diff --git a/src/pybamm/models/submodels/active_material/loss_active_material.py b/src/pybamm/models/submodels/active_material/loss_active_material.py
index ffba064d03..23a907b9cb 100644
--- a/src/pybamm/models/submodels/active_material/loss_active_material.py
+++ b/src/pybamm/models/submodels/active_material/loss_active_material.py
@@ -1,5 +1,5 @@
 #
-# Class for varying active material volume fraction, driven by stress
+# Class for varying active material volume fraction
 #
 import pybamm
 
diff --git a/src/pybamm/models/submodels/particle_mechanics/crack_propagation.py b/src/pybamm/models/submodels/particle_mechanics/crack_propagation.py
index 11dd8baf27..e0d21deecf 100644
--- a/src/pybamm/models/submodels/particle_mechanics/crack_propagation.py
+++ b/src/pybamm/models/submodels/particle_mechanics/crack_propagation.py
@@ -37,7 +37,7 @@ def __init__(self, param, domain, x_average, options, phase="primary"):
     def get_fundamental_variables(self):
         domain, Domain = self.domain_Domain
         phase_name = self.phase_param.phase_name
-        if self.x_average is True:
+        if self.x_average:
             if self.size_distribution:
                 l_cr_av_dist = pybamm.Variable(
                     f"X-averaged {domain} {phase_name}particle crack length distribution [m]",
@@ -47,8 +47,10 @@ def get_fundamental_variables(self):
                     },
                     scale=self.phase_param.l_cr_0,
                 )
+                l_cr_dist = pybamm.SecondaryBroadcast(
+                    l_cr_av_dist, f"{domain} electrode"
+                )
                 l_cr_av = pybamm.size_average(l_cr_av_dist)
-                l_cr_dist = pybamm.PrimaryBroadcast(l_cr_av_dist, f"{domain} electrode")
             else:
                 l_cr_av = pybamm.Variable(
                     f"X-averaged {domain} {phase_name}particle crack length [m]",
diff --git a/tests/unit/test_models/test_full_battery_models/test_lithium_ion/base_lithium_ion_tests.py b/tests/unit/test_models/test_full_battery_models/test_lithium_ion/base_lithium_ion_tests.py
index f728c12219..96af25766b 100644
--- a/tests/unit/test_models/test_full_battery_models/test_lithium_ion/base_lithium_ion_tests.py
+++ b/tests/unit/test_models/test_full_battery_models/test_lithium_ion/base_lithium_ion_tests.py
@@ -529,6 +529,7 @@ def test_well_posed_psd_mechanics(self):
         options = {
             "particle size": "distribution",
             "particle mechanics": "swelling and cracking",
+            "surface form": "algebraic",
         }
         self.check_well_posedness(options)
 
diff --git a/tests/unit/test_models/test_full_battery_models/test_lithium_ion/test_mpm.py b/tests/unit/test_models/test_full_battery_models/test_lithium_ion/test_mpm.py
index f69e96f7b5..c548aba490 100644
--- a/tests/unit/test_models/test_full_battery_models/test_lithium_ion/test_mpm.py
+++ b/tests/unit/test_models/test_full_battery_models/test_lithium_ion/test_mpm.py
@@ -78,21 +78,6 @@ def test_nonspherical_particle_not_implemented(self):
         with pytest.raises(NotImplementedError):
             pybamm.lithium_ion.MPM(options)
 
-    def test_loss_active_material_stress_negative_not_implemented(self):
-        options = {"loss of active material": ("stress-driven", "none")}
-        with pytest.raises(NotImplementedError):
-            pybamm.lithium_ion.MPM(options)
-
-    def test_loss_active_material_stress_positive_not_implemented(self):
-        options = {"loss of active material": ("none", "stress-driven")}
-        with pytest.raises(NotImplementedError):
-            pybamm.lithium_ion.MPM(options)
-
-    def test_loss_active_material_stress_both_not_implemented(self):
-        options = {"loss of active material": "stress-driven"}
-        with pytest.raises(NotImplementedError):
-            pybamm.lithium_ion.MPM(options)
-
     def test_reversible_plating_with_porosity_not_implemented(self):
         options = {
             "lithium plating": "reversible",
@@ -101,11 +86,6 @@ def test_reversible_plating_with_porosity_not_implemented(self):
         with pytest.raises(pybamm.OptionError, match="distributions"):
             pybamm.lithium_ion.MPM(options)
 
-    def test_stress_induced_diffusion_not_implemented(self):
-        options = {"stress-induced diffusion": "true"}
-        with pytest.raises(NotImplementedError):
-            pybamm.lithium_ion.MPM(options)
-
     def test_msmr(self):
         options = {
             "open-circuit potential": "MSMR",
@@ -186,25 +166,3 @@ def test_ec_reaction_limited_not_implemented(self):
         }
         with pytest.raises(NotImplementedError):
             pybamm.lithium_ion.MPM(options)
-
-
-class TestMPMWithMechanics:
-    def test_well_posed_negative_cracking_not_implemented(self):
-        options = {"particle mechanics": ("swelling and cracking", "none")}
-        with pytest.raises(NotImplementedError):
-            pybamm.lithium_ion.MPM(options)
-
-    def test_well_posed_positive_cracking_not_implemented(self):
-        options = {"particle mechanics": ("none", "swelling and cracking")}
-        with pytest.raises(NotImplementedError):
-            pybamm.lithium_ion.MPM(options)
-
-    def test_well_posed_both_cracking_not_implemented(self):
-        options = {"particle mechanics": "swelling and cracking"}
-        with pytest.raises(NotImplementedError):
-            pybamm.lithium_ion.MPM(options)
-
-    def test_well_posed_both_swelling_only_not_implemented(self):
-        options = {"particle mechanics": "swelling only"}
-        with pytest.raises(NotImplementedError):
-            pybamm.lithium_ion.MPM(options)