Low-amplitude Loading Research Articles

Fatigue life prediction method for AISI 316 stainless steel under variable-amplitude loading considering low-amplitude loading below the endurance limit in the ultrahigh cycle regime

To investigate the effect of cycle ratio of combined high- and low-amplitude loading corresponding to loading above and below the fatigue limit on fatigue life (cumulative fatigue damage), fatigue tests were conducted under repeated two-step amplitude loading condition with varying cycle ratios (the number of high amplitude cycles was kept at 10 cycles, the number of low amplitude loading cycle was set at four different values 103, 104, 5×104, and 10/105 cycles in one block) using AISI 316, a type of commercial austenitic stainless steel. The test results indicate that low-amplitude loading following high-amplitude loading contributed to the fatigue damage and affected fatigue life. Furthermore, both the EN (European) and KHK (The high pressure gas safety institute of Japan) standards, which describe predictive approaches for variable-amplitude loading conditions that consider stress levels below the fatigue limit in an S–N curve based on the oblique hyperbola model, gave inaccurate evaluation of fatigue life because of the effect of cycles of low-amplitude loading below the fatigue limit in one block. Fatigue damage at saturation in one block contributed by low-amplitude was proved. A reasonable predictive approach that considering the number of cycles of low-amplitude loading in one block using linear cumulative fatigue damage rule has been established based on repeated two-step amplitude loading test results, and its precision was confirmed through random-amplitude loading tests.

A new fatigue damage accumulation model considering loading history and loading sequence based on damage equivalence

In this study, a new nonlinear fatigue damage accumulation model is proposed to consider the effects of loading history and loading sequence under multi-level stress loading based on the Miner–Palmgren rule and S-N curve. By using damage equivalence, the new model is simplified and another form of the model is given. This model improves the application of the traditional Miner–Palmgren rule, by considering not only the loading sequence effect but also the loading history effect. The methods for calculating the degree of safety of specimens and cumulative damage of low-amplitude loads are also presented. Applicability of the new model is validated by predicting the fatigue life of 16Mn and 45 steel specimens under two-level stress loading. Further validation is carried out for the case of 41Cr4 and Aluminum alloys 6082 T6 under multi-level stress loading, and the strengthening and damaging effect of low-amplitude loads is considered. Comparing with the Miner–Palmgren rule and some new models, this new model gives more accurate and reliable prediction.

Damage accumulation of bovine bone under variable amplitude loads

Stress fractures, a painful injury, are caused by excessive fatigue in bone. This study on damage accumulation in bone sought to determine if the Palmgren-Miner rule (PMR), a well-known linear damage accumulation hypothesis, is predictive of fatigue failure in bone. An electromagnetic shaker apparatus was constructed to conduct cyclic and variable amplitude tests on bovine bone specimens. Three distinct damage regimes were observed following fracture. Fractures due to a low cyclic amplitude loading appeared ductile (4000μϵ), brittle due to high cyclic amplitude loading (>9000μϵ), and a combination of ductile and brittle from mid-range cyclic amplitude loading (6500 –6750μϵ). Brittle and ductile fracture mechanisms were isolated and mixed, in a controlled way, into variable amplitude loading tests. PMR predictions of cycles to failure consistently over-predicted fatigue life when mixing isolated fracture mechanisms. However, PMR was not proven ineffective when used with a single damage mechanism.

A novel dual-frequency loading system for studying mechanobiology of load-bearing tissue

In mechanobiological research, an appropriate loading system is an essential tool to mimic mechanical signals in a native environment. To achieve this goal, we have developed a novel loading system capable of applying dual-frequency loading including both a low-frequency high-amplitude loading and a high-frequency low-amplitude loading, according to the mechanical conditions experienced by bone and articular cartilage tissues. The low-frequency high-amplitude loading embodies the main force from muscular contractions and/or reaction forces while the high-frequency low-amplitude loading represents an assistant force from small muscles, ligaments and/or other tissue in order to maintain body posture during human activities. Therefore, such dual frequency loading system may reflect the natural characteristics of complex mechanical load on bone or articular cartilage than the single frequency loading often applied during current mechanobiological experiments. The dual-frequency loading system is validated by experimental tests using precision miniature plane-mirror interferometers. The dual-frequency loading results in significantly more solute transport in articular cartilage than that of the low-frequency high-amplitude loading regiment alone, as determined by quantitative fluorescence microscopy of tracer distribution in articular cartilage. Thus, the loading system can provide a new method to mimic mechanical environment in bone and cartilage, thereby revealing the in vivo mechanisms of mechanosensation, mechanotransduction and mass-transport, and improving mechanical conditioning of cartilage and/or bone constructs for tissue engineering.

Development of a Program-Loading Spectrum for the Accelerated Durability Test of Lower Control Arm

Abstract This study aims to develop a program-loading spectrum for the accelerated durability test of automotive components, with the lower control arm of a suspension as an example. Spectrum development is divided into three parts, namely, load spectrum acquisition, spectrum preprocessing, and program-loading spectrum development. Based on the strengthening property of structural low-amplitude loads under constant and random loadings, a program-loading spectrum-development method considering the strengthening effect of low-amplitude loads (SLAL) below the fatigue limit is proposed. Meanwhile, an equivalent strengthening methodology is considered during spectrum development. The number of loading cycles is decreased, and the bench test time is notably reduced by adjusting strengthening loads to the most effective level. The bench test proved that the influence of SLAL during spectrum development can simulate actual load conditions more effectively and precisely. Thus, this method can accurately predict fatigue failure from the durability test and can efficiently reduce durability test time.

Durability Testing Method of a Hub-Reducer System Based on the Shanghai Standard Road Driving Cycle

Abstract Electric vehicles are equipped with electric-drive wheel systems, so the development of matching hub reducers with proper durability is important. Based on the Shanghai standard road-driving cycle, this paper forecasts the load spectrum for key components of the hub-reducer system. After considering the material strengthening under low-amplitude loads, the reducer system running-in test specification is issued. This paper also develops an electric-drive system durability test specification on the basis of the Miner linear cumulative damage theory, and estimates the hub-reducer durability mileage. The bench test for durability shows that the first round of the prototype design satisfied the durability mileage required, and the bending-fatigue strength of the gear is sufficiently reliable, which gives possibilities for further lightweight designs. This method can be applied for the lightweight designing of other key automotive components. It also provides empirical evidence for developing automotive products that meet the standard road-driving-cycle conditions.

Dynamical measurement system for wind turbine fatigue load

A measurement system specially used in wind turbine fatigue load assessment is developed based on Labview platform and Control Area Network (CAN). By applying CAN bus communication technology, the system can perform data automatic acquisition, data stable transmission and data real-time monitoring. By adopting the technology of virtual instrument modular design, the system is designed to analyze the wind turbine mechanical load levels against wind and power, equivalent loads and lifetime fatigue loads, etc. Considering the effects of small load strengthening, low amplitude load damaging and multilevel load interaction, a novel fatigue lifetime prediction model is proposed to obtain more accurate and reliable prediction of blade fatigue life. With the developed measurement system, the in-field load measurements are performed and the results showed the system has satisfactory accuracy and good adaption, convenient operation, high integration, low cost and great practicality to load measurement of large wind turbine. And based on the proposed model the fatigue life of WT blade can be estimated more trustworthily and reliably.

Complication on the spectrum of automotive swing arm due to low-amplitude loads

A program load spectrum compiling method for automotive components was introduced with swing arm as example. Based on pseudo-damage and statistical analyses, combined with structural coaxing property, a spectrum compiling method considering the coaxing effects is put forward. The new method overcomes the deficiency of the traditional methods, in which only the damage effect of low amplitude loads is considered. Meanwhile, referred to the equivalent damage rule between damage loads, the equivalent coaxing rule between coaxing loads is explored. By transferring the coaxing loads to the most effective level, the bench tests show that the total cycling blocks decline sharply under self-made specification. The influence of strengthening loads is proved to be taken into account by deleting the useless loads during the spectrum compiling process. Moreover, the real service condition of automotive components by simulation can be faster and more real, and it is significant to predicate the failure mode of durability tests and to estimate service life of automotive components more accurately.

Quantitative Acoustic Emission Fatigue Crack Characterization in Structural Steel and Weld

The fatigue crack growth characteristics of structural steel and weld connections are analyzed using quantitative acoustic emission (AE) technique. This was experimentally investigated by three-point bending testing of specimens under low cycle constant amplitude loading using the wavelet packet analysis. The crack growth sequence, that is, initiation, crack propagation, and fracture, is extracted from their corresponding frequency feature bands, respectively. The results obtained proved to be superior to qualitative AE analysis and the traditional linear elastic fracture mechanics for fatigue crack characterization in structural steel and welds.

Piezoelectric Driven Testing Facilities to Research the Very High Cycle Fatigue Regime*

Abstract Cyclic low amplitude loading and a huge number of cycles characterizes the Very High Cycle Fatigue (VHCF) regime. The challenge in research of the Very High Cycle Fatigue regime is to apply this large number of cycles to fracture in an acceptable time frame. For this reason, it is essential to use a machine, which is able to operate at high frequencies of cycles. Two testing machine concepts with piezo actuators are presented in this study. In the first concept, a high performance piezo stack actuator is presented, in which the specimen and the load cell are mechanically assembled in series. This set-up applies forces up to 10 kN and testing frequencies up to 1000 Hz. The second testing facility is a hybrid testing system, which consists of an inertial mass actuator and a servo hydraulic actuator connected in parallel. Both systems are capable of testing normal specimen dimensions and provide the possibility to work with variable as well as constant amplitude loading.

New Combined High and Low-Cycle Fatigue Model to Estimate Life of Steel Bridges considering Interaction of High and Low Amplitudes Loadings

A new model is proposed in this paper for the estimation of the life of bridges subject to damage caused by high cycle fatigue combined with low cycle fatigue taking account of the interaction of high and low amplitude loadings. High cycle fatigue is caused by normal routine traffic (low amplitude) loading while low cycle fatigue is caused by extreme (high amplitude) loading situations produced by such as earthquakes. The model mainly consists of a new damage indicator and a new strain-life fatigue curve. Total strain is treated as the damage variable. The proposed model predictions were verified by comparing with fatigue test results for four materials reported in the literature. The proposed model was then applied to estimate the fatigue life of a bridge member subject to combined high and low-cycle fatigue damage caused by normal traffic and by earthquake loadings. The results of the case study, confirm the importance and applicability of the proposed model.

Microscopic Mechanism of Strengthening Under Low-Amplitude Loads Below the Fatigue Limit

Using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, some microscopic experiments and analyses were performed to explain the microscopic mechanism of strengthening under low-amplitude loads below the fatigue limit (SLAL). The experimental results show that the microscopic mechanism of SLAL could be dislocation-accumulation and grain boundary strengthening to low-strength material without strengthening by surface heat treatment. After SLAL, the microstructure imperfections can be improved and the fatigue resistance can be enhanced. Macroscopically, the fatigue strength and life of structures can be obviously improved and increased.

Effect of spinal manipulation on sensorimotor functions in back pain patients: study protocol for a randomised controlled trial

BackgroundLow back pain (LBP) is a recognized public health problem, impacting up to 80% of US adults at some point in their lives. Patients with LBP are utilizing integrative health care such as spinal manipulation (SM). SM is the therapeutic application of a load to specific body tissues or structures and can be divided into two broad categories: SM with a high-velocity low-amplitude load, or an impulse "thrust", (HVLA-SM) and SM with a low-velocity variable-amplitude load (LVVA-SM). There is evidence that sensorimotor function in people with LBP is altered. This study evaluates the sensorimotor function in the lumbopelvic region, as measured by postural sway, response to sudden load and repositioning accuracy, following SM to the lumbar and pelvic region when compared to a sham treatment.Methods/DesignA total of 219 participants with acute, subacute or chronic low back pain are being recruited from the Quad Cities area located in Iowa and Illinois. They are allocated through a minimization algorithm in a 1:1:1 ratio to receive either 13 HVLA-SM treatments over 6 weeks, 13 LVVA-SM treatments over 6 weeks or 2 weeks of a sham treatment followed by 4 weeks of full spine "doctor's choice" SM. Sensorimotor function tests are performed before and immediately after treatment at baseline, week 2 and week 6. Self-report outcome assessments are also collected. The primary aims of this study are to 1) determine immediate pre to post changes in sensorimotor function as measured by postural sway following delivery of a single HVLA-SM or LVVA-SM treatment when compared to a sham treatment and 2) to determine changes from baseline to 2 weeks (4 treatments) of HVLA-SM or LVVA-SM compared to a sham treatment. Secondary aims include changes in response to sudden loads and lumbar repositioning accuracy at these endpoints, estimating sensorimotor function in the SM groups after 6 weeks of treatment, and exploring if changes in sensorimotor function are associated with changes in self-report outcome assessments.DiscussionThis study may provide clues to the sensorimotor mechanisms that explain observed functional deficits associated with LBP, as well as the mechanism of action of SM.Trial registrationThis trial is registered in ClinicalTrials.gov, with the ID number of NCT00830596, registered on January 27, 2009. The first participant was allocated on 30 January 2009 and the final participant was allocated on 17 March 2011.

Analysis on Low Amplitude Loading Fatigue Life Strengthening of the Rear-Axle Housing in a Farm Transport Vehicle

Experimental studies were carried out on real rear-axle housing in a farm transport vehicle to investigate the influence of low amplitude preloading on structure life，in order to explore increase potential of rear-axle housing in a farm transport vehicle fatigue life. It is proved that a strengthening area exists in low amplitude loading area. Fatigue lives of rear-axle housings have been increased in some extent with preloading within this area. It can make fatigue life double at most. Based on physical tests, the best strengthening load region of rear-axle housing in a farm transport vehicle was estimated. It provides some technical supports for increasing fatigue life of real rear-axle housing.

Fatigue Life Estimation Considering Damaging and Strengthening of Low amplitude Loads under Different Load Sequences Using Fuzzy Sets Approach

In this study, based on the Miner rule, a new linear damage accumulation rule is proposed to consider the strengthening and damaging of low amplitude loads with different sequences using fuzzy sets theory. This model improves the application of the traditional Miner rule, by considering not only the damaging and strengthening of low amplitude loads, but also the load sequence effects. To apply the proposed model, the law of selecting membership functions for different load spectra is found, and different membership functions are investigated to show the important influence on estimating fatigue life. Applicability of the method is validated by comparing with the experimental data. It is also found that the predicted fatigue life by the proposed method is more accurate and reliable than that by the traditional ones.

Cumulative Strengthening Effect Model Based on the Low-amplitude Training Load

According to the load characteristics of vehicle structural parts during operation,the drive shaft strength evolution under low-amplitude training loads is studied.Through coaxing effect experiment of smooth specimens,the coaxing stress range of shaft material 40Cr stress is determined.The influences of load amplitude and load times on the residual fatigue strength of structural parts are analyzed.The results show that the best coaxing effect of structural strength exists under certain conditions.By orthogonal analysis and processing of test data,the relation model of strengthening or damage effect and equivalent symmetrical stress and load times is fitted.Furthermore,based on this model and the work load spectrum of auto drive shaft,a fatigue cumulative strengthen model in work process is built through the integration of strengthening and damage effect.In accordance with matching the design stress range with the best coaxing effect,a new method of lightweight design is proposed with the aim of cumulative strengthening effect of low amplitude loading,so as to provide a theoretical basis for auto structural parts lightweight design.

Lightweight Design of Automobile Drive Shaft Based on the Characteristics of Low Amplitude Load Strengthening

Lightweight Design of Automobile Drive Shaft Based on the Characteristics of Low Amplitude Load Strengthening

Surface Hardness Changes after Strengthening under Low-Amplitude Loads

This document investigated the changes of the surface hardness of the front axle and transmission gear after strengthening under low-amplitude loads below the fatigue limit (SLAL). The experimental results showed that the strength changes of the front axle after strengthening under low-amplitude loads was almost proportional to hardness, but it was inverse ratio for the transmission gear. There was no difference in the change direction of surface hardness, caused by strengthening or damaging. The reason of surface hardness changes could be due to that the microstructure of material was improved after being strengthening under low-amplitude loads below the fatigue limit.

Study on vital static properties of fine blanking of GFRP composites with that of conventional drilling

Among the high performance engineering materials, fiber-reinforced plastics play an important role. The present work is concerned with the comparison of vital static strength properties of fine blanking with conventional drilling on hand lay-up made glass fiber-reinforced plastic (GFRP) laminates of four different reinforcement lay-up sequences such as unidirectional [0/0]n, angle ply [0 ± 45]ns, quasi-isotropic [0/45/90]ns, and cross-ply [0/90]n. Observation includes tensile and flexural bending strengths of the specimens without hole and with hole by conventional drilling and fine blanking. In this work, an endeavor has been made to simulate the service conditions to determine their effect on the response of composite laminates. Detailed studies on GFRP composites when subjected to different loading environments such as static loading, particularly tensile loading, and low frequency high amplitude (fatigue) loading were carried out. The response of the composite laminates to these service environments has been evaluated in terms of flexural strength and modulus. From the tensile study, it was observed that by inserting a hole at center by drilling, the strength was reduced to one third, and by inserting a hole at center by fine blanking, the strength was increased nearly 20% than that of drilling. Apart from this, the flexural test conducted on polymeric composite specimens showed that an exposure to low frequency and high amplitude loading enhances the flexural strength up to certain duration of exposure, beyond which, due to accumulation of damage within the composites, the flexural strength reduces with number of cycles. This can be attributed to possible strain-induced stiffening of fibers and interface.

Strengthening and damaging under low-amplitude loads below the fatigue limit

The strengthening and damaging of low-amplitude loads below the fatigue limit are investigated by single tooth bending for the transmission cylindrical gear under constant low-amplitude loading and program loading. The experimental results show that the strengthening effects of low-amplitude loads are very obvious under constant low-amplitude loading, while are weakened under program loading. The strengthening loads under constant low-amplitude loading maybe have not the strengthening effects for the influence of overload in program loading, but the less low-amplitude loads could exhibit the strengthening effects. The strengthening effects of low-amplitude loads in program loading are influenced remarkably by the cycles of low-amplitude loads.