Failure analysis of helical suspension springs under compressor start/stop conditions

During testing of compressors under start/stop conditions, several helical suspension springs failed. The ensuing failure investigation showed that the springs failed due to fatigue. The analysis showed that during start/stop testing the springs would undergo both a lateral and axial deflection, greatly increasing the torsional tresses on the spring. To understand the fatigue limits under these test conditions, a bench test was used to establish the fatigue strength of the springs. The bench tests showed that the failed springs had an unacceptable surface texture that reduced the fatigue life. Based on an understanding of the compressor motion, a Monte Carlo model was developed based on a linear damage theory to predict the fatigue life of the springs during start/stop conditions. The results of this model were compared to actual test data. The model showed that the design was marginal even for springs with acceptable surface texture. The model was then used to predict the fatigue life requirements on the bench test such that the reliability goals for the start/stop testing would be met, thus reducing the risk in qualifying the compressor.     

Fatigue behaviour of technical springs

Technical Springs belong to the components with the highest cyclic and superposed static load. Nevertheless they have to fulfill the requirements of lightweight constructions. This is only possible, if high strength materials with special properties are carefully manufactured to well designed springs and special additional treatments are carried out, which impose an advantageous residual stress profile in the surface layers of the springs. After a short historical view on the spring research activities in Professor Thums area, some aspects of the fatigue behaviour of hot formed parabolic leaf springs are presented. Then the fatigue properties of cold formed helical compression springs made of different steel spring wires are discussed. Finally some results on the fatigue behaviour of helical springs at a very high number of load cycles are reported.     

Multiaxial fatigue and failure analysis of helical compression springs

Multiaxial fatigue criteria are applied to the analysis of helical compression springs. The critical plane approaches, Fatemi–Socie and Wang–Brown, and the Coffin–Manson method based on shear deformation, were used to predict fatigue lives of the springs under constant amplitude loading. Experimental fatigue lives are compared with the multiaxial fatigue criteria predictions. The stress analysis was carried out in the finite element code ANSYS, and the multiaxial fatigue study was performed using the fatigue software nCode. A failure analysis was conducted in order to determine the fatigue crack initiation point and a comparison of that location with the most damaged zone predicted by the numerical analysis is made. The Fatemi–Socie critical plane approach gives a good prediction of fatigue life. While the Wang–Brown criterion overestimates spring fatigue life, the Coffin–Mason model gives conservative results.     

Fatigue assessment and failure analysis of shot‐peened leaf springs

The paper deals with the fatigue and failure analysis of serial shot‐peened leaf springs of heavy trucks emphasizing on the influence of thermal treatment and shot peening on fatigue life. Experimental stress–life curves are determined by investigating smooth specimens subjected to fully reversed rotating bending conditions. These test results are compared to corresponding ones determined from cyclic three‐point bend tests on shot‐peened serial leaf springs in order to reveal the influence of the applied thermal treatment and shot peening process on the fatigue life of the high‐strength steel used for leaf spring manufacturing, dependent on the load level. Microstructure, macro‐ and micro‐hardness analyses are performed to support the analyses and explain the effects resulting from the certain shot peening process on the surface properties of the high‐strength spring steel under investigation. The assessment of the fatigue results reveals nearly no life improvement due to the manufacturing, emphasizing the necessity for mutual adjustment of shot peening and thermal treatment parameters to take account for life improvement.     

Magnesium alloy defectology AZ91D high‐pressure die cast and influence on the fatigue behaviour

The fatigue results of a high‐pressure die cast of AZ91D magnesium alloy revealed the presence of different types of casting defects, which account for the large scattering in the number of cycles until failure. In this paper, this magnesium alloy has been analysed, and in an effort to reproduce the same surface and material conditions exhibited in automotive service components, the fatigue test samples were manufactured using a die that employs the same casting process and equipment. To examine the fracture surface of all the fatigue tests, a scanning electron microscope was used, and the source of the failure, so as to relate fatigue life with casting defect type, was identified. Five casting defect types that influence the fatigue behaviour were observed and classified: (a) isolated pores (blowholes), (b) micro‐porosity areas, (c) circular shrinkage cavities associated with the contraction and geometry of the casted specimen, (d) surface burrs associated with the die‐casting mould and (e) the presence of oxides or inclusions.     

Spring stiffnesses of composite circular springs with extended flat contact surfaces under unidirectional line-loading and surface-loading configurations

Theoretical expressions based on the principle of minimum potential energy are presented which describe the stiffnesses of mid-surface symmetric, woven composite circular springs with extended flat contact surfaces subject to unidirectional line and surface-loading configurations. Three-dimensional finite element analysis has been employed to study the transverse shearing effects. Eighteen woven fibre/epoxy composite circular springs with extended flat contact surfaces in different radii and thicknesses were fabricated and tested. Comparison studies of the results obtained from both the analytical and numerical models are made with experimental data, and the results are found to be satisfactory. The semi-included angle of the flat contact surface is vital parameter to spring stiffnesses of the composite spring.     

Strain gauge performance during fatigue testing

Strain gauges are used extensively to measure the changes in magnitude and distribution of dynamic stresses during fatigue testing. Over 200 000 fatigue cycles at high strains (4000 ± 2000/u/m) have to be withstood under certain conditions such as when evaluating the performance of heavy vehicle springs. In this work, various types of commercial strain gauges have been evaluated to determine the most appropriate installation for severe applications.
The best results were obtained using a constantan metal foil element strain gauge with phenolic glass backing.
Tests on heavy vehicle, multi-leaf, Taperlite springs, manufactured by Tinsley Bridge Limited (formerly BSC Light Products), have demonstrated the suitability of the chosen system and showed that the dynamic stresses remained essentially constant during fatigue testing.     

Processing and property assessment of NiTi and NiTiCu shape memory actuator springs

Among the multifarious engineering applications of NiTi shape memory alloys (SMAs), their use in actuator applications stands out. In actuator applications, where the one‐way effect (1WE) of NiTi SMAs is exploited, SM components are often applied as helical coil springs. Ingots are generally used as starting materials for the production of springs. But before SM actuator springs can be manufactured, the processing of appropriate wires from NiTi ingots poses a challenge because cold and hot working of NiTi SMAs strongly affect microstructure, and it is well known that the functional properties of NiTi SMAs are strongly dependent on their microstructure. The objective of the present paper is therefore to produce binary Ni₅₀Ti₅₀ and ternary Ni₄₀Ti₅₀Cu₁₀ SMA actuator springs, starting from ingots produced by vacuum induction melting. From these ingots springs are produced using swaging, rolling, wire drawing and a shape‐constraining procedure in combination with appropriate heat treatments. The evolution of microstructure during processing is characterized and the mechanical properties of the wires prior to spring‐making are documented. The mechanical and functional characteristics of the wires are investigated in the stress‐strain‐temperature space. Finally, functional fatigue testing of actuator springs is briefly described and preliminary results for NiTi and NiTiCu actuator springs are reported.     

圆管-方管混凝土T型节点疲劳性能试验研究

基于热点应力法的管节点疲劳性能研究思路中,热点应力集中系数和疲劳寿命是描述焊接管结构节点疲劳性能的两个主要方面。对8个支管为圆管主管为内填充C50等级混凝土的方管节点试件（CT1～CT8）进行热点应力试验以确定其热点应力集中系数（SCF）,在此基础上进行疲劳试验以确定其疲劳寿命（N3）,并将试验结果与具有相同几何尺寸和受力条件的圆管-方管节点试验结果以及相关规范进行比较分析。研究表明,支管受轴向荷载下,圆管-方管混凝土节点较对应空钢管节点而言,节点区刚度分布有明显改善,前者SCF最大值均小于后者SCF最大值,具有更好的疲劳性能;已有规范中关于空钢管节点疲劳性能的S-N曲线不适用于进行圆管-方管混凝土T型节点的疲劳性能分析。     

矩形钢管K型节点复合型应力强度因子计算方法研究

相比圆形钢管桁架,矩形钢管桁架在施工方面具有一定的技术经济优势,并已广泛应用于桥梁工程中,鉴于断裂力学法评估该类结构疲劳性能的需要,该文探讨矩形钢管K型节点在支管拉压平衡荷载作用下的应力强度因子计算方法。提出带表面裂纹矩形钢管K型节点有限元建模方法,并与试验进行验证;通过参数分析,研究节点和裂纹几何参数对节点几何修正系数Y的影响;多元回归分析拟合得到矩形钢管K型节点应力强度因子计算公式及其修正后的设计计算公式,并通过算例分析给出基于断裂力学的钢管节点剩余疲劳寿命评估方法。结果表明：有限元结果与试验结果比值均值为1.012,变异系数为0.034,两者最大差值仅为5.5%,表明有限元计算结果可靠;节点几何参数2γ、τ与Y呈正相关,θ与Y呈负相关,其原因主要在于节点相贯线处刚度和受拉荷载变化,改变了裂纹尖端应力场,从而影响了裂纹扩展速率,裂纹几何参数c/a与Y呈正相关,但影响不明显,a/t₀与Y呈负相关;提出的应力强度因子计算公式与有限元计算结果吻合良好,且修正后用于设计的计算公式具有95%的可靠度;圆形钢管节点应力强度因子高于矩形钢管节点,平均提高24.9%,说明在相...     

汽车橡胶密封条挤出口模结构分析及其实验验证

首先通过分析橡胶材料流变实验数据,确定采用Bird-Carreau本构模型;然后应用Polyflow软件对某实际EPDM橡胶密封条产品挤出过程中挤出口模流道结构对挤出的影响进行了有限元分析。结果表明,口模流道某些部位(如窄缝区域)进行适当的加宽有利于使挤出速度及熔体压力分布更均匀,挤出产品形状和实际形状更接近。     

Fatigue testing and non‐destructive characterization of AlSi9Cu3(Fe) die cast specimens by computer tomography

AlSi9Cu3(Fe) aluminum alloy fatigue test specimens were produced by high pressure die casting (HPDC) and vacuum‐assisted die casting (VPDC) techniques. Non‐destructive material tests (NDT) have been performed on cast specimens by computed tomography (CT). Uniaxial fatigue tests with two stress ratios of R = ?1 and R = 0.1 have been performed in the high cycle fatigue (HCF) regime, and the CT results were reassigned after the fatigue test in order to identify the origin of the failure. The aim of this paper is to establish a relationship between the CT result and fatigue failure of die cast specimens. The location and the size of the casting defect determine the specimen fatigue life. It has also been found that the fatigue life is determined not only by the size of the defect but also by its location with respect to the position of the highly stressed area. The results can be used to judge the applicability of cast parts after non‐destructive testing.     

Uniformity Differentiation Analysis. Part 1: Powder Deposition Characteristics in Rectangular Shallow Die Using Feed Shoe with Circular Cross Section

The process of die filling is a significant unit operation in many industries. Inhomogeneity of distribution such as in mass, bulk density, and pressure might cause many tablet and compact quality issues, such as lamination, capping, and distortion. No systematic investigations have been done to evaluate pressure distribution characteristics in dies with small aspect ratios (ratio of fill height to characteristic cross section dimension <0.5). In order to evaluate pressure distribution during filling of shallow dies, a circular cross section feed shoe was used at two speeds. The deposition characteristics of two powders with differing particle characteristics were studied in this research. The second-generation pressure deposition tester (PDT-II) was used to measure the two powders' pressure distribution characteristics. A battery powder mixture (BPM) and microcrystalline cellulose (Avicel PH102) were used to fill a rectangular shallow die 32 × 30 mm in dimension and 6.5 mm deep. Feed shoe speeds of 20 and 100 mm/s were used to fill the die. Symmetry analysis, variance metrics, Gini coefficient, and uniformity analysis were used to quantify the deposition characteristics. The results showed that (1) the contour plot was the most reliable method for measuring powder deposition characteristics; (2) the leeward and forward pressure distrbution comparisons generated a larger symmetry index than the front and back pressure distribution; (3) based on contour plot analysis, BPM at 100 mm/s feed shoe speed resulted in the most uniform pressure distribution (64% uniformity at +/?20 dm (decimeter) resolution) among all; (4) feed shoe speed greatly influenced pressure distribution uniformity inside the die; and (5) the high stress zone was mostly observed in forward and back regions inside the die.     

矩形截面非圆柱螺旋弹簧的模态分析

对矩形截面非圆柱(锥形、桶形、双曲形)螺旋弹簧的自由振动问题进行了研究。在弹簧的运动微分方程中,首次考虑了簧丝截面的翘曲变形对固有频率的影响。采用改进的Riccati传递矩阵法对包括14个自由度的一阶变系数常微分方程组进行了求解。为了证明理论的有效性,对两端固支矩形截面非圆柱螺旋弹簧的固有频率进行了求解,同时给出了各种参数变化对两端固支矩形截面锥形弹簧固有频率的影响。计算表明,翘曲变形对矩形截面非圆柱螺旋弹簧的固有频率有着重大的影响,在自由振动分析中必须加以考虑。     

Comparison of different statistical models for description of fatigue including very high cycle fatigue

In this paper the fatigue behaviour of welded joints and helical compression springs are analysed using two different statistical models. The data consist of results from low cycle, high cycle and very high cycle fatigue and different number of investigated specimens. In particular, the software program ProFatigue is used for derivation of the probabilistic S–N field from experimental fatigue data. The program, based on a former regression Weibull model, allows the estimation of the parameters involved in the S–N field model, providing an advantageous application of the stress based approaches in the fatigue design of mechanical components. The results obtained are compared with the customary Wöhler-curve, represented as a straight line in a double-logarithmic scale. Application to probabilistic assessment of cumulative damage and further program enhancement can be now envisaged.     

Measuring the load eccentricity in helical compression springs

An experimental device able to detect the load eccentricity in helical compression springs is described. The theory underlying the resolution of measured bending strains of the apparatus into effective load eccentricity is discussed. Laboratory results are offered for heavy duty springs and compared to theoretical estimates retrieved from the literature.     

Carbon fibre coil springs

Details are given of an investigation into the replacement of a conventional steel coil spring in the suspension system of a production car by a coil spring in fibre reinforced plastics. The development of the design and manufacturing techniques is described and the results of static and fatigue testing of prototype springs in carbon fibre reinforced plastics are discussed.     

Studies on thermal fatigue in 3Cr2W8V die steel by positron lifetime and hardness measurements

Studies on thermal fatigue in 3Cr2W8V die steel have been carried out by using positron lifetime and microhardness measurements. It is found that the mean positron lifetime and the microhardness exhibit periodic up-and-down variations with an increase in the number of fatigue cycles. The experiments indicate that in the process of thermal fatigue there are two opposing effects on variation of defects: microdeformation and dynamic recovery and recrystallization.     

Thermal stresses in aluminium alloy die casting dies

The aim of this research is to analyze the influence of Aluminium Alloy die casting parameters, die material, and die geometry on in-service tool life. An innovative immersion testing apparatus is developed, at which Aluminium Alloy die casting is simulated. It enables controlled thermal fatigue cycling. Special specimens with different edge geometry and specimens with maraging steel welds deposited by Gas Tungsten Arc (GTA) welding are prepared. They are subjected to cyclic heating in bath of molten Aluminium Alloy 226 and cooling in bath of water-based lubricant. The specimens are continuously internally cooled with cold water. The microstructure, hardness profile, and the surface cracks developed are periodically analyzed after completion of a particular number of cycles. Temperature transients at different locations of the specimens are measured and used in calibration of finite element model (FEM). The computation of transient stresses is performed by developed FEM. The influence of immersion test parameters, material, specimen edge geometry, and thickness of maraging steel surfacing welds on thermal stresses is studied. To improve thermal fatigue testing efficiency, a specimen of particular geometry and immersion test parameters are developed based on finite element analysis. The results showed significant differences in produced thermal stresses for analyzed materials, test parameters, and edge geometries. Maraging steel is found to be superior material for die casting dies, due to generation of lower stresses.