1,721,027 research outputs found
A new statistical software for the estimation of P-S-N curves in presence of defects: statistical models and experimental validation
Full text linkIt is well-known in the literature that fatigue failures originate from the weakest element within the component loaded volume. In particular, for metallic materials the fatigue crack typically nucleates from the most critical surface defect in the High-Cycle-Fatigue (HCF) region; whereas, it generally nucleates from the most critical internal defect in the Very-High-Cycle-Fatigue (VHCF) region, at stress amplitudes below the so-called 'transition stress amplitude'. Therefore, regardless of the fatigue region, the P-S-N curves must necessarily take into account the random distribution of the critical defect size. However, in the literature, there are few statistical models that can model the dependency between the fatigue life and the defect size or that can take into account the presence of different failure mechanisms (e.g., surface crack nucleation in HCF and internal crack nucleation in VHCF). In the present paper, a new software for the estimation of the P-S-N curves is proposed. The statistical P-S-N models recently proposed by the authors and implemented in the software are described in the paper. The procedure for the parameter estimation is also explained in detail and the software is finally validated with experimental datasets collected by the authors or available in the literature
LCF-HCF strain–life model: Statistical distribution and design curves based on the maximum likelihood principle
Full text linkIn the paper, the statistical distribution of the total strain and the fatigue life in the low cycle fatigue (LCF)–high cycle fatigue (HCF) life range is analytically derived starting from the Coffin–Manson and Morrow model. The maximum likelihood principle is exploited for parameter estimation, thus allowing to consider both failures and runout specimens. A straightforward procedure for the estimation of the design curves based on the likelihood ratio confidence lower bound has been also developed. The proposed model has been validated with literature datasets obtained by testing steel and aluminum alloys, proving its effectiveness and representing a reliable alternative to the currently adopted literature models
Resonance Frequency as an Indicator of the Damage in Carbon Composite Plates: Analysis on Composites Prepared with Conventional and Sustainable Resins Subjected to Impact Tests
Full text linkThis paper experimentally investigates the impact response of composite laminates made with conventional and bio-based epoxy resin. Drop tower impact tests were conducted at varying energy levels, including repeated low-energy impacts, to evaluate perforation resistance. The laminates’ residual strength and damage tolerance were assessed using the Damage Index (DI) and by analysing the resonance frequency variations through the Impulse Excitation Technique (IET). The study demonstrates a strong correlation between the DI and the resonance frequencies of the specimens, suggesting that IET can effectively track damage progression in composite laminates. Bio-based resin laminates exhibited higher energy absorption at perforation and lower damage progression during repeated impacts due to the higher ductility of the resin. This method of using resonance frequencies to assess impact damage progression directly in composite laminates throughout the IET technique has not been previously reported in the literature
Experimental and Numerical Investigation of a Lattice Structure for Energy Absorption: Application to the Design of an Automotive Crash Absorber
Full text linkIn this work, an experimental and numerical analysis of a lattice structure for energy absorption was carried out. The goal was to identify the most influencing parameters of the unit cell on the crushing performances of the structure, thus guiding the design of energy absorbers. Two full factorial plans of compression tests on cubic specimens of carbon nylon produced by fused deposition modeling (FDM) were performed. The factors were the beam diameter and the number of unit cells. In the first factorial plan, the specimen volume is constant and the dimensions of the unit cell are varied, while the second factorial plan assumes a constant size of the unit cell and the volume changes in accordance with their number. The results showed that the specific energy absorption increases with the diameter of the beam and decreases with the size of the unit cell. Based on these results, a crash absorber for the segment C vehicle was designed and compared with the standard component of the vehicle made of steel. In addition to a mass reduction of 25%, the improved crushing performances of the lattice structure are shown by the very smooth force-displacement curve with limited peaks and valleys
Size-effect in Very High Cycle Fatigue: A review
Full text linkIt is well-known that fatigue failures in VHCF originate from defects present within the risk-volume. Moreover, experimental results have shown that the larger the specimen size and the risk-volume, the shorter is the VHCF life. This is known as size-effect and is generally due to the statistical increment of the defect size with the material volume. In the paper, size-effect in VHCF is critically revised. The experimental results obtained by testing specimens with different sizes are analyzed and the methodologies proposed to model size-effect are discussed, with the aim of guiding the future research on size-effect in VHCF
Impact Response of Carbon Fiber Composites: Comparison Between Epoxy and Bio-Based Epoxy Matrices
Full text linkSize-Effects on the VHCF Response of Flat Metallic Specimens for Automotive Applications: Analysis of Fatigue Data with a Method Based on the Stress Gradient
No full textThe design against fatigue failures at very high number of cycles (VHCF) is fundamental to guarantee the integrity of components used in structural applications (aerospace, energy production, automotive). Experimental tests to assess the VHCF response of materials are generally carried out on small specimens with sizes that are significantly different from those of the components to be designed. Size-effect, which is widely known to affect the VHCF response, must therefore be properly modeled and accounted when components are designed to ensure their structural integrity. Size-effects in VHCF have been generally investigated by testing specimens with circular cross-sections and modeled by considering the probabilistic increment of the defect size with the loaded volume. In the present paper, ultrasonic fatigue tests have been carried out on hourglass flat specimens and larger dog-bone flat specimens to investigate size-effects. One aluminum alloy and four steels used for automotive applications have been tested. The experimental results have been analyzed with an innovative statistical model based on the weakest-link principle and on the stress gradient within the specimens, which does not require the size of the defect at the origin of the fatigue failure, generally not available if the specimen fails from the surface. Size-effects were found to significantly influence the VHCF response of the investigated materials, further confirming that it is strongly material dependent, even for flat specimens. A general rule for size-effect in VHCF of flat specimens was not found, proving that it must be properly experimentally verified and safely accounted when designing large components against VHCF failure
Static strength of brittle materials under multiaxial nonuniform stress states: A novel statistical model for assessing size effects
No full textIn this paper, a model for the assessment of size effects on quasi-static strength of brittle materials subjected to applied load inducing a nonuniform stress distribution within the material volume is proposed. An equivalent stress computed through finite element analysis is introduced in the model to account for the effect of the stress gradient within the loaded volume. The defect size distribution is modelled, differently from the literature, with the largest extreme value distribution. The proposed model has been validated with datasets available in the literature and obtained through bending tests on concrete specimens. For each type of test, the model permits to properly assess size effects. Due to its general formulation, a model for the strength variation with respect to the loaded volume, regardless of the testing type, was also obtained, further proving its effectiveness
Residual properties in damaged laminated composites through nondestructive testing: A review
Full text linkThe development of damage tolerance strategies in the design of composite structures constitutes a major challenge for the widespread application of composite materials. Damage tolerance approaches require a proper combination of material behavior description and nondestructive techniques. In contrast to metals, strength degradation approaches, i.e., the residual strength in pres-ence of cracks, are not straightforwardly enforceable in composites. The nonhomogeneous nature of such materials gives rise to several failure mechanisms and, therefore, the definition of an ulti-mate load carrying capacity is ambiguous. Nondestructive techniques are thus increasingly re-quired, where the damage severity is quantified not only in terms of damage extension, but also in terms of material response of the damaged region. Based on different approaches, many nonde-structive techniques have been proposed in the literature, which are able to provide a quantitative description of the material state. In the present paper, a review of such nondestructive techniques for laminated composites is presented. The main objective is to analyze the damage indexes related to each method and to point out their significance with respect to the residual mechanical perfor-mances, as a result of the working principle of each retained technique. A possible guide for future research on this subject is thus outlined
Influence of testing velocity on mechanical strength and failure modes of single-lap joints made with hot-melt adhesive and polyolefin substrates
No full textThe mechanical behaviour of adhesive joints under dynamic loadings is an active area of research due to their significant industrial applications. Furthermore, the absence of a unique adopted standard for the study of bonded joints under impact loading increases the scientific and practical interests in this topic. In this work, the static and the dynamic responses of adhesive joints, bonded with a polyolefin hot-melt adhesive (HMA), were experimentally investigated by means of Single Lap Joint (SLJ) specimens, tested using a tensile machine and an instrumented pendulum. The substrates used in this activity were made of a polypropylene copolymer with 10% in weight of talc. In the last decades, HMAs have been used in automotive application for bonding internal and external plastic components. Indeed, HMAs are capable to bond several kinds of materials, including plastics that are difficult to bond with other adhesives. The results of the mechanical tests show that there is a clear influence of the loading rate on the force-displacement diagram and on the maximum force for the tested adhesive. Failure modes were finally analysed and compared. A change in the failure mode was noticed: in quasi-static tests the SLJs failed cohesively within the adhesive, whereas in dynamic tests the SLJs failed adhesively, with low energy absorption
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