Non-impact Fatigue Research Articles

Design of a continuously repeated impact method with constant amplitude based on Hopkinson bar: Principle and impact fatigue life testing

The lack of reliable testing methods and standards hinders systematic research on the impact fatigue performance of materials. The aim of developing an impact fatigue test method is to generate repeated impact loads with a constant amplitude, which is difficult to achieve using various existing impact test devices. This study investigated the principles of cyclic loading-resetting and constant-amplitude loading based on the split Hopkinson pressure bar (SHPB) system, established a novel impact fatigue test device, and verified the developed method by testing the impact fatigue life of a Ti-6Al-4V alloy. Electromagnetic valves, laser sensors, and a vacuum pump were used in the developed device to launch and reset the striker bar; a servo motor and laser sensors were used to reset the loading bars and specimen, and the entire system was controlled by a programmable controller to achieve continuously repeated loading. To achieve constant-amplitude loading, the conditions required to ensure that the specimen moves away from the incident bar before the returning stress wave reloads the specimen was derived. It was found that a constant-amplitude repeated impact on the specimen could only be achieved through the precise design of the geometric configuration and material of the loading bars. This method is considerably simpler than various existing methods based on energy absorption and is more applicable to impact fatigue tests. The verification tests showed that the highest loading frequency of the device was 0.5 Hz, the loading rate exceeded 105 kN/s, and the amplitude error of the repeated impact loads did not exceed 2.45 %. Under loads with the same amplitude, the impact fatigue life of a Ti-6Al-4V alloy was considerably shorter than the non-impact (low strain rate) fatigue life, which indicates the necessity of investigating the strain rate effect on the fatigue performance of materials.

Impact fatigue behavior of laser metal deposition 316L stainless steel with an automated impact fatigue system based on split Hopkinson bar technique

Repeated impact subjecting on the key components of equipment is always characterized by extremely high loading rate loading, while plastic deformation of most metals depends on loading rate. To ensure the reliability and security of these components, it is necessary to understand the loading rate effect on the fatigue performance. In this study, an automated impact fatigue system based on split Hopkinson bar technique was newly developed to investigate the materials’ impact fatigue behavior. Impact fatigue behavior and non-impact fatigue behavior of laser metal deposition 316L stainless steel were analyzed. The fatigue life decreases with the increasing loading rate, implying the significant loading rate dependence. The compression and fracture toughness tests combined with fractography analysis at high and low loading rates were also conducted to reveal the underlying mechanism of the difference between impact and non-impact fatigue life. Finally, microstructure of the laser metal deposition 316L stainless steel after impact and non-impact fatigue was characterized. It can be concluded that the fatigue crack is apt to initiate under impact fatigue due to the high density of dislocation entanglement, while under non-impact fatigue, a large number of twins are beneficial to delay the fatigue crack initiation.

Estimating durability of steels at repeated bending impacts

Many components are subjected to repeated impacts, or in some cases these impacts can appear as additional loads. Repeated impacts define a fatigue phenomenon known under the name of Impact Fatigue. Because the strain rate changes the material characteristics it is to expect that the material properties at impact fatigue to be different in regard to those obtained at non-impact fatigue. This paper presents a classification of repeated impact tests, and starting from this a series of parameters used for durability estimation will be analyzed. The high number of parameters used by different authors creates difficulties in comparison the different laboratories results. The importance of the shape and dimensions of specimens, and the stiffness of supports are highlighted. In order to avoid these influences the authors proposed an experimental technique, based on testing of Charpy specimens, in similar conditions as single impact test. A new parameter η is proposed in order to correlate the durability at repeated impacts with the Charpy V Notch (CVN) impact energy.

Models for Estimating the Durability of Repeated Impacts

Many components from industry are subjected to repeated impacts, or in some cases these impacts can appear as additional loads. Repeated impacts define a fatigue phenomenon known under the name of Impact Fatigue. Because the strain rate changes the material characteristics it is to expect that the material properties at impact fatigue to be different in regard to those obtained at non-impact fatigue. First studies at repeated impacts were made at the middle of 19th Century, but the progress in this field is not as fast as non-impact fatigue, due to experimental difficulties and the lake of standards for impact fatigue tests. This paper presents a classification of repeated impact tests, and starting from this a series of parameters used for durability estimation will be analyzed. The high number of parameters used by different authors creates difficulties in comparison the different laboratories results. The importance of the shape and dimensions of specimens, and the stiffness of bearing are highlighted. In order to avoid these influences the authors proposed an experimental technique, based on testing of Charpy specimens, in similar conditions as single impact test. The paper presents a series of results obtained for additional impacts overlapped to a cyclic load.

衝撃荷重下における原子炉用黒鉛の曲げ強度および疲労強度特性

The graphite components in HTR (High Temperature gas-cooled Reactor) are subjected to impact force when an earthquake occurs. It is important from a viewpoint of seismic safety design to investigate the difference between impact and nonimpact strengths.Both bending strength and bending fatigue strength tests, therefore, were carried out under impact and nonimpact loads on two kinds of graphite materials, that is, isotropic and near-isotropic ones. The impact response analyses, in which a beam model was used by taking account of the contact behavior between the specimen and a tap, were performed to evaluate the relationships among impact energy, impact force and impact stress.The main conclusions obtained are summarized as follows;(1) A beam model taking account of the contact behavior through the Hertzian theory is applicable to describe the impact behavior.(2) The bending strength of graphite is independent of strain rate in the range 10-6 to 5l/s.(3) The strength of graphite is lower in impact fatigue than in nonimpact fatigue, irrespective of kind of graphite and specimen volume.

Impact fatigue characteristic of martensitic stainless steel and the relationship between the impactive fatigue crack growth behavior and the dynamic stress-strain response of marerial.

It has already been indicated, as a general trend in impact fatigue, that the impact fatigue strength is lower than the ordinary fatigue strength, and that the crack growth rate under impact fatigue is higher than that under non-impact fatigue, excepting a few materials with relatively high static strength. The first aim of this study is to clarify the impact fatigue behaviors of high-strength alloy steels with the ultimate tensile strength of about 1800 MPa, and the second is to evaluate the crack growth behavior under impact fatigue load in connection with the dynamic stress-strain response. For these purposes, a series of impact fatigue tests were first carried out on martensitic stainless steel, and then single impact tests were performed to obtain dynamic stress-strain relation-ships of the martensitic stainless steel and the other three steels whose crack growth behavior in impact fatigue were already obtained by the authors. The S-N property of the martensitic stainless steels is similar to the other steels but the fatigue crack growth rate under impact fatigue load is equivalent to that under ordinary fatigue load. An interesting finding is that the crack growth behavior of each material under impact fatigue load is well correlated with the dynamic stress-strain response of that material.

CJ90-18. A study of crack propagation in impact fatigue and non-impact fatigue : By Yu Jieet al. (Xi'an Jiaotong University, Xi'an, China). Chinese J. Mech. Engng26, No. 2 (April, 1990), pp. 14–18

Organic amendment (OA) has positive effects on rice yield in literature, but few studies have reported positive effects on rice yield trend in a region, which need to be distinguished for the development of sustainable agriculture in terms of management of organic resources available in agroecosystems. The objectives of the present study were to examine the long-term effects of OA on rice yield, yield trends and soil fertility change under the same double rice cropping system at three experimental sites established in 1980s in subtropical China. Organic amendments at the sites covered different strategies in use of locally available green manure, rice straw and farmyard manure to partially substitute inorganic fertilizers at different doses of recommended N, P and K in combination at Jinxian, Jiangxi Province, to partially substitute inorganic N at rates of 30%, 50% and 70% of the recommended rate at Nanchang, Jiangxi Province, and to fully substitute single element of inorganic N, P and K at Qiyang, Hunan Province. The actual rice yield showed no significant time trends at Jinxian and Nanchang and negative trends in the treatments with low N application rates at Qiyang. Analysis of the relative yield trends showed significant positive effects of OA on the yield trends for either first or second rice crop in the treatments except for those without sufficient nutrient supply. The positive effects can be attributed mainly to the increased soil organic carbon and soil nutrient capacity due to the long-term application of OA. The results suggest that the organic amendments strategies should be encouraged as partial substitution of inorganic fertilizers for the consideration long-term yield trends and soil fertility.

Impact bending fatigue and impact response behavior of a nuclear-grade graphite beam

Repeated impact bending tests have been carried out on a fine-grained Isotropic graphite for HTTR under constant energy condition. Each specimen whose length differed was centrally impacted by an instrumented pendulum. The simple one-dimensional model taking the Hertzian contact stiffnesss into consideration has been applied to evaluate the relationship between the maximum stress of specimen generated by impact, the impact force, and the impact energy. It has been confirmed that the endurance curves in terms of impact energy are dependent on the length of the specimen, but the one in terms of the evaluated maximum stress is independent of that, and the strength of graphite is lower in impact fatigue than in nonimpact fatigue.

Fatigue crack growth and microscopic crack opening behaviour under impact fatigue load

The fatigue crack growth rates and crack opening behaviours were observed under impact and non-impact fatigue loads for high strength, low alloy HT-60 steel and austenitic SUS316 stainless steel, which had almost the same ultimate tensile strength of about 600 MPa. Particular attention was paid to the microscopic crack opening behaviour near the crack tip under two load conditions, evaluated using a fine grid method. The results showed no difference between the crack growth rates under impact or non-impact fatigue loads in the case of HT-60 steel, but in the case of SUS316 steel the crack growth rate under impact fatigue was much higher than that under non-impact fatigue. Such a difference between the two steels could be correlated with the microscopic crack opening behaviour under two load conditions; in the case of SUS316 steel, both the effective stress intensity range at the crack tip and the effective crack opening displacement behind the crack tip were larger for the crack formed by impact fatigue load than for the crack formed by non-impact load, whereas in the case of HT-60 steel, the difference between the values under impact and non-impact fatigue loads was significantly smaller.

Impact bending fatigue behavior of an isotropic nuclear graphite for HTTR.

Repeated impact bending tests were carried out on isotropic nuclear graphites for HTTR. Each specimen whose length differed was centrally impacted by an instrumented pendulum. The simple one-dimensional model taking the Hertzian contact theory into consideration has been applied to evaluate the relationship between the maximum stress of the specimen generated by impact, the impact force and the impact energy. It has been confirmed that the endurance curves in terms of impact energy is dependent on the length of specimen but the one in terms of the evaluated maximum stress is independent of that, and the strength of graphite is lower in impact fatigue than in non-impact fatigue.

衝撃荷重下での疲労き裂進展挙動と微視的開口挙動

This paper describes the fatigue crack growth and crack opening behaviors in impact fatigue of two alloy steels; one is a high strength low alloy steel HT-60 and the other is an austenitic stainless steel SUS316. Both of the alloy steels have almost the same ultimate tensile strength of about 600MPa. Major attention was paid to the microscopic crack opening behavior under impact fatigue load measured by means of the fine grating method. The results of the crack growth tests indicated that the crack growth rate in impact fatigue is higher than that in non-impact fatigue for SUS316 alloy steel, but not for HT-60 alloy steel. Such a difference in crack growth behavior between two steels was correlated to the following microscopic crack opening behavior of these steels: In the case of SUS316 alloy steel, both of the effective stress intensity range at a crack tip and the effective crack opening displacement behind the crack tip are larger for the crack propagated under repeated impact load than those for the crack under non-impact load, but in the case of HT-60 alloy steel, the difference in those values measured under impact and non-impact loads is relatively small comparing to the case of SUS316 alloy steel.

The influence of impact velocity on the impact fatigue crack propagation behavior in stainless steel, SUS316.

The fatigue crack growth behavior in austenitic stainless steel, SUS316, under repeated impact loading was investigated by means of fracture mechanics. The crack growth rate of a solution heated specimen was greater than that of an annealed specimen for both non-impact fatigue and impact fatigue. The effect of impact velocity on the crack growth rate depended on the Kmax level. The crack growth rate during repeated impact loading was greater than that in the sinusoidal stress wave. The fracture appearance at a low ΔK level was structurally sensitive modes, like intergranular cracking, regardless of repeated impact and sinusoidal loading. The dominant fracture appearance at a high ΔK level during repeated impact loading was the ductile mode, like striation.

重ね合せ接着継手の衝撃疲労強度

Adhesive bonded lap joint is fundamental to many adhesive joint design in industry. Hence, to investigate the effects of basic factors, such as over-lap length, on its fatigue strength is important to establish the design standard for the lap joint.In this study, the effect of over-lap length on fatigue strength was investigated using a single lap joint bonded with an epoxy-polyamide adhesive under impact and non-impact stress conditions.From the comparison of the non-impact fatigue strength τmax and the static shear strength τb by taking the over-lap length as a parameter, it was revealed that the longer was the over-lap length, the lower was the non-impact fatigue strength, τmax, and also the value of τb became lower at the same degree as that of τmax, as given by the following correlation. τmax/τb=1.71Nf-0.12 where Nf is the number of stress cycles to failure.However, when the impact fatigue strength on the S-N diagram was compared with the non-impact one, it was observed that the former showed more rapid lowering trend with increasing number of stress cycles than the latter. Furthermore, with increasing the over-lap length, or with increasing the stress concentration factor, the impact fatigue strength decreased more rapidly than the non-impact fatigue strength.

Study on Crack Growth Behavior in Impact Fatigue : Part III, Fatigue Crack Growth Behaviors of 2017-T3 Aluminum Alloy

Impact fatigue crack growth behaviors were investigated by using 2017-T3 aluminum alloy, one of the representative f.c.c. metals for mechanical structure, as an experimental material under the stress ratios R=0 and -1. Results are discussed in comparison with those obtained under non-impact fatigue done under the same stress ratios. It is made clear, on log da/dN-log Kmax diagram, that da/dN in impact fatigue is higher than that in non-impact fatigue under both stress ratio conditions. Furthermore, such a discrepancy in da/dN due to the difference in the stress condition does not disappear in the evaluation done based on the effective stress intensity factor range ΔKeff. And, peculiarities of impact fatigue crack growth behavior obtained in this study well resemble those for low carbon steel demonstrated previously by the authors. It is concluded that the effect of the compressive load in impact fatigue loading of R=-1 on da/dN is smaller than that in non-impact fatigue.

Effect of Microstructure on Fatigue Strength and Resistance to Fatigue Crack Propagation in Quenched and Tempered Cr-Mo Alloy Steel

The influence of microstructure on fatigue strength and resistance to fatigue crack propagation has been investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel, Then, the impact fatigue tests were carried out by using a rotating disk type impact fatigue testing machine, These results were compared with those for non-impact fatigue tests, The results obtained are summarized as follows.(1) For the non-impact fatigue, the fatigue strength and the resistance to fatigue crack propagation were markedly influenced by the microstructure. In other words, the 600°C tempered material had the most superior property of crack propagation resistance, while this material showed the most inferior resistance to crack initiation among the materials tested. The 300°C tempered material has a considerably inferior resistance to both the initiation and the propagation of fatigue crack because of low temperature temper brittleness. It was concluded from the present work that the temper treatment at 450°C was one method to improve the resistance to both the initiation and the propagation of fatigue crack.(2) For the impact fatigue, the resistance to crack propagation increased with decreasing tempering temperature. This result was not in agreement with the one described above for the non-impact fatigue. If the impact fatigue fracture behavior of component is estimated from the results of non-impact fatigue, it may involve a considerable risk.(3) The crack growth rate for impact fatigue was larger than that for non-impact fatigue. This difference increased with increasing tempering temperature. This behavior could be explained by the authors' concept of the equivalent stress ratio, in which the effect of impact loading is replaced with the stress ratio in non-impact fatigue.

高強度鋼の衝撃疲労および衝撃でない疲労のき裂進展速度の比較について 衝撃負荷と応力比および強度レベルの関係

The impact fatigue tests were carried out by using a rotating disk type impact fatigue testing machine. The influence of impact loading, stress ratio and yield strength on fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered 0.50% carbon steel. Then, the results of impact fatigue tests were compared with those of nonimpact fatigue tests. The results obtained are summarized as follows.(1) For the 423K tempered material, the dominant fracture appearance at impact fatigue was the dimple mode and the cleavage facet while that at non-impact fatigue was the intergranular facet. Such a fracture appearance was called the monotonic one. It was apparent that the acceleration of crack growth rate resulted from the rapid increase in the area percentage of monotonic fracture appearance regardless of the impact fatigue and the non-impact fatigue.(2) For the other tempered materials except the 423K tempered one, the striation was the dominant fracture appearance. However, the fracture appearance of the 573 and 873K tempered ones contained some inclusions and precipitation particles among the striation.(3) The effect of impact loading, Eac, defined by the ratio of impact fatigue striation spacing, Simp, to non-impact fatigue one, Snon, approached to about 1 as the yield strength, σys increased. The effect of impact loading could also be replaced by the equivalent stress ratio, Req. Then, the Req has related to σys as follows, Req=1-α·Eac-0.5=1-1.88×10-3·α·σys0.84.The Req decreased with increasing σys. This tendency was similar to the effect of stress ratio in non-impact fatigue.(4) The impact fatigue crack growth rate at a given ΔK level was larger than that in non-impact fatigue. This difference in crack growth rate increased with increasing tempering temperature. This fact could be well explained by the concept of equivalent stress ratio.

THE INFLUENCE OF TEST TEMPERATURE ON IMPACT FATIGUE CRACK GROWTH BEHAVIOR IN QUENCHED AND TEMPERED Cr-Mo ALLOY STEELS WITH DIFFERENT PRIOR AUSTENITE GRAIN SIZES

Abstract— Impact fatigue tests were carried out using a rotating-disk type impact fatigue testing machine. The influence of prior austenite grain size, ductile-brittle transition temperature and test temperature on impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varied from 8–3 to 25-4 μm. The results in impact fatigue tests were compared to those under non-impact conditions. The crack growth rates associated with striation formation were insensitive to the change in prior austenite grain size, ductile-brittle transition temperature and test temperature regardless of impact and non-impact fatigue. When the material was in the brittle condition, impact fatigue gave rise to a transition from striation formation to intergranular and cleavage cracking. Such a transition will result in the acceleration of crack growth rate. The Paris Law exponent values in impact fatigue were reasonably expressed by the ratio of test temperature to ductile-brittle transition temperature.

Impact Fatigue Crack Growth Behavior at Low Temperatures

The impact fatigue tests were carried out using a rotating disk type impact fatigue testing machine. The influence of ductile-brittle transition temperature and test temperature on the impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varried from 8.3 to 25.4μm, and the results of impact fatigue tests were compared with those of non-impact fatigue tests. The results obtained were as follows.(1) The impact fatigue and non-impact fatigue crack growth rates associated with striation formation at stage IIb were insensitive to the change in ductile-brittle transition temperature and test temperature.(2) In impact fatigue, it was found that the impact loading gave rise to transition of dominant fracture appearance from striation formation at stage IIb to intergranular cracking and cleavage at stage IIc as the Charpy impact value was decreased.(3) In impact fatigue, the transition of dominant fracture appearance from exclusively striation formation to intergranular cracking and cleavage was found to result in the acceleration of crack growth rate. The relation between the m value in Paris's equation and the ratio of test temperature, T, to ductile-brittle transition temperature, TM, was expressed as follows.m=1.12(T/TM)-1.99.

衝撃プログラム荷重下における疲労強度および疲労き裂の発生，進展について

As one of the investigations to reveal the effect of impact load on the fatigue behavior, i.e., fatigue life, fatigue crack initiation and propagation, a series of fatigue tests was conducted with a cantilever type rotating bending fatigue testing machine equipped with an impact loading device under two sorts of program load patterns; one is the impact program load condition in which the maximum bending stress followed by the transitional decaying vibrational load appears by an impact load at the rise time of the primary stress and another the ordinary periodical 2-step program load condition.Here, annealed 0.23%C carbon steel specimens with a small hole on the plane of the minimum sectional area were used to observe the behavior of fatigue crack initiation and propagation during the fatigue process. The experimental results were discussed from two points of view; the equivalent number of overstressing cycles and the cumulative damage in the modified Miner's rule concept.From the comparison of the equivalent number of overstressing cycles for crack initiation with that for fatigue failure, it was concluded that the impact program load pattern like the one in this experiment is more detrimental to propagate fatigue cracks than to initiate those. Such a fatigue behavior in the impact program condition is also recognized from the discussion based on the cumulative damage in the modified Miner's rule concept, that is, the ratio of the cumulative damage for crack initiation to that for fatigue failure takes generally a higher value than the value in nonimpact fatigue load conditions.

FAILURE MECHANISMS IN IMPACT FATIGUE OF METALS

Abstract Impact fatigue tests were performed with smooth and notched specimens of low carbon steels under various impact loading conditions. The characteristic failure mechanisms in impact fatigue was discriminated by comparison with those in non-impact, ordinary fatigue. The fatigue life of smooth specimens was uniquely related to the range of plastic strain at the middle of the fatigue life in both impact and non-impact fatigue, although the characteristics of micro-structural deformation and cyclic stress-strain relationships were markedly different. The growth rate of a fatigue crack in impact fatigue of notched specimens was higher than that in non-impact fatigue when compared at the same stress intensity factor. Fractographic observations with scanning electron microscopy and the X-ray diffraction technique revealed more abundant cleavage facets and a smaller spread of the plastic zone beneath the fracture surface made by impact fatigue. Both nucleation and propagation lives in notched specimens were much shorter in impact fatigue than in non impact fatigue when compared at the same values of nominal stress and stress concentration factor.