A case for associational resistance: Apparent support for the stress gradient hypothesis varies with study system

The bending test is one of the most important tests that demonstrates the advantages of functional gradient (FGM) materials, thanks to the stress gradient across the specimen depth. In this research, the flexural response of functionally graded polymeric composite material (FGM) is investigated both experimentally and numerically. Fabricated by a hand lay-up manufacturing technique, the unidirectional glass fiber reinforced epoxy composite composed of ten layers is used in the present investigation. A 3-D finite element simulation is used to predict the flexural strength based on Hashin’s failure criterion. To produce ten layers of FGM beams with different patterns, the fiber volume fraction ( Vf%) ranges from 10% to 50%. A comparison between FGM beams and conventional composite beams having the same average Vf% is made. The experimental results show that the failure of the FGM beams under three points bending loading (3PB) test is initiated from the tensioned layers, and spread to the upper layer. The spreading is followed by delamination accompanied by shear failures. Finally, the FGM beams fail due to crushing in the compression zone. Furthermore, the delamination failure between the layers has a major effect on the rapidity of the final failure of the FGM beams. The present numerical results show that the gradient pattern of FGM beams is a critical parameter for improving their flexural behavior. Otherwise, Vf% of the outer layers of the FGM beams, i.e. Vf% = 30, 40, or 50%, is responsible for improving their flexural strength.

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Finite Element Modeling of Residual Stress at Joint Interface of Titanium Alloy and 17-4PH Stainless Steel

Metals ◽

10.3390/met11040629 ◽

2021 ◽

Vol 11 (4) ◽

pp. 629

Author(s):

Nana Kwabena Adomako ◽

Sung Hoon Kim ◽

Ji Hong Yoon ◽

Se-Hwan Lee ◽

Jeoung Han Kim

Keyword(s):

Residual Stress ◽

Finite Element ◽

Stainless Steel ◽

Finite Element Modeling ◽

Equivalent Stress ◽

Stress Gradient ◽

Joint Interface ◽

Element Modeling ◽

Actual Experiment ◽

Maximum Equivalent Stress

Residual stress is a crucial element in determining the integrity of parts and lifetime of additively manufactured structures. In stainless steel and Ti-6Al-4V fabricated joints, residual stress causes cracking and delamination of the brittle intermetallic joint interface. Knowledge of the degree of residual stress at the joint interface is, therefore, important; however, the available information is limited owing to the joint’s brittle nature and its high failure susceptibility. In this study, the residual stress distribution during the deposition of 17-4PH stainless steel on Ti-6Al-4V alloy was predicted using Simufact additive software based on the finite element modeling technique. A sharp stress gradient was revealed at the joint interface, with compressive stress on the Ti-6Al-4V side and tensile stress on the 17-4PH side. This distribution is attributed to the large difference in the coefficients of thermal expansion of the two metals. The 17-4PH side exhibited maximum equivalent stress of 500 MPa, which was twice that of the Ti-6Al-4V side (240 MPa). This showed good correlation with the thermal residual stress calculations of the alloys. The thermal history predicted via simulation at the joint interface was within the temperature range of 368–477 °C and was highly congruent with that obtained in the actual experiment, approximately 300–450 °C. In the actual experiment, joint delamination occurred, ascribable to the residual stress accumulation and multiple additive manufacturing (AM) thermal cycles on the brittle FeTi and Fe2Ti intermetallic joint interface. The build deflected to the side at an angle of 0.708° after the simulation. This study could serve as a valid reference for engineers to understand the residual stress development in 17-4PH and Ti-6Al-4V joints fabricated with AM.

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Facilitation Effects of Haloxylon salicornicum Shrubs on Associated Understory Annuals, and a Modified “Stress-Gradient” Hypothesis for Droughty Times

Plants ◽

10.3390/plants9121726 ◽

2020 ◽

Vol 9 (12) ◽

pp. 1726

Author(s):

Nasr H. Gomaa ◽

Ahmad K. Hegazy ◽

Arafat Abdel Hamed Abdel Latef

Keyword(s):

Species Richness ◽

Water Availability ◽

Stress Gradient ◽

Growing Season ◽

Annual Species ◽

Negative Effects ◽

Growing Seasons ◽

Dry Seasons ◽

The Impact ◽

Haloxylon Salicornicum

Perennial shrub-annual plant interactions play key roles in desert regions influencing the structure and dynamics of plant communities there. In the present study, carried out in northwestern Saudi Arabia, we examined the effect of Haloxylon salicornicum shrubs on their associated understory annual species across four consecutive growing seasons, along with a record of the seasonal rainfall patterns. We measured density and species richness of all the annual species in permanent quadrats located beneath individual shrubs, as well as in the spaces between shrubs. During wet growing season H. salicornicum shrubs significantly enhanced the density and species richness of sub-canopy species, whereas in the relatively dry seasons they exerted negative effects on the associated species. In all growing seasons, the presence of shrubs was associated with enhanced soil properties, including increased organic carbon content, silt + clay, and levels of nutrients (N, P and K). Shrubs improved soil moisture content beneath their canopies in the wet growing season, while in the dry seasons they had negative effects on water availability. Differences in effects of H. salicornicum on understory plants between growing seasons seem due to the temporal changes in the impact of shrubs on water availability. Our results suggest the facilitative effects of shrubs on sub-canopy annuals in arid ecosystems may switch to negative effects with increasing drought stress. We discuss the study in light of recent refinements of the well-known “stress-gradient hypothesis”.

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Effect of intrinsic stress gradient on the effective mode-I fracture toughness of amorphous diamond-like carbon films for MEMS

Journal of the Mechanics and Physics of Solids ◽

10.1016/j.jmps.2007.05.013 ◽

2008 ◽

Vol 56 (2) ◽

pp. 388-401 ◽

Cited By ~ 29

Author(s):

K JONNALAGADDA ◽

S CHO ◽

I CHASIOTIS ◽

T FRIEDMANN ◽

J SULLIVAN

Keyword(s):

Fracture Toughness ◽

Stress Gradient ◽

Carbon Films ◽

Intrinsic Stress ◽

Mode I ◽

Diamond Like Carbon ◽

Mode I Fracture ◽

Mode I Fracture Toughness

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Stress gradient and structural properties of atmospheric and reduced pressure deposited polysilicon layers for micromechanical sensors

Sensors and Actuators A Physical ◽

10.1016/0924-4247(95)01061-0 ◽

1995 ◽

Vol 51 (1) ◽

pp. 9-12 ◽

Cited By ~ 7

Author(s):

M.S. Benrakkad ◽

J.M. Lopez-Villegas ◽

J. Samitier ◽

J.R. Morante ◽

M. Kirsten ◽

...

Keyword(s):

Structural Properties ◽

Stress Gradient ◽

Reduced Pressure ◽

Micromechanical Sensors

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A Chemomechanical Model for Stress Evolution and Distribution in the Viscoplastic Oxide Scale During Oxidation

Journal of Applied Mechanics ◽

10.1115/1.4032796 ◽

2016 ◽

Vol 83 (5) ◽

Cited By ~ 5

Author(s):

Hailong Wang ◽

Shengping Shen

Keyword(s):

Oxide Scale ◽

Maximum Stress ◽

Stress Gradient ◽

Oxidation Time ◽

Stress Evolution ◽

Oxide Interface ◽

Temperature Oxidation ◽

Substrate Interface ◽

Oxide Substrate ◽

Chemomechanical Model

Using the location-dependent growth strain, a chemomechanical model is developed for the analysis of the stress evolution and distribution in the viscoplastic oxide scale during high-temperature oxidation. The problem of oxidizing a semi-infinite substrate is formulated and solved. The numerical results reveal high compressive stress and significant stress gradient. The maximum stress is at the oxide/substrate interface and the minimum stress at the oxygen/oxide interface in short oxidation time, while the maximum stress is no longer at the oxide/substrate interface in long oxidation time. The stress evolutions at different locations are also presented. The predicted results agree well with the experimental data.

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