Fatigue Strength Reduction Factor Research Articles

Fatigue Strength of Carbon-Steel Castings

This paper summarizes the results of fatigue tests of round specimens of carbon-steel castings 10, 35, 50 and 100 mm in diameter. The specimens were tested under rotating bending, pulsating tension, and completely reversed tension and compression in order to investigate the effects of various defects on the fatigue strength. The various defects were all deliberately made and included artificially drilled holes, blowholes, and shrinkage cracks. The fatigue strength, the fatigue ratio, and the fatigue strength reduction factor were obtained from the fatigue test results of specimens having these various defects, and the fatigue strength of steel castings was compared with that of steel forgings. The following results were obtained. (1) The fatigue strength, the fatigue strength reduction factor, and the size effect on the fatigue strength are smaller in the case of steel castings than in the case of steel forgings. (2) The fatigue strength of specimens having artificially drilled holes is approximately equal to that of specimens having blowholes of the same size. (3) In the case of small blowholes (d/D<0.2) the size effect is extremely small. (4) There is a correlation between the defect ratio, obtained from radiographic inspection or fatigue fracture of the specimens, and the fatigue strength. (5) From these fatigue test results, we obtained the endurance limit diagram of carbonsteel castings under direct stress.

Notch Effect on Strain-Controlled Low Cycle Fatigue Strength of 80kg/mm2 High Tensile Steel

This paper deals with the results of the fatigue test on hourglass type and round notched specimens of 80 kg/mm2 high strength steel. Strain controlled low cycle fatigue tests were carried out to investigate the notch effect on the fatigue strength reduction factor and the life ratio in the low cycle range. Two kinds of hourglass type and four kinds of round notched ones were tested. The theoretical stress concentration factor were varied in five conditions.The following conclusions were obtained : (1) The natural fracture strain in static tension, ef, is decreased as the theoretical stress concentration factor, Kt, increased. The degree of decrease of ef is large in the range of 1 to 1. 6 of Kt. (2) The relation between total true strain amplitude and number of cycles to the initiation of visible crack on the surface of specimen, Nc, was obtained as eta=CeNc-ke=CpNp-kp. (3) The values of C and k can be determined in the Kt-eta and Kt-C curves in the range of 1 to 4 of Kt value. For the condition that Kt is equal to 1. 00 the eta-Nc relation was obtained as eta=0. 36 Nc-0.50+0. 0054 Nc-0.087.(4) The relation between propagation life and failure life is represented as, Nf-Nc=0. 607 Nf0.89.(5) The fatigue strength reduction factor, Kf, e, was defined as the ratio of total true strain amplitude of smooth specimen (K=1.00) to that of notched specimen. Kf, e is always higher than K in the low cycle range. (6) The life ratio was defined as the ratio of failure life of smooth specimen (K=1. 00) to that of notched specimen. Life ratio increase logarithmicly with increasing of Kt value.

Some Experimental Studies on the Fatigue Strength of 7075-T6 Aluminium Alloy having Artificial Defects

We have already reported on the effect of intermetallic compound CrAl7 on the fatigue strength of extruded 7075-T6 aluminium alloy when the compound (about less than 2mm wide) is situated at the central part of the surface of unnotched specimen. This study was performed on the fatigue strength of the alloy specimens having square or hole-type artificial defects at their central surface, and compared with the above mentioned results.The Schenck's plane-vibrating fatigue tester has been used for fatigue experiments and the measurements of fatigue strengths have been made at 107 cycles. The shape, size and depth of artificial defects are shown in Fig. 2, Tables II and III. The square-type defects are made by electric-discharge method and hole-type defects are made by drilling process. The bottom of each defect is flattened.The fatigue strengths and fatigue strength reduction factors are shown in Figs. 6, 7, 9 and 10, respectively, and the fatigue strength of the specimen having an intermetallic compound is also indicated in Table IV.The fatigue strengths of the specimen having intermetallic compounds approximately correspond to the values of square-type 0.1mm depth and the fatigue strengths of hole-type defect specimens are higher than that of square-type defect specimens. Accordingly, the intermetallic compound may be acceptable as a square-type artificial defect, and the depth of the defect affects the fatigue strength more than the width of defect.

Design of Pressure Vessels for Low-Cycle Fatigue

Methods are described for constructing a fatigue curve based on strain-fatigue data for use in pressure vessel design. When this curve is used, the same fatigue strength-reduction factor should be used for low-cycle as for high-cycle conditions. When evaluating the effects of combined mean and alternating stress, the fatigue strength-reduction factor should be applied to both the mean and the alternating component, but then account must be taken of the reduction in mean stress which can be produced by yielding. The complete fatigue evaluation of a pressure vessel can be a major task for the designer, but it can be omitted, or at least drastically reduced, if certain requirements can be met regarding design details, inspection, and magnitude of transients. Although the emphasis in this paper is on pressure vessel design, the same principles could be applied to any structure made of ductile metal and subjected to limited numbers of load cycles.

On Non-Propagating Cracks in Fatigue of Metals

A new hypothesis for formation of non-propagating cracks under fatigue of metals is proposed. The hypothesis is based on coaxing effect and strain hardening effect and others, which the related material receives at the tip of a fatigue crack. Due to these effect, the material at the tip of fatigue crack is considered to be stronger at inner parts than at near surface. The fatigue strength for formation of cracks and that for destruction are considered. A critical value of the fatigue strength reduction factor, which means the minimum value of it for formation of non-propagating crack, is also considered. The relation between the critical fatigue strength reduction factor and the fatigue strength of two kinds above mentioned is obtained from the hypothesis proposed. The reduced relations are compared with the results of experiments made on rotating bending and repeated torsion, and a good agreement is noticed.

2Al-2Mn-Ti合金および工業用純チタニウムの高温疲労強度について

Titanium and its alloy are widely used as structural materials because of their excellent corrosion-resistance and heat-resistance properties.This report deals with the unnotched and notched fatigue strengths of 2 Al-2 Mn-Ti alloy and some commercially pure titaniums in a high temperature range from 20°C to 600°C, as well as the internal heating phenomenon at a room temperature of commercially pure titanium which affects their fatigue strength.The high temperature fatigue tests were conducted on Ono's high temperature fatigue tester (3000rpm) and a Krouse high speed high temperature fatigue tester (5000rpm). The fatigue tests at a room temperature are conducted on Ono's fatigue tester and a Schenck's vibrating fatigue tester.The high temperature mechanical properties of the specimens are shown in Fig. 3-7 and the notched and unnotched fatigue strengths in a high temperature at 107 cycles are also shown in Fig. 14, respectively.The unnotched and notched fatigue strengths of 2 Al-2 Mn-Ti alloy decreased remarkably from the values of room temperature up to 100°C, and in a range from 150° to 300°C, those rates steadily changed, and in the case of further temperature, those rates increased again.The unnotched fatigue strength of commercially pure titaniums decreased remarkably with the increase of temperature up to 200°-300°C, and those rates changed smoothly in further temperatures. The high purity specimen shows lower fatigue strength than that of lower purity specimen. The reduction percentage of fatigue strengths and fatigue strength reduction factor (3.0 theoretical stress concentration) of each specimen are shown in Fig. 15 and 16.In the high temperature fatigue tests, an unnotched specimen tested in high stress levels induced internal heating phenomenon and the specimen changed their colour due to generated heat. The authors presumed the temperature of generated heat from the degree of their discolouration of specimen and examined the modified S-N curves as shown in Fig. 17.Internal heating phenomenon at a room temperature of commercially pure titanium is more remarkable as to the high purity material than in the case of lower purity, and the maximum temperature of generated heat is related to repeated stress levels as shown in Fig. 22, and also the size effect of specimen influences on their phenomenon as shown in Fig. 27.

On Non-Propagating Cracks in Fatigue of Metals

A new hypothesis for formation of non-propagating cracks at fatigue of metals is proposed. The hypothesis is based on coaxing effect and strain hardening effect and others, which the related material receives at the tip of a fatigue crack. Due to these effects, the material at the tip of fatigue crack is considered to be stronger at inner parts than at near surface. The fatigue strength for formation of cracks and that for destruction are considered. A critical value of the fatigue strength reduction factor, which means the minimum value of it for formation of non-propagating crack, is also considered. The relation between the critical fatigue strength reduction factor and the fatigue strength of two kinds above mentioned is obtained from the hypothesis proposed. The reduced relations is compared with the results of experiments made on rotating bending and repeated torsion, and good agreement is noticed.

複合ローラチェーンの疲労強度について

Fatigue tests on the three strand roller chains having various fitting conditions between middle link plates and pins were made. Results are as follows:1. The fatigue strength of treble strand M-type roller chains (standard) having clearance between middle link plates and pins is twice as that of single strand chains.2. The fatigue strength of treble strand P-type roller chains having interference between middle link plates and pins is 2.4 times as large as that of single strand chains.3. The endurance ratio of multiple strand roller chains is smaller than that of single strand roller chains.4. The fatigue strength reduction factor due to increase of strands is nearly independent to the number of cycles to estimate the fatigue strength.5. The fatigue strength reduction factor takes the smallest value at P-type. M-type is the next. The value is the largest at L-type.6. The load distribution for the each strand seems to be nearly uniform at P-type, considering from the form of destruction. In the other types, it seems to be not uniform.

Fatigue Strength of Steel Specimens with Artificial Cracks

In this paper are described fatigue tests carried out to determine the fatigue strength of artificially cracked specimens made from mild steel, 0.45% C steel, 13Cr steel and Ni-Cr steel under rotary bending. Axially cracked specimens gave considerably lower fatigue strengthreduction factors, compared with that of partial circumferentially cracked or round cracked specimens, which have the value 2.4-3.4 when crack length or depth is nearly equal to 2 mm. The strength-reduction factor of a specimen with semicircular groove containing axial or circumferential cracks is equal to the product of the strength-reduction factor of the notch itself and that of the crack itself. And then the reduction factor depending on the diameter of artificially cracked specimens is nearly equal to 1.1. The shot peening process gave an increased fatigue strength of 18% for mild steel and 30% for Ni-Cr steel specimens with artificial cracks.

On the fatigue behaviour of commercially pure titanium (2nd report)

The effects of mean stress on the fatigue properties of annealed and 50 per cent cold rolled commercially pure titanium sheets were studied on Schenck's vibrating sheet bending machine. The notch effects on the fatigue strength of annealed rods were studied on Ono's rotary bending machine.Following results were obtained:(1) The endurance limit diagram in bending is shown in Fig. 4. To construct a diagram, the experimental points under the mean stress are plotted along a line drawn through the complete reversal stress amplitude and the nominal tensile strength.(2) The relation between theoretical stress concentration factor and fatigue strength reduction factor is shown in Fig. 6.(3) In the case of 3.1 value of theoritical stress concentration factor, the fatigue strength reduction factor of annealed commercially pure titanium rod is approximately the same as that of 14S-T6 and 24S-T4 aluminium alloys.

表面圧延による疲労強度向上に関する研究

The investigation was carried out to obtain the most suitable condition of surface rolling for the purpose to increase the rotating bending fatigue limit of filleted or V-notched specimen.A surface rolling apparatus was designed specially for this investigation.The results obtained are as follows:-(1) The most suitable surface rolling pressure and the increasing rate of fatigue limit increase with the sharpness of notch.(2) The limiting value of increasing rate of fatigue limit is (β-1)×100% at the filleted specimen, where β is the fatigue strength reduction factor.(3) The maximum increasing rate of fatigue limit is 133% at the V-notched specimen with the 0.4mm radius of notch.(4) The influence of number of rolling on the increase of fatigue limit was made clear.