Creep crack growth characteristics of polypropylene film at various temperatures

Creep crack growth rates were measured using centrally cracked tension specimens of thin polypropylene film at various temperatures and stress levels. The creep crack growth rates were correlated with the stress intensity factor. The experimental results showed that there is the region of the minimum constant crack growth rate which occupies more than 75% of the total creep failure life. This steady or constant creep crack growth rate depends on the test temperature and the initial stress intensity factor. The constant creep crack growth rate characteristics were analyzed on the basis of the Arrhenius type thermally activated process. It is found that creep crack growth behavior is closely related to the crack tip opening displacement and the creep zone size.     

Numerical investigation of creep crack growth in cross‐weld CT specimens. Part II: influence of specimen size

A numerical investigation of the influence of specimen size on creep crack growth in cross‐weld CT specimens with material properties of 2.25Cr1Mo at 550 °C is performed. A three‐dimensional large strain and large displacement finite element study is carried out, where the material properties and specimen size are varied under constant load for a total of eight different configurations. The load level is chosen such that the stress intensity factor becomes 20 MPa √m regardless of specimen size. The creep crack growth rate is calculated using a creep ductility‐based damage model, in which the creep strain rate ahead of the crack tip perpendicular to the crack plane is integrated taking the degree of constraint into account. Although the constraint ahead of the crack tip is higher for the larger specimens, the results show that the creep crack growth (CCG) rate is higher for the smaller specimens than for the larger ones. This is due to much higher creep strain rates ahead of the crack tip for the smaller specimens. If, on the other hand, the CCG rate is evaluated under a constant C * condition, the creep crack growth rate is found to be higher for the larger specimens, except when the crack is located in a HAZ embedded in a material with a lower minimum creep strain rate; then, the creep crack growth rate is predicted to be higher for the smaller specimen. In view of these results, it is obvious that the size effect needs to be considered in assessments of defected welded components using results from CCG testing of cross‐weld CT specimens.     

Creep crack propagation in austenitic stainless steel at elevated temperatures

Creep crack propagation behavior at high temperature was investigated for type 304 stainless steel. The present experiment reveals that creep crack propagation is explained better in terms of K than in terms of σ_net. The rate of the creep crack propagation is represented by the Arrhenius equation. The activation energy is higher in the present experiment compared with the case of fatigue. Main crack extends by means of joining micro-cracks initiated at vicinity of the main crack tip. Creep rupture is occured when the value of stress intensity factor reaches its critical value which increases with decreasing temperature but independent of stress level. It is found that the creep rupture time is expressed as a function of initial stress and initial crack size, and good agreement is obtained between observed and calculated times to rupture.     

Time-dependent damage in carbon fibre-reinforced polymer composites

Time-dependent damage (matrix cracks) evolution in AS4/3501-6 cross-ply laminates was studied using constant strain rate and constant stress tests. First ply failure stress and strain as well as the matrix crack density at a given stress level were found to be strongly dependent on strain rate. Matrix crack density increased with creep time at a constant stress level. The compliance and creep rate of the laminate increased in the presence of these cracks. These results emphasize the importance of the knowledge of time-dependent damage evolution in a lamina/laminate of a polymer composite for reliable prediction of creep and creep rupture.     

Deterministic and probabilistic lifetimes from kinetic crack growth—generalized forms

The lifetime prediction methodology developed here is an addendum to and a generalization of that given earlier by Christensen and Miyano [Christensen RM, Miyano Y (2006) Int J Frac 137:77–87]. The previous results were not sufficiently general to model some of the results in the intermediate time ranges. The present results still retain the kinetic crack formalism but include more general forms that are in accordance with data. This new method admits both deterministic and probabilistic forms. Specific applications are given for creep rupture and constant strain rate programs. Possible applications are for any materials types whose very long term creep rupture behavior takes a power law form.     

Relationship between lifetime under creep and constant stress rate for polymer-matrix composites

The present article reviews two existing theoretical approaches for creep failure criteria of viscoelastic materials. One criterion is based on the continuum damage mechanics (CDM) and the other is based on the fracture mechanics extended to viscoelastic materials. Although both theoretical frameworks are based on different physical concepts, the deduced lifetime expressions turn out to be equivalent even though its parameters have different physical interpretation. It is proved that both theoretical frameworks, when extended to variable stress loading cases, imply the linear cumulative damage (LCD) law. Additionally the relationship obtained between the creep–rupture and constant stress rate until failure is very simple. Moreover this simple relationship is obtained independently by two different cumulative damage laws, which do not obey the LCD law, and by experimental evidence using published data for two different polymer-matrix composites (PMC). Finally a micromechanical model, used for creep–rupture of unidirectional composites, is extended for constant stress rate until failure to corroborate the simple relationship obtained between the creep–rupture and constant stress rate until failure.     

Durability of polymer matrix composites: Viscoelastic effect on static and fatigue loading

The structural applications of polymer matrix composites (PMC) demand lifetimes of 15, 25 and 50 years. However, the mechanical properties of these composites have a time dependent nature, i.e. strength and stiffness are time-dependent due to the hereditary nature (viscoelasticity) of polymers. In this context lifetime models for viscoelastic materials, i.e. energy-based criteria and fracture mechanics extended to viscoelastic media, are revised. These models are applied to predict the lifetime of composite materials under special cases of constant load (creep rupture) and constant stress rate to failure. It is verified that these lifetime theories predict similar relationship between creep failure and constant stress rate failure strength. Alternative approaches based on Strength Evolution Integral [Reifsnider KL, Stinchcomb WW. A critical element model of the residual strength and life of fatigue–loaded composite coupons. In: Hahn HT, editor. Composite materials: fatigue and fracture (ASTM STP 907). Philadelphia (PA): American Society for Testing and Materials; 1986. p. 298–313; Reifsnider KK, Case SC, Duthoi J. The mechanics of composite strength evolution. Compos Sci Technol 2000; 60:2539–46; Reifsnider KK, Case SC. Damage tolerance and durability in material systems. Wiley-Interscience; 2002] and on Linear Damage Accumulation (LCD) law confirm these results. In addition the LCD law was found to be generally unsatisfactory except for the special case of constant stress rate to failure. Accordingly this result validates the accelerated methodology proposed by [Miyano Y, McMurray M, Enyama J, Nakada M. Loading rate and temperature dependence on flexural fatigue behavior of a satin woven CFRP laminate. J Compos Mater 1994;28(13):1250–60; Miyano Y, Nakada M, McMurray MK, Muki R. Prediction of flexural fatigue strength of CRFP composites under arbitrary frequency, stress ratio and temperature. J Compos Mater 1997;31(6):619–38; Miyano Y, Nakada M, Kudoh H, Muki R. Prediction of tensile fatigue life for unidirectional CFRP. J Compos Mater 2000;34(7):538–50; Miyano Y, Nakada M, Sekine N. Accelerated testing for long-term durability of GFRP laminates for marine use. Compos: Part B 2004;35:497–502; Miyano Y, Nakada M, Sekine N. Accelerated testing for long-term durability of FRP laminates for marine use. J Compos Mater 2005;39(1):5–20], which is based on LCD law, to characterize long-term creep failure of polymer composites based on the constant stress rate failure strength curves.Finally a new formulation is proposed, based on Strength Evolution Integral, to predict of fatigue failure load for an arbitrary load ratio.     

Failure of brittle polymers by slow crack growth

The variation of crack velocity (V) with stress intensity factor (K _I) at the tip of a crack has been measured for an epoxy resin containing 42% by volume of irregularly-shaped silica particles. It has been found that at crack velocities above 10^–5 m sec^–1 the crack propagates primarily through the silica particles, whereas at velocities below this value, failure occurs primarily by particle pull-out. This variation in fracture mode is accompanied by a corresponding change in slope of the V(K) curve. Using data obtained from creep rupture experiments and the derived V(K) relationship, it has been possible to estimate the size of the inherent flaw in the composite. This was found to be approximately twice the average particle diameter which is also equal to the size of the largest particles (140 m). Fracture of the unfilled epoxy resin and the effect of environment upon slow crack growth in the composite have also been investigated.     

Plastic zone formation and fatigue crack extension during high cyclic bending of steels

In repeated high cyclic bending, with constant load amplitude, the size and the shape of the plastic zone preceding the propagating crack is controlled by local structural conditions near the tip rather than by stress intensity. No significant correlations were found between the experimentally determined sizes of plastic zone and the theoretically predicted values of Liu and Rice. The plastic zone sizes ahead of the propagating crack cannot be simply expressed as proportional to the rate of fatigue crack propagation, though a simple relationship exists between the rate and the stress intensity factor. The relationship given by Paris, dl/dN = QΔKⁿ, describes the rate of crack propagation only in a limited range of relative crack length, x < 0.5. The extent of this range depends on the structure and on the level of applied cyclic stress. Beyond this range, the Paris equation could not be applied and the crack propagation cannot be related to the stress intensity factor.     

Fatigue fracture of nylon polymers

The influence of glass fibres on the fatigue crack propagation rates of injection-moulded nylons has been determined. In contrast to previous results for unreinforced nylons, the cracking kinetics are independent of the oscillating load frequency. The fact that the crack growth rate per cycle is constant, when expressed in terms of the time under load, demonstrates that the contribution of creep crack extension is minimized by the glass fibres. Thus a true fatigue process is suggested for the fatigue fracture of the reinforced system, even when the glass fibres are preferentially aligned parallel to the crack growth direction. A complicating factor in characterizing the fatigue resistance of the glass-reinforced nylons is the tremendous influence of fibre orientation on crack growth rate. It is shown that the anisotropy problem can be handled by simply expressing the crack growth rate data in terms of the strain energy release rate rather than the usual stress intensity factor representation. Results for four different glass-filled nylons show that the diverse crack growth rates for cracking parallel versus perpendicular to the glass-fibre axes collapse on to individual strain energy release rate curves. Each single relationship therefore characterizes the fatigue fracture of the filled material and furthermore permits a prediction of the cracking rates for any glass-fibre orientation based upon the expected change in modulus. Finally it is demonstrated that the increased stress dependence of fatigue crack propagation (slope of the Paris plot) in filled nylons can be duplicated in unfilled samples under certain conditions. It is concluded that the fatigue fracture mechanism is matrix dominated in these chopped glass-fibre reinforced materials.     

Hold-time effects on high temperature fatigue crack growth in Udimet 700

Crack growth behaviour under creep-fatigue conditions in Udimet 700 has been studied, and the crack growth data were analysed in terms of the stress intensity factor as well as theJ-integral parameter. Crack growth behaviour is shown to depend on the initial stress intensity level and the duration of hold-time at the peak load. For stress intensities that are lower than the threshold stress intensity for creep crack growth, the crack growth rate decreases with increase in hold time even on a cycle basis, da/dN, to the extent that complete crack arrest could occur at prolonged hold times. This beneficial creep-fatigue interaction is attributed to the stress relaxation due to creep. For stress intensities greater than the threshold stress intensity for creep crack growth, the growth rate on a cycle basis increases with increase in hold time. For the conditions where there is no crack arrest, the crack growth appears to be essentially cycle-dependent in the low stress intensity range and time-dependent in the high stress intensity range. Both the stress intensity factor and theJ-integral are shown to be valid only in a limited range of loads and hold-times where crack growth rate increases continuously.     

Creep Crack Growth in Polypropylene Film with Various Crack Lengths at Diverse Stresses and Temperatures

Creep crack growth rates were measured using centrally cracked tension specimens of thin polypropylene film with different crack lengths at various stresses and temperatures. The creep crack growth rates were correlated with the stress intensity factor. There was the region of the minimum constant crack growth rate which occupied more than 70% of the total creep failure life. This constant creep crack growth rate characteristics were analyzed on the basis of the stress-dependent Arrhenius type thermally activated process.     

Investigation on the fatigue crack behavior of Zr702/TA2/Q345R composite plate with a crack normal to interface

In this paper, the effects of maximum load, load ratio, and average load on fatigue crack propagation of Zr702/TA2/Q345R composite plate with a crack normal to the interface are studied by experiment and finite element method. When crack propagates to the interface from the compliant material side, the crack growth rate decreases to the minimum at first. After crack penetrates through the interface, the fatigue crack growth rate accelerates continuously. When crack propagates to the interface from the stiff material side, the fatigue crack growth rate generally increases with the crack length. Regardless of the direction of crack growth, the increase of load ratio will weaken the difference of crack growth rate near the interface caused by material property mismatch. Finite element results show that elastic modulus mismatch significantly changes the variation of the stress intensity factor amplitude. All results demonstrate that crack growth rate is dependent on the competition of the stress intensity factor amplitude, the fatigue crack growth rate in the corresponding material, and the interface strength.     

Tensile creep behavior of a SiC-fiber/SiC composite at elevated temperatures

The tensile creep behavior of a SiC-fiber-reinforced SiC composite has been investigated in argon at temperatures of 1000–1300°C. The apparent stress exponents for creep of the composite and the apparent activation energies for creep increase with decrease in stress. The threshold stress approach can be used to treat the data. Creep of the CVI–SiC matrix controls the creep of the composite. The relationship between creep rate and the time to rupture can be described by the Monkman–Grant equation which provides a method of life prediction. The Larson–Miller parameter can also be used for creep-life prediction of the composite when the appropriate constant is selected.     

热循环蠕变试验法评价НЦУ铌合金表面复合防护涂层的性能

为进一步提高铌合金的高温性能,在НЦУ铌合金上分别制备了3种涂层:料浆法Si-Fe-Cr-Ti涂层,料浆法Si-B-Ti涂层以及以等离子喷涂Mo Si2为内层、硼和硅扩散层为外层的复合涂层,测定了他们在空气介质中的1 400℃等温蠕变和1 400~250℃热循环蠕变特征和持久寿命。试验表明:等离子喷涂/扩散复合涂层的持久寿命、断裂塑性和蠕变速率显著优于其他2种涂层,复合涂层表面出现蠕变裂纹后,当下阶段的蠕变速率几乎没有改变,大于70%的持久寿命服役在涂层表面有裂纹的情况下;应力较高下,等温蠕变速率高于热循环蠕变者;应力较低下,等温蠕变速率低于热循环蠕变者;持久寿命与蠕变速率对应,但规律相反;复合涂层中,涂层呈多层和多孔隙形式,阻止了裂纹扩展;复合涂层含有能够愈合缺陷的易熔化合物,不仅能填充裂纹,而且使得涂层中生成的裂纹顶端圆滑。     

Creep crack propagation at elevated temperature in a heat-resistant steel

The creep crack propagation behaviour of a 25 Cr-20 Ni heat-resistant steel at 1103 to 1163 K has been studied using a CT-specimen with a thickness of 3 to 9 mm. With increasing specimen thickness, the crack growth rates increase in the thickness range 6 to 9 mm but remain almost constant in the range 3 to 6 mm. The temperature dependence of crack growth rates can be related to a thermally activated process of creep crack propagation. A creep mechanism is suggested to be the rate controlling process of creep crack propagation. The activation energy of creep crack propagation increases with increasing stress intensity factor. The effect of microstructure on crack growth rates shows that the as-cast specimen has a much higher crack growth rate than specimens pre-aged for 1500 to 8000 h and the specimen aged for 5000 h has the optimum crack propagation resistance. The characteristics of creep crack propagation are explained by the variation of microstructure with ageing, especially the size, distribution and stability of secondary carbides and the morphology of eutectic carbides.     

Nano-mechanics based modeling of lifetime distribution of quasibrittle structures

The statistics of structural lifetime under constant load are related to the statistics of structural strength. The safety factors applied to structural strength must ensure failure probability no larger than 10^-6, which is beyond the means of direct verification by histogram testing. For perfectly brittle materials, extrapolation from the mean and variance to such a small tail probability is no problem because it is known that the Weibull distribution applies. Unfortunately, this is not possible for quasibrittle materials because the type of cumulative distribution function (cdf) has been shown to vary with structure size and shape. These are materials with inhomogeneities and fracture process zones (FPZ) that are not negligible compared to structural dimensions. A probabilistic theory of strength of quasibrittle structures failing at macro-crack initiation, which can be experimentally verified and calibrated indirectly, has recently been deduced from the rate of jumps of atomic lattice cracks governed by activation energy barriers. This paper extends this nano-mechanics based theory to the distribution of structural lifetime. Based on the cdf of strength and a power law for subcritical crack growth rate, the lifetime cdf of quasibrittle structures under constant loads is derived. The lifetime cdf is shown to depend strongly on the structure size as well as geometry. It is found that, for the creep rupture case, the mean structural lifetime exhibits a very strong size effect, much stronger than the size effect on the mean structure strength. The theory also implies temperature dependence of the lifetime cdf. For various quasibrittle materials, such as industrial ceramics and fiber composites, it is demonstrated that the proposed theory correctly predicts the experimentally observed deviations of lifetime histograms from the Weibull distribution.     

Creep crack initiation at a free edge of an interface between submicron thick elements

To clarify the mechanics of time-dependent crack initiation at an interface edge in submicron thick elements due to creep, delamination experiments are conducted using a micro-cantilever bend specimen with a tin/silicon interface edge. After the specimen time-dependently deforms under a constant load, a delamination crack is initiated at the Sn/Si interface edge. In addition, the steady state creep property of Sn is estimated by performing an inverse analysis using a finite element method based on creep deformation experiments conducted for different specimens. Stress analysis using the obtained creep property reveals that stress and strain rate singularities exist at the Sn/Si interface edge under creep deformation. The intensity of the singular field time-dependently increases as the creep region expands, and eventually it becomes a steady state. The stress and strain rate intensities at the steady state correlate well with the crack initiation life, which indicates that the singular stress field near the interface edge governs the creep crack initiation.