The Fracture Toughness of Adhesive-Bonded Joints

Fracture mechanics is related to adhesion theory and the testing of adhesive-bonded joints in the lap-shear configuration. The complexity of the stress field necessitates the strain energy release rate approach, which is followed to derive the relation for a lap-shear sample: G_c = P_c²/4b (dC/da). G_c is the fracture toughness (critical strain energy release rate), P_c is the breaking or crack instability load, a and b are crack lengths and widths, respectively, and C is the sample compliance for the Tap-shear sample with a crack of these dimensions at each loading edge. It was found that G_c ranged from 1.18 to 1.42 with an average value of 1.34 in.-lb./in.² for epoxy bonded aluminum strips (EPON 934 and Alcald 2024-T3). Evidence, in the form of photoelastic stress patterns, suggesting that crack extension occurs in the opening mode in lap-shear samples is presented and discussed.     

Fracture Toughness of Adhesive Joints

To define the influence of the processing variables on the resistance of epoxy joints to brittle crack extension during short loading times, the fracture toughness, g_ic, of the joints was measured as a function of the following variables:

1. Hardener type (TEPA vs. HHPA)

2. Ratio of hardener to resin content

3. Post-cure temperature

and 4. Joint geometry (thickness and width)

It was found that the toughness of the TEPA hardened system varied by a factor of four-to-one as the ratio of hardener to resin content and post-cure temperature varied within what might be considered reasonable limits for manufacturing. The toughness of the HHPA hardened system varied only over the middle half of this same range.

For both systems, toughness increased with joint thickness over the range of 2 to 50 mils.     

The Influence of Adherend Topography on the Fracture Toughness of Aluminium-Epoxy Adhesive Joints in Humid Environments

The adherend surface topography has a dramatic effect on the durability of structural bonds formed between aluminium and an epoxy adhesive. Systematic changes in the micro-roughness of an aluminium adherend were achieved using an ultra-milling technique to prepare surfaces with topographies ranging from ultra-flat to a sawtooth profile with a base angle of approximately 60° and a peak-to-valley depth of 10 μm. The fracture toughness of double-cantilever beam specimens, stressed in the Mode 1 direction and exposed to a humid environment was found to change by a factor of approximately one hundred as the sawtooth profile angle was increased. These changes in fracture toughness may be accounted for through complex interrelationships between moisture diffusion and interphase mechanical properties, each with a strong dependence on the surface micro-topography.     

The Influence of Adherend Topography on the Fracture Toughness of Aluminium-Epoxy Adhesive Joints in Humid Environments

The adherend surface topography has a dramatic effect on the durability of structural bonds formed between aluminium and an epoxy adhesive. Systematic changes in the micro-roughness of an aluminium adherend were achieved using an ultra-milling technique to prepare surfaces with topographies ranging from ultra-flat to a sawtooth profile with a base angle of approximately 60° and a peak-to-valley depth of 10 μm. The fracture toughness of double-cantilever beam specimens, stressed in the Mode 1 direction and exposed to a humid environment was found to change by a factor of approximately one hundred as the sawtooth profile angle was increased. These changes in fracture toughness may be accounted for through complex interrelationships between moisture diffusion and interphase mechanical properties, each with a strong dependence on the surface micro-topography.     

The Fracture Toughness of Epoxy-glass Bead Composites

Abstract

The plane strain fracture toughness of epoxy resins and glass bead filled epoxy composites has been investigated. It was found that the energy required for fracture depended primarily on the ability to dissipate energy in the polymer phase. At low temperatures, where the epoxy was relatively brittle, the addition of glass beads increased the fracture energy and induced roughness in the otherwise smooth fracture surface. At higher temperatures and/or increased catalyst concentration, the unfilled epoxy became more ductile, its fracture surface became rougher, and its fracture energy was increased. When the epoxy was ductile, the addition of beads tended to decrease the fracture energy because of a reduction of the amount of polymer on the fracture surface.

Adhesion of the matrix to the glass beads was only important when the polymer was ductile. Improved adhesion permitted the beads to constrain polymer flow and decrease the fracture energy. Poor adhesion permitted flow around the beads which required additional energy for crack propagation. At low temperatures, where the matrix was brittle, the additional constraints caused by adhesion appeared to make little difference.

Water absorption resulted in plasticizing the polymer, destroying the interface, and probably destroying the polymer near the interface. Short term immersion increased the toughness because of the additional ductility. Long term immersion tended to reduce the toughness. An effective coupling agent minimized this reduction, thereby showing that improved adhesion can improve the environmental stability and extend the useful life of the material.     

Improvement of the Adhesive Fracture Toughness of Bonded Aluminum Joints Using E-Glass Fibers at Cryogenic Temperature

Fracture toughness and crack resistance of aluminum adhesive joints were measured at the cryogenic temperature of ?150°C, with respect to the orientation and volume fraction of the E-glass fibers in the epoxy adhesive. Cleavage tests on the DCB (Double Cantilever Beam) adhesive joints were performed using two different test rates of 1.67 × 10^?2 and 8.33 × 10^?4 mm/s to observe the crack propagation trends. From the experiments, it was found that the DCB joints bonded with the epoxy adhesive reinforced with E-glass fibers not only showed a stable crack propagation with a low crack propagation speed, but also higher fracture toughness and crack resistance than those of the DCB joints bonded with the unreinforced epoxy adhesive at a cryogenic temperature of ?150°C.     

High-Temperature Fracture Toughness of Sapphire

The fracture toughness of sapphire with crack propagation parallel to the basal plane was measured from 1200° to 1500°C. Fracture surfaces of near c -axis fibers tested in tension were used to determine the values. The toughness was constant and equal to 1.4 ± 0.1 Mpa^.m^1/2 over the entire temperature range.     

Fracture Toughness of Metaphosphate Glasses

The fracture toughnesses and Young's elastic moduli of several metaphosphate glasses were measured by the indentation technique. The results show that the normal glasses lie along a K_Ic vs E line between (0.4, 40) and (0.7, 60) MPa-m¹¹² and GPa, respectively. The abnormal ZnO and MgO metaphosphates appear to be tougher and are above the line of the normal glasses.     

Degradation of Mode-I Fracture Toughness of CFRP Bonded Joints Due to Release Agent and Moisture Pre-Bond Contamination

An experimental investigation of the effects of pre-bond contamination on Mode-I fracture toughness of carbon fiber reinforced plastic (CFRP) bonded joints is presented in this paper. Two pre-bond contamination scenarios were considered; namely, the silicon-based release agent and moisture. The two contamination scenarios were realized in one of the composite substrates prior to bonding. The common characteristic of the two contamination scenarios is that they lead in the formation of defects in the form of weak bonds that cannot be detected by conventional non-destructive testing techniques. The contamination effects on Mode-I fracture toughness of the bonded joints were investigated by conducting mechanical tests on double-cantilever beam specimens and comparing the results with relative measurements taken from reference specimens. Prior to mechanical testing, the bonding quality of the specimens was tested using ultrasonic C-scan inspection. Both the release agent and moisture are found to significantly degrade the Mode-I fracture toughness of the joints. For the release agent, the effect was more significant for silicon concentrations over 5 at%; a complete lack of adhesion was observed for silicon concentrations over 7 at%. At low values of relative humidity, there was a small increase in Mode-I critical energy release rate while at larger values there is a decrease which reaches 26% for the higher relative humidity percentage. The results from the Non-Destructive Testing (NDT) tests verify the inability of conventional NDT to detect the defects resulting at the interface between the contaminated adherend's surface and the adhesive for both contamination scenarios.     

Short-Crack Fracture Toughness of Silicon Carbide

The fracture toughness of four different silicon carbides was measured using single-edge precracked beam (SEPB) and indentation/strength techniques. Two were development grades with similar microstructures and chemistries, and yet exhibited different fracture modes. The grade that exhibited a predominantly intergranular fracture had an SEPB fracture toughness (6.4 MPa√m) 88% higher than the one that showed primarily a transgranular fracture (3.4 MPa√m). The higher fracture toughness was associated with a modest increase in average strength (25%), although there was a significant increase in the Weibull modulus (11–32). Fracture toughness at short crack lengths was assessed by an indentation method that used fracture strengths, crack lengths at fracture, and a new method of estimating the constant δ that characterizes the residual driving force of the plastic zones based on the stable growth of the indentation cracks from the initial ( c ₀) to the instability ( c ^*) lengths. The results showed a rising crack-growth-resistance behavior for the grade exhibiting intergranular fracture, while the grade showing transgranular fracture had a flat crack-growth resistance. Tests on two commercial grades of silicon carbide showed similar behaviors associated with the respective fracture modes.     

Fracture Toughness of Chemically Vapor-Deposited Diamond

The fracture toughness of chemically vapor-deposited diamond is estimated by a Vickers indentation method. Freestanding diamond films of 400-μm thickness are produced with plasma-enhanced chemical vapor deposition and highly polished for indentation testing. Indentation testing was performed with a microhardness tester using a load range of 5 to 8 N. The average fracture toughness is estimated as 5.3 ± 1.3 MPA · m^1/2.     

High-Temperature Fracture Toughness of Duplex Microstructures

The temperature dependence of the fracture toughness of ceramics exhibiting duplex microstructures was studied relative to their single-phase constituents using two test methods: bend testing of chevron-notched beams, and the indentation-crack-length technique. The two materials systems studied were Al₂O₃: c -ZrO₂(Y) and A1₂O₃:Y₃A1_SO₁₂ (YAG), and the testing temperature ranged from room temperature to 1200°C. The study showed that in both systems the duplex materials showed higher toughness values than their single-phase constituents above 800°C. This result was attributed to the contribution of low-energy interphase boundaries to the overall composite toughness. Indentation crack length measurements gave toughness values and trends comparable to those determined by the chevronnotched beam method. By comparing the results of the two test methods it was possible to demonstrate that the indentation calibration constant (ξ) shows no significant temperature or material dependence. For the zirconia-containing materials, however, indentation at elevated temperatures is accompanied by significant localized plasticity, which suppressed the radial cracking. Under such conditions, some caution is warranted, since localized plasticity can lead to an overestimation of the fracture toughness.     

Dynamic Fracture Toughness of Piezoelectric Ceramics

Both dynamic and static three‐point bending fracture testings of 32 piezoelectric ceramic samples were performed in four different poling directions. A modified split Hopkinson pressure bar method with a polyvinylidene fluoride (PVDF)‐based gauge was utilized for the dynamic experiments. The loading rate greatly influenced the fracture toughness in two ways: the dynamic fracture toughness values were much higher than those of the static fracture toughness, and unlike the static fracture toughness, the influence of poling direction on the dynamic fracture toughness was not obvious.     

Effective Fracture Toughness of Microcracked Materials

Explicit analytical formulas are derived for the stress intensity factors at the tips of a main crack and of a microcrack for the two-dimensional case of a collinear microcrack. This configuration is used to derive an estimate of the toughness degradation due to microcracks linking up with an advancing main crack. The implications of this estimate for theoretical predictions of the toughening due to stress-induced micro-cracking are discussed.     

Fracture Toughness of Spray-Dried Powder Compacts

The strengths and fracture toughness values were measured for alumina powder compacts containing two different binder systems. Diametral compression was used to measure both the tensile strength and the fracture toughness (through-thickness notch). This methodology was very useful in linking processing parameters, such as binder choice and compaction stress, to the quality of the green bodies. Observations of the compact structure before and after fracture showed that the binders segregated to the region between the spray-dried granules. The presence of the excess binder in this region was linked to both the failure mode and the creation of secondary cracks.     

Mixed-Mode Fracture Toughness of Ceramic Materials

An experimental technique whereby pure mode I, mode II, and combined mode I-mode II fracture toughness values of ceramic materials can be determined using four-point bend specimens containing sharp, through-thickness precracks is discussed. In this method, notched and fatigue-precracked specimens of brittle solids are subjected to combined mode I-mode II and pure mode II fracture under asymmetric four-point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I-mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al₂O₃. Multiaxial fracture mechanics of the Al₂O₃ ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness K _Ic, the mode III fracture initiation toughness is 2.3 times higher than K _Ic. The predictions of fracture toughness and crack path based on various mixed-mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed-mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.     

Fracture Toughness of a Beta-SiC Whisker

Tensile fracture from an internal defect allowed measurement of the fracture toughness, K_Ic³ and fracture surface energy, y, for a single-crystal beta-SiC whisker with (111) crystallographic orientation in the tensile direction. Values of K_Ic=3.23 MPa.m^1/2 and γ=9.06 J/m² were obtained.