The area under load versus the load-line displacement record represents the sum of the stored potential energy, the released elastic energy, and the dissipated plastic energy necessary to grow a crack through a test specimen. When partitioned as presented in this paper, these energy components can be evaluated in terms of current elastic-plastic fracture parameters. The authors show that the elastic energy release rate, G, as calculated by measuring the appropriate component from the test record, is identical to G calculated from Kq. The plastic energy dissipated, as measured directly from the load-displacement record, represents a much higher value than would be indicated by current J-type calculations. In all cases examined, it is further shown that energy released or dissipated is strongly dependent on the size of the test specimen and the size of the initial crack length. Further, while the rolling orientation and prestraining of the steels investigated affect the plastic energy dissipation rate, there seems to be little effect on the elastic energy release rate.

The strain energy density (SED) method of determining the yield strength of adhesives and other materials that is based on the fundamental principles of mechanics is described in this paper. The results from two sets of tests are reported. In the first set, the yield strength of multiple specimens made of the adhesive FM 300K was determined in the dry/cold, moderate, and humid/hot environments; the strain rates were 10−2/s, 10−4/s, and 10−6/s. In the second set, single specimens of five other adhesives were tested in the dry/cold and humid/hot environments; the strain rate was 10−4/s. Fifty-three out of the 60 specimens have yield strengths calculated with the SED method that are equal to or lower than the values obtained with the 0.2% offset method suggested in the ASTM specifications. The SED method is most valuable when the stress-strain curve of the material exhibits a gradual transition from elastic to the inelastic behavior typical of polymers, aluminum, and some very high strength steels. To ensure the safety of bonded structures, the yield strength of adhesives should be determined with the SED method, which is based on the fundamental principles of mechanics, instead of on an arbitrary estimate of strain offset. Although applied in this paper to stress-strain curves of adhesives, the SED method may also discern the onset of yielding and instability in columns and beams. To ensure that the yield strength of adhesives for bonding load-bearing structures is not systematically overestimated, the authors recommend using the SED method in lieu of the 0.2% offset method.

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This paper describes a method of measuring the tensile properties of a bulk adhesive — including modulus of elasticity, yield stress and strain, and tensile strength and strain. The authors developed the method unaware of ASTM Test Method for Tensile Properties of Thin Plastic Sheeting (D 882). Not surprisingly, requirements founded on common sense are the same in the ASTM Standard as in the present study. Augmenting ASTM D 882, this paper offers suggestions on (1) building tensiometers for low-load tests including a load cell and displacement-controlled loading apparatus, (2) building environmental chambers housing multiple tensiometers per chamber, (3) controlling relative humidity and temperature, (4) casting adhesive sheets, (5) cutting strip specimens from the sheets and preparing them for testing, (6) conditioning the specimens, (7) applying displacements with a computer-controlled stepper motor, and (8) acquiring data automatically. The need to carefully control the environment and strain rate is shown with stress-strain curves that were measured at different relative humidities (15, 50 and 85%), temperatures (-15, 30 and 50 °C) and initial strain rates (10-2, 10-4, 10-6 and 10-8 s-1). Five series of tests were conducted to determine the effects of batch, loading method, environment, strain rate, and adhesive. The results show that the testing system is highly reliable. Stress-strain curves of replicate specimens tested in the same environment and at the same strain rate are highly reproducible. The testing system is suited particularly well for thin film specimens of any material whose tensile properties are affected by relative humidity and temperature such as epoxy, acrylic, gelatin, photographic emulsion, and many kinds of paints and plastics.