Estimation of load history by residual stress relaxation

Focusing on the impact of machining on structural integrity and fatigue life of components the surface and subsurface properties are of major importance. It is well known that machining induced residual stresses have a significant influence on the fatigue life of a component. Due to thermal and mechanical loads during a product's life cycle these stresses relax, which is undesired in most cases. The presented approach utilizes relaxations due to mechanical load to estimate the load history of a component. It is intended to qualify residual stress relaxation as a load sensor and to determine the limits of this approach. Therefore, it is demonstrated, how the residual stress state induced by turning of AISI 1060 determines the critical load causing relaxation. Subsequently, the influence of load stress and the number of load cycles is used to build up a model. The presented approach accesses load information from mass production components. Until now, this information is typically limited to prototypical developments or high price parts equipped with external sensors. One application of life cycle data is condition-based maintenance. This technology allows to extend service intervals and prevent a premature replacement of undamaged components. Thus, cost and resource efficiency are augmented. It is demonstrated that based on the changes of residual stress, possible mechanical loads and number of load cycle combinations can be identified. The changes are used to estimate the experienced loads.DFG/CRC/65

Cyclic behavior and microstructural stability of ultrafine-grained AA6060 under strain-controlled fatigue

AbstractThe strain-controlled fatigue behavior of AA6060, a precipitation hardening aluminum alloy, was investigated in ultrafinegrained (UFG) conditions after severe plastic deformation (SPD) by equal-channel angular pressing (ECAP). Two as-processed conditions, representing different stages of strain hardening and grain refinement as well as a ductility-optimized condition, achieved by a combined ECAP and aging treatment were considered. Low-voltage scanning transmission electron microscopy on samples stopped at characteristic stages of the fatigue process was applied to investigate the microstructural development. The as-processed as well as the optimized condition showed cyclic softening, which was found to be dependent on the amount of prestrain induced by ECAP processing. This is linked to dynamic recovery processes in the microstructure, indicated by a clearer distinction of grain boundaries and a reduction of dislocations in the grain interior. For all applied plastic strain amplitudes, ranging from Δεpl/2=1×10−3 to 5×10−3, the fatigue life of the ductility-optimized condition did not reach that of the severely work-hardened counterpart. For explaining this unexpected result, the differing (size-dependent) effectiveness of precipitates for the pinning of dislocations during cyclic loading was considered

Grinding of riblets with "beaver tooth" multi-layer tools

To reduce friction in turbo machinery components, riblets are induced on compressor blades or pump impellers. Here, the grinding process enables a higher productivity in machining of riblet structures compared to knurling, laser or milling operations. Usually, profiled grinding tools are used to create such structures inspired by sharkskin. Unfortunately, conventional grinding tools have to be dressed continuously to keep the desired profile in the circumferential surface. To avoid the time-consuming dressing process and to enable a self-sharpening effect, an innovative multi-layer tool concept is developed. The tool consists of two types of thin polyimide layers. The first type contains abrasives and the second is a support layer without abrasives. These layers are piled alternately in a special manufacturing process and act like a monolithic tool in grinding process. The aim of the investigations presented in this paper is to find an optimal parameter setting to produce riblet structures productively by using the self-sharpening effect. The optimal setting allows a grinding process without any dressing process by using a large part of the grinding tool volume. At first, the manufacturing process is focused to create clearly divided support and abrasive layers of the grinding tool. Furthermore, the investigation shows the relationship between grinding parameters and the setback of the supporting layer in the middle of the tool. This setback is important for the creation of riblet structures in the surface of AISI 420 workpieces

Finite strain viscoplasticity with nonlinear kinematic hardening: phenomenological modeling and time integration

This article deals with a viscoplastic material model of overstress type. The model is based on a multiplicative decomposition of the deformation gradient into elastic and inelastic part. An additional multiplicative decomposition of inelastic part is used to describe a nonlinear kinematic hardening of Armstrong-Frederick type. Two implicit time-stepping methods are adopted for numerical integration of evolution equations, such that the plastic incompressibility constraint is exactly satisfied. The first method is based on the tensor exponential. The second method is a modified Euler-Backward method. Special numerical tests show that both approaches yield similar results even for finite inelastic increments. The basic features of the material response, predicted by the material model, are illustrated with a series of numerical simulations.Comment: 29 pages, 7 figure

Low-Cycle Fatigue of Ultra-Fine-Grained Cryomilled 5083 Aluminum Alloy

The cyclic deformation behavior of cryomilled (CM) AA5083 alloys was compared to that of conventional AA5083-H131. The materials studied were a 100 pct CM alloy with a Gaussian grain size average of 315 nm and an alloy created by mixing 85 pct CM powder with 15 pct unmilled powder before consolidation to fabricate a plate with a bimodal grain size distribution with peak averages at 240 nm and 1.8 μm. Although the ultra-fine-grain (UFG) alloys exhibited considerably higher tensile strengths than those of the conventional material, the results from plastic-strain-controlled low-cycle fatigue tests demonstrate that all three materials exhibit identical fatigue lives across a range of plastic strain amplitudes. The CM materials exhibited softening during the first cycle, similar to other alloys produced by conventional powder metallurgy, followed by continual hardening to saturation before failure. The results reported in this study show that fatigue deformation in the CM material is accompanied by slight grain growth, pinning of dislocations at the grain boundaries, and grain rotation to produce macroscopic slip bands that localize strain, creating a single dominant fatigue crack. In contrast, the conventional alloy exhibits a cell structure and more diffuse fatigue damage accumulation

Ultrafine grained plates of Al-Mg-Si alloy obtained by Incremental Equal Channel Angular Pressing : microstructure and mechanical properties

In this study, an Al-Mg-Si alloy was processed using via Incremental Equal Channel Angular Pressing (I-ECAP) in order to obtain homogenous, ultrafine grained plates with low anisotropy of the mechanical properties. This was the first attempt to process an Al-Mg-Si alloy using this technique. Samples in the form of 3 mm-thick square plates were subjected to I-ECAP with the 90˚ rotation around the axis normal to the surface of the plate between passes. Samples were investigated first in their initial state, then after a single pass of I-ECAP and finally after four such passes. Analyses of the microstructure and mechanical properties demonstrated that the I-ECAP method can be successfully applied in Al-Mg-Si alloys. The average grain size decreased from 15 - 19 µm in the initial state to below 1 µm after four I-ECAP passes. The fraction of high angle grain boundaries in the sample subjected to four I-ECAP passes lay within 53-57 % depending on the examined plane. The mechanism of grain refinement in Al-Mg-Si alloy was found to be distinctly different from that in pure aluminium with the grain rotation being more prominent than the grain subdivision, which was attributed to lower stacking fault energy and the reduced mobility of dislocations in the alloy. The ultimate tensile strength increased more than twice, whereas the yield strength - more than threefold. Additionally, the plates processed by I-ECAP exhibited low anisotropy of mechanical properties (in plane and across the thickness) in comparison to other SPD processing methods, which makes them attractive for further processing and applications