D. Löhe

INVESTIGATION OF SIZE-EFFECTS IN MACHINING WITH GEOMETRICALLY DEFINED CUTTING EDGES

The miniaturization of cutting processes shows process specific size-effects like the exponential increase of the specific cutting force k c with decreasing depth of cut h. Experiments were carried out in an orthogonal turning process. The influence of different process parameters on the results was investigated separately to identify process specific size-effects. Two materials were studied: a normalized steel AISI 1045 and an annealed AISI O2. To complement the experiments, parameter variations were performed in two-dimensional, thermo-mechanically coupled finite element simulations using a rate-dependent material model and analyzed by similarity mechanics. The influence of rounded cutting-edges on the chip formation process and the plastic deformation of the generated surface were determined numerically. The complex physical effects in micro-cutting were analyzed successfully by finite element simulations and compared to experiments.

Formation of tribochemical films and white layers on self-mated bearing steel surfaces in boundary lubricated sliding contact

Abstract Formation of tribochemical layers and microstructural surface alterations during mild reciprocating sliding wear of AISI 52100 steel were studied under boundary lubricated conditions using a non-additive hydrocarbon lubricant. At the onset of relative motion no friction-induced chemical processes could be detected due to the protecting natural oxide film on the contacting steel surfaces. After a few sliding cycles, chromium carbides were pulled out of the ball surface leading to removal of the protecting layers and to the initiation of fast tribochemical reactions on the contact area. After about 1000 sliding cycles, so-called white layers developed on the ball surface showing greater carbon contents and hardness than the bulk material. This white layer formation could be attributed to high local pressures and subsequent grooving of the surface by wear debris. It was shown that the lubricant acted as a carbon donor owing to the adsorption of cracked hydrocarbon molecules on the steel surface. Adsorbed carbon atoms diffused into the bulk material and promoted the mechanically-induced phase transformation to the white layers.

Thermal-mechanical and isothermal fatigue of IN 792 CC

Abstract The cyclic deformation and lifetime behaviour of the cast Ni-base superalloy IN 792 CC was investigated both under thermal-mechanical fatigue (TMF) and isothermal fatigue (IF) conditions. During TMF the phase relations between temperature and mechanical strain were in-phase and out-of-phase, respectively. For both phase relations a similar cyclic deformation behaviour is observed. In all cases out-of-phase TMF causes tensile mean stresses, whereas in-phase TMF leads to compressive mean stresses. At T max below 800 °C out-of-phase cycling results in smaller lifetimes than in-phase loading. In spite of the rather high compressive mean stresses developing at T max above 800 °C, at these temperatures in-phase loading causes shorter lifetimes than out-of-phase TMF. This effect is due to the different damage mechanisms caused by in-phase and out-of-phase loadings: at higher T max considerable intergranular damage caused by in-phase loading reduces the lifetime below the respective values measured during out-of-phase TMF, after which no intergranular damage could be detected. A comparison of the TMF data with the cyclic deformation and lifetime behaviour under IF conditions shows that the materials reactions under TMF cannot be assessed satisfactorily by the results obtained from isothermal fatigue tests.

INFLUENCE OF FRICTION AND PROCESS PARAMETERS ON THE SPECIFIC CUTTING FORCE AND SURFACE CHARACTERISTICS IN MICRO CUTTING

For the production of small quantities of micro devices, machining is a low cost alternative to lithographic processing techniques. However, machining shows process specific size-effects upon miniaturization to the micrometer regime. Hence, the orthogonal turning process is chosen to study the influence of process parameters like uncut chip thickness h, cutting velocity vc and cutting edge radius rβ on the cutting force and the surface plastification by two-dimensional, thermo-mechanically coupled finite element simulations. A rate-dependent plasticity law is used for investigation of a normalized medium carbon steel (AISI 1045). Furthermore, the characteristics of the influences of the different parameters are analyzed mathematically by similarity mechanics. In particular, the frictional effects on the cutting process are studied in detail using a friction coefficient μ based on experimental results, and the influences of the process parameters on the cutting force and the plastic deformation of the surface layer are determined numerically. These results are compared with experimental measurements. The specific cutting forces are analyzed and discussed in detail. Size-effects observed experimentally are also found by numerical simulations.

Isothermal, thermal–mechanical and complex thermal–mechanical fatigue tests on AISI 316 L steel—a critical evaluation

Abstract Results are presented and compared with strain-controlled ‘isothermal fatigue’ (IF), strain-controlled ‘thermal–mechanical fatigue’ (TMF) and ‘complex thermal–mechanical fatigue’ (CTMF) tests. The IF and TMF tests were carried out with a common single-specimen testing system while the CTMF tests were performed with a two-specimen testing system which allows the interaction between a ‘hot’ and a ‘cold’ location of a component, e.g. the outer and the inner side of a cooled turbine blade, to be simulated. The test samples were made from the austenitic steel AISI 316 L. The complex interaction between both specimens in a CTMF test results in cyclic creep to negative total strains, which does not occur in strain-controlled IF and TMF tests. The cyclic deformation behaviour under CTMF conditions is not only determined by the hardening or softening of the material investigated, but even to a larger extent by the interaction of the specimens within the CTMF system. However, the microstructural changes under CTMF conditions can be correlated with the respective observations made under IF and TMF loadings.

Mechanical Properties of Compound-Extruded Aluminium-Matrix Profiles under Quasi-Static Loading Conditions

Compound-extruded unidirectionally reinforced lightweight profiles are a novel class of materials for the realisation of load-bearing structures. They may be fabricated in a flexible and rapid near-net-shape process. The authors present investigations of the reinforcing effect of wires in compound-extruded aluminum profiles under quasi-static tension and compression. In particular, the compounds were characterized by metallographic examinations focusing on the fracture morphology. Furthermore, specimens subject to compression tests were examined using micro computer tomography (µ-CT) and light microscopy (LM). It is shown, that the mechanical properties of wire-reinforced profiles are improved under both positive and negative quasi-static loads in comparison to non-reinforced profiles.

Fracture mechanical evaluation of the effects of grinding residual stresses on bending strength of ceramics

Abstract In order to evaluate the effects of grinding residual stresses on bending strengths, the near-surface distributions of grinding residual stresses of differently ground Al 2 O 3 samples were determined non-destructively by means of depth-resolved X-ray residual stress analyses. It was found that the compressive surface residual stresses increase with increasing single grain cutting depth. In parallel, an increase of the four-point bending strength of the samples was observed. Its magnitude was compatible with the increase of bending strengths expected from the residual stress intensity of the grinding residual stresses acting on the failure critical defects.

Finite Element Simulation of the Residual Stress States after Shot Peening

In a three-dimensional Finite-Element-Simulation of shot peening, a combined isotropickinematic viscoplastic material description was introduced in order to describe the cyclic softening effects during peening. After verifying the model in the simulation of push-pull tests at different strain amplitudes it could be used for the shot peening simulation. The simulated residual stress profile is compared with experimental results determined by X-ray diffraction and with simulated results of a simpler isotropic viscoplastic material model.

The Fatigue Behavior of an Aluminium Foam Sandwich Beam Under Alternating Bending

Aluminium foam-sandwiches which are applied in car bodies, e.g., as side impact protection structures, are loaded not only by quasistatic, but also by cyclic forces. If these fatigue loadings induce damage of the foam-sandwich structure, the stiffness, strength and impact behaviour may be adversely affected. The present study shows results of bending fatigue tests on an Aluminium foam-sandwich material that was manufactured by a powder metallurgy process with succesive rolling and foaming. The sandwich had a foam core with 10 mm and outer sheet layers with thickness 1 mm. Alternating bending tests with normal stresses parallel to the sheet plane were realised using a servoelectric fatigue testing machine. The sinusoidal loading was momentum controlled with a load ratio of R = –1 and frequencies up to 50 Hz. The fatigue limit was calculated from 18 tests using the stair case method with an ultimate number of cycles of 107. The cyclic deformation behaviour within the HCF- and the LCF-regime was determined from hysteresis loops of the bending moment versus the bending angle which were measured at selected load cycles during each test.The material shows pronounced cyclic hardening at the beginning of the fatigue loading due to work hardening processes mainly within the sheet layers. Afterwards, a cyclic neutral behaviour occurs until the end of the test. Damage by fatigue crack initiation generally starts within the sheet layers, mostly near large and deep pores within the gauge length. Subsequently, the cracks propagate firstly within the sheet layers and after that through the foamed core of the sandwich perpendicular to the bending axis.

Effect of superimposed high cycle fatigue loadings on the out-of-phase thermal-mechanical fatigue behaviour of CoCr22Ni22W14

The thermal-mechanical fatigue (TMF) behaviour of CoCr22Ni22W14 was investigated under total strain controlled out-of-phase (OP) experiments without and with superimposed high cycle fatigue (HCF) loadings. The minimum temperature T min was 200 °C and the maximum temperature T max was varied between 750 and 1200 °C. The mechanical strain amplitude e me a, during pure TMF tests was kept equal to the thermal strain amplitude e th 1 and the superimposed HCF amplitude e HCF a,t was varied between 0.05 and 0.2%. In both loading conditions cyclic hardening is observed, which is the less pronounced the higher T max is. Only at T max = 1200 °C, cyclic softening appears after cyclic hardening in the first cycles as a result of creep damage accumulation. With increasing superimposed HCF amplitudes, the cyclic deformation behaviour is obviously more and more determined by the superimposed HCF loadings. Due to the dynamic relaxation processes at higher temperatures, tensile mean stresses develop during all TMF tests performed. Under TMF-OP conditions a significant lifetime reduction is observed as a result of superimposed HCF loadings. This lifetime reduction increases with growing HCF amplitudes and may approach 90% of the fatigue lifetime obtained from pure OP experiments. For each T max the dependence between the total strain amplitude (e a.t = e me a.t + e HCF a.t ) and the number of cycles to failure can be described as a potential function (e a.t = A × N B b ) with an exponent b which decreases with increasing T max and which depends on the material properties at different temperatures.