laser-shock peened over an area of 25 x 25 mm. The LSP treatment was performed at the CSIR National Laser Centre (South Africa) using a Quanta Ray Pro-270 laser system from Spectra Physics. This was accomplished without a sacrificial ablative coating [4] using a laser power intensity of 3 GW/cm in conjunction with a laser spot size of 1.5 mm that was applied in a raster pattern with spot overlap as indicated in the schematics of Table 1. Differences in the LSP treatment of the samples pertain to the laser spot coverage densities as summarised in Table 1. In all cases the laser track lines were along the plate widths. Table 1: Parametric variation of spot density in samples Sample name G H I A (reference) Spot density 100 spots per cm 500 spots per cm 2000 spots per cm Untreated Principal sample (and strain) directions are defined in terms of the plate geometry as longitudinal (L) parallel to the long dimension of the plate which coincides with the rolling direction, transverse (T) across the plate length, and normal (N), normal to the plate surface. The as-received parent material plate was used as stress reference that was investigated with the same setup and measurement protocol. MPISI experimental procedure. MPISI is a constant wavelength diffraction instrument that delivers a 1.659 Å wavelength neutron beam onto the instrument CoR when diffracting from the (331) plane of a silicon multiwafer single crystal monochromator at a fixed take-off angle of 83.5°. This places the diffraction angle from the (311) planes of aluminium at approximately 85.5° as measured with the 2D neutron detector. A matchstick-shaped gauge volume was established with the primary beam aperture width set at 0.6 mm and 15 mm in height, with the secondary aperture width at 0.6 mm this rendered a nominal gauge volume of 5.4 mm. Both apertures were positioned 20 mm from the CoR to minimize beam divergence and precisely aligned by means of through-thickness intensity scans from an aluminium pin located at the CoR. Fig. 2a-d shows the experimental setup and measuring directions of the samples. Alignment of the sample with respect to the neutron gauge volume was achieved by performing through-wall-thickness intensity scans from the back surfaces (opposite to the peened faces) of all samples and determining the surface position using the procedures described in [5]. Alignment accuracies are estimated to be better than ±10 μm. Strain measurements have been performed through the sample thickness in the central part of the LSP treatment patch as line scans by applying different measurement point densities. Close to the peened surfaces measurements were done in 0.2 mm steps, as overlapped gauge volumes, over the first millimeter to capture the near-surface peening effect, followed by step sizes of 0.5 mm to cover the remainder of the sample thickness. A total of 11 points (per strain direction) were thus measured through thickness. In order to compensate for spurious strains resulting from partially submerged gauge volumes at the sample surfaces, each sample was rotated 180° in the horizontal plane and the depth-dependence of the strain component re-measured at corresponding positions to the first measurements. The d-values at each corresponding position were averaged for the 0o and 180° L T Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 413-418 doi: http://dx.doi.org/10.21741/9781945291173-70 415 orientations as described in [6]. Data acquisition times for all data points were 3600 s that rendered Δd/d better than 10. (a) (b) (c) (d) Fig.1: Schematic depictions of the orientations and through-thickness measurement regions (a). Photographs showing the sample setups on MPISI for the measurement of the three principal strain directions normal (b), longitudinal (c) and transverse (d). The longitudinal and transverse components were measured with the sample in transmission geometry whereas the normal component was measured in reflection geometry. KOWARI experimental procedure. KOWARI is a constant wavelength diffraction instrument with a variable take-off angle. A take-off angle setting of 79° delivered 1.76 Å wavelength neutrons from the (400) refection of a silicon monochromator that put the (311) reflection of aluminium at ~90o. A nominal gauge volume of 0.2 x 15 x 0.2 mm was established with the primary and secondary apertures positioned at 10 mm from the CoR to minimize the beam divergence. The same instrument and sample setup procedures as used on MPISI were employed, though with faster measurement times. Through-thickness points were measured with regular 0.2 mm steps resulting in 16 overall points plus one additional point on the very surface. Data acquisition times of 1000 s for the T-component and 1500 s for the L-component were applied to compensate for texture related intensity variations in the plates, and 1200 s for the N-component. Acquisition times were increased by 50% for the first measurement position (closest to the peened surfaces) where data were taken with 1⁄2 submerged gauges. This gave diffraction peak intensities to provide Δd/d better than 10 . At each depth, measurements were taken by stepwise translation of the sample parallel to the surface at each depth (equivalent to “oscillating the sample”) over a distance of 2 mm in 0.5 mm steps to improve the grain statistics. This protocol is depicted in the measurement grid shown in Fig. 2a-c. The 5 data points at each depth were averaged in the data analysis. (a) (b) (c) Fig. 2: Depiction of the measurement grid employed for the longitudinal (a) and transverse (b) component measurements on KOWARI. A cross sectional view of the in-plane and throughthickness measurement grid is given in (c). Results A reasonable assumption for thin plates is that the stress normal to the surface σ⊥ is zero, from which the in-plane stress (σ//) component can be determined by enforcing a bi-axial stress condition σ// = 1 1⁄2S2 (hkk) � d// − d ⊥ d0 � (1) where 1⁄2S2(hkl) is the corresponding diffraction elastic constant for the material and reflection, d// (dL or dT) and d⊥ (dN) respectively the in-plane and normal components of the lattice plane spacing at Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 413-418 doi: http://dx.doi.org/10.21741/9781945291173-70 416 the corresponding depth positions. The values for the diffraction elastic constants of aluminium used are S1(311) = -5.158 TPa and 1⁄2S2 (311) = 19.574 TPa as determined from the single crystal elasticity constants using the Kröner approximation. Under the plane-stress

Residual Stresses Associated with the Production of Coiled Automotive Springs

Cold coiling of high tensile steel rod into helical coil springs for the automotive industry is a new technique being implemented amongst spring manufacturers worldwide. To characterise this coil production process, the neutron strain scanning technique has been employed to non-destructively elucidate the influence production stages have on the tri-axial residual stress state. Samples investigated represented key production steps in the cold-coil forming process: Cold coiling; Tempering; Hot setting; Hot peening; Shot peening. Investigations revealed that the stress field was axi-symmetrical, that the dominant variation in all samples occurred along the hoop direction (helical circumference), whilst the radial and axial stresses are substantially lower. Accurate two-dimensional stress maps of the rod cross section have been compiled revealing key features associated with the cold coiling step. Comparison of the stress fields after each production step revealed altered stress values. The final shot peening process stage not only reduced stress concentrations at the internal bore, but contributed to the establishment of favourable surface residual stress conditions that enhance the fatigue life of the final product.