Resonant fatigue testing of riveted joints

A special test facility was constructed for evaluating the fatigue strength of turbine-blade components under steady pullout and vibratory bending loadings. The use of mechanical resonance for these applications was particularly attractive in that failure in inaccessible areas could be detected by the change in natural frequency of the specimen. The sensitivity of the method allowed determination of a fatigue crack as early in life as possible. The application of this facility to the determination of the fatigue strength of a type of riveted connection employed on certain types of blades is described in this paper. A steady load representing the centrifugal force was applied hydraulically through a long flexible pullrod which in combination with the test specimen was excited laterally at its resonant frequency by an electromagnetic shaker. An accelerometer provided closed-loop control on both vibration amplitude and resonant frequency through special circuitry. Fatigue-strength reduction factors at 10⁷ cycles were determined for three types of riveted test specimens in air at room temperature. These values exceeded the elastic stress-concentration factors estimated from earlier photoelastic tests for all three cases.     

Analysis of mechanical joints in wood

Stresses and strains associated with single-fastener mechanical joints in wood are determined numerically and experimentally. Effects of friction, and variations in joint geometry and ratio of pin-to-hole diameter are evaluated. The results obtained provide design information for single- and multiple-bolt connectors, indicating that superior designs would use close-fitting pins and suggesting that optimum end distance should be eight times the hole radius.     

Mesoscale fatigue failure of welded joints of low-alloy steel

The fatigue failure kinetics for welded joints of low-alloyed steel is investigated by analyzing the displacement vector field under high-cyclic loading. Failure accumulation in the heat-affected zone at the mesoscale is found to possess a multi-stage character that is associated with the formation and evolution of mesostructure deformation and fragmentation. Identified are the pertinent parameters that described the mechanical state of welded joints under load.     

Dynamic tensile strength of composite laminate joints fastened mechanically

The tensile strength of composite joints is determined under dynamic loading conditions. The composites are laminates made from hybrid fiber reinforced plastic (HFRP) and carbon fiber reinforced plastic (CFRP). Three different mechanically fastened joint configurations are considered: they are the pin-connected, single-lap and double-lap type. The joint strength under dynamic load is found to be lower than that under quasi-static load. The pin-connected joint was the weakest. Investigated also are the influence of geometric parameters for pin-connected HFRP laminate joints. The results shed light on how to improve the bearing strength of mechanical joints when encountering dynamic loads.     

The improvement of fatigue strength by edge treatment

This paper presents a new approach to improving the fatigue strength of sheet material by edge treatment. The improvement is achieved by inducing a compressive residual stress, by means of a mechanical procedure called edge dimpling, to a finite length along the edge. The new technique can be utilized to increase the fatigue resistance of plates with large-scale stress concentrations such as an unstressed access-door opening into a wing structure, stress concentrations at the corners of window doublers or long, wide slots in the structures.     

Problems of fatigue strength definition for shell structures

Stresses caused by oscillations of shell structures may vary significantly even for the same oscillation form. Therefore, during fatigue tests it is very important to solve the loading problem for these structures and further evaluate their fatigue strength statistical parameters. A new method for solution of these difficult problems is proposed in this paper. The basic idea is to take into account relative load values of fatigue tests instead of often used absolute load values. The fatigue loads are reproduced on a test bed by the same means by which the dynamic loads were determined. Levels of fatigue loads used in the tests are based on the individual dynamic loads for each of the structures investigated. The basic principles are described for axial fans used in railway transport. Axial fan blades are a characteristic example of shell structures.     

A simpliied fatigue assessment method for transverse illet welded joints

Under the as-welded condition the fatigue crack initiation period was considered nonexistent and Linear Elastic Fracture Mechanics(LEFM) was used to calculate fatigue strength for a range of weld geometries. Fracture mechanics assessment of welded joints requires accurate solutions for stress intensity factor(SIF). However, the solutions for the SIF of complex welded joints are dificult to determine due to the complicated correction factors. Three methods for SIF prediction are discussed on illet welded specimens containing continuous or semi-elliptical surface cracks, including the traditional correction method Mk, the approximate correction method Kt, and the suggested additional crack size method(ac+ae).The new additional crack parameter ae is used to replace the stress concentration effect of weld proile Mk, which simpliies the calculation process. Experimental results are collected to support fatigue strength assessment of the additional crack size method.     

Using enclosing ellipsoids in multiaxial fatigue strength criteria

We propose in this paper a scalar measure for the amplitude of shear stress variations based on the definition of an enclosing hyper-ellipsoid of minimum Frobenius norm. This equivalent shear amplitude is combined with the maximal mean stress, in both linear and quadratic functions, leading to two alternative fatigue criteria. The predictions of these criteria are compared with experimental data in order to validate the proposed shear amplitude measure. The crucial issue concerning the feasibility of the computation of the minimum norm ellipsoid is discussed and a practical algorithm is proposed and demonstrated.     

复杂节点冰激疲劳分析的高效谱分析方法

冰激疲劳是威胁渤海湾导管架平台安全的一个重要因素.由于疲劳分析需要考虑所有可能环境工况下的热点应力计算,因而往往是一个复杂而费时的过程,特别对于具有复杂节点的大型结构.本文提出了处理不规则复杂节点的冰激疲劳分析的高效谱分析方法,采用既可以考虑结构整体又可以模拟局部节点细节的混合有限元模型,并结合虚拟激励法来计算热点应力的功率谱密度,在保证精度的前提下大大提高了疲劳分析效率.最后通过一个实际平台的疲劳分析实例表明了方法的可行性和实用性.本文提出的方法也可以应用于受循环载荷作用的其他结构疲劳分析中.     

Strength of adhesive-bonded lap joints with respect to change of temperature and fatigue

The theoretical analyses of the stress distribution in bonded joints are of little help to the practical designer. An approximate solution yields an expression for the “average” ultimate shear stress in terms of two constants. This paper shows how these constants can easily be determined experimentally and gives test results which show their temperature dependence and their variation under fatigue-loading conditions. The constants are essential characteristics of a given adhesive and can be used to design any glued joint using this adhesive.     

Integrated numerical–experimental analysis of interfacial fatigue fracture in SnAgCu solder joints

In ball grid array (BGA) packages, solder balls are exposed to cyclic thermo-mechanical strains arising from the thermal mismatch between package components. Thermo-mechanical fatigue crack propagation in solder balls is almost always observed at the chip side of the bump/pad junction. The objective of the experimental part of this study is to characterize the bump/pad interface under fatigue loading. Fatigue specimens are prepared by reflowing Sn3.8Ag0.5Cu lead-free solder alloy on Ni/Au substrates. Obtained results show that fatigue damage evolution strongly depends on the microstructure. Applied strain and solder volume both have an influence on the fatigue damage mechanism. In the numerical part of the study, fatigue experiments are modeled using the finite element technique. A cohesive zone approach is used to predict the fatigue damage evolution in soldered connections. Crack propagation is simulated by an irreversible linear traction–separation cohesive zone law accompanied by a non-linear damage parameter. Cohesive zone elements are placed where failure is experimentally observed. Damage evolution parameters for normal and tangential interaction are scrutinized through dedicated fatigue tests in pure tensile and shear directions. The proposed cohesive zone model is quantitatively capable of describing fatigue failure in soldered joints, which can be further extended to a numerical life-time prediction tool in microelectronic packages.     

Dynamic analysis of planar rigid-body mechanical systems with two-clearance revolute joints

In this paper, the behavior of planar rigid-body mechanical systems due to the dynamic interaction of multiple revolute clearance joints is numerically studied. One revolute clearance joint in a multibody mechanical system is characterized by three motions which are: the continuous contact, the free-flight, and the impact motion modes. Therefore, a mechanical system with n-number of revolute clearance joints will be characterized by 3 ⁿ motions. A slider-crank mechanism is used as a demonstrative example to study the nine simultaneous motion modes at two revolute clearance joints together with their effects on the dynamic performance of the system. The normal and the frictional forces in the revolute clearance joints are respectively modeled using the Lankarani–Nikravesh contact-force and LuGre friction models. The developed computational algorithm is implemented as a MATLAB code and is found to capture the dynamic behavior of the mechanism due to the motions in the revolute clearance joints. This study has shown that clearance joints in a multibody mechanical system have a strong dynamic interaction. The motion mode in one revolute clearance joint will determine the motion mode in the other clearance joints, and this will consequently affect the dynamic behavior of the system. Therefore, in order to capture accurately the dynamic behavior of a multi-body system, all the joints in it should be modeled as clearance joints.     

The effect of preload on fatigue strength of residually stressed specimens

This paper investigates the effect of preload on fatigue-strength improvements gained by compressive residual stresses induced mechanically by dimpling. Experiments on 2024-T3 aluminum-alloy specimens of two different types with preload ratios up to 1.2 and subsequent statistical analysis of data show that preload can be slightly beneficial or detrimental. The results are discussed with reference to residualstress redistribution by preload.     

Radiation effects on creep rupture and fatigue strength of pure aluminum

Paper describes an experimental investigation on the effect of nuclear radiation on the creep rupture and fatigue strength of pure aluminum for the ratios of alternating stress to mean stress ranging from zero to infinity. It was found that not only was the strength of the material tested affected by radiation over a range of ratios of stresses, but also the brittle-ductile transition was influenced.Based upon the experimental results obtained, an analytical model was developed to correlate creep rupture, fatigue strength and radiation effect for various ratios of stresses. This information may prove to be valuable in dealing with the stress analysis of nuclear reactors.Paper was presented at 1966 SESA Annual Meeting held in Pittsburgh, Pa., on November 6–9.     

含铝纤维复合炸药的能量输出和力学强度

通过空中爆炸实验,研究了铝纤维对爆炸能量输出的影响,结果表明:wAl=0.20的TNT/Al,冲击 波压力峰值为TNT 的1.19倍,TNT 当量为TNT 的1.29倍;wAl=0.20的RDX/Al,冲击波压力峰值为 RDX的1.20倍,爆热为TNT的1.64倍,是RDX的1.31倍。通过抗压实验,研究了铝纤维对炸药力学强度 的影响,结果表明,铝纤维能增强TNT炸药的力学强度,破坏应力为6.8MPa,应变为0.043。铝纤维对炸药 能量和力学强度的双重增强特性,可以为现代高性能炸药设计提供参考。     

An estimation method of bearing strength of bolted joints in fibre reinforced composite

Some researchers have estimated the strength of bolted joints in fibre reinforced composite, using simple and efficient engineering procedures. However, for these procedures the effect of clamping due to the strength of bolted joints is not considered. In this paper, a method is presented for predicating critical bearing strength of single-hole bolted joints in composite on the basis of observing and analysing the results of experiments. The clamping effect of bolts is considered. The calculated results correspond to the test data on Glaphic/Epoxy laminates.     

Cohesive zone model for high-cycle fatigue of adhesively bonded joints under mode I loading

A cohesive zone model adequate for simulating the behaviour of adhesively bonded joints subjected to high-cycle fatigue and pure mode I loading is presented. The bilinear cohesive zone law with linear softening relationship was considered. The main advantage of the proposed formulation is the use of a unique damage parameter accounting for cumulative damage resulting from static and fatigue loading. The method was implemented in a user subroutine of the commercial finite element software Abaqus®. Two-dimensional numerical simulations of the double cantilever beam test using different representative combinations of the modified Paris law coefficients were performed. It was verified that the results of the model simulate with excellent agreement the several Paris laws used as input, thus demonstrating the good performance of the method as a predictive tool.     

Improving fatigue strength of bainite/martensite dual-phase steels in very high cycle fatigue regime by refining microstructures

Very high cycle fatigue behaviors of two bainite/martensite dual-phase steels were investigated. One of the steels was cyclic rapid heat treated and its microstructures were refined. Fatigue strength of the steel is 225 MPa higher than that without refining. Observation of fracture surfaces show that the fatigue cracks initiate at bainites for non-refined steel and at non-metallic inclusions for the refined steel. The size of inclusions is much smaller than that of bainites which results in the improvement of fatigue strength.     

Fatigue strength of welded joints based on local, semi-local and nominal approaches

Analysis based on the so-called “local approach” is made to estimate the fatigue strength of welded joints. Numerical analyses or strain gauges are employed for finding the stress and/or strain state in the vicinity of the weld toe. The notch stress intensity factor (NSIF) approach applied to fillet welded joints, as far as the opening angle between the weld and the main plate surface is constant (e.g. 135°, typical for many fillet welds), is able to rationalise the fatigue strength data both for different joint geometries and absolute dimensions. The NSIF approach has been previously developed as an extension of the Linear Elastic Fracture Mechanics (LEFM) to open V-notches and is based on the exponential local stress field around the V-notch tip. Several different “local approaches”, although simpler and more practical than the NSIF, are based on the stress (or strain) values determined beyond the exponential local one. To distinguish such approaches from the NSIF based one, we define the former as semi-local or nominal approaches while the latter is a local approach. The paper underlines that the local approaches, differently from the other ones, are able to unify in a single scatter band the fatigue strength data obtained from welded joints having different geometry and absolute dimensions.