Zum thermischen Abbau von Kugelstrahl-Eigenspannungen bei vergütetem 42 CrMo 4

Thermal relaxation of shot-peening-induced residual stress in quenched and tempered steel 42 CrMo 4 Thermal relaxation of shot-peening-induced macro and micro residual stress in a quenched and tempered state of the 42 CrMo 4 steel is investigated with X-rays and is described by the Avrami-approach. The parameters of this relationship can now be determined more exactly using a new, iterative method. By means of the parameters determined, the relaxation of macro and micro residual stress is evaluated and compared. The influence of annealing time and temperature on the dependence of macro residual stress on distance to surface is modelled using the Avrami-approach. The transient relaxation of macro residual stress during heating up to the temperature required can be determined by applying the stress-transient-method which will be presented here. It turns out that not the elapsed heating time but rather the relaxation of residual stress during heating up determines the further relaxation of residual stress.     

Zum Wechselverformungsverhalten von vergütetem 42 CrMo 4 bei zweistufiger Zug-Druck-Beanspruchung

Cyclic deformation behaviour of quenched and tempered 42 CrMo 4 (AISI 4140)at two-block push-pull-loading The behaviour of steels in the course of two- and multi-block cyclic loading has been investigated up to now almost exclusively regarding the fatigue life. According to this, only a few papers exist, dealing with the cyclic deformation behaviour at two- and multi-block-push-pull-loading. Therefore, in stress- and total strain-controlled experiments with a single change of the amplitude (two-block-experiments) and multiple changes between two blocks of different lengths and amplitudes (multi-block-experiments) the cyclic deformation processes have been investigated for the quenched and tempered steel grade 42 CrMo 4 (equivalent to AISI 4140). Using the data of stress- and strain-Wöhler-curves determined in usual fatigue tests, damages defined according to Miner's rule were adjoined to the blocks. The Miner-damages at failure observed in the two-black-experiments with changes from high to low amplitudes were smaller than one and at inverse changes of amplitudes larger that one. In contrast to this, in multi-block-experiments no universally valid correlations were observed between the Miner-damages at failure and the test-parameters applied. At all tests cyclic work-softening was observed as in single-step-experiments. However, work-softening processes at high amplitude loadings yield to much larger plastic strain amplitudes after changing to smaller amplitudes than in single-step tests. Contrarily, in multi-block-tests work-softening at higher amplitude loadings reduces with decreasing block-length and increasing portion of the blocks with the smaller amplitude. This is attributed to effects of static strain-ageing. Total-strain-controlled two-block cyclic deformation experiments yield to similar effects as in stress-controlled tests. However, the behaviour at high strain-amplitudes was influenced by distinct work-softening in the first cycles and by self-unloading due to the applied strain-control, which promotes quasi-stabilization-effects.     

Ermüdungsverhalten von normalisiertem 42CrMo4 unter Torsionsbeanspruchung bei Raumtemperatur

Room Temperature Fatigue Behaviour of a Normalized Steel SAE 4140 in Torsion Cyclic deformation behaviour of a normalized steel SAE 4140 in shear strain-controlled torsion is characterized by cyclic softening and cyclic hardening. If mean shear stresses are superimposed to an alternating shear stress, cycle-dependent creep occurs, and the number of cycles to failure decreases. In shear strain-controlled torsional loading, mean stresses are observed to relax nearly to zero within a few cycles. Fatigue life is not influenced by mean shear strains.     

Versuchsführungsbedingte Unterschiede im zyklischen Verformungsverhalten von Vergütetem 42 CrMo 4 bei Spannungs- und Totaldehnungskontrolle

Differences in the cyclic deformation behaviour of quenched and tempered steel 42 CrMo 4 (AISI 4140) due to stress- and strain-control Cyclic stress-strain-curves and Manson-Coffin-plots of quenched and tempered steel 42 CrMo 4 (AISI 4140) strongly depend on whether they are determined under stress- or total-strain-control. At total-strain-controlled experiments, this is caused on the one hand by comparatively high initial stress-amplitudes which lead to distinctive cyclic worksoftening. On the other hand, the occurring differences in the evolution of inhomogeneous deformation patterns at both types of loading, which can be recorded by means of photoelasticity and microscopy, lead to differently distributed plastic deformations and to different integral values of plastic strain.     

Lokale Last- und Eigenspannungsverteilungen an rißbehafteten Oberflächen nach Korrosionsermüdung von vergütetem Ck 45

Distributions of Loading and Residual Stresses in Cracked Surfaces of Corrosion Fatigued Quenched and Tempered Steel Ck 45 Corrosion fatigue in the active state leads to a great number of short cracks and corrosion pits in the surface of metallic materials. This paper deals with the resulting distributions of loading and residual stresses in the case of bending fatigue. The material investigated was the quenched and tempered CBN-ground steel Ck 45. Results presented clearly indicate that stress distributions around cracks and corrosion pits are very inhomogeneous. Corrosion pits lead to a relaxation of manufacturing induced residual stresses. In the loaded state as a consequence of notch effects, maximum stresses are observed, which explains the importance of corrosion pits as crack initiation sites.     

Rechnerischer Festigkeitsnachweis der Ermüdungstragfähigkeit von vergüteten und einsatzgehärteten Stirnrädern

The work presents a calculative proof of strength for compensated and case hardened spur gears taking into account the different stress conditions in the tooth root and on the tooth flank. The basis of the proof of stregth is the local comparision of the occured stresses and the allowed stresses in the spur gear. The complex stress condition on the tooth flank is illustrated. On the flank occours a three dimensional stress condition with turnig principial stress systems. Furthermore the influences on this stress condition of residual stresses and of the contact of technical surfaces are discussed. Especially the described complex stress condition on the tooth flank asks for a calculation with an adequate criterion of failure. For this purpose a variant of the Schubspannungsintensitätshypothese (SIH) is proposed. The applicability of the proposed model is verified with numerous results of experiments. Some of the calculations are presented here. Altogether there is a good conformity between the test results and the calculated results especially for the ammount of the fatigue limit, the kind of damage (e.g. tooth root breackage, pitting, tooth breackage in the region of the flank) and the position of the beginning of the damage.     

Werkstoffmechanische Beanspruchungs- und Lebensdaueranalyse zweistufig schwingbeanspruchter Biegeproben aus 42CrMo4

Strees-strain- and fatigue life analysis on specimens of 42CrMo4 under two-step bending Fatigue test results from unnotched specimens of 42CrMo4 under constant amplitude- and two-step-bending were analyzed using both the Local Strain Approach (LSA) and the Nominal Stress Concept (NSC). These analyses were carried out in order to compare the predictive accuracy of both concepts. As a first approximation it was assumed that only the materials ultimate tensile strength or the experimentally attained constant amplitude life curve at R = 0 respectively is known. Within the scope of LSA two damage parameters were applied. The parameter P_J considering mean stresses and load sequence effects and the parameter P_SWTconsidering mean stresses only. For the above mentioned assumptions the predictions of the LSA using the parameter P_SWT are slightly better than those obtained by the NSC. A pronounced improvement can be attained using the parameter P_J. However, compared to the predictions by the LSA in combination with P_SWTand cyclic material properties, the fatigue life calculated by the NSC using experimental constant amplitude loading curves are closer to the experimental results. Based on these findings, which can be interpreted as trends valid for other cases as well, the frequent use of the parameter P_J within the framework of the LSA is recommended. If available, experimentally attained constant amplitude loading curves should be used to adjust the P-Woehler curves.     

Messungen der Rißschließeffekte an Al 8090-T6 und 42CrMo4

Two different materials, a quenched and tempered steel with German designation 42CrMo4 and an Al-Li 8090-T6, were used to investigate the influence of crack closure on the lower part of the loading cycle. The usually measured closure parameter K_op was determined, too. But, the primary target of the present investigation was the lowest K_min-value experienced by the crack front in the tension-tension fatigue regime. This lowest K_min was denoted as K_min. In addition, the modification of ΔK to a lower value ΔK_red due to crack closure during the lower part of a fatigue cycle has been determined. It was found that for 42CrMo4 the influence of crack closure on K_min or ΔK is negligible, while for the Al-Li 8090-T6 closure can substantially alter K_min and ΔK. The two materials investigated can – in respect to their closure effects – be considered as the two boundary conditions enclosing the range of structural materials.     

Einfluß positiver und negativer Mittelspannungen auf das zyklische Spannungs-Dehnungsverhalten von normalisiertem Ck 45

Influence of Positive and Negative Mean Stresses on the Cyclic Stress-Strain Behaviour of Normalized Steel SAE 1045 In technical parts cyclic loads are often superimposed by static load components. In such cases, mean stresses influence the cyclic deformation behaviour and produce cyclic creep effects. Single step push-pull fatigue tests were carried out with normalized specimens of the steel SAE 1045. Using cyclic deformation curves and cyclic creep curves, the influence of different mean stresses will be discussed.     

Inhomogene Verformungserscheinungen bei Schwingbeanspruchung des Vergütungsstahls 42 CrMo 4

Inhomogeneous Deformation Processes During Cyclic Loading of Quenched and Tempered Steel SAE 4140 The cyclic deformation behavior of a normalized steel SAE 4140 in the loading range σ_a < R_eS is characterized by softening and hardening phenomena. Cyclic softening processes are combined with inhomogeneous plastic deformations and the propagation of a fatigue Lüdersband. The following hardening process occurs homogeneously. At stress amplitudes σ_a < R_p0.2 the quenched and tempered SAE 4140 worksoftens continuously until crack initiation. Plastic deformation processes are extremely inhomogeneous and are concentrated on small fatigue zones. The inhomogeneity increases with decreasing tempering temperatures und stress amplitudes.     

Aggregatzustände und Molekülbewegungen bei Kunststoffen

States of Aggregation and Molecular Movements in Plastics The molecular structure of amorphous polymers is discussed. Possibilities for molecular movements in the different states of aggregation are described and the relaxation phenomena are in vestigated which are responsible for the transition between the states of aggregation. The essentials of the mechanical behaviour of amorphous polymers in the various states of aggregationare described in relation to molecular structure.     

Ermüdung des Stahles 42CrMo4 unter mehrachsiger,phasenverschobener Beanspruchung

Fatigue of the Quenched and Hardened Steel 42CrMo4 under Multiaxial In- and Out-of-Phase Loading Two kinds of specimens out of the quenched and hardened steel 42CrMo4 (similar SAE 4140) were tested by combined tension-torsion in- and out-of-phase loading. The specimens batches differed in the residual stress state. Under cyclic, stress controlled loading an elastic behaviour is registrated until 50% of the life time. Then a rapid softening occurs, which correlates with the von-Mises equivalent stress in the case of in-phase loading. At high stress amplitudes, the residual stresses may be neglected. The lifetime is best described by the fatigue criterion of Zenner which considers the integral average of the stress state in every plane. This stress state is described by a linear combination of the shear stress amplitude and the normal stress amplitude. Below the cyclic yield strength, the residual stresses must be taken into account as static stresses. The comparison of the local residual stress distributions is possible by using the weakest link model of Heckel, which is described in detail.