Fatigue behaviour of PMMAsilica acrylic casting dispersions

Abstract

Fatigue tests were performed on central hole notched specimens made of acrylic casting dispersions particulate composites. Three compositions with different volume fraction of filler and different particle size were analysed. The tests were conducted at two temperatures and two stress ratios. Constant and block variable loading amplitudes were applied to the specimens using a servohydraulic machine. An equivalent stress was used to correlate the fatigue lifetimes for the variable amplitude histories. Fatigue crack propagation tests were also performed at room temperature and for stress ratio R=0. The increase of particle size reduces the total fatigue lifetimes due to a reduction of the initiation life. Fatigue crack propagation rate increases with the increase of particle size. As the temperature and the stress ratio increase the fatigue strength decreases.     

Effect of bolt clamping force on the fracture strength of mixed mode fracture in an edge crack with different sizes: Experimental and numerical investigations

To investigate the effect of bolt clamping force, resulting from torque tightening, on the mixed mode fracture (I and II) strength and effective geometry/loading factor of an edge crack with different lengths, experimental and numerical studies have been carried out. In the experimental part fracture tests were conducted on three batches of simple edge crack and bolt torque tightened with 3.5 and 7 N m edge crack at three different crack sizes of Poly methyl-methacrylate (PMMA) rectangular plate. The specimens’ fracture strength was obtained using a tensile test machine at different loading angles by employing a modified Arcan fixture. In numerical part, finite element simulations were employed to model the three test specimen batches at the three crack lengths to obtain their stress intensity geometry/loading factors. The results show that the bolt tightening torque significantly reduces the effective geometry/loading factor, and thus increases the joint fracture strength compared to bolt-less simple edge crack specimens. However, the bolt clamping effect on increasing the fracture strength was almost the same for different crack lengths.     

STRESS-RATIO EFFECT OF FATIGUE CRACK PROPAGATION IN A BIAXIAL STRESS FIELD

Fatigue crack propagation tests were conducted under conditions of equibiaxial, uniaxial and shear loading by using a cruciform specimen in a servo hydraulic testing machine. The effect of non-singular stress cycling on the fatigue crack propagation rate was examined based on the observation of crack opening behavior. The crack propagation rate was significantly influenced by the non-singular stress parallel to the crack when it was correlated to the stress intensity range. The crack closure behavior was greatly affected by the non-singular stress. The crack propagation rate was uniquely correlated to the effective range of the stress intensity factor except for the case of completely reversed shear loading where significant plasticity was detected. The crack opening displacement range was concluded to be a parameter controlling the crack propagation rate for all the stress conditions examined in the present experiments. Some discussion is made on the effect of material anisotropy on fatigue crack propagation in a biaxial stress field.     

Fatigue crack propagation in tensile residual stress fields of welded joints under fully compressive cycling

Fatigue crack propagation rates in centre-crack-typed transverse butt-welded joints were measured at a constant stress intensity factor range obtained by decreasing the applied and mean loads on test specimens. The propagation rate was maintained constant except at extremely compressed stress ratios. Fatigue crack propagation properties under compressive loading were found to be similar to those under tensile loading. Only under highly compressive cycling did crack propagation rates decrease.     

Influence of oxidation on fatigue crack initiation and propagation in turbine disc alloy N18

Fatigue crack initiation and propagation behaviour in subsolvus heat treated turbine disc alloy N18 has been assessed in air and vacuum at 650 and 725 °C under three-point loading. Fatigue crack initiation processes have been evaluated using single edge U-notch specimens under a 1-1-1-1 trapezoidal loading waveform along with interrupted tests at 650 °C to allow intermittent observations of the notch surface. The results show apparent grain boundary (GB) oxidation can occur under an oxygen partial pressure of 10⁻²–10⁻³ Pa. Cracks mainly initiate from grain boundaries or γ/γ′ interfaces due to the formation and subsequent cracking of Cr-rich and/or Co-rich oxides, and occasionally initiate from surface pores. Fatigue life in these tests appears to be dominated by this crack initiation process and is significantly reduced by increasing temperature and/or application of an oxidizing environment. Crack growth tests conducted under 1-1-1-1 and 1-20-1-1 loading waveforms indicate that oxidation significantly degrades the crack growth resistance of N18 and is associated with more intergranular fracture surface features. Additional oxidation effects on propagation caused by higher temperature or prolonging dwell time appear limited, whereas a prolonged dwell period seems to instead promote additional creep process, which further enhance crack growth, especially at higher temperature.     

Fatigue crack propagation under variable amplitude loading in PMMA and bone cement

Fatigue failure of PMMA bone cement is an important factor in the failure of cemented joint replacements. Although these devices experience widely varying loads within the body, there has been little or no study of the effects of variable amplitude loading (VAL) on fatigue damage development. Fatigue crack propagation tests were undertaken using CT specimens made from pure PMMA and Palacos R bone cement. In PMMA, constant amplitude loading tests were carried out at R- ratios ranging from 0.1 to 0.9, and VAL tests at R = 0.1 with 30% overloads every 100 cycles. Palacos R specimens were tested with and without overloads every 100 cycles and with a simplified load spectrum representing daily activities. The R- ratio had a pronounced effect on crack propagation in PMMA consistent with the effects of slow crack growth under constant load. Single overloads caused pronounced crack retardation, especially at low da/dN. In Palacos R, similar overloads had little effect, whilst individual overloads at low da/dN caused pronounced acceleration and spectrum loading retarded crack growth relative to Paris Law predictions. These results demonstrate that VAL can have dramatic effects on crack growth, which should be considered when testing bone cements.     

Fatigue crack growth of a railway wheel

Typically, fatigue crack propagation in railway wheels is initiated at some subsurface defect and occurs under mixed mode (I–II) conditions. For a Spanish AVE train wheel, fatigue crack growth characterization of the steel in mode I, mixed mode I–II, and evaluation of crack path starting from an assumed flaw are presented and discussed.Mode I fatigue crack growth rate measurement were performed in compact tension C(T) specimens according to the ASTM E647 standard. Three different load ratios were used, and fatigue crack growth thresholds were determined according to two different procedures. Load shedding and constant maximum stress intensity factor with increasing load ratio R were used for evaluation of fatigue crack growth threshold.To model a crack growth scenario in a railway wheel, mixed mode I–II fatigue crack growth tests were performed using CTS specimens. Fatigue crack growth rates and propagation direction of a crack subjected to mixed mode loading were measured. A finite element analysis was performed in order to obtain the K_I and K_II values for the tested loading angles. The crack propagation direction for the tested mixed mode loading conditions was experimentally measured and numerically calculated, and the obtained results were then compared in order to validate the used numerical techniques.The modelled crack growth, up to final fracture in the wheel, is consistent with the expectation for the type of initial damage considered.     

A STUDY ON FATIGUE CRACK GROWTH UNDER OUT-OF-PHASE COMBINED LOADINGS

Abstract— Fatigue tests were performed on thin-walled tubular specimens of S45C steel under tension-compression, pure torsion, in-phase and out-of-phase axial-torsional loadings. The relationship between cracking behaviour and stress components on the crack plane was investigated. Measurement of microcrack density showed that microcracking was governed predominantly by the shear stress amplitude acting on the crack plane for all loading conditions. The failure crack was formed by coalescence of many cracks initiated near the maximum shear planes. The cracks grew turning their orientation to the direction perpendicular to the maximum normal stress. The transition of crack orientation occurred at relatively longer crack lengths at a higher stress ratio. The crack growth behaviour for all loading modes can be correlated using an equivalent strain intensity parameter based on shear and normal strains on the crack plane.     

Fatigue crack growth in heat-treated aluminium alloys

Fatigue crack propagation tests have been performed in several heat-treated aluminium alloys under constant amplitude loading. All experiments were performed, in load control, in a servo-hydraulic closed-loop mechanical test machine. The tests were carried out using middle tension, M(T), specimens. The influence of stress ratio and thickness were analysed. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. A strong stress ratio and material dependence effects on the fatigue crack growth were observed. These effects are discussed in terms of the different dominant closure mechanism. The crack growth behaviour of heat-treated aluminium alloys depends mainly on whether the dominant closure mechanism is plasticity-induced or roughness-induced. The enhancement of roughness-induced closure promotes higher crack growth resistance in these alloys. Roughness-induced closure dominates crack closure in aluminium alloys age hardened by naturally ageing and also artificially aged alloys with higher contents of Mn and Cr elements. In alloys aged hardened by artificially ageing and simultaneously with a lower content of these alloying elements plasticity-induced closure is dominant.     

Rißausbreitung bei Leichtbauwerkstoffen

Fatigue Crack Propagation of High Strength Alloys Investigations of the crack propagation behaviour under variable amplitude loading conditions show a strong influence of sequence effects. The fatigue crack propagation as a consequence of changes in the loading conditions is not linear. New continuum mechanical analyses enable an interpretation of the influence of sequence effects on fatigue crack propagation by considering the plastic deformations and displacements around the crack tip and their correlation to the crack closure behaviour. In order to enable a direct investigation of the crack propagation and crack closure behaviour in the scanning electron microscope a special loading equipment was designed. The investigations led to the following results:

• there existed only a weak correlation between the crack propagation rates and mechanisms at the side surfaces and on the fracture surfaces of the specimens,
• the crack propagation behaviour was significantly influenced by the microstructural constitution of the alloy,
• the continuum mechanical analyses could be corroborated in the tests.

For the tests the high strength aluminum alloys 2024-T3 and X-7075 were applied.     

Stress intensity factors for surface cracks at countersunk holes

Fatigue crack growth from countersunk fastener holes loaded in remote tensile loading was studied using the transparent polymer PMMA. A single edge corner crack at the bottom of the plate and a single internal surface crack at the sharp intersection between the bore and the countersink were induced in the PMMA specimens by pre-cracking. The specimens were then fatigue tested under constant amplitude remote tensile loading and the ‘back-calculation’ method was used to determine stress intensity factors at several crack front locations. When variations in fatigue crack closure were taken into account, the experimental stress intensity factors agreed well with the computational results at selected crack fronts.     

Face/core debond fatigue crack growth characterization using the sandwich mixed mode bending specimen

Face/core fatigue crack growth in foam-cored sandwich composites is examined using the mixed mode bending (MMB) test method. The mixed mode loading at the debond crack tip is controlled by changing the load application point in the MMB test fixture. Sandwich specimens were manufactured using H45 and H100 PVC foam cores and E-glass/polyester face sheets. All specimens were pre-cracked in order to define a sharp crack front. The static debond fracture toughness for each material configuration was measured at different mode-mixity phase angles. Fatigue tests were performed at 80% of the static critical load, at load ratios of R = 0.1 and 0.2. The crack length was determined during fatigue testing using the analytical compliance expression and verified by visual measurements. Fatigue crack growth results revealed higher crack growth rates for mode I dominated loading. For specimens with H45 core, the crack grew just below the face/core interface on the core side for all mode-mixities, whereas for specimens with H100 core, the crack propagated in the core or in the face laminate depending on the mode-mixity at the debond crack tip.     

Characterization of magnesium spot welds under tensile and cyclic loadings

Resistance spot welds of a magnesium alloy were characterized in terms of microstructure, hardness and monotonic and cyclic properties. Microstructural features in base metal and different zones in the weld region were discussed and the mechanical behavior of spot welds in tensile–shear configuration was studied. Effects of welding parameters were investigated on the micro- and macro-scale characteristics of magnesium spot welds. To this end, five sets of spot weld specimens were prepared, utilizing different welding parameters. The effect of cyclic loading was studied on microstructure and hardness of the base metal and the weld region, and it was shown that microstructural features do not change remarkably under cyclic loading. Fatigue crack initiation and propagation behavior was discussed for different specimen sets under both low and high cyclic loads. Fatigue cracks under high cyclic loading initiated close to the nugget edge, and decreasing the cyclic load nucleated the cracks farther from the nugget.     

Experimental investigation of fatigue crack growth behavior of GH2036 under combined high and low cycle fatigue

Fatigue crack growth rates have been experimentally determined for the superalloy GH2036 (in Chinese series) at an elevated temperature of 550 °C under pure low cycle fatigue (LCF) and combined high and low cycle fatigue (CCF) loading conditions by establishing a CCF test rig and using corner-notched specimens. These studies reveal decelerated crack growth rates under CCF loading compared to pure LCF loading, and crack propagation accelerates as the dwell time prolongs. Then the mechanism of fatigue crack growth at different loadings has been discussed by using scanning electron microscope (SEM) analyses of the fracture surface.     

Investigation of cracks found in helicopter longerons

Four cracked longerons, containing a total of eight cracks, were provided for study. Cracked regions were cut from the longerons. Load was applied to open the cracks, enabling crack surface examination. Examination revealed that crack propagation was driven by fatigue loading in all eight cases. Fatigue crack initiation appears to have occurred on the top edge of the longerons near geometric changes that affect component bending stiffness. Additionally, metallurgical analysis has revealed a local depletion in alloying elements in the crack initiation regions that may be a contributing factor. Fatigue crack propagation appeared to be initially driven by opening-mode loading, but at a crack length of approximately 0.5 in. (12.7 mm), there is evidence of mixed-mode crack loading. For the longest cracks studied, shear-mode displacements destroyed crack-surface features of interest over significant portions of the crack surfaces.     

Thermoelasticity and CCD analysis of crack propagation in AA6082 friction stir welded joints

The advantages of friction stir welding (FSW) process compared to conventional fusion welding technologies have been clearly demonstrated in recent years. In the present study, AA6082 FSW joints were produced by employing different welding parameters. The principal aim of this work is to apply thermoelastic stress analysis (TSA) to study crack propagation characteristics of friction stir welded aluminum sheets, during cyclic fatigue tests. The crack propagation experiments were performed by employing single edge notched specimens; fatigue tests were performed under tension with load ratio R = 0.1. All the mechanical tests were conducted up to failure. The TSA measurement system allowed crack evolution to be observed in real-time during fatigue cycles and stress fields to be derived on the specimens from the measured temperature variation. The thermoelastic data were used to analyse principal stresses and principal strains on the specimens surface and the crack growth rate during tests. In addition, it was possible to evaluate all the joints defects effects, as a function of welding parameters, correlating effects on different crack growth rate and instabilities. The achieved results were compared with those obtained by classical CCD camera monitoring of crack front propagation during cyclic loading and all the results were validated by employing finite element analysis performed with ABAQUS software.     

裂纹止裂技术及裂纹动态扩展规律研究

为了对动态荷载作用下水泥粉煤灰砂浆的裂缝动态扩展行为进行研究,提出了一种大尺寸带V型底边的半圆边裂纹(SECVB)试件,其V形底部具有止裂功能。SECVB试件的V形底部设计为180°,150°和120°三个角度。采用落锤冲击装置进行了冲击试验,并使用裂纹扩展计(CPG)用于测量裂纹扩展的相关参数。利用有限差分程序AUTODYN对裂纹扩展行为进行了数值模拟,并用有限元程序ABAQUS计算了裂纹的动态应力强度因子(DSIF);根据CPG测量的裂纹萌生时间和扩展时间来确定临界应力强度因子。试验和数值模拟结果表明,SECVB试件适合于研究动态荷载作用下水泥粉煤灰砂浆的裂纹扩展行为和止裂行为。在裂纹扩展过程中,裂纹可能在一段时间内止裂,并且裂纹在起始时刻的断裂韧度高于裂纹扩展时的断裂韧度。     

FATIGUE OF THICK-SECTION COLD-EXPANDED HOLES WITH AND WITHOUT CRACKS

Abstract— Fatigue tests under spectrum loading were conducted to evaluate hole cold-expansion in thick-section open-hole aluminium alloy specimens, some of which contained residual fatigue cracks before cold expansion. Cold expansion resulted in an increase in life by a factor of about 7. Small residual fatigue cracks did not inhibit the effectiveness of the cold-expansion process, indicating that it may not be essential to remove such cracks prior to hole cold-expansion. The increase in life is primarily associated with a reduced crack propagation rate compared with that for cracks from non-cold-expanded holes. Cold-expanded hole fractures displayed a marked disparity in crack depths adjacent to the two faces of the specimens. Considerable differences were evident in crack depths and fatigue crack areas at failure between cold-expanded and non-cold-expanded hole specimens. These findings have ramifications in the damage tolerance assessment of aircraft structures.     

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of HT80 steel at several stress ratios

Fatigue crack propagation rates and threshold stress intensity factors were measured for welded joints and base metal by using 200 mm wide centre-cracked specimens. The fatigue crack propagation properties of welded joints were similar in spite of the different zones in which the cracks propagated (ie, in the heat-affected zone and in the weld metal) and the different welding process used (submerged arc welding and gas metal arc welding). They were, however, inferior to those of the base metal. It was revealed by observation of the crack closure that the fatigue cracks were fully open during the whole range of loading, due to the tensile residual stress distribution in the middle part of the welded joints. This observation also explains the lack of a stress ratio effect on the fatigue crack propagation properties of welded joints, and their inferiority to those of the base metal.     

Fatigue crack propagation under in‐phase and out‐of‐phase biaxial loading

Fatigue damage characteristics of aluminium alloy under complex biaxial loads such as in‐phase and out‐of‐phase loading conditions and different biaxiality ratios have been investigated. The effects of microscale phenomena on macroscale crack growth were studied to develop an in‐depth understanding of crack nucleation and growth. Material characterization was conducted to study the microstructure variability. Scanning electron microscopy was used to identify the second phase particles, and energy dispersive X‐ray spectroscopy was performed to analyse their phases and elements. Extensive quasi‐static and fatigue tests were conducted on Al7075‐T651 cruciform specimens over a wide range of load ratios and phases. Detailed fractography analysis was conducted to understand the crack growth behaviour observed during the fatigue tests. Significant differences in crack initiation and propagation behaviour were observed when a phase difference was applied. Primarily, crack retardation and splitting were observed because of the constantly varying mode mixity caused by phase difference. The crack growth behaviour and fatigue lives under out‐of‐phase loading were compared with those under in‐phase loading to understand the effect of mixed‐mode fracture.