Impact Fatigue Tests Research Articles

Influence of Stress Mode on Failure Micromechanisms of Nodular Cast Iron

The paper deals with comparison of the failure micromechanisms of nodular cast iron at static, impact and fatigue stress. Several specimens of ferrite-pearlitic nodular cast irons with different content of ferrite in a matrix were used for metallographic analysis, mechanical tests and microfractographic analysis. Mechanical properties were found out by static tensile test, impact bending test and fatigue tests. The microfractographic analysis was made with use of scanning electron microscope on fracture surfaces of the specimens fractured by these mechanical tests. Fracture surfaces of analysed specimens are characteristic of mixed mode of fracture. Failure micromechanism of nodular cast irons is dependent on mode of stress.

An experimental method to quantify the impact fatigue behavior of rocks

Fatigue failure is an important failure mode of engineering materials. The fatigue behavior of both ductile and brittle materials has been under investigation for many years. While the fatigue failure of ductile materials is well established, only a few studies have been carried out on brittle materials. In addition, most fatigue studies on rocks are conducted under quasi-static loading conditions. To address engineering applications involving repeated blasting, this paper proposes a method to quantify the impact fatigue properties of rocks. In this method, a split Hopkinson pressure bar system is adopted to exert impact load on the sample, which is placed in a specially designed steel sleeve to limit the displacement of the sample and thus to enable the recovery of the rock after each impact. The method is then applied to Laurentian granite, which is fine-grained and isotropic material. The results demonstrate that this is a practicable means to conduct impact fatigue tests on rocks and other brittle solids.

J1010203 強震動時の鋼材の材料強度と耐震裕度 : ひずみ速度依存性の影響評価に基づく一考察([J101-2]耐震・免震・制震 耐震(2))

Dynamic effect of impact loading such as seismic motion was researched in the viewpoint of seismic margin of steel structures. Impact fatigue tests were conducted from extremely low cycle to low cycle using cycle-loading test apparatus of impact-tensile load and the influence of dynamic strain on material strength of carbon steels. As the test results, the dynamic fatigue lives depended on strain amplitude not strain rate, suggesting the fatigue failure by seismic motion was not affected by strain rate. The strength for impact load includes viscosity stress caused by strain rate. Therefore, the test results and discussion suggested the viscosity stress corresponded to implicit seismic margin in the current design on the basis of stress.

Analysis of failure modes under nano-impact fatigue of coatings via high-speed sampling

Multiple nano-impact testing of coatings has recently been significantly exploited for fatigue testing of hard coatings. Using high-speed sampling, dynamic impact fatigue testing can provide significantly more information about fatigue deformation of coatings during cycling. Here, the additional analysis parameters provided by dynamic impact fatigue have been utilised to uniquely identify delamination (without fracture) and failure during cycling of a sol–gel coating on stainless steel. This has been used to map the evolution of fatigue damage as a function of impact velocity and cycles.

An investigation on the impact fatigue characteristics of valve leaves for small hermetic reciprocating compressors in a new automated test system

ABSTRACTThis paper presents an investigation on the impact fatigue characteristics of valve leaves that are prevalently used in hermetic reciprocating compressors especially for the household type refrigerators. A unique automated impact fatigue test system has been designed and produced, which enables to carry out impact fatigue tests of the compressor valve leaves under the desired impact velocities. The test system incorporates a noncontact actuation, a data acquisition system and an acoustic‐based damage detection technique, which continuously monitors the health of the structure. The damage detection system allows parametrical investigation on the impact fatigue life by detecting any possible damage and automatically terminating the test. The investigation relates the impact fatigue lifetime of the valve leaves with the impact velocity, asymmetrical impact, operation temperature, material type (carbon strip steel, stainless strip steel and new stainless strip steel grade) and tumbling operation duration. The observations show that the cracks have initiated from the edges of the valve leaf where is in contact with the valve plate. Subsequently, the cracks initially have propagated in the radial direction inwards the center of the impact area. Various failure cases have been resulted in by either a single crack or inter‐related multiple cracks. Microscopic and metallographic observations have been performed on the specimens to enhance the understanding of the damage mechanisms. The investigation and introduced test system guide the design optimization of the valve leaves in terms of compressor performance due to the energy consumption and lifetime of the valve leaf.

Delamination-Crater Interaction in Damage of Glass/Epoxy Composite Plates Subjected to Impact Fatigue

The use of composite materials is increasing although their behavior under impact fatigue loading remains rather unknown. This study is to assess the evolution of damage, especially delamination and crater, in a composite Glass/Epoxy woven fabric, using repeated impact tests at low energy levels (<10J). Both types of damage that arise and grow within the material cannot be independent from each other. Our objective in this work is to establish the interaction between two damages (delamination and crater) on laminate damage, and understand the contribution of each of them in the different phases through which passes the composite before perforation. To do this, impact fatigue tests are carried out on composite plates and measures of the crater size (diameter and depth) and the size of the delaminated area (diagonals from a diamond shape) are collected for different numbers of impacts and impact energies. A question worth asking; can we foster one of these damages over the other? especially when we are interesting to the “structure applications”, where one "prefers" perforation to delamination (while completing correctly the function's intended to the structure), or “shielding applications”, where one "prefers" the delamination to perforation. Although the range of impact velocities is not the same, it is still interesting to consider the synergy between these two damages at low impact velocities, always in the case of “structure applications” and “shielding applications”.

Fatigue at Nanoscale: An Integrated Stiffness and Depth Sensing Approach to Investigate the Mechanisms of Failure in Diamondlike Carbon Film

Nanoscale impact fatigue tests were conducted to comprehend the relative fatigue performance and failure modes of 100 nm thick diamondlike carbon (DLC) film deposited on a 4 in. diameter Si (100) wafer of 500 μm thickness. The nanofatigue tests were performed using a calibrated TriboIndenter equipped with Berkovich indenter in the load range of 300–1000 μN. Each test was conducted for a total of 999 fatigue cycles (a low cycle fatigue test). Contact depth in this load range varied from 10 to 30 nm. An integrated contact stiffness and depth sensing approach was adapted to understand the mechanisms of fatigue failure. The contact depth and stiffness data indicated some peculiar characteristics, which provided some insights into the mechanisms of cohesive and adhesive failure in thin films. Based on the contact stiffness and depth data, and surface observations of failed DLC films using atomic force microscope and scanning probe microscopy, a five-stage failure mechanism is proposed. The failure of films starts from cohesive failure via cracks perpendicular to the film/substrate interface, resulting in a decrease in contact depth with number of fatigue cycles and no appreciable change in contact stiffness. This is followed by film delamination at the film/substrate interface and release of elastic stored energy (residual stress) resulting in an increase in contact stiffness. Finally, as the film breaks apart the contact stiffness decreases with a corresponding increase in contact depth.

Mechanical properties of polypropylene-fiber reinforced concrete after gamma irradiation

Fiber reinforced concretes (FRCs) exhibit property improvement caused by the fibers. By using gamma radiation we have further improved mechanical properties of hydraulic concretes elaborated with Portland cement, water, silica sand, marble and polypropylene (PP) fibers. Compression strength, compression modulus, impact strength and dynamic elastic modulus were determined. Impact fatigue testing is a convenient way to evaluate non-irradiated concretes. We find improvement of the strength and elastic modulus – dependent on PP fiber concentration, marble particle sizes and the applied dose. Both the compressive strength and the elastic modulus are the highest for concrete with 1.5 vol% of PP fibers. The compressive strength value at that PP fibers concentration, the average marble size of 1.4 mm and irradiated at 50 kGy is higher by 19% with respect to non-irradiated concrete. For 9.5 mm marbles the analogous improvement amounts to 25% but for the dose of 10 kGy.

Crack Propagation in a Toughened Epoxy Adhesive under Repeated Impacts

Adhesives are being increasingly used in structural applications, especially in aerospace, automotive and naval structures, making their structural integrity an important issue. In-service loading histories of such structures usually contain low-energy impacts, repetition of which can significantly affect their performance. This paper deals with the behaviour of the toughened epoxy adhesive FM73 under repeated impacts, known as impact fatigue. Izod impact fatigue tests were performed on FM73 specimens in order to study the evolution of damage and to characterise this via measurable parameters, such as the maximum force and the contact time. A finite element model was developed to simulate the impact tests and this was used to calculate the dynamic strain energy release rate, which was compared with that determined using a simple analytical method. A relationship between the maximum dynamic strain energy release rate and impact fatigue crack growth rate was established that was used as the basis of an impact fatigue crack growth law.

Crack Propagation in a Toughened Epoxy Adhesive under Repeated Impacts

Adhesives are being increasingly used in structural applications, especially in aerospace, automotive and naval structures, making their structural integrity an important issue. In-service loading histories of such structures usually contain low-energy impacts, repetition of which can significantly affect their performance. This paper deals with the behaviour of the toughened epoxy adhesive FM73 under repeated impacts, known as impact fatigue. Izod impact fatigue tests were performed on FM73 specimens in order to study the evolution of damage and to characterise this via measurable parameters, such as the maximum force and the contact time. A finite element model was developed to simulate the impact tests and this was used to calculate the dynamic strain energy release rate, which was compared with that determined using a simple analytical method. A relationship between the maximum dynamic strain energy release rate and impact fatigue crack growth rate was established that was used as the basis of an impact fatigue crack growth law.

Combinative influence of impact and pressing forces on coating failure behaviour

In this work, a ball-on-plate impact fatigue test was proposed as an experimental technique to investigate the failure behaviour of coating–substrate systems under simulated stamping force conditions. Each impact cycle consisted of an impact force and a pressing force which could be adjusted by changing the distance between the impact ball and the plate surface and by regulating air pressure in the air cylinder of the impact tester, respectively. Two PVD (CrN and TiAlN) and one CVD (TiC) coatings on D2 substrates were tested at combinations of different impact/pressing loads (i.e., 200 N/400 N, 400 N/400 N and 600 N/400 N) for 10,000 cycles. It was found that the sizes of the impacted craters were linearly increased with the impact forces and were more dependent on the substrate than the coatings. The possible failures of the tested coatings included not only cohesive and adhesive failure modes but also fatigue cracking. The CrN coating was the best against the failures at all load combinations. While the TiC coating only showed a small degree of chipping after the test at the highest impact/pressing load, all cohesive and adhesive failures and fatigue cracks could be observed on the TiAlN coating even at the lowest impact/pressing load condition.

Development of a Novel Microimpact-Fatigue Tester and Its Application to Impact Reliability of Lead-Free Solder Joints

Drop-impact forces cause portable electronic devices to fail. Assessment of resistance against drop-impact force is required to predict life of an electronic package. Several methods have been used to evaluate impact-fatigue life and other advanced methods have been proposed. However, such conventional impact tests require excessive time and cost. In this paper, a novel micro-impact-fatigue tester is developed to overcome such drawbacks of conventional methods. A newly developed impact-fatigue apparatus directly applies impact force to solder joints and measures deformation of the solder joints. The impact-fatigue test apparatus consists of an electromagnetic actuator, an impact-pin, a load-cell, a displacement sensor, and a main frame. Electromagnetic actuator produces a repeatable impact force with a changeable amplitude and pulse duration. Impact-fatigue apparatus was used to test reliability of a lead-free solder (96.5Sn4.0Ag0.5Cu). An evaluation of impact-fatigue life was performed over a wide range of 1-10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> cycles with various applied forces ranging from 40 N to 110 N. Two failure modes were observed in a section inspection. First type of failure was a mixed mode failure, where a bulk solder failure and interface failure coincide and the relation between the impact load and fatigue life is almost linear. Stress-based life-prediction model is proposed for the mixed mode failure. The second type of failure, interface failure between the Ni(P) layer and the solder, occurs under a high-load condition. Fatigue life is shorter in the second type of failure than in the mixed mode failure. Brittleness of the interface reduces the impact-fatigue life.

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.

Design of an Impact Loading Machine Based on a Flywheel Device: Application to the Fatigue Resistance of the High Rate Pre-straining Sensitivity of Aluminium Alloys

This paper presents a device that has been designed for tensile loading at medium impact rates (up to 103 s–1) and for performing either interrupted or failure tests. This machine allows us to apply prescribed pre-straining to the specimen, and then apply subsequent loading histories such as impact fatigue. Two specimen loading systems are considered, which make it possible to carry out tests with various ranges of force and various durations of time. A multi-CCD camera system is triggered by a chosen threshold from the force signal. The system is dedicated to the displacement measurement and gives both qualitative and quantitative information about the stretching mechanism leading to fracture. To illustrate the performance of the device, experimental results concerning impact tensile tests at a strain rate of about 300 s–1 are presented, as well as consecutive impact-fatigue tests on two aluminium alloys.

Effect of Microstructure on Impact Fatigue Resistance and Impact Wear Resistance of Medium Cr-Si Cast Iron

Abstract A great amount of iron grinding balls in tube mills have been consumed. Under this impact abrasive wear working condition, the failure of wear resistant alloying white irons grinding balls is mainly caused by fatigue spalling. The impact wear resistance of martensitic high chromium cast iron (Cr of 15%) is not high sometimes, but its cost is not low. Thus, medium Cr-Si wear resistant cast iron is recommended. The influence of the iron on impact fatigue resistance and impact wear resistance is pronounced. Ball-on-ball impact fatigue test and high stress impact wear test of the grinding balls have been carried out. The results show that the impact fatigue resistance (IFR) and impact wear resistance (IWR) of medium Cr-Si cast iron are superior to those of martensitic high chromium cast iron (Cr of 15%). The main reasons are that (1) the stress in medium Cr-Si cast iron is released in the as-cast state; (2) the matrix is fine pearlite with better toughness and plasticity; (3) the pearlite is more stable compared with a retained austenite under repeated impact load and less phase transformation can take place; (4) high silicon content improves the morphology of eutectic carbide; (5) there is no secondary carbide which results in less crack sources. All these factors are beneficial to improvement of impact fatigue spalling resistance. The eutectic carbide M7C3 is the main constituent to resist wear.

620 アクリル系接着剤の繰り返し衝撃負荷に対する引張強度

The impact fatigue strength of the acrylic adhesive under impact tensile loading was evaluated by adding impact cyclic loads to a tubular butt joint. The material of adherends was aluminium alloy A5052-H34,and the acrylic adhesive Hardloc M-372-20 was used. First, the combined loading test was performed in order to evaluate the static strength of the adhesive. As a result, the value of fracture stress under shear load is stronger than the value of fracture stress under tensile load. As the adhesive layer thickness become thick, the value of fracture stress become low. In the impact fatigue test, the thickness of the adhesive layer was 0.1mm. As a result, the maximum incidence stress becomes low, the number of impact loads to a fracture will increase. In cace the insident stress is 9MPa, the specimen did't fracture even if it was subjected to the impact loads 1 million times. It was considered that the fatigue limit is 9MPa.

高Ｃｒ白鋳鉄溶湯によるサーメット鋳ぐるみ材料の高温圧縮衝撃疲労特性

In order to improve the high-temperature wear resistance of white cast iron, a cermet powder layer mixed with Ni and/or Cr powder was inserted by molten 27 mass% Cr white cast iron. The microstructures of surface and vertical cross-section of the inserted layer were examined to investigate castability of the molten cast iron. High-temperature wear resistance of the inserted layer was examined by means of a high-temperature compressive impact fatigue test. The results obtained are as follows. (1) The inserted layer with 50 vol. % Ni powder or 25 vol. % Ni and 25 vol. % Cr powder shows satisfactory metallographic structure, though the inserted layer with 50 vol. % Cr powder includes some micro pores and voids. (2) The wear resistance decreases with increasing temperature for whole the inserted layers. The impact fatigue strength of the layer with 50 vol. % Cr powder is highest in whole the inserted layers at higher temperature side. (3) However, the inserted layer with 50 vol. % Cr powder shows many cracks connecting with the micro pores and voids after the impact fatigue test. Therefore, it is important to make an inserted layer without any micro pores and voids by using smaller sized cermet powder.

Modelling of damage and failure of glass/epoxy composite plates subject to impact fatigue

An investigation has been carried out to study the impact fatigue damage of glass/epoxy laminated composites. Accumulation of damage, such as matrix cracking, delamination and fibre breakage, with repeated impact of the composite material may reduce the overall stiffness. These damage modes have been combined in a very complicated way to describe damage growth and fracture. A model is proposed for characterising the damage as a function of the normalised impact number. The scalar variable D, which characterises the material damage, is written as a function of the life duration β, using a modified form of the Mankowsky empirical law [Int J Solids Struct 32(11) (1995) 1607]. The macroscopic failure mode and the internal damage in laminated specimens of glass/epoxy as a consequence of impact fatigue are analysed at different levels of incident impact energy. The impact fatigue tests have been conducted on an apparatus built in our laboratory.

An observation of crack initiation and early crack growth under impact fatigue loading

Abstract The micro-characteristics of crack initiation and early crack growth of low carbon steels under low cyclic impact fatigue loading were studied. A split Hopkinson’s bar was used in the impact fatigue test, and SEM was used to examine the evolution of the surface on the post-test specimen. It is revealed that the most favourable position of crack initiation is grain boundaries. On the other hand grain boundaries are an effective obstruction for crack propagation. It is easy for cracks to initiate and grow in grains with straight slip bands, but difficult in grains with cross slip bands. The damage caused by micro-pits growing and linking up are quite shallow.

Effect of carbon and hydrogen on physical and mechanical properties and servicing characteristics of structural steels

The effects of chemical composition, melting process (open-hearth, electric arc furnace), melting unit operation volume (2 × 10 2 kg, 3 × 10 4 kg, 25 × 10 4 kg), deformation process (band, wheel) and heat treatment process on the properties of carbon steels have been investigated. The results of determining the inclination of steels no. 30, 50, and У7 to brittle fracture as well as the results of bending fatigue, impact fatigue and thermal fatigue tests have shown the advantage of steel no. 50, which is characterized by minimum solubility of hydrogen. It was found that periodically there were some low values of impact toughness at 293 K for open-hearth processed steel with carbon content above 0.6%. Structure and properties of high-carbon steel after heating below A I have been investigated. The application of micro-structural analysis allowed to explain the reduction in KCU value by the internal pressure of molecular hydrogen and methane in steel.