Crack initiation and crack propagation under thermal cyclic loading

Theoretical and experimental investigations of crack initiation and crack propagation under thermal cyclic loading are presented. For the experimental investigation a special thermal fatigue test rig has been constructed in which a small circular cylindrical specimen is heated up to a homogeneous temperature and cyclically cooled down under well defined thermal and mechanical boundary conditions by a jet of cold water. At the end of the cooling phase the specimen is reheated to the initial temperature and the following cycle begins. The experiments are performed with uncracked and mechanically precracked specimens of the German austenitic stainless steel X6CrNi 1811.

In the crack initiation part of the investigation the number of load cycles to initiate cracks under thermal cyclic load is compared to the number of load cycles to initiate cracks under uniaxial mechanical fatigue loading at the same strain range as in the cyclic thermal experiment. The development of initiated cracks under thermal cyclic load is compared with the development of cracks under uniaxial mechanical cyclic load.

In the crack propagation part of the investigation crack growth rates of semi-elliptical surface cracks under thermal cyclic loading are determined and compared to suitable mechanical fatigue tests made on compact-tension and four-point bending specimens with semi-elliptical surface cracks. The effect of environment, frequency, load shape and temperature on the crack growth rate is determined for the material in mechanical fatigue tests.

The theoretical investigations are based on the temperature distribution in the specimen, which is calculated using finite element programs and compared to experimental results. From the temperature distribution, elastic and elastic-plastic stress distributions are determined taking into account the temperature dependence of the material properties. The prediction of crack propagation relies on linear-elastic fracture mechanics. Stress intensity factors are calculated with the weight function method and crack propagation is determined using the Paris relation.

To demonstrate the quality of the crack growth analysis the experimental results are compared to the prediction of crack propagation under thermal cyclic load.     

FATIGUE UNDER CYCLIC COMPRESSIVE LOAD

For ultra-high strength steels and aluminium alloys, a fatigue crack could initiate from a notch tip under cyclic compressive load. The threshold value for fatigue crack initiation under compressive load can be as great as four times that under tensile load. The crack grew at a decreasing rate until eventually it stopped growing altogether under cyclic compressive load with a maximum length of 0.2-0.5 mm. If the minimum compressive load was near zero, i.e. compression to zero load cycling, the threshold value was near that under tensile loading and the compressive fatigue crack could continue to grow; however, the crack growth rate under compression to zero load fatigue was 10–100 times less than that under the tensile fatigue loading.     

Uniaxial and biaxial flexural fatigue of glass reinforced inter-penetrated network polymer composites

Conclusions Flexural fatigue of uniaxially and biaxially stressed IPN/glass mat composites was investigated using four point bend (4PB) and concentrically loaded (CL) specimen geometries. Regions of nearly constant bending moment between the inner spans of a 4PB beam and within the inner annulus of a CL circular plate yield quasi-uniform uniaxial and biaxial stress, respectively, on the tensile faces. The specimen dimensions were optimized for both loading geometries to give: (1) reduced specimen deflection through maximizing the ratio of the induced tensile stresses to the applied load, (2) minimized contact stresses by maximizing the induced stress with respect to the unit contact load, and (3) a large material volume exposed to the maximum cyclic stress (i.e., statistical fracture initiation).A power model was used to analyze the fatigue data for the 4PB and CL specimens. Both IPN composite materials studied fatigued more rapidly under the more severe loading conditions imposed by the CL specimen geometry.Fractography revealed that debond fracture was the dominant damage process for both geometries. The initial debond cracks were uniformly distributed throughout the stressed regions, confirming the presence of nearly uniform tensile stress. Damage localization followed after further cycling and was characterized by a locally high debond fracture density, fiber fracture, and always occurred where several glass strands crossed near the specimen surface. Final specimen failure resulted from the preferential growth of dominant cracks through the specimen thickness.     

Fatigue growth behavior for surface crack in welding joints under combined compressive and bending stresses

It has been demonstrated by experiments that crack can grow under cyclic compressive loading. However, it is difficult to observe and describe accurately by mathematical methods. In addition, cracks may close under compressive loading, which also increases the complexity of the problem. The fatigue growth behavior for surface cracks under biaxial loadings was studied by fatigue tests of HTS-A steel. According to experimental evidences, it is concluded that the transverse compressive stress not only changes the fracture morphology but also affect crack propagation life. Considering the influence of the compressive stress, this paper proposed an equivalent SIF and crack growth model subjected to compressive and bending stresses on the basis of McEvily formula. Finally, comparisons are made between prediction results and experimental data.     

Analysis of the fatigue failure of a mountain bike front shock

We present an analysis of a mountain bike front shock failure. The failure of the 1-year-old shock occurred catastrophically as the bike was ridden off of a 1-m drop. The failure was the result of fast fracture through both shock tubes at the location where the tubes were press fit into the shock upper crown. Examination of the fracture surfaces of the tubes revealed regions of fatigue crack growth that nearly penetrated the entire thickness of both tubes. An estimate of the forces during use, coupled with stress analysis, revealed three stresses near the fracture site—axial compression, bending, and hoop stresses. During operation, the axial compressive stress is negligible while the hoop and bending stresses are significant. Based on fracture mechanics, and an estimate of the bending stress from a 1-m drop, it is confirmed that the fatigue cracks present on the fracture surface were large enough to induce fast fracture. Prior to the existence of the fatigue cracks, the stresses were magnified locally near the fracture site by a significant stress concentration caused by the sharp transition from the shock tube to the crown. The fatigue cracks initiated at a circumferential location in the tube commensurate with high tensile bending stress and the stiffest region of the crown (highest stress concentration). Based on the evidence, the most probable cause of the bike shock fatigue failure was the shock design, which facilitated high local stresses during use.     

Fatigue crack retardation and closure in polymethylmethacrylate

The presented results are from an investigation of crack surface profiles and the influence of intermittent overloads on fatigue crack growth in polymethylmethacrylate, a transparent polymer. Fatigue cracks were grown in compact tension specimens under conditions of constant range in stress intensity factor. Tensile overloads were found to perturb subsequent crack growth in polymethylmethacrylate under certain conditions.

Examination of monochromatic light interference fringe patterns emanating from the fatigue cracks indicates that the crack perimeter was closed at zero load. The crack surfaces were displaced, however, in the interior of the specimen at zero load. Measurements of the crack opening displacements during loading revealed that a significant tensile load was required to displace the portions of the crack surfaces which Were initially closed. These observations are discussed in light of the Elber concept of crack closure.     

含多处损伤宽板螺接搭接件疲劳寿命研究

对含多处损伤（Multiple Side Damage,MSD）宽板搭接件做了等幅疲劳试验和断口分析,得到搭接件的疲劳寿命和孔边MSD裂纹的形成特点、裂纹前沿形状及扩展历程。结果表明,搭接件的疲劳破坏具有一定的隐蔽性,其疲劳寿命的绝大部分消耗在螺栓头下裂纹扩展阶段,当孔间裂纹出现首次连通时,搭接件剩余寿命约为总寿命的0.7%~9.4%。基于有限元软件FRANC2D/L和裂纹扩展分析软件AFGROW,建立了考虑钉载、第二弯矩和孔间裂纹干涉等影响因素的含MSD宽板搭接件疲劳寿命计算模型,并对孔边多裂纹的扩展寿命进行了计算分析。计算结果与试验结果的对比表明,该文所建寿命计算模型具有一定的精度,能满足工程需要,计算结果和结论可作为该类结构损伤容限设计的参考依据。     

The effect of initial notch shape of compact specimen on fatigue pre-cracking and fracture toughness testing

The circular notched compact specimens, along with standard specimens having straight or chevron notch are provided for fatigue and fracture toughness testings in order to study the crack observation capability during fatigue pre-cracking, skewness of the crack front, and the resulting fracture toughness K_Q. The test results indicated that circular notched specimens significantly facilitate the crack observation during fatigue testing as the cracks initiate on both surfaces of the specimen. No remarkable differences were observed on geometries of the fatigue crack front obtained and the resulting fracture toughness among these three types of specimen. The macroscopic observation of beach marks on the fracture surfaces revealed that, in the present material Ti-6Al-4V (ELI), the advance of only 1.3% of the whole crack length corresponded to the load level at which fracture toughness K_Q was evaluated.     

SHEAR FATIGUE CRACK GROWTH: A LITERATURE SURVEY

A fatigue crack is often initiated by a localized cyclic plastic deformation in a crystal where the active slip plane coincides with the plane of maximum shear stress. Once a crack is initiated, the crack will propagate on the maximum shear plane for a while and, in the majority of the cases, will eventually change to the plane of the applied tensile stress. The “shear” and “tensile” modes of fatigue crack propagation are termed stage I and stage II fatigue crack growth. They are also known as mode II and mode I fatigue crack growth. However, the mechanism of the tensile mode fatigue crack propagation is shear in nature. Considerable progress has been made recently in the understanding of mode II fatigue crack growth. This paper reviews the various test methods and related data analyses. The combined mode I and mode II elastic crack tip stress field is reviewed. The development and the design of the compact shear specimen are described and the results of fatigue crack growth tests using the compact shear specimens are reviewed. The fatigue crack growth tests and the results of inclined cracks in tensile panels, center cracks in plates under biaxial loading, cracked beam specimens with combined bending and shear loading, center cracked panels and the double edge cracked plates under cyclic shear loading are reviewed and analyzed in detail.     

Effects of repeated loading on the fracture behavior of notched TiAl specimens

Notched specimens of a fully lamellar TiAl alloy and a duplex TiAl alloy were in situ tensile tested with repeated loading-unloading-reloading processes in a scanning electron microscope (SEM). The step-by-step processes of initiation and extension of the main crack were captured by SEM. The fracture surfaces were observed and one-sector-to-one-sector related to the crack extension. Effects of loading-unloading damage on the fracture behavior were evaluated by combining the pictures of propagating crack configurations, corresponding fracture surfaces and the load locus. The results revealed the following events: (1) at the elastic regime, the loading-unloading process had negligible effect on the fracture behavior produced by subsequent reloading; (2) at the plastic regime, even at a value much lower than that of the preload, the reload extended further the existing main crack; (3) after a heavy loading-unloading process, the main crack extended and resulted in final fracture at a value of the reload, which was lower than that of the preload and (4) microcracks produced in the loading-unloading process had minor effects on the fracture behavior.     

Failure study of connecting shafts of a plug screw feeder in a paper production plant

The connecting shafts of two plug screw feeder units of a paper production plant failed within a relatively short period after the production start of the plant. To investigate the reasons for these failures, material characterizations of the broken shafts were performed. The program included tensile, impact and hardness tests and metallographical and fractographical investigations. Fracture surfaces of the failed shafts were studied using light and electron microscopy. Obvious striations and beach marks could be observed by fractographic examination. The fracture features observed on the fracture surfaces reveal that (a) the shafts had experienced fatigue crack growth before the final failure; (b) the crack growth initiated from a sharp undercut at the threaded part of the shaft; (c) the fatigue driving forces have combined torsional and bending loading, having torsional as the governing load.     

Composite-to-metal tubular lap joints: strength and fatigue resistance

The axial strength and fatigue resistance of thick-walled, adhesively bonded E-glass composite-to-aluminum tubular lap joints have been measured for tensile and compressive loadings. The joint specimen bonds a 63 mm OD aluminium tube within each end of a 300 mm long, 6 mm thick E-glass/epoxy tube. Untapered, 12.5 mm thick aluminium adherends were used in all but four of the joint specimens. The aluminum adherends in the remaining four specimens were tapered to a thickness of 1 mm at the inner bond end (the bond end where the aluminum adherend terminates). For all loadings, joint failure initiates at the inner bond end as a crack grows in the adhesive adjacent to the interface. Test results for a tension-tension fatigue loading indicate that fatigue can severely degrade joint performance. Interestingly, measured tensile strength and fatigue resistance for joints with untapered adherends is substantially greater than compressive strength and fatigue resistance.The joint specimen has been analyzed in two different ways: one approach models the adhesive as an uncracked, elastic-perfectly plastic material, while the other approach uses a linear elastic fracture mechanics methodology. Results for the uncracked, elastic-plastic adhesive model indicate that observed bond failure occurs in the region of highest calculated stresses, extensive bond yielding occurs at load levels well below that required to fail the joint, and a tensile peel stress is generated by a compressive joint loading when the aluminum adherends are untapered. This latter result is consistent with the observed joint tensile-compressive strength differential. Results of the linear elastic fracture mechanics analysis of a joint with untapered aluminum adherends are also consistent with the observed differential strength effect since a mode I crack loading is predicted for a compressive joint loading. Calculations and a limited number of tests suggest that it may be possible to selectively control the differential strength effect by tapering the aluminum adherends. The effect of adherend material and thickness on fracture mechanics parameters is also investigated. The paper concludes by examining the applicability of linear elastic fracture mechanics to the joints tested.     

Microstructure and fracture in three dimensions

A high resolution three dimensional (3D) scanning technique called X-ray microtomography was used to measure internal crack growth in small mortar cylinders under compressive loading. Tomographic scans were made at different load increments in the same specimen. 3D image analysis was used to measure internal crack growth during each load increment. Load–deformation curves were used to measure the corresponding work of the external load on the specimen. Fracture energy was calculated using a linear elastic fracture mechanics approach using the measured surface area of the internal cracks created. The measured fracture energy was of the same magnitude that is typically measured in concrete tensile fracture. A nominally bilinear incremental fracture energy curve was measured. Separate components for crack formation energy and secondary toughening mechanisms are proposed. The secondary toughening mechanisms were found to be about three times the initial crack formation energy.     

Subcritical crack-growth and lifetime behavior of glass and SiG under static load

Crack initiation and subcritical crack growth in glass sheet and SiC bar specimen under static loading were investigated to study the failure process. It has been demonstrated that the lifetime process of brittle materials involves three possible forms of crack growth: subcritical crack growth, partly subcritical crack growth and instantaneous fracture without subcritical crack growth. Curves of upsilon -K obtained in step-by-step static fatigue tests and in constant loading rate tests showed different trends for borosilicate glass sheet. alpha -SiC that is generally considered immune to mechanical fatigue effect and environmental attack was also tested under static loading and the lifetime was measured. The results showed that the threshold load to damage effect was over 80% of the initial strength for the SiC.     

SEMI-ELLIPTICAL FATIGUE CRACK GROWTH UNDER ROTATING OR REVERSED BENDING COMBINED WITH STEADY TORSION

Abstract— An analysis of the influence of steady torsion loading on fatigue crack growth rates under rotating or reversed bending is presented. Mixed-mode (I + III) tests were carried out on cylindrical specimens in DIN Ck45k steel and results are compared for two different testing machines: rotary bending and reversed bending obtained by cyclic Mode I (Δ K ₁) with or without superimposed static Mode III ( K _III) loading, simulating the real conditions on power rotor shafts where many failures occur. The growth and shape evolution of semi-elliptical surface cracks, starting from a chordal notch on the cylindrical specimen surface, was measured for several Mode III/ Mode I ratios. Results have shown that the steady Mode III loading superimposed on the cyclic mode I leads to a significant reduction in the crack growth rates. It is suggested that this retardation is related to an increase of plastic zone size near the cylindrical surface in association with the interlocking of rough fracture surfaces, friction and fretting debris, leading to a decrease of the ΔK effective at the crack tip profile due to the "crack closure effect". This work provides a contribution to a better understanding of crack growth rates under mixed-mode load conditions thereby allowing one to predict remaining lifetimes and to estimate the risks of pre-cracked rotor shafts.     

Subcritical Crack-Growth and Lifetime Behavior of Glass and SiC under Static Load

［1］H.F.Hardath: ‘A Review of Cumulative Damage‘ paper for AGARD, USA, 1965. ［2］P. Ostojic: J. Mater. Sci., 1995, 30, 3011. ［3］T.L.Anderson: Fracture Mechanics, CRC Press, USA,1995. ［4］K.Reifsnider and J.Duthoit:Concept of Fatigue Strength in Brittle Polymer and Ceramic Composite,in Fatigue‘99, eds. X.R.Wu and Z.G.Wang, 1999, 3,1647. ［5］S.M.Wiederhorn, E.R.Fuller and R.Thomson: Met.Sci, 1980, 14, 450. ［6］S.J.Dill, S.J.Bennison and R.H.Dauskardt: J. Am. Ceram. Soc., 1997, 80, 773. ［7］J.Mencik: Strength and Fracture of Glass and Ceramics, Elsevier, Amsterdam, 1992 . ［8］J.A.Salem, J.L.Shannon and R.C.Bradt: J. Am. Ceram. Soc., 1989, 72(1), 20. ［9］R.H.Dauskardt, D.B.Marshal and R.O.Ritchie: J. Am.Ceram. Soc., 1990, 73(4), 893.     

试验机压辊断裂原因分析

在对10CrNi4MoV钢原始态焊缝板状试样进行四点弯曲疲劳试验的过程中,试验机一上压辊发生了断裂。分别从压辊的材质、受力状态和断口形貌等方面对压辊的断裂原因进行了分析。结果表明：压辊发生断裂是因为试验中改变了压辊设计时的受力状态,使得压辊薄弱部位产生了应力集中,在外载荷周期作用下,最终成为疲劳裂纹源,加之压辊材料对裂纹比较敏感,最终导致了疲劳断裂。提出了重新设计压辊时应采取的措施。     

Effects of microstructural through‐thickness non‐uniformity and crack size on fatigue crack propagation and fracture of rolled Al‐7075 alloy

Some of the fatigue tests performed using the standard compact tension (CT) and a non‐standard specimen made of rolled 7075 aluminium alloy exhibit fatigue crack growth (FCG) lagging in a small region along the crack front. Through‐thickness microstructural evaluation shows that material grains in this region did not flatten as much as other regions. In the non‐standard specimen, surface cracks are either grown under fatigue loading or broken under monotonically increasing quasi‐static loads at different crack sizes. The aforementioned lagging also exists in a narrow region of 3‐D FCG for specimens with microstructural through‐thickness non‐uniformity. A more important feature for this type of specimen with surface crack is the deflection of fast fracture direction into the grain interfaces, namely from L‐T orientation to S‐L and S‐T directions. It is proved that this is due to significant levels of second principal stresses near the free surface for small cracks and lower fracture toughness of the material in S‐L and S‐T directions.     

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

A crack propagation criterion based on ΔCTOD measured with 2D‐digital image correlation technique

The fatigue cracks growth rate of a forged HSLA steel (AISI 4130) was investigated using thin single edge notch tensile specimen to simulate the crack development on a diesel train crankshafts. The effect of load ratio, R, was investigated at room temperature. Fatigue fracture surfaces were examined by scanning electron microscopy. An approach based on the crack tip opening displacement range (ΔCTOD) was proposed as fatigue crack propagation criterion. ΔCTOD measurements were carried out using 2D‐digital image correlation techniques. J‐integral values were estimated using ΔCTOD. Under test conditions investigated, it was found that the use of ΔCTOD as a fatigue crack growth driving force parameter is relevant and could describe the crack propagation behaviour, under different load ratio R.